• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 1109 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

BQ76PL455A-Q1: Incorrect voltages on VM and VREF pin

BQ76PL455A-Q1: Cell balancing

Niranjan Tarle
Prodigy 100 points

Part Number: BQ76PL455A-Q1

Hi,

I have successfully connected the BQ76PL455A-Q1 self-designed board to the Bq76PL455 GUI. I can see the voltages of all 6 cells connected to the PCB on the GUI. When trying to balance the cells, this is what I observed-

Cell No. Cell Voltage MOSFET Gate Voltage w.r.t to GND(Balancing OFF) V MOSFET Gate Voltage w.r.t to GND(Balancing ON) V Vgs V
1 3.122 0 2.82 2.82
2 3.158 3.08 6.14 3.06
3 3.169 6.31 7.83 1.52
4 3.062 9.4 11.82 2.42
5 3.177 12.57 14.14 1.57
6 3.619 15.61 18.78 3.17

The datasheet of the MOSFET 2V7002K  says that the threshold voltage VTH is (2.3V max and typ. 1V), so the MOSFETS corresponding to cells 3, 4, 5 do not turn ON, but these are directly controlled by the EQ pin of BQ76 IC. The GUI shows that the cell is balancing when I click on "balancing ON" but in actual, the Vgs is less than the threshold voltage, so the MOSFET doesn't power up. I don't know what to do. I need your help getting ahead!  

Regards,

Niranjan

  • 0
    TI__Genius 10785 points

    HI Niranjan,

    Vgs is not respect to GND. It should be respect to the cell negative voltage.

    What is column VgsV?

    Are you using actual cells or resistor divider?

    If you are using cells then make sure you are not using the resistor divider.

    Roger

  • 0 in reply to Roger Hwang
    Prodigy 100 points

    Hi Roger,

    Thanks for the quick reply.

    I am using 18650 SAMSUNG 25R cells for testing and there is no resistor divider involved. 

    Yes, I perfectly understand that the Vgs should be w.r.t to cell negative voltage. The (VgsV) is the cell voltage measured between the Gate and the Source of the MOSFET where the source is directly connected to the cell's negative terminal. The Vgs should be well above 2.8V, but as you can see it's not. I was unable to find the error due to which each MOSFET has a different Vgs voltage. For reference, I am attaching the table again. I need your help.

    Cell No. Cell Voltage MOSFET Gate Voltage w.r.t to GND(Balancing OFF) V MOSFET Gate Voltage w.r.t to GND(Balancing ON) V Vgs V
    1 3.122 0 2.82 2.82
    2 3.158 3.08 6.14 3.06
    3 3.169 6.31 7.83 1.52
    4 3.062 9.4 11.82 2.42
    5 3.177 12.57 14.14 1.57
    6 3.619 15.61 18.78 3.17

    Regards,

    Niranjan

  • 0 in reply to Niranjan Tarle
    TI__Genius 10785 points

    Niranjan,

    MOSFET 2V7002K should be turn on with your cell input voltage.

    1. Can you go to datasheet figure 25 and make sure unused cells are connected to together to highest cell?

    2. Can you take few snap shuts for GUI and show me the register maps?

    3. Did anybody reviewed your schematic?

    Roger

  • 0 in reply to Roger Hwang
    Prodigy 100 points

    Hi Roger,

    1. Yes, the unconnected cells are connected to the Highest cell. In my case the cell 6 positive terminal.

    2. The screenshot of the GUI 

    3. No, my schematic wasn't reviewed by anyone from TI. I am attaching the schematic for your reference. 

    5582.BMS schematic.pdf

    Regards,

    Niranjan

  • 0 in reply to Niranjan Tarle
    TI__Genius 10785 points

    Hi Niranjan,

    Your EQ circuit looks okay.

    Couple of things to double confirm.

    1. Remove the LED and try.

    2. Just double check that FET is properly soldered.

    Remove the LED for debugging and let me know how it goes.

    Roger

  • 0 in reply to Roger Hwang
    TI__Genius 10785 points

    Hi Niranjan,

    I am going to close this thread but let us know if you are not making progress.

    Roger