Power Devices and Circuits [304186]

Mr. M. K. Bhosale
Assistant Professor
Dept. Of Electronics and Telecommunication Engg,
Sinhgad Institute of Tecnology, Lonavala

mbhosale.sit@sinhgad.edu
Cell. +91 7588628575

1
Prof.M.K.Bhosale SIT Lonavala
 Unit-I: Power Devices
Contents: Construction, Steady state characteristics & Switching characteristics of SCR, Construction, Steady state
characteristics Power MOSFET & IGBT. SCR ratings: IL, IH, VBO, VBR, dv/dt, di/dt, surge current & rated current.
Gate characteristics, Gate drive requirements, Gate drive circuits for Power MOSFET & IGBT,opto isolator driving
circuits for SCR. Series and parallel operations of SCR‘s. Applications of above power devices as a switch .
Unit Objectives :On completion the students will be able to:
1) To introduce students to different power devices to study their construction, characteristics and turning on circuits.
2) To give an exposure to students of working & analysis of controlled rectifiers for different loads, inverters, DC
choppers, AC voltage controllers and resonant converters.
3) To study the different motor drives, various power electronics applications like UPS, SMPS, etc. and some protection
circuits
Unit outcomes : On completion the students will be able to
1) ) Design & implement a triggering / gate drive circuit for a power device
2) Understand, perform & analyze different controlled converters.
3) Evaluate battery backup time & design a battery charger.

Outcome Mapping:
PEOs: ii,iii POs:ii,iii COs: 3 PSOs:3

Books used
T1. M. H. Rashid, ―Power Electronics circuits devices and applications, PHI 3rd edition
R4. P. S. Bimbhra, ―Power Electronics‖, Khanna Publishers, Delhi. Ranjan Bose, ―Information Theory coding and
Cryptography, McGraw-Hill, 2nd Ed.

Prof.M.K.Bhosale SIT Lonavala

 Introduction
Gate Cathode

+
n
19
10 cm
-3 + 19
n 10 cm
-3
 10m


J3 - 17 -3
p 10 cm 30-100m


J2
–
n
13
10 -5 x 10 cm
14 -3 50-1000m

J1
p
+
17
10 cm
-3
 30-50m
19 -3
p 10 cm

Anode

Prof.M.K.Bhosale SIT Lonavala

 Silicon Controlled Rectifier
• A Silicon Controlled Rectifier (or Semiconductor Controlled Rectifier) is a four layer solid state device that
controls current flow
• The name “silicon controlled rectifier” is a trade name for the type of thyristor commercialized at General
Electric in 1957

• The basic purpose of the SCR is to function as a switch that can turn on
• or off small or large amounts of power.

Prof.M.K.Bhosale SIT Lonavala

Prof.M.K.Bhosale SIT Lonavala
SCR V-I characteristics.

Prof.M.K.Bhosale SIT Lonavala

Prof.M.K.Bhosale SIT Lonavala
silicon-controlled rectifier (SCR) equivalent

Prof.M.K.Bhosale SIT Lonavala

Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
 SCR ratings

Surge current rating:

Maximum possible surge current device can withstand without damaging itself.

di/dt rating:
Maximum tolerable rate of rise of anode current.

dv/dt rating:
It is the maximum rate of rise of anode to cathode voltage that will not the SCR with
gate terminal open.

Maximum Squared Current integral (∫i2dt): This rating in terms of A2S is a measure
of the energy the device can absorb for a short time (less than one half cycle of power
frequency). This rating is used in the choice of the protective fuse connected in series
with the device.

Prof.M.K.Bhosale SIT Lonavala

SCR ratings

Prof.M.K.Bhosale SIT Lonavala

Prof.M.K.Bhosale SIT Lonavala
 Power Device : GTO

• Construction,
• VI characteristics
• Switching characteristics

Prof.M.K.Bhosale SIT Lonavala

 GTO : Introduction

• GTO stands for “Gate Turn off Thyristor”. It is a

bipolar semiconductor switching device that
includes three terminals namely anode, cathode
& gate like a conventional thyristor. It has the
capacity of gate turn off. This device is used to
turn ON and OFF the main current supply through
a gate drive circuit.

