A switching mode power supply and a method of operating the power supply in a power save mode. The switching mode power supply includes a first PWM controller and a second PWM controller that are driven by different driving voltages and control first and the second voltages to be output, respectively, a first transformer that is controlled by the first PWM controller to output the first voltage and having a primary coil, a secondary coil to induce the first voltage, and an auxiliary winding, and a rectifier that rectifies and smoothes a current flowing through the auxiliary winding of the first transformer, generates a power save mode voltage based on the respective driving voltages of the first and the second PWM controllers, and supplies the power save mode voltage to the first and the second PWM controllers. Accordingly, the power save mode is operated using a voltage difference without requiring an extra controller.