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BQ77PL900: Application using N-channel MOSFET

Karl Nissen
Prodigy 10 points
Part Number: BQ77PL900

Hi - is there an application note that shows how to use the BQ77PL900 part with N-channel enhancement MOSFETs? I'm working my way through the notes at the moment so I may not have located this information yet. The notes and the evaluation board appear to use P-channel enhancement parts.

Thanks,

Karl

  • 0
    Prodigy 10 points

    Sorry - that should be the datasheet for the BQ77PL900 and the documentation for the evaluation board.

  • 0 in reply to Karl Nissen
    TI__Guru* 80543 points

    Hi Karl,

    Are the MOSFETs for balancing or are you looking to use N-channel for the protection FETs? I do not think there are examples for N-channel for the protection FETs (requires FET drivers to drive voltages above battery stack voltage to enable Nch).

    Best regards,

    Matt

  • +1 in reply to Matt Sunna
    Prodigy 10 points

    Hi Matt,

    Sorry I wasn't clear on this - it's the protection MOSFETs that control charge and discharge to and from the cells. 

    A little bit of background - I'm trying to work out why a Gazelle Innergy electric bike battery won't turn on to either charge or discharge. It uses AOB266L protection MOSFETS to control charge and discharge and these are N-channel enhancement devices. The same battery uses the BQ77PL900 to drive the protection circuitry. In most electric bike batteries the negative side of the battery and charger are switched enabling the use of N-channel enhancement MOSFETs and a V(GS) of the correct polarity. N-channel devices have higher charge mobility and lower resistance than P-channel devices so the heat produced at high current drain is less so N-channel enhancement MOSFETs tend to be used exclusively in this application.

    i don't have any technical documentation or a circuit diagram for the battery so I'm trying to work out the control circuitry. V(GS) on both MOSFETs is 0 volts (or millivolts) and if I can work out why I should be able to get the battery working again.

    I think you may have answered one question I had about the board - there appears to be a DC-DC converter on the board which maybe is used to generate a V(GS) voltage higher than the battery voltage to be able to switch the N-channel protection MOSFETs on and off.

    Photo of the BMS board

    You can see the BQ77PL900 to the right of the balance resistors in the middle of the board and the protection MOSFETs at the bottom left (B+) and bottom right (P+). The ferrite cored inductor just above the P+ protection MOSFET could be part of a DC-DC converter to provide the necessary raised V(GS) voltage (above that of the available terminal voltage from the cells) to the protection MOSFETs.

    Thanks,

    Karl