Power electronics devices and circuits

Second Edition

Power
Electronics
Devices and Circuits
V. Jagannathan


Power Electronics
Devices and Circuits
SECOND EDITION

V. Jagannathan
Professor and Head
Department of Electrical and Electronics Engineering
Coimbatore Institute of Technology
Coimbatore

New Delhi-110001
2011


POWER ELECTRONICS: Devices and Circuits, Second Edition
V. Jagannathan
© 2011 by PHI Learning Private Limited, New Delhi. All rights reserved. No part of this book may
be reproduced in any form, by mimeograph or any other means, without permission in writing from
the publisher.

ISBN-978-81-203-4196-8
The export rights of this book are vested solely with the publisher.
Seventh Printing (Second Edition)

L

L

November, 2010

Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus,
New Delhi-110001 and Printed by Baba Barkha Nath Printers, Bahadurgarh, Haryana-124507.


Contents
Preface

xi

1. Introduction

1–19

1.1
1.2
1.3
1.4
1.5
1.6

What is Power Electronics?
1
History

1
Power Electronics Applications
2
Power Semiconductor Devices and Their Classifications
3
Power Semiconductor Devices: Characteristics and Ratings
5
Ideal and Real Switches: Comparison of Characteristics
7
1.6.1 Ideal Switch Characteristics
7
1.6.2 Desirable Characteristics of a Real Switch
7
1.6.3 Power Loss Characteristics of an Ideal Switch
7
1.6.4 Power Loss Characteristics in a Real Switch
8
1.7 Power Electronic Systems
10
1.8 Types of Power Electronic Circuits/Converters
11
1.9 Merits and Demerits of Power Electronic Converters
12
1.10 Recent Developments
12
Summary
13
Solved Examples
14
Review Questions

18
Problems
18

2. Power Switching Devices and their Characteristics
2.1
2.2

Preliminaries
20
Power Diodes
20
2.2.1 Diode V–I Characteristics
21
2.2.2 Diode Reverse Recovery Characteristics
22
2.2.3 Types and Ratings of Power Diodes
22
2.2.4 Series and Parallel Operation of Diodes
23
iii

20–108


iv
2.3

2.4

2.5
2.6

2.7

2.8

2.9

2.10

2.11

2.12

2.13

Contents

Thyristors
24
2.3.1 Structure, Symbol, and V–I Characteristics
24
2.3.2 Transistor Analogy
26
2.3.3 Thyristor Turn-on Methods
27
2.3.4 Thyristor Turn-off Methods
30
Switching Characteristics of Thyristors

30
2.4.1 Switching Characteristics during Turn-on
30
2.4.2 Switching Characteristics during Turn-off
32
Thyristor Gate Characteristics
33
Thyristor Commutation Methods
35
2.6.1 Natural Commutation
35
2.6.2 Forced Commutation
35
Thyristor Protection
39
2.7.1 Over Voltage Protection
40
2.7.2 Suppression of Overvoltages
40
2.7.3 Overcurrent Protection
41
2.7.4 Snubber Circuits
44
Thyristor Ratings
44
2.8.1 Anode Voltage Ratings
45
2.8.2 Current Ratings
46
2.8.3 Surge Current Rating

49
2.8.4 I2t Rating
49
2.8.5 di/dt Rating
50
Series and Parallel Operation of Thyristors
50
2.9.1 Series Operation
51
2.9.2 Parallel Operation
53
Triggering of Thyristors
55
2.10.1 Triggering of Thyristors in Series
55
2.10.2 Triggering of Parallel Connected SCRs
57
Heat Sinks, Heating, Cooling and Mounting of Thyristors
2.11.1 Thermal Resistance
58
2.11.2 Thyristor Heat Sinks
59
Thyristor Trigger Circuits
59
2.12.1 RC Firing Circuits
59
2.12.2 Synchronized UJT Triggering (or Ramp Triggering)
2.12.3 Ramp and Pedestal Triggering
62
2.12.4 Pulse Transformers

63
Other Thyristor Devices
64
2.13.1 TRIAC
64
2.13.2 DIAC
65
2.13.3 LASCR
66
2.13.4 Programmable Unijunction Transistor (PUT)
67
2.13.5 Silicon Unilateral Switch (SUS)
67
2.13.6 Reverse Conducting Thyristor (RCT)
67
2.13.7 GTO (Gate-Turn-Off) Thyristor
68

57

61


Contents

2.14 Power Transistors
73
2.14.1 Bipolar Junction Transistor (BJT)
73
2.15 Power MOSFET

78
2.16 Comparison of MOSFET and BJT
82
2.17 Insulated Gate Bipolar Transistor (IGBT)
82
2.17.1 Basic Structure
83
2.17.2 Equivalent Circuit
84
2.17.3 Operation Models
85
2.17.4 Output Characteristics
85
2.17.5 Transfer Characteristics
86
2.17.6 Switching Characteristics
86
2.17.7 Latch-up
87
2.17.8 Safe Operating Area (SOA)
87
2.17.9 Applications
87
2.18 MOS Controlled Thyristor (MCT)
88
2.19 Typical Rating of High Power Devices
88
2.20 Driver Circuits for Gate Commutation Devices
2.20.1 GATE Drive Circuits for Power MOSFET
2.20.2 Driver Circuits for MOSFET

90
2.20.3 Driver Circuits for IGBT
91
2.20.4 Base-Drive Circuits for Power BJT
92
2.20.5 GATE Drive Circuits for GTO
92
Solved Examples
93
Review Questions
104
Problems
108

