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Management of the Difficult Pediatric Airway Narasimhan
V Latin American Congress on Biomedical Engineering CLAIB 2011 May 16 21 2011 Habana Cuba Sustainable Technologies for the Health of All 1st Edition V. M. Ribera Mascarós
The export rights of this book are vested solely with the publisher.
Seventh Printing (Second Edition) 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
1.1What is Power Electronics? 1
1.2History 1
1.3Power Electronics Applications 2
1.4Power Semiconductor Devices and Their Classifications 3
1.5Power Semiconductor Devices: Characteristics and Ratings 5
1.6Ideal and Real Switches: Comparison of Characteristics 7
1.6.1Ideal Switch Characteristics 7
1.6.2 Desirable Characteristics of a Real Switch 7
1.6.3Power Loss Characteristics of an Ideal Switch 7
1.6.4Power Loss Characteristics in a Real Switch 8
1.7Power Electronic Systems 10
1.8Types of Power Electronic Circuits/Converters 11
1.9Merits 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
2.1Preliminaries 20
2.2Power Diodes 20
2.2.1Diode V–I Characteristics 21
2.2.2Diode Reverse Recovery Characteristics 22
2.2.3Types and Ratings of Power Diodes 22
2.2.4Series and Parallel Operation of Diodes 23
2.3Thyristors 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
2.4Switching Characteristics of Thyristors 30
2.4.1 Switching Characteristics during Turn-on 30
2.4.2 Switching Characteristics during Turn-off 32
2.5Thyristor Gate Characteristics 33
2.6Thyristor Commutation Methods 35
2.6.1Natural Commutation 35
2.6.2Forced Commutation 35
2.7Thyristor Protection 39
2.7.1Over Voltage Protection 40
2.7.2 Suppression of Overvoltages 40
2.7.3 Overcurrent Protection 41
2.7.4Snubber Circuits 44
2.8Thyristor Ratings 44
2.8.1Anode Voltage Ratings 45
2.8.2Current Ratings 46
2.8.3Surge Current Rating 49
2.8.4 I2t Rating 49
2.8.5 di/dt Rating 50
2.9Series and Parallel Operation of Thyristors 50
2.9.1Series Operation 51
2.9.2Parallel 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
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
8.1Preliminaries 335
8.2The 8051 Microcontroller 336
8.2.1The 8051 Pin Configuration 337
8.2.28051 Architecture 339
8.2.3Memory Organization 339
8.2.4The Special Function Register 340
8.2.5Timers/Counters 341
8.2.6The Serial Interface 341
8.2.7The Interrupts 341
8.2.8The Power Control Register (PCON) 342
8.3The Instruction Set 342
8.3.1 Addressing Modes 342
8.3.2Arithmetic Instructions 343
8.3.3Logical Instructions 343
8.3.4Data Transfer Instructions 344
8.3.5Boolean Instructions 345
8.3.6The Program Branching and Machine Control Instructions 345
8.3.7Instruction Timing 346
8.4Interfacing 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
8.4.4Interfacing a Digital to Analog Converter 349
8.4.5 Interfacing a Relay and an Optocoupler 349
8.4.6 Interfacing a Pulse Transformer 351
8.5Applications 352
8.5.1SCR Triggering 352
8.5.2Cycloconverter 354
8.5.3Fault Diagnosis in Three-phase Thyristor Converters Using Microcontroller 357
8.6ASICs for Motor Control Applications 360
8.6.1Need for DSP Based Motor Control 360
8.6.2Motor 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 supply (UPS). The last chapter relates the conventional power semiconductor devices to most advanced integrated circuit fabrication technology. The microprocessor-based control and
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
Introduction
1.1WHAT 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.
1.2HISTORY
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.
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.
1.3POWER 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 Some applications of power electronics
S. No. Area
1.Aerospace
Applications
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.TelecommunicationBattery 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 systemsHigh voltage dc (HVDC) transmissions, excitation systems, VAR compensation, static circuit breakers, fans and boiler-feed pumps, and non-conventional energy systems (solar, wind).
1.4POWER 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.Power BJTs
2.Power MOSFETs
3.Insulated Gate Bipolar Transistors (IGBTs)
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.Pulsed gate requirements (SCR and GTO).
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.Unipolar voltage withstanding capability (BJT, MOSFET, and IGBT).
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).
2.Bidirectional current capability (TRIAC).
These devices may either be voltage controlled or current controlled. The voltage controlled devices are:
1.Power MOSFET
2.IGBT, etc.
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 devices
Capability to block forward voltage
Significant capability to block reverse voltage
Reverse conduction
Type of forward on switching control Is control available for switching OFF forward current? Is control available for reverse conduction?
control available for switching OFF
C – Continuous signal. L – Latching signal.
1.5POWER 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 s.
The fast-recovery type diodes with relatively small reverse recovery times (0.1–5 s) 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 6000V, 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 s.
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 4000V, 3000A.
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 10kHz 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 0.5–1.5V. If the base drive voltage is withdrawn, the transistor switches into nonconduction (or off) state.
Power MOSFETs are used in high-speed power converters and are available at a relatively low power rating in the range of 1000V, 50A 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 1200V, 400A. They are acceptable to frequencies up to 20kHz. Characteristics and symbols of important power devices are shown in Table 1.3.
Table 1.3 Symbols and characteristics of important devices
1.6IDEAL 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.
1.6.1Ideal
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.
1.6.2Desirable 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.
1.6.3Power 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
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).
Fig. 1.1 Characteristics of an ideal switch: (a) voltage and current waveforms and (b) power loss.
1.6.4Power
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).
