Switching power supply unit having a regulator circuit and electronic apparatus using the same

A switching power supply unit comprising:

• a transformer having a primary winding, a first secondary winding, and a second secondary winding;
• a first output circuit connected to the first secondary winding and outputting a first output voltage constant-voltage-controlled by feedback control provided on a primary side of the transformer;
• a second output circuit connected to the second secondary winding to output a second output voltage;
• and
• a regulator circuit connected to the second output circuit to perform constant-voltage-control over the second output voltage so as to output as a third output voltage;
• wherein
• a load current flowing through the first output circuit is adjusted according to the third output voltage such that the second output voltage is maintained at a desired value or more;
• and
• the regulator circuit includes a first impedance element connected in series to the second output circuit, a second impedance element connected between an output terminal of the first output circuit and a control terminal of the first impedance element, and a voltage control circuit controlling the impedance of the second impedance element such that the third output voltage is maintained constant.