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Minimum Q-factor in receiver coil for bq51050B

Thomas Stauffer18
Prodigy 20 points
Other Parts Discussed in Thread: BQ51050B

According to the standard the Q-factor of the receiver coil should be at least 77.

According to the TIDA-00329 a the Würth 760308101303 coil would meet these requirements. But according to the datasheet of Würth the coil only has a Q-Factor of 25 (which I have no idea how the calculated it, because based on their other datasheet value the Q-factor would be somewhat above 77 in an ideal case, and below 77 if you're unlucky).

But when I look on e.g.

www.mouser.ch/.../

Many of this coils are far away from a Q-factor of 77.

So I wonder if we still can use a bq51050B with one of this coils.

  • 0
    TI__Prodigy 145 points
    Thomas

    You could be ok using a lower Q coil, except that it may effect your system efficiency. Please make sure the coil complies with voltage gain requirements that the datasheet shows.
  • 0
    Intellectual 310 points
    The minimum Q factor of 77 is for the entire resonant circuit, not just the receiver coil. With the correct capacitor you should have a Q factor that is more than large enough.
  • 0 in reply to Sammy Peiren62
    Prodigy 20 points

    @Sammy: The wireless power specification states that the minimum requirement for Q is 77, while Q is defined ONLY for the coil. Check p. 120. I myself made a mistake here, the frequency to calculcate Q is 1 MHz and not 100 kHz as I first (wrongly) assumed.

    For everyone else:

    The 2 documents from TI:

    slyt570.pdf
    TI-Designing-a-Qi-compliant-receiver-coil.pdf

    Do not specify a specific gain factor, but talk enough about the topic that it is helpful for me to verify if my design meets the requirements.