A current-fed DC/DC converter for the efficient charging of HV capacitors in mobile applications

This paper suggests a current-fed double inductor push-pull converter for the use as a preamplifier stage of a constant current charger for HV capacitors. With this converter it is possible to offer a wide input voltage range for the use in mobile applications with a 24 V or a 48 V battery. A novel snubber technique for current-fed converters is introduced, so that transistor conduction losses can be reduced significantly. The main part of the snubber is an active transformer short-circuit which eliminates the negative impact of the transformer leakage on the MOSFETs Drain-Source Voltage, by forcing the beginning of the commutation process earlier. Due to the reduction over the overvoltages MOSFETs with smaller Breakdown Voltage can be used leading to smaller On-Resistances. In a prototype the conduction losses were reduced to 55 %. The introduction of the transformer short-circuit can be achieved easily and inexpensive by replacing two diodes of the output rectifier bridge with MOSFETs. Additionally a small signal model of the current-fed double inductor push-pull converter is derived. The HV capacitor load profile demands particular requirements to the control, which will be closer observed. It will be shown that the best solution is a feed-forward of the link current to the input stage control. Otherwise a constant control deviation of the link voltage can occur.