Multi-mode Control of the Energy Transfer Unit in Capacitively-coupled Hybrid Battery Packs for Enhanced Auxiliary DC Bus Regulation

Recently, a novel capacitively-coupled hybrid battery energy storage architecture has been presented to achieve reduced size and weight for electric vehicles (EVs). Due to capacitive coupling, an energy transfer unit (ETU) is typically employed to balance energy between the energy-dense and the power-dense battery packs. The ETU may also be used to regulate the vehicle’s low-voltage (LV) auxiliary direct current (dc) bus using low-power converters such as dual-active-bridge (DAB) dc-dc converters. Existing strategies use the DAB converters connected to the energy-dense pack to regulate the LV auxiliary dc bus voltage and the DAB converters connected to the power-dense pack to maintain an average state of charge in the power-dense pack. This approach results in lost regulation of the LV dc bus in EVs when encountering high dynamic or heavy auxiliary loads or when confronted with increased power requirements stemming from augmented auxiliary power systems. To address the challenge, this paper proposes a novel multi-mode control strategy for the ETU that delivers enhanced LV dc bus regulation under various operating scenarios. The effectiveness of the multi-mode control strategy under different load conditions and transients has been verified through hardware results of a 500 W ETU prototype consisting of two DAB converters with a 12 V auxiliary dc bus.