JPS6416260A - Switching power device - Google Patents

Abstract

PURPOSE:To effect protecting operation stably, by a method wherein a voltage at the connecting point of a primary winding and a switching element is taken out through a capacitor to stop the driving of the switching element when an overvoltage is detected. CONSTITUTION:A switching transistor (Tr) 3 is connected to a power source input terminal 1, on which a DC voltage VIN is impressed, through the primary winding 2p of a power source transformer 2 while the transistor 3 is driven by a pulse, whose width is modulated by a driving circuit 11. A DC voltage VOUT is outputted from the secondary winding 2s of the transformer 2 through a rectifying diode 6 and the like. The voltage VOUT is inputted into the driving circuit 11 through an error amplifier 12, a photocoupler 15 and a comparator 20. In this case, an overvoltage detecting circuit 30 is connected to the connecting point A of the primary winding 2p of the transformer 2 and the transistor 3 through a capacitor 29 to control the operation of a switching circuit 22. According to this method, a tertiary winding for detecting an overvoltage becomes unnecessary and the overvoltage may be detected at the primary side of the transformer 2.