电气转换switching.doc

20 Soft Switching In addition to the resonant circuits introduced in Chapter 19? there has been much interest in reducing the switching loss of the PWM converters of the previous chapters. Several of the more popular approaches to obtaining soft switching in buck? boost? and other converters? are discussed in this chapter. Mechanisms that cause switching loss are discussed in Chapter 4? including diode reverse recovery? semiconductor output capacitances? and IGBT current tailing. Soft switching involves mitiga- tion of one or more of these switching loss mechanisms in a PWM converter. The energy that would oth- erwise be lost is recovered? and is transferred to the converter source or load. The operation of a semiconductor device? during a given turn-on or turn-off switching transition? can be classified as hard- switched? zero-current switched? or zero-voltage switched. Operation of diodes and transistors with soft switching is examined in Section 20.1. In particular? it is preferable to operate diodes w i th zero-voltage switching at their turn-off transitions? and to operate MOSFETs with zero-voltage switching during their turn-on transitions. However? zero-voltage switching comes at the expense of increased conduction loss? and so the engineer must consider the effect of soft switching on the overall converter efficiency. Resonant switch converters are a broad class of converters in which the PWM switch network of a conventional buck? boost? or other converter is replaced with a switch cell containing resonant ele- ments. These resonant elements are positioned such that the semiconductor devices operate with zero- current or zero-voltage switching? and such that one or more of the switching loss mechanisms is reduced or eliminated. Other soft-switching approaches may employ resonant switching transitions? but other- wise exhibit the approximately rectangular waveforms of hard-switched converters. In any case? the resulting hybrid converter combines the properti