Online Presentation

on

“Unit-I”

Presented by
Dr. Vadthya Jagan,
M. Tech & Ph. D (IIT Roorkee),
Associate Professor,
Department of Electrical and Electronics Engineering,
Vignana Bharathi Institute of Technology.
2-Nov-21 1
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Contents…..
✓ Concept of Power Electronics
✓ Scope and Applications

✓ Types of Power Converters

✓ Advantages and Disadvantages of Power Electronic Converters

✓ Power Semiconductor Switches and their V-I characteristics

✓ (a) Power Diodes, (b) SCR, (c) Power MOSFET, and (d) Power IGBT

✓ Operating Principle and Characteristics of SCR

✓ Firing Circuits of SCR

✓ Methods of SCR Commutation

✓ Thyristor ratings and protection

✓ Series and Parallel Operation of SCR

✓ Gate drive circuits for IGBT and MOSFETs

2-Nov-21 Department of Electrical and Electronics Engineering 2

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Concept of Power Electronics

Power Electronics belongs to partly power engineers and partly
electronics engineers.

Power engineering is mainly concerned with generation, transmission,

distribution and utilization of electric power/energy at high efficiency.

Electronics engineering is guided by distortion less production,

transmission, and reception of data and signals of very low power level (few
watts/milli watts), without much consideration to the efficiency.

Power electronics is a subject that concerns the applications of

electronic principles into situations that are rated at power level rather than
signal level.
Source: Power Electronics – Dr. P. S. Bimbhra

2-Nov-21 Department of Electrical and Electronics Engineering 3

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Scope and Applications

Scope of Power Electronics
(a) Switch-mode (dc) power supplies and uninterruptible power supplies
Advances in microelectronics fabrication technology have led to the
development of computers, communication equipment, and consumer
electronics, all of which require regulated dc power supplies and often
uninterruptible power supplies.
(b) Energy conservation

(c) Process control and factory automation

Source: Power Electronics – Dr. P. S. Bimbhra

2-Nov-21 Department of Electrical and Electronics Engineering 4

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Scope and Applications (cont..)

Scope of Power Electronics
(d) Transportation

(e) Electro-technical applications.

Welding, electroplating, and
induction heating.

(f) Utility-related applications.

2-Nov-21 Department of Electrical and Electronics Engineering 5

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Scope and Applications (cont..)

Applications of Power Electronics
(a) Residential

Refrigeration and freezers, Space heating, Air conditioning, Cooking, Lighting,

Electronics (personal computers, other entertainment equipment).
(b) Commercial
Heating, ventilating, and air conditioning, Central refrigeration, Lighting,
Computers and office equipment Uninterruptible power supplies (UPS),
Elevators.

(c) Industrial
Pumps, Compressors, Blowers and fans, Machine tools (robots), Arc furnaces,
induction furnaces, Lighting, Industrial lasers, Induction heating, Welding.
Source: Power Electronics – Ned Mohan, T. M. Undeland, W. P. Robbins

2-Nov-21 Department of Electrical and Electronics Engineering 6

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Scope and Applications (cont..)

Applications of Power Electronics
(d) Transportation
Traction control of electric vehicles, Battery chargers for electric vehicles,
Electric locomotives, Street cars, trolley buses, Subways, Automotive
electronics including engine controls.
(e) Utility systems
High-voltage dc transmission (HVDC), Static var compensation (SVC),
Supplemental energy sources (wind, photovoltaic), fuel cells, Energy storage
systems, Induced-draft fans and boiler feedwater pumps.
(f) Aerospace
Space shuttle power supply systems, Satellite power systems, Aircraft
power systems.
(g) Telecommunication Battery chargers, Power supplies (dc and UPS)

2-Nov-21 Department of Electrical and Electronics Engineering 7

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Quiz-1
1. Electronic Devices (BJT, MOSFET, Diode, and IGBT) will
operate at
(a) Low Power and High Frequency
(b) Low Power and Low Frequency
(c) High Power and Low Frequency

(d) High Power and High Frequency

2. Power Electronic Devices will operate at

(a) Low Power and High Frequency
(b) Low Power and Low Frequency
(c) High Power and Low Frequency

(d) High Power and High Frequency

2-Nov-21 Department of Electrical and Electronics Engineering 8

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Types of Power Converters

Power Electronics can be defined as a branch of electrical engineering
devoted to conversion and control of electric power, using electronic converters
based on semiconductor power switches.

Source: Power Electronics – Dr. P. S. Bimbhra

2-Nov-21 Department of Electrical and Electronics Engineering 9

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Types of Power Converters (cont..)

Power Electronic Converter

2-Nov-21 Department of Electrical and Electronics Engineering 10

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Types of Power Converters (cont..)

Types:
(a) AC to DC Converters
(i) Diode Rectifier
(ii) Phase Controlled Rectifier
(b) DC to DC Converters

(c) DC to AC Converters

(d) AC to AC Converters
(i) AC Voltage Controllers
(ii) Cycloconverter
(e) Static Switches

2-Nov-21 Department of Electrical and Electronics Engineering 11

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Types of Power Converters (cont..)

(a) AC to DC Converters
Converts fixed ac input voltage into a fixed
(i) Diode Rectifier dc voltage. The input voltage may be single-
phase or three phase.
Vac =Vin = Vmsinwt Vo
PEC
Vm
Fixed AC Fixed DC Vavg
0 π 2π 3π wt π
(Constant voltage and (Constant voltage) 0 2π 3π wt
constant frequency)

Applications:
1. Electric traction,
2. Battery charging,
3. Electroplating,
4. Electrochemical processing
5. Power supplies,
6. Welding and
7. Uninterruptible power supply (UPS) systems.
2-Nov-21 Department of Electrical and Electronics Engineering 12

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Types of Power Converters (cont..)

(a) AC to DC Converters
Converts fixed ac input voltage into a
(ii) Phase Controlled Rectifier variable dc voltage.

Vac =Vin = Vmsinwt

Vo
PEC
Vm
Vavg
π Fixed AC Variable DC
0 2π 3π wt
(Variation in voltage)0 π 2π 3π wt
(Constant voltage and
constant frequency)

Commutation: Line or Natural commutation

Applications:
1. DC drives,
2. Metallurgical and chemical industries,
3. Excitation systems for synchronous machines etc.,
2-Nov-21 Department of Electrical and Electronics Engineering 13

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Types of Power Converters (cont..)

(b) DC to DC Converters Converts fixed dc input voltage into a
variable dc voltage and vice versa.

PEC
VDC V0
Fixed DC Variable DC V0,avg
VDC ton toff
(Constant voltage) (Variation in magnitude of voltage)
0 t
0 t T

Commutation: Forced commutation

Applications:
1. DC drives,
2. Subway cars,
3. Trolley trucks,
4. Solar Photovoltaic systems
5. Battery-driven vehicles etc.,
2-Nov-21 Department of Electrical and Electronics Engineering 14

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Types of Power Converters (cont..)

(c) DC to AC Converters An inverter converts fixed DC voltage to a
variable AC voltage. The output may be a
variable voltage and variable frequency.
PEC V0
VDC

VDC
Fixed DC
Variable AC
(Constant voltage) 0 wt
(Variation in voltage magnitude)
0 t

Commutation: Forced commutation

Applications:
1. Variable speed a.c. motor drives.
2. Induction Heating.
3. Uninterruptible power supplies (UPS).
4. High voltage d.c. transmission lines and
5. Battery-vehicle drives.
2-Nov-21 Department of Electrical and Electronics Engineering 15

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Types of Power Converters (cont..)

