SWITCHING POWER SUPPLY DEVICE

PROBLEM TO BE SOLVED: To enable supply of appropriate load current and overcurrent protecting operation, by varying the operating point for overcurrent protection according to two rated input voltage ranges of high and low kinds. SOLUTION: An input voltage detection circuit portion 5 detects whether direct-current input voltage, rectified by a diode bridge 3, is not less than or less than the predetermined Zener voltage of a Zener diode ZD1, and outputs the result of the detection. An operating point switching circuit 15 operates as follows: when the detection voltage of the input voltage detection circuit portion 5 is less than the Zener voltage (within the lower rated input voltage range), its transistor TR2 is turned off. Thus, the operating point varying circuit 15 disconnects the parallel connection of a second overcurrent sensing resistor R5 with a first overcurrent sensing resistor R2 of the input voltage detection circuit portion 6, to set a first operating point for overcurrent protection. When the detection voltage of the input voltage detection circuit portion 5 is not less than the Zener voltage (within the higher rated input voltage range), its transistor TR2 is turned on. Thus, the operating point varying circuit 15 connects the second overcurrent-sensing resistor R5, in parallel with the sensing resistor R2 to set a second operating point for overcurrent protection which is higher than the first operating point for overcurrent protection. COPYRIGHT: (C)2004,JPO