In this paper, a simple and efficient Sigma DC-DC converter is proposed and its optimal design has been studied. The integrated transformer includes the forward windings for the efficient transferring of the majority power in the way of ZCS resonance, and the flyback windings for voltage regulation. Based on the proper design of the flyback leakage inductance, the voltage of the main switch can reach zero through the resonance between the leakage inductor of the flyback windings and the paralleled capacitor of the main switch before its turn on. Therefore, the energy stored in the leakage inductor of the flyback windings can be recycled to further improve system efficiency. A 140W prototype is designed to verify the theoretical analysis, and the experimental results show that ZVS of the main switch is realized and the efficiency of the prototype can reach 96% with the optimal design.
