A converter for converting an input voltage (U.sub.i) between a first
supply terminal (1) and a second supply terminal (2) to an output voltage
(U.sub.0), which converter includes switching means (S.sub.0) which, in
the operating state of the converter, are alternately switched on and off
under the control of a control signal (V.sub.cntrl), an inductive element
(T) which, in conjunction with the switching means (S.sub.0), forms a
series circuit which is coupled between the first supply terminal (1) and
the second supply terminal (2), a control circuit (CNTRL) for supplying
the control signal (V.sub.cntrl), and evaluation means (EVMNS) for
evaluating a voltage (U.sub.S) across the switching means (S.sub.0), which
voltage (U.sub.S) exhibits ringing, and for supplying the control circuit
(CNTRL) with information causing the switching on of the switching means
(S.sub.0) during a given valley of the voltage (U.sub.S) across the
switching means (S.sub.0). The converter remains locked to said valley for
as long as the frequency of the control signal (V.sub.cntrl) is within a
given frequency window. The converter also includes a frequency control
circuit (FC) for controlling the frequency of the control signal
(V.sub.cntrl) within the frequency window. If the frequency reaches the
lowest frequency or the highest frequency of the frequency window, the
information is adapted in such a way that the converter is locked to
another valley of the voltage (U.sub.S) across the switching means
(S.sub.0), so that the frequency of the control signal (V.sub.cntrl)
remains within the frequency window.