89
89

3. AC to DC Converters
3.1
3.2
3.3

v

Preliminaries
109
The Principle of Phase Control
110
Converter Classifications
113

3.3.1 Single-phase Half Wave Thyristor Rectifier with RL Load
114
3.3.2 Single-phase Half Wave Thyristor Rectifier with RL Load and
Free-wheeling Diode
116
3.3.3 Single-phase Half Wave Thyristor Rectifier with RLE Load
117
3.4 Single-phase Full Wave Thyristor Converters
118
3.4.1 Single-phase Full Wave Mid-point Thyristor Converter
118
3.5 Single-phase Full Wave Bridge Converters
120
3.5.1 Single-phase Bridge Rectifier Connected to Resistance Load
120
3.5.2 Series RL Load
121
3.5.3 RL Load with Free-wheeling Diode
122
3.6 Full Wave Bridge Rectifier Feeding RLE Load
122
3.7 Single-phase Semi-converter
124
3.8 Calculation of Active and Reactive Power Inputs
125
3.9 Effect of Load Inductance
127
3.10 Three-phase Thyristor Converter Circuits
127
3.10.1 Three-phase Half Wave Converter

128

109–166


vi

Contents

3.10.2 Three-phase Full Converters
129
3.10.3 Line Commutated Three-phase Inverter
133
3.10.4 Three-phase Semi-converters
134
3.11 Effect of Source Impedance on the Performance of Converters
3.11.1 Single-phase Full Converter
136
3.11.2 Three-phase Full Converter Bridge
138
3.12 Dual Converters
139
3.12.1 Dual Converter without Circulating Current
141
3.12.2 Dual Converter with Circulating Current
141
3.13 Single Phase Series Converters
142
3.13.1 Two Semiconverters in Series
142

3.13.2 Two Single Phase Full Converters in Series
144
3.13.3 Twelve-pulse Converters
146
3.14 Gating Circuits
147
3.15 Cosine Firing Scheme
147
Solved Examples
149
Review Questions
161
Problems
164

135

4. AC to AC Converters

167–196

4.1
4.2

Preliminaries
167
AC Voltage Controllers
167
4.2.1 Types of AC Voltage Controllers
168

4.3 Methods of Voltage Control
170
4.3.1 Single-phase AC Voltage Controller Supplying R Loads
(Phase Control)
170
4.3.2 Single-phase AC Voltage Controller Supplying R Loads
(Integral Cycle Control)
172
4.4 Single-phase Voltage Controller Supplying RL Loads
173
4.5 Three-phase AC Voltage Controller
176
4.6 Single-phase Transformer Tap Changer
178
4.7 Cycloconverters
180
4.7.1 Principle of Operation
181
4.7.2 Single-phase to Single-phase Cycloconverter Feeding RL Load
4.7.3 Three-phase to Single-phase Cycloconverters
184
4.7.4 Three-phase to Three-phase Cycloconverter
187
4.8 Output Voltage Equation
188
4.9 Effect of Source Inductance
189
Solved Examples
190
Review Questions

194
Problems
195

5. DC to DC Converters (Choppers)
5.1
5.2

Preliminaries
197
Principle of Chopper Operation

183

197–248
197


Contents

vii

5.3

Control Schemes
199
5.3.1 Constant Frequency Scheme
199
5.3.2 Variable Frequency Scheme
199

5.3.3 Current Limit Control (CLC)
200
5.4 Step Up Choppers
200
5.5 Chopper Circuits: Classification
202
5.6 Steady State Time–Domain Analysis of Type A Chopper
206
5.7 Thyristor Based Chopper Circuits
208
5.7.1 Voltage Commutated Chopper
209
5.7.2 Current Commutated Chopper
212
5.7.3 Load Commutated Chopper
214
5.8 Multiphase Choppers
215
5.9 Switch Mode Power Supplies (SMPS)
217
5.10 Switch Mode DC–DC Converter (without Isolation)
218
5.10.1 Buck Converter
218
5.10.2 Boost-type Converter
220
5.10.3 Buck Boost Converter
223
5.10.4 Cuk Converters
225

5.11 Switch Mode DC–DC Converter (with Isolation)
225
5.11.1 Fly Back Converter
226
5.11.2 Push–Pull Converter
227
5.11.3 Half-bridge Converter
228
5.11.4 Full-bridge Converter
229
5.12 Resonant Converters
230
5.12.1 Zero-current Switching Resonant Converters
231
5.12.2 Zero-voltage Switching Resonant Converters
236
5.12.3 Comparison between ZCS and ZVS converters
240
Solved Examples
241
Review Questions
246
Problems
247

6. Inverters
6.1
6.2
6.3

6.4

6.5

Preliminaries
249
Classification
249
Parallel Inverters
250
6.3.1 Basic Parallel Inverter
250
6.3.2 Modified Parallel Inverter
252
Series Inverters
253
6.4.1 Basic Series Inverter
253
6.4.2 Modifications of Series Inverter
255
Single-phase Bridge Voltage Source Inverter
256
6.5.1 Single-phase Half Bridge Inverter
256
6.5.2 Single-phase Full Bridge Inverter
259
6.5.3 Steady State Response of Single-phase Inverters

249–298

260


viii
6.6

Contents

Force
6.6.1
6.6.2
6.6.3

Commutated Thyristor Inverter
261
McMurray Inverter (Auxiliary Commutated Inverter)
261
Modified McMurray Full Bridge Inverter
263
McMurray–Bedford Half Bridge Inverter (Complementary Impulse
Commutated Inverter)
264
6.7 Three-phase Bridge Inverters
267
6.7.1 Three-phase Inverter under 180° Mode Operation
268
6.7.2 Three-phase Inverter under 120° Mode Operation
271
6.8 Voltage Control in Single-phase Inverters
274