(d) AC to AC Converters
These converter circuits convert fixed ac
(i) AC voltage controller voltage directly to a variable ac voltage at
the same frequency.
Vac =Vin = Vmsinwt
PEC
Vm V0 I0
Fixed AC Input Variable AC Output I0
0 π 2π 3π wt (Constant voltage and (Variation in voltage only π
0 2π 3π wt
constant frequency) with constant frequency)

Commutation: Line or Natural commutation

Applications:
1. Speed control of poly-phase induction motors.
2. Domestic and Industrial Heating.
3. Light controls.
4. On-load transformer tap changing and
5. Static Reactive Power Compensators.
2-Nov-21 Department of Electrical and Electronics Engineering 16

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Types of Power Converters (cont..)

(d) AC to DC Converters These circuits convert input power at one
frequency to output power at a different
(ii) Cycloconverter frequency through one-stage conversion

Vac =Vin = Vmsinwt V0

PEC V0 I0
Vm I0
Fixed AC Input Variable AC Output
π 2π 3π 4π wt(Constant voltage and (Variation in voltage 0 π 2π 3π 4π wt
0
constant frequency) and frequency)
fs=50Hz f0=25Hz

Applications:
1. Aerospace
2. Slow-speed large ac drives like rotary kiln,
3. Induction heating etc..
(e) Static Switches: The power semiconductor devices can operate as static
switches or contactors. Static switches possess many advantages over mechanical
and electromechanical circuit breakers. Depending upon the input supply, the
static switches are called ac static switches or dc static switches.
2-Nov-21 Department of Electrical and Electronics Engineering 17

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Quiz-2
3. Commutation can be defined, as the process of
(a) Making the SCR ON
(b) Making the SCR OFF
(c) Making the SCR ON and OFF

(d) None of the above

4. From the following Power Electronic Converters, which

converter uses forced commutation circuit
(a) AC-DC Converter
(b) DC-DC Converter
(c) AC voltage controller

(d) All the above

2-Nov-21 Department of Electrical and Electronics Engineering 18

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Advantages of Power Electronic Converters

The advantages possessed by power-electronic systems are as under:
(i) High efficiency due to low loss in power-semiconductor devices.
(ii) High reliability of power electronic converter systems.
(iii) Long life and less maintenance due to the absence of any moving
parts.
(iv) Fast dynamic response of the power electronic systems as
compared to electromechanical converter systems.
(v) Small size and less weight result in less floor space and therefore
lower installation cost.
(vi) Mass production of power-semiconductor devices has resulted in
lower cost of the converter equipment.
Source: Power Electronics – Dr. P. S. Bimbhra
2-Nov-21 Department of Electrical and Electronics Engineering 19

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Disadvantages of Power Electronic Converters

(i) Power electronic converter circuits have a tendency to generate
harmonics in the supply system as well as in the load circuit.
(ii) AC to DC and AC to AC converters operate at a low input power
factor under certain operating conditions. In order to avoid a low
power factor, some special measures have to be adopted.
(iii) Power electronic controllers have low overload capacity, These
converters must, therefore, be rated for taking momentary
overloads. As such, cost of power electronic controller may
increase.
(iv) Regeneration of power is difficult in power electronic converter
systems.
2-Nov-21 Department of Electrical and Electronics Engineering 20

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Power Electronic Systems

Power Electronic
Source Motor Load
Converter

Control Sensing
Unit Unit

Input
commond
Fig.1.1: Block diagram of a typical power electronic system.
Source: Power Electronics – Dr. P. S. Bimbhra

2-Nov-21 Department of Electrical and Electronics Engineering 21

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Quiz-3
5. Power-electronic equipment has very high efficiency, because
(a) The devices always operate in active region
(b) The devices never operate in active region
(c) The devices traverse active region at high speed and stay at the two
states, ON and OFF
(d) Cooling is very efficient. (1)

6. Match the devices given below with the

circuit symbols on right hand side and give the
(2)
correct answer from the codes given below:
(A) MOSFET (B) IGBT (C) SCR (D) BJT
(3)

(4)

2-Nov-21 Department of Electrical and Electronics Engineering 22

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Power Semiconductor Switches

Based on (i ) turn-on and turn-off characteristics, (ii) gate signal requirements
and (iii) degree of controllability, these power semiconductor switches are
.
broadly classified into three groups such as:

1. Uncontrolled Switches: On and off states controlled by the power circuit.

e.g: Diodes

2. Fully-controlled Switches: Turned on and off by control signals. e.g: BJT,

MOSFET, IGBT, gate turn off (GTO) thyristors ect..,

3. Semi-controlled Switches: Turned on by a control signal but must be

turned off by the power. e.g: SCR

Source: Power Electronics – Dr. P. S. Bimbhra

2-Nov-21 Department of Electrical and Electronics Engineering 23

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Uncontrolled Switches
Diode
A IA K

Depending on their applications, the uncontrolled diodes are classified

.
into two types:
Depletion Layer
(1)Signal Diode, and
(2)Power Diode
The p-n Junction
A p-n junction is formed when p -type
semiconductor is brought in physical, contact
with n-type semiconductor.
A p-region has greater concentration of holes
whereas n-region has more electron-
concentration. In p-region, free holes are called
majority carriers and free electrons minority
carriers. In n-region, free electrons are called
majority carriers whereas free holes are called
minority carriers.
2-Nov-21 Department of Electrical and Electronics Engineering 24

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Signal Diode
Diode
A K
A IA K
P N

.
IA
VAK
(a) IA (b) Forward voltage drop, VAK
Forward Conduction
State
Reverse
leakage current
VBR
VAK -VAK
-VAK o VAK
o Cut-in voltage
Reverse Blocking
State

(d) - (c)
IA -IA
Fig. 1.2: signal diode (a) structure (b) circuit symbol (c) practical V-I characteristics (d) ideal characteristics

2-Nov-21 Department of Electrical and Electronics Engineering 25

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Power Diode and V-I characteristics

Diode
A A IA K
- +
K
+
P N N

IA .
Drift
(b) IA
region
Forward voltage drop, VAK
(a) Forward Conduction
State
Reverse
VAK leakage current
-VAK VBR
o
-VAK VAK
o
Cut-in voltage
Reverse Blocking
State
-
IA
(c)
(d) -IA
Fig. 1.3: Power diode (a) structure (b) circuit symbol (c) practical V-I characteristics (d) ideal characteristics

2-Nov-21 Department of Electrical and Electronics Engineering 26

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Power Diode and its waveforms

Vac =Vin = Vmsinwt
Vm

Diode 0 π 2π 3π 4π wt
A K
I0 V0
VD V0
I0
I0
230V, 50Hz
1-Φ ACAC
Vac Vin
R Load V0 V0(avg)
Supply 0 π 2π 3π 4π wt

VD
1:1
Transformer
0 π 2π 3π 4π wt
(1-2)V Drop
(a)
(b)

Fig. 1.4: Single-phase half wave rectifier (a) circuit diagram with R load (b) waveforms.

2-Nov-21 Department of Electrical and Electronics Engineering 27

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Types of Power Diodes

According to their reverse recovery characteristics.

1. General purpose diodes

.
2. Fast recovery diodes
3. Schottky diodes.

1. General purpose diodes:

Reverse recovery time: about 25µs
Current rating: 1Amp to several thousand amperes
Voltage rating: 50V to 5kV
Applications: Battery charging, electric traction, welding and UPS.
Source: Power Electronics – Dr. P. S. Bimbhra

2-Nov-21 Department of Electrical and Electronics Engineering 28

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Type of Power Diodes (cont..)

