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BQ76942: External mosfets for cell balancing

Richard Cooke
Expert 1530 points
Part Number: BQ76942
Other Parts Discussed in Thread: BQ76952

Hello,

We need a BMS circuit to handle an 8S LiFePO4 battery pack.  We'd like to use your BQ76942 chip but have questions regarding the external mosfets used for cell balancing.  Can I use 8 N-Ch mosfets?  Or do I need to use N-Ch for cells 1-5 and P-Ch for cells 6 - 8?  I'm asking as we've used a competitor's chip (ISL94202) in the past and they required the N-Ch (cells 1-5) and P-Ch (cells 6 - 8).

Also What type of Zener diode should we use at the gates of the external balancing mosfets?  Page 5 of SLUAA81 (Cell Balancing with BQ76952, BQ76942 Battery Monitors) shows the zeners but does not describe them.

Thanks,

Richard

  • +1
    TI__Prodigy 720 points

    HI Richard,

    Yes, you can use 8 N-Ch mosfets instead of combining N-Ch and P-Ch. 

    For the Zener diode, these are to protect against transients which may occur if there is a short across the pack. You will want a zener diode with a breakdown voltage (Vz) of less than that of the FET max gate voltage. We recommend a zener voltage (Vz) of 5.6V.

    Hope this helps.

    Best,

    Andrew