Circulating current reduced three level DC-DC converter with two transformers for battery chargers

This paper introduces an improved isolated three-level (TL) DC-DC converter for battery charger. Conventional TL DC-DC converters usually have one transformer for isolation. Simultaneously, large circulating current freewheels at the primary of the transformer, which lowers the efficiency. A TL DC-DC converter with two transformers to reduce the circulating current is proposed in this paper for battery chargers. Like the conventional TL DC-DC converters, there is no addition switches at the primary of the transformers. The two transformers are connected in series. The flying capacitor of the converter is split into two, and the middle node of the flying capacitors are connected to the neutral point of the two transformers. The corresponding secondary windings of the transformers are also connected in series. Cooperating with four-diode rectifier stage, the primary circulating current can be reduced. The ZVS of the lagging switches is determined by the energy stored in a magnetizing inductor rather than leakage inductance. Because of the ZVS and reduced circulating current, the efficiency of the proposed converter is better than the conventional one. In the end, the performance of the proposed converter is verified by a 1kW prototype.