A bi-directional dc-dc converter is provided that may provide voltage conversion for two separate electrical power systems. The two electrical power systems may have different functions, electrical requirements and power transfer directions. The bi-directional dc-dc converter may include back-to-back bi-directional dc-dc converter circuits isolated from each other by a transformer. Multiple such dc-dc converters may be connected in parallel to increase power capability. Phase shift pulse width modulation (PWM) may be used to switch the parallel dc-dc converters so as to decrease both voltage ripple and current ripple. The number of dc-dc converters may be modified to meet the different needs of various electric power systems. A single bi-directional dc-dc converter of the invention may be employed to provide electric power conversion for multiple electric power systems, even where those systems have different power requirements and different power transfer directions.