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 = 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
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,
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 +
‘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:
where V = initial voltage of the switch, I = final current in the switch and ton = turn-on period. Similarly, during turn-off,
The total energy loss/cycle is equal to sw onoff 1 =() 6 JVItt joules.
The average power loss or power dissipation due to switching losses is equal to: sw onoff 1 =() 6 PVItt f 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,
A similar analysis for the turn-off period gives,
Therefore, the total energy loss/cycle is swonoff
The average switching power dissipation at a switching frequency f is given by sw onoff 11 ()watts
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
Therefore, Ts is
The energy dissipation of the switch during its static ON-state in one switching cycle will be onoff 1 () 2
Therefore, the average static power dissipation at a frequency f will be
1.7POWER 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. Control unit Digital circuit Power electronic converter Load Command Main power source Feedback signal
Fig. 1.3 Block diagram of a typical power electronic system.
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.
1.8TYPES 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
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.
1.9MERITS AND DEMERITS OF POWER ELECTRONIC CONVERTERS
Merits
1.High efficiency due to low loss in power semiconductor devices.
2.High reliability of power-electronic components and converter systems.
3.Long life and less maintenance due to the absence of moving parts.
4.Fast dynamic response compared to electromechanical converter systems.
5.Small size and less weight result in less floor space and therefore, lower installation cost.
6.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.
1.10RECENT 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
the new circuits. It is easier to study the circuit using simulation compared to accomplishing the same in the laboratory on a hardware breadboard. Simulation packages such as SPICE and SABER are popular amongst power electronic engineers and academicians for their usefulness in the analysis, design, and education.
The demand for sophistication and growth for power electronics is expected to come from the following areas:
1.SMPS and UPS,
2.Energy conservation,
3.Process control and factory automation (Robotics),
4.Transportation,
5.Electro-technical applications such as welding, electroplating, and induction heating, and
6.Utility related applications, for example, HVDC.
Power electronic modules
A power electronic converter may require two, four, or more semiconductor devices depending upon the circuit configuration. Power modules consisting of two, four or six devices are, at present, available. Thus, a power electronic converter can be assembled from power modules instead of from individual semiconductor devices. Gate drive circuits for power modules are also commercially available. As a result of these developments, intelligent modules have come in the market.
Intelligent module, also called smart-power, is state-of-the-art power electronics and it consists of power module and a peripheral circuit. The peripheral circuit comprises an interface of power module with the input/output through proper isolation from low-voltage signal and high-voltage power circuit, a drive circuit, protection and diagnostic circuitry against maloperation such as excess current, over voltage and so on, microcomputer control, and controlled power supply. The user has merely to connect the existing supply and the load terminals to the smart-power. At present, intelligent modules are being used extensively in power electronics.
SUMMARY
The study of power electronics encompasses many fields within electrical engineering including power systems, solid state electronics, analog and digital control and signal processing, electromagnetic fields, and more. A combination of the knowledge of these diverse fields makes the study of power electronics challenging as well as interesting. Advances in these fields are bound to improve the prospects for further growth with emergence of new applications in power electronics. Next chapter is devoted to study of power semiconductor devices beginning with power diodes. Chapters 3, 4, 5, and 6 discuss various forms of power electronic converters and their operations. Chapter 7 is devoted to brief discussion on popular applications areas of power electronic converters while the last chapter describes the microcontroller based control and protection circuits.
SOLVED EXAMPLES
EXAMPLE 1.1 Typical switching waveforms for a real switch are given in Fig. 1.4.
(i)Show that switch-on energy loss in this case is given by
where V = OFF state voltage
I = ON state current ton = Turn-ON time.
(ii)For the same switch, obtain the expression for the switch-off energy loss as offoff 1 6 JVIt
where toff = Turn-OFF time
(iii)Find the expression for average switching power loss.
(iv)Also obtain the expression for instantaneous peak power loss during switch-on and the instant, it occurs.
Solution
(i)Assume linear variation for voltage and current during ton and toff. The OFF-state voltage is V and the ON-state current is I. The switching frequency is f Hz. The switch is ideal as far as its static performance is considered, i.e. the switch conduction loss is zero as its ON-state resistance is zero. Figure 1.4 shows the voltage across the switch and the current through it as functions of time.
1.4
Energy loss during turn-on:
and current waveforms (E 1.1).
Taking t = 0, as the starting of turn-on proccess, v and i during turn-on are expressed as
The instantaneous power will be
The energy loss during ton is obtained by integrating p, from t = 0 to t = ton, Therefore,
(ii)Energy loss during turn-off:
i The total energy loss due to switching in one cycle is: onoffJJJ
(iii)For a switching frequency f, the average power dissipation in the switch will be: sw onoff 1 ()watts. 6 PVIttf
(iv)To determine the instantaneous maximum power dissipation, during ton 2 on on 12 dp t VI
Pmax, the instantaneous peak power loss occurs at t = ton/2
EXAMPLE 1.2 In case I = 80 A, V = 220 V, ton = 1.5 µs, and toff = 4 µs for switching waveforms shown in Fig. 1.4, find the energy loss during switch-on and switch-off intervals. Find also the average power loss in the switch for a switching frequency of 2 kHz.
Average power loss= (4.4 + 11.73) × 2000 × 10–3 = 32.26 W.
EXAMPLE 1.3 In case I = 100 A, V = 200 V, ton = 1.5 µs and toff = 4 µs. Find peak instantaneous power dissipation.
EXAMPLE 1.4 Derive an expression for energy loss during turn-on transistion and turn-off transistion for a switch whose V and I curves are given in Fig. 1.5. The switch is assumed to be nonideal both during static and dynamic conditions.
Fig. 1.5 Voltage and current waveforms (E 1.4).
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