6.8.1 External Control of the AC Output Voltage
274
6.8.2 External Control of the DC Input Voltage Through Variable DC Link
6.8.3 Internal Control of the Inverter Voltage
276
6.8.4 Pulse Width Modulated Inverters
277
6.9 Voltage Control of Three-phase Inverter
281
6.10 Harmonic Reduction in the Output Voltage
282
6.10.1 Harmonic Reduction by Transformer Connections
282
6.10.2 Harmonic Reduction by Multiple Commutation in Each Half Cycle
6.11 Current Source Inverter
286
6.11.1 Single-phase Capacitor Commutated Current Source Inverter
with R Load
286
6.11.2 Single-phase Auto-sequential Commutated Inverter (One-phase ASCI)
6.12 Three-phase Current Source Inverter
288
Solved Examples
289
Review Questions
296
Problems
298

7. Power Controllers: Their Applications

7.1
7.2
7.3

7.4
7.5

7.6
7.7

Preliminaries
299
DC Motor Speed Control
299
7.2.1 Principle of Speed Control
300
Phase Controlled Converters
301
7.3.1 Single-phase DC Drives
303
7.3.2 Three-phase DC Drives
308
7.3.3 Dual Converter Drives
311
Chopper Controlled DC Drives
312
AC Drives
315
7.5.1 Induction Motor Drives
315

7.5.2 Speed Control by Stator Voltage Control
316
7.5.3 Variable Voltage Variable Frequency Control
317
7.5.4 Speed Control by Chopper Controlled Rotor Resistance
7.5.5 Slip Power Recovery Control
319
Synchronous Motor Control
320
Static Circuit Breakers
320
7.7.1 DC Circuit Breakers
321
7.7.2 AC Circuit Breakers
321

275

284

287

299–334

318


Contents

HVDC Transmission

322
7.8.1 Types of HVDC Lines
323
7.8.2 Converter Station
324
7.9 Static Var Systems
325
7.9.1 Thyristor Controlled Reactor-fixed (TCR) Capacitor
326
7.9.2 Thyristor Switched Capacitor–Thyristor Controlled Reactor (TSC–TCR)
7.10 Uninterrupted Power Supply (UPS)
327
7.10.1 On-Line UPS
327
7.10.2 Off-Line UPS
329
7.10.3 Salient Features of an On-Line Inverter
330
7.10.4 Inverters
331
7.10.5 Transfer Switch
331
Solved Examples
332
Review Questions
334

ix

7.8

8. Microcontroller Based Control and Protection Circuits
8.1
8.2

8.3

8.4

8.5

Preliminaries
335
The 8051 Microcontroller
336
8.2.1 The 8051 Pin Configuration
337
8.2.2 8051 Architecture
339
8.2.3 Memory Organization
339
8.2.4 The Special Function Register
340
8.2.5 Timers/Counters
341
8.2.6 The Serial Interface
341
8.2.7 The Interrupts
341
8.2.8 The Power Control Register (PCON)

342
The Instruction Set
342
8.3.1 Addressing Modes
342
8.3.2 Arithmetic Instructions
343
8.3.3 Logical Instructions
343
8.3.4 Data Transfer Instructions
344
8.3.5 Boolean Instructions
345
8.3.6 The Program Branching and Machine Control Instructions
8.3.7 Instruction Timing
346
Interfacing the 8051 Microcontroller
346
8.4.1 Interfacing External Memory
346
8.4.2 Interfacing an Input/Output Device
347
8.4.3 Interfacing an Analog to Digital Converter
348
8.4.4 Interfacing a Digital to Analog Converter
349
8.4.5 Interfacing a Relay and an Optocoupler
349
8.4.6 Interfacing a Pulse Transformer
351

Applications
352
8.5.1 SCR Triggering
352
8.5.2 Cycloconverter
354
8.5.3 Fault Diagnosis in Three-phase Thyristor Converters Using
Microcontroller
357

326

335–364

345


x

Contents

8.6

ASICs for Motor Control Applications
360
8.6.1 Need for DSP Based Motor Control
360
8.6.2 Motor Control Peripherals
361
Review Questions

364

References
Index

365
367–371


Preface
Rapid developments in power electronics during the last few decades have revolutionized the art
of power modulation and control. Today, power semiconductor devices and converters using these
devices can handle high voltages and currents at high speeds. Their applications in different areas
are ever increasing aided by the use of sophisticated digital systems like microcontrollers and
computers. It is felt that this ever-growing subject, power electronics, must be learnt by students with
clarity and ease. It is therefore written in a simple straightforward style emphasizing the core
concepts underlying various power electronics circuits without delving deep into complex,
circuitous and mathematical elaborations. This book is expected to serve as a student-friendly text
to the undergraduate students of electrical and electronics engineering. It can also be used as a
textbook for one-semester course in power electronics.

