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BQ51003: Low-Power Minimum Q for Rx Coil

Charles Geber
Prodigy 50 points
Part Number: BQ51003

The bq51003 datasheet calls for an Rx coil with a minimum Q of 77.  To create a very small product, I would like to use the TDK WR121210 (12mm) coil, but at (L=8.32uH and R=950mOhms), its Q comes out to be only 55.  If my product only needs < 0.5W, would the bq51003 work with this coil?  Thanks...

  • 0
    TI__Guru* 75906 points
    The problem with the coil is the L value, too low.
    Small RX coils will have low coupling factors resulting in low system gain. To offset this the RX coil L needs to be increased.
    A 12mm coil is very small will be difficult to get good performance. Would recommend starting with something in the 30uH range. If you can increase coil size that would help.
  • 0 in reply to Bill Johns
    Prodigy 50 points
    Thanks for the prompt reply, Billl...the Wurth 760308101217 coil is (24.2uH 1.2ohms); from Equation (7) of the bq51003 data sheet, Q calculates to 127. However, the Wurth datasheet indicates Q=13 @ 125Khz. Equation (7) utilizes a frequency of fD=1MHz. Does the 10X frequency value explain the 10X Q factor? Does this coil seem workable with the bq51003? Thanks, Chuck Geber
  • 0 in reply to Charles Geber
    TI__Guru* 75906 points
    This one is better, the L is higher but I have had bad luck with coils this small. The size is the problem, coupling factor will be low. Looking at some old test data I have k is below 0.2 for this coil. Best test case was with a 20mm TX coil, non-standard k=0.193.
    Compared with a 19mm round coil in the same test k=0.600.
    The risk is high if a standard off the shelf TX is used, standard coil is 50mm.

    Regarding the Q, yes this a function of the test freq 125kHz vs 1MHz.
  • 0 in reply to Bill Johns
    Prodigy 50 points
    Thanks again Bill - I am now looking at a 17mm round Wurth coil (Q=293@1MHz) or a 20mm round TDK coil (Q=362@1MHz)
  • 0
    Prodigy 50 points
    Hi Bill - I have seen implementations with an alignment magnet in the center of the receiving coil....is it acceptable to put a metal-cased component in the center of the coil? Both of these seem counter-intuitive to me....thanks!
  • 0 in reply to Charles Geber
    TI__Prodigy 320 points

    Charles,

    I am covering for Bill while he is out of office. Having any kind of metal at the center of the coil or anywhere were it can be exposed to the magnetic field is not advisable since it will decrease power transfer efficiency and the field can heat up the metal considerably. Any metal component at the center of the coil would need to be shielded with some ferrite material.

    Best regards,

    Norelis