Control circuit for controlling a switch based on a voltage of an auxiliary winding in a switching power supply

The present invention refers to a control circuit and an associated method of controlling the output voltage of a primarily controlled power switching supply with a primary-sided switch and a transformer with an auxiliary winding, in which after opening the primary-sided switch an auxiliary voltage for indicating the output voltage is induced. The induced voltage is supplied to a control circuit as control variable. In order to provide a control circuit, which needs as few external components as possible, which can be manufactured at low cost and which has a possibly low need of space, a driver output of the control circuit is connected to a control input of the primary-sided switch and the control circuit is adapted such that it adjusts the switching frequency of the primary-sided switch depending on the auxiliary voltage, wherein a constant factor of this dependency is selected such that the output current of the switching power supply adopts a predetermined value, and wherein the driver output is further connected to the input of a comparator for determining the current through the primary-sided switch.