Power Electronics

Outline
I. What is power electronics?

II. The history

III. Applications

IV. A simple example

V. About this course

I. What is power electronics?
1) Definition

2) Relation with information electronics

3) The interdisciplinary nature

4) Position and significance in the human society

1) Definition
• Power Electronics:
is the electronics applied to conversion and control of
electric power.

Range of power scale :

milliwatts(mW) megawatts(MW) gigawatts(GW)

• A more exact explanation:

The primary task of power electronics is to process and

control the flow of electric energy by supplying voltages
and currents in a form that is optimally suited for user
loads.
 Power Electronic Systems

What is Power Electronics ?

A field of Electrical Engineering that deals with the application of power
semiconductor devices for the control and conversion of electric power

sensors
Input
Source Power Electronics Load
- AC Converters
- DC Output
- unregulated - AC
- DC
POWER ELECTRONIC
CONVERTERS – the heart
of power a power
Reference Controller electronics system
 Power Electronic Systems
Important: very low
power loss
Why Power Electronics ? (efficient: 9x%, Inverters)

sensors
Input
Source Power Electronics IDEALLY LOSSLESS
Load!
- AC Converters
- DC Output
- unregulated - AC
- DC

Reference Controller
 Power Electronic Systems

Why Power Electronics ?

Other factors:
• Improvements in power semiconductors fabrication
• Power Integrated Module (PIM), Intelligent Power Modules
(IPM)

• Decline cost in power semiconductor

• Advancement in semiconductor fabrication

• ASICs • FPGA • DSPs
• Faster and cheaper to implement complex algorithm
 Relation with multiple disciplines

Systems& Signal
Circuit Control theory processing
theory
Control Simulation &
computing
Electric theory Power
machines electronics electronics
electronics

Power electr Solid state

systems physics
oni
Electromagnetics

8
Conversion of electric power
Electric Other names for electric
Power Power Power
input output power converter:
Converter -Power converter
-Converter
Control -Switching converter
input -Power electronic circuit
-Power electronic converter

Two types of electric power Changeable properties in conversion

DC(Direct Current) Magnitude

Frequency, magnitude,
AC (Alternating Current)
number of phases
Classification of power converters
Power
output
Power DC AC
input
AC to AC converter
AC AC to DC converter ( Fixed frequency : AC controller
(Rectifier) Variable frequency: Cycloconverter
or frequency converter)

DC DC to DC converter DC to AC converter
(Chopper) (Inverter)
 Power electronic system
Generic structure of a power electronic system

Power Power Power

input output
Converter

Control input
Feedforward/Feedback Feedback/Feedforward
Controller
( measurements of input signals ) ( measurements of output signals )
Reference
(commanding)

Control is invariably required.

Power converter along with its controller including the
corresponding measurement and interface circuits, is
also called power electronic system.
Typical power sources and loads for a
power electronic system

Power input Power output

Source Power Load
Vi ii Converter io Vo

-Electric utility -Electric Motor

Feedback/
-battery -light
-other electric energy source Feed forward
-power converter -heating
Controller -power converter
Reference -other electric or
electronic equipment

• The task of power electronics has been recently

extended to also ensuring the currents and power
consumed by power converters and loads to meet
the requirement of electric energy sources.
 2) Relation with information electronics
• A Classification of electronics by processing object
Information electronics: to process information
Electronics
Power electronics: to process electric power

Other classifications of electronics

Vacuum electronics: using vacuum devices,
e.g, vacuum tubes devices
Electronics
Solid (Solid state) electronics: using solid state devices,
e.g, semiconductor devices

Physical electronics: physics,material,fabrication,

and manufacturing of electronic
Electronics devices
Applied electronics: application of electronic
devices to various areas
3) The interdisciplinary nature
William E. Newell’s description
Electronics Power

Sta er e
po
i ts

w
tic quip
cu

&r
,cir

ot a ent
es
Power

ti n
vic

m
g
De
Electronics

Continuous,
discrete
Control

Power electronics is the interface between

electronics and power.
Relation with multiple disciplines

Systems & Signal

Control theory processing
Circuit
Simulation &
theory
computing

Electric Power
machines Electronics
electronics

Power Solid state

systems physics
Electromagnetics

• Power electronics is currently the most active

discipline in electric power engineering worldwide.
 Smart Power System

04/06/23 16
 4) Position and significance
in the human society
• Electric power is used in almost every aspect and everywhere
of modern human society.

• Electric power is the major form of energy source used in

modern human society.

• The objective of power electronics is exactly about how to

use electric power, and how to use it effectively and
efficiently, and how to improve the quality and utilization of
electric power.

• Power electronics and information electronics make two

poles of modern technology and human society——
information electronics is the brain,and power electronics is
the muscle.
 Power Electronics Components
• Power electronics combines power, electronics and
control.
– It may be defined as the application of solid-state
electronics for the control and conversion of electric
power.
 II. The history
Applicat
Applica ion of
fast-switching
Invention of fully-controlled
Thyristor semiconductor
devices
GTO
GTR IGBT
Mercury arc rectifier Power diode Power MOSFET Power MOSFET
Vacuum-tube rectifier Thyristor Thyristor Thyristor
Thyratron (microprocessor) (DSP)
1900 1957 mid 1970s late 1980s

Pre-history 1st phase 2nd phase 3rd phase

• The thread of the power electronics history precisely follows and matches
the break-through and evolution of power electronic devices
III. Applications
• Industrial
• Transportation
• Utility systems
• Power supplies for all kinds of electronic equipment
• Residential and home appliances
• Space technology
• Other applications
 Examples of Some
Applications

04/06/23 21
Relation with multiple disciplines

22
Device Applications
 Importance
Increasing applications of Power Electronic Equipment in Power Systems
Availability of high power
semiconductor devices Control Center
Decentralized renewable Central Power
Solar Power Plants
energy generation sources Station
 CHP House
Increased power transfer
Combined Heat and Power Wind Power Plants
with existing transmission Plant (CHP)  Village
system Factory  Commercial
Commercial Building Building
Effective control of power House
flow needed in a Apartment Building
Micro-Turbine
deregulated environment  Hospital
Fuel Cell
Norms for Power quality  Smart House  Commercial
 Performance Building
Building

Future Power System

24
 Power Ratings of Devices

04/06/23 25
 Ratings of Power Devices

04/06/23 26
 Devices Symbols and Characteristics

04/06/23 27
 Devices Symbols and Characteristics

04/06/23 28
 Control Characteristics of Devices

04/06/23 29
 Control Characteristics of Devices

04/06/23 30
 Switching Characteristics

04/06/23 31
Industrial applications

• Motor drives
• Electrolysis
• Electroplating
• Induction heating
• Welding
• Arc furnaces and ovens
• Lighting
Transportation applications

• Trains & locomotives

• Subways
• Trolley buses
• Magnetic levitation
• Electric vehicles
• Automotive electronics
• Ship power systems
• Aircraft power systems
 Utility systems applications
• High-voltage dc transmission(HVDC)
• Flexible ac transmission(FACTS)
• Static var compensation & harmonics
suppression: TCR, TSC, SVG, APF
• Custom power & power quality
control
• Supplemental energy sources :
wind, photovoltaic, fuel cells
• Energy storage systems
Power supplies for electronic equipment

• Telecommunications

• Computers

computer
server
• Office equipment

• Electronic instruments

• Portable or mobile
electronics
Telecommunication
Residential and home appliances

• Lighting
• Heating
• Air conditioning
• Refrigeration & freezers
• Cooking
• Cleaning
• Entertaining
Applications in space technology
• Spaceship power
systems

• Satellite power systems

• Space vehicle power

systems
Other applications
• Nuclear reactor
control

• Power systems for

particle accelerators

• Environmental
engineering
Trends
• It is estimated that in developed countries now 60% of
the electric energy goes through some kind of power
electronics converters before it is finally used.

Power electronics has been making major

contributions to:
--better performance of power supplies and better control of
electric equipment
reduction of energy consumption leads to less pollution
--energy saving
reduction of pollution produced by power converters
--environment protection
direct applications to environment protection technology
• Power source
• Controllers (PLC, Motion controller, robot controller, HMI controller)
• Motors
• Inverters and motor amplifiers (pack, drives)
• Sensors
• Robot (6 servo motors)
• Conveyor
• Camera
• Sucker
• HMI
• Bus (CC Link, Ethernet, Profibus, …)
• Accessories (buttons, alarming lights, safety sensor, …)
 IV. A simple example
A simple dc-dc converter example

How many methods:

Important: very low 1.??
power loss Input source:100V 2.??
(efficient: 9x%, Inverters) 3.??
Output load:50V, 10A, 500W
How can this converter be realized?
Dissipative realization

Resistive voltage divider

Dissipative realization
Series pass regulator:
transistor operates in active region
Use of a SPDT switch

SPDT: Single pole double throw

 The switch changes the dc voltage level

Advantages PWM
1) Very low power loss P =U*I
-ON: U = 0 => P = 0
-OFF: I = 0 => P = 0
2) Device is ON (possibly max) for short time =>
Disadvantages PWM increase over rate (LED)
1) EMF & noise due to switching 3) Digital form => easy for MCU, PLC
2) Damage to some attached devices 4) Take advantage of peak part load efficiency at
(bearing) due to pulsating current certain voltage => On for load at that certain voltage
5) Respond much more quickly (fast switching
between ON  OFF => change DC level much faster
=> control and dynamic response.
Addition of low pass filter
Addition of (ideally lossless) L-C low-pass filter, for
removal of switching harmonics:

Choose filter cutoff frequency f0 much smaller than

switching frequency fs
This circuit is known as the “buck converter”
Addition of control system for
regulation of output voltage
 Major issues in power electronics
• How to meet the requirement of the load or gain better
control of the load

• How to improve the efficiency

--for reliable operation of power semiconductor devices
--for energy saving

• How to realize power conversion with less volume, less

weight, and less cost

• How to reduce negative influence to other equipment in the

electric power system and to the electromagnetic
environment
Power Electronics
Lecture Outline

50
 Power Electronic Devices
• The power Electronic devices provides the utility of
switching.

• The flow of power through these devices can be

controlled via small currents.

• Power electronics devices differ from ordinary

electronics devices in terms of their characteristics.

51
 Power Electronic Devices
• Power Semiconductor Devices can be classified into
three groups according to their degree of
controllability.

– Diodes (on and off controlled by power circuit)

– Thyristors (latched on by control signal but must be
turned off by power circuit)
– Controllable Switches (turned on and off by control
signal)

52
 Diode

• A p-n junction diode is formed by placing p and n type

semiconductor materials in intimate contact on an
atomic scale.
 The PN Junction in Steady State

Metallurgical
Na Junction Nd
+ + + + + +
- - - - - -
- - - - - - + + + + + +
P - - - - - - + + + + + + n
- - - - - -
+ + + + + +
- - - - - -
+ + + + + +

Space Charge
ionized Region ionized
acceptors donors

E-Field
_ _
+ +
h+ drift = h+ diffusion e- diffusion = e- drift
Reverse
55

Bias
Forward Bias

56
Diode Characteristics

57
 Diode Equation

Where,
Is = Reverse saturation current ( Amps)
v = Applied forward voltage across the device
(volts)
q = Change of an electron
k = Boltzmann's constant
T = Temperature in Kelvin

58
 Power Diode
• Power semiconductor diode
is the “power level” counter
part of the “low power signal
diodes”.

• The symbol of the Power

diode is same as signal level
diode. However, the
construction and packaging is
different.

59
 Power Diode
• Power dides are required to carry up to several KA of current
under forward bias condition and block up to several KV
under reverse biased condition.

• Large blocking voltage requires wide depletion layer.

• This requirement will be satisfied in a lightly doped p-n

junction diode of sufficient width to accommodate the
required depletion layer.

• Such a construction, however, will result in a device with high

resistively in the forward direction.

• If forward resistance (and hence power loss) is reduced by

increasing the doping level, reverse break down voltage will
reduce. 60
 Power Diode
• These extreme requirements call for important structural changes
in a power diode which significantly affect their operating
characteristics.
• This apparent contradiction in the requirements of a power diode is
resolved by introducing a lightly doped “drift layer” of required
thickness between two heavily doped p and n layers.

61
 Switching Characteristics of Power Diodes
• Power Diodes take finite time to make transition from reverse bias
to forward bias condition (switch ON) and vice versa (switch OFF).

• Behavior of the diode current and voltage during these switching

periods are important due to the following reasons.

– Severe over voltage / over current may be caused by a diode switching

at different points in the circuit using the diode.

– Voltage and current exist simultaneously during switching operation of

a diode. Therefore, every switching of the diode is associated with
some energy loss. At high switching frequency this may contribute
significantly to the overall power loss in the diode.

62
 Turn On Characteristics
• Diodes are often used in
circuits with di/dt limiting
inductors.

• The rate of rise of the

forward current through
the diode during Turn ON
has significant effect on
the forward voltage drop
characteristics.

63
 Turn On Characteristics
• It is observed that the forward
diode voltage during turn ON
may transiently reach a
significantly higher value V Fr
compared to the steady state
voltage drop at the steady
current IF. VF ~ 1V when diodes
fully ON

• Forward recovery time, tFR is the

time required for the diode
voltage to drop to a particular
value after the forward current
starts to flow. Unlike normal diodes, VFr can be
much bigger than VF ~ 1V64during
transient from OFF->ON
 Turn Off Characteristics
• The diode current does not Unlike normal diodes, IRR big enough
comparable to IF during transient
stop at zero, instead it grows period => can cause damage

in the negative direction to Irr

called “peak reverse recovery
current” which can be
comparable to IF.

• Voltage drop across the diode

does not change appreciably
from its steady state value till
the diode current reaches
reverse recovery level.
Unlike normal diodes, VRR can be
larger than PIV of normal diodes 65
during transient => cause damage
 Turn Off Characteristics
• The reverse recovery
characteristics shown is
typical of a particular type of
diodes called “normal
recovery” or “soft recovery”
diode.
• The total recovery time (trr)
in this case is a few tens of
microseconds.

66
 Turn Off Characteristics
• This is acceptable for line frequency rectifiers (these diodes
are also called rectifier grade diodes).

• High frequency circuits (e.g PWM inverters) demand faster

diode recovery.

67
 Types of Diodes
• Depending on the application requirement various types
of diodes are available.

– Schottky Diode

– Fast Recovery Diode

– Line Frequency Diode

 Types of Diodes

– Schottky Diode

– These diodes are used where a low forward voltage drop

(typically 0.3 v) is needed.
– These diodes are limited in their blocking voltage
capabilities to 50v- 100v.
 Types of Diodes

– Fast Recovery Diode

– These diodes are designed to be used in high frequency

circuits in combination with controllable switches where a
small reverse recovery time is needed.

– At power levels of several hundred volts and several

hundred amperes such diodes have trr rating of less than
few microseconds.
 Types of Diodes

– Line Frequency Diode

– The on state of these diodes is designed to be as low as

possible.

– As a consequence they have large trr, which are acceptable

for line frequency applications.

