This book is intended as a teaching textbook for advanced undergraduate and postgraduate courses in power electronics. The reader is presumed to have a background in mathematics, electronic signal devices and electric circuits that would be common in the early years of first degree courses in electrical and electronic engineering. It is the writers' experience that engineering students prefer to learn by proceeding from the particular to the general and that the learning route be well illustrated by many worked examples. Both of these teaching practices are followed here and a lot of problems are also included for attempt and solution at the ends of most chapters.

About one half of the text was written while the principal author (W.S.) was on study leave at the Department of Electrical and Computer Engineering, University of Wisconsin, Madison, Wisconsin, USA. His grateful thanks are acknowledged to the stimulating company of Professor Donald Novotny and Professor Tom Lipo during this period of sabbatical scholarship, sponsored by the Fulbright Commission.

It has become evident in recent years that the reign of the silicon controlled rectifier member of the thyristor family, as the universal semiconductor power switch, is drawing to a close. Except in very high power applications the technology of the immediate future lies with three-terminal, control electrode turn-off devices such as the gate turn-off thyristor (GTO), the bipolar power transistor and the field effect power transistor (FET).

THE PRINCIPLES OF RECTIFICATION

The process of electrical rectification is where current from an a.c. supply is converted to a unidirectional form before being supplied to a load. Although unidirectional, the load current may pulsate in amplitude, depending on the load impedance. With resistive loads the load voltage polarity is fixed. The polarity of the voltage across series-connected load inductance elements may vary during the load current cycle.

In a rectifier circuit there are certain electrical properties that are of interest irrespective of circuit topology and impedance nature. These properties can be divided into two groups, (i) on the supply side, and (ii) on the load side of the rectifier, respectively. When the electrical supply system has a low (ideally zero) impedance, the sinusoidal supply voltages remain largely undistorted even when the rectifier action causes nonsinusoidal pulses of current to be drawn from the supply. For the purposes of circuit analysis one can assume that semiconductor rectifier elements, such as diodes and thyristor devices, are ideal in that they are dissipationless and have zero conducting voltage drop.

A study of rectifier circuits is basically a study of waveforms. No energy is stored within a rectifier so that there is a constant connection between the currents and voltages on the a.c. side and the current and voltage on the d.c. side. In rectifier calculations the essential requirement is to obtain an accurate physical picture of the operation and then establish circuit equations that are valid for the particular condition.

THREE-PHASE INDUCTION MOTOR WITH SINUSOIDAL SUPPLY VOLTAGES

A three-phase induction motor contains a three-phase distributed winding that is housed in slots on the stationary part of the motor, usually called the stator. The rotating part of the machine, or rotor, also contains either a distributed three-phase winding or a cage of interconnected copper bars that serve as rotor winding conductors. When the rotor contains a distributed winding the three phases of this winding are connected to three slip rings on the motor shaft and the motor is known as a wound-rotor machine or slipring machine. When a cage of copper bars is used these bars are electrically connected by end rings inside the rotor, no electrical connection can be made to them and the motor is known as a squirrel-cage motor or, more simply, a cage motor.

One set of three-phase windings is connected to a three-phase voltage supply and this set becomes the primary or excitation (field) windings. With a slip-ring motor either the stator or the rotor windings may act as primary windings, although invariably the stator is used. With a cage motor only the stator windings can be used as primary windings. The other set of motor windings, known as secondary windings, is not connected to the electrical supply but is closed on itself. There is no electrical connection between the primary windings and the secondary windings but these are linked magnetically, as in a transformer.

POWER FLOW CONTROL BY SWITCHES

The flow of electrical energy between a fixed voltage supply and a load is often controlled by interposing a controller, as shown in Fig. 1.1. Viewed from the supply, the apparent impedance of the load plus controller must be varied if variation of the energy flow is required. Conversely, seen from the load, the apparent properties of the supply plus controller must be adjusted. From either viewpoint, control of power flow can be realised by using a series-connected controller with the desired properties. If a current source supply is used instead of a voltage source supply, control can be realised by the parallel connection of an appropriate controller. For safety reasons the latter technique is rarely adopted.

The series-connected controller in Fig. 1.1 can take many different forms. In a.c. distribution systems where variability of power flow is a secondary requirement, transformers are often the prevalent interposing elements. The insertion of reactive elements is inconvenient because variable inductors and capacitors of appropriate size are expensive and bulky. It is easy to use a series-connected variable resistance instead, but at the expense of a considerable loss of energy. Viewing from the load side, loads that absorb significant electric power usually possess some form of energy ‘inertia’. This allows amplitude variations created by the interposed controller to be effected in an efficient manner.

