Switching power supply unit

摘要

A switching power supply unit includes first and second switching circuits which alternately turn first and second switching elements on and off with an intermediate period in which both are turned off, and energy is stored in a primary winding of a transformer during an ON period of the first switching element and the energy is discharged from a secondary winding of the transformer during an OFF period of the first switching element. A control circuit includes an OFF period extending circuit connected to a control terminal of the first switching element, such that a transistor remains turned on for a desired period even after the energy has been discharged from the secondary winding, extending the OFF period of the first switching element for the desired period. In a light load operation, a phototransistor in the second control circuit is turned on so that the ON period of the second switching element will be shorter than the time required to discharge the energy from the secondary winding.