A single-switch quasi-resonant converter is proposed to obtain high efficiency. Using a variable switching frequency control, this converter is continuously operated at the critical conduction mode for soft switching of the power semiconductor switch. By the resonance, the proposed converter reduces the turn-on loss of the switch without additional active switches and alleviates the reverse-recovery losses of the rectifying diodes. Furthermore, the voltage stress of the rectifying diodes due to the output series resonant circuit is clamped to the output voltage. Experimental results for the 48-V/160-W converter at the variable switching frequency are obtained to show the performance of the proposed converter.