Zero-voltage-switching push-pull forward three-level converter for low-voltage fuel cell applications

An efficient DC-DC converter is needed as the interface between a low-voltage fuel cell source and a constant DC bus for inverter operation. Zero-voltage-switching (ZVS) push-pull forward (PPF) converter is very fit for low input-voltage application. However, the output-voltage variation range of the fuel cell is wide, and the PPF converter cannot realize ZVS under wide input-voltage range. In order to solve this problem, this paper proposes a ZVS PPF three-level (TL) converter, which integrates the advantages of the TL and PPF converters. The proposed converter can operate in three-level and two-level modes. All the switches can realize ZVS under all input-voltage range. Four switches sustain only half of the input voltage, which can be clamped by the input divided capacitors; the others sustain one and a half of the input voltage, which can be clamped by the flying capacitor and clamping capacitor. As the input voltage is low, the freewheeling diodes connected with three-level legs are substituted for the MOSFETs to reduce the conducting loss, and thus efficiency can be increased. Compared with the conventional two-level converter, voltage stress of the rectifier diodes is reduced. Operating principle and control strategy are illustrated. Realization of ZVS and duty-cycle loss are analyzed. Finally, the simulation results verify the theoretical analysis.