Abstract: This invention relates to a primary-controlled switched-mode power supply of the type of a free-running flyback converter, which comprises a transformer with a primary-side winding, a secondary-side winding and at least one auxiliary winding. The switched-mode power supply comprises a primary-side switch, which is connected to the primary-side winding, in order to interrupt a current flow through the primary-side winding, a freely oscillating circuit for the generation of switching pulses, which drive the primary-side switch, and a circuit for generating an image voltage between the terminals of the auxiliary winding, in order to generate an image voltage, which on the primary side forms a voltage to be regulated on the secondary side.

Abstract: The present invention relates to a control circuit for controlling the synchronous rectifier switch of a switched mode power supply, which comprises a primary-side switch and a transformer with a primary winding, a secondary winding and a secondary-side auxiliary winding. A voltage tapping point is provided between the secondary winding and the auxiliary winding, the control circuit being adapted to be connected to a connection of the auxiliary winding and to said voltage tapping point. The control circuit is adapted to be connected to the control terminal of a switch and the output voltage is adapted to be tapped parallel to the secondary winding. The control circuit controls a secondary-side switch for short-circuiting the connection between the output terminal and the voltage tapping point and an RC element for providing positive and negative control pulses having a predefined time constant. Due to the use of the RC element, a control circuit can be provided at an extremely reasonable price.