The Book
The text begins with an introductory chapter on the area of power electronics with discussions
ranging around the characteristics and ratings of power semiconductor devices. Further, the chapter
gives a bird’s eye view of various types of converter circuits along with their major applications.
Chapter 2 details the underlying principle of operation of practical power semiconductor devices
such as power diodes, thyristors and devices like DIAC, TRIAC and LASCR belonging to thyristor
family. An elaborate treatment of gate-commutated devices like GTOpower BJT, power MOSFET,
and IGBT is also presented in the chapter 2. Chapters 3, 4 and 5 unlock the operating principles
of various types of converters, — ac to dc converters, ac to ac converters and dc to dc converters

(choppers and SMPS). These chapters integrate within themselves different methods of phase,
frequency and voltage control for achieving high level of performance. Analysis of each class of
converter circuit is undertaken leading to the evaluation of their performance parameters.
Chapter 6 provides an in-depth coverage of all inverter types such as parallel, series,
single phase bridge type, three phase bridge type and current source inverters, laying emphasis on
voltage and waveform control. Power controllers and their applications form the subject matter of
Chapter 7. This chapter outlines the dc and ac drives, HVDC transmission and uninterrupted power
supply (UPS). The last chapter relates the conventional power semiconductor devices to most
advanced integrated circuit fabrication technology. The microprocessor-based control and
xi


xii

Preface

protection circuits have enhanced the quality of power modulation and control. This chapter not
only gives an overview of various microcontroller chips but also considers their applications in
triggering and fault diagnosis.
Each chapter is accompanied by adequate number of solved problems, review questions and
problems involving both short and lengthy answers and solutions. The solved problems are so
chosen that going through them reinforces the understanding of the basic concepts.

Acknowledgements
This book would not have been possible but for the timely assistance received from many quarters.
The author likes to express his heartfelt thanks to the correspondent and to the principal of
Coimbatore Institute of Technology, Coimbatore, for their support and encouragement throughout
the preparation of the book.
The author expresses his gratitude to his colleagues, Prof. R. Shanmuga Sundaram and
Prof. S. Uma Maheswari for their invaluable help during the writing of the book.

The author wishes to thank his wife and children for the understanding, encouragement and
patience exhibited by them during the preparation of the book.
The author is extremely grateful to PHI Learning for coming forward to undertake publication
of this book and his special thanks are due to its editorial and production departments.
The author would appreciate feedbacks from the readers of this book towards further
improvements of its content and presentation.
V. Jagannathan


1
Introduction
1.1 WHAT IS POWER ELECTRONICS?
Power electronics deals with the applications of solid state electronic devices in the control and
conversion of electric power. It may be regarded as the technology that links two major areas of
electrical sciences, namely electric power and electronics. The concepts of power control and
conversion have undergone revolutionary changes with the emergence of power electronics. In the
areas of speed control of dc and ac motor drive systems, for example, schemes using solid state
power converters have successfully replaced conventional methods such as Ward–Leonard system of
speed control.
Power electronics is based primarily on the switching of the power semiconductor devices. With
the development of power semiconductor technology, the new devices such as power MOSFET with
better characteristics were introduced while the power-handling capabilities and the switching
speeds of the earlier power devices such as Silicon Controlled Rectifier (SCR) have improved
tremendously at the same time. The development of microprocessors/microcomputers technology has
had a great impact on the control strategies for the power semiconductor devices. In fact, modern
power electronics equipment uses power semiconductors as the muscle power with microprocessors/
microcomputers contributing the necessary brainpower and intelligence.
During the past three decades, power electronics has registered phenomenal growth to occupy
an important place in modern technology. It is now used in a wide variety of high-power products,
including heat controls, light controls, motor controls, power supplies, and High Voltage Direct

Current (HVDC) transmission systems.

1.2 HISTORY
The history of power electronics dates back to the year 1900 when the mercury arc rectifiers were
introduced. Then the metal tank rectifier, grid-controlled vacuum-tube rectifier, ignitron, phanotron,
and thyratron were introduced one after another. These were the devices employed for power control
until the 1950s.
The first revolution in electronics occurred in the year 1948 when the silicon transistor was
invented at Bell Telephone Laboratories by Bardeen, Brattain, and Schockley. Most of present day’s
advancements in electronic technology are traceable to this particular invention.
1


2

Power Electronics: Devices and Circuits

The second electronics revolution is said to have occurred in the year 1958 when the General
Electric Company, successfully developed a first commercial four-layer device called thyristor. The
advent of thyristor heralded the arrival of power semiconductor era. Since then, many different types
of power semiconductor devices and conversion techniques have been introduced. The
microelectronics revolution that followed enabled the processing of large chunk of information at
incredible speeds. The power electronics revolution had gained enough momentum by 1980s and
1990s which is entirely due to this phenomenon. Now it is possible to convert and control large
amount of power with relative ease at high efficiencies.

1.3 POWER ELECTRONICS APPLICATIONS
In general, a power electronic converter is a static device that converts one form of electrical power
to another form such as ac to dc, dc to ac, and so on. Conventional power controllers using
thyratrons, mercury-arc rectifiers, magnetic amplifiers, rheostatic controllers, and so forth have been

replaced by power electronic converters using power semiconductor devices in almost all
applications. The development of new power semiconductor devices and new circuit topologies
using them for improved performance have opened up a wide field of new applications for power
electronic converters. Their continuously falling prices have also contributed to these phenomena to
a large extent. The use of power semiconductor devices in conjunction with microprocessors/
microcomputers has further enhanced the capabilities of the power electronic converters.
Table 1.1 shows some important applications of power electronics. The power ratings of power
electronics systems range from a few watts in the case of lamps to several hundred megawatts in
HVDC transmission systems.
Table 1.1
S. No.

Area

1.

Aerospace

2.

Commercial

3.

Industrial

4.

Residential

5.
6.

Telecommunication
Transportation

7.

