| 摘要 |
PROBLEM TO BE SOLVED: To provide a switching power-supply device that allows reducing the capacitance of a capacitor supplying a power-supply voltage without causing start-up failure.SOLUTION: A switching power-supply device includes: a control circuit 3a controlling on and off of a switching element Q10; and a second rectifier smoothing circuit (a diode D10 and a capacitor C10) rectifying and smoothing a voltage generated at a tertiary winding P2 of a transformer T and supplying the resultant voltage to the control circuit 3a as a power-supply voltage Vcc. The switching power-supply device further includes a start-up circuit 1a supplying a first constant current Ia to the control circuit 3a as a start-up current Ist when the power-supply voltage Vcc is lower than or equal to a first threshold voltage V1 lower than a start-up voltage Von (a stop voltage Voff) of the control circuit 3a and is over the start-up voltage Von, and supplying a second constant current Ib larger than the first constant current Ia to the control circuit 3a as the start-up current Ist when the power-supply voltage Vcc is over the first threshold voltage V1 and is lower than or equal to the start-up voltage Von. |
