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BQ76942: OCC Protection---Charge pump voltage leakage via DISCHG FET gs

Fabio Gou
Intellectual 2970 points
Part Number: BQ76942

The  customer  is using BQ76942 test OCC (Over current charging function).

When OCC happen, the CHG FET will be closed, but the Discharge FET will keep turning on.

Then the charger is removed. The pack+ voltage will keep higher than BAT+.

Because the DSG pin voltage will leak to PACK+ via DISCHG FET GS 10M resistor.

So the Pack+ voltage will keep the high voltage state and  it will not recover from OCC if there is no load in the pack+ and pack-.

Is there any good solution?

  • 0 in reply to Fabio Gou
    TI__Genius 17370 points

    Hi Fabio,

    It sounds like the DSG FET is turning on due to the body diode protection features of the device. This normal and expected during a charging current when there are no other faults present that would turn of DSG.

    I recommend removing D10 and having the PACK pin connected to PACK+ with only R37. The diode is likely interfering with the charger removal detection.

    Regards,

    Max Verboncoeur

  • 0 in reply to Maxwell Verboncoeur
    TI__Intellectual 2970 points

    Hi Max, the customer have removed D10. It has the same issue.

    I guess this should be a common issue.

    Because from DSG →10M→discharge fet (in half on state)→charge fet (MΩ，in off state) this power path,

    the voltage of discharge fet (S pin) and Pack+ pin will be larger than battery. It is a reasonable phenomenon?

    Is there any good solution to solve this?

    Thanks

  • 0 in reply to Fabio Gou
    TI__Genius 17370 points

    Hi Fabio,

    Could you send your .gg config file?

    Also, R34 should be 10MOhms, not 1MOhm.

    Regards,

    Max Verboncoeur

  • 0 in reply to Maxwell Verboncoeur
    TI__Intellectual 2970 points

    Hi MAX, the customer is using 10M in their board.

    this issue is very common. Because we can explain this from Hardward design perspective.

    The DSG pin voltage= Bat voltage+10V.

    DSG →10M resistor →discharge FET (in half on state)→charge fet (MΩ，in off state).

    In this power path, the Pack+ pin voltage will be higher than Battery absolutely.

    Could you pls ask Terry or Arelis about this issue? I guess there is similar issue in the before.

    Whether we have some suggestions?

  • +1 in reply to Fabio Gou
    TI__Genius 17370 points

    Hi Fabio,

    I think I see what you mean now. When the DSG FET is on and PACK+ is floating, the small amount of leakage current through the GS resistor of the DSG FET can pull PACK+ above the BAT voltage.

    There are two recovery methods for the OCC protections however (see below). 

    • The PACK pin voltage dropping below the BAT voltage minus a programmable threshold. This method likely won't work without a load present between the PACK+ and PACK- terminals since there is nothing to pull PACK+ to PACK-. You could add a small load to prevent PACK+ from being pulled up through the GS resistor, but that would add some leakage current.

    • The detected charge current dropping below a programmable threshold for a programmable amount of time. This method is likely the method that will work better for you without a load.

    Regards,

    Max Verboncoeur