Utility systems

Some applications of power electronics
Applications
Space shuttle power supplies, satellite power supplies, aircraft
power systems.
Advertising, heating, airconditioning, central refrigeration,
computer and office equipment, uninterruptible power supplies
(UPS), switched mode power supplies (SMPS), elevators, light
dimmers, and flashers.
Arc and industrial furnaces, blowers and fans, pumps and
compressors, industrial lasers, transformer-tap changers, rolling
mills, textile mills, excavators, cement mills, and welding.
Airconditioning, cooking, lighting, space heating, refrigerators,
electric-door openers, dryers, fans, personal computers, other
entertainment equipment, vacuum cleaners, washing and sewing
machines, light dimmers, food mixers, electric blankets, and
food-warmer trays.
Battery chargers, power supplies (dc and UPS).
Battery chargers, traction control of electric-vehicles, electric
locomotives, streetcars, and trolley buses automotive electronics.
High voltage dc (HVDC) transmissions, excitation systems, VAR

compensation, static circuit breakers, fans and boiler-feed pumps,
and non-conventional energy systems (solar, wind).


Introduction

3

1.4 POWER SEMICONDUCTOR DEVICES AND THEIR
CLASSIFICATIONS
Ever since the silicon controlled rectifier (SCR), the first thyristor, came into existence late in the
year 1957, a wide variety of power semiconductor devices were developed during the three decades
that followed this invention. Until the year 1970, the SCR and other power semiconductor devices
of the thyristor family such as TRIAC and DIAC had been exclusively used for power control
applications in industries. The applications of other important power semiconductor devices that
include power BJT, power MOSFET, and so forth to power control problems began in 1970.
Power semiconductor devices can be classified into three groups according to their degree of
controllability. These groups have been briefly described here:
Group I includes uncontrolled power semiconductor devices such as diodes. These are called
uncontrolled devices because their ON and OFF states are not dependent on the control signals but
on supply and load circuit conditions.
Group II devices are partially controllable. These include devices that are triggered into
conduction by control signals but are turned off by the load circuit or by the supply. Such devices
include thyristors such as line commutated SCR, force commutated SCR, light activated SCR,
TRIAC, DIAC, and more.
Group III devices can be turned on and off by control signals. This category of fully
controllable devices includes:
1.
2.
3.

4.

Power BJTs
Power MOSFETs
Insulated Gate Bipolar Transistors (IGBTs)
Gate Turn Off Thyristors (GTOs)

These devices are also referred to as gate controlled devices or gate commutation devices.
Group II and group III devices can also be classified—according to the gate signal requirements
as under:
1.
2.

Pulsed gate requirements (SCR and GTO).
Continuous gate signal requirements (BJT, MOSFET, and IGBT).

The classification of the devices can also be done on the basis of voltage withstanding capability
as under:
1.
2.

Unipolar voltage withstanding capability (BJT, MOSFET, and IGBT).
Bipolar voltage withstanding capability (SCR, GTO).

And on the basis of current conduction capability as under:
1.
2.

Unidirectional current capability (SCR, GTO, BJT, MOSFET, IGBT, and Diode).
Bidirectional current capability (TRIAC).

These devices may either be voltage controlled or current controlled. The voltage controlled
devices are:
1.
2.

Power MOSFET
IGBT, etc.


4

Power Electronics: Devices and Circuits

The current controlled devices are:
1. Thyristors
2. Power BJTs, etc.
The important features of these devices are summarized in Table 1.2

Bipolar power transistor

Power MOSFET

Insulated gate bipolar
transistor (IGBT)

Thyristor (SCR)

Asymmetrical SCR

Reverse conducting
thyristor

Gate turn off thyristor
(GTO)
Reverse blocking type

GTO without reverse
voltage blocking
capability

Triac

MOS controlled thyristor
(MCT)

Properties of power semiconductor switching devices

Power diode

Table 1.2

Capability to
block forward
voltage
Significant
capability to
block reverse
voltage













































Reverse
conduction























































L

L

C – Continuous signal.
L – Latching signal.



Not applicable

Not applicable

C




Not applicable

L


Not applicable

Is control
C
available
for reverse
conduction?

Is control
available
for
switching
OFF
reverse
current?





Not applicable

C






Not applicable

L

Not applicable

Is control
available
for
switching
OFF
forward
current?

C

No control

Type of
forward
on
switching
control


Introduction

1.5

5

POWER SEMICONDUCTOR DEVICES: CHARACTERISTICS AND
RATINGS

A diode is a two-layer p-n junction semiconductor device with two terminals, namely anode and
cathode. If a forward voltage that makes the anode potential greater than that of the cathode, is
applied, the device starts conducting and behaves essentially as a closed switch. For a reverse
voltage, the diode does not conduct but behaves as an open switch blocking the reverse voltage.
Power diodes are of three types: general purpose, high speed (or fast recovery), and Schottky types.
General-purpose diodes are available up to 3000 V, 3500 A. They are useful for low frequency
applications since their reverse recovery times are comparatively large around 25 ms.
The fast-recovery type diodes with relatively small reverse recovery times (0.1–5 ms) are useful
in high frequency circuits. The rating of fast recovery diodes can go up to 3000 V, 1000 A.
Schottky diodes have low on-state voltage drop and very small recovery time, typically
nanoseconds. They are suitable for very high frequency circuits operating from low voltages. Their
ratings are limited to 100 V, 300 A and the forward voltage drop of a power diode is very low,
typically 0.3 V.
A thyristor has three terminals, namely an anode, a cathode, and a gate. Unlike the diode, which
conducts only after its anode to cathode voltage exceeds the cut-in voltage, the thyristor will conduct
only when a small current is passed through the gate terminal to the cathode. This means that the
gate controls the beginning of conduction in thyristor. But once a thyristor attains the conduction
state, the gate loses its control since the thyristor continues to conduct even after the removal of gate
supply. When a thyristor is in a conduction mode, the forward voltage drop is very small, typically
0.5–2 V. A conducting thyristor can be turned off by making the potential of the anode equal to or
less than the cathode potential. The line-commutated thyristors are turned off due to the sinusoidal
nature of the input voltage and forced-commutated thyristors are turned off by an extra circuit
employed called commutation circuitry. Natural or line-commutated thyristors are available with
ratings up to 6000 V, 3500 A.

