A power supply device includes a transformer including primary winding and secondary winding, a sensor configured to sense at least one of output voltage and output current in the power supply device, a pulse width modulation (PWM) controller configured to perform PWM switching so that voltage is selectively provided to the primary winding based on one of the sensed output voltage and output current, and a regulator configured to stop PWM switching of the PWM controller and keep the PWM switching in stopped state when at least one of the sensed output voltage and output current exceeds a preset value.