Prof.M.K.Bhosale SIT Lonavala

 GTO : Introduction
• It is special type of Thyristor called the Gate Turn Off Thyristor.
• A solid-state semiconductor device like a thyristor is not a
completely controlled switch.
• Thus, it is difficult to utilize in the application like the conversion
of DC-AC & DC-DC circuit.
• An expensive, as well as a bulky communication circuit, must be
used to deactivate the thyristor.
• To overcome this problem, the GTO (Gate Turn-Off Thyristor)
device is used which is fully controlled .
• It is a current-controlled device similar to a normal
thyristor.

Prof.M.K.Bhosale SIT Lonavala

 GTO : Introduction
• To activate the GTO into the mode of conduction, a small
positive gate current is required as well as through a
negative pulse on the gate terminal; and it is capable of
being switched off. In the following image, it includes
double arrows on it which differentiate the thyristor from
the ordinary thyristor. These arrows mainly specify the
flow of current in the bidirectional throughout the gate
terminal.

Gate Turn-Off
Thyristor symbol

Prof.M.K.Bhosale SIT Lonavala

 GTO : Introduction
• To deactivate the GTO, it uses a high gate current.
Alternatively, in the conduction state, thus thyristor works
like a normal thyristor including a small ON condition
voltage drop.
• The switching speed of this gate turn-off thyristor is
faster as compared to normal thyristor and also it has high
current and voltage ratings as compared with power
transistors.

Prof.M.K.Bhosale SIT Lonavala

 GTO : Introduction
• A gate turn off thyristor is a pnpn device. In
which it can be turned ON like an ordinary SCR
by a positive gate current. However it can be
easily turned off by a negative gate pulse of
appropriate magnitude.

Prof.M.K.Bhosale SIT Lonavala

 GTO :V-I Characteristics
• The Gate Turn Off thyristor V-I characteristics are related to a CT or
conventional thyristor. The GTO’s latching current is more than a CT. For
GTO, the latching current is 2A whereas, for a CT, it ranges from 100 mA –
200 mA. The V-I characteristics of GTO are shown below.
• characteristics mainly include four regions or modes like forward blocking,
forward conduction, reverse blocking & reverse conduction.

Prof.M.K.Bhosale SIT Lonavala

 GTO :V-I Characteristics
• The Gate Turn Off thyristor V-I characteristics are related to a CT or
conventional thyristor. The GTO’s latching current is more than a CT. For GTO,
the latching current is 2A whereas, for a CT, it ranges from 100 mA – 200 mA.
The V-I characteristics of GTO are shown below.
• characteristics mainly include four regions or modes like forward blocking,
forward conduction, reverse blocking & reverse conduction.

A symmetric GTO
has a high reverse
blocking capability
while asymmetric
GTO has a small
reverse blocking
capability as
shown in figure.

Prof.M.K.Bhosale SIT Lonavala

 GTO : Switching Characteristics
• A basic gate drive circuit for a GTO is shown in the figure below.
For turning-on a GTO, first transistor TR1 is turned on, this in
turn switches on TR2 to apply a positive gate-current pulse to
turn on GTO. For turning off the GTO, the turn-off circuit should
be capable of outputting a high peak current. Usually, a thyristor
is used for this purpose. The turn-off process is initiated by
gating thyristor T1. When T1 is turned on, a large negative gate
current pulse turns off the GTO.

Prof.M.K.Bhosale SIT Lonavala

 Gate turn-on

• The turn-on process for a GTO is similar to that of a

conventional thyristor.
• Gate turn-on time for GTO is made up of delay time, rise
time, and spread time like a thyristor.
• Further, turn-on time in a GTO can be decreased by
increasing its forward gate current as in a thyristor.

