SMPS with low load switching frequency stabilization

摘要

A switch-mode power supply has a control device with an input for a control voltage which is coupled to a secondary voltage and an output for controlling a transistor that is connected in series with a primary winding of a switch-mode power supply transformer. The control device additionally includes a device for inhibiting the switching transistor from switching on again when a control error that is formed in the control device is less than a reference value. The control signal input and the output of the control device are coupled through a coupling element having a low-pass filter characteristic. A switching-on pulse for the switching transistor is consequently superimposed on the control voltage, with a time delay. If the load to be supplied is small (standby mode), the switching of the switching transistor is inhibited until the switching-on pulse which is coupled to the control voltage has decayed. A stable switching frequency slightly above the audibility limit can be set for the standby mode. The power loss is accordingly low.