| 摘要 |
PURPOSE:To prevent a current not smaller than a rated value from flowing in an AC power supply line, when a switching power supply recovers from its momentary interruption. CONSTITUTION:When an AC voltage V0 is applied to AC input terminals 54, 55 and a switching power supply is brought into a steady state, a voltage V2 exceeds the Zener voltage of a Zener diode Z1, and a transistor Q1 is turned on, and transistors Q2, Q3 are turned off. Then, a starting capacitor C3 of a starting circuit 5 is charged and a switching control circuit 4 is operated. Hereupon, if the AC voltage V0 is cut off, an output voltage V1 of a rectifying- smoothing circuit 1 is lowered, and the divided voltage V2 of an AC voltage sensing circuit 7 begins to decrease. When the voltage V2 gets lower than the Zener voltage, the transistor Q1 is turned off, and the transistors Q2 Q3, are turned on. Thereby, the charged charge of the capacitor C3 is discharged quickly, and the switching control circuit 4 stops operating. As a result, even though the AC voltage V0 is applied again to the terminals 54, 55, no current not smaller than a rated value flows in an AC power supply line. |
