DC Characteristics and Stability 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 switched-off interval can freewheel either through the shorted secondaries of the transformer or through a separate freewheeling 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 half-wave mode. As a result the incremental gain of the power stage of these converters vary in an extremely wide range with input voltage and load. Consequently, it is not possible to achieve a stable feedback control with high loop gains using conventional compensation techniques. The addition of a FWD not only linearizes the dc characteristics but also results in a reduction of current and voltage stresses on semiconductor devices and an improved efficiency.