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BQ25756: BQ25756 High power levels

David Spilker
Prodigy 20 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25758, LM5171,

Tool/software:

Hello,

We are interested in using the BQ25756 chip to try and create a bidirectional 100 Amp DC/DC.

We see the BQ25756 chip seems to be limited by the battery charging sense resistor (5m @ 10 amp max) and the input sense of 2m at 20 A. We would like to see about increasing this, is it possible to decrease the sense resistor values with just less accuracy? 

We would also be open to paralleling multiple chips. Our concern would be, could they parallel and operate in reverse mode? Possibly by setting a master at a slightly higher voltage than the slave chips to force them into constant current mode under a draw?

Thank you in advance for any clarification you can provide.

V/r

-David

  • 0
    TI__Genius 14340 points

    Hello David,

    Thanks for working with us on this. Just to clarify, the BQ25756 has a max charge current of 20A. Also, for a DC/DC converter, I'd recommend using the BQ25758. The BQ25758 also has an output current of 20Amps.

    It is not possible to decrease the output sense resistor. The output sense resistor must be 5mΩ for the BQ2575X family.

    David Spilker said:
    Our concern would be, could they parallel and operate in reverse mode?

    Yes, the chargers could operate in parallel in forward and reverse mode. I recommend using an I2C mux to communicate with the chargers and an external clock to synchronize the ICs.

    Let me know if you have any more questions.

    Best Regards,
    Ethan Galloway

  • 0
    TI__Genius 12775 points

    Hi David,

    What is the input and output voltage?

    Do you really need a buck-boost converter?

    If not, you may consider LM5171 which supports 60A and you can stack two IC to support 100A.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Prodigy 20 points

    Hello, and thank you for the replies.

    We need to be able to operate in a bidirectional fashion from a wide operating range and a 28V battery source.

    We also need MPPT support, so that is why we were interested in the BQ25756. How does the BQ25756 handle multiple chips in parallel with MPPT? I would imagine there could be challenges with multiple chips running MPPT sweeps on a panel.

    Also, do you have any supporting documentation for running multiple chips in parallel by chance?

    Thank you again for the support! 

    David

  • 0 in reply to David Spilker
    TI__Genius 14340 points

    Hello David,

    The BQ25756 ICs can't charge with MPPT from the same solar panel. The BQ25756 ICs can charge a battery in parallel if the BQ25756 ICs are powered from individual solar panels.

    We are current writing documentation about using the BQ25756 and BQ25756E in parallel. This document should be out in a few months. For your application, you'll need to use an I2C mux to talk to all the chargers.

    By the way, what's the input voltage from the solar panel?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 20 points

    Hi Ethan,

    Is there anymore to running them in parallel other than the running the sync pins off a clock and an I2C mux?

    Solar panel should be around 65VOC and 52 VMP. But the range could also vary depending on application.

    Thanks!

  • 0 in reply to David Spilker
    TI__Genius 14340 points

    Hello David,

    The sync pin and an I2C mux is all that's needed.

    Keep in mind that the BQ25756 ICs can charge in parallel from a single power source that's not a solar panel, but they can't charge in parallel from a single solar panel because of the way the MPPT algorithm works

    You can read more about MPPT from the FAQ: e2e.ti.com/.../faq-bq2575x-central-faq-page

    Let me know if you have any other questions.

    Best Regards,
    Ethan Galloway