For MHz GaN-based power converters, zero-voltage switching (ZVS) is one of the critical strategies to avoid excessive switching losses of hard-switching. Active clamp flyback (ACF) is one of the most commonly used isolated topologies with the capability of zero-voltage turn on when the switching sequence of the clamping switch is appropriately controlled. However, sharp current drops of leakage inductor current occur after the turn-off actions of the clamping switch, especially in high frequency high step-up applications. This leads to insufficient energy to discharge the junction capacitance of the main switch to achieve zero-voltage turn on. In this paper, the reasons for the current drops and ZVS loss are explained in detail. The related parameters are then analyzed with quantitative derivations. Then, a resonant voltage-doubling ACF converter is proposed with the capability of ZVS in full-load range, optimized transformer designs, and low ringing. The circuit modes and the conditions of ZVS for both switches are discussed, along with the design considerations of the key parameters. All the proposed analyses are verified on a 1 MHz 300W resonant voltage-doubling ACF converter prototype with 28 V input and 180V output.
 

gallium nitride,zero-voltage switching (ZVS),high frequency,flyback,resonant