OUTPUT OVERCURRENT PROTECTING CIRCUIT AND CONSTANT- VOLTAGE SWITCHING POWER SOURCE CIRCUIT HAVING THE SAME

摘要

PROBLEM TO BE SOLVED: To prevent an overcurrent from flowing to a DC voltage output side when the DC voltage output side of a constant-voltage switching power source circuit is short-circuited.;SOLUTION: When the load connected to a DC output voltage side is changed from an infinity to '0', the ON duty of a high-frequency pulse output from a switching transistor Q1 becomes long. When the ON duty of the pulse becomes a certain length, the voltage of the ON duty of the voltage induced in a second primary side winding P2 is lowered. The discharge current of a capacity C23 is reduced so that the output voltage of a 'CR integrated circuit' is raised to exceed the Zener voltage of a Zener diode ZD21 for specifying an operating voltage, the output voltage of the 'CR integrated circuit' is output to the anode side with the result that a 'switching transistor OFF circuit' is operated. The gate of the transistor Q1 is discharged through a diode D21, a transistor Q21 and a transistor Q22; and the transistor Q1 is lowered to a voltage to be turned OFF.;COPYRIGHT: (C)2004,JPO