Power Electronics

1. Chapter 22 Power Electronics • Introduction • Bipolar Transistor Power Amplifiers • Classes of Amplifier • Four-layer Devices • Power Supplies and Voltage Regulators

2. 22.1 Introduction • 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

3. 22.2 Bipolar Transistor Power Amplifiers • When designing a power amplifier we normallyrequire 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

4. 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)

5. 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

6. 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

7. Driving a push-pull stage

8. Distortion in push-pull amplifiers

9. Improved push-pull output stage arrangements

10. 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.

11. 22.3 Classes of Amplifier • Class A • active device conducts for complete cycle of input signal • example shown here • poor efficiency(normally lessthan 25%) • low distortion

12. Class B • active devices conductsfor half of the completecycle of input signal • example shown here • good efficiency(up to 78%) • considerable distortion

13. Class AB • active devices conductsfor more than half butless than the completecycle of input signal • example shown here(with appropriate Rbias) • efficiency depends on bias • distortion depends on bias

14. Class C • active devices conductsfor less than half thecomplete cycle ofinput signal • example shown here • high efficiency(approaching 100%) • gross distortion

15. 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

16. 22.4 Four-layer Devices • 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

17. The thyristor • a four-layerdevice with apnpn structure • three terminals:anode, cathodeand gate • gate is thecontrol input

18. Thyristor operation • constructionresembles twointerconnectedbipolar transistors • turning on T2 holds on T1 • device thenconducts untilthe current goesto zero

19. Use of a thyristor inAC power control • once triggered the deviceconducts for the remainderof the half cycle • varying firing timedetermines output power • allows control from 0-50%of full power

20. Full-wave powercontrol using thyristors • full-wave controlrequired two devices • allows control from0-100% of full power • requires two gatedrive circuits • opto-isolation oftenused to insulatecircuits from AC supply

21. The triac • resembles a bidirectionalthyristor • allows full-wave controlusing a single device • often used with abidirectional triggerdiode (a diac) to producethe necessary drive pulses • this breaks down at aparticular voltage and fires the triac

22. A simple lamp-dimmer using a triac

23. 22.5 Power Supplies and Voltage Regulators • Unregulated DC power supplies

24. Regulated DC power supplies

25. Voltage regulators

26. Switch-modepower supplies • uses a switchingregulator • output voltage iscontrolled by theduty-cycle of theswitch • uses an averagingcircuit to ‘smooth’output

27. An LC averaging circuit

28. Using feedback in a switching regulator

29. 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