Amplitude variations of the controller may be exchanged for a fractional time variation of connection and disconnection from the supply.

Amplification

The single most important function in electronics can be expressed in one word: amplification. This is the process whereby the power of a signal is increased in magnitude. A simple mechanical example of amplification is provided by the power steering system on cars and commercial vehicles, where a small force applied to the steering wheel by the driver is amplified hydraulically to produce the force required to move the front wheels of the vehicle. Here is the basic feature of an amplifier: a small input signal is used to control a more powerful output signal. The extra power is drawn from some external energy source, the latter being the vehicle engine in this instance.

The earliest example of electrical amplification is the electromagnetic relay, invented by Joseph Henry in 1835, and used by Samuel Morse to increase the power of weak telegraph signals. It was the relay which made possible the first long-distance telegraph line, from Baltimore to Washington, which was opened in 1844. As can be seen in fig. 1.1, the weak incoming signal is used to operate an electromagnet which attracts an armature and closes electrical contacts; these contacts then switch a powerful outgoing signal which is transmitted to the next leg of the line. The dots and dashes of the strong output signal are thus a faithful replica of the weak input. Relays are still used extensively in power switching systems, but are generally being superseded by electronic methods.

The digital world

At the heart of today's computer-based electronic systems lies the elementary transistor on/off switch. The simplicity and reliability of semiconductor switching have led circuit design into the digital world where the signals represent numbers, and circuit functions are expressed by logic and arithmetic. Even applications involving analogue inputs and outputs, such as audio recording, are rapidly becoming digital, by using analogue to digital and digital to analogue converters to convert signal voltages into numbers and back from numbers to voltages again, thereby achieving greater fidelity in the recording process.

We shall see as this chapter unfolds that the binary numbering system, working to a base of 2 instead of the base of 10 used in the familiar decimal system, is ideally suited to electronic implementation. The simple ‘ons’ and ‘offs’ of electronic switches represent the noughts and ones of binary numbers. Digital electronics is therefore free from many of the critical features of analogue circuits such as distortion and drift, the basic gate circuits being essentially simple. This simplicity has the advantage that several million such circuits can be packed onto one ‘chip’ in an integrated circuit microcomputer.

These final two chapters lead from the electronic fundamentals so far described into the world of microcomputers and the vital software which controls them. We begin by examining the way that logical operations can be carried out using ordinary switches and then see how simple transistor circuits can expand into arithmetical adders, memories, counters, timers and finally the computer itself.

Introduction

The subject of impedance matching is frequently surrounded by an undeserved aura of mystery. When a circuit fails to work as well as expected, impedance mismatch is often blamed in a rather vague sort of way, but correcting the problem may be regarded more as witchcraft than science. The purpose of this chapter is to outline the principles and practice of impedance matching and, in so doing, to attempt to dispel any associations of mystery and magic!

Input impedance

Any electrical device which requires a signal for its operation has an input impedance. Just like any other impedance (or resistance in d.c. circuits), the input impedance of a device is a measure of the current drawn by the input with a certain voltage across it.

For example, the input impedance of a 12 V light bulb rated at 0.5 A is 12/ 0.5Ω, or 24 Ω. The bulb is a clear example of impedance because we know that there is nothing but a filament to consider. The input impedance of a circuit such as bipolar transistor amplifier might seem to be more complicated. At first sight, the presence of capacitors, resistors and semiconductor junctions in a circuit makes the input impedance difficult to assess. However, any input circuit, however complicated, may be resolved into the simple impedance shown in fig. 5.1.

Anyone who is interested in technology is aware of the importance of electronics. Despite its all-pervading influence, however, electronics retains a strong element of mystery for many people who are otherwise well informed on technical matters. In this book, I aim to provide a practically-based explanation of the subject to try to dispel the mystery.

In the study of electronics, I have always found practical experience to be an invaluable stimulant and confidence-builder. There is enormous satisfaction in ‘lashing up’ a new circuit on the test bench and seeing it work for the first time. As an experimental physicist, I have never regarded electronic design as an end in itself, but rather as a valuable tool in research and development. The practical approach is therefore of primary importance in the electronics content of the UMIST Pure and Applied Physics degree course. Experience of teaching electronics to people of various backgrounds has encouraged me in the belief that a book based on a practical viewpoint can be valuable at all levels from school, through college or university, to the industrial laboratory. This book aims to cover a wide range of circuit building bricks, analogue and digital, discrete and integrated. It is from these bricks that an elaborate system is constructed, whether it be a colour television set or a computer. Sufficient practical information is given to enable the reader to construct the circuits and test them in the laboratory.

