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BQ76920: External balance circuit not apparently working

Nicholas Belanger
Prodigy 95 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ76930

I have a board with a BQ76920, and external balancing circuit that was designed after http://www.ti.com/lit/ug/slvu925c/slvu925c.pdf

When setting balancing bits, and reading back the register to confirm that the bits are set, there is no apparent difference in current being drawn from that cell (scoping 1 ohm shunt) even though we should see between a 30-40 mA increase. Upon further probing, the FET never appears to be set. The source-drain voltage is that of the cell, and the gate voltage was about equal to the source voltage. This stayed steady as the balance bit for the corresponding flipped on and off.

  • 0
    Prodigy 95 points

    Update: The the FETs corresponding with other cells (at least 2 and 3) are being driven and balancing is working as expected, but the gate of FET 1 stays high even as the bit switches on and off.

  • 0
    TI__Guru* 81034 points

    Hi Nicholas,

    The cell balancing should work. Be sure to use a low impedance probe. Otherwise, I assume you are able to read and write from the I2C of the device with no issue?

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 95 points

    Hi Matt,

    We are able to read to the I2C of the device, read the voltage etc. We are also able to balance the cells 2,3,4 and 5. However, cell 1 does not balance. We probed the gate of the Q10B and found out that the gate to source voltage was close to zero. We measured the voltages between Probe A and B points on the image below when the balancing was enabled for cell 1 and found it to be exactly the same when balancing was disabled.

    This was however, not the case for cell 2 (Probe B and C), cell 3,4 and 5.

    Is this a damaged BQ chip or are there other variables we need to debug ?

    Please note that though the image below shows BQ76930, we are using the BQ76920 with very similar circuit.

  • 0 in reply to Nicholas Belanger
    TI__Guru* 81034 points

    Hi Nicholas,

    If you are following the same circuit for the BQ76920, you should not see this issue. I think it may be good to replace the device and make the measurements again. If you continue to observe the issue after changing the device, please send your schematic so we can check if there is anything that may have been missed.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Guru* 81034 points

    Hi Nicholas,

    Let me know if you have any update on this issue.

    Thanks,

    Matt