2. Fast recovery diodes:
Reverse recovery time: about 5µs or less
Current rating:. 1Amp to several thousand amperes
Voltage rating: 50V to 3kV
Applications: Choppers, Commutation circuits, SMPS, Induction heating
3. Schottky diodes:
Current rating: 1Amp to 300 amperes
Voltage rating: limited to 100Volts
Applications: High-freq. Instrumentation and Switching power supplies.
As compared to p-n junction diode, a Schottky diode has (i) lower cut-in voltage,
(ii ) higher reverse leakage current and (iii) higher operating frequency.
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Quiz-4
7. The manufacturer provided the details on the data sheet as
voltage rating as 100V, current rating as 80A and switching time
as 0.2µs
(a) General purpose diode, (b) Schottky diode,
(c) Fast recovery diode, (d) Signal diode.

8. The diode will be in ON-state, when anode is connected to ……….

and cathode is connected to ……….. terminal of the supply
(a) Negative, Negative, (b) Positive, Positive,
(c) Positive, Negative, (d) Negative, Positive.
9. The diode will block………………….
(a) Forward voltage, (b) Reverse voltage,
(c) Both forward and reverse voltage, (d) None of the above.

2-Nov-21 Department of Electrical and Electronics Engineering 30

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Diode as a Switch

Diode Conductor

A IA K A IA K
. Low Resistance

ON – Short Circuited
VAK
Forward Biased Condition VAK = 1 to 2V

Diode Insulator
A IA K A K
IA = 0
High Resistance
OFF – Open Circuited
VAK
Reverse Biased Condition VAK = Finate

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SCR (Silicon Controlled Rectifier)

A A

P P
. J1 J1
N N N
VAK SCR J2
J2
P P P
G J3 G J3
G IA N N

K K

THYRISTOR = THYRatron tube + transISTOR

Fig. 1.5: SCR (a) circuit symbol (b) structure (c) interconnection of two transistors

Source: Power Electronics – Dr. M. D. Singh

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Operating Principle of SCR

A A

P P
J1 – Forward biased . J1 – Reverse biased
N N
Open Open
J2 – Reverse biased J2 – Forward biased
P P
A
G J3 – Forward biased G J3 – Reverse biased
N N

P
J1 – Forward biased K
K N
Closed
Forward blocking state J2 – Reverse biased Reverse blocking state
P
– OFF State G J3 – Forward biased – OFF State
High resistance N
High resistance

K
Forward conducting state – ON State Low resistance
2-Nov-21 Department of Electrical and Electronics Engineering 33

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Static V-I Characteristics of SCR

IA

Load
Forward voltage drop, VT

Forward Conduction State

SCR . A
V S vAK
R1 IG2>IG1>IG
IL IG2
G K Reverse
leakage current
IG1 I =0
G
IH VAK
Vg -VAK VBR
o VAK2 VAK1 VBO
OP
VAK2<VAK1<VB0
Reverse Blocking State Forward Blocking State

Thyristor has three basic modes of operation: Forward

leakage current

1) Reverse blocking (off-state) mode,

2) Forward blocking (off-state) mode and
3) Forward conduction (on-state) mode.
-IA
Fig. 1.6. (a) Elementary circuit for obtaining thyristor V-I characteristics (b) Static V-I characteristics of thyristor.

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Static V-I Characteristics of SCR (cont..)

IA

1) Reverse blocking (off-state) mode:

A Forward voltage drop, VT

Forward Conduction State

.

P IG2>IG1>IG
J1 – Reverse biased Reverse IL IG2
N IG1 I =0
Open leakage current G
J2 – Forward biased IH VAK
-VAK VBR
P
G J3 – Reverse biased o VAK2 VAK1 VBO
N OP
VAK2<VAK1<VB0
Reverse Blocking State Forward Blocking State
Forward
leakage current

The device behaves like two diodes

which are connected in series with
-IA
reverse voltage applied across them.

2-Nov-21 Department of Electrical and Electronics Engineering 35

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Static V-I Characteristics of SCR (cont..)

IA

2) Forward blocking (off-state) mode:

A Forward voltage drop, VT

Forward Conduction State

.

P IG2>IG1>IG
J1 – Forward biased Reverse IL IG2
N IG1 I =0
Open leakage current G
J2 – Reverse biased IH VAK
-VAK VBR
P
G J3 – Forward biased o VAK2 VAK1 VBO
N
OP
VAK2<VAK1<VB0
Reverse Blocking State Forward Blocking State
Forward
leakage current

-IA

2-Nov-21 Department of Electrical and Electronics Engineering 36

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Static V-I Characteristics of SCR (cont..)

IA

3) Forward conduction (on-state) mode.

A Forward voltage drop, VT

Forward Conduction State

.
P
J1 – Forward biased IG2>IG1>IG
N
J2– Reverse biased Reverse IL IG2
IG1 I =0
Closed P leakage current G
IH VAK
G J3 – Forward biased -VAK VBR
N
o VAK2 VAK1 VBO
OP
VAK2<VAK1<VB0
Reverse Blocking State Forward Blocking State
K Forward
leakage current
A thyristor can be brought from forward
blocking mode to forward conduction mode
by turning it on by applying (i) a positive
gate pulse between gate and cathode or (ii)
a forward breakover voltage across anode
-IA
and cathode.

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Static V-I Characteristics of SCR (cont..)

IA

Forward voltage drop, VT

Forward Conduction State

.
VBO
Forward break over voltage

IG3>IG2>IG1>IG
Reverse IL IG3 IG2 IG1
leakage current IG=0
VAK1 IH
-VAK VBR VAK
VAK2 o VAK3 VAK2 VAK1 VBO
OP
Reverse Blocking State
VAK3<VAK2<VAK1<VB0
Forward Blocking State
VAK3 Finger voltage Forward
leakage current

o a IG2
IG1 IG3
IG=0 Gate current

-IA

2-Nov-21 Department of Electrical and Electronics Engineering 38

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Static V-I Characteristics of SCR (cont..)

Latching Current: Minimum value of anode current
above which the SCR goes to conduction state without IA

any gate signal.

. Forward voltage drop, VT
Holding Current: Minimum value of anode current
Forward Conduction State

below which the SCR goes to OFF state.

IG2>IG1>IG
IA Reverse IL IG2
IG1 I =0
leakage current G
IH VAK
-VAK VBR
o VAK2 VAK1 VBO
OP
VAK2<VAK1<VB0
-VAK VAK Reverse Blocking State Forward Blocking State
Forward
o leakage current

(b) - (a)
IA -IA
Fig. 1.7: (a) practical V-I characteristics (b) ideal V-I characteristics

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Quiz-5
10. When a thyristor is reverse biased, the number of blocked
p-n junction is
(a) 1, (b) 2,
(c) 3, (d) 4.

11. In a thyristor, the ratio of holding current to latching currentis

(a) 0.4, (b) 1,
(c) 2.5, (d) 4.

12. The SCR in OFF state, will block………………….

(a) Forward voltage, (b) Reverse voltage,
(c) Both forward and reverse voltage, (d) None of the above.

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SCR as a Switch
SCR
GClosed
Conductor
A IA K
K
A IA Low Resistance
.
VAK ON – Short Circuited

Forward Biased Condition VAK = 1 to 3V

SCR
Open Insulator
G IA
A K A K
IA = 0 High Resistance
VAK OFF – Open Circuited

Forward Biased Condition VAK = Finate

SCR
G
Open Insulator
A IA K
A K
IA = 0
High Resistance
VAK OFF – Open Circuited
Reverse Biased Condition VAK = Finate
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Quiz-7
16. The most efficient gate-triggering signal for SCR is
(a) a steady dc level, (b) a short duration pulse ,
(c) a high-frequency pulse train , (d) a low-frequency pulse train.

