A single-stage power factor correction power supply has two transformers: a main transformer and an auxiliary transformer (fly-back transformer). The main transformer transfers energy from the primary circuit to the secondary circuit. The auxiliary transformer is used to correct input current waveform. A fill-valley circuit is used to stored energy when the input voltage is higher than the voltage across bulk capacitors in the fill-valley circuit and to release energy when the input voltage is lower than the voltage across the bulk capacitors in fill-valley circuit. A small capacitance value capacitor is used to improve input current waveform.