Problems 61

(b) Derive analyliLiil espressiuns for Ihe converter output voUjge and for the efficiency.

(c) For V = 0, plot V/V vs. О over Ihe range 0 < D < l,for (/) RJfi = 0.01, and {ii] RJR = 0.0,5,

(d) For Vp = 0, plot Ihe converter efficiency over the range 0<D< I, for (() R,JR =0.01, and (ii) RJR = 0,05,

This page intentionally left blank

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 implementation can depend on the power processing function being perfttrmed. 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 predicted by the ideal-switch analysis of the prevtt>iis chapters-an example is the discontinuous ct>ndiiction 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 t

(SPST) switches, represented ideally in Fig. 4.1. So, although we often draw converter schematics using ideal single-pole double-throw ISPDT) switches as in Fig. 4.2(a), the schematic of Fig. 4.2(b) containing SPST switches is more realistic. The realization of a SPOT 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 t>r in the off state, leading to behavior not predicted by the SPOT switch of Fig. 4.2(a). In addition, it is possible for the switch state to depend on the applied voltage or current wavefonns-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

Fig. 4.1 .SRST .switch, with defined voltage and curreiu polarities.