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BQ76940: All balancing MOSFETs run simultaneously

Pawel Wozniak
Prodigy 30 points
Part Number: BQ76940

Hi,

Recently I've made a new version of a BMS for a 10S battery based on a schematic by earlier designer. The problem that I got is that after powering the BMS all but one of the balancing MOSFETs are ON. The one that is not on is on first cell. Given that the battery has 3.73V on each cell, the voltages between VC0-VC1 and so on are just the same. From the software side all balancing is disabled. Any obvious mistakes that I made ?

  • 0
    TI__Genius 10830 points

    HI Pawel,
    Today is Labor Day in the USA. One of our engineers will come back to you in the next few days after we return.

  • 0 in reply to David Hien
    TI__Guru 54715 points

    Hi Pawel,

    The schematic looks normal except the VC1 capacitor is recommended to connect to GND rather than VC0.  Several designs have the connection you show without problems.  The connection should not cause the condition you describe.  For the external balance FETs to be on VC1 through VC4 pins would need to be pulled high and something similar on the upper groups.  I don't know of a behavior which would do this.  Suggestions would be to check the register values again writing cell balance to 0, shut down and re-boot the part, check the pin voltages at the IC, and check the FETs for damage.   On a new assembly process of course check for shorts.  On a new design look for unexpected metal on the layout or pin swaps particularly on the FETs.