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BQ51051B: bq51051B charging slowly

zach dickerson
Prodigy 150 points
Part Number: BQ51051B

Hello, our Qi Charger is very slow.  We set it to 1500mA charging speed and it will not go past 700mA.  We tried a variety of inductive coils, 18W 5V supply and 15W Qi chargers, nothing will go past 700mA when we measure current with multimeter across the battery positive terminal while charging.  We have played with sizing of R43 and it does not seem to matter, I can only make it slower but not faster.

Are there any obvious mistakes or flaws in our layout or schematic?  

This is the last part of our PCB that is not working properly.  Most of the chips I'm using are from TI.

  • 0
    TI__Guru 71166 points

    Hello

    The schematic looks pretty good.

    On the PCB, thermal pad under device is it connected to a large thermal plane?

    '

  • 0 in reply to Bill Johns
    Prodigy 150 points

    Here are all the layers of the 4 layer.  The Qi Chip is stiched to the GND layer, but the layer directly under the chip is the 3.3V power plane, and opposite side of PCB has other chips (double sided 'PCB).

    I have short leads to the JST header for the Qi cables with 24AWG wire 2 inches long maybe.

    I am not sure what the nominal inductor is, or what next steps are to trouble shoot at this point. I need to get to 1.3-1.5A charging speed or I can't use this chipset in production.

  • 0 in reply to zach dickerson
    TI__Guru 71166 points

    The thermal plane looks OK. 

    Also check for temp rise of the device to be on the safe side.

    Which coil are you using?

    Check the ILIM and FOD circuit -- Remove R40, Increase R41 (R-FOD), Decrease R43.  Shift resistance from R43 to R41.

  • 0 in reply to Bill Johns
    Prodigy 150 points

    Hello,

    I tried R1 at 6, 8, 10, 12, 20, 75, 82, 100 ohm

    and Rfod at 200,--------------,180,133,133ohm 

    so, mostly tried to stay at R1+Rfod = 220-210 ohm (assuming Kilm = 321 at 1.5A charge)

    For my test, I have 4-18650 Li-Ion battery in parallel with no protect circuit. I cut the + wire (about 18AWG), and have my multimeter clamp on the wires and test amps.  They oscillate (if that's normal), between 1.0 amp and 1.3A.  Sometimes see 1.4A briefly if position just right on the charger pad.  Is this even a correct test procedure to measure?

    with thermal gun, I measure 120-125F at hottest location on PCB< average around 95F, at room temp 72F, while charging at rate above.

    For  Inductors, I have several, I have best charge rates with 25uH   150 mOhms   815-AWCCARX350300101  //NOT ideal as it's costly, big and seems low stocking


    Mfr. #: 760308201
    Desc.: Wireless Charging Coils Wireless Charging Coils WE-WPCC Rcvr Extnded 10uH 0.16Ohm Qi Qual

    Mfr. #: IWAS3010AZEB130KF1
    Desc.: Wireless Charging Coils Wireless Charging Coils Receiver 13uH 10%

    Mfr. #: IWAS4532AGEB140KF1
    Desc.: Wireless Charging Coils Wireless Charging Coils Receiver 14uH 10%

    Mfr. #: WR221230-36M8-G
    Desc.: Wireless Charging Coils Wireless Charging Coils 22X12X0.87mm Oval Rx coil unit

    Mfr. #: WR444030-16F3-G
    Desc.: Wireless Charging Coils Wireless Charging Coils RX WIRELESS CHARGING COIL 44 x 40 x 0.6mm

    Mfr. #: WR483265-15F5-G
    Desc.: Wireless Charging Coils Wireless Charging Coils RX Wireless Charging Coil 48X32X1.25MM

    Mfr. #: WR111180-49F5-G
    Desc.: Wireless Charging Coils Wireless Charging Coils Wrless Charging Coil 31.8uH 1.34ohm

    Mfr. #: WR202010-18M8-ID
    Desc.: Wireless Charging Coils Wireless Charging Coils Wearable Wireless Rx Coil 20x20x0.1mm

    Mfr. #: WR121210-27M8-ID
    Desc.: Wireless Charging Coils Wireless Charging Coils Wearable Wireless Rx Coil 12x12x0.1mm

    Mfr. #: WMRT149A-1
    Desc.: Wireless Charging Coils Wireless Charging Coils Wireless charger Rx antenna 6.9nh

    Mfr. #: AWCCA-RX350300-101
    Desc.: Wireless Charging Coils Wireless Charging Coils RxCoil,1 coil 25uH 1.05A @ 19V

    Mfr. #: WR555540-16K2-FS2
    Desc.: Wireless Charging Coils Wireless Charging Coils Rx Wireless Charging Coil 15.8uH 0.10ohm

    Mfr. #: 146179-2001
    Desc.: Wireless Charging Coils Wireless Charging Coils NuCurrent Chg Coil Tri-Mode 1.40mm Thck

  • 0 in reply to zach dickerson
    TI__Guru 71166 points

    Sorry for the delay getting back to you on this one.

    Are you still seeing thermal rise and limited output current?

  • 0 in reply to Bill Johns
    Prodigy 150 points

    No, the last post is where the design status remains.  It's currently too slow charge rate for us to put into production.

  • 0 in reply to zach dickerson
    TI__Guru 71166 points

    Another issue that could cause thermal rise is the AC field from TX entering the PCB and battery.  Solution is to add shielding between RX coil and PCB. I have used Wurth Electronics 354002 before with good results, available at Digikey.

    Also see FAQ on Wireless power RX Overheating:

    e2e.ti.com/.../905835