Comparison between different types of Diodes

General Purpose Fast Recovery Schottky Diodes

Diodes Diodes
Up to 6000V & Up to 6000V and Up to 100V and
3500A 1100A 300A
Reverse recovery Reverse recovery Reverse recovery
time – High time – Low time – Extremely
low.

trr  25 s trr  0.1 s to 5 s trr  a few nano sec

72
 Comparison between different types
of Diodes

General Purpose Fast Recovery Schottky Diodes

Diodes Diodes
Turn off time – Turn off time – Low Turn off time –
High Extremely low
Switching Switching Switching
frequency – Low frequency – High frequency – Very
(Max 1KHz) (Max 20KHz) high.
(Max 30KHz)
VF  0.7 to 1.2V VF  0.8 to 1.5V VF  0.4 to 0.6V
73
Diode Circuits

74
Lecture Outline

75
 tRR and IRR Calculations
• In practice, a design engineer frequently needs to calculate tRR and IRR .
• This is in order to evaluate the possibility of high frequency switching.
• As a thumb rule, the lower tRR the faster the diode can be switched.

76
 Example-1
• The manufacturer of a selected diode gives the rate of fall of the
diode current di/dt=20 A/μs, and its reverse recovery time trr
=5μs. What value of peak reverse current do you expect?

SOLUTION. The peak reverse current is given as:

The storage charge QRR is calculated as:

77
 Example-1 (contd…)

Hence,

78
 Example-2
• The current waveform passing through a diode switch in a switch
mode power supply application is shown in following figure. Find
the average, rms, and the peak current.

SOLUTION. The current pulse duration is shown to be 0.2 ms within a period

of 1 ms and with a peak amplitude of 50 A. Hence the required currents are:

79
 Snubbers

• In general, snubbers are used for:

– turn-on: to minimise large overcurrents through the device at turn-on

– turn-off: to minimise large overvoltages across the device during turn-off.

– Stress reduction: to shape the device switching waveform such that the
voltage and current associated with the device are not high
simultaneously.

• Switches and diodes requires snubbers. However, new generation of IGBT,

MOSFET and IGCT do not require it.

80
 Snubber Circuits for Diode
• Snubber circuits are essential for diodes used in switching
circuits.
• It can save a diode from overvoltage spikes, which may arise
during the reverse recovery process.

• A very common snubber circuit

for a power diode consists of a
capacitor and a resistor
connected in parallel with the
diode as shown in following
figure.

81
 Snubber Circuits for Diode
• When the reverse recovery current decreases, the capacitor by virtue of its
property will try to hold the voltage across it, which, approximately, is the
voltage across the diode.
• The resistor on the other hand will help to dissipate some of the energy
stored in the inductor, which forms the IRR loop. The dv/dt across a diode
can be calculated as:

82
 Snubber Circuits for Diode

• Usually the dv/dt rating of a diode is given in the manufacturers

datasheet. Knowing dv/dt and the RS , one can choose the value
of the snubber capacitor CS.

• The RS can be calculated from the diode reverse recovery

current:

• The designed dv/dt value must always be equal or lower than the
dv/dt value found from the datasheet.

83
 Series and Parallel Connection of Power Diodes
• For specific applications, when the voltage or current rating of a
chosen diode is not enough to meet the designed rating, diodes
can be connected in series or parallel.
• Connecting them in series will give the structure a high voltage
rating that may be necessary for high-voltage applications.

84
 Series and Parallel Connection of Power Diodes

• If a selected diode cannot match the required current

rating, one may connect several diodes in parallel.
• In order to ensure equal current sharing, the designer must
choose diodes with the same forward voltage drop
properties.

85
 Diode With RC Load
• Following Figure shows a diode with RC load.
• When switch S1 is closed at t=0, the charging current that flows
through the capacitor and voltage drop across it are found from

86
 Diode With RL Load
• Following Figure shows a diode with RL load.
• When switch S1 is closed at t=0, the current through the
inductor is increased

87
 Diode With RL Load
• The waveform shows when t>>T, the voltage across
inductor tends to be zero and its current reaches maximum
value.

• If an attempt is made to
open S1 energy stored in
inductor (=0.5Li2) will be
transformed into high
reverse voltage across
diode and switch.

88
 Example#3
• A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The
capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0
determine:

– Peak Diode Current

– Energy Dissipated in resistor

– Capacitor voltage at t=2 μs

89
 Example#3
• A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The
capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0
determine:

– Peak Diode Current

– Energy Dissipated in Resistor

90
 Example#3
• A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The
capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0
determine:

– Capacitor voltage at t=2 μs

91
 Freewheeling Diode
• If switch S1 is closed a current is established through the load,
and then, if the switch is open, a path must be provided for
the current in the inductive load.

• This is normally done by

connecting a diode Dm, called a
freewheeling diode.

92
 Freewheeling Diode
• The circuit operation is divided into two modes.
• Mode 1 begins when the switched is closed.
• During this mode the current voltage relation is

93
 Freewheeling Diode
• Mode 2 starts when the S1 is opened and the load current
starts to flow through Dm.

94
 Freewheeling Diode
• The waveform of the entire operation is given below.

S1 Closed S1 Open
95
 Rectification
• Converting AC (from mains or other AC source) to DC power
by using power semiconductor devices is called
rectification.

• Two Categories
– Uncontrolled Rectifiers
– Controlled Rectifiers

96
 Properties of an Ideal Rectifier
• It is desired that the rectifier present a resistive load to
the ac power system.
• This leads to
– Unity power factor
– ac line current has same wave shape as voltage

• An ideal rectifier should have η = 100%, Vac = 0, RF

= 0, TUF = 1, HF = THD = 0, and PF = PDF = 1.

97
 Uncontrolled Rectifiers
• In most power Electronic systems, the power input is
in the form of a 50Hz or 60Hz sine wave ac voltage.
• The general trend is to use inexpensive diode rectifiers
to convert ac into dc in an uncontrolled manner.

98
Rectifier Performance Parameters

99
Rectifier Performance Parameters

I S2  I S21 I S2
THDi   1
I S21 I S21

100
 Transformer Utilization Factor (TUF)
• The transformer utilization factor (TUF), which is a measure
of the merit of a rectifier circuit, is defined as the ratio of the
dc output power to the transformer volt–ampere (VA) rating
required by the secondary winding

• where Vs and Is are the rms voltage and rms current ratings of
the secondary transformer.

101
 Peak Inverse Voltage (PIV)
• Peak inverse voltage is an important parameter in the design
of rectifiers.

• PIV is the maximum voltage that appears across the diode

during its blocking state.

102
 Total Harmonic Distortion (THD)
• This is a measure of the distortion of a waveform, which
characterized the difference between the total rms ac
current ( secondary current Is) and fundamental component
of ac source current (Is1), which can be defined by
decomposing the secondary current into Fourier series.

I S2  I S21 I S2
THDi   1
I S21 I S21

• In the case of pure sinusoidal source current I s=Is1, therefore

HF=0.

103
Single Phase Half Wave Uncontrolled Rectifier
• A single Phase half wave rectifier is the simplest type
and is not normally used in industrial or domestic
applications.

104
 Single Phase Half Wave Rectifier

• Although output voltage is

D.C, it is discontinuous and
contains Harmonics.
105
1-Phase Half Wave Rectifier Performance Parameters

106
1-Phase Half Wave Rectifier Performance Parameters

107
 Example 4: The rectifier shown in figure has a pure
resistive load of 10Ω. Determine (a) The efficiency, (b)
Form factor (c) Crest Factor (d) Ripple factor (e)
Transformer Utilization Factor (f) PIV

Solution

Vm 285
Vodc    90.7V Vm 285
 3.141 Vorms    142.4V
2 2
Vm Vm
I odc   9.07 A I orms   14.25 A
R 2R 108
 Example-4
(a) Efficiency

Podc Vodc I odc (c) Crest Factor

 
Porms Vorms I orms Vm 285
CF   2
Vorms 142.4
90.7  9.07
 100  40.06%
142.4 14.2

(b) Form Factor (d) Ripple Factor

Vm
Voac
Vorms RF   FF 2  1
FF   2 Vodc
Vodc Vm
 RF  1.57 2  1  1.211

FF   1.57
2 109
 Example-4
(e) Transformer Utilization Factor

• The poor TUF of a half-wave rectifier signifies that the

transformer employed must have a 3.496 (1/0.286) VA rating
in order to deliver 1W dc output power to the load.
.
• If the transformer rating is 1 KVA (1000VA) then the half-wave
rectifier can deliver 1000 X0.287 = 287 watts to resistance
load.

• In addition, the transformer secondary winding has to carry a

dc current that may cause magnetic core saturation.

• As a result, half-wave rectifiers are used only when the current

110
 Example-4
(f) Peak Inverse Voltage (PIV)

111
 Example-4(Conclusion)
• Taking into account the obtained rectifier parameters we conclude
that this type of rectifier is characterized with bad parameters
presented by :
1. Low (poor) transform utilization 28.6%, which means that the
transformer must be 1/0.286=3.49 times larger that when it is
used to deliver power from a pure ac voltage.
2. Low ( poor) rectification efficiency = 40.5%
3. Presence of current dc component in the secondary current
causing additional losses ( winding and core heating).
4. High ripple factor (1.21), which means that a filter with large
capacitance is required for smoothing the output voltage,
therefore this yield high capacitor starting current problem.
• Therefore this type of rectifier is rarely used due to the weakness in
quality of it's power and signal parameters.
112
 Exercise#1
• A diode whose internal resistance is 20Ω is to supply power to
a 100Ω load from 110V(rms) ac source. Calculate (a) peak load
current (b) the dc load current (c) the rms load current (d) TUF (e)
TUF when Rf=0Ω (f) Conclusion.

Solution:
Given a half-wave rectifier circuit Rf =20Ω, RL=100Ω
Given an ac source with rms voltage of 110V
Therefore the maximum amplitude of sinusoidal input is given
by

113
 Exercise#1

(d) TUF

114
Exercise#1

115
 Exercise#2
• An AC supply of 230V rms is applied to a half wave rectifier circuit through a transformer of turn ratio 5:1. Assume the diode is an ideal one. The load
resistance is 300Ω. 
• Find
– (a) peak load current
– (b) the dc load current
– (c) the rms load current
– (d) TUF
– (e) PIV
– (f) FF
– (g) RF
– (h) power delivered to load

116
Half Wave Diode Rectifier With R-L Load

117
 Half Wave Diode Rectifier With R-L Load

• The average output voltage is given by

• The average output current is given by

118
 Half Wave Diode Rectifier With R-L Load

• The addition of a freewheeling diode

• The average dc voltage varies

proportionately to [1 -
cos(π + σ)].

• This can be made maximum by

decreasing σ (ideally σ = 0 ).

• We can make  σ = 0 with the

addition of a freewheeling diode
given by Dm as shown with the
dotted line.

119
 Single Phase Full Wave Rectifier
• A full-wave rectifier converts an ac voltage into a pulsating
dc voltage using both half cycles of the applied ac voltage.

• In order to rectify both the half cycles of ac input, two

diodes are used in this circuit. Both diodes feed a common
load With the help of a center-tap transformer.

• A center-tap transformer is the one which produces two

sinusoidal waveforms of same magnitude and frequency but
out of phase with respect to the ground in the secondary
winding of the transformer.

120
 Single Phase Full Wave Rectifier
• Each half of the transformer
with its associated acts as a
half wave rectifier.

121
 
1 2Vm
Vodc   Vm sin t dt 
 0 

2 Vm
I odc 
 R


1 Vm
Vorms  
 m
V sin  t 2
dt 
 0 2

Vm
I orms 
2 R

122
 Example 5. The rectifier in shown in figure has a purely
resistive load of R Determine (a) The efficiency, (b) Form
factor (c) Ripple factor (d) Crest Factor (e) TUF (f) PIV

2Vm 2 110
Vodc    70.06V
 
Vodc 2:1

I odc   7A
R 10Ω

Vm
Vorms   77.78V
2
Vorms
I orms   7.77 A
R
123
 Example-5
Podc Vodc  I odc 70.06  7
    81.05%
Poac Vorms  I orms 77.78  7.77

Vorms 77.78
FF    1.11
Vodc 70.06

2 2
RF  FF  1  1.11  1  0.483

124
 Example-5
• The average TUF in centre-tap full-wave rectifying circuit is
determined by considering the primary and secondary winding
separately. 

• There are two secondary windings here. Each secondary

is associated with one diode. This is just similar
to secondary of half-wave rectifier. Each secondary has
TUF as 0.287.

125
 Exercise-3
• A Full-Wave rectifier circuit is fed from a transformer having a
center-tapped secondary winding. The rms voltage
from end of secondary to center tap is 30V. if the diode
forward resistance is 5Ω and that of the secondary is 10Ω for
a load of 900Ω, Calculate:

1. Power delivered to load

2. Ripple Factor
 
3. Efficiency at full-load
 
4. TUF

126
 Exercise-4
• A Full-wave rectifier circuit uses two silicon diodes with a
forward resistance of 20Ω each. A dc voltmeter connected
across the load of 1kΩ reads 55.4volts. Calculate
 
1. Rms value of load current
2. Average voltage across each diode
3. Ripple factor
4. Transformer secondary voltage rating

127
 Exercise-5
• A 230V, 60Hz voltage is applied to the primary of a 5:1 step
down, center tapped transformer used in the Full-wave
rectifier having a load of 900Ω. If the diode resistance and the
secondary coil resistance together has a resistance
of 100Ω. Determine:
 
1. dc voltage across the load
2. dc current flowing through the load
3. dc power delivered to the load
4. ripple factor

128
 Single Phase Full Wave Bridge Rectifier
• Instead of using centre-
tapped transformer we could
use four diodes.

129
Single Phase Full Wave Bridge Rectifier

130
 Single Phase Full Wave Bridge Rectifier
• Advantages of Bridge rectifier circuit:
– No center-tapped transformer is required
– The TUF is considerably high
– PIV is reduced across the diode.

• Disadvantages of Bridge rectifier circuit:

– The only disadvantage of bridge rectifier is the use of four diodes
as compared to two diodes for center-tapped FWR. 
– This reduces the output voltage

131
 Example 6 single-phase diode bridge rectifier has a purely
resistive load of R=15 ohms and, VS=300 sin ωt and unity
transformer ratio. Determine (a) The efficiency, (b) Form factor, (c)
Ripple factor, (d) Input power factor.

1

2 Vm 2 Vm
Vdc   Vm sin t dt   190.956 V I dc   12.7324 A
0   R
 1/ 2
1  Vm
Vrms    Vm sin t  dt 
2
  212.132 V
  0  2
Pdc Vdc I dc
   81.06 % Vrms
Pac Vrms I rms FF   1.11
Vdc
2
Vac Vrms  Vdc2 2
Vrms 2
RF 
Vdc

Vdc
 2
 1  FF  1  0.482 The PIV=300V
Vdc

Re al Power VS I S cos 
Input power factor =  1
Apperant Power VS I S
132
 Exercise-6
• A bridge rectifier uses four identical diodes having forward
resistance of 5Ω and the secondary voltage of 30V (rms).
Determine the dc output voltage for IDC=200mA and the value of
the ripple voltage.

133
 Exercise-7
• In a bridge rectifier the transformer is connected to 220V, 60Hz
mains and the turns ratio of the step down transformer is
11:1. Assuming the diode to be ideal, find:

1. Idc
2.  voltage across the load
3. PIV assume load resistance to be 1kΩ

134
 Three Phase Supply
• 4 wires
– 3 “active” phases, A, B, C
– 1 “ground”, or “neutral”
• Color Code
– Phase A Red
– Phase B Black
– Phase C Blue
– Neutral White or Gray

• Three phase voltages with respect to Neutral.

