a switched mode power supply includes a primary coil , a switch element , a secondary circuit , and a control circuit . the switch element is wired in series with the primary coil , for applying a dc voltage to the primary coil according to a control signal . the secondary circuit is coupled to the primary coil with output terminals providing an output voltage . the control circuit prepares the control signal with a feedback signal dependent on the output voltage supplied to the control circuit . the control circuit includes a signal generation circuit to create the control signal and a protection circuit . the protection circuit has the effect that no control signal is delivered to the switch element , if the feedback signal reaches the value of a first reference signal after a first period after starting the control circuit .