Field of a "pancake" coil in QI charger

[Joseph M. Zias]

TL;DR Summary

What is the magnetic field of a coil used in a QI charger?

Attached is a photo the the primary transformer coil of a QI (I believe pronounced CHEE) wireless charger, as used for charging a cell phone. I know the fields of solenoids but what would be the magnetic field structure of this. By-the-way, a similar coil was used on old AM radio sets.

 

[tech99]

The magbnetic field is in the form of a "doughnut". The same form as for a single wire loop.

 

[Baluncore]

You can think of a pancake coil as having a flux similar to a very short solenoid.

Just as a solenoid can be modeled as one single turn of a cylindrical current sheet, a pancake coil can be modeled as a single turn of a flat current sheet.

The magnetic field between each pair of turns is cancelled, so for a pancake coil, the magnetic flux passes inside the inner turn and outside the outer turn.

 

[Merlin3189]

I don't know about these. Would I be right in guessing that the pancake coil, apart from being flat and thus easier to fit, would also accommodate a range of secondary coil sizes and also be more tolerant of mis-alignment?

BTW I didn't know what you meant by this sort of coil in AM radios. They would be too low inductance for tuning the AM bands, IMO, though I had seen them used in ATU's for matching.
But I started in the 1950's. You are going back to the 1920's. I attach pics of a crystal set design. The text admits it has poor selectivity, as one would expect using a small inductance and the heavy damping of a crystal set.

 

[Klystron]

Summary:: What is the magnetic field of a coil used in a QI charger?

Attached is a photo the the primary transformer coil of a QI (I believe pronounced CHEE) wireless charger, as used for charging a cell phone. I know the fields of solenoids but what would be the magnetic field structure of this. By-the-way, a similar coil was used on old AM radio sets.

The magnetic field is in the form of a "doughnut". The same form as for a single wire loop.

Given a simplified analogy between a transformer coil and a loop antenna, the magnetic (B) field of a single loop of the coil becomes easier to visualize.

Loop antennae in various guises appeared in early radio sets particularly in direction finding rigs. As a point of departure, one can model solenoid magnetic fields by vertical stacking and the 'pancake coil' via concentric loops. Perhaps too basic for the topic of Qi chargers.

Thanks for an interesting post. I learned quite a bit today about Qi charging technology.

 

[Baluncore]

Would I be right in guessing that the pancake coil, apart from being flat and thus easier to fit, would also accommodate a range of secondary coil sizes and also be more tolerant of mis-alignment?

Yes, the flux passes through the hole in the centre, which is far from the outer flux path, so alignment is easy, and orientation is not important. The pancake is flat so the other coupled coil can be close and well coupled.

What is the black plane behind the pancake in post #1 picture ?

 

[Tom.G]

Referring to pancake coils,

They would be too low inductance for tuning the AM bands, IMO, though I had seen them used in ATU's for matching.

I recall seeing a few table-top vacuum tube radios with a pancake coil mounted on the inside of the back cover. They were wound with cotton covered wire, perhaps 24ga. If I recall correctly they had three leads, so were tapped.

Manufactured probably around the 1930's to '40's.

 

[Baluncore]

There is another coil very similar to a pancake coil. Early wireless receivers sometimes used a loop antenna or coil made in the form of a spiders-web. It was wound on an odd number of pins or slots which reduced inter-turn capacitance and AC resistance.
https://commons.wikimedia.org/wiki/File:Spider_coil.jpg

The coil could be tuned, but mostly it was used as a separate loop antenna that was connected to a tap or primary on the tuned RF front-end. Some loop antennas were hinged so they could be swung independently to peak a signal, or to reject an interfering station in a particular direction.

 

[Joseph M. Zias]

Great responses - thank you.

 

[sophiecentaur]

Early wireless receivers sometimes used a loop antenna

That stuff was all superceded by ferrite rod antennae, which are much better in receivers. (Not suitable for high magnetic fields, required for power transfer) So I don't think that a lot of work would have been done in that direction for a few decades. But the near field of an rf coil is not the same as the radiated field. Design would probably be more like transformer design, rather than antenna design, for the sort of frequencies that seem to be used for QI style circuits (around 100kHz).
Something that always confuses me is the talk of High Q resonances when the purpose of this sort of system is to transfer a lot of power. I would have thought that would imply high 'losses' and consequent low Q.

 

[Baluncore]

Something that always confuses me is the talk of High Q resonances when the purpose of this sort of system is to transfer a lot of power. I would have thought that would imply high 'losses' and consequent low Q.

Your analysis is correct for the primary of a charger that is in use, but not for the time the charger is operating, but not being used. During that idle period, maximum Q implies minimum quiescent power. The high Q disappears as expected when the secondary load is applied.

 

[sophiecentaur]

During that idle period, maximum Q implies minimum quiescent power.

Right; cheers for that. But systems are usually 'intelligent' enough these days to be able to work on minimum power as long as there's no power drawn. Obvs any circuit would need to be pretty efficient so losses would need to be low(ish)
But my issue is more with the (mis-sold) idea of long distance power transmission with high Q coupling, which really doesn't make sense to me. That's a bit more of the Tesla legacy, I think.

Actually, the idea of contactless charging is quite attractive to me but none of the family's stable of IOS devices has contactless charging facilities so I'm a bit sour grapes about the idea. It's clearly the way to go, though. Charging motor cars would be particularly attractive when there's only on-street parking. Contactless charging stations all along the roadside would be the only solution.

 

[Baluncore]

Charging motor cars would be particularly attractive when there's only on-street parking. Contactless charging stations all along the roadside would be the only solution.

Or bicycles to subvert the dominant paradigm between the homes and private garages.

 

[sophiecentaur]

Or bicycles to subvert the dominant paradigm between the homes and private garages.

The problem with bicycles (and walking) is that we are where we are. Vast numbers of people have distant jobs that are only accessible by car and vast numbers of elderly people are really not in a position to cycle or walk, even for short journeys. (Average age is constantly increasing too)

Jobs and homes could be reconciled in the long term. I used to walk to my teaching job for many years but many of the staff lived miles away. You would need to include high levels of use of public transport in your alternative model. There's an unbelievable amount of resistance from the majority of men in the UK to using public transport.

I think technology could provide solutions long before the culture could change to eliminate the problems. Humans can be infuriating!