SWITCHING POWER SUPPLY

PURPOSE:To keep a load voltage at a specified value by controlling a switching power supply, when the value obtained by detecting through an overvoltage sensing circuit the output obtained by detecting and amplifying the difference between output and reference voltages is more than or less than said specified value. CONSTITUTION:The value Vc obtained by division of the output voltage of a rectifier and smoothing circuit 4 through resistances 6 and 7 is inputted to an error amplifier 8 and compared with a reference voltage 9, and an error is amplified. The output of said amplifier 8 and that of an oscillator circuit 10 are compared by a pulse width modulation circuit 11 to open and close a switching element 3 via driver circuit 12, to generate AC voltage on the secondary side of a voltage conversion transformer 2 and to rectify and smooth 4 said AC voltage to output DC. When an electric current to a load 5 increases/decreases, a divided voltage Vc decreases/increases and the output of said error amplifier 8 increases/decreases to cause the pulse width to increase/ decrease and to hold the output voltage constant. When the output voltage increases extraordinarily, the output of said error amplifier 8 becomes excessive and an overvoltage sensing circuit 13 operates to cut OFF the driver circuit 12. In this manner, an operating voltage is maintained within a certain range.