Three Phase Half Wave Rectifier
 Three Phase Half Wave Rectifier
5 / 6
3 3 3 Vm
Vdc 
2  / 6Vm sin t dt  2  0.827Vm
3 3 Vm 0.827  Vm
I dc  
2  R R

5 / 6
3 1 3 3
 V sin t 
2
Vrms  m dt   Vm  0.8407 Vm
2 /6
2 8
0.8407 Vm
I rms 
R

PIV  3 Vm 137
 Example 7 The rectifier shown in following figure is
operated from 460 V 50 Hz rms supply at secondary side
and the load resistance is R=20 . If the source
inductance is negligible, determine (a) Rectification
efficiency, (b) Form factor (c) Ripple factor (d) Crest
Factor (e) Peak inverse voltage (PIV) of each diode.

138
 Example-7
• Phase to neutral voltage is given by
460
VS   265.58 V
3
• Peak voltage now can be calculated as

Vm  265.58  2  375.59 V
• Average value of load voltage and current now can be calculated as

3 3 Vm
Vdc   0.827 Vm  310.6V
2
3 3 Vm 0827 Vm
I dc    15.5 A
2 R R
139
 Example-7
• RMS value of load voltage and current

Vrms  0.8407 Vm  315.5V

0.8407 Vm
I rms   15.77 A
R

• (a) Rectifier efficiency

Pdc Vdc I dc
 
Pac Vrms I rms

Vdc I dc 310.6 15.5

   96.7 %
Vrms I rms 315.5 15.77
140
• (b) Form Factor
Example-7
Vrms 315.5
FF    1.01
Vdc 310.6

• (c) Ripple Factor

RF  FF 2  1  0.18
• (d) Crest Factor
Vm 375.59
CF    1.19
Vrms 315.5
• (e) PIV

PIV  3 Vm  650.54V
141
 Three Phase Bridge Rectifier
• Three Phase bridge rectifier is
very common in high power
applications because they have
the highest possible
transformer utilization factor
for a three-phase system.
• It can operate with or without
transformer and give six-pulse
ripple on the out.

• Diodes D1, D3, D5 will conduct

when the supply voltage is most
positive.
• Diodes D2, D4, D6 will conduct
when the supply voltage is most
negative.
142
1 cycle

143
144
 Three Phase Bridge Rectifier
2 / 3
6 3 3 Vm
Vdc 
2 /3
3Vm sin t dt 

 1.654Vm

3 3 Vm 1.654  Vm
I dc  
 R R

2 / 3

  
6 2 3 9 3
Vrms  3Vm sin t dt   Vm  1.655 Vm
2 /3
2 4
1.655 Vm
I rms 
R

PIV  3 Vm 145
 Example 8 The 3-phase bridge rectifier is operated from 460 V
50 Hz supply and the load resistance is R=20ohms. If the
source inductance is negligible, determine (a) The efficiency,
(b) Form factor (c) Ripple factor (d) Crest Factor (e) Peak
inverse voltage (PIV) of each diode .

3 3 Vm
Vdc   1.654Vm  621.226 V

3 3 Vm 1.654Vm
I dc    31.0613 A
 R R
3 9 3
Vrms   Vm  1.6554 Vm  621.752 V
2 4

1.6554 Vm
I rms   31.0876 A
R
(a) The efficiency
Example-8
Pdc Vdc I dc
   99.83 %
Pac Vrms I rms
(b) Form factor

Vrms
FF   1.00084
Vdc
(c) Ripple factor

RF  FF 2  1  0.04
(d) Crest Factor

3Vm 650.55
CF    1.04
Vrms 621.75
(e) Peak inverse voltage (PIV) of each diode PIV  3 Vm  650.54
Comparison of Diode Rectifier

148
 Thyristor Types
Some of the more major types:
• Shockley diode
• SCR
– Phase-control Thyristors
– Fast-switching Thyristors
• Triac, Diac
• Silicon controlled switch (SCS)
• Reverse-conducting Thyristors (RCTs).
• Static induction Thyristors (SITHs).
• Bidirectional Phase-controlled thyristors (BCT)
• LASCR (Light activated SCR)
• Gate Turn-off thyristors (GTO)
• FET-controlled thyristors(FET-CTH)
• MOS Turn-off thyristors (MTO)
• MOS-controlled thyristors (MCT)
• MTO - MOS Turn Off Thyristor
• ETO - Emitter Turn Off Thyristor
• GATT - Gate Assisted Turn Off Thyristor
 Four Semiconductor Layer (pnpn)
- Thyristors -
• Four semiconductor layer (pnpn) devices with a control
mechanism are known as thyristors.
• This include Shockley diode, silicon controlled rectifier
(SCR), diac, triac, silicon controlled switch (SCS), …
• They act as open circuits capable of withstanding a certain
rated voltage until they trigger.
• When trigger, they turn on and become low resistance
current path and remains so, even after the trigger is
removed, and will go off if the current is reduced to a
certain level or until they are trigger off.
• Usage: mainly used in industrial applications where power
control and switching are needed such as lamp dimmers,
motor speed control, ignition systems and charging
 Introduction
• One of the most important type of power
semiconductor device.

• Compared to transistors, thyristors have lower on-state

conduction losses and higher power handling capability.

• However, they have worse switching performances than

transistors.

• Name ‘thyristor’, is derived by a combination of the

capital letters from THYRatron and transISTOR.
152
 Introduction
• Thyristors are four-layer pnpn power semiconductor
devices.

• These devices switch between conducting and

nonconducting states in response to a control signal.

• Thyristors are used in timing circuits, AC motor speed

control and switching circuits.

153
 Thyristors
• Bell Laboratories were the first to fabricate a silicon-based
thyristor.

• Its first prototype was introduced by GE (USA) in 1957.

• Later on many other devices having characteristics similar to

of a thyristor were developed.

• These semiconductor devices are SCR, SCS, Triac, Diac, PUT,

GTO, e.t.c.

• This whole family of semiconductor devices is given the

name thyristors. 154
 Thyristor/ SCR
• SCR is a three terminal, four layers solid state
semiconductor device, each layer consisting of
alternately N-type or P-type material, i.e; P-N-
P-N,

• It can handle high currents and high voltages,

with better switching speed and improved
breakdown voltage .
A K

155
 Silicon-Controlled Rectifier

• Note: Gate requires small positive pulse for short duration to turn SCR on. Once the device is
on, the gate signal serves no useful purpose and can be removed.
 Silicon-Controlled Rectifier (SCR)
• SCR is a switching device for high voltage and current operations.
• SCR is most popular of thyristor family due to its
Fast switching action, small size and high voltage and current ratings.
• It’s another four layer pnpn device with three terminals,
anode, cathode, and gate.
• SCR is turned on by applying +ve gate signal when anode
is +ve with repect to cathode.
• In on state it’s act as short between A and K and small
forward resistance.
• SCR is turned off by interrupting anode current.
• In off state, it act ideally as an open circuit between A and K,
and high resistance.
• Some application are motor control, time delay, heater control,
relay control and phase control.
Silicon-Controlled Rectifier

SCR Equivalent Circuit

 Thyristor/ SCR
• Thyristor can handle high currents and high voltages.

• Typical rating are 1.5kA & 10kV which responds to 15MW

power handling capacity.

• This power can be controlled by a gate current of about 1A

only.

• Thyristor acts as a bistable switch.

– It conducts when gate receives a current pulse, and continue
to conduct as long as forward biased (till device voltage is
not reversed).
– They stay ON once they are triggered, and will go OFF only if
current is too low or when triggered off.
159
 Phase Control Thyristor
• These are converter thyristors.
– Used for rectifying line-frequency voltages and currents, low on-state
voltage and large voltage and current range
• The turn-off time tq is in the order of 50 to 100sec.
• Used for low switching frequency.
• Commutation is natural commutation
• On state voltage drop is 1.15V for a 600V device.
• They use amplifying gate thyristor.
 Phase Control Thyristors

Used for rectifying line-frequency voltages and currents,

low on-state voltage and large voltage and current range
 Fast Switching Thyristors
• Also called inverter thyristors.
• Used for high speed switching applications.
• Turn-off time tq in the range of 5 to 50sec.
• On-state voltage drop of typically 1.7V for 2200A,
1800V thyristor.
• High dv/dt and high di/dt rating.
Fast switching Thyristors
 Thyristor/ SCR Operation

• When the anode voltage is made

positive with respect to the cathode,
junctions J1 and J3 are forward biased
and junction J2 is reverse biased.

• The thyristor is said to be in the forward

blocking or off-state condition.

• A small leakage current flows from

anode to cathode and is called the off-
state current.

164
 Thyristor/ SCR Operation
• If the anode voltage VAK is increased to a
sufficiently large value, the reverse biased
junction J2 would breakdown.
• This is known as avalanche breakdown and the
corresponding voltage is called the forward
breakdown voltage VBO.
• Since the other two junctions J1 and J3 are already
forward biased, there will be free movement of
carriers across all three junctions.
• This results in a large forward current and the
device is now said to be in a conducting or on-
state.  
• The voltage drop across the device in the on-
state is due to the ohmic drop in the four layers
and is very small (in the region of 1 V). 165
Thyristor/ SCR

Latching
current IL

166
Real SCR Characteristic Curve

Latching
current IL

167
Silicon-Controlled Rectifier
Turning the SCR on
 Silicon-Controlled Rectifier
SCR Characteristics
• SCR has a horizontal voltage swing. Voltage
across SCR VF is high before it fires, but then
it drops significantly once it begins conducting.
SCR only conducts in one direction.
• SCR will on if voltage anode to cathode >=
forward breakover voltage V(BR)F. In this
instance the gate current IG can be 0.
• More IG1, IG2 is applied, less V(BR)F1, V(BR)F2, V(BR)F3
is required.
 Thyristor Conduction
ig vs
ia

+ + t
vs vo vo
_ _
t

ig

 t

• Thyristor cannot be turned off by applying negative gate current. It can only
be turned off if Ia goes negative (reverse)
– This happens when negative portion of the of sine-wave occurs (natural
commutation),

• Another method of turning off is known as “forced commutation”,

– The anode current is “diverted” to another circuitry.
 Thyristor Operating modes
Thyristors have three modes :

• Forward blocking mode:

Only leakage current flows,
so thyristor is not
conducting.
• Forward conducting mode:
large forward current flows
through the thyristor. Latching
current IL
• Reverse blocking mode:
When cathode voltage is
increased to reverse
breakdown voltage ,
Avalanche breakdown
occurs and large current
flows.
171
 Important characteristics
Latching Current IL
•This is the minimum anode current required to maintain
the thyristor in the on-state immediately after a thyristor
has been turned on and the gate signal has been removed.
•If a gate current greater than the threshold gate current is
applied until the anode current is greater than the latching
current IL then the thyristor will be turned on or triggered.

Holding Current IH
•This is the minimum anode current required to maintain
the thyristor in the on-state.
•To turn off a thyristor, the forward anode current must be
reduced below its holding current for a sufficient time for
mobile charge carriers to vacate the junction.
172
 Important characteristics
Reverse Current IR
•When the cathode voltage is positive with respect to the
anode, the junction J2 is forward biased but
junctions J1 and J3 are reverse biased. The thyristor is said to
be in the reverse blocking state and a reverse leakage current
known as reverse current IR will flow through the device.

Forward Breakover Voltage VBO

•If the forward voltage VAK is increased beyond VBO , the
thyristor can be turned on. But such a turn-on could be
destructive. In practice the forward voltage is maintained
below VBO and the thyristor is turned on by applying a
positive gate signal between gate and cathode.
173
 Turn-on Characteristics

Moment when
SCR start “ON”

ton  td  tr
When IG rise to enough current

to turn SCR ON

174
174
 Turn-off Characteristics
VAK
tC
tq

IA
di
C o m m u ta tio n
A n o d e cu rre n t dt
b e gin s to
d e cre a se Re co ve r y R e co m bin a tio n

t1 t2 t3 t4 t5

t q = d e vice o ff tim e
tr r tg r
t c = circu it o ff tim e
tq 175
175
tc
• https://www.electronics-tutorials.ws/power/thyristor-circuit.html
• SCR =“ON” if inject a small trigger pulse of current 1mA to 50mA into (G) when
SCR is in forward direction (UAK>0) for regenerative latching to occur.
– Once trigger pulse applied, diode between G & K is ON => VGK ~ 0.7V even SCR is not
yet ON
• Generally, trigger pulse ~ several us but the longer the Gate pulse => the
faster the turn-“ON” SCR.
– Once triggered and fully conducting, UAK~ 1.0V for all values of IAK up to its rated value.
• Once SCR “ON”, SCR continues to conduct even without G signal until IAK < IH
and it automatically turns-“OFF”.
– Unlike BJT and FET’s, SCR cannot be used for amplification or controlled switching.
– in DC circuits and some highly inductive AC circuits the current has to be artificially
reduced by a separate switch or “turn off” circuit.
• SCR are specifically designed for use in high-power switching applications and
do not have the ability of an amplifier.
– SCR operate only in a switching mode like ON/OFF switch.
 The SCR can be turned on at its gate terminal.

With a dc Load Anode

source, the
SCR stays
on after
Cathode
it is gated. current
Load
Gate

Gate pulse Time

occurs here
177
 With an ac Load Anode

source, the
SCR turns
off at the
Cathode
zero-crossing. Gate

off

on
current
Load

Gate pulse Time

occurs here
Turns off here
178
The gate can Load Anode

be pulsed for
each positive
alternation.
Cathode
Gate

current
Load

Time

179
 The average Load Anode

load current
can be
decreased
Cathode
by gating Gate
the SCR later.
current
Load

Time

180
…. and later. Load Anode

Cathode
Gate

current
Load

Time

181
…. or, not Load Anode

at all.

Cathode
Gate

current
Load

No gate pulses: ILoad = 0

0
Time

182
 Parameters and Specifications
• Instantaneous Forward Gate Current
– Instantaneous current flowing between gate and cathode terminals in a direction to
forward bias the gate junction.
• Instantaneous Forward Gate Voltage
– Instantaneous forward voltage between gate and cathode terminals with anode terminal
open.
• DC Gate Trigger Voltage
– Gate voltage with IGT (DC gate trigger current) flowing but prior to start of anode
conduction.
• DC Gate Trigger Current
– Forward gate current required to trigger a thyristor at stated temperature conditions.
• Peak Reverse Gate Voltage
– Maximum allowable peak reverse voltage between the gate terminal and the cathode
terminal.
• Peak Gate Power Dissipation
– Maximum instantaneous value of gate power dissipation.
 Parameters and Specifications
• Average Gate Power Dissipation
– Maximum allowable value of gate power dissipation averaged over a full cycle.
• Holding Current (Gate drive)
– Value of Instantaneous Forward Current below which thyristor returns to forward
blocking state after having been in forward conduction under stated temperature
and gate termination conditions.
• Latching Current (after remove Gate drive)
– Value of minimum anode current to remain in the on-state after removal of the
gate trigger pulse under specified condition.
• Instantaneous Reverse Blocking Current
– Instantaneous anode current at stated conditions of negative anode voltage,
junction temperature, and gate termination.
• Instantaneous Forward Blocking Current
– Instantaneous anode current at stated conditions of forward blocking voltage,
junction temperature, and gate termination.
THYRISTOR GATE CHARACTERISTICS
 SCR Characteristics & Ratings
• Forward- breakover voltage, VBR(F): voltage at which SCR enters (ON) region.
• Latching current, IL: Minimum IAK to maintain SCR “ON” immediately after SCR starts “ON” and G
signal has been removed.
• Holding current, IH: Once SCR “ON”, SCR continues to conduct even without G signal until IAK < IH
and it automatically turns-“OFF” => I H is current level below that SCR enter OFF
• Gate trigger current, IGT: value of gate current to switch SCR on.
• Average forward current, IF (avg): maximum continuous IAK(dc) that SCR can withstand.
• Reverse-breakdown voltage, VBR(R): maximum reverse voltage before SCR breaks into avalanche.
 Thyristor turn-ON methods

• Thyristor turning ON is also known as Triggering.

