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BQ24725A: ACFET get burned during start-up

Edward Lee
Intellectual 1655 points
Part Number: BQ24725A

Hi Team,

        My customer uses several bq24725a on their board.

        They found the ACFET get burned during start-up in some of them.

        After capturing the waveform of ACDRV, CMSR, Vin and the current of Q1, they observed the Vin drops before the inrush current occurs.

        This will induce up to 50A inrush current and get Q1 burned.

  

        Is it possible the wrong parameter of ACFET like lower vth or high gain (Ids vs vgs) causes the short through during start-up?

        For now, it seems that after replacing the ACFET with lower gain one, the waveform is back to normal and the inrush current is reduced to 7A.

        It's kind of you give us some directions.

Thanks

Sincerely,

Edward

  • 0
    TI__Guru**** 165886 points

    HI Edward,

    The FET is used as a switch so I do not see how its small signal gain affects performance.  I suspect is related to the FETs safe operating area and the soft start turn on time induced by ACDRV and the additional capacitance from gate to source.  The FET datasheet should have a curve showing the amount of current for a specified time.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 1655 points

    Hi Jeff,

        Thanks for your prompt reply. 

        The 50A inrush current lasting for 30us with the VDS of ACFET around 15V really hits the SOA of FET.

        I will help customer to enlarge the capacitance between gate and source of ACFET to see any improvement.

    Thanks

    Sincerely,

    Edward

  • 0
    TI__Mastermind 25800 points

    Hey Edward,

    If you are seeing such a high inrush current, it likely means the BATFET was still in the process of turning off while the input ACFET/RBFETs were turning on. 

    There has to be sufficient turn-on delay between the BATFET turn-off and ACFET/RBFET turn-on. 

    As such, if if there is too much delay on the BATFET turn-off, or too little delay on the ACFET/RBFET turn-on, it is likely that a scenario like this will arise.

    Regards,

    Joel H