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BQ77915: Problem during Short Circuit

Pankaj Yadav1
Prodigy 120 points
Part Number: BQ77915

Hi,

I are facing Mosfet burnout problem during Short Circuit, please let me know correct values for Rdsg,Rgs & Rchg,Rgs for Mosfet details attached.

I am using in 2 BQ7791501 in 10S configuration & using total 4 Mosfets, 2 for charging & 2 for discharging in parallel.

Everything was working good earlier, but now we have to replace Mosfets due to availability issues. 

Is Rdsg=4.5k & Rgs=10Mohm ok?

Is Rchg=1k in series with Protection Circuit(Diode in Parallel 1M resistance) & Rgs=3Mohm ok?

Also Using 220E on each Mosfet Gate 

Also Using Rld=450k,

& 2 100nF Capacitors in series on Source to Drain of Mosfet.

Please recommend any changes required

Also using RC filter on Sense Resistor.

  • 0
    TI__Guru 54715 points

    Hi Pankaj,

    Unfortunately it is not obvious what will fix the issue.  Since the design apparently worked well before the FETs were changed it is apparently some FET characteristics.  Your component values seem ok but the new FETs may need some adjustment.  You will likely have to test many times to find the issue which is bothering them, it can be very frustrating.

    At short circuit the FETs will heat, if RDS on is higher on the new FET it will be hotter and may be at risk.  Of course check the current capability of the FETsjl

    The Ciss will act with the RDSG (4.5k in the picture) above to set the turn off.  For the new FETs you may need a faster turn off. 

    A fast turn off can excite the cell inductance and cause a large voltage rise on the cells, PACK+ and through the short to PACK-.  The FET may go into avalanche and must take the power/energy if it does.  Sometimes too fast of a turn off is bad for the system, but too slow will heat the FETs.   If the PACK+ voltage spike is very high at the SCD event it may be switching too fast and breaking the FETs from too much avalanche energy. The 4.5k RDSG may need to be larger.  If there is no voltage spike at turn off  the FETs may be switching too slowly heating and burning, the 4.5k RDSG may need to be smaller.  Check the FET ratings compared to the measurements during the SCD event.

    Hope this helps identify the issue.