chap20 - 20 Soft Switching In addition to the resonant c i...

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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 with 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 properties of the resonant switching network and the parent hard-switched PWM converter. Soft-switching converters can exhibit reduced switching loss‚ at the expense of increased con- duction loss. Obtaining zero-voltage or zero-current switching requires that the resonant elements have large ripple; often‚ these elements are operated in a manner similar to the discontinuous conduction
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762 Soft Switching modes of the series or parallel resonant converters. As in other resonant schemes‚ the objectives of designing such a converter are: (1) to obtain smaller transformer and low-pass filter elements via increase of the switching frequency‚ and/or (2) to reduce the switching loss induced by component nonidealities such as diode stored charge‚ semiconductor device capacitances‚ and transformer leakage inductance and winding capacitance. The resonant switch and soft-switching ideas are quite general‚ and can be applied to a variety of topologies and applications. A large number of resonant switch networks have been documented in the
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chap20 - 20 Soft Switching In addition to the resonant c i...

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