Chapter 5: Power Electronics: Devices and Circuits

Power electronics is an enabling technology for all electrical and electronic apparatus requiring electric power to drive. Over the past twenty years, the power electronics industry has grown tremendously. Its growth is a result from increasing demand of reliable, efficient, compact and cost effective power supplies for telecommunication, computer, and motor drive industries as well as for medical equipments and military use.
Nội dung trích xuất từ tài liệu:
Chapter 5: Power Electronics: Devices and CircuitsChapter 5Power Electronics: Devices andCircuits5.1 IntroductionPower electronics is an enabling technology for all electrical and electronic apparatusrequiring electric power to drive. Over the past twenty years, the power electronicsindustry has grown tremendously. Its growth is a result from increasing demand of re-liable, efficient, compact and cost effective power supplies for telecommunication, com-puter, and motor drive industries as well as for medical equipments and military use.This growth is facilitated by the significant improvement in semiconductor technologyin which smaller packaging and higher power handling devices have been marketed. Inresponse to the advancement in semiconductor and magnetics technology, power elec-tronics researchers and engineers have strived to thoroughly employ these technologiesthrough new circuit design and topologies, optimized control and packaging techniques,in order to meet the industry demands. Power electronics is all about using electronic devices and circuits with storage el-ement to control the level of voltage and current, either in the form of AC or DC. Powerelectronics circuits are switching converters with periodic switching actions to processthe electrical energy to meet the design specification. Apart from semiconductors, in-ductor and transformer are the critical magnetic components in the power switchingconverter. Their functions such as storage element, power splitting, and safety isolation 835. Power Electronics: Devices and Circuits 84will be explained in detail in Chapter 6. In order to control power, some form of controltechniques are needed and will be discussed in Chapter 7. This chapter we will begin with the semiconductor devices used for power con-verters. We then analyze the basic DC/DC converters at the steady state. That is,the output voltage and current are at stable condition. Finally, the operation of gatedriver, which drives the transistor, is presented.5.2 Electrical Energy Conversion by SwitchingThe characteristics of power conversion by power electronics converters are summarizedas follows: 1. Electrical energy can be generated, transmitted and converted to a form that is suitable for the load we are interested in. 2. Power Electronics concerns conversion and processing of electrical energy by power semiconductor devices and storage elements. 3. Power Electronics technologies enable great efficiency enhancement, tremendous size and weight reduction of electrical equipment. 4. Power Electronics technologies are based on switching on and off the power source by power semiconductors. The electrical energy conversion process can be pre- cisely controlled in a manner far much better than electromechanical devices. 5. Power Electronics applications include power supplies for computers, communi- cation equipment, machine drives, lighting, automobile and many applications. 6. Electrical energy conversion can be classified into the following four categories : AC to AC, AC to DC, DC to DC, and DC to AC.5. Power Electronics: Devices and Circuits 85 Figure 5.1: Four categories of electrical energy conversion.Figure 5.2: Diode: (a) Symbol, (b) I-V characteristic, (c) idealized characteristics.Sources: Mohan 1995 [2].5. Power Electronics: Devices and Circuits 86 Figure 5.3: Diode switching characteristics. Sources: Mohan 1995 [2].5. Power Electronics: Devices and Circuits 875.3 Power Semiconductor Devices as Switches5.3.1 DiodesA diode performs as a switch. It is driven by the voltage applied across its two terminals:anode and cathode. Fig 5.2(a) shows the symbol of a diode. A is the anode, the positiveterminal. K is the cathode, the negative terminal. When a diode is forward biased, vdis positive (i.e. potential at A is higher than K ), the arrow shows the direction of thediode current iD . When the diode conducts, a small forward voltage drop denoted asVF is established and the magnitude is usually around 1V. When the diode is reversebiased, it is blocked and the diode current becomes slightly negative. This is due to thecontribution of reverse saturation current. For example, 1N4004 has a reverse currentof 50µA. And this reverse current is of temperature-dependent; when temperature ishigher the reverse current is increased and vice versa. The reverse voltage applied ona diode has a limit. Beyond the limit the diode will breakdown and becomes a shortcircuit. This limit is usually called the peak inverse (or reverse) voltage. Another interesting fact of semicondu ...