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Coil design for BQ51003 in a textile implementation

Guillaume Levant
Prodigy 80 points
Other Parts Discussed in Thread: BQ25101, BQ51003, BQ25100, TIDA-00318, TIDA-00762

Hi,

I am designing a product that is incorporated in a fabric dressing, and I am thinking of using the BQ51003 with a BQ25101 to charge a 100 mAh LiPo battery.

The company that makes the dressing can embroider some conductive wire that are in copper, gold or silver for example, of many different thickenesses.

What we would try to do is to embroider the receiver coil directly in the fabric dressing, which two edges can be soldered directly on the PCB.

I would like to know if there is a design guide that could help me to check if what we are trying to do is possible, and how to give the specifications of the coil drawing to my partners ?

I know my question can sound very strange, but it would be easier for the integration of our product if we can do it.

Best regards,

Guillaume

  • 0
    TI__Genius 14890 points

    Guillaume,

    Very interesting application!

    The key to the coil will be its inductance.  It will also need shielding material.  We have several documents that may help - but I'm not sure any would match exactly what you're looking for.  Start at ti.com/wirelesspower.  Look at the Application Notes under the Technical documents tab.  The "3Q 2012 Issue Analog Applications Journal" has a section on designing a coil.

    There are also many other coil options that may be interesting, including PCB coils and flexible coils.

    Copper is the main metal used for coils. I have seen aluminum as well.  

    If you have not seen it, we have a reference design with the bq51003 + bq25100 (TIDA-00318) that may be what you're looking for.

    Another consideration is the distance from the coil to the transmitter that would be used to deliver the power.  How would you charge the system?

    I am certain we can get a working solution out of this.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 80 points
    Dick,

    Thanks for your answer, I'll have a look on the different articles you mentioned !

    To charge the device we plan to use a Qi compliant charger that are saled by others companies like IKEA for example. The distance would be small (only a few millimeters) because the device will be put directly on the charger surface.

    Is there a way to simulate or calculate accurately the inductance and DC resistance of a coil with its material and dimensions ?

    Regards,

    Guillaume
  • 0 in reply to Guillaume Levant
    TI__Genius 14890 points

    Guillaume,

    The inductance of a coil is influenced by many factors.  A calculation can be done, but that is only the beginning.  TI does not have tools available.  You can search online for "planer inductor" and you should find some information.

    Look at section 9.2.1.2.2 Series and Parallel Resonant Capacitor Selection in the bq51003 datasheet.  An update to this datasheet was published last night, so please reload the datasheet to make sure its release date is June 2016.  This discusses the measurement of Ls' which is the inductance value in the final solution.  This is the key to the value.  Generally, the inductance values increase in the power transfer environment.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 80 points

    Dick,

    From what I understand from the datasheet and other publications, the inductance of the receiver coil must be as close as possible of the transmitter coil's inductance, and the Q factor must be above 77, correct ?

    On our side, one of the key issue of the choice of the inductor is also its size, because or product is quite small (less than 20mm large), so I mad a few research today and I found this inductor of Wurth :

    But there are a few things I don't understand in the datasheet :

    In the BQ51003 datasheet page 27, the following formula is used to calculate the quality factor : Q = (2pi*frequency*inductance)/(DC resistance)

    It is also said that this Q should be above 77 at 1 MHz.

    Now when we check the Q curve of the coil datasheet, the Q is about 22 at 1 MHz, but when we calculate the Q with the formula, using the worst cas values, the Q is 97.75 at 1 MHz.

    What could make a so huge difference between these two different values of the quality factor ?

    Guillaume

  • 0 in reply to Guillaume Levant
    TI__Genius 14890 points

    Guillaume,

    The inductance of the RX does not need to match that of the TX.  Efficiency is best if they are the same physical size, but most applications do not have that.  The RX coil is usually smaller in size than the TX coil.

    Another consideration is the Qi-certification or not.  Will you create your own transmitter?  The 20mm coil can work with the standard Qi transmitters.  We do have reference designs using smaller TX coils.  Since Qi specifies the TX coil, these reference designs can not be certified to the Qi specification.  If curious, look at TIDA-00334, TIDA-00415 and TIDA-00762.

    I will look into the Q-factor deltas.  

    The 1.2 ohm DCR on the coil is quite high.  It would lead to power loss (heating), but with lower power, may not be an issue except for efficiency.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 80 points
    Dick,

    The device have to be certified to the Qi certification, unfortunately we don't have enough time to design a Qi transmitters.
    I saw two other coils but on Wurth's site, some are listed as Qi compliant and others as "works with Qi Rx IC's", which I think means that they cannot be used to design a Qi certified product.
    The Qi compliant coil is this one :
    katalog.we-online.de/.../760308101214.pdf
    The coil that "works with Qi Rx IC's" is this one :
    katalog.we-online.de/.../760308101220.pdf

    The DC resistances are lower but the same thing with the Q-factor happens... I don't understand with above a certain frequency, the Q factor decreases. Mays because of parasitic capacitances ?

    Best regards,

    Guillaume