Designing a Qi-compatible coil into a rigid PCB - Page 1
Forum section: electronics / beginners

author: krcm0209
author_rank: Contributor
timestamp: December 20, 2019, 04:17:18 pm
content: I am in the first stages of project conceptualization.I am attempting to design a wireless power coil into a rigid PCB which can be used to power LEDs and maybe a power efficient micro on the same board. I am aiming for the overall size of the board to be roughly 3.5 x 2 inches. (8.89 x 5.08 cm)In my research up to now, I have seen this board which may seem to indicate that what I am trying to achieve is possible:https://www.seeedstudio.com/Wireless-Charging-PCB-Module-5-12V-p-1913.htmlI have noticed that the board in the link above doesn't appear to have any additional copper layers or components behind the coil, and I am wondering why. I can understand why you may not want to have random traces routed behind the coil without some sort of shielding, but what about putting the coil on the bottom layer of a 4 layer board and putting a solid ground layer immediately behind the coil? Would that allow for 2 normal layers of SMD components and circuity on the top 2 layers without too much worry?My biggest concern with this project is that I do not know where to begin researching how to design the coil itself. If possible, I would prefer to grab an open-source coil design and integrate it, but short of that, I could try to imitate a similar design. I would intend to connect the coil to some SMD ICs related to handling Qi power delivery. I have found this resource, maybe this is best?https://www.mouser.com/pdfDocs/TI-Designing-a-Qi-compliant-receiver-coil.pdfIt seems like this could be a good IC candidate?http://www.ti.com/lit/ds/symlink/bq51013b.pdfI'm not looking to give too much away, but the general idea is to end up with a business card sized board which can power some LEDs using a Qi wireless charger.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2019, 07:00:45 pm
content: The field from the coil permeates the space around it, including through the coil itself.You cannot have solid inner layers, because that shorts out the field. Instead of field going through the coil, it's restricted to the front surface, and extends about the same height over the board -- that is, a few board thicknesses and trace widths, give or take.Whereas the open coil has a field about comparable to the min(width, length) of the coil, extending out from both faces of the coil.You can add a ferrite plate to one side, shunting field past that face; this allows more circuitry to be placed there (behind the shield, on an adjacent PCB). The field on the other side is reduced somewhat in size, covering maybe half the width and distance; in exchange for that, the up-close coupling is very good indeed, to the point where two such half-shielded coils can be used as a regular transformer (if still a bit poor) without needing a resonant network.Qi I think takes this route, and goes with resonant anyway for added efficiency.Or you can place circuitry off to the side, as long as it doesn't take up much area compared to the coil area (there's still some field out here, outside of the coil edge), and preferably has solid copper under it to shield itself (which appears to be what seeed has done on that board). If you're strapped for board area, you can put the circuitry in the center hole instead, which again shorts out some of the field, but as long as the remaining hole area is greater than the shielded area, the impact on performance won't be too bad.Note that metal shields have the effect of reducing the coil's inductance, while ferrite shields increase it.Coil design is noncritical. The frequency is tuned by capacitors, which can be swapped out to suit the coil (preferably, you'll have several footprints in parallel so you can get the right total without resorting to oddball values). The impedance match is fairly arbitrary, being determined as much by the distance between tx/rx, as by the coil dimensions and the coupling network. If you don't have the tools to calculate or measure this matching, you're probably just as well off simply tuning it and assuming it's fine. It's not going to be efficient or powerful by any means, but it doesn't need to be (we're talking less than a watt, no?).Probably 3-4 turns around the outside of a business card would be enough, and you should be able to squeeze a small (CR1220?) cell (if necessary) and NFC chip (13.56MHz) in the middle. More turns for lower frequencies, of course.You'll have a harder time doing Qi, as the ferrite (and consequent increase in inductance, allowing practical performance at low frequencies) is pretty much obligatory. (Old style passcards are 125kHz and have many turns of fine wire embedded in them, visible with a bright light or x-ray. Good luck printing that on a PCB. The resistance of all that wire would give poor efficiency for a charger, but might still be workable without ferrite.)Tim

author: Smokey
author_rank: Super Contributor
timestamp: December 23, 2021, 07:38:31 am
content: Anyone been successfully with something like this?

author: wizard69
author_rank: Super Contributor
timestamp: December 25, 2021, 01:14:51 am
content: Quote from: krcm0209 on December 20, 2019, 04:17:18 pmWell I'm not sure what you have found so far but I really think you should contact the association handling the Qi standard. As for coil design the document you referenced seems to cover coil requirements in some detail so you should be able to design from there. As for the issue of a ground plane that would likely not work. You might get buy with some circuity. As for other items you likely will have to model and test. For example on many cell phones the Qi coil is near displays and or batteries so engineering likely designed with the interference from that stuff in mind.