Abstract: A circuit for the conversion of an unregulated input voltage (UE) into a regulated DC output voltage (UA) is specified, with which the input voltage (UE) can assume both a higher and a lower value than the desired output voltage (UA). In addition to a set/actual value comparator (SIV), a current simulation (SNB) is provided, which simulates the current (JL) in the storage choke (L2) during charging as capacitor voltage, a common driver circuit (TRS), designed as switched constant current source, for both semiconductor switches (V1, V2), an input voltage monitoring (ESU), which switches off the switched-mode regulator when there is too low an input voltage (UE) and an output monitoring circuit (ASU), which reduces the set value of the regulating circuit on crossing in the downward direction of a selectable threshold.

Abstract: In a free-oscillating converter for the current supply of a monitor, the switching frequency of an electronic switch is positioned midway between two consecutive multiples of the line frequency of the monitor, which obviates the necessity for a technically and financially expensive synchronization between switching frequency and line frequency. In the operating range of the converter, it is sufficient to adhere to the switching frequency with an accuracy of a few hundred Hz (f.sub.s .apprxeq.1.5+x).multidot.f.sub.z, where x.epsilon. .sub.o, f.sub.s =switching frequency and f.sub.z =line frequency.

Abstract: A switch controller has a regulating path formed of an inductance and a smoothing capacitor. A DC potential at the smoothing capacitor is connected to an amplifier which serves for amplifying a difference between this DC potential and a prescribed reference potential. A following pulse-width modulator for the control of a switching transistor is connected to the amplifier. In accordance with the invention, an auxiliary regulating path is connected parallel to the regulating path. The auxiliary regulating path is designed such that it has a maximum phase rotation of -90.degree. in its phase response.