A new single-stage-isolated ac-dc converter that can guarantee both high efficiency and high power factor is proposed. It is based on a new dc-dc topology that has prominent conversion ratio similar to that of boost topology so that it is adequate to deal with the universal ac input. In addition, since it utilizes the transformer more than others based on the general flyback topology, the size of whole power system can be reduced due to the reduced transformer. Moreover, the voltage stresses on the secondary rectifiers can be clamped to the output voltage by adopting the capacitive output filter and clamp diode, and the turn-off loss in the main switch can be reduced by utilizing the resonance. Furthermore, since this converter operates at the boundary conduction mode, the line input current can be shaped as the waveform of a line voltage automatically and the quasi-resonant zero-voltage switching can be obtained. Consequently, it features higher efficiency, lower voltage stress, and smaller sized transformer than other topologies. A 100W prototype has been built and tested as the validation of the proposed topology. Copyright (C) 2010 John Wiley & Sons, Ltd.