Stability analysis of push-pull and bridge-type zero-current-switched quasi-resonant converters

In the buck-derived, push-pull and bridge-type topologies, the output inductor current during the switch-off interval can freewheel either through the shorted secondaries of the transformer or through a separate freewheeling diode. It is shown that the former method results in highly nonlinear DC voltage conversion ratio characteristics for zero-current-switched quasi-resonant converters operating in half-wave mode. As a result, the incremental gain of the power stage of these converters varies in an extremely wide range with input voltage and load. Consequently, it is not possible to achieve stable feedback control with high-loop gains using conventional compensation techniques. The addition of a freewheeling diode is shown to linearize the DC characteristics, reduce current and voltage stresses on semiconductor devices, and improve efficiency.