Power Electronics Chapter 22 Introduction Bipolar Transistor Power

Power Electronics Chapter 22 § Introduction § Bipolar Transistor Power Amplifiers § Classes of Amplifier § Four-layer Devices § Power Supplies and Voltage Regulators Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 1

Introduction 22. 1 § Amplifiers that produce voltage amplification or current amplification also produce power amplification § However, the term power amplifier is normally reserved for circuits whose main function is to deliver large amounts of power § These can be produced using FETs or bipolar transistors, or using special purpose devices such as thyristors and triacs Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 2

Bipolar Transistor Power Amplifiers 22. 2 § When designing a power amplifier we normally require a low output resistance so that the circuit can deliver a high output current – we often use an emitter-follower – this does not produce voltage gain but has a low output resistance – in many cases the load applied to a power amplifier is not simply resistive but also has an inductive or capacitive element Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 3

§ Current sources and loads – when driving a reactive load we need to supply current at some times (the output acts as a current source) – at other times we need to absorb current (the output acts as a current sink) Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 4

– the circuit above is a good current source but a poor current sink (stored charge must be removed by RE) – an alternative circuit using pnp transistors (below) is a good current sink but a poor current source Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 5

§ Push-pull amplifiers – combining these circuits can produce an arrangement that is both a good current source and a good current sink – this is termed a push-pull amplifier Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 6

§ Driving a push-pull stage Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 7

§ Distortion in push-pull amplifiers Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 8

§ Improved push-pull output stage arrangements Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 9

§ Amplifier efficiency – an important consideration in the design of power amplifiers is efficiency – efficiency determines the power dissipated in the amplifier itself – power dissipation is important because it determines the amount of waste heat produced § excess heat may require heat sinks, cooling fans, etc. Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 10

Classes of Amplifier 22. 3 § Class A – active device conducts for complete cycle of input signal – example shown here – poor efficiency (normally less than 25%) – low distortion Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 11

§ Class B – active devices conducts for half of the complete cycle of input signal – example shown here – good efficiency (up to 78%) – considerable distortion Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 12

§ Class AB – active devices conducts for more than half but less than the complete cycle of input signal – example shown here (with appropriate Rbias) – efficiency depends on bias – distortion depends on bias Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 13

§ Class C – active devices conducts for less than half the complete cycle of input signal – example shown here – high efficiency (approaching 100%) – gross distortion Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 14

§ Class D – in class D amplifiers the active devices are switches and are either ON or OFF – an ideal switch would dissipate no power § since either the current or the voltage is zero – even real devices make good switches – amplifiers of this type are called switching amplifiers or switch-mode amplifiers – efficiency is very high Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 15

Four-layer Devices 22. 4 § Although transistors make excellent switches, they have limitations when it comes to switching high currents at high voltages § In such situations we often use devices that are specifically designed for such applications § These are four-layer devices – these are not transistors, but have a great deal in common with bipolar transistors Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 16

§ The thyristor – a four-layer device with a pnpn structure – three terminals: anode, cathode and gate – gate is the control input Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 17

§ Thyristor operation – construction resembles two interconnected bipolar transistors – turning on T 2 holds on T 1 – device then conducts until the current goes to zero Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 18

§ Use of a thyristor in AC power control – once triggered the device conducts for the remainder of the half cycle – varying firing time determines output power – allows control from 0 -50% of full power Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 19

§ Full-wave power control using thyristors – full-wave control required two devices – allows control from 0 -100% of full power – requires two gate drive circuits – opto-isolation often used to insulate circuits from AC supply Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 20

§ The triac – resembles a bidirectional thyristor – allows full-wave control using a single device – often used with a bidirectional trigger diode (a diac) to produce the necessary drive pulses – this breaks down at a particular voltage and fires the triac Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 21

§ A simple lamp-dimmer using a triac Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 22

Power Supplies and Voltage Regulators 22. 5 § Unregulated DC power supplies Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 23

§ Regulated DC power supplies Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 24

§ Voltage regulators Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 25

§ Switch-mode power supplies – uses a switching regulator – output voltage is controlled by the duty-cycle of the switch – uses an averaging circuit to ‘smooth’ output Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 26

§ An LC averaging circuit Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 27

§ Using feedback in a switching regulator Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 28

Key Points § Power amplifiers are designed to deliver large amounts of power to their load § Bipolar circuits often use an emitter follower circuit § Many power amplifiers use a push-pull arrangement § The efficiency of an amplifier is greatly affected by its class § While transistors make excellent switches, in high power applications we often use special-purpose devices such as thyristors or triacs § A transformer, a rectifier and a capacitor can be used to form a simple unregulated supply § A more constant output voltage can be produced by adding a regulator. This can use linear or switching techniques Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 22. 29