This paper proposes an active clamp flyback (ACF) converter with GaN devices. Compared to Si FETs, when the main GaN FET is at full zero voltage switching (FZVS), maximum switching loss reduction can be achieved. Thus, the use of dynamic resonant period control (DRPC) technique to achieve fast ZVS can reduce 87% of the transformer leakage inductance energy loss on the active resonant circuit (ARC). When the load changes, the DRPC technique dynamically adjusts the auxiliary switch on-time to prevent large voltage stress on the primary side components. At full load, the leakage energy loss can be reduced, thereby transferring more energy to the secondary side with an efficiency of up to 94%.