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To narrow down the switching-frequency-variation range of an LLC resonant converter, asymmetric pulsewidth 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.