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BQ51050B: Qi charging erratic behavior

Benjamin Eymard
Prodigy 30 points
Part Number: BQ51050B

Hello everyone,

I am trying to use the BQ51050B to charge a small battery (450mAh) with the Amazon basic charger link as described bellow : 

The circuit seems to work just as intended when the battery is not in place, but as soon as I put the battery, the behavior become erratic (sometimes it charges, sometimes it doesn't, sometimes the charger indicates END_OF_CHARGE). I'm guessing, this is because of the metal within in the battery that the BQ detects and tries not to overheat by stopping the charge.

I was thinking to add a layer of ferrite sheet, such as link, in between the coil and battery. Do you know if I can expect a more stable behavior with this addition ?

Also, as I'm working with relatively small Ibulk (400mA), do you know if the FOD calibration can help ?

Thank you

Benjamin

  • 0
    TI__Guru* 75486 points

    Hello Benjamin

    Yes adding the shielding material is a good first step, Wurth material will work.  AC field from the TX is probably entering the battery and appearing as a Foreign Object, could trip fault or heat up battery.

    FOD calibration would possibly help, what value resistors are you using for R-FOD and R-os, can you send schematic?

  • 0 in reply to Bill Johns
    Prodigy 30 points

    Hello Bill,

    Thank you for your answer. As you advised I've ordered a ferrite sheet, hopping it will solve my issue.

    Concerning the R-FOD and the R-OS resistance,  I've set them respectively to 200Ω and 10kΩ.

    The used circuit is as described :

    To meet the desired charging current, we only changed C7/C23 and C8/C18 that correspond to C1 and C2 in the datasheet to 95nF and 992pF.

    Thank you

    Benjamin

  • 0 in reply to Benjamin Eymard
    TI__Guru* 75486 points

    Hello Benjamin

    Couple of things:

    1.) R-FOD is 200 ohm, that is OK but 225 would give more margin. 

    2.) R-os is 10k, this is a little low may want to try 20k.

    3.) Coil tuning C1 value is 200nF, this would work for about a 15uH coil.  Which coil are you using .

  • 0 in reply to Bill Johns
    Prodigy 30 points

    Hello Bill,

    Thank you for your answer. Concerning the mentioned points :

    1) Ok, I'll try with a 225Ω if the ferrite sheet does not mitigate my issue. Would 250/300/350 give me more margin if the 225 is not enough ?

    2) Thank for the information. I couldn't find any information about R-os selection in the datasheet. Could you point where I could find some ?

    3) As I mentioned, we changed C1 for 95nF and C2 for 992pF. We are using a 27uH coil, with L' measured around 26.4uH when all the parts are together. 

    Thanks

    Best regards

  • 0 in reply to Benjamin Eymard
    TI__Expert 5645 points

    Hi Benjamin, 

    Since Monday (9/5/22) is a federal holiday, we will get back to you on Tuesday.

    Thank you for your understanding. 

    Regards, 

    Arelis Guerrero 

  • +1 in reply to Arelis Guerrero
    TI__Guru* 75486 points

    Hi Benjamin

    1) "Would 250/300/350 give me more margin if the 225 is not enough ?" -- Bill J - 225 would be enough.

    2) "I couldn't find any information about R-os selection in the datasheet. Could you point where I could find some ?" -- Bill J - This info is in the FOD Calibration procedure see below link.

    https://www.ti.com/lit/zip/sluc577

    3) "As I mentioned, we changed C1 for 95nF and C2 for 992pF. We are using a 27uH coil, with L' measured around 26.4uH when all the parts are together. " -- Bill J -- This would be good.

  • 0 in reply to Bill Johns
    Prodigy 30 points

    Hello Bill,

    I just received the ferrite sheet and it seems that is enough to fix my issues. I think I'll stick to this solution for now.I'll keep the information you gave me in case I stumble across a similar issue.

    Thanks

    Best regards

    Benjamin