chap04 - 4 Switch Realization We have seen in p revious...

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Switch Realization We have seen in previous chapters that the switching elements of the buck, boost, and several other dc-dc converters can be implemented using a transistor and diode. One might wonder why this is so, and how to realize semiconductor switches in general. These are worthwhile questions to ask, and switch imple- mentation can depend on the power processing function being performed. The switches of inverters and cycloconverters require more complicated implementations than those of dc-dc converters. Also, the way in which a semiconductor switch is implemented can alter the behavior of a converter in ways not pre- dicted by the ideal-switch analysis of the previous chapters—an example is the discontinuous conduction mode treated in the next chapter. The realization of switches using transistors and diodes is the subject of this chapter. Semiconductor power devices behave as single-pole single-throw (SPST) switches, represented ideally in Fig. 4.1. So, although we often draw converter schematics using ideal single-pole double-throw (SPDT) switches as in Fig. 4.2(a), the schematic of Fig. 4.2(b) containing SPST switches is more realistic. The realization of a SPDT switch using two SPST switches is not as trivial as it might at first seem, because Fig. 4.2(a) and 4.2(b) are not exactly equivalent. It is possible for both SPST switches to be simultaneously in the on state or in the off state, leading to behavior not predicted by the SPDT switch of Fig. 4.2(a). In addition, it is possible for the switch state to depend on the applied voltage or current waveforms—a familiar example is the diode. Indeed, it is common for these phenomena to occur in converters operating at light load, or occasionally at heavy load, leading to the discontinuous conduction mode previously mentioned. The converter properties are then significantly modified. How an ideal switch can be realized using semiconductor devices depends on the polarity of the voltage that the devices must block in the off state, and on the polarity of the current that the devices 4
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64 Switch Realization must conduct in the on state. For example, in the dc–dc buck converter of Fig. 4.2(b), switch A must block positive voltage when in the off state, and must conduct positive current when in the on state. If, for all intended converter operating points, the current and blocking voltage lie in a single quadrant of the plane as illustrated in Fig. 4.3, then the switch can be implemented in a simple manner using a tran- sistor or a diode. Use of single-quadrant switches is common in dc–dc converters. Their operation is dis- cussed briefly here. In inverter circuits, two-quadrant switches are required. The output current is ac, and hence is some- times positive and sometimes negative. If this current flows through the switch, then its current is ac, and the semiconductor switch realization is more complicated.
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This note was uploaded on 01/17/2010 for the course EL 5673 taught by Professor Dariuszczarkowski during the Spring '09 term at NYU Poly.

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chap04 - 4 Switch Realization We have seen in p revious...

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