Light Activated SCRs (LASCR) are suitable for high voltage power systems especially HVDC.
They are available up to 6000 V, 1500 A with a switching speed of 200–400 ms.
GTOs are gate-turned-off thyristors. They are turned on by applying a short positive pulse to the
gate as in SCRs but are turned off by the application of short negative pulse to their gates. Hence,
these do not require any separate commutation circuit. GTOs are very attractive for forced
commutation converters and are available up to 4000 V, 3000 A.
TRIACs are widely used in all types of simple heat controls, light controls, and in ac switches
mostly in low power ac applications. The characteristics of TRIACs are similar to two thyristors
connected in antiparallel and having only one gate terminal. The current flow through a TRIAC can
be controlled in either direction.
A DIAC is a gateless TRIAC designed to breakdown at a low voltage.
High-power bipolar transistors are commonly used in power converters at a frequency below
10 kHz and are effectively applied in the power ratings up to 1200 V, 400 A. A bipolar transistor
has three terminals, namely base, emitter, and collector. It is normally operated in common-emitter
configuration as a switch. As long as the base of an NPN-transistor is at a higher potential than the
emitter and the base current is sufficiently large to drive the transistor to the saturation region, the
transistor stays on, provided the collector-emitter junction is properly biased. The forward
conduction drop lies in the range 0.5–1.5 V. If the base drive voltage is withdrawn, the transistor
switches into nonconduction (or off) state.


6

Power Electronics: Devices and Circuits

Power MOSFETs are used in high-speed power converters and are available at a relatively low
power rating in the range of 1000 V, 50 A at a frequency range of several tens of kilohertz. Power
MOSFETs are voltage controlled devices unlike transistors that are current controlled. Similar to
transistors that need continuous supply of base current to keep it in the ON state, MOSFETs also
require the continuous application of gate source voltage of appropriate magnitude in order to remain

in the ON state.
IGBTs are voltage-controlled power transistors. They are inherently faster than BJTs but not as
fast as MOSFETs. They are suitable for high voltage and high current applications up to 1200 V,
400 A. They are acceptable to frequencies up to 20 kHz. Characteristics and symbols of important
power devices are shown in Table 1.3.
Table 1.3 Symbols and characteristics of important devices
Devices

Symbols
A

Characteristics

ID

ID

K

Diode

VAK

0
VAK

Thyristor

IA

G

IA

K

A

TRIAC
A

VAK

0
IA

B

IA

Gate triggered

Gate triggered
VAK

0

G

Gate triggered

IA

Gate triggered

LASCR
A

IA
IB

NPN BJT

IC

C

IBn

B

IBnI>
> IBt
BnIB1

IB1
C

IE
IC

VGSn > VGS1
VGSn

IC

VGS1
VT

IE
ID

0
ID

G
S

VCE

0

G
E
D

N-Channel
MOSFET

VAB
VAK

0
IC

E

IGBT

K

G

0

VGSn > VGS1

VCE
VGS0
VGS1 > VGSn
VGSn
VDS


Introduction

7

1.6 IDEAL AND REAL SWITCHES: COMPARISON OF
CHARACTERISTICS
An ideal switch is one that possesses ideal characteristics like zero resistance when ‘ON’ and infinite

resistance when ‘OFF’. Further, the transitions from OFF to ON and the reverse is expected to take
place instantaneously in an ideal switch. Practical or real switches exhibit a deviation from these
ideal properties by having finite but very small ‘ON’ state resistances and finite but very large ‘OFF’
state resistances, and very small OFF and ON transition times.
Power semiconductor devices are used essentially as switching elements in most of the power
converter circuits. They are ideal substitutes for mechanical switches. The performance of a switch
is assessed by its behaviour under static as well as its dynamic conditions. If a switch remains in its
OFF state or ON state it is said to be in static condition. A dynamic condition prevails in the switch
when it is moving from one state to another. High power conversion efficiencies would result if these
switches behave like ideal switches both under static as well as dynamic conditions.

1.6.1

Ideal Switch Characteristics

Following are the features of an ideal switch:
(a) ON state resistance = 0, leading to zero forward voltage drop while in conduction state.
(b) OFF state resistance = ¥, resulting in zero leakage current while blocking forward as well
as reverse voltages under OFF state.
(c) Capacity to conduct infinitely large current and to withstand infinitely large forward as well
as reverse voltages.
(d) Ability to switch instantaneously from OFF to ON and from ON to OFF state.
(e) No power requirement to control the switch.
(f) Easy control.
Above features ensure zero conduction loss and zero switching loss in an ideal switch, even if
the switch handles large power at high voltage and high current conditions. While no real power
semiconductor switches have these ideal properties, efforts are continuously made to take the real
switch features closer to those of ideal switch.

1.6.2

Desirable Characteristics of a Real Switch

The desirable qualities of a real switch are:
(a) The device conducts large currents with negligibly small voltage drops across them.
(b) They must be able to block high forward as well as reverse voltages when OFF with
negligibly small leakage currents.
(c) Very small turn ON and turn OFF times so that the device can operate at high frequencies.
(d) Suitable for parallel and series operations under high current and high voltage conditions.
(e) High operating temperatures.
(f) Long life.

