This invention relates to a circuit arrangement for forming a direct voltage from an essentially sinusoidal input voltage comprising a filter for suppressing high-frequency interference signals, a switched mode power supply part which is composed of a diode, a coil, a capacitor and a transistor and is coupled to the filter through a rectifier, while its elements are arranged so that in the conductive state of the transistor the diode is cut off and the coil current flows at least through the transistor and in the cut-off state flows through the diode and a parallel arrangement of a load and the capacitor, and a pulse generator which forms from the input voltage switching pulses for the transistor, whose frequency is varied uniformly in time between a minimum frequency at the maximum value of the rectified input voltage and a maximum frequency at the minumum value.
Such a circuit arrangement is known from German Offenlegungsschrift No. 26 52 275 and may be used, for example, as an electric ballast unit for gas discharge lamps. The filter is connected to an alternating voltage mains and derives therefrom a substantially sinusoidal current. The transistor is controlled so that the transient duration is constant, that is to say that the transistor is switched on again when the coil current reaches the value zero. The variation in time of the frequency of the pulses controlling the transistor is parabolic during a half cycle of the input voltage. The maximum value of the frequency is reached at the minimum value of the input voltage and the minimum value of the frequency is reached at the maximum value of the input voltage.
Depending upon the switching processes in the combinatorial circuit part, interference voltages are superimposed on the alternating voltage. These interference voltages occur above the minimum frequency of the pulses and become manifest in a disturbing manner especially in the low frequency range. Such interference voltages are reduced to given tolerable values by the filter connected between the input of the circuit arrangement and the rectifier. In general, such a filter is composed of a coil and a capacitor.
Permissible values for such interference voltages are defined in the VDE standard 0871. The maximum interference voltages prescribed in the VDE standard 0871 are to be taken into account in the proportioning of the filter. It has been found that especially the interference voltages at low frequencies determine the size of the coil of the filter.