Closed-loop Control of MOSFET Gate Voltage for Charge Balance in a Smart Li-ion Battery Cell

A smart battery (SB) cell integrates a half-bridge topology with Li-ion cells using very low on-resistance MOSFETs. The half-bridge provides additional degrees of freedom such as bypassing or inserting the cell in a pack depending on the cell state-of-charge (SoC) and state-of-health (SoH). In this paper, a new operation mode is proposed where the gate voltage of the bottom MOSFET is controlled in closed-loop to provide additional current path, which can be made independent of the charge/discharge current magnitude. This controllable additional current through the bottom MOSFET provides a new approach to balance the SoCs of the batteries without the requirement of a full-scale dc-dc converter. The current magnitude can be controlled based on the amount of SoC unbalance and the time required to balance. The parameters of the analog controller to get the desired transient response are determined. Since this operation mode introduces additional power loss, a thermal analysis is performed to optimize the balancing current and the temperature rise. The results are verified using the real device models in a spice simulation environment.