Introduction

The thermionic valve was the first active (amplifying) element in electronics. Although obsolete for most small-scale amplification, the valve still finds a place where high voltages must be handled or high-power high-frequency signals are involved. In addition, the inherent characteristics of valve audio amplifiers are popular with many audiophiles. In particular, the graceful behaviour of the circuits near overload can give a subjectively clean sound at high levels. It is therefore useful for the electronic engineer to have at least a rudimentary knowledge of the valve and its circuitry. This chapter gives a brief account of valve (vacuum tube) circuits, including a description of the one thermionic device which is still extensively used: the cathode-ray tube.

Thermionic emission

In the early 1880s, Thomas Edison, having developed the carbon-filament lamp, turned his attention to the blackening of the glass bulb which occurred after some hours of use. In an attempt to intercept some of the blackening particles, he sealed a metal plate inside one of his lamps and was surprised to find that, if the plate was made positive with respect to the filament, it drew a current. For twenty years, no one was to know that this ‘Edison effect’ current was due to electrons emitted by the hot filament being captured by the positively charged plate. The term thermionic emission was coined to describe this thermal liberation of free particles, literally thermal ions.

Introduction

Our introduction to the transistor in chapter 1 used the device as a switch to turn a light bulb on and off. This is the simplest possible example of amplification, where the signal has only two voltage levels coresponding to the ‘on’ and ‘off’ conditions. Such a signal is shown in fig. 10.1; here, the two levels are zero and +5 V, the transition between the levels being virtually instantaneous so that the waveform is rectangular. Signals from radiation detectors such as the Geiger–Müller tube are of this form, whilst the enormous field of digital electronics is concerned entirely with trains of rectangular pulses. It is now common practice to transmit and record ordinary analogue signals in digital form because the signal has then only two levels and the various distortions which may occur in transmission and recording are readily corrected. This chapter is concerned with the handling of pulses and the modifications and distortions which may occur to them in various circuits.

‘Squaring’ a waveform

Any repetitive waveform may be converted to a rectangular wave merely by driving a simple voltage amplifier so hard that it is driven alternately into cut-off and saturation. Fig. 10.2 shows a circuit that will do this. If the input is fed with a sine wave of at least 5 V r.m.s. amplitude, the output will be a rectangular wave suitable for use with the various pulse-shaping experiments in this chapter.

Introduction

The integrated circuit (IC) is clearly the building brick of electronic circuits. We have already had a flavour of IC applications when looking at power supply regulators. Now we turn towards the full range of IC capabilities. A glance through a component distributor's catalogue reveals a seemingly limitless range of ICs for virtually every conceivable function. An IC may contain from a dozen transistors to a million depending on the application, together with all necessary resistors, diodes etc. The intimate thermal connection achieved by fabricating all the components on one chip of silicon generally leads to excellent stability and predictability in use.

The understanding of discrete components gained from earlier chapters will be found essential to the proper interfacing of ICs: in fact even today very few analogue circuit applications dispense totally with discrete semiconductors. However, the IC designer has today relieved the circuit designer of much of the ‘donkey work’. In addition, the small size and low power consumption of ICs has made possible products like the Camcorder and hand-held GPS position-fixing satellite receiver.

This chapter deals with applications of linear ICs. They are designed to handle analogue signals, which carry their information in terms of amplitude and waveshape. Most audio and radio signals come into this category; they are distinct from the standard binary pulses of digital circuits which are discussed in chapter 13.

Power sources

The necessary d.c. supplies for electronic circuits may be drawn from batteries or obtained by rectification of the a.c. mains. Batteries have the advantage of portability and complete absence of a.c. components in their output. There is, however, a danger of leakage if exhausted batteries are accidentally allowed to stay too long in equipment; this may endanger many hundreds of pounds worth of circuitry through corrosion damage.

The e.m.f. of a battery is not usually constant throughout its life, that of the common low cost zinc chloride and alkaline batteries, falling from 1.6 V to 1.3 V over the useful life of the cell. Mercury oxide cells have a much better e.m.f. characteristic, remaining at 1.3 V over virtually the whole of their life and then falling off rapidly so that there is no doubt when the end of their life is reached; they are, however, expensive. Silver oxide cells have a similarly constant e.m.f. of 1.55 V.

Rechargeable nickel–cadmium (NiCd) cells are available in an enormous range of sizes, ranging from tiny ‘button’ cells to the large batteries used for electric traction. The smaller sizes are usually hermetically sealed so that there is no risk of leakage and no need for topping up. NiCd batteries and the newer nickel-metal-hydride (NiMH) type make an ideal power source for portable electronics, since the need for battery replacement is avoided; the charger may be incorporated into the instrument giving facilities for mains or battery operation.