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PCB coil to replace hand wound coil for inductive charging

PCB coil to replace hand wound coil for inductive charging

PCB coil to replace hand wound coil for inductive charging

(OP)
We want to replace a wire would coil (parameters and photo see attachment) by a PCB coil as transmitter coil for inductive charging.
We designed the coil according to the original dimensions (except thickness of conductive path).
Unfortunately, the receiver coil does not detect charging current.

Do you have any tipps, what to consider or to modify?

Many thanks in advance!

RE: PCB coil to replace hand wound coil for inductive charging

Lots of questions here:
1. Are the diameters given correct? The inner diameter and outer diameter do not seem to be correct. Measuring with a mm scale says that OD is more like 76 mm (if ID is 20). I disregard the inner/outer diameter text, which obviously cannot be correct..
2. The much thinner (assume 35 microns width) means around 20 times higher DC resistance in the PCB coil. And that is usually crucial to Q value. Which in its turn is important in a system where there is a parallel capacitor to support heavy current in the charging coil (uses resonance). With such a high resistance, there will not be much of a resonance in the circuit.
3. Did you measure the emitted field and the frequency? Some systems try and find the resonant frequency but many don't. So it could be worth-while to check what your actual frequency is. With original coil as well as with the new one. Also, measure flux in the center of both coils. That may give you a clue as how to continue - or if this version is feasible at all.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: PCB coil to replace hand wound coil for inductive charging

Sorry about ID/OD. Scaling error. OD looks more like 38 mm.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: PCB coil to replace hand wound coil for inductive charging

What's the blue stuff in the middle of the coil? You used blue to color the connecting trace on the back side, but the entire coil appears to have its backside covered with the same copper layer.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: PCB coil to replace hand wound coil for inductive charging

I thought it was a hole. But if it is copper, then it will short most of the magnetic field.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: PCB coil to replace hand wound coil for inductive charging

(OP)
On the backside of the coil, there is the cross connection as well as a copper layer not linked to the cross connection to work as EMC protection.

RE: PCB coil to replace hand wound coil for inductive charging

Quote (OP)

to work as EMC protection.
Isn't this counter productive for a device intended to radiate electro-magnetic energy?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: PCB coil to replace hand wound coil for inductive charging

As bad as a shorted turn in a transformer!

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: PCB coil to replace hand wound coil for inductive charging

"On the backside of the coil, there is the cross connection as well as a copper layer not linked to the cross connection to work as EMC protection."

That's what I thought; you just turned an inductor into a skinny capacitor. As a test, you could Dremel tool the cladding on the backside and see if that works.


TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: PCB coil to replace hand wound coil for inductive charging

(OP)
Many thanks for the recommendations! I will remove the backside and check if this is the root cause.

My understanding is that a EMC is required for the coil.
What do you recommend to solve this topic in a thin way?

RE: PCB coil to replace hand wound coil for inductive charging

You might try a design with the same coil on both sides and use a single jumper wire to get back from the center.

Normal PCB copper thickness is called "1 ounce" (an amount of copper metal spread over a standard area - 1ft2). Most places will allow up-thicknesses to 4 or 6 ounce. 1oz results in 0.0014" thickness.

Do note, there are specially board houses that will supply up to 200oz! That is copper more than a 1/4" thick.

And no you cannot leave the cladding on the back of the board regardless of the EMI fears.

Keith Cress
kcress - http://www.flaminsystems.com

RE: PCB coil to replace hand wound coil for inductive charging

(OP)
Do you think, there is an opportunity to integrate the faraday box (as the PCB has a thickness of few microns only, it would look more like a Faraday sheet) into the PCB design?
Or is a certain distance of the Faraday box from the coil required to avoid the phenomena explained in the comments above?

RE: PCB coil to replace hand wound coil for inductive charging

PCB is usually 1.6 mm and Cu clad is 35 microns for 1 Pound/Square-foot.

Forget about EMC until you got a working device.
(Non-P.C. to say so. But still what you need to do)

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: PCB coil to replace hand wound coil for inductive charging

It appears from https://www.comsol.com/paper/download/442891/maier... that the useful EM field extends at least to 1/2 the diameter of the coil vertically. That means for your 40-mm diameter, your Faraday box top and bottom cannot be much closer than about 20 mm from the PCB without affecting the performance.

You could play with http://coil32.net/online-calculators/pcb-inductor-... and see if a smaller coil can do the job. According to the calculator, something with 1/2 the dimensions and 17.6 turns will result in the same inductance.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: PCB coil to replace hand wound coil for inductive charging

Three solutions to EMC;
1.- Clean up your wave form to reduce or eliminate unwanted frequencies.
2.- Faraday shield. The device being charged must be inside the Faraday shield.
The Faraday shield will stop wanted emissions as well as unwanted emissions.
3.- Live with it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: PCB coil to replace hand wound coil for inductive charging

(OP)
Many thanks for the great recommendations!

Waross: can you please provide some more info how to clean up the wave form?

RE: PCB coil to replace hand wound coil for inductive charging

I am not the best person to ask but I am sure that others will help out.
I would look at the waveforms on the scope and identify the frequencies that I did not want. Then decide if there is enough radiated energy to worry about. (As in,- Can you live with it?)
When you have identified the problem frequencies, start asking questions about band pass filters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: PCB coil to replace hand wound coil for inductive charging

The system that the OP asks about is probably using a resonant circuit. Like the Qi (CHEE) standard. Dimensions are about the same.

If that is the case, the frequency is around 140 kHz and careful switching should produce a minimum of EMI. So EMI is not the main problem. What worries me is that there is no charging at all (receiver doesn't react) and that means that something else is wrong. The coil resistamce or the inductance most probably.

The original coil seems to have a core and is not just a coil former. Have you tried to see if it is magnetic? That would explain a few things. If it is, then you need to have one, too. That may be what you think is an EMI shield. It is not. It is part of the transformer action.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: PCB coil to replace hand wound coil for inductive charging

Quote (Skogsgurra)

What worries me is that there is no charging at all (receiver doesn't react) and that means that something else is wrong.

If his original design this was tested on had that backplate on the PCB, it wouldn't charge due to practically zero radiated emissions making it above the PCB.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: PCB coil to replace hand wound coil for inductive charging

Sure. But if he needs a core as well, it wouldn't help much to remove the shorted turn from the transformer.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: PCB coil to replace hand wound coil for inductive charging

(OP)
Hello IRstuff,

I tried to dremel the shield layer away. Unfortunately, as it`s a flex PCB, the layers are so close to each other that I was not able to do it without damaging the coil.
I will need another production run without shield.

RE: PCB coil to replace hand wound coil for inductive charging

Your photo of the original coil looks like it's made of Litz wire, hundreds of separately insulated conductors in parallel to mitigate skin effect at high frequencies, so Q may very well be a factor.

RE: PCB coil to replace hand wound coil for inductive charging

If it's flex, use an X-acto knife to cut it out and peel it away from the other layers... some judicious heat can help here (delamination of flex due to high heat is a common re-work mistake, so make it work for you).

Dan - Owner
http://www.Hi-TecDesigns.com

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