General constraints for inductive power transfer

I'm trying to get a feel for the pertinent issues that would drive the design of a wireless power coupler.

[I state it as a "wireless power coupler" and not a "charger" even though there will be a storage device on the "far side"; but, the item essentially remains in situ semi-permanently (the wireless aspect simply to *allow* it to be temporarily moved, as required; it is almost always *used* while in the charging position)]

Coil diameter is practically limited to something less than

6 inches. Gap between coils would be O(1-2 inches). Alignment/placement of devices won't be well constrained; there won't be a closely-fitting mechanical guide that ensures "ideal" alignment.

Efficiency wrt thermal issues is a concern; a few degrees C is about all I could tolerate above ambient.

So, how do I go about ball-parking the sorts of available power I can move across the gap "on average". I.e., I'm trying to get a feel for my power budget on both sides of the air gap. And, which parameters give me the biggest bang-for-the-buck (e.g., getting the coils closer seems to be the biggest improvement)

[Any numbers on charge times -- from which power transfer rates could be extrapolated -- for wireless *phone* chargers? Noting, of course, that they are a far more favorable physical arrangement]
Reply to
Don Y
Loading thread data ...

The typical wireless phone chargers transfer between 2.5 and 7.5 watts. The ones that I'm currently working with cause a temperature rise of 10C or so.

My intuition tells me that that you can't get away from the temperature rise without really minimizing the gap -- to the point of having the transformer core exposed. But I haven't run any numbers, or done any experiments in that regard.

--
www.wescottdesign.com
Reply to
Tim Wescott

Sure...

What kind of limits are you needing to comply with?

CE limits on magnetic field intensity? Induced voltage? Convection cooling (thermal)?

With that kind of separation, I would expect a coupling factor over 0.1, perhaps as high as 0.5 with optimal pole pieces. Shouldn't be a big deal!

If your only limit is thermal, I would expect nice fine Litz would get you a few hundred watts transferred (for reactive power in the low kVAs). Think induction cooker, but with a somewhat lower temperature limit and better matched load.

Alternately, if you find it useful to have a bit more distance, the efficiency and power limit can stand to drop a bit. Like, to the tune of low tens of watts, efficiency under 50%, and nearly equal coil spacing to diameter. Greater distances get harder and harder to implement, so that distance ratios of "several" become impossible (except for gimmicky press releases).

I don't know what the AC field intensity limits are, offhand. You'll want to look that up if this is any kind of commercial thing.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
 Click to see the full signature
Reply to
Tim Williams

I'm looking at a much lower power level -- perhaps an order of magnitude less (my question is designed to tell me how MUCH less I should target).

Reply to
Don Y

Awhile back I had a task to inductively pass data from one PCB to another...

The coupling relations shown on the schematic seem to match reality pretty well.

(Someone here, can't remember who, pointed me to a link from whence I made the model.) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
 Click to see the full signature
Reply to
Jim Thompson

A six-inch coil will have significant field six inches away, and that (AC) magnetic field will lose energy in any nearby metal objects. So, you want to use smaller diameters, mechanically designed so that no conductive objects clutter the active zone.

Then, consider Tesla's trick, of making the receive coil part of a resonant circuit: this will greatly increase energy transfer. The drive coil will typically be driven off-resonance, for low quiescent current, and when the receiver comes into range, the drive will pull to the resonant frequency and you can ramp up the power (knowing it is going somewhere useful).

A Sonicare toothbrush uses a drop-into-socket design to encourage good alignment, and seems to work well.

At the more complex end, the Qi standard seems useful: a typical driver is and there are some COTS devices available.

Reply to
whit3rd

We have successfully developed a working prototype of inductive power coupler, and the following are general constraints that we have observed during the design/ prototyping stages. First of all are you thinking of resonant power transfer or the plain jane version ?

  1. transmitter oscillator should be simple but brawny -- use oscilloscope to check that oscillations are indeed occurring
  2. Inductive reactance is directly proportional to both coil inductance and operating frequency.
  3. Too much gap between secondary and primary coils reduces magnetic coupling between the two - note that the coils are an air core transformer and lack of ferrite or metal core is a handicap. Hope this helps. .
Reply to
dakupoto

There's too much "positional tolerance" to support smaller coils; there would be many cases where they'd never "line up" (even approximately). Hence my goal of keeping the coil as large as possible.

"Nearby" conductive objects can be controlled.

Yes.

A key design constraint is that it must accommodate *poor* alignment (see comment re: coil diameter)

Thanks!

Reply to
Don Y

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.