A power supply circuit includes a rectifying unit for outputting a rectified input voltage, a transformer for receiving the voltage and outputting an induced voltage based on the input voltage, a first smoothing unit for smoothing the induced voltage to output a first output voltage, a detecting unit for outputting an error signal corresponding to the difference between the first output voltage and a reference voltage, and an insulating unit for receiving the error signal and outputting a signal corresponding to the error signal. The input and output terminals of the insulating unit are insulated. The circuit also includes a controlling unit for outputting a control signal for selectively inputting the input voltage into the transformer on basis of the signal from the insulating unit, and also includes a switching unit connected to the transformer for selectively inputting the input voltage into the transformer, based on the control signal.