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This paper proposes a new asymmetrical duty cycle control method for a three-port bidirectional DC-DC converter with two current-fed ports interfacing with low voltage battery and ultracapacitor in a fuel cell vehicle. Along with the phase shift control managing the power flow between the ports, asymmetric duty cycle is applied to each port to maintain a constant DC bus voltage at low voltage side, which as a result will achieve wide zero-voltage-switching (ZVS) range for each port under varied ultracapacitor and battery voltages. The ZVS range analysis of different duty cycle control methods as well as the circulation power loss between the ports have been analyzed. In addition, the power flow design featuring the reduced coupling factors between the ports have been developed for the three-port bidirectional DC-DC converter. A fuel cell vehicle power train including a 2.5 kW three-port DC-DC converter interfacing a 12 V battery and ultracapacitor was built in the laboratory. The proposed asymmetrical duty cycle control and power flow design was implemented and verified on the hardware test bed under urban driving cycle. The experimental results validated that proposed asymmetrical duty cycle method has higher efficiency than other methods; furthermore, they also validated the reduced coupling factor between phase shift control and duty cycle control.