A power supply apparatus includes a main converter, an auxiliary winding switching circuit and a pulse width modulation controller. The main converter includes a transformer. The transformer includes a primary side winding, a secondary side winding and an auxiliary winding. The auxiliary winding includes a first winding and a second winding. According to a secondary side voltage, the first winding induces to generate a first voltage, and the second winding induces to generate a second voltage. The first voltage is greater than the second voltage. The auxiliary winding switching circuit sends the first voltage to the pulse width modulation controller when the first voltage is not greater than a predetermined voltage. The auxiliary winding switching circuit sends the second voltage to the pulse width modulation controller when the first voltage is greater than the predetermined voltage.