Electrical power from an input voltage supply is converted to first and second output voltages of opposite polarities using a single inductor (L) and only four principal switches (S1, S2, S4, S6). In contrast to known circuits, none of the switches is exposed to voltages greater than the input voltage (V1). In a first type of charging cycle (FIG. 5(a)-(c)), the first output voltage (V2+) is obtained from the input voltage supply through the inductor. In a second type of charging cycle (FIG. 5 (d)-(f)), the second output voltage (V2-) is obtained from the first output voltage via the intermediate step of storing energy in the same inductor as is used in the first type of charging cycle. Auxiliary switches (S7a, S7b) can be operated in wait states between cycles of the first and second type.