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Series and parallel resonant capacitor selection

Ed Rapp
Prodigy 160 points
Other Parts Discussed in Thread: BQ51013

We are designing a small wearable sensor that is using the WPC standard to charge the battery.  We have the bq51013 eval board but we are using a smaller receiving coil (about 25mm x 14 mm).  I want to calculate the capacitor values for the resonance circuit so I used the formulas in the bq51013 app note.  (Which agrees with the WPC instructions!)

 As a sanity check, I reversed the equation with the eval board capacitior values and the receiving inductor valves from the Vishay data sheet.  Vishay claims the inductance is 9.7 uH which agrees with our measurement (10 uH).  However, when I used the cap values on the eval board (183 nF and 1900 pF for C1 and C2 respectively), the computed values for Ls' = 13.8 uH and Ls = 13.4 uH.

 I realize the Ls' is the receiver coil inductance on the receiving coil test fixture which explains the higher inductance but Ls should be the free air inductance.  I expected the free air inductance to be closer to the published (and measured) inductance.  Since TI went to the trouble to put multiple capacitors in parallel on the eval board to "tweek" the values, I suspect that the resonant frequency is important.

 The bq51013 eval board works too well to have missed the inductance by that much. What am I missing?

Ed

  • 0
    TI__Guru 71146 points

    Values that I measured on my coils here were Ls’ = 14.0uH and Ls = 11.5uH .

    We have found that performance is better with the Cd value set to high end of the specification.  Target range for this tuning is 900kHz to 1.1MHz, so the Cd value is selected to be closer to 1.1MHz.

    Evaluate the start up behavior with your coil to determine if tuning for 1.0MHz will work best for your application.

    Also note that metal around the coil will impact the inductance and final coil tuning should be done in the product configuration or stack up.  We have seen that the battery will have a significant impact on inductance values.  

    If your product will NOT be WPC certified and NOT interoperable with other transmitters the Cd value tuning is less critical.   

    Bill

  • 0 in reply to Bill Johns
    Prodigy 160 points

    Bill,

    Thanks, your answer was a big help.  The capacitor equations compute the values on the demo board with your inductor values and the higher frequency (1.1 MHz).

    Ed

  • 0 in reply to Bill Johns
    Intellectual 855 points

    Dear Bill.

    Can you let me know what is the meaning of the start up behavior?

    I understand Fs (100Khz) is used for power transfer. But I cannot find why Fd is 1Mhz and how it is used for the start up behavior.

    Our customer design RX Coil using FPCB. They want to simulate the coil and to evaluate real material.

  • 0 in reply to avraka
    TI__Guru 71146 points

    For start up info see figure 6 in bq51013EVM-725 User’s Guide.  

     

    The 1MHz resonant point is used for detection of the RX by a moving coil type of transmitter.  This transmitter is used almost exclusively in Japan.