Prof.M.K.Bhosale SIT Lonavala

Gate turn-on

Prof.M.K.Bhosale SIT Lonavala

 Gate turn-off
• The turn-off characteristics of a GTO are different
from those of an SCR. Before the initiation of the
turn-off process, a GTO carries a steady current Ia,
as shown in fig.
• The figure shows a typical turn-off dynamic
characteristic for a GTO.
• The total turn-off time tq is subdivided into three
different periods; namely the storage period (ts), the
fall period (tf), and the tail period (tt). In other
words,
• tq = ts + tf + tt

Prof.M.K.Bhosale SIT Lonavala

 Gate turn-off
• At the time t = ts+ tf, there is a spike in voltage due to
abrupt current change.
• After tf anode current ia and anode voltage Va, keep
moving towards their turn-off values for a
time tt called tail time. After tt, anode current reaches
zero value and Va undergoes a transient overshoot due to
the presence of Rc, Cs, and then stabilizes to its off-state
value equal to the source voltage applied to the anode
circuit. Here Rs and Cs are the snubber circuit parameters.
The turn-off process is complete when tail current reaches
zero. The over shoot voltage and tail current can be
decreased by increasing the size of Cs, but a compromise
with snubber loss must be made. The duration
of tt depends upon the device characteristics.

Prof.M.K.Bhosale SIT Lonavala

 Gate turn-off
voltage and current wave-forms during turn-off of a GTO

Prof.M.K.Bhosale SIT Lonavala

 GTO :Advantages
• The GTO has outstanding switching characteristics
• The configuration of the GTO circuit has less weight and size than
the thyristor circuit unit.
• A commutation circuit is not required, hence cost, weight and
size can be reduced.
• The switching speed of GTO is high as compared with SCR.
• Less maintenance
• The current surge capacity is similar to an SCR.
• The blocking voltage capacity of GTO is high
• di/dt ratings are more at turn ON
• Efficiency is high

Prof.M.K.Bhosale SIT Lonavala

 GTO :Disadvantages
• Gate drive circuit losses, as well as ON-state voltage drop, is more
• The structure of GTO is multi-layered, so the gate triggering
current value is high as compared to the conventional thyristor.
• High losses of Gate drive circuit
• The voltage drop of ON state across the gate turn off thyristor is
more.
• The latching & holding current’s magnitude is high as compared
to SCR
• The latching current value is 2A whereas, for an SCR, it ranges
from 100 mA to 500 mA.
• As compared with SCR, the triggering current of GTO is high

Prof.M.K.Bhosale SIT Lonavala

 GTO :Applications
• GTO is used in many applications because of many benefits as
compared to another thyristor like outstanding switching
characteristics, less maintenance and no require of commutation
circuit, etc.
• In choppers as well as inverters, It is used as the main control
device.
• AC drives & DC drives
• DC circuit breakers
• Induction heating
• Used in traction applications because of less weight
• Low power applications
• AC stabilize power supplies
• Used in drive systems like rolling mills, machine tools & robotics.

Prof.M.K.Bhosale SIT Lonavala

Comparison of GTO and SCR

Prof.M.K.Bhosale SIT Lonavala

Types of MOSFET

Prof.M.K.Bhosale SIT Lonavala

COSTRUCTION OF MOSFET

Prof.M.K.Bhosale SIT Lonavala

Steady state (V-I)characteristics of MOSFET

Prof.M.K.Bhosale SIT Lonavala

GATE DRIVE CIRCUIT FOR MOSFET

Prof.M.K.Bhosale SIT Lonavala

 Questions:

Prof.M.K.Bhosale SIT Lonavala

 Power Device : IGBT
• Construction
• VI characteristics
• Transfer Characteristic
• switching characteristics

Prof.M.K.Bhosale SIT Lonavala

 IGBT : Introduction
• The IGBT is a power switching transistor which
combines the advantages of MOSFETs and BJTs
for use in power supply and motor control
circuits

Typical IGBT

Prof.M.K.Bhosale SIT Lonavala

Insulated Gate Bipolar Transistor(IGBT)

• BJT has low on state power loss but as switch it is slower.

