PURPOSE: A power supply device including a protection circuit is provided to prevent the loss of a pattern by bypassing a surge voltage or current to a ground. CONSTITUTION: A rectifier(10) rectifies and smooths utility power. A switching unit(20) switches the rectified power. A transformer(30) has a primary winding and a secondary winding. A stabilizing unit(40) stabilizes the transformed power. A photocoupler includes an input unit and an output unit. If an overcurrent flows in a feedback signal line between the switching unit and the output unit of the photocoupler, the protection unit bypasses the overcurrent to the ground.
