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Fuel cells in the kW range have a high output current and low output voltage. To connect them to the mains, a DC-DC converter with a high voltage gain and an inverter are needed. In this paper an investigation of the voltage-fed full bridge dc-dc converter with high voltage gain is presented. A detailed analysis of the different operating modes, the semiconductors' current and voltage stress, the switching losses and the transformer stress is given. The influence of diverse parameters like winding ratio, leakage inductance and output inductance on the operating behaviour, hence voltage gain and efficiency is stated out. The analytical results are compared with measurements taken from a 1.2 kW prototype. Finally, design criteria for the dc-dc converter are given to realise a high voltage gain and high efficiency.