A Hierarchical ZVS Battery Equalizer Based on Bipolar CCM Buck-Boost Units

Conventional battery equalizers rely on unipolar continuous-conduction mode of power electronic converters. This causes substantial energy losses during the switching transitions. To cope with this issue, this paper proposes a novel hierarchical battery equalizer based on bipolar continuous-conduction mode buck-boost units. In the proposed structure, the inductor current is controlled to enter into the negative region at the end of each switching period. Thus, the body diode of power MOSFET provides a freewheeling path for the inductor current during the dead band. This ensures zero-voltage switching of both power MOSFETs. Therefore, the switching losses are significantly reduced. Meanwhile, the integral of the inductor negative current is precisely controlled to minimize the circulating current. This guarantees both minimized conduction losses and suitable equalization speed. The operation principle and control strategy of the equalizer are analyzed. The comparison of equalization speed between different equalizer is presented. An experimental prototype to balance four series-connected Lithium-ion battery cells is implemented. Both simulation and experimental results validate the functionality and analysis of this battery equalizer.