17. Once SCR starts conducting a forward current, its gate loses
control over
(a) anode circuit voltage only, (b) anode circuit current only,
(c) anode circuit voltage and current, (d) anode circuit voltage, current and time.
18. In a thyristor
(a) Latching current IL is associated with turn-off process and holding current IH
with turn-on process,
(b) both IL and IH are associated with turn-off process
(c) IH is associated with turn-off process and IL with turn-on process,
(d) both IL and IH are associated with turn-on process.
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Turn-OFF/Commutation methods of SCR

The tum-off of a thyristor means bringing the device from forward-
conduction state to forward-blocking state. The thyristor turn-off requires that
(i) its anode current falls below the holding current and (ii) a reverse voltage is
.
applied to thyristor for a sufficient time to enable it to recover to blocking state.
Commutation is defined as the process of turning-off a thyristor.
Types of Commutation:
1) Class-F (Line commutation) Natural commutation
2) Class-A (Load commutation)
3) Class-B (Resonant-pulse commutation)
4) Class-C (Complementary commutation) Forced commutation
5) Class-D (Impulse/voltage commutation) and
6) Class-E (External pulse commutation)
Source: Power Electronics – Dr. M. D. Singh

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Turn-OFF methods of SCR (cont..)

(1) Class-F commutation: This type of commutation is also known as natural
SCR
G
commutation or line commutation.
A K
I0 Vac = Vmsinwt
VT .
Vm
1-Φ AC
R Load V0 π
Supply, AC
Vac=Vmsinwt 0 2π 3π 4π wt
50Hz Gate
Pulse, T Firing angle
α α

0
Applications: V0 V0
I0
wt
I0
1.Phase-controlled rectifiers, V0(avg)
0 π 2π 3π 4π wt
2.Line commutated inverters,
VT
3.AC voltage controllers, and
0 π 2π 3π 4π wt
4.Step-down cycloconverter. (1-2)V Drop

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Turn-OFF methods of SCR (cont..)

(2) Class-A commutation:
This type of commutation is also known as load commutation/resonant
commutation/current commutation/self commutation.
.
A A
T T
K i0G K i0G
L
VDC VDC L
C

RL C RL
(a) (b)

Fig. 1.8: Class-A commutation (a) load in series with capacitor (b) load in parallel with capacitor

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Turn-OFF methods of SCR (cont..)

Design Considerations: A
A
VDC = VT + VL + VC + VRload VT T
VT T
VT = 1. to 2Volts
VDC = VL + VC + VRload K i0G K i0
di(t ) 1 VL L
VDC = L +  i (t )dt + Ri (t ) VL L
dt C VDC VDC
d d 2i (t ) 1 d VC VC C
VDC (t ) = L 2
+ i (t ) + R i (t ) VDC = Constant C
dt dt C dt
d 2i(t ) 1 d VLoad RL
0=L 2
+ i(t ) + R i(t ) VLoad RL
dt C dt
2
d i (t ) 1 R d
+ i (t ) + i(t ) = 0
dt 2 LC L dt
The above equation is second order differential equation, therefore the roots for
the above equation is Source: Power Electronics – Dr. M. D. Singh

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Turn-OFF methods of SCR (cont..)

VDC

Design Considerations: A VDC

0 t
d 2i (t ) R d 1 VT
+ i (t ) + i(t ) = 0 T Gate α=0
pulse
dt 2 L dt LC Firing angle
.
−b  b − 4ac
2 K i0
m1, m 2 = 0 t
2a VL L VC
2 VDC
R R  1 
−    − 4  VC C t
L L  CL 
m1, m2 = I0
2
0 t
2 VLoad RL
R  R   1  VT
m1, m2 = −    −  =  
2L  2 L   CL 
0 t
R
 =−
(1-2)V Drop

2L For the network to be in underdamped case system

2
 R   1 
2

 = w =    −  R   1  4L
   <  R<
 2 L   CL   2 L   CL  C
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Turn-OFF methods of SCR (cont..)

d 2i (t ) R d 1
Design Considerations: + i (t ) + i(t ) = 0
dt 2 L dt LC
The above equation is second order diff. eq’n, therefore the solution for this equation is
i (t ) = .e t ( A1 cos wt + A2 sin wt )
V  VDC R 
− t 
i (0+) = i (0−) = 0 A1 = 0; A2 = DC i (t ) = e

 2L 
sin wt 
L  L 
 
For the above equations, thyristor current becomes zero at wt=π t= =
w 2
 R   1 
  − 
Differentiating above equation w.r.t t and substituting wt=π  2 L   CL 
 R 
d −  V
i(t ) = −e  2 L  w DC
dt L

VDC = VL + VC + VRload
d
VDC = VL + VC VC = VDC − VL VC = VDC − L i (t )
dt
 R 
−  VDC   R 
−  
VC = VDC + Le  2L  w
VC = VDC  1 + e  2 L  w 
L  

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Turn-OFF methods of SCR (cont..)

(3) Class-B commutation: self commutation/resonant pulse commutation.

Operating Principle: Mode 1: (T – OFF)

.
A A
T L L
T -OFF

K G
K
C C
VDC VDC
i0 i0
RL RL
Charging current of capacitor, IC

Capacitor may charges to VDC

(a) (b)
Fig. 1.9: Class-B commutation (a) circuit configuration and (b) circuit in Mode-1.

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Turn-OFF methods of SCR (cont..)

(3) Class-B commutation:

Operating Principle:
Mode 2: (T – ON) . Mode 3: (T – ON)
A A
T -ON L T -ON L

K K
C C
VDC VDC
i0 i0
RL RL
SCR carries two currents:
1) Discharging capacitor current, ID Net current= IL-IC

(a) 2) Load current, IL (b) End of this mode, T – OFF

Fig. 1.10 : Circuit configuration of class-B commutation in (a) Mode-2 and (b) Mode-3.

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Turn-OFF methods of SCR (cont..)

Design Considerations:
A
VDC = VL + VC + VRload
T -OFF L
R = low. resistance
VDC = VL + VC
K
di (t ) 1 C
VDC = L +  i (t )dt
dt C
VDC
d d 2i (t ) 1 i0
VDC (t ) = L + i (t ) VDC = Constant
dt dt 2 C RL
d 2i(t ) 1
0=L + i(t )
dt 2 C
2
d i (t ) 1
+ i(t ) = 0
dt 2 LC
The above equation is second order differential equation, therefore the roots for
 1 
−0  ( 0) − 4
2

the above equation is −b  b − 4ac

2

m1, m 2 =  CL 
m1, m2 =
2a 2
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Turn-OFF methods of SCR (cont..)

Design Considerations: m1, m2 = 
1
=  
CL
1
 = 0,  = w = 
. CL
d 2i (t ) 1
+ i(t ) = 0
dt 2 LC
The above equation is second order differential equation, therefore the solution for this
equation is i (0+) = i (0−) = 0 A1 = 0; A2 = VDC
C
i (t ) = e t ( A1 cos wt + A2 sin wt )
L
C C
i (t ) = VDC sin wt i (t ) = iC ( peak ) sin wt where, iC ( peak ) = VDC
L L
For this commutation, the time taken by the SCR to get into reverse biased
condition is approximately equal to one quarter period of the resonant circuit and
the peak discharge current is assumed as twice the load current
 C
toff = LC I C ( peak ) = 2 I L = VDC
2 L
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Turn-OFF methods of SCR (cont..)

(4) Class-C commutation: It is also known as complementary commutation.
Operating Principle: w. r. t. input supply, both the thyristors are in forward
biased condition. Mode 0: (TA and TM – OFF)
.

RL R RL R

C C
VDC VDC V =0
C

A A A A

TM TA TM - OFF TA - OFF

K G K G K K

(a) (b)
Fig. 1.11: Circuit configuration of (a) class-C commutation (b) circuit during Mode-0
Source: Power Electronics – Dr. M. D. Singh
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Turn-OFF methods of SCR (cont..)

