PURPOSE: To reduce the loss of a switching element by detecting output voltage, output current or both and by comparing said output and a current flowing through said switching element to control the switching.

CONSTITUTION: An output voltage divided by detection resistances 6, 7 is compared with a reference voltage 5 and inversion-amplified by an error amplifier 4. The output of said amplifier 4 is compared with the output of a resistance 8 for detecting a current flowing through a switching element 2 by a comparator 9. When the current of said switching element 2 exceeds the output of said error amplifier 4, a control transistor 3 is saturated by the output of said comparator 9 and the switching element 2 is turned OFF. At the same time, the output of a transformer 1 is generated on the secondary side to supply a load with a current and to charge a capacitor with electricity. When the current on the secondary side becomes zero, said transformer is reversed to apply a voltage to the gate of said switching element 2 to saturate side element so that a current flows through said transformer 1.