A unified analysis of converters with resonant switches

Quasi-resonant converters are a recently introduced family of single-switch resonant converters featuring zero-current or zero-voltage switching. Recognition of the topological structure uniting these converters — and the PWM converters on which they are based — leads to general models of their dc and low-frequency ac behavior. An expression is derived that yields the dc conversion ratio of a quasi-resonant converter in terms of the well-known conversion ratio of the underlying PWM topology. A small-signal, low-frequency dynamic model is developed whose parameters also incorporate the PWM conversion ratio. The dc and ac models reveal that any quasi-resonant converter with a full-wave resonant switch has dc and low-frequency behavior identical to that of its PWM parent, with switching frequency control replacing duty ratio control. Converters with half-wave resonant switches act more like PWM converters under current programming or discontinuous conduction mode, exhibiting lossless damping in the small-signal model and output resistance at dc.