• With anode is positive with respect to cathode, a thyristor

can be turned ON by any one of the following techniques :

– Forward voltage triggering         

– Gate triggering
– dv/dt triggering
– Temperature triggering
– Light triggering

187
 Forward Voltage Triggering
• When breakover voltage (VBO) across a thyristor is exceeded
than the rated maximum voltage of the device, thyristor turns
ON.

• At the breakover voltage the value of the thyristor anode

current is called the latching current (IL) .

• Breakover voltage triggering is not normally used as a

triggering method, and most circuit designs attempt to avoid
its occurrence.

• When a thyristor is triggered by exceeding VBO, the fall time of

the forward voltage is quite low (about 1/20th of the time
taken when the thyristor is gate-triggered).

188
 Gate Triggering
• Turning ON of thyristors by gate triggering is simple and
efficient method of firing the forward biased SCRs.

• In Gate Triggering, thyristor with forward breakover

voltage (VBO), higher than the normal working voltage is
chosen.

• Whenever thyristor’s turn-ON is required, a positive gate

voltage b/w gate and cathode is applied.

• Forward voltage at which device switches to on-state

depends upon the magnitude of gate current.
– Higher the gate current, lower is the forward breakover
voltage .
189
 Gate Triggering
• Turning ON of thyristors by gate triggering is simple and
efficient method of firing the forward biased SCRs.

190
 dv/dt triggering
• With forward voltage across anode & cathode of a thyristor, two
outer junctions (A & C) are forward biased but the inner junction
(J2) is reverse biased.
• The reversed biased junction J2 behaves like a capacitor because of
the space-charge present there.
• As p-n junction has capacitance, so larger the junction area the
larger the capacitance.
• If a voltage ramp is applied across the anode-to-cathode, a current
will flow in the device to charge the device capacitance according to
the relation:

• If the charging current becomes large enough, density of moving

current carriers in the device induces switch-on.
• This method of triggering is not desirable because high charging
current (Ic) may damage the thyristor.
191
 Temperature Triggering
• During forward blocking, most of the applied voltage appears
across reverse biased junction J2.
• This voltage across junction J2 associated with leakage current
may raise the temperature of this junction.
• With increase in temperature, leakage current through
junction J2 further increases.
• This cumulative process may turn on the SCR at some high
temperature.
• High temperature triggering may cause Thermal runaway and
is generally avoided.

192
 Light Triggering
• In this method light particles (photons) are made to
strike the reverse biased junction, which causes an
increase in the number of electron hole pairs and
triggering of the thyristor.
• For light-triggered SCRs, a slot (niche) is made in the
inner p-layer.
• When it is irradiated, free charge carriers are
generated just like when gate signal is applied b/w
gate and cathode.
• Pulse light of appropriate wavelength is guided by
optical fibers for irradiation.
• If the intensity of this light thrown on the recess
exceeds a certain value, forward-biased SCR is turned
on. Such a thyristor is known as light-activated SCR
(LASCR).
• Light-triggered thyristors is mostly used in high-
voltage direct current (HVDC) transmission systems.
193
 Thyristor Gate Control Methods
• An easy method to switch ON a SCR into conduction is to
apply a proper positive signal to the gate.

• This signal should be applied when the thyristor is forward

biased and should be removed after the device has been
switched ON.

• Thyristor turn ON time should be in range of 1-4 micro

seconds, while turn-OFF time must be between 8-50 micro
seconds.

• Thyristor gate signal can be of three varieties.

– D.C Gate signal
– A.C Gate Signal
– Pulse
194
 Thyristor Gate Control Methods
D.C Gate signal: Application of a d.c gate signal causes the flow of
gate current which triggers the SCR.
– Trigger angle <= 90 deg
– Disadvantage is that the gate signal has to be continuously applied, resulting in
power loss.
– Gate control circuit is also not isolated from the main power circuit.
– S2 high power switch

195
 Thyristor Gate Control Methods
A.C Gate Signal: In this method a phase - shifted a.c voltage derived from the
mains supplies the gate signal.
– Instant of firing can be controlled by phase angle control of the gate signal.
– During positive cycle, C charges up via R1 while SCR OFF. G is activated only when VA
has risen enough (depending on RC constant) to turn ON D1 and C starts discharge
via G & K, turning SCR “ON”.
– Increasing R1 will delay GTAK and ITAK, causing a lag in SCR conduction time => SCR
conduction can be controlled between 0 and 180 deg (50% efficiency)

Trigger angle: 0 to 180 deg

Trigger angle <= 90 deg (RC to delay/lengthen trigger angle) 196
 Thyristor Gate Control Methods
Pulse: Here the SCR is triggered by the application of a positive pulse of
correct magnitude.
– For Thyristors it is important to switched ON at proper instants in a
certain sequence.
– This can be done by train of the high frequency pulses at proper
instants through a logic circuit.
– A pulse transformer is used for circuit isolation.

197
 Thyristor Commutation
• Commutation: Process of turning off a conducting thyristor

• SCR cannot be turned OFF via the gate terminal.

• It will turn-off only after the anode current is negated either

naturally or using forced commutation techniques.

• Therefore, commutation can be classified as

–Natural commutation
–Forced commutation

198
 Line Commutation (Natural Commutation)
• Occurs only in AC circuits.
• Natural Commutation of thyristor takes place in
– AC Voltage Regulators
– Phase controlled rectifiers
– Cycloconverters

199
Thyristor Turn-Off: Line-Commutated Thyristor Circuit

200
 Forced Commutation
• Applied to d.c circuits.

• If a thyristor is used in a DC circuit, when first turned on, it will stay

on until the current goes to zero. To turn off the thyristor it is
possible to use a Forced commutation circuit. The circuit creates a
reverse voltage over the thyristor (and a small reverse current) for a
short time, but long enough to turn off the thyristor.

201
 1. Firing angle (how much:
30 deg)
2. Shape of pulse (square)
- Vol level
- Current
- Pulse width
(other if any)
3. IC555???

1. AC->DC:
comparator =>
square wave
2. Input to Gate
Gate control and drive
drive/Amp is AC power
3.
Gate
Control
 Analog Circuit: Generate Gating Signals
• Analog circuit
Analog Circuit: PWM Signal Generation
•
Analog Circuit Example for Gating Signals
In the figure, the first stage is a control transformer for stepping down the line voltage. The secondary
of the transformer is fed to the zero-cross detector.
• In systems where the ac supply has high impedance, the non-sinusoidal current of the bridge causes
distortion of the line voltage. The result of the distortion of the line voltage waveform is multiple zero
crossings and zero-cross jitter.
– The latter means that the zero-cross frequency fluctuates. The former means that more than one zero
crossing may occur around the zero-cross of the fundamental of the line voltage. Multiple zero-cross and
zero-cross jitter could result in unsymmetrical firing of the bridge => distortion of the line voltage =>
harmonic instability.
– To prevent un-symmetry, the zero-cross detector is equipped with hysterisis comparators and a PLL. The
former eliminates multiple zero crossings. The latter stabilizes and locks the frequency of the detection pulses
to the line fundamental zero-crossings.
• The firing pulse generator utilizes the zero-cross
pulses of the previous stage to generate a periodic
ramp. The ramp is reset and commences rising with
the onset of each zero-cross pulse. This ramp is
compared with a reference voltage proportional to
the desired ignition delay angle. A short firing pulse
is generated at the moment the ramp exceeds the
reference voltage.
• The final stage is the gate drive circuit. This stage amplifies, shapes and delivers the firing pulses to
the gate of the thyristors. In the majority of applications galvanic isolation is needed between the
drive circuit and the thyristor gate. This isolation may be provided through pulse transformers, opto-
couplers, or optical fiber cable (for light triggered thyristors, LTSCR).
 Analog Circuit Example for Gating Signals
• In lower comparator, the negative line voltage is compared with the zero
level. Thus, this comparator is high, when line voltage is negative & low,
when the line voltage is positive. The upper comparator compares the
positive line voltage with the level of 0.7V=> pulse width. Thus, the output
of this comparator is high when line voltage exceeds 0.7V & low otherwise.
• The output of both comparators is "ORed" through two diodes. The
combined output is a negative notch at the arrival of a zero crossing. The
negative notch is inverted through a transistor and a short pulse is obtained
at the output of the stage, which is synchronous to the line zero-cross as
shown in the figures below.
Need one shot circuit
(monostable) to create
pulse of certain width
Alpha trigger
angle Need one shot circuit
(monostable) to create
pulse of certain width
 Analog Circuit Example for Gating Signals
• In the phase control circuit, a -6V supply is connected to the input of an integrator, through a
potentiometer. The output of the zero-cross detector activates a MOS-FET switch that is
connected across the terminals of the integrator capacitor. The capacitor voltage increases
linearly as it integrates the constant input to the integrator. At the arrival of the zero-cross
pulse, the MOS-FET is activated and the capacitor terminals are shorted. Thus, the voltage of
the capacitor is decreased to almost zero. At the end of the zero-cross pulse, the MOS-FET is
deactivated and the capacitor commences charging again. This produces the periodic ramp as
in the figure.

• The integrator ramp is compared with a variable reference voltage to produce the firing
pulses. These pulses are amplified before delivered to the gate drive circuit. The gate drive
circuit consists of four pulse transformers delivering the firing pulse between the gate and
cathode of the bridge thryristors. A transistor drives the transformers. When the firing pulse
arrives, the transistor saturates essentially connecting the primary of the transformer to the
+6V supply. The diode across the primary is needed to circulate the magnetizing current of the
transformer, when the pulse is removed and the transformer is essentially disconnected from
the supply. All pulse transformers are driven simultaneously by the firing circuit. Thus, the four
thyristors receive firing pulses simultaneously, however, only the thyristors with forward
voltage with ignite.
 Digital Circuit: Generate Gating Signals
• Advantages:
– Stable, very robust to the noise
– Easy to be controlled by CPU/DSP
– Reduce the load of CPU/DSP
• Disadvantages:
– Expensive hardware
– High frequency counter
Digital Circuit: Generate Gating Signals
 Digital Circuit: Generate Gating Signals
• Software programming
– At t1, the number in free running counter (TCNT) matches
the number of the output compare register (OC1), an
interrupt request will be made.
– Interrupt service routine
• Determine the type of interval (gap, or pulse)
• Set the output pin to logic “0” for gap interval or logic “1” for pulse
interval.
• Calculate the time interval for following T period
• Clear the interrupt flag and return
– If the calculation in the interrupt service routine can not
be completed during Tg, the program will crash. What
shall we do?
Zero Crossing Detection Circuit
 Silicon-Controlled Rectifier
Application – On-Off Control of Current
Assuming the SCR is initially off. SW1 close,
provide a pulse of current into the gate. SCR on so
it conduct current to load. Remain in conduction
even after the momentary conduct of SW1 is
removed if the IA =>than IH.
 Silicon-Controlled Rectifier
Application – On-Off Control of Current
When SW2 is momentary closed, IA reduced to
below IH. SCR off.
In this circuit SW1 is pressed momentarily to turn
the SCR on and SW2 is pressed momentarily to
turn it off.
 Silicon-Controlled Rectifier
Application -
Rain Fall Detector and Burglar Alarm
Single-phase Thyristor Half-Bridge Rectifier
 Silicon-Controlled Rectifier
Application – Half Wave Power Control
• Application in lamp dimmer, electric heater,
electric motor.
• Vac are applied across terminal A and B. RL
represents the resistance of load (heating
element or lamp element). R1 limits the current.
R2 is potentiometer (it sets the trigger level
for the SCR).
 Silicon-Controlled Rectifier
Application – Half Wave Power Control
• By adjusting R2, SCR can be made to trigger at any point
on the positive half cycle of ac waveform (0 to 900)
• When trigger at beginning, it conducts for
approximately 1800 and maximum power is delivered to
load.
 Silicon-Controlled Rectifier
Application – Half Wave Power Control
• When trigger at near peak of positive half cycle, it
conducts for approximately 900 and less power is
delivered to load.
• When input goes negative, SCR off and diode is used to
prevent negative ac voltage from being applied to the
gate of SCR.
 Silicon-Controlled Rectifier
Application: Over -Voltage protection Circuit –Crow Bar Circuit
• Vout dc from regulator is monitor by zener, D1 and
resistive voltage divider R1 and R2. The Vout max is set by
zener voltage, if this voltage exceeded, D1 conducts and
voltage divider produces an SCR trigger voltage. SCR on
and connected across the line voltage. SCR current
causes the fuse to blow, thus disconnecting the line
voltage from power supply.
 SCR Applications - dc motor control
• SCRs are used in a variety of power control
applications.
• One of the most common applications is to use
it in ac circuits to control a dc motor or
appliance because the SCR can both rectify and
control.
• The SCR is triggered on the positive cycle and
turns off on the negative cycle.
• A circuit like this is useful for speed control for
fans or power tools and other related
applications.
 SCR Applications- crowbar circuits
• Another application for SCRs is in
crowbar circuits (which get their name
from the idea of putting a crowbar
across a voltage source and shorting it
out!)
• The purpose of a crowbar circuit is to
shut down a power supply in case of
over-voltage.
• Once triggered, the SCR latches on.
• The SCR can handle a large current,
which causes the fuse (or circuit
breaker) to open.
 DIAC (diode for alternating current)
• The DIAC is a five-layer device trigger diode that conducts current only after its
breakdown voltage has been exceeded momentarily.
• When this occurs, the resistance of the diode abruptly decreases, leading to a sharp
decrease in the voltage drop across the diode and, usually, a sharp increase in
current flow through the diode.
• The diode remains "in conduction" until the current flow through it drops below a
value characteristic for the device, called the holding current.
• Below this value, the diode switches back to its high-resistance (non-conducting)
state.
• This behavior is bidirectional, meaning typically the same for both directions of
current flow .
– their terminals are not labeled as anode and cathode but as A1 and A2 or MT1
("Main Terminal") and MT2.
• Most DIACs have a breakdown voltage around 30 V.
• DIACs have no gate electrode, unlike some other thyristors they are commonly used
to trigger, such as TRIACs.
• diac is normally used in ac circuits
• The drawback of the diac is that it cannot be triggered at just any point in the ac
power cycle; it triggers at its preset breakover voltage only. If we could add a gate to
the diac, we could have variable control of the trigger point, and therefore a greater
degree of control over just how much power will be applied to the line-powered
device.
DIAC (diode for alternating current)
 TRIAC (Triode for Alternating Current)
• Triac is five layer device that is able to pass current bidirectionally
and is therefore behaves as an a.c. power control device.
• In triac , the main connections are simply named main terminal 1 (MT1)
and main terminal 2 (MT2).
• The gate designation still applies, and is still used as it was with the SCR.
• The useful feature of the triac is that it not only carries current in either
direction, but the gate trigger pulse can be either polarity regardless of
the polarity of the main applied voltage.
• The gate can inject either free electrons or holes into the body of the
triac to trigger conduction either way.
– So triac is referred to as a "four-quadrant" device.
• Triac is used in an ac environment, so it will always turn off when the
applied voltage reaches zero at the end of the current half-cycle.
• If we apply a turn-on pulse at some controllable point after the start of
each half cycle, we can directly control what percentage of that half-
cycle gets applied to the load, which is typically connected in series with
MT2.
• This makes the triac an ideal candidate for light dimmer controls and
motor speed controls. This is a common application for triacs.
 Triac operation
• The triac can be considered as two thyristors connected in antiparallel as
shown in Fig .
• The single gate terminal is common to both thyristors.
• The main terminals MT1 and MT2 are connected to both p and n regions of
the device and the current path through the layers of the device depends
upon the polarity of the applied voltage between the main terminals.
• The device polarity is usually described with reference to MT1, where the
term MT2+ denotes that terminal MT2 is positive with respect to terminal
MT1.
The Gate Turn-Off Thyristor (GTO)
GTOs Schematic representation
 SCR Gate Drives
Problems & Solutions
 Outline
• Introduction
• Voltage Divider Triggering
• RC Triggering
• Double RC Triggering
 Introduction
• The popular terms used to describe how SCR is operating are
conduction angle and firing delay angle.

