Patent ID: 7375990

Claim:
A switching power device switching electric current supplied to a primary side winding of a switching transformer to rectify and smooth AC power generated respectively in first, second, and third secondary side windings of the switching transformer and supply DC power, comprising: a self-oscillation type switching unit having a first switching device and a second switching device, wherein the second switching device variably controls an applied voltage to a control terminal of the first switching device, and switching the electric current supplied to the primary side winding of the switching transformer to generate the AC power respectively in the first, second, and third secondary side windings of the switching transformer; a short-circuit detecting unit detecting a short-circuit of an output side of an electronic circuit connected to the first secondary side winding of the switching transformer depending on whether or not a plurality of diodes connected in series are electrically conducted; a voltage detecting unit outputting a switching control signal in accordance with the voltage of the DC power obtained by rectifying and smoothing the AC power generated in the second and third secondary side windings of the switching transformer and a signal from the short-circuit detecting unit; a feedback unit optically transmitting the switching control signal from the voltage detecting unit to a control terminal of the second switching device of the switching unit; and a stopping unit stopping the switching operation of the switching unit in accordance with the switching control signal from the voltage detecting unit when the short-circuit of the output side of the electronic circuit is detected by the short-circuit detecting unit and, holding a stopping state of the switching operation of the switching unit; wherein a condenser is connected to the output side of the electronic circuit; and wherein a constant-voltage device is connected between the third secondary side winding and the voltage detecting unit.