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BQ76940: FETs die after first recovery time while doing SC test when using bq76200 , bq76940 and bq78350 in Battery Management System

Medd
Prodigy 45 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350, BQ76200,

Hi;

I have a Costum BMS which consist of bq76200 , bq76940 and bq78350 ICs. My problem is that All FETs are burning while making short circuit test. The problem is occuring when IC tries to exit from recovery mode. In other words, When SC occurs, FETs shut down the output and wait for 5sec ( bq78350 recover time ). After that, while SC condition is still present, FETs are burning(Short) and fuse is blowing.

I have 7 CHG and 7 DSG parelel FETs and CHG/DSG resistors for bq76200 are 100 ohm. The protection level for bq78350 ( ASCD) is in the minumum level. ( 70us, 176 A ). 

If you further interest, I can share the Schematic design file.

Thank you.

  • 0
    TI__Guru* 80963 points

    Hi,

    This number of FETs may have very high capacitance depending on the FET. The FETs may not be turning on fast enough or the BQ76200 may be getting pulled into a UVLO condition cycling the FETs. If you have  pre-discharge FET, it may help (this requires the BQ78350-R2 firmware which enables a PDSG pin). Are you using separate gate resistors for each FET? The charge pump capacitor also may need to be larger depending on the FET capacitance. 

    This application report on using multiple FETs in parallel is very helpful. Not sure if you have already seen this: https://www.ti.com/lit/an/slva729a/slva729a.pdf

    Best regards,

    Matt