Patent ID: 7545654

Claim:
Control circuit for controlling the output voltage and/or the output current in a switching power supply controlled by the primary side, the switching power supply comprising: a transformer with a primary-sided (PW) and a secondary-sided (VS) main winding, wherein electrical energy pulses are transmitted from the primary-sided to the secondary-sided main winding, a primary-sided switch (T 1 ) for switching on and off the primary current through the primary-sided main winding in response to a control signal (G) of the control circuit, a primary-sided auxiliary winding (HW), in which a voltage pulse is induced after switching off the primary-sided switch, wherein the level of the voltage pulse depends on a load at the secondary side, a sample and holding device (S&H) for sampling and storing a level of the voltage pulse for generating a control variable, wherein the control signal is generated on the basis of the control variable, wherein the control circuit comprises a primary current comparator ( 102 ), which determines a switch-off point in time of the primary-sided switch depending on the primary current, and the control variable is fed back to the primary current comparator such that the switch-off point in time of the primary-sided switch can be influenced additionally depending on the control variable such that the duration during which the primary current flows through the primary-sided main winding is reduced differently.