(4) Class-C commutation: Mode 2: (TA – ON)
Operating Principle:
Mode 1: (TM – ON) .
RL R
C
RL R VDC
C A A
VDC TM - OFF TA - ON
A A SCR TM carries two currents:
TA - OFF 1) Charging capacitor current, I K K
TM - ON C

K K 2) Load current, IL
V = −V
V =V
C DC

(a)
C DC
(b)

Fig. 1.12: Equivalent circuits of class-C commutation in (a) Mode-1 and (b) Mode-2.

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Turn-OFF methods of SCR (cont..)

Design Considerations:
VDC = VRload + VC + VTA VTA = 1 to 2volts during its on period

VDC = VRload + VC .
RL R
1
VDC = RL i (t ) +  i(t )dt C
C
VDC
Applying the Laplace Transform to the above equation A A
VDC 1 VDC  1 
= RL I ( S ) + I (S ) = I ( S )  RL +  TM - OFF TA - ON
S CS S  CS 
Apply inverse Laplace Transform to the above equation
K K
V
i (t ) = DC et /( R C )
L
(1)
RL
When TM is conducting, the capacitor is charged to dc supply voltage VDC
through R. Now, when TA is triggered, a voltage twice the supply voltage is applied to
2VDC t /( R C )
the RLC series circuit. So the current through this circuit is i (t ) = e L
(2)
RL
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Turn-OFF methods of SCR (cont..)

Design Considerations:
RL R
VDC = VRload + VTM VTM = VDC − VRload
C
. 2VDC t /( R C ) VDC
VTM = VDC − RL i (t ) i (t ) = e L
A A
RL
TM - OFF TA - ON
2V
VTM = VDC − RL DC et /( R C )L

VTM = VDC − 2VDC e t /( RL C )

RL K K
VTM = VDC (1 − 2et /( R C ) )
L
(3)
For turning off of the main SCR TM, the capacitor voltage VC should be same as the
voltage across the TM VTM = VC (4) VC = VDC (1 − 2et /( R C ) )
L

0 = VDC (1 − 2e 0 = 1 − 2e 1 = 2e 0.5 = e
toff /( RL C )
At t= toff, VC = 0
toff /( RL C ) toff /( RL C ) toff /( RL C )
) (5)

toff
Apply logarithmic on both sides toff = 0.6931RL C C = 1.44
RL

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Numerical Problems
Problem 1.1: For the class-C commutation, the dc source voltage VDC = 120V and
the current through Rload and R is 20A. The turn-off time of both the SCRs is 60µs.
Calculate, the value of commutating
. capacitance C for successful commutation.

Solution: The resistances Rload, = R = VDC/I = 120/20 = 6Ω

toff
Now, we have the relation for C for successful commutation as C = 1.44
RL

60*10−6
C = 1.44 RL R
6
C
VDC
C = 14.4  F A A

TM TA

K G K G

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Numerical Problems
Problem 1.2: For class-C commutation, the dc voltage Edc = 220V and the

current through R1 and R2 is 25A. The turn-off time of both the SCRs is
50μsec. Calculate the value
.
of commutating capacitor C for successful
commutation. Ans: 8.18μF
toff
C = 1.44
RL

50*10−6
C = 1.44
8.8

C = 8.18*10−6

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Turn-OFF methods of SCR (cont..)

(5) Class-D commutation: It is also known as voltage/impulse/parallel
capacitor/auxiliary commutation.
Operating Principle: w. r. t. input supply, both the thyristors are in forward
.
biased condition. Mode 0: (T & T – OFF)
TM A M
G
TM -OFF
A K
A K
C
TA G C
TA - OFF
A
VDC K K RL VDC K A K RL
VC = 0
D
D - OFF
A L A L

(a) (b)
Fig. 1.13: Configuration of (a) class-D commutation circuit and (b) circuit during Mode-0
Source: Power Electronics – Dr. M. D. Singh
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Turn-OFF methods of SCR (cont..)

(5) Class-D commutation:
Operating Principle: w. r. t. input supply, both the thyristors are in forward
biased condition. .

T
Mode 1: (TA – ON)
M -OFF
TM -OFF
A K
A K
C
C VC = VDC
TA - ON
TA - OFF
VDC K A K RL A
VDC K K RL
D - OFF
D - OFF
A L
VC = VDC A L

(a) End of this mode, TA & TM – OFF (b)

Fig. 1.14: Configuration of class-D commutation circuit during Mode-0

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Turn-OFF methods of SCR (cont..)

(5) Class-D commutation:
Operating Principle:
Mode 2: (TM – ON) SCR TM. carries two currents: Mode 3: (TA – ON)
TM -ON
1) Discharging capacitor current, IC
A K
TM -ON 2) Load current, IL
TM -ON C
A K TA - ON
A K
C
TA - OFF C VC = −VDC VDC K A K RL
A K TA - OFF D - OFF
VDC K RL
VDC K A K A L
D - ON RL
A L D - OFF
A L
End of this mode, TA & TM – OFF
(a) (b)
Fig. 1.15: Configuration of class-C commutation circuit (a) during Mode-2 and (a) during Mode-3

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Turn-OFF methods of SCR (cont..)

✓ With the firing of auxiliary thyristor TA, a reverse voltage VC is suddenly
applied across TM; therefore this method of commutation is also called
as voltage commutation.
.

✓ With sudden appearance of reverse voltage across TM, its current is

quenched; in fact the current momentarily reverses to recover the stored
charge of TM, as an auxiliary thyristor TA is used for turning off of the
main thyristor TM. This type of commutation is also known as auxiliary
commutation.
✓ When thyristor, TA is turned ON, capacitor gets connected across TM to
turn it OFF, so this type of commutation is also called as parallel
capacitor commutation.
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Turn-OFF methods of SCR (cont..)

Design Considerations:
q q = i *t
(a) Designing of commutating capacitor: C=
V
i *t iL * tOFF
C= . C = TM -ON
V VDC A K

The magnitude of the commutating capacitor C

TA - ON
depends upon the load current, input voltage A
VDC K K
and turn-off time of TM RL
D - OFF

(b) Designing of commutating inductor: A L

The design of inductor depends upon the two main criteria.

i) The maximum permissible value of the capacitor current IC whenever the
main thyristor TM in the ON state.
ii) The time duration during which the capacitor voltage returns to the
current polarity for the turn-off state of TM
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Turn-OFF methods of SCR (cont..)

Design Considerations:
(b) Designing of commutating inductor:
.
V
iC ( peak ) = DC (1)
wr L
1
oscilation frequency, wr = rad / sec (2)
LC TM -ON
VDC A
=
K
iC ( peak )
1 C
L
LC TA - OFF

VDC K A K RL
C
iC ( peak ) = VDC D - ON
L
A L
2
V 
L  C  DC 
 I L max 

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Turn-OFF methods of SCR (cont..)

(6) Class-E commutation: It is also known as external pulse commutation.
In this commutation method, a reverse voltage is applied to the current
carrying thyristor from an external pulse source.
.
Operating Principle: w. r. t. input dc supply, the thyristor T are in forward
biased condition. Mode 0:
T T-off
G
A K
A K
C
C
PULSE VC = VDC PULSE
GENERATOR GENERATOR

VDC VDC

PULSE EP PULSE
EP
TRANSFORMER TRANSFORMER
RL RL

(a) (b)
Fig. 1.16: Configuration of (a) class-E commutation circuit and (b) circuit during Mode-0
Source: Power Electronics – Dr. M. D. Singh
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Turn-OFF methods of SCR (cont..)

