A DC voltage transformer includes an oscillator, which can be controlled in terms of a characteristic such as its phase and/or frequency, and a modulation circuit that provides a periodic or random control signal to the oscillator. A power switch is connected in series with an inductance between two power supply potentials, the power switch being switched between open and closed states. A first smoothing capacitor is connected directly between the two power supply potentials, and a second smoothing capacitor is arranged in series with a Zener diode, with both the capacitor and diode being connected in parallel with the main terminals of the power switch. A modulated reference signal form the oscillator is connected to the control input of the power switch. The modulation of the reference signal can be of any type, for example a periodic modulation that is a periodic variation of the phase and/or the period of the reference signal made possible by for example a sine wave. The pulse duty factor of the reference signal is used to establish the relation between an input voltage and an output voltage of the transformer.