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BQ77910 External MOSFET for Battery Balancing

Reid Yamura
Prodigy 10 points

Hi,

I am currently looking at the BQ77910 data sheet and I want to use external MOSFETs for cell balancing (for better heat dissipation and higher balancing currents) as mentioned on pg. 24.  However it mentions for me to contact TI for a circuit example.  I contacted TI and they told me to post the question on here. 

I plan to use this circuit for 9 Lipo batteries in series that will have an end charge voltage of around 3.4V.

Any suggestions would be appreciated.

Thanks,

Reid

  • 0
    TI__Guru 54715 points

    The bq77910 does not have a separate control pin for an external balance transistor, the gate voltage for the external FET is developed across the Rvcx resistor.  A p-channel FET example is shown in the figure 3010.pch_ext_FET.pdf  N-channel circuit examples are in and . The external circuit will be the same, the device balancing control algorithm will vary with the part. Rvcx should typically be 1k maximum to reduce voltage sensing errors, refer to recommendations in the specific device data sheets.

    Since the bq77910 balancing is only voltage based, balancing during low current charge is typically desired. Balancing during charge avoids capacity reduction of the pack. Balancing during low current avoids balancing based on current induced voltages from cell resistance variations. With the external FET typically balancing is done when half the cell voltage is well above the FETs gate-source threshold voltage. Select a balancing mode and threshold appropriate for your system requirements.

  • 0 in reply to WM5295
    TI__Expert 3170 points

    Are there any additional updates on external cell balancing with the BQ77910?  

  • 0 in reply to George Peterson
    TI__Guru 54715 points

    Not at this time. The external balance circuit might be made up of a small resistor and FET.  The FET could be N or P channel since the pins pull together when the internal balance FET turns on, simply connect to the appropriate pin.  Refer to the documents referenced above for a basic N channel example.  A bipolar transistor might also be used.

    Even though the external balance current may not be set much higher than the internal current, having the larger input resistors will improve the transient filtering to the bq77910 input pins.