being reached according to the present invention circuit 15 controls switch k so that it is only closed at the beginning and at the end of each positive halfwave of voltage vac during periods t 0 t 1 at halfwave beginnings and t 2 t 3 at halfwave ends switch k u2032 is open during these periods between times t 0 and t 1 diodes d 1 and d 3 are forward biased accordingly the current flowing through switch k charges capacitor c 1 fig4 d as well as capacitor c 3 fig4 b however capacitor c 2 continues its discharges into the load not shown connected thereacross fig4 c at time t 1 circuit 15 turns off switch k after detecting that voltage v has reached the desired value v 0 capacitor c 1 starts discharging as it provides voltage v to a load the load supply power is then provided by capacitors c 1 and c 2 the supply voltage across capacitor c 3 however remains constant between t 1 and t 4 and between t 5 and t 2 at a time t 4 slightly subsequent to time t 1 circuit 15 turns switch k u2032 on this results in a discharge of capacitor c 3 into capacitor c 2 by path c 3 k u2032 c 2 d 2 thus the voltages across capacitors c 2 and c 3 are balanced preferably a circuit not shown in fig3 a adapted to ensuring a constant current discharge is provided in series with switch k u2032 the turn on delay of switch k u2032 with respect to the turning off of switch k times t 1 to t 4 aims at avoiding any risk of simultaneous conduction of these.