Abstract: A blocking oscillator contains a switching transistor which is driven by an integrated circuit. This circuit receives its supply voltage during the blocking phase from a winding at the primary side. A further winding at the primary side supplies the regulating variable for the integrated circuit during the flow phase. The supply input of the integrated circuit is connected by way of a further transistor and a diode to the control winding and a Zener diode biases the base of the further transistor. In the start-up phase in which the voltage of the supply winding (control winding) is built up, the circuit supplies the required operating voltage and switches off again in the steady state of the power unit.