Clamping for current-fed de/dc converters with recovery of clamping energy in fuel cell inverter systems

Current-fed dc/dc converters like current-fed full bridge converters or boost converters with autotransformer, which are appropriate solutions in medium power fuel cell systems, need clamping circuits to protect the main semiconductors against overvoltages. Depending on the stray inductances of the inductive components the power drawn by the clamping circuit can be up to 5-10% of the converter's transferred power. This paper describes methods to recover the energy of the clamping circuit (sometimes called "active clamping") and to feed it directly into the converter's secondary side. These methods are shown for the current fed full bridge converter and the boost converter with autotransformer. The operating behaviour of the clamping circuits is shown and analysed. Also the semiconductor losses of the switches in the clamping circuit are derived for both converters. The general operating behaviour and the analysis is verified by simulation results and by practical results of two laboratory setups with a rated power of 20 kW. Practical issues of the clamping circuit like the non-ideal reverse recovery behaviour of the MOSFET's body-diode are also covered in this paper.