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BQ25750: VBAT_OV_STAT in no load condition

Albert Tournilhac
Prodigy 30 points
Part Number: BQ25750
Other Parts Discussed in Thread: BQSTUDIO

Tool/software:

Hello,

I'm using the BQ25750 to make a 12S NMC battery charger : 50V/15A

I'm able to charge correctly at different current values but in no load condition the VBAT_OV_STAT is trigged.

Do you have any recommendations to avoid this condition ? Below my schematic, thanks for the help.

  • 0
    TI__Mastermind 31185 points

    Hello Albert,

    Thanks for being patient with this. I'll get back to your question later this week.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 31185 points

    Hello Albert,

    VBAT_OV triggering here makes sense. With no load, the output voltage is quickly rising from 0V all the way to 50V.

    I've got a few questions to help with this behavior:

    • What register settings are you using?
    • Does setting the ICHG current as low as possible prevent VBAT_OV from triggering?
    • Can you detect whether or not a load is present when the BQ25750 starts up?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 30 points

    Hello Ethan,

    Thanks for your reply, here some answers : 

    • What register settings are you using?

    In my setup, I'm using BQStudio to test the buck-boost. After power-on, I disable the watchdog as well as EN_PFM.

    Then I set ITERM to 250mA and ICHG to a current from 250mA to 8A depending to the test I want to do. 

    The VBAT_OV_STAT is trigged even if the load is removed (or the BMS opening the battery), or when the current drop below ITERM at the end of the charge.

    • Does setting the ICHG current as low as possible prevent VBAT_OV from triggering?

    I tried to set all the currents as low as possible (ICHG = 400 mA, ITERM = 250 mA, IPRECHG = 250 mA), but It not seems to help.

    • Can you detect whether or not a load is present when the BQ25750 starts up?

    Yes, because we monitor the battery state by its BMS. But It's the end charge which concern me as described above.

    It's seems that adding a big capacitor helps, but It needs to be minimum around 5 mF. Then, if you have another tip I'll take It.

    Rgds,

  • 0 in reply to Albert Tournilhac
    TI__Mastermind 31185 points

    Hello Albert,

    Thank you for the new information. I have a few more suggestions that may help the behavior:

    • Can you set EN_PFM=0? We recommend setting PFM below 0 when the termination current is less than 2A.
    • Can you set EN_TERM=0? This should make the charger continuously operate and keep a steady DC voltage once the battery charge voltage is reached.

    Let me know if this works for your system.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 30 points

    Hello Ethan,

    The EN_PFM was already at 0, I noticed it not goes well in the CV condition.

    Set EN_TERM at 0 is a good solution to solve my end of charge problem thanks.

    Althrought It doesn't works in no load condition (open circuit when battery is removed or when the BMS open the battery protection circuit for some reasons)

    I noticed something interresting though: in this case of no load, I have not the VBAT_OV_STAT when I set the EN_PFM=1 AND EN_TERM=0.

    Does this seems odd to you ?

    Thanks for your help.

  • +1 in reply to Albert Tournilhac
    TI__Mastermind 31185 points

    Hello Albert,

    I don't think is odd. Now that I think about it, the system is using the BMS to terminate the charging current so the BQ25750 doesn't need to set the PFM bit to 0.

    The charger is more efficient at no load / low load conditions with EN_PFM=1. This could make the charger less likely to overshoot and cause VBAT_OV.

    Let me know if setting EN_PFM=1 and EN_TERM=0 is a workable solution for your system.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 30 points
    ​Hello Ethan,

    Ok I think we nailed the parameters which helps to avoid the error. I'll try different combinations to see which one is the best.

    Thanks for your help.

    I have another quick question not related to this topic : even if the lower set frequency is 200khz in the datasheet, can I still go under ? I'm trying to reduce switching power losses.