PURPOSE: To decrease the power loss when a main switching transistor (TR) is turned off by applying a changed component of an output at the turning-off of the main switching TR to an input winding of a driving transformer from a secondary winding of a main transformer.

CONSTITUTION: When a base current ib of the main switching TR4 is stopped at a time t4, since a rectifier 8 is nonconductive, the changed component of the secondary voltage of the main transformer 3 is generated in an input winding (a), an applied voltage V1 is increased, a voltage is generated also in a winding (b) and a large reverse bias voltage between a base and an emitter of the TR4 is applied thereto. Thus, even if an oscillatory voltage is generated in the winding (b) of the drive transformer 5 because of a sudden change in the reverse base current ib introduced from the base of the main switch TR4, a large negative bias voltage is guaranteed for a voltage Vb between the base and emitter of the TR4, a current ic is dropped suddenly as shown in solid lines and stopped at a time t5. Then the trailing time of the current ic is reduced remarkably from t3 to t5.