• MOSFET has high on state power loss but as switch it is
faster.
• Combined BJT & MOSFET monolithically on the silicon
wafer to develop a new device that will have qualities of both.
• New device named IGBT (Insulated Gate Bipolar Transistor)
• Other names are IGT (Insulated Gate Transistor), COMFET
(Conductivity Modulated FET).

Prof.M.K.Bhosale SIT Lonavala

 IGBT : Construction
• Insulated gate bipolar transistor is a three terminal, trans
conductance device that combines an insulated gate N-
channel MOSFET input with a PNP bipolar transistor output
connected in a type of Darlington configuration.
• As a result the terminals are labelled
as: Collector, Emitter and Gate.
• Two of its terminals (C-E) are associated with the
conductance path which passes current, while its third
terminal (G) controls the device.

Prof.M.K.Bhosale SIT Lonavala

IGBT : Construction

Prof.M.K.Bhosale SIT Lonavala

Vertical oriented structure to maximize the area for current flow.
• This will reduce resistance & hence on state power loss.
• The main difference in structure of IGBT as compared to
MOSFET is P+ layer form’s drain of IGBT.
• N- drift layer to improve its breakdown voltage capacity.
• IGBT made up of N+ buffer layer & they termed as asymmetric.
• This layer improves operation by means
– It reduce on state voltage drop across device.
– Shortens the turn off times.
• Buffer layer reduce reverse blocking capacity of the IGBT.
• By buffer layer IGBT classified into two types
– Non punch through IGBT (n+ layer is absent)
– Punch through IGBT (n+ later is present)

Prof.M.K.Bhosale SIT Lonavala

IGBT :Static V-I characteristics

Prof.M.K.Bhosale SIT Lonavala

IGBT :Static V-I characteristics

Prof.M.K.Bhosale SIT Lonavala

i) Cut off region:
In this region no collector current flows w.r.t. VCE through IGBT due to
VGE <VGET and IGBT remains off.
ii) Ohmic / linear region:
In this region VCE < (VGE - VGET), collector current flows through IGBT i.e. it
becomes on. The collector current changes w.r.t. VCE & VGE. Due to high
collector current and low collector voltage, the power IGBT is always
operated as a switch in linear region.
iii) Pinch-off / saturation region:
In this region VCE > (VGE - VGET) and IGBT conducts. The collector current
remains constant w.r.t. VCE & changes with VGE. IGBT is used in this
region
for voltage amplification.
Prof.M.K.Bhosale SIT Lonavala
 IGBT :Transfer Characteristics
• Figure below shows the transfer characteristic
of IGBT, which is exactly same as PMOSFET. The
IGBT is in ON-state only after VGE is greater than
a threshold value VGET.

Prof.M.K.Bhosale SIT Lonavala

 IGBT :Static V-I characteristics

• The graph is similar to that of a BJT except that the

parameter which is kept constant for a plot is VGE
because IGBT is a voltage controlled device unlike
BJT which is a current controlled device. When the
device is in OFF mode (VCE is positive and VGE <
VGET) the reverse voltage is blocked by J2 and when
it is reverse biased, i.e. VCE is negative, J1 blocks the
voltage.

Prof.M.K.Bhosale SIT Lonavala

IGBT : Switching Characteristics

Prof.M.K.Bhosale SIT Lonavala

 IGBT : Switching Characteristics
• Turn on time ton is composed of two components as
usual, delay time (tdn) and rise time (tr). Delay time is
defined as the time in which collector current rises
from leakage current ICE to 0.1 IC (final collector
current) and collector emitter voltage falls from VCE to
0.9VCE. Rise time is defined as the time in which
collector current rises from 0.1 IC to IC and collector
emitter voltage falls from 0.9VCE to 0.1 VCE.

