LLC resonant converter with asymmetric PWM for hold-up time

To narrow down the switching frequency variation range of an LLC resonant converter, asymmetric pulse-width modulation (APWM) of the hold-up time is proposed. During the hold-up time, the modulation method of an LLC resonant converter is changed from frequency modulation (FM) to APWM. Since the LLC resonant converter achieves a higher gain with APWM than it does with FM at the same switching frequency, the switching frequency variation range is reduced. Therefore, an optimal design of the magnetic components is possible, resulting in a high efficiency under normal operation and a high power density. The proposed control scheme is applied to an 85 W LLC resonant converter.