Isolated switching power source

In an isolated switching power source for controlling a voltage, comparing a voltage signal corresponding to an output voltage with a reference voltage to cause to generate a control signal according to a difference between the reference voltage and the voltage signal, by feeding back the control signal to a control circuit in an input side through a photo-coupler, wherein a feedback means comprises:

• a plurality of resistors, connected in series between a positive terminal and a negative terminal of the power source output, for obtaining the voltage signal corresponding to the output voltage;
• a bipolar transistor element for generating the control signal by comparing the reference voltage with the voltage signal corresponding to the output voltage, connecting a base of the transistor element to a connection point of the resistors and an emitter of the transistor element to a source of the reference voltage;
• and
• a photo-coupler comprising a light emitting element connected between an emitter of the transistor element and one terminal of the power source output, the light emitting element being used as the sole source of the reference voltage, in means for feeding back the control signal;
• and
• wherein the reference voltage is a forward dropping voltage of the photo-coupler light emitting element.

In an isolated switching power source for controlling a voltage, comparing a voltage signal corresponding to an output voltage with a reference voltage to cause to generate a control signal according to a difference between the reference voltage and the voltage signal by feeding back the control signal to a control circuit in an input side through a photo-coupler, wherein a feedback means comprises:

• a plurality of resistors, connected in series between a positive terminal and a negative terminal of the power source output, for obtaining the voltage signal corresponding to the output voltage;
• a field effect transistor element for generating the control signal by comparing the reference voltage with the voltage signal corresponding to the output voltage, connecting a gate of the transistor element to a connection point of the resistors and a source of the transistor element to a source of the reference voltage;
• and
• a photo-coupler comprising a light emitting element connected between the source of the transistor element and one terminal of a power source output, the light emitting element being used as the sole source of the reference voltage, in means for feeding back the control signal;
• and
• wherein the reference voltage is a forward dropping voltage of the photo-coupler light emitting element.