A power supply apparatus includes a bulk capacitor, a voltage dropper unit and a pre-charging capacitor. The voltage dropper unit is electrically connected to the bulk capacitor. The pre-charging capacitor is electrically connected to the voltage dropper unit. When the power supply apparatus receives an input direct current voltage, a voltage of the bulk capacitor is provided with a first voltage, and the pre-charging capacitor is charged so that the pre-charging capacitor is provided with a second voltage, and the second voltage is greater than the first voltage. When the voltage of the bulk capacitor is less than a predetermined voltage, the second voltage of the pre-charging capacitor is converted voltage by the voltage dropper unit to provide for extending a hold up time of the power supply apparatus.