A dc-dc converter employs a series resonant circuit (23,25) which is driven by square waves of current. By adjusting the frequency of the square waves of current above the resonant point or by holding the frequency constant and varying the current supplied to the converter, the voltage on the resonant components can be stepped up and controlled. Switching losses in the inverter devices (6) are low since the inverter feeds a lagging load which allows the use of lossless snubbers (15,17,19,21).