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bq77908A: OCD/SCD fault recovery possible with partial load?

John M
Expert 2135 points
Other Parts Discussed in Thread: BQ77908A

Can the bq77908A recover from a OCD/SCD fault if the load has not been disconnected entirely but just dropped significantly in value?

We have an application where we'd like to shut off the discharge FETs at 100A-200A (depending on pack configuration) but have the FETs turned back on as soon as the load drops back down to its regular operating level, anywhere from 5A-30A. The load would not be disconnected, only reduced, after a short circuit or overcurrent event. SOR would be set to zero.

After reading the datasheet it seemed that the load needed to be completely disconnected for recovery to occur but I was hoping, if that's true, that a workaround existed.

  • 0
    TI__Guru 54715 points

    When the bq77908A turns off the FET, current will stop unless the FET goes into avalanche mode.  Recovery from overcurrent with SOR = 0 is determined by the voltage on DPCKN.  Normally DPCKN will go high on overcurrent and removal of the load will let it pull back down to normal voltage.  The device does not have a way to sense current with the FET off, and it does not have a way to sense current below a threshold.

    If you manipulate DPCKN, you can control when the part recovers.  If you hold DPCKN low such as happens with a large capacitive load, the part will recover very quicly and will cycle the FET. Similarly with SOR = 1 you could ground DPCKN and use CHGST for recovery, but check the fault conditions affected.   Since the part does not communicate a status of the fault, it could be difficult for a controller to know the cause of the protection, but it could attempt recovery in a suitable pattern for the characteristics of the load, such as limiting re-tries and time between them to a time known not to overheat the FETs.

  • 0 in reply to WM5295
    Expert 2135 points

    Looks like what we want to do will be problematic, if not practical at all. We'll carefully consider the options you mentioned and see what might work best for our application. Thank you for your time and information!