Abstract: The present invention relates to a control circuit for controlling the output power of a primary-controlled switched mode power supply unit, which comprises a primary switch and a transformer with an auxiliary winding. Within the auxiliary winding, voltage pulses are induced by primary-sided switching operations, which can be taken into account when controlling the output power. In order to provide an improved control and an enhanced flexibility with respect to the operational parameters, the sampling instance is determined based on the duration of the voltage pulse at the auxiliary winding during a previous switching cycle. In order to detect a defective contact between the auxiliary winding and the control circuit in a particularly simple and secure manner, the negative voltage pulse at the auxiliary winding occurring when the switch is closed may be used for confirming a proper connection.