DC characteristics and stability of push-pull and bridge-type zero-current-switched quasi-resonant converter

In the buck-derived, push-pull, and bridge-type topologies the output inductor current during the switched-off interval can free-wheel either through the shorted secondaries of the transformer or through a separate free-wheeling diode (FWD). 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 the half-wave mode. As a result the incremental gain of the power stage of these converters varies with input voltage and load over an extremely wide range, so that it is not possible to achieve stable feedback control with high loop gains using conventional compensation techniques. It is shown that the addition of a FWD not only linearizes the DC characteristics but also reduces current and voltage stresses on semiconductor devices and improves efficiency.