A blocking-oscillator-type converter circuit works with a transformer that has only one primary winding and one secondary winding. The transformer contains no reaction coupling winding for the blocking oscillator.The control voltage for the oscillator is derived from the primary voltage of the transformer, that is, during the free-running phase.Furthermore, a voltage monitoring circuit is provided that works independent of the oscillator, inhibiting the output voltage of the oscillator when the voltage on the output of the blocking-oscillator-type converter is too large.A current monitoring circuit works independent of the oscillator and the voltage monitoring circuit and inhibits the pulses for the power transistor when the current on the output side exceeds a specified amount.