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BQ76930: Blancing circuit issue

Sebo
Intellectual 385 points
Part Number: BQ76930

Hi,

we are facing an issue with our cell-balancer. Cell balancing works perfectly for all cells except one.

The 4th cell (VC5b to VC6) is never balanced. By measurement we could see that external balancing fet can't be controlled by the bq. We assume there is an error in our circuit design.

Following you can see our cell-balancing circuit. If i am right, cell 4 can't be balanced because the gate of the FET is not connected to VC6 via a capacity. (VC5b to VC6) I just don't understand why the balancing Fet is connected this way in "Top 10 design considerations". According to TI this should work.  I have also seen this on other documents about the bq76930.  But when looking at other shematics that are provided here in the Forum, they connected this FET not like we did.

Am i right with this? I would be really thakfull if a user could take a look at our circuit. Thanks in advance!

This is our balancing circuit .  I also added a Picture of the shematic from "top 10 design considerations".

Thanks in advance and best regards :)

Sebastian

  • 0
    Intellectual 385 points

    Seems like my pictures where not uploaded correctly.

    Here is the circuit:

    Here is an example from "top 10 design considerations"

  • 0 in reply to Sebo
    TI__Guru 54715 points
    Hi Sebastian,
    I think your circuit looks correct, I don't see a reason it would not balance. During balancing VC5B and VC6 should pull together turning on the FET. Be sure there are no leakages across particularly C12 and D3. Also be sure the values are correct particularly for R11 and R19. Of course check the solder connections.
  • 0 in reply to WM5295
    Intellectual 385 points

    Hi WM5295,

    i allways appreciate your answers :)

    Thanks for replying that fast. I will check your suggestions and measure if vc6 is pulled to vc5b. Maybe there is an internal problem at the bq. (Next time i'll do this before asking hehe)

    I'll Report as soon we did some measurements.

    Kind Regards

    Sebastian

  • 0 in reply to Sebo
    TI__Guru 54715 points
    Hi Sebastian,
    Glad the suggestions are helpful. Remember that VC5b must come up and VC6 must come down to a mid-point so that there is sufficient gate voltage to turn on Q6. If VC6 pulls down to VC5b they will be together but the balancing will not be effective since Q6 will not come on. A wrong value, damaged component, or high resistance solder joint could do that.