Abstract: A high efficiency, low voltage, high current, half-bridge, series resonant, multiphase, DC-DC power supply includes a combination of a boost type pre-regulator and a “base-sub” half-bridge, series resonant converter unit. The base-sub converter unit includes two half-bridge converters, connected in parallel to the boost type pre-regulator, and having outputs that are combined to produce the required output power. The DC-DC power supply has inherently a Zero Voltage and a Zero Current switching mode feature. The output voltage is the sum of the input voltage and of an additional (“Delta”) voltage, whose amplitude varies in order to insure that the output voltage is fixed and stable. In operation, the boost type pre-regulator converts only the additional “Delta” voltage, leading to a significant increase in efficiency of the boost type pre-regulator, and of the entire power supply.