A transformer has a primary winding connected between a pair of dc input terminals via a switch, a secondary winding connected between a pair of output terminals via a first rectifying and smoothing circuit, and a tertiary winding connected to a second rectifying and smoothing circuit for feeding a switch control circuit. Included in the switch control circuit is a switch control pulse generator circuit whereby the switch is driven to provide a constant dc voltage at the output terminals. A mode selector circuit is connected to the switch control pulse generator circuit to cause the switch to be driven continuously under normal load, and at intervals under light load. The intermittent driving of the switch is suspended, and the switch driven continuously, in the event of a drop in the output voltage of the second rectifying and smoothing circuit to a predefined value during operation in light load mode.