| 摘要 |
PROBLEM TO BE SOLVED: To reduce the capacitance of a smoothing capacitor, by a method wherein the detection of the power supply of a commercial power supply by a power-failure detection circuit is performed independently of the detection of a power-supply voltage value after a rectifying and smoothing operation by a voltage detection circuit, the reference value in the output circuit of a power-failure notice signal is controlled variably, and the output timing of the power-failure notice signal is changed. SOLUTION: This power-supply apparatus is provided with a power-failure detection circuit wherein a voltage, in a point B, which is obtained by rectifying and smoothing a commercial power supply Vin by using a diode bridge circuit DB1 and a smoothing capacitor C1 is detected so as to be compared with a reference voltage Vx by a comparator COMP1, and a power-failure detection signal ACDWN* is generated. In the power-supply apparatus, a circuit which is composed of a resistance 3 and a bidirectional photocoupler Q4 and which detects the AC voltage, in a point A, of the commercial power supply Vin is installed. When a power failure is detected, the reference voltage Vx of the comparator COMP1 is changed to a higher value. As a result, the capacitance of the smoothing capacitor C1 is reduced, and the production cost and the size of the powersupply apparatus can be reduced.
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