3.16 Demystifying active-clamp flyback loop compensation
Current-mode controlled Active clamp flyback (ACF) converter achieves soft switching to boost full-load efficiency at a higher switching frequency operation, and the burst-mode operation effectively maintains its light load efficiency over stringent regulations. However, due to the resonance nature of ACF switching current waveform, it is challenging to characterize the small-signal property of the current feedback loop analytically. Besides, when a low-ESR output capacitor or a second order filter is used, the instability of the burst control loop may happen and result in disturbing audible noise and increased output ripple. Therefore, a modeling technique for ACF is developed in this paper, which provides useful guidelines on the loop compensation clearly to accommodate the dynamic of the current feedback loop. Besides, a novel ripple compensation technique and a simple serial damping method are proposed to stabilize the burst control loop in various output filter designs. Finally, the above studies and control methods are supported with the simulation and experimental verification on a 45W ACF with state-of-the-art GaN devices designed for high-density notebook adapters.