1.6.3

Power Loss Characteristics of an Ideal Switch

In case of an ideal switch, power loss during its working is zero, because of its ideal characteristics.
It has zero resistance during “ON” state and infinite resistance during “OFF” state. The voltage drop


8

Power Electronics: Devices and Circuits

across the switch is zero while “ON” and the current through the device is zero during “OFF” state.
Further since the transitions from ON to OFF and OFF to ON are instantaneous the switching losses
are also zero. Device voltage and current waveforms are shown in Fig. 1.1(a) for the ideal switch.
Power loss in the switch is zero during ON state, OFF state and also during the transition from one
state to the other. This is depicted in power loss waveform shown in Fig. 1.1(b).
isw vsw

vsw



isw

Psw
Psw = 0

isw

t

t

Switch
OFF

Switch
ON
(a)

(b)

Fig. 1.1 Characteristics of an ideal switch: (a) voltage and current waveforms and (b) power loss.

1.6.4

Power Loss Characteristics in a Real Switch

Real power semiconductor switches suffer from very small conduction losses and switching losses
due to non-ideal features like finite, though very small, ON state resistances and very small OFF state
resistances. The switching times are also finite, though very small, of the order of microseconds. The
switching losses become considerable portion of the total device losses in devices like power
MOSFETs operating at very high frequencies.
A simple circuit employing a real switch is shown in Fig. 1.2(a). The switch is assumed to
possess ideal static characteristics, so that there is no static ON state and OFF state losses. However,
because of the non-ideal dynamic characteristics, there would be switching losses. The switch
voltage and switch current waveforms are shown in Fig. 1.2(b). The switching loss curve is also
shown in Fig. 1.2(b).
SW

V

R

±

vsw

vsw
isw
Psw
vsw

Psw
v

v

t1
(a)

Fig. 1.2

isw

Psw

t2

t3

t4

t

(b)

Power loss in the real switch: (a) circuit and (b) voltage and current waveforms and power
loss curve.

The switch of Fig. 1.2(a) is turned ON at t = t1. Prior to t = t1 the switch is in the forward blocking
state. During the turn-on operation that takes place from t1 to t2, the voltage across the switch reduces
from the initial value V to zero. During the same period, the current through the switch rises from


Introduction

9

zero to the static ON state value, I. The current waveform represents the instantaneous value of the
switch current during the turn-on transition. During this period, there is power dissipation inside the
switch. The instantaneous value of this power loss is given by the curve shown in Fig 1.2(b) as the
product of the instantaneous values of voltage and current. Depending on the nature of the current
and voltage waveforms, during the transition, the peak power can reach relatively large magnitudes.
The energy dissipated in this turn-on process can be assumed to be equal to the area under power
loss/power dissipation curve.
Turn-off switching operation takes place from t3 to t4 as shown in Fig 1.2(b). During this
transition, switch voltage rises from zero to V, (the supply voltage) as the current falls from I to zero.
Transition periods, Ton and Toff are not equal in power semiconductor switches though Toff is
generally larger.
The total energy, Jsw dissipated in a switching cycle is given by the sum of the areas under
power loss wave form during turn-on and turn-off. Therefore,
Jsw = Jon + Joff

(1.1)

where Jon and Joff represent switching energy loss during turn-on and turn-off, respectively.
The average power loss in watts is given by
Psw = (Jon + Joff)f = Jswf

(1.2)

‘f ’, is the switching frequency in Hz.
If the linear variation is assumed for voltage and current waveforms during turn-on and turnoff transitions as shown in Fig. 1.4, an expression for Jon can be obtained as:
1
J on = VIton
(1.3)

6
where V = initial voltage of the switch, I = final current in the switch and ton = turn-on period.
Similarly, during turn-off,

1
J off = VItoff
6

(1.4)

The total energy loss/cycle is equal to
1
J sw = VI (ton  toff ) joules.
6
The average power loss or power dissipation due to switching losses is equal to:
1
Psw = VI (ton  toff ) f
6

where ‘f ’ is the switching frequency.
If the static performance of the switch is also non-ideal, i.e. ON-state resistance of the switch
is finite of the order of a few ohms, resulting in a small conduction voltage drop vf then, Jon, taking
into account the above voltage drop, vf,
1
1
VIton  V f Iton
6
3
A similar analysis for the turn-off period gives,
J on

J off

1
1
VItoff  V f Itoff
6
3

(1.5)

(1.6)


10

Power Electronics: Devices and Circuits

Therefore, the total energy loss/cycle is
Jsw

J on  J off

1
È1
Ø
ÉÊ 6 VI  3 V f I ÙÚ (ton  toff ) joules

(1.7)

The average switching power dissipation at a switching frequency f is given by
Psw

1
È1
Ø
ÉÊ 6 VI  3 V f I ÙÚ (ton  toff ) f watts

(1.8)

ON-state power loss
The power loss during ON-state of the switch is calculated as follows:
For a given duty cycle k, ‘ON’ time of the switch is
Ton = static-on time Ts 

1
(ton  toff )
2

(1.9)

Therefore, Ts is

Ts

k 1
 (ton  toff )
f 2

(1.10)

The energy dissipation of the switch during its static ON-state in one switching cycle will be
Ëk 1
Û
V f I Ì  (ton  toff ) Ü
Íf 2
Ý
Therefore, the average static power dissipation at a frequency f will be
J

Pstatic

(1.11)

Ëk 1
Û
V f I Ì  (ton  toff ) Ü f
Íf 2
Ý

(1.12)

The total loss in watts = Pstatic + Psw

(1.13)

1.7 POWER ELECTRONIC SYSTEMS
The block diagram shown in Fig. 1.3 depicts a typical power electronic system. Major system
components are shown in various blocks and an ac or a dc supply may be used as a main power
source.