– Conduction angle is the number of degrees of an ac cycle during which

the SCR is turned ON.

– Firing delay angle is the number of degrees of an ac cycle that elapses

before the SCR is turned ON.

• Of course, these terms are based on the notion of total cycle

time (3600)
 Introduction
• An SCR is fired by a short burst of current into the gate (IG).

• The amount of gate current needed to a fire particular SCR is

symbolized as IGT.

• Most SCRs require current between 0.1 and 50mA.

• Since there is a standard pn-junction between gate and

cathode, voltage between these two terminals (VGK) must be
slightly greater than 0.7 volt.
 Example-1
• For the circuit shown in figure below, what voltage is required at
point X to fire the SCR? The gate current needed to fire 2N3669
is 20mA under normal conditions.
Solution

• The voltage between point X and

cathode must be sufficient to forward
bias the junction between X and K (0.7V).
• And also at least cause 20mA to flow
from 150Ω resistor.
• For 20mA current to flow in XG branch
we need

• Therefore,
 Gate Control Circuits
• Gate Control Circuit Design
• Consideration must be given to the following points when designing
gate control circuits.

• The gate signal should be removed after the thyristor has been
turned on. A continuous gate signal will increase the power
loss in the gate junction.

• No gate signal should be applied when the thyristor is reversed

biased. If a gate signal is applied under these conditions, the
thyristor may fail due to an increased leakage current.

• The width of the gate pulse must be greater than the time
required for the anode current to rise to the holding current. In
practice, the gate pulse width is made wider than the turn-on
time of the thyristor.
238
 Gate Control Circuits
• A simple type of gate control circuit (triggering circuit) is shown
in following figure.

• When SW is closed, there

will be current into the
gate when supply voltage
goes positive.

• Firing delay angle is

determined by setting of
R2.
 Gate Control Circuits
• One disadvantage of this simple triggering circuit is that the firing delay angle is
adjustable is only from about 00 to 900
• (IGTmax is at 90%, after that it will go down => if trigger angle > 90 deg, it will conduct before 90 deg or not conduct)

• This can be understood by referring to

following figure.
 Example-2
• For following figure assume that the supply is 115V rms,
IGT=15mA, and R1=3KΩ. The firing delay is desired to be 20o. To
40Ω
what value should R2 be adjusted? B

Solution 3KΩ

• At 20o instantaneous
supply voltage is

• Voltage drop across Load

 Example-2
• Total resistance in the
gate lead is given by

40Ω

3KΩ

• Therefore, R2 is
 Example-3
• For following figure assume that the supply is 115V rms,
IGT=15mA, and R1=3KΩ. The firing delay is desired to be 30o. To
what value should R2 be adjusted?
Solution
40Ω

• At 30o instantaneous
supply voltage is 3KΩ

• Voltage drop across Load

 Example-3
• Total resistance in the
gate lead is given by
40Ω

3KΩ

• Therefore, R2 is
 Example-4
• For following figure assume that the supply is 115V rms,
IGT=15mA, and R1=3KΩ. The firing delay is desired to be 60o. To
what value should R2 be adjusted?
Solution
40Ω

• At 30o instantaneous
supply voltage is 3KΩ

• Voltage drop across Load

 Example-4
• Total resistance in the
gate lead is given by
40Ω

3KΩ

• Therefore, R2 is
 Example-5
• For following figure assume that the supply is 115V rms,
IGT=15mA, and R1=3KΩ. The firing delay is desired to be 90o. To
what value should R2 be adjusted?
Solution
40Ω

• At 90o instantaneous
supply voltage is 3KΩ

• Voltage drop across Load

 Example-5
• Total resistance in the
gate lead is given by
40Ω

3KΩ

• Therefore, R2 is
 Example-6
• For following figure assume that the supply is 115V rms,
IGT=15mA, and R1=3KΩ. The firing delay is desired to be 150o. To
what value should R2 be adjusted?
Solution
40Ω

• At 150o instantaneous
supply voltage is 3KΩ

• Voltage drop across Load

 Example-6
• Total resistance in the
gate lead is given by
5Ω

3KΩ

• Therefore, R2 is

• R2 is same as it was for firing angle of 30o. Therefore with this

circuit arrangement it is not possible to fire SCR beyond 90o.
 Example-7
• For following figure assume that the supply is 115V rms,
IGT=15mA, and R1=3KΩ. The firing delay is desired to be 10o. To
what value should R2 be adjusted?
Solution
40Ω

• At 10o instantaneous
supply voltage is 3KΩ

• Voltage drop across Load

 Example-7
• Total resistance in the
gate lead is given by
40Ω

3KΩ

• Therefore, R2 is

• Cannot have firing angle of 10o. For extended firing angle R1 can be
made smaller.
 Example-8
• For following figure assume that the supply is 115V rms,
IGT=15mA, and R1=3KΩ. The firing delay is desired to be 18o. To
what value should R2 be adjusted?
Solution
40Ω

• At 15o instantaneous
supply voltage is 3KΩ

• Voltage drop across Load

 Example-8
• Total resistance in the
gate lead is given by

40Ω

3KΩ

• Therefore, R2 is
 Conclusion
• The value of resistor R2 is increasing as firing angle is
further delayed.

S. No Firing Angle R2
1 10o -1.21KΩ
2 18 160Ω
Range of 3 20o 600Ω
Firing 4 30o 2.3KΩ
Angles The same 30deg
5 60 o
9.3KΩ => Firing angle is
6 90o 7.7KΩ only 30 deg even
150 deg is
7 150o 2.3KΩ requested
 RC Triggering Circuits
• The simplest method of improving gate control is to add a
capacitor at the bottom of the gate lead resistance as shown in
following figure.

• Advantage of this circuit

is that the firing delay
angle can be adjusted
past 90o.
 RC Triggering Circuits
• This can be understood by focusing on the voltage across
Capacitor C.
• When the ac supply is –ve, the
reverse voltage across SCR is
applied to RC triggering circuit,
charging the capacitor –ve on
top plate and +ve on bottom
plate.
• When the supply enters its
positive half cycle, the forward
voltage drop across SCR tends
to charge C in opposite
direction.
• However, voltage buildup in
new direction is delayed until
the –ve charge is removed.
 RC Triggering Circuits
• The idea can be extended to achieve even extended firing
angles by modifying the circuit slightly.

• A resistor has been inserted into

the gate lead, requiring the
capacitor to charge higher than 0.7
V to trigger the SCR.

• With the resistor in place,

capacitor voltage must reach a
value large enough to force
sufficient current (IGT) through the
resistor.
 RC Triggering Circuits
• The firing delay angle can further be extended by the use of
double RC network as shown in following figure.

• The delayed voltage across C1 is

used to charge C2 resulting in even
further delay in building up the
gate voltage.
– When apply Vsupply, VC1 charge first,
causing triggering delay compared to
no C1 and when Vsupply applied directly
to G through resistors => then VC1 is
used to charge VC2, causing another
triggering delay
 Triggering
• 50Hz sine wave takes 1/50 seconds to complete one cycle.
 RC Triggering Circuits
• Capacitors in RC triggering circuits usually fall in the range from
0.01µF to 1µF.

• For the given capacitor sizes minimum firing

delay angle (maximum load current) is set by
fixed resistors R1 and R3.

• The maximum firing angle (minimum load

current) is set mostly by variable resistor R2.

• When these gate control circuits are used

with 50Hz AC supply, the time constant of
the RC circuit should fall in the range of 1-
20ms.
 RC Triggering Circuits
• For single RC circuit of fig (a) the product (R 1+R2)C1 should fall in the
range 1ms to 20ms.

Fig(a)

• For double RC circuit of fig(b) (R 1+R2)C1 should fall in that range and
R3C2 should also fall in that range.

Fig(b)
 Example-9
• For the circuit shown in following figure approximate the R1, R2
and R3 to give wide range of firing adjustment.
 Example-9

• Minimum time constant occurs in RC

network-1 when R2 is set to minimum.
 Example-9

• Maximum time constant occurs in RC

netwrok-1 when R2 is set to maximum.
 Example-9

• Time constant of RC netwrok-2 is 2ms.

 Example-9
• Minimum and maximum firing angles
are (1ms  360o/20ms = 18o)
 Example-10
• For the circuit shown in following figure, to what value the
potentiometer be set to obtain a firing delay angle of 120o.
 Silicon-Controlled Switch (SCS)
• SCS is similar to the SCR in construction with the exception being
the SCS has two gates.
• It can be turned on and off by using either terminal. Normally the
SCR is available in power ratings lower than SCR and has faster
switching time than SCR.
 Silicon-Controlled Switch (SCS)
• Either gate can fire SCS. To start, assume that Q1 and
Q2 are off and not conducting.
• A positive pulse on the cathode gate drive Q2 into
conduction and provide a path for Q1 IB. When Q1 on, its
IC provide IB to Q2, thus sustaining the on state of SCS.
SCS only conducts in one direction.
 Silicon-Controlled Switch (SCS)
• SCS can also be turned on with negative pulse on anode
gate. This drives Q1 into conduction, in turn provide IB
for Q2. Once Q2 on, it provide a path for Q1 IB, thus
sustaining the on state.
• To turn off, a positive pulse is applied to anode gate.
This rev-bias BE junction of Q1 and turn it off. Q2 in
turn cut-off and SCS cease conduction. It also can turn
off using negative pulse on cathode gate.
 Silicon-Controlled Switch (SCS)
• Another method for turn off SCS is using
switching method, to reduce anode current
below holding value. BJT acts as a switch to
interrupt the anode current.
• SCS is being used in digital application such as
counter, register and timing circuits.
Single-phase Thyristor Half-Bridge Rectifier
Single-phase Thyristor Half-Bridge Rectifier
 DiAc and Triac
• Both  the  Diac  and  the Triac  are  types  of Thyristors
that  can  conduct  current  in  both  directions
(bilateral).   They  are  four‐layer  devices.
• Diac and triac unlike the SCR will conduct in both
directions making it ideal for ac applications. Diac has
two terminals, while triac has a third terminal, which is
the gate for triggering.
• Diac function basically like two parallel 4-layer diodes
(Shockley diodes) turned in opposite direction. The triac
function basically like two parallel SCR turned in
opposite directions with a common gate terminal.
• Diac turns on when breakover voltage is reached in
either direction.
 Diac
• Diac is also a breakover type device. It’s has
two terminals A1 and A2. When breakover
voltage reach conduction occur with either
polarity across the two terminals.
 Diac
• Once breakover occurs, current direction
depending on the polarity of the voltage across
the terminal. The device turn off when the
current drops below the holding value. The
breakover voltage is approximately symmetrical
for a positive and a negative breakover voltage.
 Diac
• When Diac is biased, the pnpn structure from
A1 to A2 (positive direction) provide the same
operation as 4-layer diode. In equivalent circuit
Q1 and Q2 are fwd-bias, Q3 and Q4 are rev-bias.
The other way around if Diac is biased from A 2
to A1.
 DIAC Applications
• Diacs are used primarily for triggering of triacs.
• Some of the circuit applications of diac are
– Lamp Dimmer
– Heat Control

282
 Triac
• Triac is basically a diac with a gate terminal.
Triac can be turned on by a pulse at the gate
and does not require breakover voltage to
initiate conduction, as Diac.
• Basically triac can be though as two SCR
connected in parallel and in opposite directions
with a common gate terminal.
• Unlike SCR, triac can conduct current in either
direction when it is trigger on, depends on the
polarity of the voltage across A1 and A2
terminals.
 Triac
* Breakover potential
decrease as the gate
current increase (as SCR).

* Triac cease to conduct

when IA drop below
specified value of IH. The
only way to turn off the
triac is to reduce the
current to a sufficiently
low level.

•Ι +  Mode = MT2 current positive (+ve), Gate current positive (+ve)

•Ι –  Mode = MT2 current positive (+ve), Gate current negative (-ve)
•ΙΙΙ +  Mode = MT2 current negative (-ve), Gate current positive (+ve)
•ΙΙΙ –  Mode = MT2 current negative (-ve), Gate current negative (-ve)
Modes Ι– and ΙΙΙ+ are, however, less sensitive configurations requiring a greater gate current
Phase controlled bidirectional switching with
Triacs
 Triac
• When fired by the gate or by exceeding the
breakover voltage, the Triac conducts in both
directions.
• Triac being used in AC power control circuits.
Example-1

287
 Example-1
• Solution

a) How will you trigger the triac by only +ve voltage?

288
 Example-1
• Solution

b) How will you trigger the triac by only -ve voltage?

VA = - (VF+VGT)
= - 2.7V

289
Example-2

290
 Example-2
• Solution

DIAC: ON when VA >= VBO =>

create current to turn TRIAC ON
- make TRIAC less sensitive to
voltage change applied to its gate
- And provide a sharp trigger
current pulse to fully turn-ON the
Triac

291
Example-3
At the start of each cycle, C1 charges up via VR1 until
VC1 is sufficient to trigger Diac ON (triac gate pulse
starts) => C1 to discharge into the gate of the triac
turning Triac “ON”.

Once triac is ON, it shorts out the gate triggering phase

control circuit => no gate triac current (triac gate pulse of
very short time ends) => the triac takes control for the
remainder of the half-cycle.
Triac turns-OFF automatically at the end of the half-
cycle and the VR1 – C1 triggering process starts again
on the next half cycle.

