The paper presents a dc-link voltage control scheme by which the power losses associated with the power electronic converters of a series hybrid electric vehicle (HEV) powertrain are reduced substantially. A dc-link commonly connects the three powertrain branches associated with series HEVs, presently interfaced by a three-phase rectifier, a three-phase inverter, and a dual-active bridge (DAB) dc-dc converter. Dynamic efficiency models of the converters are developed, and a methodology is proposed by which the dc-link voltage is varied with respect to its default value, based on the ratio between the battery and dc-link voltages. The voltage control scheme introduced varies the phase shift between the gating signals of the two DAB converter bridges, proportionally to the ratio of converter input voltage to output voltage referred to the transformer primary. This level of instantaneous control forces the converter to operate in boost mode when the battery charges and buck mode when the battery discharges, allowing the converter to persistently avoid hard switching losses over its entire operating range. The control scheme is tested in simulations with a full HEV model by comparing its performance with constant voltage and unity voltage conversion ratio PI control schemes. The scheme proves most effective for vehicles with high hybridization factor driving in an urban environment.