There is provided a power supply device including: a first controller operating a pulse width modulation (PWM) control integrated circuit (IC) when a load is connected to an output terminal thereof; a second controller determining whether to operate the PWM control IC according to a change in voltage of an auxiliary capacitor connected to the first controller when a load is not connected to the output terminal; and a constant voltage circuit unit supplying a constant voltage to the first controller and the second controller when the PWM control IC operates. Power consumption can be considerably reduced when a load is not connected to the output terminal of the power supply device. Also, since elements for controlling the power supply device are implemented as a single integrated circuit (IC), a leakage current can be reduced to thus minimize power consumption. In addition, voltage can be supplied to the IC while reducing switching noise.