Because the triac requires differing amounts of gate

current in each switching mode of operation, for
example Ι+ and ΙΙΙ–, a triac is therefore asymmetrical
meaning that it may not trigger at the exact same point
for each positive and negative half cycle.

To change gate triggering angle, either change VR1

292
or change VBO of DIAC
Triac Triggering

Circuit (b) has wider control range beyond 90°

The diac popular breakover

voltage is around 32 V
TRIAC Driver: Isolated Gate Driver
 Triac
Application- Phase Control
• Here R1 controls the trigger point at which the
triac turns on for each half of the cycle.
• The off time is called delay angle and the on
time is called the conduction angle.
 Triac
Application- Phase Control
• D1 is used to provide trigger pulses to triac
gate and conduct during positive half at which
the triac trigger. A1 and G are positive with
respect to A2.

• D2 conduct during negative half cycle and R1 set

the trigger point. A2 and G are positive with
respect to A1.
 Triac Waveforms

The higher the firing angle

the lower the average power
Please note the asymmetrical
behavior in (C)
 Example of Triac Ratings
• Used in heat / light control, ac motor control circuit
• V / I rating: 1200V / 300A.
• Max. Frequency: 400Hz.
• Switching time: 200 to 400sec.
• On state resistance: 3.6m.
 TRIAC Applications
• Electrical AC power control using a Triac is extremely
effective when used properly to control resistive type
loads such as incandescent lamps, heaters or small
universal motors commonly found in portable power
tools and small appliances.

299
 DIAC Applications
– Lamp Dimmer

300
 DIAC Applications
– Heat Control

301
Example: Control Speed of AC Motor
Example: AC Load Interface to MCU
Example: AC Dimmer
Example: AC Dimmer
Example: AC Dimmer
Example: AC Dimmer
 Unijunction Transistor (UJT)
• UJT is another solid state three terminal device that can be used
in gate pulse, timing circuits and trigger generator applications to
switch and control thyristors and triacs for AC power control
applications.

309
 Unijunction Transistor (UJT)
• Equivalent Circuit: UJT’s have unidirectional conductivity and
negative impedance characteristics acting more like a variable
voltage divider during breakdown

310
 Unijunction Transistor (UJT)
• As the physical position of the p-n junction is closer to
terminal B2 than B1 the resistive value of RB2will be less than RB1.

• These two series resistances produce a voltage

divider network between the two base terminals
of the Unijunction transistor

• Since this channel stretches from B2 to B1,

when a voltage is applied across the device, the
potential at any point along the channel will be
in proportion to its position between
terminals B2 and B1.

• The level of the voltage gradient therefore

depends upon the amount of supply voltage.

311
 Unijunction Transistor (UJT)
• When used in a circuit, terminal B1 is connected to ground and
the Emitter serves as the input to the device. 

• Suppose a voltage VBB is applied

across the UJT
between B2 and B1 so that B2 is
biased positive relative to B1.

• With zero Emitter input applied,

the voltage developed
across RB1 (the lower resistance)
of the resistive voltage divider can
be calculated as:

312
 Unijunction Transistor (UJT)
• For a Unijunction transistor, the resistive ratio of RB1 to RBB is
called the intrinsic stand-off ratio (η).

• Typical standard values of η range

from 0.5 to 0.8 for most common
UJT’s.

313
 Unijunction Transistor (UJT)
• If a small positive input voltage (less than the voltage
developed across resistance RB1) is now applied to the
Emitter input terminal, the diode p-n junction is reverse
biased, thus offering a very high impedance and the device
does not conduct.

• The UJT is switched “OFF” and zero current flows.

• However, when the Emitter input voltage is increased and

becomes greater than VRB1 (or ηVBB + 0.7V, where 0.7V equals
the p-n junction diode volt drop) the p-n junction becomes
forward biased and the Unijunction transistor begins to
conduct.

• The result is that Emitter current, ηIE now flows from the

Emitter into the Base region. 314
UJT Characteristics

315
UJT Characteristics

316
 Example-1
• The intrinsic stand-off ratio for a UJT is determined to be 0.6. If
the inter-base resistance (RBB) is 10kΩ what are the values of RB1
and RB2?

Solution

• Intrinsic stand-off ratio for a UJT is given as

317
 Example-1
• Inter-base resistance (RBB) is 10kΩ

318
 Example-2
• A UJT has 10V between the bases. If the intrinsic stand off ratio
is 0.65, find the value of stand off voltage. What will be the peak
point voltage if the forward voltage drop in the pn junction
is .7V?
Solution

• Stand off voltage (VRB1) is given as

319
 Example-2
Solution

• Peak point Voltage (VP) is given as

320
Exercise-1

321
 UJT Applications
• The most common application of a Unijunction
transistor is as a triggering device for SCR’s and Triacs 

• Other UJT applications include sawtoothed generators,

simple oscillators, phase control, and timing circuits.

• The simplest of all UJT circuits is the Relaxation

Oscillator producing non-sinusoidal waveforms.

322
 UJT Relaxation Oscillator
• In a basic and typical UJT relaxation oscillator circuit, the
Emitter terminal of the Unijunction transistor is
connected to the junction of a series connected resistor
and capacitor.

323
UJT Relaxation Oscillator

324
 UJT Relaxation Oscillator

• Discharge of the capacitor occurs when VC =Vp.

• Note: VD is ignored in above equation

325
 Example-3
• The data sheet for a 2N2646 Unijunction Transistor gives
the intrinsic stand-off ratio η as 0.65. If a 100nF capacitor
is used to generate the timing pulses, calculate the timing
resistor required to produce an oscillation frequency of
100Hz.

326
 Example-3
• The timing period is given as:

• The value of the timing

resistor, R3 is calculated as:

327
UJT Motor Speed Control Circuit

328
Example-4

329
Example-4

330
Example-4

331
Example-4

332
 Outline
• Power Transistors
– Power BJT

– Power MOSFET

– IGBT

• GTO

334
 Power Transistors
• Power transistors are fully controlled semiconductor
switches.

• These are turned ON when the signal (voltage or current)

is given to control terminal.

• Types
– Power BJT
– Power MOSFET
– Insulated Gate Bipolar Junction Transistor (IGBT)

335
 Power BJT
• The symbol of the Power BJT is same as signal
level transistor.

336
 Power BJT
• The construction of the Power Transistor is  different from the
signal transistor as shown in the following figure.

• The n- layer is added in the power BJT which is known as drift

region. 337
 Power BJT
• A Power BJT has a four layer structure of alternating P and
N type doping.

• In most of Power Electronic applications, the Power

Transistor works in Common Emitter configuration.

• In power switches npn transistors are most widely used

than pnp transistors.

• The thickness of the drift region determines the breakdown

voltage of the Power transistor.
338
 VI Characteristics

In Quasi-saturation, both
junctions are forward bias. BJT
offers low resistance => power
loss is less. In this region, the
device does not go into deep
saturation. So, it can turn off
quickly.
=> higher frequency
applications.

339
 VI Characteristics
• The  VI characteristics of the Power BJT is different from signal level
transistor.

• The major differences are Quasi saturation region & secondary

breakdown region.

• The Quasi saturation region is available only in Power transistor

characteristic not in signal transistors. It is because of the lightly
doped collector drift region present in Power BJT.

• The primary breakdown is similar to the signal transistor’s avalanche

breakdown. 

• Operation of device at primary and secondary breakdown regions

should be avoided as it will lead to the catastrophic failure of the
device.

340
 Power MOSFET
A power MOSFET is a specific type of 
metal–oxide–semiconductor field-effect transistor (MOSFET) designed to
handle significant power levels. Compared to the other 
power semiconductor devices, such as an insulated-gate bipolar transistor
 (IGBT) or a thyristor, its main advantages are high switching speed and
good efficiency at low voltages. It shares with the IGBT an isolated gate that
makes it easy to drive.

The power MOSFET shares its operating principle with its low-power
counterpart, the lateral MOSFET

Power MOSFETs have much lower Ron than other low power signal types.

The power MOSFET is the most common power semiconductor device in

the world, due to its low gate drive power, fast switching speed, easy
advanced paralleling capability, wide bandwidth, ruggedness, easy drive,
simple biasing, ease of application, and ease of repair. In particular, it is the
most widely used low-voltage (that is, less than 200 V) switch. It can be
found in a wide range of applications, such as most power supplies, 
DC-to-DC converters, low-voltage motor controllers, and 
many other applications.

341
Power MOSFET

342
Power MOSFET

343
Power MOSFET

344
• IGBT is a new development in area of Power MOSFET
technology.
• Application in high voltage or high frequency => can synthesize
complex waveforms PWM, lowpass filers, switching amplifiers,
variable-frequency drives (VFDs), electric cars, trains, variable
speed refrigerators, lamp ballasts, and air-conditioners.

• This device combines into it the advantages of both MOSFET and

BJT.
• So an IGBT has high input impedance like a MOSFET and low on
state power loss like a BJT.
• Further, IGBT is free from second breakdown problem presented
in BJT.
345
 Cross-Sectional View of an IGBT

Metal

Silicon Dioxide

Metal

346
Cross-Sectional View of an IGBT

347
 IGBT
•  It is an FET integrated with a bipolar transistor in a form of
Darlington configuration.

348
 IGBT I-V Characteristics
IGBT is simply “ON” or “OFF” by activating and deactivating G. Applying a positive input
voltage signal across GE will turn “ON”, while making the input gate signal zero or slightly
negative will cause it “OFF” in much the same way as a bipolar transistor or eMOSFET.
Another advantage of the IGBT is that it has a much lower on-state channel resistance than
a standard MOSFET.

349
 IGBT
• “IGBT Transistor” has the output switching and conduction
characteristics of a bipolar transistor but is voltage-controlled
like a MOSFET.
– IGBT’s gate driver is similar to MOSFET

• IGBTs are mainly used in power electronics applications, such as

inverters, converters and power supplies, were the demands of the
solid state switching device are not fully met by power BJTs and
power MOSFETs.
• High-current and high-voltage BJTs are available, but their switching speeds
are slow, while power MOSFETs may have higher switching speeds, but high-
voltage and high-current devices are expensive and hard to achieve.

350
Antiparallel diode

351
Protection of power semiconductor devices

Protection circuits
• Overvoltage protection
• Overcurrent protection

Snubber circuits—specific protection circuits that can limit

du/dt or di/dt
• Turn-on snubber
• Turn-off snubber
 Causes of overvoltage on power semiconductor
devices

External reasons
• Overvoltage caused by operation of mechanic swithes
• Overvoltage caused by thunder lightening

Internal reasons
• Overvoltage caused by the reverse recovery of diode or
thyristor
• Overvoltage caused by the turning-off of fully-controlled
devices
Measures to protect power semiconductor devices from
overvoltage

• Lightening arrestor
• RC or RCD snubbers (will be discussed later)
• Zener diode, Metal Oxide Varistor (MOV), Break Over Diode
(BOD)
 Measures to protect power semiconductor devices from
overcurrent
• Fuse
• Circuit breaker
• Protection with current feedback control in the control
circuit
• Protection with overcurrent detection in the gate drive
circuit—the fastest measure
 Functions and classifications of snubbers
Functions
• Limiting voltages applied to devices during turn-off transients
• Limiting device currents during turn-on transients
• Limiting device current rising rate (di/dt) at device turn-on
• Limiting the rate of rise (du/dt) of voltages across devices during
device turn-off
• Shaping the switching trajectory of the device

Classifications
• According to different switching transients
– Turn-off snubber (sometimes just called snubber)
– Turn-on snubber
• According to the treatment of energy
– Power dissipating snubber
– Lossless snubber
 Operation principle of typical snubbers
Circuit configuration uCE without turn-off snubber
iC
without turn-on snubber uCE
iC
T u rn -o n with turn-off
Ri snubber snubber
Li O
VDi t
with turn-on snubber
T u rn -o ff
snubber
VDs

V Rs

Cs

L
VD
Switching trajectory
Other turn-off snubbers
Comparison Table

359
 SIMPLE MOSFET GATE DRIVER

Note: MOSFET requires VGS =+15V for turn on and 0V to turn

off. LM311 is a simple amp with open collector output Q1.

When B1 is high,
high Q1 conducts. VGS is pulled to ground.
MOSFET is off.
off
When B1 is low,
low Q1 will be off. VGS is pulled to VGG. If VGG is
set to +15V, the MOSFET turns on.
Triggering and Drive Circuit for MOSFET
 Some examples of drive circuits

Gate drive with totem-pole configuration

 Some examples of drive circuits
Electrical isolation methods

• Transformer isolation

• Opto-coupler isolation

• Isolated dc power supplies

for drive circuits
Drive Circuits for MOSFET & IGBT
Drive Circuits for MOSFET & IGBT
 A typical gate drive circuit for IGBT based on an
integrated driver chip

4 VCC 4.7k
Detection Fast recovery diode
14 circuit 1 Sensing trr¡Ü0.2s
8
1
Timer and
reset circuit
+5V 30V
14 5
Interface 3.1
circuit 5 uo 4
M57962L +15V
ui 13
Turn-off Error 1 100F
circuit
8
indicating 100F
13 6
6 VEE -10V

M57962L integrated driver chip

IGBT application examples
IGBT application examples
IGBT application examples
 Application notes
• IGBTs don't work directly with 3.3V and 5V micro-
controllers such as Arduino.
– At least 7-volts is required for turn on. The high Vce of
1.5V to 2V can waste power.
• Shouldn't be used unless it's a very high voltage circuit.
They have a high voltage drop (Vce ~2V) with low
voltage H-bridge circuits and are better used for higher
voltage switching.
 GTO (Gate Turn-off Thyristor)
• GTO can be turned-on by a gate signal, and it can also be
turned-off by a gate signal of negative polarity.

371
 GTO (Gate Turn-off Thyristor)
• Applications of GTO: They are used in

– Motor drives

– Static VAR compensators (SVCs)

– AC/DC power supplies with high power ratings

372
 GTO (Gate Turn-off Thyristor)
• Compared to a conventional SCR, the device has the
following disadvantages

– Magnitude of latching, holding currents is more. The latching

current of the GTO is several times more as compared to
conventional thyristors of the same rating. 

– On state voltage drop and the associated loss is more.

– Due to multicathode structure of GTO, triggering gate current is

higher than that required for normal SCR.

– Gate drive circuit losses are more.

373
SCR REVIEW
 SCR Ratings
(a) SCR Current Ratings
1- Maximum Repetitive RMS current Rating
• Average on-state current is the maximum average current value that can be carried by the SCR in
its on state.
• RMS value of nonsinusoidal waveform is simplified by approximating it by rectangular waveform.
• This approximation give higher RMS value, but leaves slight safety factor.
• Average value of pulse is

• Form factor is
• Knowing the form factor for given waveform, RMS current can be
obtained from

I RMS =fo(IAVE)
• Maximum repetitive RMS current is given by

I T(RMS) =fo(IT(AVE))
• Conduction angle verses form factor
Conduction angle (θ) Form factor (fo)
20° 5.0
40° 3.5
60° 2.7
80° 2.3
100° 2.0
120° 1.8
140° 1.6
160° 1.4
180° 1.3
 Conduction Angle

• Duration for which SCR is on. It is measured as shown

2- Surge Current Rating
Peak anode current that SCR can handle for brief duration.

3- Latching current
Minimum anode current that must flow through the SCR in order for it to
stay on initially after gate signal is removed.

