Patent ID: 7872880

Claim:
A switch mode power supply comprising: a first series circuit connected between positive and negative electrodes of a DC power supply, the first series circuit including a main switching device and a subsidiary switching device connected in series with each other; a transformer including a first winding on a primary side thereof and a second winding on a secondary side thereof, the first winding being disposed between the positive electrode of the DC power supply and a connection point of the main switching device and the subsidiary switching device; a rectifying and smoothing circuit, the second winding being connected to a load via the rectifying and smoothing circuit; a main control circuit to controllably turn the main switching device ON and OFF; a subsidiary control circuit to controllably turn the subsidiary switching device ON and OFF; the main control circuit and the subsidiary control circuit turning the main switching device and the subsidiary switching device ON and OFF alternately to convert a second winding voltage generated across the second winding to a DC electric voltage via the rectifying and smoothing circuit and to feed the DC electric voltage to the load; the transformer further including a third winding and a fourth winding on the primary side thereof; the main control circuit employing a third winding voltage generated across the third winding as a signal voltage to turn the main switching device ON and OFF, whereby the DC electric voltage fed to the load is set at a predetermined value; and the subsidiary control circuit including a transistor and a voltage regulator, the subsidiary control circuit controlling the transistor to apply to a control terminal of the subsidiary switching device a lower one of a fourth winding voltage generated across the fourth winding and a regulation voltage of the voltage regulator, the lower one of the fourth winding voltage and the regulation voltage being equal to or higher than zero volts.