Because of the electrochemical reaction, fuel cell has the power quality of low voltage and high current. However, a fuel-cell stack with high output voltage is difficult to fabricate and it may fail when any single cell is inactive. In addition, the output voltage is varied easily with respect to the load variation. To satisfy the requirement for a high voltage, a high-efficiency DC-DC convertor with high voltage gain is one of the essential mechanisms in fuel-cell applications. A newly designed high-step-up DC-DC convertor utilised in the proton-exchange-membrane fuel cell (PEMFC) system is constructed on the basis of voltage-clamped and soft-switching techniques for alleviating the switching and conduction losses, to increase further the conversion efficiency. Moreover, the closed-loop-control methodology is utilised in the proposed scheme to overcome the voltage-drift problem of the fuel cell under load variation. Some experimental results via a PEMFC power source of 250 W nominal rating are given to demonstrate the effectiveness of the proposed power-conversion strategy.