4- Holding Current
Minimum value of anode current, required to maintain SCR in conducting
state.
 (b) SCR Voltage Ratings

1- Peak repetitive forward blocking voltage

Maximum instantaneous voltage that SCR can block in forward direction.
2- Peak Repetitive Reverse Voltage
Maximum instantaneous voltage that SCR can withstand, without
breakdown, in reverse direction.
3- Non-repetitive peak reverse voltage
Maximum transient reverse voltage that SCR can withstand.
 (c) SCR Rate-of-Change Ratings

1- (di/dt rating)
Critical rate of rise of on-state current. It is the rate at which anode current increases and must be
less than rate at which conduction area increases.
To prevent damage to SCR by high di/dt value, small inductance is added in series with device. Vaue
of required inductance is
L>= Vp
(di/dt)max

2- dv/dt rating
Maximum rise time of a voltage pulse that can be applied to the SCR in the off state without causing
it to fire. Unscheduled firing due to high value of dv/dt can be prevented by using RC snubber circuit.
 (d) Gate Parameters
1- Maximum Gate Peak Inverse Voltage
Maximum value of negative DC voltage that can be applied without damaging the gate-cathode junction.

2-Maximum Gate Trigger Current

Maximum DC gate current allowed to turn on the device.

3- Maximum gate trigger voltage

DC voltage necessary to produce maximum gate trigger current.

4- Maximum Gate Power Dissipation

Maximum instantaneous product of gate current and gate voltage that can exist during forward-bias.

5- Minimum gate trigger voltage

Minimum DC gate-to-cathode voltage required to trigger the SCR.

6-Minimum gate trigger current

Minimum DC gate current necessary to turn SCR on.
 Comparison between different commonly
used Thyristors
• Line Commutated Thyristors available up to 6000V,
4500A.
• Ex: Converter grade (line commutated) SCR.
• V / I rating: 5KV / 5000A
• Max. Frequency: 60Hz.
• Switching time: 100 to 400sec.
• On state resistance: 0.45m.
 Example of Inverter Grade Thyristor Ratings
• V / I rating: 4500V / 3000A.
• Max. Frequency: 20KHz.
• Switching time: 20 to 100sec.
• On state resistance: 0.5m.
 Series and Parallel SCR Connections

SCRs are connected in series and parallel to extend

voltage and current ratings.

For high-voltage, high-current applications, series-

parallel combinations of SCRs are used.
 SCRs in Series
• Unequal distribution of voltage across two series SCRs.

• Two SCRs do not share the same supply voltage. Maximum voltage
that SCRs can block is V1+V2, not 2VBO.
• Resistance equalization

• Voltage equalization
• RC equalization for SCRs connected in series.
 SCRs In Parallel
• Unequal current sharing between two SCRs is shown:

• Total rated current of parallel connection is I1+I2, not 2I2.

• With unmatched SCRs, equal current sharing is achieved by adding low
value resistor or inductor in series with each SCR, as shown below.

• Value of resistance R is obtained from:

R=V1-V2
I2-I1
Current sharing in SCRs with parallel reactors
Equalization using resistors is inefficient due to
 Extra power loss
 Noncompansation for unequal SCR turn-on and turn-off times.
 Damage due to overloading

SCRs with center-tapped reactors is shown below.

 What is Commutation for SCR?
• What is Commutation?
The process of turning off an SCR is called
commutation.

It is achieved by
1. Reducing anode current below holding current
2. Make anode negative with respect to cathode

• Types of commutation are:

1. Natural or line commutation
2. Forced commutation
 SCR Turnoff Methods
1. Diverting the anode current to an alternate path

2. Shorting the SCR from anode to cathode

3. Applying a reverse voltage (by making the cathode positive with

respect to the anode) across the SCR

4. Forcing the anode current to zero for a brief period

5. Opening the external path from its anode supply voltage

6. Momentarily reducing supply voltage to zero

 (1) Capacitor Commutation

• SCR turnoff circuit using a transistor switch

• SCR turnoff circuit using commutation capacitor

• Value of capacitance is determined by:

C>= tOFF
0.693RL
(2) Commutation By External Source
 (3) Commutation by Resonance
 Series resonant turnoff circuit
 Parallel resonant turnoff circuit
(4) AC line commutation
GATE TRIGGERING/DRIVE CIRCUITS
 Power Switches Classification

 Power semiconductor devices can be

categorized into 3 types based on their
control input requirements:
• Current-driven devices – BJTs, MDs, GTOs
• Voltage-driven devices – MOSFETs, IGBTs,
MCTs
• Pulse-driven devices – SCRs, TRIACs
 Gate-Triggering Circuits
• Triggering circuits provide firing signal to turn on power switches at
precisely the correct time.
• Firing circuits must have following properties
1. Produce gate signal of suitable magnitude and sufficiently short rise time.
• The gate signal should be removed after the thyristor has been turned on. A continuous gate signal
will increase the power loss in the gate junction.
2. Produce gate signal of adequate duration.
• The width of the gate pulse must be greater than the time required for the anode current to rise to
the holding current. In practice, the gate pulse width is made wider than the turn-on time of the
thyristor.
3. Provide accurate firing control over the required range.
4. Ensure that triggering does not occur from false signals or noise
5. In AC applications, ensure that the gate signal is applied at right times (e.g., when the SCR is
forward-biased)
• No gate signal should be applied when the thyristor is reversed biased. Otherwise, the thyristor may
fail due to an increased leakage current.
6. For SCR in three-phase circuits, provide gate pulses that are 120° apart with respect to the
reference point
7. Ensure simultaneous triggering of power switches (e.g., SCRs) connected in series or in
parallel.
 What are Gating requirements
• Kind or type of application which required a switching action
• Propagation delay (speed of switching)
• Rated Voltage for Breakdown Voltage (VBO) for triggering
• Holding Current (IHo)
• The avanlache voltage
• What is gate firing network.
The firing network is the combination of two 4 layer device which
is the thyristor, for example, the diac is used to attached to the
gate terminal of GTO, TRIAC, SCR and other three terminal
thyristor device. also we can used the capacitor to delay the firing
action of the device.
 Pulse/Pulse Train Triggering Signals?
• Gate drive requirements in terms of continuous dc signal can be obtained from Fig.
4.11. But, it is common to use a pulse to trigger a thyristor. For pulse widths beyond
100 µsec, the dc data apply . For pulse widths less than 100 µsec, magnitudes of
gate voltage and gate current can be increased.
• For thyristors, higher the magnitude of gate current pulse, lesser is the time to
inject the required charge for turning-on the thyristor. Thus, SCR turn-on time can
be reduced by using gate current of higher magnitude. It should be ensured that
pulse width is sufficient to allow the anode current to exceed the latching current.
In practice, gate pulse width is often taken as equal/or greater than, SCR turn-on
time.
• Sometimes the pulses of Fig. 4.12 (a) are modulated to generate a train of pulses as
shown in Fig. 4.12 (b). This technique of firing the thyristor is called high-frequency
carrier gating. The advantages offered by this method of firing the SCRs are lower
rating, reduced dimensions and therefore an overall economical design of the pulse
transformer needed for isolating the low power circuit from the main power circuit.
 Firing Circuit – Firing Angle Control

• Firing circuit –firing angle control

 Establish relation between vc and Vt

+
iref + current vc firing  controlled Vt
- controller circuit rectifier
–
 AC-DC controlled rectifier

+
vc firing  controlled rectifier
Va
circuit
–

?
vc(s) va(s)
DC motor

The relation between vc and va is determined by the firing circuit

It is desirable to have a linear relation between vc and va

 AC-DC controlled rectifier

linear firing angle control

Vm
Input voltage
0  2 3 4

vc vt
Sawtooth compared with control signal

Results of comparison to trigger SCRs

Output voltage
 AC-DC controlled rectifier

linear firing angle control

vt vc

Vm  
 3 4
0 2
vc
 
vt
vc vt
vc
 

2Vm  vc 
Va  cos   

  vt 

A non-linear relation between Va and vc

 Firing/Triggering Control

+
vc firing  controlled rectifier
Va
circuit
–

?
vc(s) va(s)
DC motor

The relation between vc and va is determined by the firing circuit

It is desirable to have a linear relation between vc and va

 Firing/Triggering Control

Cosine-wave crossing control

Vm
Input voltage
0  2 3 4

vc vs
Cosine wave compared with vc

Results of comparison trigger SCRs

Output voltage
 Firing/Triggering Control
Example: DC drives with Controlled rectifier
Cosine-wave crossing control
Vscos(t)
cos() = vc
Vm
v 
0  2 3 4   cos 1  c 
 vs 

vc vs

2Vm v c  1  v c 
Va  coscos
   

 vs   vs 


A linear relation between vc and Va

 Firing angle control: Linear/Nonlinear

• Firing angle control linear firing angle control

vt v vc
 c  180
180  vt

2Vm  vc 
Va  cos 180 
  vt 

Cosine-wave crossing control

v c  v s cos 

2Vm v c
Va 
 vs
 AC-DC controlled rectifier

e.g. cosine wave crossing control

Monostable to create a pulse of certain width
 Firing/Triggering Control

Va is the average voltage over one period of the waveform

- sampled data system

Delays depending on when the control signal changes – normally taken as half of
sampling period
 Firing/Triggering Control

Va is the average voltage over one period of the waveform

- sampled data system

Delays depending on when the control signal changes – normally taken as half of
sampling period
 Modeling: Firing/Triggering Control

•Steady state: linear gain amplifier

•Cosine wave–crossing method
•Transient: sampler with zero order hold
converter

T
GH(s)

T – 10 ms for 1-phase 50 Hz system

– 3.33 ms for 3-phase 50 Hz system
 Modeling: Firing/Triggering Control

• Model simplified to linear gain if bandwidth (e.g.

current loop) much lower than sampling
frequency
 Low bandwidth – limited applications

• Low frequency voltage ripple  high current

ripple  undesirable
 Example: Firing/Triggering Control
Modeling of the Power Converters: DC drives with SM Converters

Vdc
Switching signals obtained by comparing
control signal with triangular wave +

Va

vtri

q
vc

We want to establish a relation between vc and Va

AVERAGE voltage

vc(s) Va(s)
? DC motor
 Example: Firing/Triggering Control
Modeling of the Power Converters: DC drives with SM Converters

Bipolar switching scheme

Vdc
vc
2vtri
-Vdc
q
vtri
+
Vdc
Vdc vA
+ VAB 
0
vc −
Vdc
vB
0
q
Vdc
vAB
v v
d A  0 .5  c d B  1  d A  0 .5  c -Vdc
2 V tri 2 V tr i

V dc
V A  VB  V AB  vc
V tr i
 Example: Firing/Triggering Control
Modeling of the Power Converters: DC drives with SM Converters

Bipolar switching scheme

v a ( s) Vdc

v c ( s) Vtri

vc(s) Vdc va(s)

DC motor
Vtri
 Example: Firing/Triggering Control
Modeling of the Power Converters: DC drives with SM Converters
Vdc
Unipolar switching scheme vc
Leg b
Vtri
+ -vc

vtri Vdc

qa
vc −
vA
Leg a

vtri

-vc qb vB

vAB

The same average value we’ve seen for bipolar !

 Example: Firing/Triggering Control
Modeling of the Power Converters: DC drives with SM Converters
Unipolar switching scheme

v a ( s) Vdc

v c ( s) Vtri

vc(s) Vdc va(s)

DC motor
Vtri
 Methods of Thyristor Turn-on/Triggering
• Thyristors can be turned On/Triggered by several methods:
– Thermal /TemperatureTurn-on (is normally avoided).
– Light as in case of Light Activated Thyristors.
– High Voltage (forward breakover voltage), it might destroy the
thyristor.
– dv/dt.
– Gate Current.
• Most commonly employed triggering method for Thyristors is “Gate Turn-On”.
In this method, a gate pulse is used to turn-On the thyristor and a circuit used
to generate gate pulse is called firing/triggering circuit.
• Three popular firing/triggering circuits:
– R Firing
– RC Firing
– UJT relaxation oscillator
 Popular Firing/Triggering Circuits

R Triggering - Resistance firing circuit RC Triggering - RC half-wave trigger circuit

 Popular Firing/Triggering Circuits

R Triggering - Resistance firing circuit RC Triggering - RC half-wave trigger circuit

Resistive phase control RC phase control

Typical  Gate  Control  Circuits

ƒ Max.  firing  angle  =  90°

ƒ iG waveform  is  idealized,
actually  it  is  zero  after
triggering
ƒ Burglar Alarm
 Use  DC  supply
 SW  is  closed  by  opening  a door, 
window,  or  light  beam
interruption
Triggering  beyond  90°

Problems:
Minimum  Firing  Angle  is  set
by  R1  +R3 •Temperature  dependence
Maximum  Firing  Angle  is  set •Inconsistent  firing  behavior
by  the  size  of  R2 when  replacing  the  SCR
Popular Firing/Triggering Circuits
 SCR Trigger Circuits using UJT Oscillator
Circuit A
Example 1: R-Firing Circuit
Example 2: R-Firing Circuit
 Types Of Gate Firing Signals

1. DC signals
2. Pulse signals
3. AC signals
(1) DC Gating Signal From Separate Source
DC Gating signals from Same Source
 Disadvantage of DC gating Signals

1. Constant DC gate signal causes gate power

dissipation

2. DC gate signals are not used for firing SCRs in AC

applications, because presence of positive gate
signal during negative half cycle would increase
the reverse anode current and possibly destroy
the device.
 (2) Pulse Signals
1. Instead of continuous DC signal, single pulse or train of pulses is
generated.
2. It provides precise control of point at which power switches (e.g.,
SCR) is fired.
3. It provides electrical isolation between power switches (e.g., SCR)
and gate-trigger circuit. Pulse transformers and opto-couplers can
be used to provide this isolation:
• Pulse transformers:
Advantages: Not need external power for operation and very simple to use
Disadvantages: Saturate at low frequency hence it
can be used only for high frequencies and the
signal can be distorted due to magnetic coupling.
• Opto-couplers:
The rise and fall times of phototransistors are
very small.
 SCR trigger circuits using UJT oscillator
Circuit A
Circuit B
Short pulse
Long pulse
Pulse train generator
Pulse train with timer
and AND gate
Triggering SCRs in Series and in Parallel
SCR trigger circuit using DIAC
SCR trigger circuit using Optocoupler
Photo-SCR coupled isolator
 Thyristor/SCR Driver: Isolated Gate
• Driver
To prevent MCU from the effect of transients due to
switching, use either a pulse transformer or opto-
isolator (recommended here with MCT2E). MCU should
be coded to generate a pulse of 10us to trigger the SCR
at an appropriate time (Later). SCR 2N6403 used in
Figure 13.24 can work up to 400V and carry load current
of 16A. The BJT transistor 2N2222 is used as the SCR
gate driver to amplify the pulse current to the required
triggering gate current of 30mA (note that most MCUs
can not source this much current).
TRIAC Driver: Isolated Gate Driver
 (3) AC Signals

Resistive phase control RC phase control

R Triggering - Resistance firing circuit RC Triggering - RC half-wave trigger circuit

