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BQ76952: Discharge FET Drive Circuit Structures

Yunus Emre
Prodigy 10 points
Part Number: BQ76952

Hello Everyone, 

I am designing BMS for 48V 100Ah battery pack. It must able conduct 100Amps current conntinuously. I am confused about discharge FET drive circuit. There is 2 different circuit. First one just used gate resistor and other one uses pmos for gate drive. What is the difference for usage? Why do we need to pmos to drive discharge FETs. Reference schematics are in the below.

Gate Resistors

PMOS

Thank You,

  • 0
    TI__Mastermind 34450 points

    Hello Yunus,

    The first and second circuit have D6/R32 and D39/R131 respectively, the Zener is to protect the FET gate-source voltage from exceeding maximum ratings during events of large transients.

    Now on to your main question, these circuits in the end do the same thing. They affect the turn-off speed of the DSG FET.

    The first circuit shown, DSG  FET turn-on speed is dependent on R39/R41. During turn-off the D10 Schottky diode will become forward-biased, allowing the turn-off speed to be faster as the DSG FET is now being discharged through R39/R41 and now D10/R40. However, this circuit is still limited by the speed of the BQ76952 driver.

    The second circuit shown, DSG FET turn-on will be dependent on the resistances of the path, during turn-off however it will be dependent on the Q28 FET, which will turn-on when the DSG pin goes low. Once ON, the Q28 FET will discharge the DSG FETs through R135.

    Best Regards,

    Luis Hernandez Salomon