The d.c. to d.c. voltage converter has a transformer (2) with a primary winding (2a), a secondary winding (26) and a reset winding (2C). The d.c. source (1) is coupled to the primary via a switching element controlled by a circuit (11). The secondary circuit has a inductance (5), rectifying diodes (6,7), smoothing inductance (8) and capacitor (9) and the connected load.$ The switching element (3) has a switching action with rise (ton) and fall (toff) times that are related to the system parameters by L greater than E. ton/ID and LM greater than 4 toff to power 2/TT to power 2 Ll. By suitable selection of values the switching speed can be increased and loss reduced.$
