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BQ25756: BQ25756 or BQ25750

Stas Shamanin
Intellectual 270 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25750, LM74930

Tool/software:

Hi!

I have drone project.

It has Fuel cell and 12s-14s battery.

Battery is needed to provide high curent (up to 300A) to the system, when fuel cell can provide only 50A.

Battery should give power automatically when fuel cell goes down (ideal diode lm5050 selected for this role)

Battery should be charged in flight (up to 20A)

Here are current schematics

I think to add BQ25756 as standalone charger from fuel cell

But it looks like BQ25750 can solve all my problems, at least it can remove TPS48110 as switch, it would be nice if it can remove also LM5050.

All I need - auto switch when fuel cell loses power to battery, and I am not sure if BQ25750 has internal engine to do so.

  • 0
    TI__Genius 16520 points

    Hello Stas,

    The BQ25750 does have an automatic switch over for when the input voltage (or fuel cell in your case) loses power. However, the BQ25750 will turn OFF the battery FETs if the switch over current is greater than 8Amps, but you can override this protection by connecting the BATFETs directly from the system to the battery.

    Can you answer a few questions for me to help you out?

    What's the input voltage of fuel cell and the charge voltage of your battery?

    300 amps is a lot of current. What FETs are you going to use to perform the power-path switch over?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Intellectual 270 points

    Hi!

    Thanks for help!

    1.1 Fuel Cell is nominal 48V output. In overpower condition it takes voltage directly from battery it is powered by (in our case it may be the same battery), but the problem is the internal path of fuel cell is designed for maximum 100A with regular fuses that burn, that is why we want to provide direct path from battery to the system. Also Fuel Cell has integrated back emf consumer, so I can connect battery throw diode and don't care about providing path to backEMF

    1.2. Design should withstand 85V max peak. I had discussion with TI team before, they suggested take this peak voltage as start point for my system.

    2. Battery is 12s default(may be 14s) So 52 (61) V max, 36 (42) Vmin  expected from battery. Fuel Cell manufacturer approved this connection (picture):

     

    3. I discussed FET issue with TI team regarding LM5050 and TPS48110, and we came to agreement, that in not frequent switching (which is expected in application) I may use 12 MOSFETs in parallel (12 for LM5050 and 12 for TPS48110 in back-to-back topology). I would be happy to use one lm74930 as I do in another project, but it has only 70V absolute max voltage.

    MOSFET is IAUT300N10S5N015

    it is 216 max nC total gate charge 1.5mOhm, by calculations in ambient 50 degrees with ambient cooling it should withstand 25A, that is why there are 12 MOSFETs.

    We thought about using less MOSFETs with external cooling, but this solution has higher weight. Weight is more critical parameter than cost, that is why I also consider put more integrated solution if it fits my application (that is motivation for BQ25756 compare to BQ25750)

    4. Can you clearify "Loses power. It means like 0V from fuel cell, or it dependent on secondary voltage (less than voltage of battery) Or I can configure like "if less than 48V switch to the battery" or "whatever has higher voltage provides power"

  • 0 in reply to Stas Shamanin
    TI__Genius 16520 points

    Hello Stas,

    Thanks for more information.

    Stas Shamanin said:
    Can you clearify "Loses power. It means like 0V from fuel cell, or it dependent on secondary voltage (less than voltage of battery) Or I can configure like "if less than 48V switch to the battery" or "whatever has higher voltage provides power

    The BQ25750 has built in over-voltage and under-voltage range. If the input power goes outside the "power good" range, the BQ25750 will switch over to using the battery.

    I think it'll be easier to use the BQ25756 for your design. The BQ25750 can't support a load of 300A on the system. You could possibly use the BQ25750's BATDRV signal to tell the power path when to switch, but we have not tested this.

    Best Regards,
    Ethan Galloway