(6) Class-E commutation: When the commutation of T is
Operating Principle: desired, a pulse of duration equal to or

Mode 1: T - ON . slightly greater than the turn-off time

T-ON specification of thyristor is applied.

A K When a pulse of voltage EP from
C
VC = VDC PULSE the pulse generator is applied to the
GENERATOR
VDC primary of pulse transformer, the
EP
PULSE
TRANSFORMER
voltage induced in the secondary
RL
appears across thyristor T as a reverse
voltage and turns it off.
Fig. 1.17: Configuration of class-E commutation circuit during Mode-1

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Turn-OFF methods of SCR (cont..)

(6) Class-E commutation: When the commutation of T is
Operating Principle: desired, a pulse of duration equal to or

Mode 2: T - OFF . slightly greater than the turn-off time

T-off
specification of thyristor is applied.
A K
C When a pulse of voltage EP from
PULSE
GENERATOR the pulse generator is applied to the
VDC
primary of pulse transformer, the
PULSE EP
RL
TRANSFORMER
voltage induced in the secondary
appears across thyristor T as a reverse
voltage and turns it off.
Fig. 1.18: Configuration of class-E commutation circuit during Mode-1

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Quiz-8
19. Commutation is a ……………… process of thyristor
(a) turn – ON, (b) turn – OFF,
(c) Both turn ON and OFF, (d) None of these.
20. Which commutation technique is used in a phase-controlled
rectifier
(a) Line commutation, (b) Load commutation,
(c) Resonant pulse commutation, (d) Forced commutation.
21. In a complementary commutation, main thyristor TM can be
turned OFF by
(a) Making use of auxiliary thyristor TA, (b) Making use of capacitor, C
(c) Making use of diode, D, (d) Bringing the current below the holding current IH.
22. For successful commutation of class-D, the equation for C is
toff RL iL *VDC
(a) C = 1.44 , (b) C = 1.44 (c) C = iL * tOFF , (d) C=
RL toff VDC tOFF
2-Nov-21 Department of Electrical and Electronics Engineering 68

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Firing Circuits for Thyristors

An SCR can be switched from off-state to on-state in several ways; these are
(i) forward-voltage triggering,
(ii) dv/dt triggering, .

(iii) temperature triggering,

(iv) light triggering and
(v) gate triggering.
The instant of turning on the SCR cannot be controlled by the first three methods
listed above. Light triggering is used in some applications, particularly in a series-
connected string. Gate triggering is, however, the most common method of turning on
the SCRs, because this method lends itself accurately for turning on the SCR at the
desired instant of time. In addition, gate triggering is an efficient and reliable method.

Source: Power Electronics – Dr. P. S. Bhimbra

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Firing Circuits for Thyristors (cont..)

Source: Power Electronics – Dr. P. S. Bhimbra

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Firing Circuits for Thyristors (cont..)

A firing circuit should fulfil the following two functions.

(i) If power circuit has more than one SCR, the firing circuit should
produce gating pulses
. for each SCR at the desired instant for
proper operation of the power circuit. These pulses must be
periodic in nature and the sequence of firing must correspond
with the type of thyristorised power controller.
(ii) The control signal generated by a firing circuit may not be able to
turn-on an SCR It is therefore common to feed the voltage pulses
to a driver circuit and then to gate-cathode circuit. A driver circuit
consists of a pulse amplifier and a pulse transformer.
Source: Power Electronics – Dr. P. S. Bhimbra

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Firing Circuits for Thyristors (cont..)

Methods of firing circuits.

(i) Resistance Firing Circuit

(ii) Resistance-Capacitance
. Firing Circuits.
(i) RC half-wave trigger circuit
(ii) RC full-wave trigger circuit
(iii) Unijunction Transistor (UJT) Firing Circuits.

Source: Power Electronics – Dr. P. S. Bhimbra

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Firing Circuits for Thyristors (cont..)

(i) Resistance Firing Circuit

As stated above, resistance trigger

.
circuits are the simplest and most economical.
They however, suffer from a limited range of
firing angle control (00 to 90°), great
dependence on temperature and difference in
performance between individual SCRs.

Fig. 1.19. Resistance firing circuit.

Source: Power Electronics – Dr. P. S. Bhimbra

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Firing Circuits for Thyristors (cont..)

(i) Resistance Firing Circuit

Fig. 1.19. Resistance firing circuit.

Fig. 1.20. Resistance firing of an SCR in a half-wave circuit

with dc load (a) No triggering of SCR (b) a = 90° (c) a. < Source: Power Electronics – Dr. P. S. Bhimbra
90~.
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Firing Circuits for Thyristors (cont..)

(ii) Resistance-Capacitance Firing Circuit
(a) RC half-wave trigger circuit

Fig. 1.21. Resistance-capacitance half-wave

firing circuit.

Fig. 1.22. Waveforms for RC halfwave trigger circuit of (a)

high value of R (b) low value of R. Source: Power Electronics – Dr. P. S. Bhimbra

2-Nov-21 Department of Electrical and Electronics Engineering 75

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Firing Circuits for Thyristors (cont..)

(ii) Resistance-Capacitance Firing Circuit
(b) RC full-wave trigger circuit

Fig. 1.23. Resistance-capacitance full wave firing

circuit.

Fig. 1.24. Waveforms for RC full-wave trigger circuit of (a)

high value of R (b) low value of R. Source: Power Electronics – Dr. P. S. Bhimbra

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Quiz-9
23. Most efficient and reliable triggering method is
(a) forward voltage triggering, (b) dv/dt triggering,
(c) Gate Triggering, (d) Thermal triggering.
24. The function of R1 in case of R-firing circuit is to

(a) Limit the gate current, (b) limit the load current,
(c) None of these, (d) both (a) and (b).
25. In a complementary commutation, main thyristor TM can be
turned OFF by
(a) Making use of auxiliary thyristor TA, (b) Making use of capacitor, C
(c) Making use of diode, D, (d) Bringing the current below the holding current IH.
26. For successful commutation of class-D, the equation for C is
toff RL iL *VDC
(a) C = 1.44 , (b) C = 1.44 (c) C = iL * tOFF , (d) C=
RL toff VDC tOFF
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Series and Parallel Operation of SCRs

Thyristor ratings indicate voltage, current, power and temperature limits
within which a thyristor can be used without damage or malfunction.
For reliable operation of a thyristor, it should be ensured that its current
.
and voltage ratings are not exceeded during its working.
One of the major disadvantages of thyristors is that they have low
thermal time constant. If a thyristor handles voltage, current and power greater
than its specified ratings, the junction temperature may rise above the safe limit and
as a result, thyristor may get damaged. Therefore, when SCRs are selected, some
safety margin must be kept in the form of choosing device ratings somewhat higher
than their normal working values. The manufacturers of thyristors make a
comprehensive list of the voltage, current, power and temperature ratings after
carefully testing the device. If SCRs are operated under these specified conditions, no
Source: Power Electronics – Dr. P. S. Bimbhra
damage will be done to SCRs.
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Series Connection of SCRs

A
(a) Static equalizing network: I

Let, ns is the no. of series connected SCRs in a string, Ibmin I1

T1 R ED
ED is the voltage drop across the first SCR, T1, ED = I1 R
E1 is the voltage drop across the
. remaining (ns-1) SCRs Ibmax I2

T2 R
Ibmin is the minimum leakage current of the SCR T1,
Ibmax is the leakage current through the remaining SCRs, T2 to Tns, ES

I +I =I +I I −I =I
1 2 b max
−I b min
I − I = I
1 2 b
I = I
b b max
−I b min
T3 R
b min 1 b max 2

