Patent ID: 7679939

Claim:
A control circuit for a switching power supply comprising a controllable switch for switching a current flowing through a first winding of a transformer and a voltage providing circuit for providing an output voltage based on a voltage generated in the second winding of the transformer, the control circuit comprising: a comparing circuit for getting a comparison signal based on a comparison between an input signal derived from a current through the switch or through the first winding and a threshold signal derived from a voltage present in the voltage providing circuit; a threshold signal modulation circuit adapted to modulate the threshold signal in such a way that the comparison signal is activated as soon as the input signal derived from the current flowing through the switch or through the first winding crosses a first threshold value, if the current flowing through the switch or through the first winding exhibits a first current slope rate, and that the comparison signal is activated as soon as the input signal of the comparing circuit describing the current flowing through the switch or through the first winding crosses a second threshold value, if the current flowing through the switch or through the first winding exhibits a second current slope rate smaller than the first current slope rate, wherein, for a given voltage present in the voltage providing circuit, the first threshold value is smaller than the second threshold value; and a switch driver for closing the switch periodically, and for opening the switch in response to an activation of the comparison signal provided by the comparing circuit.