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BQ76200: discharge Mosfet M7-M11 are damaged when do short circuit with BQ76200

Frank Xiao
Genius 12055 points
Part Number: BQ76200

Hi team,

My team is using the BQ76200 now, when they do short circuit test at pack side. When short M42 and D36, the MOS can be protected, when add M42 and D36, the discharge MOS M7-M11 will be damaged when do short circuit.

The short circuit test is 267A 200us.

 

Could you please give some suggestions?

Thanks.

  • 0
    TI__Guru 54715 points
    Hi Frank,
    The function of the DSG pin is to switch between the VDDCP level when on and the PACK pin level when off. With D36 and M42, at initial turn off the bq76200 will pull DSG to PACK-, as M42 begins to turn off it becomes a diode and pulls less on the discharge FET gates. The gates of the discharge FETs will be pulled down until they begin to turn off and PACK+ will begin to fall. However D36 will prevent the PACK pin from falling, so the bq76200 PACK and DSG pins will remain at a high voltage while the D36 and M42 diode are reverse biased and the discharge FETs operate as source followers and slowly fall toward GND. The FETs will likely heat and be damaged.
    If you can tolerate the current you might put a pull down resistor at D38. This will help the PACK pin to travel to GND as the system turns off and DSG should continue to pull down the gates through the M42 body diode.
    Some designers implement a SCR type function attached at the FETs to pull down the discharge gates, it will pull down fast but can be a problem if it is triggered unexpectedly.
    Figure 3 of www.ti.com/.../slua795.pdf shows a P-channel FET which travels with the power FET to turn off its gate. Some variation of this might be helpful in your situation, this exact circuit may not be suitable due to the propagation delay introduced. Added propagation delay would be undesired with short circuit. M42 conducting during the initial switching may avoid some of that delay.