POWER SUPPLY CIRCUIT

PURPOSE:To efficiently transmit the change of DC voltage to a control circuit and to obtain stable DC voltage by turning the change of the DC current to a digital value and transmitting it through an insulated transmission means to the control circuit. CONSTITUTION:A switching element Q1 switching the current flowing through a primary side coil 4a of a transformer 4, and a control circuit 13 controlling the switching of the switching element Q1 based on the change of the DC voltage obtained by rectifying and smoothing the voltage generated on both sides of the secondary side coil 4b of this transformer 4, are provided. A converter 9 supplies the change of the DC voltage through a photocoupler 10 and an amplifier 11 to a D/A converter 12 while converting the DC voltage, that is, a detection signal, supplied from a filter 7 through an amplifier 8 into a digital signal, and supplies the digital detection signal to a PWA modulation circuit 13 as an analog detection signal. Thus, the change of the output DC voltage can be efficiently transmitted to the control circuit 13, and the stable DC voltage can be supplied to the secondary side 4b of the transformer 4.