Active clamp flyback using GaN power IC for power adapter applications

Both high switching frequency and high efficiency are critical in reducing power adapter size. The active clamp flyback (ACF) topology allows zero voltage soft switching under all line and load conditions, eliminating all leakage inductance and snubber losses. Using enhanced mode GaN switches, an optimized active clamp flyback can enable 5-10x higher switching frequency, while improving efficiency. This paper analyzes a high frequency ACF operating in critical conduction mode. Detailed equivalent circuits and analytical models are provided to explain operation principles and current waveforms. A novel secondary side resonant concept is presented to shape the clamp current waveform in a way that significantly reduces rms current and improves synchronous rectifier operation. Using monolithic GaN power ICs, a high density 45W adapter prototype is demonstrated with excellent efficiency and EMI performance.