Ferrite for Wireless Power RX Coil Measurement

[Sebastian]

I am in the process of evaluating Wireless Power for a project. In order to calculate the cap values for the resonant circuit, the inductance of the receiver coil has to measured in a test fixture with a shield on the bottom, made of a PC44 ferrite piece from TDK Corp. that is 50mm x 50mm x 1mm.

The problem is I can't find that ferrite anywhere. Not even complete text fixtures which would probably to expensive anyway.

PS: http://www.ti.com/lit/ds/symlink/bq51013b.pdf (Page 29-30)

[T3sl4co1l]

http://www.mhw-intl.com/assets/TDK/Ferrite%20Material%20for%20HF%20Application.pdf

PC47 is listed as a current material, "under 500kHz / lowest losses" (that is, for a mu=2500 MnZn "power" ferrite with large Bmax).  I doubt it makes any difference, and common EMI materials are probably a better choice.
http://www.mouser.com/ProductDetail/LeaderTech/SB28B2100-1AB/?qs=sGAEpiMZZMtQwYqzsFSgXNSRVZO5oDvAYVPrvJPXgaY%3d
#28 is a wideband EMI ferrite, but it's probably just fine for this sort of thing.  Check a datasheet for comparison if you like.

FYI: the difference is basically fractional percent losses and inductance, unlikely enough to matter.  The magnetic field is mostly, and necessarily, going through air, so the effective permeability seen by the coil is maybe 2 -- whether the actual ferrite material itself is 1000 or 2000 makes about 2/1000ths of a difference.

Tim

[Sebastian]

Thanks for the help.
Do you think it makes a noticeable that it is 2.5mm thick instead of 1mm? I can not compare it to the right one because I can't find it anywhere.

[T3sl4co1l]

More effect, but nothing that can't be solved, say by removing a few turns to compensate.  It may not matter at all; I don't see much mention of what frequency it operates at.  I guess it's not like a 13.56MHz RFID where everything needs to be tuned just so, but rather it's quite tolerant, and perhaps necessarily so given the power coupling topology.

Tim

[Sebastian]

The device follows the QI 1.1 standard and that works at 100kHz to 205kHz.
I don't have to reach a particular impedance, so removing turns doesn't do me any good. I have to measure the actual value under the given circumstances in order to calculate the capacitor value for the series resonant circuit. (Resonant at 100kHz)
I think I maybe found a source for the ferrite. Acording to the standard it should be below the coil in transmitters, so I should be able to salvage it from one. But I am still wondering why I can't find it anywhere, whoever builds transmitters has to get them somewhere too.

[Sebastian]

I got the transmitter today and it does have the ferrite plate inside.
It is round instead of square but I think that won't make a big difference.
What worries me more is that I couldn't remove it without cracking it. It has a film on both sides so it doesn't fall apart but it is shattered. Does that change the properties much?

[T3sl4co1l]

Yes.  If you can fit the pieces together tightly, some superglue or epoxy can be used to bond them back together with minor loss in performance.

Again, because the application is mostly airgap, it's probably not a big deal.  You don't want visble gaps though, or bits moving around.

Tim

[Sebastian]

Good, thanks for the help.