There is provided a switching mode power supply including: a direct current (DC)/DC converting unit converting a DC power level; an auxiliary power supply unit discharged in a no load mode in which a load is not connected to an output terminal of the DC/DC converting unit; and a controlling unit sensing a change in impedance according to whether or not the load is connected to the output terminal to thereby determine whether or not the switching mode power supply is in the no load mode, and driving a pulse width modulation integrated chip (PWM IC) when a voltage level of the auxiliary power supply unit is at a preset level or less in the no load mode.