Prof.M.K.Bhosale SIT Lonavala

IGBT : Switching Characteristics
• The turn off time toff consists of three components,
delay time (tdf), initial fall time (tf1) and final fall time
(tf2). Delay time is defined as time when collector
current falls from IC to 0.9 IC and VCE begins to rise.
Initial fall time is the time during which collector
current falls from 0.9 IC to 0.2 IC and collector emitter
voltage rises to 0.1 VCE. The final fall time is defined
as time during which collector current falls from 0.2 IC
to 0.1 IC and 0.1VCE rises to final value VCE.

Prof.M.K.Bhosale SIT Lonavala

Requirements of gate drive of IGBT

*The driver circuit used for MOSFET are also

applicable to IGBT. The gate to source input
capacitance should be charged quickly.

*MOSFET or IGBT turns on when gate source input

capacitance is charged to sufficient level.

*The negative current should be high to turn off

MOSFET or IGBT.

Prof.M.K.Bhosale SIT Lonavala

Typical Gate Drive Circuit for MOSFET / IGBT
+ V CC

+V

RA
D
8 4 Q1
7
I
C R2
5 3
R 6 5
B G
S
2 5
1 5 Q2
R1 R3

C
C1

Prof.M.K.Bhosale SIT Lonavala

 IGBT : Advantages
• Lower gate drive requirements
• Low switching losses
• Small snubber circuitry requirements
• High input impedance
• Voltage controlled device
• Temperature coefficient of ON state resistance is
positive and less than PMOSFET, hence less On-
state voltage drop and power loss.
• Enhanced conduction due to bipolar nature
• Better Safe Operating Area

Prof.M.K.Bhosale SIT Lonavala

 Advantages of IGBT

• Combines the advantages of BJT & MOSFET

• High input impedance like MOSFET
• Voltage controlled device like MOSFET
• Simple gate drive, Lower switching loss
• Low on state conduction power loss like BJT
• Higher current capability & higher switching
speed than a BJT. ( Switching speed lower than
MOSFET)

Prof. M. Madhusudhan Rao, E&C Dept., MSRIT

Prof.M.K.Bhosale SIT Lonavala
 IGBT : Disadvantages

• Cost
• Latching-up problem
• High turn off time compared to PMOSFET

Prof.M.K.Bhosale SIT Lonavala

 Applications of IGBT
• ac and dc motor controls.
• General purpose inverters.
• Uninterrupted Power Supply (UPS).
• Welding Equipment's.
• Numerical control, Cutting tools.
• Robotics & Induction heating.

60
Prof. M. Madhusudhan Rao, E&C Dept., MSRIT
Prof.M.K.Bhosale SIT Lonavala
 IGBT : Applications
• IGBTs are widely used in medium power applications such as
dc and ac motor drives, UPS systems, power supplies, and
drives for solenoids, relays, and contractors.
• Though IGBTs are somewhat more expensive than BJTs, yet
they are becoming popular because of lower gate-drive
requirements, lower switching losses, and smaller snubber
circuit requirements.
• IGBT converters are more efficient with less size as well as
cost, as compared to converters based on BJTs.
• Recently, IGBT inverter induction-motor drives using 15-20
kHz switching frequency are finding favor where audio-noise
is objectionable.
• In most applications, IGBTs will eventually push out BJTs. At
present, the state of the art IGBTs is available up to 1200 V,
500 A.
Prof.M.K.Bhosale SIT Lonavala
Device Characteristics IGBT Power MOSFET POWER BJT

More than 1kV (Very

Voltage Rating Less than 1kV (High) Less than 1kV (High)
High)

Current Rating More than 500A (High) Less than 200A (High) Less than 500A (High)

