INTRODUCTION

The final decision regarding the choice of switching devices and ancillary components must depend on the overall design for total system realisation. Any choice between device options is likely to have an interactive effect on other components. For example, the selection of a certain switching device for use in a particular circuit may or may not require switching aids and/or sophisticated protection. A relatively inexpensive electronic switching device may require ancillary circuitry that is prohibitive in cost, volume requirement, heat generation, excessive control circuit requirements, etc.

A consideration of some of the circuit features listed in the previous paragraph may be grouped under the general heading of ‘circuit protection.’ This, in turn, may be divided between the major classifications of ‘preventive protection’ and ‘abuse protection.’

Preventive protection is required so that when a device is operational it is embedded in an environment which prevents certain secondary device ratings from being exceeded. A typical example of this is the use of snubber circuits, which is adopted to prevent spurious triggering due to transients.

Abuse protection is required to cope with fault conditions which cause the normal primary ratings of the device to be exceeded. Typical examples are overvoltage or current surges arising from short circuiting of the load. In terms of Fig. 2.2 above, the inside or circuit user shell then dilates (i.e. enlarges) to transgress the outer or absolute maximum limit shell of the particular device design.

RATING, SAFE OPERATION AREA AND POWER HANDLING CAPABILITY OF DEVICES

If the modern power control engineer is to make the correct choice of switching device for a given application it is necessary to be aware of the characteristics and limitations of the devices that are currently available. The obvious limitations to any device are its maximum voltage and current ratings, which must not be exceeded. It is hence essential to have some knowledge of device construction and fabrication.

Power handling capability (PH)

The maximum power handling capability PHmax of a semiconductor switch is related to the product of Vbus and Imax. For any semiconductor device the maximum voltage to which the device can be subjected is related to the avalanche breakdown value of the silicon p–n junction, while the maximum device current is limited to the chip current density.The maximum current density is affected by various factors, including temperature and mechanical stress.

Both the maximum voltage and maximum allowable current are affected by the impurity levels present in the chip. Moreover, they are affected inversely in such a way that any increase of one parameter will result in reduction of the other. When endeavours are made to increase a device power handling capability, limiting factors arise as with most engineering problems. A compromise has to be made as invariably the load current flows through the same junctions that have to withstand the supply voltages without unintentional avalanching.

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.