Is it realistic to transmit small amounts of wireless power with SMT inductors?

[mikeselectricstuff]

I've not experimeted extensively - series resonant is very simple and works fine as long as you don't overvolt the capacitor - can't comment on efficiency vs. other methods.

BTW one fun thing to do is use an RGB LED with 3 inductors at right angles to each others, so the colour changes withteh orientation.

[Marco]

There's also this graph which was posted:
http://www.wirelesspowerconsortium.com/data/images/1/2/figure2.jpg

And if I'm reading it correctly then if D is larger than D2 then the efficiency goes down, so it would seem to be detrimental to use transmitting coils that are larger than the receiving coils.

Taken at face value the tiny diameter of the coil in a SMD component would explode z/D, which drops it off even faster. But these numbers are almost certainly for air cored coil pairs, whereas you'd probably only want air core for the transmitter. Ferrite "pulls in" the field lines, so it's hard to say how it's all going to work out in reality.

I'd try this for the transmitting coil (if you have some litz wire you could make your own, but the ferrite film behind it helps a bit) and this as the receiver (really high inductance for the size, very high permeability ferrite). Make the transmit coil resonant with a series capacitor and connect it to a square wave generator of variable amplitude, put the smd inductor in parallel with a CG0 capacitor and a LED and see what it all does. The chip inductor probably has to point towards the transmitting coil BTW.

[T3sl4co1l]

Meh, resonance is resonance... you get as much power transfer as your circuit is set up for.

I find series resonant better for most purposes, because it complements the voltage-sourcing inverters that are easiest to build with MOSFETs.  Power control is then accomplished by varying the DC supply, or PSPWM with an H-bridge.

Conversely, parallel resonant needs a current sourcing inverter, which is inconvenient for MOSFETs, and inconvenient for power supply (you usually solve this problem by using a stinking huge inductor, and then your second problem is it's still voltage-sourcing, just over longer time scales, which makes it very vulnerable to faults).

I've not experimeted extensively - series resonant is very simple and works fine as long as you don't overvolt the capacitor - can't comment on efficiency vs. other methods.

And it's easily calculated based on driver output resistance, L and C.  Also, make sure you don't burn too much power in the driver at worst case load.  Some external resistance may be desired.

Tim

[Starlord]

So I'm still a little confused here.

Why does the size of the transmission coil make a difference? Is it because it spreads out the magnetic field to cover a wider area? But with the same power input wouldn't it be more... um... diffuse? Diluted?

Maybe that's the point? Like, if I have an airtight cylinder, and I move the piston the chamber expands and I get a partial vacuum. The air (magnetism) is spread out over a larger area, but at a lower pressure. So I then pump more air into there (increase the voltage to increase the current and thus the total power) and then I can get back to 1 atmosphere everywhere inside the cylinder.

Now onto my next question:


https://electronics.stackexchange.com/questions/201836/why-am-i-getting-odd-inductances-from-home-made-inductor

Is there any reason I couldn't make this large coil using PCB traces, since that would allow me to make this thing any size I need?  (Like, say, 41x41cm or 16x16")

I guess the inductance would be lower than with a coil, but it would be a lot easier and cheaper to make a large flat surface this way than using dozens of Qi charger coils.

But in what way does the inductance matter here? I'm guessing that the only bit that really matters is that I get the resonance frequency right? So if I want 100Khz. but I use a PCB coil, the inductance will be low requiring a larger capacitor to get that frequency? I suppose if I want to keep my surface flat having a large capacitor might be problematic, but maybe I could just use lots of smaller SMT ones in parallel to get the capacitance I need?

Is there some other effect I'm not thinking of here though of using a lower inductance coil? Increased voltage / current requirements perhaps?

[T3sl4co1l]

I'd try this for the transmitting coil (if you have some litz wire you could make your own, but the ferrite film behind it helps a bit)

A fine starting point.  L may be on the low side, but the general idea is right if nothing else -- that's just a matter of turns.

and this as the receiver (really high inductance for the size, very high permeability ferrite). Make the transmit coil resonant with a series capacitor and connect it to a square wave generator of variable amplitude, put the smd inductor in parallel with a CG0 capacitor and a LED and see what it all does. The chip inductor probably has to point towards the transmitting coil BTW.

Now, the drawings don't show which way this part is wound, so it's not clear how good it would be.  It's "unshielded", apparently, but beyond that...

If it's wound axially, then that gives L/D ~ 2, and that also means the end terminations are shorting out the field at the ends.  Which both shields it from external fields, and reduces inductance and Q significantly.

They don't even rate it for Q, only DCR.  ACR is typically several times DCR (2-10 times, say).  Probably since thin wire will be used here, core loss (and end cap electrode resistance) will dominate, which can also be expressed as ESR.  So, it's probably not all that great, electrically.

It's also not clear how high permeability is.  This is a useful ref:
https://www.seventransistorlabs.com/Images/Rod_Core_Pressman_Billings_Morey.png
For L/D ~ 2, mu_eff doesn't increase much for mu_material > 10: it could be low-mu powdered iron, or high-mu ferrite, and you wouldn't know the difference.  (Hopefully they'd use a lower mu material, so that core loss isn't terrible?)

Tim

[Marco]

AFAICS with wire wound chip inductors the caps will always shield the magnetic path a bit. The multilayer inductors are aligned differently but they are baked in ferrite, that's far better shielding.

[T3sl4co1l]

There are myriad kinds of chip inductor.  The ones with bottom pads, and a vertical winding axis, are a good example.  Maybe not as practical for signal coupling.

Lots of axial wirewound chips without end caps exist: http://www.coilcraft.com/1812cs.cfm of course these only go up to 33uH, and you'll have a hard time finding larger with air core.  (They have a cored series in the size up to 1mH, but it's also shielded.)

Tim