Switching power supply using a single isolating element to facilitate multiple functions

摘要

A switching power supply comprising a main converter that turns ON and OFF a DC current applied to a primary winding of a main transformer by a main switching element and outputs a main output voltage to a secondary circuit by rectifying and smoothing current induced in a secondary winding; an error amplifier generating an error voltage between the main output voltage and a first reference voltage; an over-voltage protection circuit that generates an over-voltage protection signal when the main output voltage exceeds a second reference voltage; a PWM circuit to control a drive signal for the main switching element to minimize error voltage outputted by the error amplifier; and a shut-down execution circuit that stops the main converter operation when the over-voltage protection signal is generated; wherein are provided a device to superimpose the over-voltage protection signal on the error signal of the main output voltage induced in the secondary winding and to transmit the superimposed signal to the primary circuit after isolating the superimposed signal with a photo-coupler; and a primary control circuit that detects an error signal transmitted via the photo-coupler by dividing the error signal into three levels of stop range, normal operation range, and abnormal operation range, and rapidly stop operation of the main converter when the error signal value is deflected to the stop range or the abnormal operation range due to failure of the photo-coupler, thereby resulting in improved reliability of the switching power supply.