Patent ID: 8036003

Claim:
A switching power supply device comprising: a switching element which performs an on-off operation to turn on and off a direct current voltage to be applied to a primary side main winding; a primary side control circuit which controls the on-off operation of the switching element; a transformer having the primary side main winding, a secondary side winding and a primary side auxiliary winding, the secondary side winding being adapted to induce a voltage that is to be rectified and output to a secondary side circuit from the switching power supply device, the primary side auxiliary winding being adapted to supply a power supply voltage to the primary side control circuit; a feedback circuit which transmits a deviation of the voltage induced by the secondary side winding to the primary side control circuit; and a short-circuiting switch which short-circuits a part of the feedback circuit; wherein when an abnormal condition occurs in the secondary side circuit, the short-circuit switch is changed to an ON state to reduce a voltage induced by the primary side auxiliary winding and stop an operation of the primary side control circuit and an operation of the switching element.