A.Q. Huang, Recent Developments of Power Semiconductor Devices, VPEC Seminar Proceedings, Pp. 1-9, Sep- N. Mohan and T. Undeland, Power Electronics: Converters, Applications, and Design, New York: John Wiley J. Wojslawowicz, Ruggedized transistors emerging as power MOSFET standard-bearers, Power Technics Mag- azine, pp 29-32, January 1988. Further Information B M. Bird and K G. King, An Introduction to Power Electronics, New York: Wiley-Interscience, 1984 R Sittig and P. Roggwiller, Semiconductor Devices for Power Conditioning, New York: Plenum, 1982. V.A.K. Temple, " Advances in MOS controlled thyristor technology and capability, Power Conversion, pp 544-554,Oct.1989 B W. Williams, Power Electronics, Devices, Drivers and Applications, New York: John Wiley, 1987. 30.2 Power Conversion Kaushik rajashekara Power conversion deals with the process of converting electric power from one form to another. The power ctronic apparatuses performing the power conversion are called power converters Because they contain no moving parts, they are often referred to as static power converters. The power conversion is achieved using ower semiconductor devices, which are used as switches. The power devices used are SCRs(silicon controlled rectifiers,or thyristors), triacs, power transistors, power MOSFETs, insulated gate bipolar transistors(IGBTs) and MCTs(MOS-controlled thyristors). The power converters are generally classified as 1. ac-dc converters(phase-controlled converters) 2. direct ac-ac converters(cycloconverters) 3. dc-ac converters(inverters) 4. dc-dc converters(choppers, buck and boost converters) AC-DC Converters The basic function of a phase-controlled converter is to convert an alternating voltage of variable amplitude and frequency to a variable dc voltage. The power devices used for this application are generally SCRs. The average value of the output voltage is controlled by varying the conduction time of the SCRs. The turn-on of the SCR is achieved by providing a gate pulse when it is forward-biased. The turn-off is achieved by the commutation of current from one device to another at the instant the incoming ac voltage has a higher instantaneous potential than that of the outgoing wave. Thus there is a natural tendency for current to be commutated from the outgoing to the incoming SCR, without the aid of any external commutation circuitry. This commutation process is often referred to as natural commutation. A single-phase half-wave converter is shown in Fig. 30.9. When the SCR is turned on at an angle a, full supply voltage(neglecting the SCR drop) is applied to the load. For a purely resistive load, during the positive half cycle, the output voltage waveform follows the input ac voltage waveform. During the negative half cycle the SCR is turned off. In the case of inductive load, the energy stored in the inductance causes the current to flow in the load circuit even after the reversal of the supply voltage, as shown in Fig. 30.9(b). If there is no off the SCR as soon as the input voltage polarity reverses, as shown in Fig. 30.9(c). When the SCR is of o freewheeling diode D, the load current is discontinuous. A freewheeling diode is connected across the load t the load current will freewheel through the diode. The power flows from the input to the load only when the SCR is conducting. If there is no freewheeling diode, during the negative portion of the supply voltage, SCR returns the energy stored in the load inductance to the supply. The freewheeling diode improves the input c 2000 by CRC Press LLC© 2000 by CRC Press LLC A.Q. Huang, Recent Developments of Power Semiconductor Devices, VPEC Seminar Proceedings, pp. 1–9, Sep￾tember 1995. N. Mohan and T. Undeland, Power Electronics: Converters, Applications, and Design, New York: John Wiley & Sons, 1995. J. Wojslawowicz, “Ruggedized transistors emerging as power MOSFET standard-bearers,” Power Technics Mag￾azine, pp. 29–32, January 1988. Further Information B.M. Bird and K.G. King, An Introduction to Power Electronics, New York: Wiley-Interscience, 1984. R. Sittig and P. Roggwiller, Semiconductor Devices for Power Conditioning, New York: Plenum, 1982. V.A.K. Temple, “Advances in MOS controlled thyristor technology and capability,” Power Conversion, pp. 544–554, Oct. 1989. B.W. Williams, Power Electronics, Devices, Drivers and Applications, New York: John Wiley, 1987. 30.2 Power Conversion Kaushik Rajashekara Power conversion deals with the process of converting electric power from one form to another. The power electronic apparatuses performing the power conversion are called power converters. Because they contain no moving parts, they are often referred to as static power converters. The power conversion is achieved using power semiconductor devices, which are used as switches. The power devices used are SCRs (silicon controlled rectifiers, or thyristors), triacs, power transistors, power MOSFETs, insulated gate bipolar transistors (IGBTs), and MCTs (MOS-controlled thyristors). The power converters are generally classified as: 1. ac-dc converters (phase-controlled converters) 2. direct ac-ac converters (cycloconverters) 3. dc-ac converters (inverters) 4. dc-dc converters (choppers, buck and boost converters) AC-DC Converters The basic function of a phase-controlled converter is to convert an alternating voltage of variable amplitude and frequency to a variable dc voltage. The power devices used for this application are generally SCRs. The average value of the output voltage is controlled by varying the conduction time of the SCRs. The turn-on of the SCR is achieved by providing a gate pulse when it is forward-biased. The turn-off is achieved by the commutation of current from one device to another at the instant the incoming ac voltage has a higher instantaneous potential than that of the outgoing wave. Thus there is a natural tendency for current to be commutated from the outgoing to the incoming SCR, without the aid of any external commutation circuitry. This commutation process is often referred to as natural commutation. A single-phase half-wave converter is shown in Fig. 30.9. When the SCR is turned on at an angle a, full supply voltage (neglecting the SCR drop) is applied to the load. For a purely resistive load, during the positive half cycle, the output voltage waveform follows the input ac voltage waveform. During the negative half cycle, the SCR is turned off. In the case of inductive load, the energy stored in the inductance causes the current to flow in the load circuit even after the reversal of the supply voltage, as shown in Fig. 30.9(b). If there is no freewheeling diode DF , the load current is discontinuous. A freewheeling diode is connected across the load to turn off the SCR as soon as the input voltage polarity reverses, as shown in Fig. 30.9(c). When the SCR is off, the load current will freewheel through the diode. The power flows from the input to the load only when the SCR is conducting. If there is no freewheeling diode, during the negative portion of the supply voltage, SCR returns the energy stored in the load inductance to the supply. The freewheeling diode improves the input power factor