Gate triggering characteristics
 Analog Circuit: Generate Gating Signals
• Analog circuit
Analog Circuit: PWM Signal Generation
 Analog Circuit Example for Gating Signals
• In the figure, the first stage is a control transformer for stepping down the line voltage. The secondary
of the transformer is fed to the zero-cross detector.
• In systems where the ac supply has high impedance, the non-sinusoidal current of the bridge causes
distortion of the line voltage. The result of the distortion of the line voltage waveform is multiple zero
crossings and zero-cross jitter.
– The latter means that the zero-cross frequency fluctuates. The former means that more than one zero
crossing may occur around the zero-cross of the fundamental of the line voltage. Multiple zero-cross and
zero-cross jitter could result in unsymmetrical firing of the bridge => distortion of the line voltage =>
harmonic instability.
– To prevent un-symmetry, the zero-cross detector is equipped with hysterisis comparators and a PLL. The
former eliminates multiple zero crossings. The latter stabilizes and locks the frequency of the detection pulses
to the line fundamental zero-crossings.
• The firing pulse generator utilizes the zero-cross
pulses of the previous stage to generate a periodic
ramp. The ramp is reset and commences rising with
the onset of each zero-cross pulse. This ramp is
compared with a reference voltage proportional to
the desired ignition delay angle. A short firing pulse
is generated at the moment the ramp exceeds the
reference voltage.
• The final stage is the gate drive circuit. This stage amplifies, shapes and delivers the firing pulses to
the gate of the thyristors. In the majority of applications galvanic isolation is needed between the
drive circuit and the thyristor gate. This isolation may be provided through pulse transformers, opto-
couplers, or optical fiber cable (for light triggered thyristors, LTSCR).
 Analog Circuit Example for Gating Signals
• In the lower comparator, the negative line voltage is compared with the
zero level. Thus, this comparator is high, when the line voltage is negative
and low, when the line voltage is positive. The upper comparator compares
the positive line voltage with the level of 0.7V. Thus, the output of this
comparator is high when the line voltage exceeds 0.7V and low otherwise.
• The output of both comparators is "ORed" through two diodes. The
combined output is a negative notch at the arrival of a zero crossing. The
negative notch is inverted through a transistor and a short pulse is obtained
at the output of the stage, which is synchronous to the line zero-cross as
shown in the figures below.
 Digital Circuit: Generate Gating Signals
• Advantages:
– Stable, very robust to the noise
– Easy to be controlled by CPU/DSP
– Reduce the load of CPU/DSP
• Disadvantages:
– Expensive hardware
– High frequency counter
Digital Circuit: Generate Gating Signals
 Digital Circuit: Generate Gating Signals
• Software programming
– At t1, the number in free running counter (TCNT) matches
the number of the output compare register (OC1), an
interrupt request will be made.
– Interrupt service routine
• Determine the type of interval (gap, or pulse)
• Set the output pin to logic “0” for gap interval or logic “1” for pulse
interval.
• Calculate the time interval for following T period
• Clear the interrupt flag and return
– If the calculation in the interrupt service routine can not
be completed during Tg, the program will crash. What
shall we do?
Zero Crossing Detection Circuit
 Thyristor/SCR Driver: Zero-Crossing
• To determine the firing time (firing
angle), a zero-crossing circuit is
used as in Figure 13.25 or Figure
13.26 (using a transformerless
power supplies).
 Thyristor/SCR Driver: Isolated Gate
• Driver
To prevent MCU from the effect of transients due to
switching, use either a pulse transformer or opto-
isolator (recommended here with MCT2E). MCU should
be coded to generate a pulse of 10us to trigger the SCR
at an appropriate time (Later). SCR 2N6403 used in
Figure 13.24 can work up to 400V and carry load current
of 16A. The BJT transistor 2N2222 is used as the SCR
gate driver to amplify the pulse current to the required
triggering gate current of 30mA (note that most MCUs
can not source this much current).
 Thyristor/SCR Driver: Zero-Crossing
• To determine the firing time (firing
angle), a zero-crossing circuit is
used as in Figure 13.25 or Figure
13.26 (using a transformerless
power supplies).
 Thyristor/SCR Driver: Snubber Circuit
• In Figure 13.23, SCR 2N6401 has a current rating of 16A (surge load
currents up to 160A) with a maximum supply voltage of 100V. It requires
30 mA of gate current to turn on. A RC snubber circuit is advised to be
used across the SCR to avoid a large dV/dt effect (i.e., the phenomenon
that SCR may be turned on even though the voltage across the SCR is less
than forward break over-voltage when the voltage V rises very fast)
Thyristor
application &
photosensitive
control circuits
Contents:
 Ac power control using TRIAC.
 Light dimmer.
 Automatic battery charger.
 Emergency light system.
 Temperature controller.
 Fan speed regulator.
 Opto coupler.
 Burglar alarm .
 Batch counter.
 Smoke detector.
 Ac phase control using triac:
Phase control is also called as firing angle control. so phase
control is basically the control of firing angle of the triac. The
phase control is used to precisely control the amount of power
delivered to the load such as fans or lamp load etc..
Operation:
Operation in positive half cycle:
In the positive half cycle , live point (L) is positive
with respect to the neutral point (N).
The charging current for the capacitor C₁ flows
through R₁ as shown in figure.
As soon as the voltage across C₁ reaches the break-
over voltage of the Diac, it is turn on to supply gate
current for the Triad and the Triad will be turned on.
The conducting Triac is equivalent to a closed
switch. So the R₁C₁ and Diac short circuited. The
load voltage is equal to the instantaneous supply
voltage.
Operation in negative half cycle:
In the negative half cycle, the live point (L) is negative with
respect to the neutral (N).
The Triac and Diac both are in the off state. The charging current
for the capacitor C₁ flows through R₁ as shown in fig.
Voltage on C₁ is now negative. As soon as this voltage reaches the
break-over voltage of the Diac, it is turned on and supplies gate
current to the Triac.
The Triac is then turned on. The Diac and R₁C₁ circuit is short
circuited.
The load current reverses its direction and the voltage across the
load will be negative equal to the instantaneous ac supply voltage.
Thus Diac being a bi-directional device can turn on the Triac in
both the half cycles of the input ac supply. The capacitor C₁ must be
a non-polarized capacitor, being capable of charging to positive as
well as negative voltages.
 The firing angle (α) i.e. the instant at which the Triac is
turned on in the positive as well as negative half cycles of
the ac supply voltage can be controlled by making the
resistance R₁ variable.
 This variable resistance would then decide the charging rate
of capacitor C₁ and hence firing angle (α) in each half cycle.
As the control circuit operates directly on AC supply, it is
automatically synchronized with the supply. The load
voltage waveforms for a resistive load are shown above.
Light dimmer using triac:
Light dimmer
using triac:
Battery charger:
Waveforms of
battery charger:
Operation:
 The input transformer T is a step down transformer reduces the
230V AC mains 15V.
 The secondary voltage rectified by the full wave rectifier circuit.
The zener diode Z1 maintains a constant voltage 15V, at point “x”
 The rectifier voltage waveform at “A” .The dotted line in this figure
indicates the battery voltage.
 When voltage at point A is greater than the battery voltage the
SCR1 is forward biased and can conduct if the gate junction is a
forward biased.
 Thus SCR1 conducts from P to R as shown in figure and charges 12
volt battery connected in the circuit.
 As the battery accumulates more and more charge, the dotted line
goes up and the point P and R come closer to Q in figure, thus a
reducing the conduction time for SCR1, and hence increasing the
charging time , of battery.
 When the battery is fully charged say about 14 volts the cathode of
SCR1 is at 14V and the gate is at 14.3V. This difference of 0.3V
between the gate and cathode can not forward biased as gate
junction and will not be triggered. Thus the battery is cut off from
the supply and charging will stop automatically.
Emergency light system:
The basic emergency lightning system is as shown in figure. This
system includes the facility to charge the 6 volt battery and switches
automatically from the AC supply failure takes place.
Operation:
Mode 1 (when AC supply is on) :
The diode D1and D2 along with the center tap transformer T1
from full wave rectifier. They provide DC voltage for the 6V lamp
load when the AC supply is ON. Diode D3 and R1 supply the battery
charging current which can be varied by R1. The anode gate of SCR1
is kept at the battery voltage. While the cathode of SCR1 is kept at a
higher potential by C1. Therefore as long as the AC supply is ON, the
SCR1 remains reverse biased.
Mode 2 (when AC supply is OFF):
As soon as the AC supply is interrupted, the output of the
rectifier formed by D1 and D2 goes to zero. The cathode potential of
SCR1 falls below battery voltage. The gate current is supplied to
SCR1 through R3 and the SCR is triggered. This connects the 6v
battery across the lamp. When the AC input reappears the SCR1 is
turned OFF automatically and the charging of the battery will begin.
Temperature controller:
Phase control circuits may also be used for regulating
temperature. Fig. shows the connection diagram of temperature
controller.
Operation:
 It is a simple full-wave phase control circuit.
 By adjusting resistance R₁ and pot R₂ we can fix the resistance
temperature for the load.
 Z₁ is a zener diode which gives a fixed voltage across it. this voltage
appears across the thermistor R₄.
 When the voltage across the thermistor R ₄ is sufficient to charge the
capacitor C₁ to a voltage equal to or more than the break-over voltage
of the Dias, the Dias triggered and sends a trigger pulse to the gate of
the Triac.
 The Triac starts conducting, thus connecting the heater element in the
circuit.
 As the temperature increases, the thermistor resistance decreases and
as such, the voltage across capacitor C ₁ is reduced. This increases the
firing angle of the Triac thus reducing the voltage across the heater
element accordingly and consequently reduction of heat takes place.
 Gradually, a stage comes when the voltage across the capacitor C ₁
becomes insufficient to trigger the Diac and the Triac is automatically
switched off.
 This results in the disconnection of the heater element from the
circuit.
 R1 is limiting resistor.
 The function of Z₁ is to avoid the effect of the supply fluctuations
on the performance of the circuit.
 For a 220V, 50Hz A.C. supply, R₁ may be chosen 47k, 2W and the
capacitor C₁ of 0.1μF value.
 The rating of zener diode Z₁ may be decided by the break-over
voltage of the Dias. The Triac specification depends on the load to
be controlled.
Fan speed regulator using triac:
 The fan motor is a single phase induction motor, the speed of which
depends on the rms value of voltage applied to it.
 Fig shows how a triac can be used to control the speed of fan motor.
Operation:
Opto-coupler:
In electronics, an opto-isolator (or optical isolator, optocoupler,
photocoupler, or photoMOS) is a device that uses a short optical
transmission path to transfer a signal between elements of a circuit,
typically a transmitter and a receiver, while keeping them electrically
isolated — since the signal goes from an electrical signal to an optical
signal back to an electrical signal, electrical contact along the path is
broken.
Operation:
 A common implementation involves a LED and a phototransistor,
separated so that light may travel across a barrier but electrical
current may not. When an electrical signal is applied to the input of
the opto-isolator, its LED lights, its light sensor activates, and a
corresponding electrical signal generated at the output. Unlike a
transformer, the opto-isolator allows for DC coupling and provides
significant protection from serious overvoltage conditions in one
circuit affecting the other.
 With a photodiode as the detector, the output current is proportional
to the amount of incident light supplied by the emitter. The diode
can be used in a photovoltaic mode or a photoconductive mode.
 In photovoltaic mode, the diode acts like a current source in parallel
with a forward-biased diode. The output current and voltage are
dependent on the load impedance and light intensity.
 In photoconductive mode, the diode is connected to a supply
voltage, and the magnitude of the current conducted is directly
proportional to the intensity of light.
 An opto-isolator can also be constructed using a small incandescent
lamp in place of the LED; because the lamp has a much slower
response time than an LED, will filter out noise or half-wave power in
the input signal. In so doing, it will also filter out any audio- or higher-
frequency signals in the input. It has the further disadvantage, of
course, (an overwhelming disadvantage in most applications) that
incandescent lamps have finite life spans.
 The optical path may be air or a dielectric waveguide. The transmitting
and receiving elements of an optical isolator may be contained within a
single compact module, for mounting, for example, on a circuit board;
in this case, the module is often called an opto-isolator or opto-isolator.
 The photosensor may be a photocell, phototransistor, or an optically
triggered SCR or Triac.
 An opto-coupler, also called opto-isolator, is an electronic component
that transfers an electrical signal or voltage from one part of a circuit to
another, or from one circuit to another, while electrically isolating the
two circuits from each other.
 It consists of an infrared emitting LED chip that is optically in-line with
a light-sensitive silicon semiconductor chip, all enclosed in the same
package.
 The silicon chip could be in the form of a photo diode, photo transistor,
photo Darlington, or photo SCR.
 Burglar alarm:
Fig. shows simple burglar alarm circuit which makes enclosure
or on the door for the purpose of protection against burglary , scr
is connected in the circuit as shown in fig. limiting resistor R and
the micro switch are connected at the gate of the scr. The micro
(door) switch remains in the OFF position when the door is
closed, whereas the switch is put on automatically when the door
is opened. With the reset switch S closed, working of the circuit
as follows:
 Operation:
 Condition (1):
when the door is closed. The d.c. supply is available at
input terminals of the alarm. The micro switch being in the off
position, does not allow any signal at the gate of the scr. The
scr does not conduct in this condition and therefore return path
is not available for the current to flow through the alarm and
the alarm is not energized.

 Condition (2):
when the door is opened. micro switch in this condition,
being in the ON position allows the required signal at the scr
gate. Resistor R is for limiting the current. As soon as ,the gate
signal is received, the scr starts conducting and the alarm is
energized. For putting the alarm off, the scr is de-energized by
opening the reset switch s. the alarm may be replaced by lamp,
as shown in the dia.
 Batch counter:
Batch counter is special purpose counter, which is
used to count the number of opaque object moving on a
conveyer belt. The block diagram of a batch counter
system is shown in fig.
Operation:
 This system is used for counting the number of objects
that are passing on a conveyer belt.
 A light beam is focused continuously on LDR and the
reverse photocurrent is flowing through it.
 As soon as an object passes and interrupts the beam of
light, the photocurrent flowing through the LDR will
reduce to zero.
 Hence corresponding to every object passing through
we get low going pulse as shown in fig.
 An electronic counter counts these low going pulse and
displays them on seven segment display.
 As each counted pulse correspond to an object, the
displayed number corresponds to the number of objects.
Smoke detector:
 Operation:
Operation in presence of smoke:
When the smoke is present between the light source and LDR, it interrupt the light falling
on LDR. The resistance of LDR increases.
 This increase the base voltage of Q1 and turn it on. The collector voltage of Q1 now
reduce to a low value.
 Due to this the base voltage of Q2 will reduce to a very low value and Q2 is turn off.
 The relay is de-energized and the N.C. contact is closed to connect supply to the alarm
and the alarm will be operated to indicate the presence of smoke.

Operation in absence of smoke:

 When the smoke is not detected, the light from the light source falls on LDR without any
interruption. The resistance of LDR is low and the base voltage of transistor Q1 is low.
i.e. Q1 is in the off state. This raises the collector voltage of Q1 to Vcc
Due to high collector voltage of Q1, the base of Q2 receives a sufficiently high voltage
and Q2 is turned on.
 The collector current of Q2 flows through the relay coil to energies the relay. The N.C.
contact of the relay is open circuited and the alarm does not sound.