SWITCHING POWER SUPPLY

PROBLEM TO BE SOLVED: To provide a switching power supply capable of reducing power loss in a constant voltage circuit without increasing the number of transformer windings. SOLUTION: The auxiliary power supply of the switching power supply 1 includes: a first DC conversion means D4generating a first DC conversion voltage V1by changing the induced voltage of the auxiliary winding T3into a direct current; a second DC conversion means D3, C3boosting the induced voltage to a second DC conversion voltage V2higher than the first DC conversion voltage V1while changing the induced voltage into a direct current; and a constant voltage circuit 5 outputting a second boosted DC conversion voltage V2, while restricting the conversion voltage so as not to exceed a predetermined voltage. When the induced voltage of the auxiliary winding T3rises, and a voltage is supplied to an active filter control circuit 4 from the constant voltage circuit 5 prior to voltage supply from the first DC conversion means D4, and then the first DC conversion voltage V1exceeds the output voltage V3from the constant voltage circuit 5, the operation of the constant voltage means 5 is stopped and a voltage is supplied from the first DC conversion means D4to the active filter control circuit 4. COPYRIGHT: (C)2011,JPO&INPIT