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Electrical power system in future uninterruptible power supply (UPS) or electrical vehicle (EV) may employ hybrid energy sources, such as fuel cells and super-capacitors. It will be necessary to efficiently draw the energy from these two sources as well as recharge the energy storage elements by the DC bus. In this paper, a bidirectional isolated DC-DC converter controlled by phase-shift and duty cycle for the fuel cell hybrid energy system is analyzed and designed. The proposed topology minimizes the number of switches and their associated gate driver components by using two high frequency transformers which combine a half-bridge circuit and a full-bridge circuit together on the primary side. The voltage doubler circuit is employed on the secondary side. The current-fed input can limit the input current ripple that is favorable for fuel cells. The parasitic capacitance of the switches is used for zero voltage switching (ZVS). Moreover, a phase-shift and duty cycle modulation method is utilized to control the bidirectional power flow flexibly and it also makes the converter operate under a quasi-optimal condition over a wide input voltage range. This paper describes the operation principle of the proposed converter, the ZVS conditions and the quasi-optimal design in depth. The design guidelines and considerations about the transformers and other key components are given. Finally, a 1-kW 30~50-V-input 400-V-output laboratory prototype operating at 100 kHz switching frequency is built and tested to verify the effectiveness of the presented converter.He has authored or coauthored more than 100 papers. His research areas include switch mode power supplies, piezoelectric transformers, power factor correction, and switch mode audio power amplifiers.