Design consideration of active clamp flyback converter with highly nonlinear junction capacitance

Active clamped flyback converter performs zero voltage switching (ZVS) and recycling the leakage energy of transformer to boost the efficiency in higher frequency operation. Although it is well known that the switch-node capacitance determines the circulating energy for ZVS, the capacitance-nonlinearity impacts from each of the two primary-side switches and from the secondary synchronous rectifier have not been well understood. In this paper, the design tradeoff and system performance with different nonlinearity of junction capacitance from each switching device are investigated across the full load to light load operations, and then control strategies to overcome the capacitance nonlinearity are proposed. Then, analytical equations and design procedures are developed, considering the nonlinearity impact. Finally, the above studies and control method are supported with the experimental verifications on a 30W charger and a 45W adapter using state-of-the-art GaN and silicon FETs.