Main power
source

Command

Control
unit

Digital
circuit

Power
electronic
converter

Load

Feedback
signal

Fig. 1.3

Block diagram of a typical power electronic system.


Introduction

11

The output from the power electronic converter may be variable dc, or ac voltage, or it may be

a variable voltage and variable frequency. In general, the output of a power electronic converter
circuit depends upon the requirements of the load. For example, if the load is a dc motor, the
converter output is a variable direct voltage. In case of an induction motor, the converter output is
a variable voltage and variable frequency ac.
The feedback voltage signal may correspond to the speed if it were a rotating machine and it
is then compared with the command signal. The difference of the two, when taken through the digital
circuit components, controls the instant of turn-on of semiconductor devices forming the solid-state
power converter system. In this manner, speed of the motor can be controlled, as desired, over a wide
range with the adjustment of the command signal.

1.8 TYPES OF POWER ELECTRONIC CIRCUITS/CONVERTERS
Broadly speaking, power electronic converters (or circuits) can be classified into five types as:
1. Diode rectifiers: A diode rectifier circuit converts ac input voltage into a fixed dc voltage.
The input may be single-phase voltage or a three-phase voltage. Diode rectifiers find wide
use in electric traction, battery charging, electroplating, electrochemical processing, power
supplies, welding, and uninterruptible power supply (UPS) systems.
2. AC to DC converters (Phase-controlled rectifiers): These converters translate constant
ac voltage to variable dc output voltage. These rectifiers are also called line-commutated or
naturally commutated ac to dc converters since these rectifiers use line voltage or source
voltage for commutation. Phase-controlled converters use line-commutated thyristors. They
may be 1-phase or 3-phase converters depending on the number of the input supply phases.
These are used in dc drives, metallurgical and chemical industries, excitation systems for
synchronous machines and so forth.
3. DC to DC converters (DC choppers): A dc chopper converts fixed dc input voltage to
a variable dc output voltage. The chopper circuits use forced commutation. Thyristors
are used in high power DC choppers. For low power applications, thyristors are replaced by
power transistors, power MOSFETs, GTO thyristors, and the like. Choppers find a
wide application in dc drives, subway cars, trolley trucks, battery-driven vehicles, and many
more.
4. DC to AC converters (inverters): An inverter converts fixed dc voltage supply to ac

voltage supply. The converters of this type use the principle of Pulse Width Modulation
(PWM) to produce an output which may be a variable voltage and variable frequency
supply. Modern day inverters use power semiconductor devices such as power transistors,
power MOSFETs, and IGBTs. Forced-commutated inverters find wide use in inductionmotor drives, synchronous motor drives, induction heating, UPS, and so on. However, the
inverters used in HVDC transmission are dependent on the supply voltage for their
commutation. Hence, they are known as line-commutated inverters.
5. AC to AC converters: These convert fixed ac input voltage into variable ac output
voltage. There are two types of ac to ac converters.
(a) AC voltage controllers (AC voltage regulators): These converter circuits convert
fixed ac voltage directly to a variable ac voltage at the same frequency. AC voltage


12

Power Electronics: Devices and Circuits

controller employs two thyristors in antiparallel or a single TRIAC. Turn-off is obtained
by line commutation. Output voltage is controlled by varying the firing angle delay. AC
voltage controllers are widely used to control heating and lighting, speed control of
fans, pumps and so on.
(b) Cycloconverters: These circuits convert input power at one frequency directly to
output power at a different frequency. (These are single stage converters). Line
commutation is more common in these converters, though forced and load commutated
cycloconverters are also available. These are primarily used in slow-speed large ac
motors that drive loads like rotary kilns.

1.9 MERITS AND DEMERITS OF POWER ELECTRONIC CONVERTERS
Merits
1.
2.

3.
4.
5.
6.

High efficiency due to low loss in power semiconductor devices.
High reliability of power-electronic components and converter systems.
Long life and less maintenance due to the absence of moving parts.
Fast dynamic response compared to electromechanical converter systems.
Small size and less weight result in less floor space and therefore, lower installation cost.
Mass production of power semiconductor devices has brought down the cost of converter
equipments.

Demerits
Power-electronic converter circuits introduce harmonics into the supply and the load systems,
adversely affecting the performance of the load and the supply. In the supply system, the harmonics
distort the voltage waveform and seriously influence the performance of other equipments connected
to the same supply line. In addition, the harmonics in the supply line can also cause interference with
communication lines. It is, therefore, necessary to insert filters at the input of a converter. Other
disadvantages of the converters include:
1. 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.
2. Power-electronic controllers have low overload capacity. These converters must, therefore,
be rated for taking momentary overloads which increases the cost of power electronic
controllers.
3. Regeneration of power is difficult in power electronic converter systems.
The merits possessed by power electronic converters far outweigh their disadvantages. As a
consequence, semiconductor-based converters are being extensively employed in systems where
power flow is to be regulated. As already stated, conventional power controllers used in many
installations have already been replaced by semiconductor-based electronic controllers.

1.10 RECENT DEVELOPMENTS
With the increased availability of computers, the simulation of power electronic converters and
systems has become popular. Computer simulations are commonly used to analyze the behaviour of