18 16 khz f 1 19 37 khz and f 2 138 59 khz the converter in an embodiment of the invention may be used between frequencies f 1 and f 2 between which the gain varies from 40 to 80 decibels to enabling varying the voltage and power at the converter output the operation is shown in fig3 where the series current is as a function of time is being measured by device 20 as follows the instant when switch t 1 or t 2 begins to conduct is defined with respect to the instant x k x k 1 x k 2 or x k 3 of end of conduction of the preceding switch which corresponds to a passage to zero of the series current is along one direction or the other by counting respectively a period t k u2212 1 t k or t k 1 accordingly the start of conduction of switch t 2 is determined by counting down a duration t k that was calculated at the time of the preceding cycle during the conduction time of diode d 2 referred to as a freewheeling diode this aspect of calculating durations t is shown in the diagrams of fig4 thus fig4 a shows curves for the series current is full lines and parallel current ip broken lines upon starting a supply sequence after starting for example a radiological x ray image acquisition and in a stable mode switch t 1 for instance is conducting a calculation of duration t 4 is carried out during interval 4 and its value is counted down from the instant x 4 when the.