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BQ77905EVM-707: Doubts regarding BQ7790518

Goutam Ghosh
Prodigy 100 points
Part Number: BQ77905EVM-707

Tool/software:

Hi,

I am planning to use BQ7790518 for charging OV protection of a 8S battery pack. In the EVM, I have tested the charging scenario and made one of the CELL voltages greater than 4.2V. The circuit is shown below. My doubt is when the CHG MOSFET is OFF, there is still the battery voltage - the antiparallel diode voltage of the CHG MOSFET is seen, however the current is zero. Is it because of the MOSFET package? According to my understanding, after cutting off the voltage across the charger terminal (PACK+/PACK-) should have been OL means open.

  • 0
    Prodigy 100 points

    According to my understanding, after cutting off the voltage across the charger terminal (PACK+/PACK-) should have been OL means open.

  • 0 in reply to Goutam Ghosh
    TI__Mastermind 42060 points

    Hello Goutam,

    In an overvoltage condition, the CHG MOSFET will turn-off, this would still allow the device to discharge if needed, so the output is not fully floating, it may measure the battery voltage minus a diode drop.

    This is normal and it is how the device works, to allow discharge in this scenario.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 100 points

    Hi Luis,

    Thanks for your reply. Hope in this scenario until the device recovers ( if the voltage comes down the CHG MOSFET will turn back on), the charge current will never flow.

  • 0 in reply to Goutam Ghosh
    Prodigy 100 points

    Hi Luis,

    Also wanted to understand, how the device achives this as, you take away the Vgs of the CHG MOSFET in a scenario of OV, still the path remains conductive for reverse current direction? I have tried to replicate this scenario without the device using just a Mosfet in the negative path, every time the MOSFET turns OFF, the entire circuit is isolated.

  • +1 in reply to Goutam Ghosh
    TI__Mastermind 42060 points

    Hello Goutam,

    Yes, the gate of the CHG pin will begin to pull towards Vss, then it will be Hi-Z. So the voltage across the gate-source would essentially be 0-V. This would turn off the FET and charging current cannot flow. 

    In the other direction, it can conduct, so discharge is still allowed unless the DSG FET is turned off. The load circuit is isolated until current begins to flow through the body-diode of the DSG FET.

    Best Regards,

    Luis Hernandez Salomon