| 摘要 |
PROBLEM TO BE SOLVED: To provide a switching power supply wherein switching transistors are prevented from overheating even if an AC input voltage drops. SOLUTION: An operation stop circuit 6 is connected with a feedback winding N2, and comprises transistors Q6 and Q7 and a Zener diode D14. If the AC input voltage becomes lower than the range of control, the feedback voltage of the feedback winding N2 becomes lower than the Zener potential of the Zener diode D14, and the Zener diode D14 is turned off. As a result, the transistor Q6 is turned off, and the transistor Q7 is turned on. When the transistor Q7 is turned on, a transistor Q3 is turned on, a gate voltage is drawn out of a switching transistor Q1, and the switching transistor Q1 is turned off.
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