Soft switching

Quasi-resonant and resonant transition switching power supplies have a much more attractive radiated spectral shape. This is because the transitions are forced to be at a lower frequency by the resonant elements, hence only the low frequency spectral components are exhibited (below 30MHz). The lower rate of change during the transitions are responsible for behavior. The higher frequency spectral components are almost non existent. The near-held radiated spectrum of a quasi-resonant, hyback converter are shown in Figure E-2. The quasi-resonant and soft switching families of converters are much quieter and easier to hlter. 

The EMI characteristics will change appreciably only if the switcher IC used has soft-switching characteristics... 

In applications where high power density or thermal management is of prime importance, hard- switched converters are not feasible using conventional Si components. In these cases, resonant or quasi-resonant (also termed soft-switching ) topologies can be used. The electrical resonance is obtained through parasitic... 

Khersonsky, Y., et ah, Evolution of Soft-Switched, High-Frequency, High-Power Isolated DC-DC Converter, Conf. Record of the 2001 IEEE Industry Application Conf., Vol. 3, September 30-October 4, 2001, pp. 1851-1857. 

The soft-switch technology uses a so-called quantum tunneling composite (QTC). This composite has a remarkable characteristic that enables it to be an isolator in its normal condition and to change into a metal-like conductor when pressure is being exercised on it77. 

Soft-switched converters can be classified as (a) resonant converters, and (b) resonant (zero-current or zero-voltage) transition converters. Similar to the PWM converters, there are two types in each case, namely, single-ended and double-ended. 

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