An improved active zero voltage switching assisting circuit with lower dv/dt for DC-DC series resonant converter with constant input current

With a constant current dc power distribution system, the input voltage of each series connected power converter module varies with the load. Achieving zero voltage switching (ZVS) for the converter using passive techniques becomes a challenging and difficult task with the wide variation in the load that also changes the input voltage. Therefore, an active assisting method that adapts to the varying input voltage is needed for achieving ZVS with good efficiency over the load range. Traditional active ZVS assistance techniques either operates with higher rms currents in the main switches and the ZVS assisting branch, which further result in higher conduction losses, or result in hard switching of assisting switches, which result in high EMI due to high dv/dt, at full DC bus voltage. In this paper, an active ZVS assisting circuit is proposed which operates with low rms currents in the assisting branch with lower EMIs due to dv/dt in the assisting branch. The proposed technique is experimentally verified through testing of a series resonant converter whose input is connected to a 1 A current source and output current is regulated at 0.33 A for a full load of 250 W, operating at a switching frequency of 250 kHz.