A PWM Plus Voltage-Second Balance Control for ZVS Four-Switch Flyback Converter

The four-switch flyback (FSF) converter features low voltage stress of the power switches and zero-voltage-switching (ZVS). In this article, the clamp capacitance is properly designed to avoid multiple meeting points of the leakage inductance current and magnetizing current. Then, a pulse width modulation (PWM) plus voltage-second balance (VSB) control is proposed to turn off the clamp switch to guarantee the negative magnetizing current for realizing ZVS, and the secondary rectifier current sensor is not necessary anymore. The principle of the FSF converter is analyzed, the design considerations are provided, and the implementation of the proposed PWM plus VSB control is given. Finally, a 60-W prototype operating at 500 kHz using GaN devices is built and tested in the lab. The experimental results are provided to verify the effectiveness of the proposed control scheme.