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BQ76920: external balancig not working

Karel Salek
Prodigy 20 points
Part Number: BQ76920

Hello, 

My configuration is 4S6P battery pack. I have issue with external balancing, its working just for CELL1. Other cells are not working. 

This is my external balancing schematic. 

Mosfets are XP222N03015R-G.

When i turned on balancing on cell4 i measured these values: V(g)=12.77 V, V(source)=11 V, V(drain)=14.6 V

For the transistor typical Vgs treshold 0,8 V is fullfiled. so transistor should be in saturation mode. Bellow you can see scope picture with V(g) waveform.

The waveform lookes like its described in datasheet so i dont see problem here.

There is no voltage drop on resistor so current is not flowing.

I tryed all cells and just cell1 is balancing correctly. Can someone help what im missing here? I tryed change tranzsitors, resoldered resistors, nothing helped :(

  • +1
    TI__Guru* 81034 points

    Hi Karel,

    It looks like something is not connected correctly for your top cell balancing resistor. It will not be in the balancing path when the FET is enabled. See this reference design image for the correct connection:

    For best voltage measurement accuracy, I also suggest smaller cell input capacitors - 0.22uF is the value used in most of the reference designs. This will give a better settling time for measurements during balancing resulting in less measurement variation. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 20 points

    Hello,

    I desoldered balancing transistor and resistor, i measured the balancing path and everything seemd okay. I changed transistor and its working now. I have no problem with reading votage so i will let 2.2u here. Im just courius why transistor didnt working.

    Best regards

    Karel

  • 0 in reply to Karel Salek
    TI__Guru* 81034 points

    Hi Karel,

    Glad to hear it is working now. I am not sure why your transistor was not working - maybe since your balancing resistor is not in the correct path for the top cell it is not limiting the current.

    Matt