Self-oscillation type switching power supply having time constant circuit electronic switch an external voltage and having charging time variable in response to output voltage

摘要

A self-oscillation switching power supply including a transformer having a primary winding, a secondary winding, and a feedback winding, and further including a control winding if necessary; a switching transistor for turning on and off the current of the primary winding; a control transistor for controlling a positive feedback signal from the feedback winding or control winding to the switching transistor; and a time constant circuit which is charged by a voltage generated across the feedback winding or control winding and which supplies a control voltage to the control transistor. The self-oscillation switching power supply further comprises an electronic switch which is opened and closed in response to the positive feedback signal from the control winding or feedback winding wherein an external voltage source is connected to the time constant circuit via said electronic switch so that the charging time of the time constant circuit is varied according to an output voltage detection signal.