A power-supply includes a power-supply input. A transformer has a primary winding and a secondary winding separated galvanically from the primary winding. A controlled semiconductor switch has a control input in series circuit with the primary winding, which lies parallel to the power-supply input. A controllable control circuit for the semiconductor switch delivers a periodic control signal, which is controlled such that the output voltage on the secondary winding is kept at a desired value independent of the load, to the semiconductor switch. A voltage limiting circuit lies parallel to the input side of the primary winding. An output signal of the voltage limiting circuit acts on the switch state of the semiconductor switch to move the semiconductor switch to a stationary state, in which it generates no alternating current on the primary winding, when the voltage on the primary winding has exceeded a specified limiting value.