Input Device Voltage, Vge, 4-8V Voltage, Vgs, 3-10V Current, hfe, 20-200

Input Impedance High High Low

Output Impedance Low Medium Low

Switching Speed Medium Fast (nS) Slow (uS)

Cost HIgh Medium Low

Prof.M.K.Bhosale SIT Lonavala

Prof.M.K.Bhosale SIT Lonavala
 Requirement of a typical triggering / driver
(such as opto isolator) circuits for various power
devices

• A gate driver is a power amplifier that accepts a low-power

input from a controller IC and produces the appropriate
high current gate drive for a power device. As
requirements for power electronics continue to increase,
the design and performance of the gate driver circuitry are
becoming more important.

Prof.M.K.Bhosale SIT Lonavala

Requirement of a typical triggering / driver
(such as opto isolator) circuits for various power
devices
• Power semiconductor devices are the heart of modern power
electronics systems. These systems utilize many gated semiconductor
devices such as ordinary transistors, FETs, BJTs, MOSFETs, IGBTs, and
others as switching elements in switched-mode power supplies
(SMPS), universal power supplies (UPS), and motor drives. Modern
technology evolution in power electronics has generally followed the
evolution of power semiconductor devices.
• Power level requirements and switching frequency are increasing in
the power electronics industry. The metal oxide semiconductor field
effect transistor (MOSFET) and insulated gate bipolar transistor (IGBT)
are two of the most popular and efficient semiconductor devices for
medium to high power switching power supplies in most applications.

Prof.M.K.Bhosale SIT Lonavala

 Requirement of a typical triggering / driver
circuits for various power devices
• In high power applications, the gate of a power switch can never be driven
by the output of a logic IC (PWM controller). Because of the low current
capabilities of these logic outputs, charging the gate capacitance would
require an excessive amount of time, most likely longer than the duration
of a switching period. Hence dedicated drivers must be used to apply a
voltage and provide drive current to the gate of the power device. This can
be a driver circuit and it may be implemented as dedicated ICs, discrete
transistors or transformers. It can also be integrated within a PWM
controller IC.
• A gate driver is a power amplifier that accepts a low power input from a
controller IC and produces the appropriate high current gate drive for a
power device. It is used when a PWM controller cannot provide the
output current required to drive the gate capacitance of the associated
power device.

Prof.M.K.Bhosale SIT Lonavala

 Requirement of a typical triggering / driver
circuits for various power devices
• The gate driver circuit is an integral part of power electronics
systems. Gate drivers form an important interface between
the high-power electronics and the control circuit and are
used to drive power semiconductor devices. The output of DC-
DC converters or SMPS mainly depends on the behavior of
gate driver circuits, which means if the gate driver circuit
doesn’t drive the gate of a power device properly, the DC-DC
converter output will not be according to the design
requirement. Therefore, the design of the gate driver circuit is
critically important in the designing of power electronic
converters.

Prof.M.K.Bhosale SIT Lonavala

 Requirement of a typical triggering / driver
(such as opto isolator) circuits for various power
devices
• Gate Driver Isolation
• Gate drive circuits for power inverters and converters often require electrical
isolation for both functional and safety purposes. Isolation is mandated by
regulatory and safety certification agencies to prevent shock hazards. It also
protects low voltage electronics from any damage due to faults on the high
power side circuit and from human error on the control side. The electrical
separation between various functional circuits in a system prevents a direct
conduction path between them and allows individual circuits to possess
different ground potentials. Signal and power can still pass between isolated
circuits using inductive, capacitive or optical methods.
• Many applications of power devices (e.g., converters where high power
density and high efficiency are required) require an isolated gate drive
circuit.

Prof.M.K.Bhosale SIT Lonavala

 Requirement of a typical triggering / driver
(such as opto isolator) circuits for various power devices
• Isolation Techniques
• Basically, there are two popular techniques available to implement isolated gate
drivers: magnetic (using gate drive transformers/Pulse Transformer )
and optical (using an optocoupler).

