Power Electronics Devices and Circuits pdf pdf
Devices and Circuits Power
Electronics
Power Electronics Second Edition
Professor and Head
Department of Electrical and Electronics Engineering
Coimbatore Institute of Technology
Coimbatore
New Delhi-110001
Power Electronics
Devices and Circuits
SECOND EDITION
V. Jagannathan
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
Contents Preface xi
1. Introduction
1–19
1.1 What is Power Electronics?
1
1.2 History
1
1.3 Power Electronics Applications
2
1.4 Power Semiconductor Devices and Their Classifications
3
1.5 Power Semiconductor Devices: Characteristics and Ratings
5
1.6 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 20–108
2.1 Preliminaries
20
2.2 Power Diodes
20
2.2.1 Diode V–I Characteristics
21
2.2.2 Diode Reverse Recovery Characteristics
22
Contents
2.3 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
2.4 Switching Characteristics of Thyristors
30
2.4.1 Switching Characteristics during Turn-on
30
2.4.2 Switching Characteristics during Turn-off
32
2.5 Thyristor Gate Characteristics
33
2.6 Thyristor Commutation Methods
35
2.6.1 Natural Commutation
35
2.6.2 Forced Commutation
35
2.7 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
2.8 Thyristor Ratings
44
2.8.1 Anode Voltage Ratings
45
2.8.2 Current Ratings
46
2.8.3 Surge Current Rating
49
2
2.8.4 I t Rating
49
2.8.5 di /dt Rating
50
2.9 Series and Parallel Operation of Thyristors
50
2.9.1 Series Operation
51
2.9.2 Parallel Operation
53
2.10 Triggering of Thyristors
55
2.10.1 Triggering of Thyristors in Series
55
2.10.2 Triggering of Parallel Connected SCRs
57
2.11 Heat Sinks, Heating, Cooling and Mounting of Thyristors
57
2.11.1 Thermal Resistance
58
2.11.2 Thyristor Heat Sinks
59
2.12 Thyristor Trigger Circuits
59
2.12.1 RC Firing Circuits
59
2.12.2 Synchronized UJT Triggering (or Ramp Triggering)
61
2.12.3 Ramp and Pedestal Triggering
62
2.12.4 Pulse Transformers
63
2.13 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
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
89
2.20.1 GATE Drive Circuits for Power MOSFET
89
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
3. AC to DC Converters
109–166
3.1 Preliminaries 109
3.2 The Principle of Phase Control 110
3.3 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
Contents
4.7 Cycloconverters 180
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
4.7.1 Principle of Operation 181
4.2 AC Voltage Controllers 167
4.7.2 Single-phase to Single-phase Cycloconverter Feeding RL Load 183
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) 197–248
4.2.1 Types of AC Voltage Controllers 168
4.1 Preliminaries 167
3.10.2 Three-phase Full Converters 129
3.12.1 Dual Converter without Circulating Current 141
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 135
3.11.1 Single-phase Full Converter 136
3.11.2 Three-phase Full Converter Bridge 138
3.12 Dual Converters 139
3.12.2 Dual Converter with Circulating Current 141
4. AC to AC Converters 167–196
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
5.1 Preliminaries 197
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
249–298
6.1 Preliminaries 249
6.2 Classification 249
6.3 Parallel Inverters 250
6.3.1 Basic Parallel Inverter 250
6.3.2 Modified Parallel Inverter 252
6.4 Series Inverters 253
6.4.1 Basic Series Inverter 253
6.4.2 Modifications of Series Inverter 255
6.5 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 260
Contents
6.6 Force Commutated Thyristor Inverter 261
6.6.1 McMurray Inverter (Auxiliary Commutated Inverter) 261
6.6.2 Modified McMurray Full Bridge Inverter 263
6.6.3 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 275
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 284
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) 287
6.12 Three-phase Current Source Inverter 288 Solved Examples 289 Review Questions 296 Problems 298
7. Power Controllers: Their Applications 299–334
7.1 Preliminaries 299
7.2 DC Motor Speed Control 299
7.2.1 Principle of Speed Control 300
7.3 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
7.4 Chopper Controlled DC Drives 312
7.5 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 318
7.5.5 Slip Power Recovery Control 319
7.6 Synchronous Motor Control 320
7.7 Static Circuit Breakers 320
7.7.1 DC Circuit Breakers 321
7.8 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) 326
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
8. Microcontroller Based Control and Protection Circuits 335–364
8.2 The 8051 Microcontroller 336
8.3.6 The Program Branching and Machine Control Instructions 345
8.5.2 Cycloconverter 354
8.5.1 SCR Triggering 352
8.5 Applications 352
8.4.6 Interfacing a Pulse Transformer 351
8.4.5 Interfacing a Relay and an Optocoupler 349
8.4.4 Interfacing a Digital to Analog Converter 349
8.4.3 Interfacing an Analog to Digital Converter 348
8.4.2 Interfacing an Input/Output Device 347
8.4.1 Interfacing External Memory 346
8.4 Interfacing the 8051 Microcontroller 346
8.3.7 Instruction Timing 346
8.3.5 Boolean Instructions 345
8.2.1 The 8051 Pin Configuration 337 8.2.2 8051 Architecture 339
8.3.4 Data Transfer Instructions 344
8.3.3 Logical Instructions 343
8.3.2 Arithmetic Instructions 343
8.3.1 Addressing Modes 342
8.3 The Instruction Set 342
8.1 Preliminaries 335
8.2.7 The Interrupts 341
8.2.6 The Serial Interface 341
8.2.5 Timers/Counters 341
8.2.4 The Special Function Register 340
8.2.3 Memory Organization 339
8.2.8 The Power Control Register (PCON) 342
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 365 Index
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
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
11111 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
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 inHVDC transmission systems.
