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This circuit demonstrates a cell balancing stage for an LFP battery cell.
On the left side, the MCU provides a 3.3 V enable signal. This drives the LED of an optocoupler (modeled here with a voltage-controlled current source and a BJT). The optocoupler ensures galvanic isolation and level shifting, allowing the MCU (referenced to pack ground) to safely control a MOSFET that is referenced to an individual cell in a stacked battery pack.
On the right side, a P-channel MOSFET is connected in a high-side configuration: its source is tied to the positive terminal of the cell, and its drain is connected to a balancing resistor (15 Ω) down to the negative terminal. The resistor dissipates excess energy from the cell during balancing.
A pull-up resistor (200 kΩ) keeps the MOSFET gate normally at the same potential as the source, ensuring the MOSFET is OFF by default. When the optocoupler turns ON, it pulls the gate down toward the cell negative terminal, creating a negative Vgs and turning the MOSFET ON.
In the simulation setup, a 3.2 V source represents the cell under balancing (Cell 3), while an additional 6.4 V source represents two series-connected cells below it (Cells 1 and 2).
Thus, when enabled, the optocoupler drives the MOSFET into conduction and the balancing resistor dissipates the cell’s excess charge as heat, equalizing the voltage with the other cells in the stack.
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