Small Signal Analysis of Active Clamp Flyback Converters in Transition Mode and Burst Mode

Active clamp flyback (ACF) converters in transition mode improves the full load efficiency of high-frequency AC/DC and DC/DC power supplies with zero-voltage switching (ZVS). To minimize the impact of the circulating energy for ZVS, burst mode control is used to benefit ACF light load efficiency. However, due to the resonance nature of ACF switching current waveforms, it is difficult to characterize the small-signal properties analytically. In this paper, two new small signal models are developed, and analytical design guides are investigated. An average modeling technique for the transition mode ACF is developed which eliminates the complexity of describing the second-order resonance waveforms, and the model enables a higher system bandwidth design. A high-frequency model for burst mode is derived with the describing function method, which identifies the loop compensation impact and differentiates the stability criteria from the conventional ripple-based (V²) control. Enlightened from the models, a novel nonlinear ramp compensation method and a parallel damping technique are proposed to improve the loop stability and optimize the dynamic response. Finally, SIMPLIS simulations verify the model accuracy, and the fast transient response with the compensation methods are demonstrated on a high-density 45W GaN ACF notebook adapter.