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BQ76952: Failure events if current sensing circuit disconnects

Harry McNally
Intellectual 485 points
Part Number: BQ76952


Hello

We are reviewing our designs to consider outcomes from single BMS board failures. We can evaluate and test many protections by shorting (in analysis and actual testing) either the discharge FET (during discharge) or charge FET during charging. If the FET cannot turn off, the CUV, COV, or over-temperature PF protections will trigger.

The battery sense inputs open-wire detection allows the PF to trigger for cell voltage sensing loss.

But if one of the series resistors between the current shunt and SRP or SRN pins develops a dry joint, is there internal "open-wire" like sensing for these pins for the BQ to respond to this failure ?

I haven't been able to find a protection status and event in the registers to show this so we want to work though safety when pack current and short circuits cannot be detected.  

I am arranging to access the functional safety documents but I thought this might be useful as a forum question.

All the best
Harry

  • 0
    TI__Mastermind 42070 points

    Hello Harry,

    No, there is no "open-wire" type of feature for the sense pins.

    The functional safety documentation would describe what would happen if either SRP/SRN pins were to be floating. Off-hand, I would imagine that the voltage could be almost anything, due to leakage currents.

    Typically for secondary current protections, fuses are commonly used. 

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 485 points

    Hi Luis

    Thanks for the clear answer.

    We have a chemfuse on the BMS so secondary protection is in place for short circuit. We need to focus on the rating of that fuse for the analysis.

    The erratic current sense if SRP or SRN is floating is an interesting one.

    I had assumed the BQ would see no current so load current would flow 'unseen' through the discharge FET (on) and charge FET (off) body diode which is our default config. If the body diode heated, the BMS can disconnect on FET over-temperature. If the FET failed short earlier than that and ceased heating, then CUV PF would trigger and protect the cells.

    If charging starts and stops erratically that is a reliability issue and COV still protects the cells if the charger persists every time the BMS re-enables the charge FET.

    Thanks in the other thread for arranging access to the functional safety documents. I think I'll learn and understand more from the work that has already been done.

    All the best
    Harry