Prof.M.K.Bhosale SIT Lonavala

Requirement of a typical triggering / driver
(such as opto isolator) circuits for various power
devices

Prof.M.K.Bhosale SIT Lonavala

Requirement of a typical triggering / driver
(such as opto isolator) circuits for various power
devices

Prof.M.K.Bhosale SIT Lonavala

Opto Isolator Circuit working

If the light emitted from the LED (source) falls on the

(sensor) phototransistor base, then the (sensor gets turned
ON) transistor starts conduction. If the current passing
through the LED is made to zero or turned off, then the
transistor stops conduction.

Prof.M.K.Bhosale SIT Lonavala

 Types of Opto Isolators
There are various types of opto isolators which are classified
based on various criteria such as number of channels, isolation
voltage, type of packing, output voltage capacity, current transfer
ration (CTR), and so on.

Prof.M.K.Bhosale SIT Lonavala

Prof.M.K.Bhosale SIT Lonavala
Opto isolator driving circuit for SCR

Current Transfer
ratio=Io/Iin
=1mA/5mA=0.2

Prof.M.K.Bhosale SIT Lonavala

4N26 OptoIsolator

Prof.M.K.Bhosale SIT Lonavala

Importance of series and parallel
operations of various power devices (no
derivation and numerical).

• The power-handling capabilities of power devices are

generally limited by device area utilisation, encapsulation,
and cooling efficiency. Many high-power applications exist
where a single device is inadequate and, in order to
increase power capability, devices are paralleled to
increase current capability or series-connected to increase
voltage ratings. Extensive series connection of devices
is utilised in HVDC transmission thyristor and IGBT
modules while extensive paralleling of IGBTs is
common in inverter applications.

Prof.M.K.Bhosale SIT Lonavala

 Importance of series and parallel
operations ofconnected
• When devices are variousin series
power devices
for high-voltage
operation, both steady-state and transient voltages
must be shared equally by each individual series device.
• If power devices are connected in parallel to obtain
higher current capability, the current sharing during
both switching and conduction is achieved either by
matching appropriate device electrical and thermal
characteristics or by using external forced sharing
techniques.

Prof.M.K.Bhosale SIT Lonavala

 series operations of various power
• Due to variationsdevices
in blocking currents, junction
capacitances, delay times, on state voltage
drops, and reverse recovery for individual
power devices, external voltage equalization
networks and special gate circuits are required
if devices are to be reliably connected and
operated in series.

Prof.M.K.Bhosale SIT Lonavala

 series operations of SCR
• SCRs are available of ratings up to 10 KV and 3 KA.
But sometimes we face demand, more than these
ratings. In this case combination of more than one
SCRs is used. Series connection of SCRs meets high
voltage demand and parallel connection of SCRs
meets high current demand. These series and parallel
connection of SCR or Thyristor will work efficiently if all
SCRs are fully utilized. Although all SCRs in a string
are of same rating, their V-I characteristics differ from
one another. This leads to unequal voltage or current
division among them. Hence every SCR is not fully
utilized. So the efficiency of string is always less than
100%

Prof.M.K.Bhosale SIT Lonavala

 Series operations of SCR

With increase in the numbers of SCRs in a string voltage or

current handled by each SCR is minimized. This
phenomenon increases the reliability of the string, but
reduces the utilization of each SCR. Thus string efficiency
decreases. Reliability of string is measured by derating
factor (DRF) which is given by the expression

Prof.M.K.Bhosale SIT Lonavala

 Series operations of SCR

• Following are the problems in Series SCR

connection
1. During forward blocking & Reverse blocking
2. During turn on & turn off of SCR

