A digitally controlled DCM flyback converter with a low-volume dual-mode soft switching circuit

This paper introduces a dual-mode hybrid zero-voltage/zero-current switching (ZVS/ZCS) circuit and a complementary controller for flyback converters operating in discontinuous conduction mode (DCM). The new ZVS/ZCS solution provides soft switching feature and high efficiency throughout the whole operating range. It also requires smaller components than other soft switching solutions [1]-[6]. To achieve these benefits, the dual-mode ZVS/ZCS flyback combines a low power passive snubber and a novel secondary side circuit. The secondary side circuit provides ZCS on the primary side and ZVS on the secondary. A digital controller governs the operation of this converter. At light loads the ZVS/ZCS is disabled to minimize conduction losses. At medium and heavy loads ZVS/ZCS provides soft switching and at the same time minimizes power losses of the snubber, by capturing energy that would have been lost otherwise. Experimental results obtained with a 50W, 200KHz, universal input (85V-264V) programmable output PFC prototype demonstrate efficiency improvement of up to 15% compared to a conventional flyback PFC rectifier and about five times smaller snubber circuit.