String voltage, E = E + (n − 1) E S D s 1
E = RI
1 2
I = I − I
2 1 b

E = I R + (n − 1) I R
S 1 s 2
E = I R + (n − 1)( I − I ) R
S 1 s 1 b

E = I R + n I R − I n R − I R + I R
S 1 s 1 b s 1 b
E = n I R − I n R + I R
S s 1 b s b
Tns R

nE −E E = n E − I R (n − 1)
I R(n − 1) = n E − E R=
S s D b s
s D S
K
b s s D S
(n − 1) I Source: Power Electronics – Dr. P. S. Bimbhra
s b

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Series Connection Operation of SCRs

A

(b) Dynamic equalizing network:

I

Let, ns is the no. of series connected SCRs in a string, RD D

T1 R ED
Q - max. perm diff of the reverse recovery charge of SCRs
max C

E max - maximum permissible difference

. in the voltage RD D
T2 R
ED – permissible voltage for the fast recovery SCR, T1, C
E1 – voltage across the remaining SCRs, T2 to Tns, ES
RD D
T3 R
E = E − E
1 D max
C
String voltage, E = E + (n − 1) E S D s 1
E = E + (n − 1)( E − E )
S D s D max

E = E + n E − n E − E + E
S D s E = n E − n E + E
D s max D max S s D s max max

Q R
E = n E − E (n − 1) E (n − 1) = n E − E E =
D D
T R max ns
S s D max s max s s D S max
C C
Q  Q ( n − 1)
(n − 1) = n E − E
max C= max s

C
s nE −E s
Source: Power Electronics – K
D S
Dr. P. S. Bimbhra s D S

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Thyristor Ratings and Protection

Thyristor ratings indicate voltage, current, power and temperature limits
within which a thyristor can be used without damage or malfunction.
For reliable operation of a thyristor, it should be ensured that its current
.
and voltage ratings are not exceeded during its working.
One of the major disadvantages of thyristors is that they have low
thermal time constant. If a thyristor handles voltage, current and power greater
than its specified ratings, the junction temperature may rise above the safe limit and
as a result, thyristor may get damaged. Therefore, when SCRs are selected, some
safety margin must be kept in the form of choosing device ratings somewhat higher
than their normal working values. The manufacturers of thyristors make a
comprehensive list of the voltage, current, power and temperature ratings after
carefully testing the device. If SCRs are operated under these specified conditions, no
Source: Power Electronics – Dr. P. S. Bimbhra
damage will be done to SCRs.
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Thyristor Ratings

(a) (b)
Fig. 1.25: Thyristor (a) anode voltages during blocking state and (b) V-I characteristics of a SCR with gate open.
Some subscripts are associated with voltage ratings for convenience in identifying them.
First subscript letter indicates the direction or the state : D - forward-blocking region; R –reverse-
blocking region.
Second subscript letter denotes the operating values: W - working value; R - repetitive value ; S -
surge or non-repetitive value.
Third subscript letter: M - indicates the maximum or peak value.

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Thyristor Ratings (cont..)

A
(1) Anode Voltage Ratings:

P
J1 – Forward biased
. N
J2– Reverse biased
Open P
G J3 – Forward biased
N

K
i. VDWM – Peak working forward blocking voltage: It specifies the maximum forward-blocking
voltage that a thyristor can withstand during its working. VDWM equal to the maximum value of
the sine voltage wave.
ii. VDRM – Peak repetitive forward blocking voltage: It refers to the peak transient voltage that a
thyristor can withstand repeatedly or periodically in its forward-blocking mode. Voltage VDRM is
encountered when a thyristor is commutated or turned-off.
iii. VDSM – Peak surge (non-repetitive) forward blocking voltage: It refers to the peak value of the
forward surge voltage that does not repeat. Its value is about 130% of VDRM, but VDSM is less than
forward breakover voltage VBO.

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Thyristor Ratings (cont..)

A
(1) Anode Voltage Ratings:

P
J1 – Forward biased
. N
J2– Reverse biased
Open P
G J3 – Forward biased
N

K
iv. VRWM – Peak working reverse blocking voltage: It specifies the maximum reverse-blocking
voltage that a thyristor can withstand during its working. VDWM equal to the negative maximum
value of the sine voltage wave.
v. VRRM – Peak repetitive reverse blocking voltage: It refers to the peak reverse transient voltage
that may occur repeatedly or periodically. Voltage VRRM is encountered when a thyristor is
commutated or turned-off.
vi. VRSM – Peak surge (non-repetitive) reverse blocking voltage: It refers to the peak value of the
reverse surge voltage that does not repeat. Its value is about 130% of VRRM, but VRSM is less than
forward breakover voltage VBR.

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Thyristor Ratings (cont..)

(1) Anode Voltage Ratings:
vii. VT – On-state voltage drop: It is the voltage drop between anode and cathode with specified
forward on-state current and junction temperature. Its value is of the order of 1 to 1.5 V.
.
viii. Forward dv/dt rating: If rate of rise of forward anode-to-cathode voltage is high, thyristor may
turn on even when (a) there is no gate signal and (b) anode to cathode voltage is less than forward
breakover voltage. A high value of dv/dt, at which a thyristor just gets turned on is called
critical rate of rise of anode voltage or forward dv/ dt rating of the device. If applied du/ dt
exceeds this critical value, thyristor gets turned on. Typical values of dv/dt are 20 - 500 V/µsec.
ix. Finger voltage. It is the minimum value of forward bias voltage between anode and cathode for
turning-on the device by gate triggering. The magnitude of finger voltage is somewhat more than
the normal on-state voltage drop in the thyristor
x. VSF – Voltage safety factor: Voltage safety factor is usually taken between 2 to 3.
peak repetitive reverse voltage (VRRM ) peak repetitive reverse voltage (VRRM )
VSF = =
maximum value of input voltage (VM ) 2 *RMS value of input voltage
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Thyristor Ratings (cont..)

(2) Anode Current Ratings:
A thyristor is made up of semiconductor material, its thermal capacity is therefore quite small. Even
for short over currents, the junction temperature may exceed the rated value and the device may be
damaged. As the junction temperature
. is dependent on the current handled by a thyristor, a correct
choice of current ratings is essential for a long working life of the device.
(i) Latching Current: Minimum value of anode current above which
the SCR goes to conduction state without any gate signal.
(ii) Holding Current: Minimum value of anode current below which
the SCR goes to OFF state.
(iii) Average on-state current (lTAV): The forward voltage drop
across conducting SCR is low, therefore power loss in a thyristor
depends primarily on forward average on-state current ITAV.

Pavg = (forward on state voltage across a thyristor) * ITAV

Pavg =  (instantaneous voltage across SCR)*(instantaneous current through SCR)dt
Fig. 1.20. Variation of junction temperature with constant anode current i a

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Thyristor Ratings (cont..)

(2) Anode Current Ratings:

.
Fig. 1.26. Average on-state power dissipation Pavg as a
function of ITAV for (a ) rectangular wave and (b) half
wave sinusoid.

Fig. 1.27. Maximum allowable case temperature Tcm as a function

of ITAV for (a ) rectangular wave and (b) half wave sinusoid.

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Thyristor Ratings (cont..)

(2) Anode Current Ratings:
(iv) RMS on-state current (lRMS): By definition, for direct current, rms value IRMS or Irms = average
or dc value, Idc . Heating of the resistive elements of a thyristor, such as metallic joints, leads and
.
interfaces depends on the forward rms current Irms. The rms current rating is used as an upper
limit for constant as well as pulsed anode current ratings of the thyristor.
(v) Surge Current Rating: When a thyristor is working under its repetitive voltage and current
ratings, its permissible junction temperature is never exceeded. However, a thyristor may be
subjected to abnormal operating conditions due to faults or short circuits. In order to accommodate
these unusual working conditions, surge current rating, ITSM (peak non-repetitive on-state current),
of thyristors is also specified. A surge current rating indicates the maximum possible non-
repetitive, or surge, current which the device can withstand. Higher currents caused by non-
repetitive faults or short circuits should occur once in a while during the life span of a thyristor to
prevent its degradation.

