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BQ25750: Power Paths switching performance

Darren DeBattista1
Intellectual 525 points
Part Number: BQ25750

Tool/software:

Hi, 

As per the datasheet, the BQ25750 should be able to automatically switch the power paths. Thus, if VAC goes below the set threshold, the battery should be automatically connected to VSYS (ACFETs off and BATFETs on). Similarly, if VAC goes above a given threshold, ACFETs should turn on while BATFETs should turn off to direct VAC to VSYS while also charging the battery. In this regards, is there any data showing the performance of this switching? More specifically, are there any delays between turning off the ACFETs and turning on the BATFETs?

  • 0
    TI__Mastermind 30785 points

    Hello Darren,

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

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 30785 points

    Hello Darren,

    Just a quick heads-up, I will get back to you early next week about this. Thanks for being patient with this. I've been swamped this week.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 30785 points

    Hello Darren,

    When VAC is plugged in, it takes about 2.6ms for the ACFETs to turn-on. When VAC is removed, it takes about 1ms for the BATFETs to turn-on. Does this answer your question?

    Also, we have an FAQ page that goes over the power path switch-over. Also, you should know that the auto reverse mode normally has faster activation time at about 50us.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Intellectual 525 points

    Hi Ethan, 

    Thanks for your reply. This solves in part my question, but I have further questions:

    1. What is the power path switching methodology? Is it a break before make ie, if VAC is plugged in, will BATFETs turn off and than ACFETs turn on? If so, is there any dead time period? Are there any waveforms I can check?

    2. While charging current will be limited to 3A, my system current can go up to 20A. After reading the FAQ link you sent, I am assuming that I do have to connect the BATFETs between SYS and SRN (thus disabling BATFETs protection) and short the AC sense resistor (thus disabling the ACFETs protection). Am I right in saying this is inevitable in my case?

    3. In my application, VAC can also be a 4S battery, thus both VBAT and VAC can range between 10V - 16.8V. Can this create any issues (for example, if VAC is lower than VBAT)? This shouldn't be the case considering the table shown in the FAQ, but asking for confirmation.

    Note: I cannot use auto reverse mode for two reason; 1) my system will ideally not include an MCU, thus I cannot enable auto reverse mode and 2) As described before, SYS current can go up to 20A, which would necessitate me to design the charger to handle 20A in reverse mode. 

  • +1 in reply to Darren DeBattista1
    TI__Mastermind 30785 points

    Hello Darren,

    Thanks for letting me know about the system.

    1. The power path is break-before-make to prevent shoot through from the VAC to VBAT. The dead time is about 150µs.

    2. Yes, you will need to do this in your case to allow for a 20A system load current while the power path is switching over.

    3. No, this should not be a problem for the charger.

    Let me know if you have any other questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Intellectual 525 points

    Hello Ethan, 

    Thanks for your quick reply. This has solved the issues I faced so far.

    Regards