SLUAAP7 January   2024 BQ76905 , BQ76907

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Cell Balancing Circuit Considerations
    1. 2.1 Internal Cell-Balancing Circuit Design
    2. 2.2 External Cell-Balancing Circuit Design Using N-Channel FETs
    3. 2.3 External Cell-Balancing Circuit Design Using BJTs
  6. 3Considerations for a Host-Balancing Algorithm
  7. 4Timing Information
  8. 5Debugging Common Issues With Cell Balancing
    1. 5.1 Using a Resistor Divider as a Cell Simulator
    2. 5.2 Cell Balancing Troubleshooting
  9. 6Summary
  10. 7References

External Cell-Balancing Circuit Design Using BJTs

External FETs are suited for applications with typical 4.2V lithium ion cells since balancing is most commonly done at higher voltages during charge. For applications that need higher balancing current than the internal balancing can provide but also need to balance at lower cell voltages, external BJTs may be considered. The balancing current for an external BJT can be controlled by selecting the appropriate balance resistor (Rbal) and base resistor (Rbn). In Figure 2-5, as the internal FET is turned on inside the device, the current flowing through Rbn puts the NPN transistor into saturation.

GUID-20230517-SS0I-LSJR-V5LV-RKXVXKWSLKVV-low.svg Figure 2-5 Balancing Circuit Using External BJTs

A Zener diode is also used in this circuit to protect from pack transients similar to the FET circuits. A standard diode is also suitable to use instead of a Zener when using BJTs because it is the forward voltage of the Zener that protects the transistor. The base-emitter diode (or emitter-base diode for a PNP) will conduct in the reverse direction which will prevent the Zener from conducting.

Note: The Zener diode might not be needed since the BJT comes with a built-in diode.

In Figure 2-6, the circuit was designed with an Rn of 100Ω and Rgn of 240Ω. The Rbal resistor is set to 50Ω for a balance current of approximately 74mA through the BJT 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 89mA. An NPN transistor was selected with hFE of 30 at IC = 100 mA.

GUID-20230503-SS0I-B6FP-4P22-5L0HS8W9MX1T-low.png Figure 2-6 BQ76907 Cell Balancing With NPN BJT, Cell 7 (VC7) Rbal_voltage Approximately = 3.68V
Note: In Figure 2-6, the positive duty cycle represents the voltage across the Rbal resistor while balancing cell 7 (VC7). While the negative duty cycle represents the voltage across Rbal resistor when balancing is OFF.