A Bidirectional Cell-to-Buffer Battery Equalizer at Boundary Conduction Mode with Constant On-Time Control

Conventioanl adjacent Cell-to-cell battery equalizer is flawed with long energy-flow-path between remote battery cells, which incurs low balancing speed and low energy efficiency. To cope with this issue, this paper proposes a novel Cell-to-Buffer (C2B) battery equalizer. The structure consists of a multplexer network and a bidirectional buck-boost equalizer unit. The multiplexer network selects in the battery cell to link the equalizer unit. The equalizer unit transfers the energy between cell and buffer to achieve targeted equalization. An auxiliary battery cell serves as a buffer to reduce the voltage ratio equalizer unit, which helps to simplify the equalizer unit design. In the equalizer unit, boundary conduction mode (BCM) is achieved with zero current detection (ZCD) and constant on-time control. BCM operation facilitates the zero-voltage-switching (ZVS) of MOSFETs. Instead of absolute equalization, a preset voltage tolerance range is defined to improve the balancing speed. An experimental platform to equalize between battery cell and buffer is built and tested. The function of ZCD is validated. Battery cell voltage is equalized within a preset voltage range via buffer.