A novel resonant transition push-pull DC - DC converter

Abstract

In hard-switched converters, the switching frequency is limited by switching losses and EMI problems. However, high-switching frequency is necessary to reduce the converter size. Hence, soft switching is imperative at high switching frequency to obtain good efficiency. Push-pull converter is a preferred topology at medium power level. This paper proposes a novel resonant transition topology for push-pull converter. The proposed topology uses two additional switches and two diodes when compared to hard-switched push-pull converter. These extra switches introduce freewheeling in the primary circuit and thus enable loss-less switching. In classical push-pull converters, the transformer primary is left open during two sub-intervals in a period making the turn-on of the switch hard. The new circuit topology converts these open circuit intervals into freewheeling ones. With such a modification, all trapped energy in the core is conserved to achieve zero-voltage switching during the entire transition. Switch stress, control and small signal model are similar to hard-switched PWM converter. Idealized analysis and design methodology are explained for the push-pull converter. A prototype-300 kHz, 200 W push-pull converter validates the design method. Dynamic analysis of the push-pull converter is presented. The proposed topology can be extended to half-bridge converter also. Its circuit diagram is presented.