Patent ID: 7158391

Claim:
A switching power supply unit, comprising: an input terminal; an output terminal; a transformer provided between the input terminal and the output terminal, said transformer comprising a primary coil and a secondary coil; a primary side circuit connected to the primary coil of the transformer, said primary side circuit comprising: a main switching element driving the primary coil of the transformer; and a control circuit driving and controlling the main switching element to keep constant an output voltage of said output terminal by detecting the output voltage of said output terminal to feedback and acquiring a voltage induced in said transformer; and a secondary side circuit connected to the secondary coil of said transformer, said secondary side circuit comprising: a dummy load circuit connected to the output terminal; a detection circuit detecting a secondary coil voltage of the transformer, said detection circuit comprising a time-constant circuit having a resistor and a capacitor, and a first switch element being controlled by said time-constant circuit, said first switch element being turned off in a state of light load and being turned on in a state of an ordinary load except for the state of the light load; and a dummy load control circuit being controlled by said detection circuit, and controlling flow of current through said dummy load circuit only in a state of a light load, said dummy load control circuit comprising a capacitor charged when said first switch element of said detection circuit is in the on state, a second switch element turned on or off according to the charged voltage of the capacitor, and a third switch element turned on or off by said second switch element, said dummy load circuit turning on to flow current during the state of light load when the third switch element is turned on by turning off the second switch element and the first switch element, said dummy load circuit turning off flow of current in the state of ordinary load when the third switch element is turned off by turning on the second switch element and the first switch element, so that current flows through the dummy load circuit only in the state of the light load.