Table 1.1 Some applications of power electronicsS. No. Area Applications
1. Aerospace Space shuttle power supplies, satellite power supplies, aircraft power systems.
2. Commercial Advertising, heating, airconditioning, central refrigeration, computer and office equipment, uninterruptible power supplies (UPS), switched mode power supplies (SMPS), elevators, light dimmers, and flashers.
3. Industrial Arc and industrial furnaces, blowers and fans, pumps and compressors, industrial lasers, transformer-tap changers, rolling mills, textile mills, excavators, cement mills, and welding.
4. Residential 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.
5. Telecommunication Battery chargers, power supplies (dc and UPS).
6. Transportation Battery chargers, traction control of electric-vehicles, electric locomotives, streetcars, and trolley buses automotive electronics.
7. Utility systems High voltage dc (HVDC) transmissions, excitation systems, VAR
Introduction
1.4 POWER SEMICONDUCTOR DEVICES AND THEIR CLASSIFICATIONS
Power semiconductor devices can be classified into three groups according to their degree of controllability. These groups have been briefly described here:
4. 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. Power MOSFET
2. Bidirectional current capability (TRIAC). These devices may either be voltage controlled or current controlled. The voltage controlled devices are:
2. Bipolar voltage withstanding capability (SCR, GTO). And on the basis of current conduction capability as under: 1. Unidirectional current capability (SCR, GTO, BJT, MOSFET, IGBT, and Diode).
1. Unipolar voltage withstanding capability (BJT, MOSFET, and IGBT).
2. 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. Pulsed gate requirements (SCR and GTO).
3. Insulated Gate Bipolar Transistors (IGBTs)
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.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.
1. Power BJTs
controllable devices includes:
Group III devices can be turned on and off by control signals. This category of fully
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 II devices are partially controllable. These include devices that are triggered into
2. Power MOSFETs
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
Table 1.2 Properties of power semiconductor switching devicesr r o o r e r st st o p g se la R
) ) y ri st g si T o in er
C t y T R ri e E v g in h an ip ct
B y d F C S k y t u re tr r h S b in c )
G al ) d
io d it
(S o t k t r c e Td O te n u lo il
e (I r ic O c st ff a ll a o b C r r o o tr T b w M c ri lo o o a ri e o h g e e o st y r p tr e b it T w d (G g (M p e st rn a ri m n th e a o rs u c r te si y o w w m e rs ltP t n h la o la e o c a O T o sy te P ev v v S
T tr su a e A R ip
O G G In R B
M Capability to block forward voltage Significant capability to block reverse voltage Reverse conduction Type of
C forward on switching
L control Is control C available for switching OFF forward L current? l o tr n
Is control C available co for reverse
No conduction? le le le le le le le b b b b b b b
L a a a a a a a lic lic lic lic lic lic lic p p p p p p p p p p p p p p
Is control t a t a t a t a t a t a t a available o o o o o o o
C N N N N N N N for switching
OFF reverse current? L C – Continuous signal.
L – Latching signal.
Introduction
1.5 POWER SEMICONDUCTOR DEVICES: CHARACTERISTICS AND
RATINGSA 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
Power Electronics: Devices and Circuits
I A Gate triggered
I C
I E C G E
I D
G D A KI A G A B
I A G A K
I A G
V AK
V AK Gate triggered
V GS 1 > V GSn
V AK
I A Gate triggered Devices Symbols Characteristics Diode
Thyristor TRIAC LASCR NPN BJT
IGBT N-Channel MOSFET
Table 1.3 Symbols and characteristics of important devicesV GSn > V GS1
V AB
V GSn
V GS
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.
I E C B
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.
V AK A K
I D
V AK
I D
I A Gate triggered
I C
I B E
I D
V CE
I C
I Bn
I B 1 I Bn > I B 1 V CE
I C
V GSn
V GSn > V GS 1 V GS 1 V T
V DS
I B n > I B t
Introduction
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
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).
v i sw sw v sw i sw P sw P sw = 0 i sw Switch t t Switch
OFF 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 v sw sw P P sw sw i
sw i
sw P sw±
V R v v v sw t t t t t 1 2 3 4
(a) (b)
Fig. 1.2 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 = t . Prior to t = t the switch is in the forward blocking
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1
state. During the turn-on operation that takes place from t to t , the voltage across the switch reduces
1
2
Introduction
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 t to t as shown in Fig 1.2(b). During this
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4 transition, switch voltage rises from zero to V, (the supply voltage) as the current falls from I to zero.
Transition periods, T and T are not equal in power semiconductor switches though T is
on off off generally larger.
The total energy, J dissipated in a switching cycle is given by the sum of the areas under
sw
power loss wave form during turn-on and turn-off. Therefore,
J = J + J (1.1)
sw on off