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Thyristor Ratings (cont..)

(2) Anode Current Ratings:
(vi) I2t Rating: This rating is employed in the choice of a fuse or other protective equipment for
thyristors. The rating in terms of amp2-sec specifies the energy that the device can absorb for a
.
short time before the fault is cleared. It is usually specified for overloads lasting for less than, or
equal to, one-half cycle of 50 or 60 Hz supply.
(vii) di/dt Rating: This rating of a thyristor indicates the maximum rate of rise of current from
anode to cathode without any harm to the device. When a thyristor is turned on, conduction starts
at a place near the gate. This small area of conduction spreads to the whole area of junction. If the
rate of rise of anode current (di/dt) is large as compared to the spreading velocity of carriers across
the cathode junction, local hot spots will be formed near the gate connection on account of high
current density. This causes the junction temperature to rise above the safe limit and as a
consequence, SCR may be damaged permanently. Therefore, a limit on the value of di/dt at turn-
on is specified in amperes per microsecond for all SCRs.
Typical values of di/dt are 20 to 500 A/µsec.
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Thyristor Protection
Reliable operation of a thyristor demands that its specified ratings are not exceeded. In
practice, a thyristor may be subjected to over voltages or over currents. During SCR turn-on,
di/dt may be prohibitively large. There may be false triggering of SCR by high value of dv/dt.
A spurious signal across gate-cathode
. terminals may lead to unwanted turn-on. SCRs are
very delicate devices, their protection against abnormal operating conditions is, therefore,
essential.

Fig. 1.28. Thyristor protection circuit. Source: Power Electronics – Dr. P. S. Bimbhra

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Thyristor Protection (cont..)

(a) di/dt protection. When a thyristor is forward biased and is turned on by a gate pulse, conduction of
anode current begins in the immediate neighborhood of the gate-cathode junction, Thereafter, the current
spreads across the whole area of junction. The thyristor design permits the spread of conduction to the
whole junction area as rapidly as possible.
. However, if the rate of rise of anode current, i.e. di/dt, is large as
compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on
account of high current density. This localized heating may destroy the thyristor. Therefore, the rate of rise
of anode current at the time of turn-on must be kept below the specified limiting value. The value of di/dt
can be maintained below acceptable limit by using a small inductor, called di/dt inductor in series with the
anode circuit. Typical di/dt limit values of SCRs are 20-500 A/µsec.
(b) dv/dt protection. The rate of rise of suddenly applied voltage across thyristor is high, the device
may get turned on. Such phenomena of turning-on a thyristor, called dv/dt turn-on must be avoided as it
leads to false operation of the thyristor circuit. For controllable operation of the thyristor, the rate of rise of
forward anode to cathode voltage dv/dt must he kept below the specified rated limit. Typical values of dv/dt
are 20 - 500 V/µsec. False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit
in parallel with the device.
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Thyristor Protection (cont..)

RS CS

Snubber circuit: A snubber circuit consists of a series combination S T G

A K
of resistance RS and capacitance CS, in parallel with the thyristor. L
O
VDC A
D
.

di
VDC = ( RS + RL )i + L (1)
dt

The solution to the above eq (1) is

VDC L
i= e − ( t / ) (2) where, =
( RS + RL ) ( RS + RL )
Differentiating eq (2) w. r. to t
di VDC 1
= e − ( t / ) * di VDC − ( t / )
= e (3)
di
=
VDC
e − ( t / ) *
1 dt ( RS + RL ) L
dt L
dt ( RS + RL )  ( RS + RL )
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Thyristor Protection (cont..)

RS CS

Snubber circuit: A snubber circuit consists of a series combination S T G

A K
of resistance RS and capacitance CS, in parallel with the thyristor. L
O
VDC A
D
.

di VDC − ( t / )
= e (3)
dt L

 di  VDC
L=
VDC
At t = 0; i= imax from eq(3)   =  di 
Voltage across SCR is Va = RS*i
 dt  max L
 
 dt  max
dVa di  dVa   di 
= RS  dVa  L  dV 
 = RS  
VDC
dt dt    = RS RS = * a 
 dt  max  dt  max  dt  max L VDC  dt  max
L
RS = 2
CS
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Thyristor Protection (cont..)

(c) Overcurrent Protection: Thyristors have small thermal time constants.
Therefore, if a thyristor is subjected to overcurrent due to faults, short circuits or surge
currents ; its junction temperature
. may exceed the rated value and the device may be
damaged. There is thus a need for the overcurrent protection of SCRs. As in other
electrical systems, overcurrent protection in thyristor circuits is achieved through the
use of circuit breakers and fast-acting fuses.

(d) Gate Protection: Gate circuit should also be protected against over voltages
and over currents. Over voltages across the gate circuit can cause false triggering of
the SCR. Overcurrent may raise junction temperature beyond specified limit leading
to its damage. Protection against over-voltages is achieved by connecting a zener
diode ZD across the gate circuit. A resistor R2 connected in series with the gate
circuit provides protection against over currents.
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Thyristor Protection (cont..)

(d) Gate Protection: A common problem in thyristor circuits is that they suffer
from spurious, or noise, firing. Turning-on or turning-off of an SCR may induce
trigger pulses in a nearby SCR. Sometimes transients in a power circuit may also
.
cause unwanted signal to appear across the gate of a neighboring SCR. These
undesirable trigger pulses may turn on the SCR leading to false operation of the
main SCR. Gate protection against such spurious firing is obtained by using
shielded cables or twisted gate leads. A capacitor C1 and a resistor R1 are also
connected across gate to cathode to bypass the noise signals .
(e) Heat Sink: The heat produced in a thyristor by electrical loss is dissipated to
ambient fluid (air or water) by mounting the device on a heat sink. When heat due to
losses is equal to that dissipated by the heat sink, steady junction temperature is
reached. Thyristor heating and hence its junction temperature rise is dependent
primarily on current handled by the device during its working.
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Thyristor types and their V-I characteristics

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Thyristor types and their V-I characteristics

2-Nov-21 Department of Electrical and Electronics Engineering 97

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Thyristor types and their V-I characteristics

2-Nov-21 Department of Electrical and Electronics Engineering 98

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Transistor types and their V-I characteristics

2-Nov-21 Department of Electrical and Electronics Engineering 99

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
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Text Books & References

Edition and
Reference Title Author Publisher
Year
Tata
M. D. Singh, 2nd Edition,
T1 Power Electronics McGraw-Hill
K. B. Kanchandani 2008
Education
th
4 Edition,
T2 Power Electronics P. S. Bimbhra Khanna
2006
Power Electronics: Ned Mohan, T. M.
3rd Edition,
R1 Converters Applications Undeland, W.P. Wiley
2015
and Design Robbins,
Power Electronics-
4th Edition,
R2 Circuits, Devices and M. H. Rashid Pearson
2017
Applications
Springer
Fundamentals of Power R. W. Erickson, 2nd Edition, Science &
R3
Electronics D. Maksimovic 2001 Business
Media

2-Nov-21 Department of Electrical and Electronics Engineering 100

Prepared by: Dr. Vadthya Jagan – M. Tech & Ph. D (I.I.T Roorkee)
 Stay Home!...............Stay Safe!!............Stay Connected!!!

Think beyond the boundaries……

2-Nov-21 Department of Electrical and Electronics Engineering 101