The flyback converter is one of the most attractive isolated converters in small power applications because of its simple structure. However, it suffers from a high device stress and a large transformer size. To relieve these drawbacks, a high-efficient pulsewidth modulation resonant single-switch isolated converter is proposed. The proposed converter derives the power using the resonance between transformer leakage inductor and secondary capacitor without a large filter inductor. Therefore, the switch turn-OFF loss and snubber loss are reduced by a sinusoidal-shaped current. Moreover, it has a reduced voltage stress on the secondary diodes without a dissipative snubber and a smaller transformer size. Therefore, it features a simple structure, low cost, and high efficiency. The operational principle and characteristics of the proposed converter are presented and verified experimentally with a 300 similar to 400 V(dc) input, 70 V(dc)/1.5 A output prototype converter.