For applications that need higher cell balancing current, external FETs are often used. When using external FETs, the cell input resistors can be increased to the maximum recommended value of 1kΩ. Increasing the resistor size will help to provide enough voltage across the gate of the FET. In Figure 2-3, as the internal FET is turned on inside the device, the current flowing through R_n-1 provides the V_GS for the external FET.

Figure 2-3 Balancing Circuit Using External N-channel FETs

Care must be taken to select an external FET with a low R_DSON defined at low V_GS. For example, say the minimum cell balance voltage is 3.9V. The external FET can have an R_DSON defined at or below 3.9V x 100 / (100 + 100 + 80) = 1.39V.

A Zener diode is needed to protect the external FET gate from pack transients. For example, in the event of a short across the pack in a 7-cell battery, Cell 7 would have approximately 28V across R_n during the event and the opposite transient at the release of the short circuit. The gate voltage can be connected through a resistor to limit the current when the diode conducts. (During normal operation the Zener will not conduct).

For Figure 2-3, the circuit was designed with an R_n of 100Ω and R_gn of 1kΩ. The R_bal resistor is set to 50Ω for a balance current of approximately 77mA through the external FET at 4V. At this cell voltage, an additional approximately 15mA of current flows through the internal FET of the device for a total balancing current of close to 92mA. An N-channel MOSFET was selected with an R_DSON defined for low V_GS down to 1.4V.

Figure 2-4 BQ76907 Cell Balancing With N-Channel FETs, Cell 7 (VC7) Rbal_voltage Approximately = 3.86V