Integrating an energy storage system and choosing an architecture solution that best fits the application requires extensive concept design and evaluation in the case of hybrid electric propulsion. Hybrid electric vehicles are being designed to use a high-voltage DC bus (> 300 VDC). To supply high power to electrical subsystems normally requires variable-voltage bidirectional DC/DC converters to interface between the main storage battery pack and the high-voltage bus. Possible architectures include (a) direct battery pack connection to the high-voltage bus, (b) a single bidirectional DC/DC converter with a high-voltage battery pack, and (c) multiple sets of low-voltage battery modules each in series with a low-voltage, highcurrent DC/DC converter with the entire arrangement connected in a parallel configuration to the high-voltage bus.

There are pros and cons in each option in terms of redundancy, reliability, safety, and cost. Use of a DC/DC converter has a potential to improve the overall efficiency, reliability, and safety for a compact system as compared to the direct connection scenario. DC/DC converters must be optimized for high power density. An optimal thermal management system is required since the proposed DC/DC converter designs incorporate recent advances in semiconductor technology that accommodate the increases in power density and efficiency. Achieving a high power density in DC/DC converters creates some technical issues that must be taken into account. These include materials stability at the high operating temperatures and electromagnetic compatibility (EMC) considerations.