A power supply apparatus includes a current modulation unit, a voltage converting unit, a current detector, and a control unit. The current modulation unit is used for receiving an input voltage and a control signal, generating an output current and adjusting the value of the output current according to the control signal. The voltage converting unit, coupled to the current modulation unit, converts the output current into the output voltage. The current detector, coupled to the voltage converting unit and the output terminal, detects the current flowing through the load and outputs a current detection result. The control unit, coupled to the current modulation unit, the current detector and the output terminal, receives the current detection result and the output voltage, and generates the control signal according to the current detection result and the output voltage.