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BQ79656-Q1: CBFETs turn on/off time

Yuto Kitamura
Expert 6485 points
Part Number: BQ79656-Q1
Other Parts Discussed in Thread: BQ79758-Q1, BQ79616

Tool/software:

Dear team,

Would I know the CBFETs turn-on/ turn-off time?

Sometimes when we operate in auto cell balancing mode, cell balancing stops. We suspects it is because turn-on time is shorter than turn-off time it might shoot thorough the current via odd and even cells, and stops the operation.

In the auto cell balancing mode, does it make dead time to prevent the shoot through current?

Please give me your thoughts if you have any idea of the reason of this behavior.

Best regards,
Yuto Kitamura

  • 0
    TI__Expert 7000 points

    Yuto,

    There is a typical 150us turn on and turn off time of the CBFET.
    You can add a ~200us blanking time between the odd CB sequence and the even CB sequence.

    Best,

         Quentin

  • 0 in reply to Quentin Silic
    TI__Expert 6485 points

    Hi Quentin,

    Thank you for your comments, that makes sense.

    Quentin Silic said:
    There is a typical 150us turn on and turn off time of the CBFET.

    >> Can we find this specification in datasheet or any other documents? If so, please let us know.

    Quentin Silic said:
    You can add a ~200us blanking time between the odd CB sequence and the even CB sequence.

    >> Is it already added in auto cell balancing mode? If not, would you tell me how or which register should be set to configure that?

    Best regards,
    Yuto

  • 0 in reply to Yuto Kitamura
    TI__Expert 7000 points

    Kuto,

    Unfortunately this is not listed in the datasheet. 

    Auto cell balancing:
    If you run auto cell balancing, you run the risk of this occurring, you would need to disable 1 cell for them all not to turn on together.

    Manuel cell balancing:

    If you run manual cell balancing, you can balance all cells, and you con control the time between odd and even to implement the 200us blanking time. 

    Best,

         Quentin

  • 0 in reply to Quentin Silic
    TI__Expert 6485 points

    Hi Quentin,

    Quentin Silic said:
    Auto cell balancing:
    If you run auto cell balancing, you run the risk of this occurring, you would need to disable 1 cell for them all not to turn on together.

    >> Do you mean disable 1 cell by setting CB_CELLx_CTRL register(0x0318-0x0327) in a cycle and will change the disabled cell in the next cycle?

    Quentin Silic said:

    Manuel cell balancing:

    If you run manual cell balancing, you can balance all cells, and you con control the time between odd and even to implement the 200us blanking time. 

    >> Do you recommend manual cell balancing rather than auto cell balancing?

    Q: Is BQ79758-Q1 improved this feature in auto cell balancing mode, which mean blank time is added by any chance?

    Best regards,
    Yuto Kitamura

  • 0 in reply to Yuto Kitamura
    TI__Expert 7000 points

    Yuto,

    1. If you are doing auto cell balancing and you disable at least 1 cell, then you will not have all FETs turn on at the same time. Just do not configure it for cell balancing. Keep the CB_TIME at 0. 

    2. I would recommend using manual cell balancing, and separate the functions of odd cell balancing and even cell balancing by ~200us. 

    Yes, BQ797xx you can use auto cell balancing with all channels. You just need to adjust the blanking time on BQ79718 using the PWM function, but BQ79616 does not have this functionality. 

    Best,

         Quentin

  • 0 in reply to Quentin Silic
    TI__Expert 7000 points

    Yuto,

    Just adding some more information here.

    Because each CBFET has a turn on/off time of ~150us, please double that for the CBFET that is tuning on and the one that is turning off and add some margin. I recommend extending the time to 500us. 

    Best,

         Quentin

  • 0 in reply to Quentin Silic
    TI__Expert 6485 points

    Hi Quentin,

    I understand we need 500us blanking time for adding margin.

    Quentin Silic said:
    Just do not configure it for cell balancing. Keep the CB_TIME at 0. 

    >> There is no CB_TIME register on BQ79656. Was I below correct?

    Yuto Kitamura said:
    >> Do you mean disable 1 cell by setting CB_CELLx_CTRL register(0x0318-0x0327) in a cycle and will change the disabled cell in the next cycle?

     

    Quentin Silic said:
    Yes, BQ797xx you can use auto cell balancing with all channels. You just need to adjust the blanking time on BQ79718 using the PWM function, but BQ79616 does not have this functionality. 

    >> Did your PWM mean the datasheet of BQ79758-Q1 6.3.4.2.3: PWM settings? I thought it controls balancing current but does it actually PWMing CB FETs and as a result averaging the CB current?

    According to the 6.5.3.2.40 BAL_CTRL1, the PWM cycle is 100ms. So to add >500us blanking time, can we set at maximum 98.437%, so that we can add >1.5625 blanking time. (not only odd -> even or even -> odd change but every PWM cycle.)

    Best regards,
    Yuto

  • +1 in reply to Yuto Kitamura
    TI__Expert 7000 points

    Yuto,

    Let's continue discussion via email. 
    Email me at q-silic@ti.com 

    Best,

         Quentin