Patent ID: 8289740

Claim:
A voltage reduction detection circuit that detects a voltage reduction in an AC power supply for a switching power supply, the switching power supply converting AC power fed from the AC power supply into DC power, then switching the DC power passing across a primary winding of a switching transformer to generate AC power in a secondary winding of the switching transformer, and then rectifying and smoothing the AC power to output a predetermined DC voltage, the voltage reduction detection circuit comprising: a first rectifier diode having an anode connected to an output terminal of the secondary winding; a second rectifier diode connected in parallel with the first rectifier diode and having a cathode connected to the output terminal; and resistors arranged to divide a voltage between a cathode side of the first rectifier diode and an anode side of the second rectifier diode, wherein the voltage reduction in the AC power supply is detected based on variation in a voltage resulting from voltage division by the resistors.