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BQ51013B: Small RX Coil (as TIDA-00881) not compatible with all TX Coil ?

Part Number: BQ51013B
Other Parts Discussed in Thread: TIDA-00881

Hello,

I made my board with a BQ51013B.

All the schematic is manly base on the TIDA-00881. I used the same Rx Coil (Wurth 760308101214) to can copy value for all capacitor around the coil from the ref design.

But actually, my board is not dected by the TX QI transmitter (a final product one) that i have.

Maybe, as this RX coill is small, it is not compatible with all TX QI transmitter ?

  • The coil should work on TI Qi transmitter, this is how it was tested here.
    Check the coil tuning, C1 / C2 / C6. The value of Ls' coil inductance will change when installed in the case with battery and mounted on TX coil. See section 9.2.1.2.2 in data sheet.
  • Which Tx evaluation Module i should buy to can test my design ?
    Only tx evaluation module is needed ? Or Rx part can be usefull ?
  • Hello, my problem seems very similar to this post: https://e2e.ti.com/support/power_management/wireless_power/f/693/p/329192/1149091#1149091

    But i am already using a retail Qi charger.

    Here some screenshoot of my signal :

    one of the AC :

    And the RECT (that is totally similar the other post)

  • Hello Fabien

    The first scope shot shows a freq of 175kHz, this is what I expect for the digital ping freq.  The 2ed scope shot shows a digital ping every 500mS and duration of 80mS this is also what I expect from the TX.  Now on the RX side this should generate enough voltage to exceed UVLO on V-rect and start the RX, typically 2.7V.

    It looks like voltage at V-rect is close but not high enough to start the RX.  Problem could be coil or coil tuning. As a test reduce the distance between RX and TX coil to see if it will start.

  • ok, i will try that, but RX and TX are already very close. All my screenshot was done on my side (RX).

    The coil is the one referenced by the TIDA-00881, and the coil tuning comes from this source also. Maybe on my board this tuning is not totally correct.
    So the tuning can be the source of the low voltage seen on my AC and RECT pin ?

  • By putting each coil really close (pressing them), i success (sometimes.... really sometimes) to have RECT established a really short time.

    Higher pulse voltage on RECT by pressing coil :

    And sometimes.... RECT is established, briefly :

    So tuning problem ?

  • This looks better we see the that the voltage is higher, 5V range more than enough to exceed UVLO. Also the noise on pulse about 40mS in is the RX sending the first packets of info. But TX did not decode and digital ping was not extended. The lower scope capture we can see power transfer started.

    I would investigate the RX coil tuning, key is the RX coil inductance value in position on the TX coil and in RX case, this is the Ls' value.
  • Hi, some news of my investigation.

    With a bigger coil the power is established more easily. 

    But i think i have another problem.

    Since Vout is established to 5V. The power stop immediatly and so VRect is also falling.

    See the screenshot below. Yellow is Vrect and Blue is Vout. What can explain thiis problem on Vout ?

    But sometimes the problem is not s syncrhone between the two signal. in the next screenshot, Vrect stay a little more (this time Vrect is blue and Vout is yellow....)

  • I am always trying to find why my clone of the TIDA-00881 is not working....

    Here a screenshoot of a working receiver (Vrect and Vout) on the frist line and my receiver on the second one...

    The Vrect seems really different, no ? Why my Vout is established earlier ?

  • A couple of things are going on. The Vrect voltage is at 7V with digital ping at 175kHz, that is high. Little tuning is done to the operating point and 5V output is enabled. But shortly after output is enabled TX shuts down, we see a pulse on V-rect that is probably end power transfer (EPT).
    A couple of things will trigger EPT after power up:
    1.) check TS-CTRL connection, 10k resistor to ground should work
    2.) EN1 / 2 should be grounded
    3.) AD pin should be grounded
    4.) Possibly ILIM / FOD connection or resistors
  • Well done.

    The problem was on TS-CTRL connection.

    The Capacitor put in parallel of the thermistor seems to give me some problem with this 2 function pin.

    By removing C21, all now is ok.

    BUT.... :-( All is ok wit a bigger coil that the wanted one.

    The problem with my small coil (Wurth 760308101214) is always here : not enough resonance.

    But all the capa around the coil are exactly the one use on the TIDA-00881, ref design using the same coil !

    I have exactly the same value, see below :