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Original question:

BQ76200: Turn OFF time of DSG mosfet is greater than expected

BQ76200: Abnormal discharger waveform

Gabriel Wang1
Genius 3825 points
Part Number: BQ76200

Dear Expert

As for this waveform ,yellow is mosfet Vds waveform ,green is Vgs output waveform . When the MOS is turned off, the waveform is abnormal.

Attached schemetic:

  • 0
    TI__Guru 54715 points

    Hi Gabriel,

    The schematic for the FET array looks rather fuzzy, so I don't know if I see correctly.  It looks like you have options for 2 different FET types.  The common drain point looks labeled OUT?, the pack output name looks cutoff.  The turn off circuit Q34 looks like it references OUT+, not sure if that is the common drain point or the P+..  Be sure the circuit is referenced to the proper potential.

    When BQ76200 DSG turns off it will pull toward the PACK pin level.  This should turn on Q34 which you want to pull the power FET gates to the source level.  It looks like your Q34 may cut off before the gate is discharged. AO3401 has a low Vgsth, but also has a 12V Vgs abs max limit.  You might make sure the FET has not broken.  

    I am a little confused with the yellow VDS trace, when the power FET(s) turn off I would expect VDS to increase.when VGS went low.  Perhaps I am misunderstanding the question.