Patent ID: 7577003

Claim:
A switching power source comprising: a switching element being connected in series with a primary winding of a transformer connected to a DC power source; a first rectifying/smoothing circuit configured to rectify and smooth AC power induced by a secondary winding of the transformer; a second rectifying/smoothing circuit configured to rectify and smooth AC power induced by an auxiliary winding of the transformer and to provide an internal power source; an output detection circuit configured to detect an output voltage that is provided from the first rectifying/smoothing circuit to a load; and a control circuit configured to control an ON-period of a pulse signal supplied to the switching element according to a feedback voltage from the output detection circuit, the control circuit including, an overcurrent detection circuit configured to detect whether or not an overcurrent exceeding a predetermined reference value is passed through the switching element, and a constant-current drooping control circuit configured to select one of a first constant current and a second constant current smaller than the first constant current according to an overcurrent detection result from the overcurrent detection circuit and to superpose a selected current on the feedback voltage from the output detection circuit, wherein the control circuit controls the ON-period of the pulse signal supplied to the switching element according to a resultant feedback voltage provided by a feedback voltage superpose circuit.