Prof.M.K.Bhosale SIT Lonavala

Series operations of SCR
During forward blocking & Reverse blocking

Prof.M.K.Bhosale SIT Lonavala

 series operations of SCR
During forward blocking & Reverse blocking

• So we can see from the diagram, for same leakage

current, unequal voltage division takes place. Voltage
across SCR1 is V1 but that across SCR2 is V2. V2 is much
less than V1. So, SCR2 is not fully utilized. Hence the string
can block V1 + V2 =5KV+3KV= 8 KV, rather than 10 KV and
the string efficiency is given by = 80%.
n=(v1+v2)/v1*No of SCR=8KV/2*5KV=80%
• To improve the efficiency a resistor in parallel with every
SCR is used. The value of these resistances are such that
the equivalent resistance of each SCR and resistor pair will
be same. Hence this will ensure equal voltage division
across each SCR.
• This resistance is called static equalizing circuit.

Prof.M.K.Bhosale SIT Lonavala

 Series operations of SCR
During forward blocking & Reverse blocking

Prof.M.K.Bhosale SIT Lonavala

 Series operations of SCR
During turn on & turn off of SCR
• But this resistance is not enough
to equalize the voltage division
during turn on and turn off. In
these transient conditions, to
maintain the equal volume
across each device a capacitor
is used along with resistor in
parallel with every SCR. This is
nothing but snubber ckt which
also known as dynamic
equalizing circuit. An additional
diodes can also be used to
improve the performance of
dynamic equalizing circuit.

Prof.M.K.Bhosale SIT Lonavala

 Parallel operations of SCR
• It is common practice to parallel devices in order to
achieve higher current ratings or lower conducting
voltages than are attainable with a single device.
• When current required by the load is more than
the rated current of single thyristor , SCRs are
connected in parallel in a string .

• Following are the problems in Parallel SCR

connection
1. Different dynamic resistance
2. Unequal current through SCR(Thermal runaway)
3. Different current during turn on & turn off of SCR
Prof.M.K.Bhosale SIT Lonavala
 Parallel operations of various power devices

• Dynamic Resistance RT=dVak/dIa

• I2<I1 Unequal current due to different on state resistance of SCR

Anode Current, Ia

Anode to Cathode voltage

, Vak

Prof.M.K.Bhosale SIT Lonavala

 Parallal operations of various power devices
• When the operating current is more than the individual current ratings
of SCRs then we use more than one SCRs in parallel. Due to different
V-I characteristics SCRs of same rating shares unequal current in a
string. Let a string consists of two transistors in parallel as shown in fig.
1 and their current rating by 1 KA. From the V-I characteristics of the
devices it can be seen that for operating volume V, current through
SCR1 is 1 KA and that through SCR2 is 0.8 KA. Hence, SCR2 is not
fully utilized here. Though the string should withstand R KA
theoretically it is only capable of handling 1.8 KA. So, the string
efficiency is = 90%.

Solution 1
Unequal current due to different on
state resistance of SCR it can be
compensated by connecting resistance
R in series with each SCR as shown in
figure (Static equalization circuit)

Prof.M.K.Bhosale SIT Lonavala

Parallel operations of various power devices

• Solution 2
• Due to unequal current division when current through
SCR increases, its temperature also increases which in
turn decreases the resistance. Hence further increase in
current takes place and this is a cumulative process. This
is known as thermal ‘run away’ which can damage the
device. To overcome this problem SCRs would be
maintained at the same temperature. This is possible by
mounting them on same heat sink.

Prof.M.K.Bhosale SIT Lonavala

 Parallel operations of various power devices
Different current during turn on & turn off of SCR
• Add inductor in series with SCR which will work as di/dt
protection circuit that will give equal amount of current
through SCR during turning on & turning SCR
• When we provide parallel connection of SCR for dynamic
problem during ON & Off of SCR that we can equalize by
adding di/dt protection is called dynamic equalization ckt.

Prof.M.K.Bhosale SIT Lonavala

Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
Prof.M.K.Bhosale SIT Lonavala
Applications of above power devices as a switch

Prof.M.K.Bhosale SIT Lonavala

Thank You

Prof.M.K.Bhosale SIT Lonavala