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BQ51050B: Communication issue with selfmade Tx and Rx coils

Beat Schilbach
Prodigy 10 points
Part Number: BQ51050B

Hello

I have to analyze a almost finished system with a small handheld device and its related docking station. Both coils Tx (17 windings, ca. 20mm diameter) and Rx (15 windings, ca. 15mm diameter) are customized. Based on below measured inductance values, the coupling factor is 0.72 

LRx,stray = 11.8 µH (with LCR-Meter @ 10kHz)

LRx (=L’S) = 25.3 µH (with LCR-Meter @ 10kHz)

LS = 19.62 µH (with LCR-Meter @ 10kHz)

LTx,stray = 8.114 µH (with VNA @ 100 kHz)

LTx = 17.363 µH (with VNA @ 100 kHz)

The transmitters resonant frequency is 100 kHz (S11 measurement with VNA). The original assembly of the handheld device (C = 350 nF, L 25.3 uH) results in a resonant frequency of ca. 55 kHz. I corrected the value to 100 kHz by setting the Cs to 100 nF by using the formulas of the datasheet

With the original assembly, the communication peaks on Vrect are visible while in the corrected version, those peaks are only visible if the battery output is not active. Increasing the comm caps does not help.

 

My questions are:

1) Don’t have I to take the mutual inductance only for calculating the series-resonant capacitor Cs?

2) Why does’nt the communication work anymore after putting the receiver resonant frequency to 100 kHz?

3) Why do I get a positive voltage increase on V_Rectifier while communication is active?

Thanks to anyone for helping me

  • 0
    TI__Guru* 75606 points
    I will send you more info offline regarding the test results.

    My questions are:

    1) Don’t have I to take the mutual inductance only for calculating the series-resonant capacitor Cs?--No, only L&C value. The TX is tuned with 6.3uh coil and 400nF of low ESR C (COG/NPO). The RX is tuned with Ls' value and Cs the currents are lower in RX tank and X7R caps can be used.

    2) Why does’nt the communication work anymore after putting the receiver resonant frequency to 100 kHz?--This is hard to say.

    3) Why do I get a positive voltage increase on V_Rectifier while communication is active?--V-Rect + Comm will change with load and a Boost at low load is common and Buck with high load is common. As load change the position of a 2ed resonance point moves causing this affect. Also as Cs is decreased the the value of Comm caps should be reviewed.
  • 0 in reply to Bill Johns
    Prodigy 220 points

    Hello Bill

    My colleague is busy with another project and I am taking over this problem. It's still the same as before x-mas.

    I ordered a new TI USB-GPIO device to be able to use the tool you send Beat and read if there is any communication, as Beat could not measure anything on the scope.

    I am new to this topic and need to familiarize with it first. But already a few question marks popped up while talking to Beat:

    1. Can you please respond to his PM from 18.December regarding the bigger inductance we got and if the chip might have a problem with this?

    2. although I will check if there is really any communication (although as written above it could not be seen on the scope) I asked myself if there would be no communication at all, should the TX not stop the charging? Beat told me something aboug 1.6s from the G-standart.

    3. the Ccom caps, the recommendation I read is for the usual inductor sizes. As we have a much bigger inductor as recommended, could the problem be due to the wrong Ccomm cap sizes that are not well chosen for this big inductor?

    4. Somewhere you wrote or told  Beat that the V_Rectifier voltage should not drop that much at the begining and stay high at around 5V? Did we understand this correct? I would expect the drop down to the max. battery voltage that will happen at a specific current. If the battery is totally empty I would expect the voltage to drop down to around 3.3-3.6V at the begining. I would guess this is acceptable right?

    Thanks for getting in touch with me.

    Cheers

    Michel

  • 0 in reply to Michel Buchs
    TI__Guru* 75606 points
    I will contact you and we can discuss this off-line.