Is it realistic to transmit small amounts of wireless power with SMT inductors? - Page 1
Forum section: electronics / projects

author: Starlord
author_rank: Frequent Contributor
timestamp: May 27, 2017, 03:02:10 am
content: Let's say I wanted to wirelessly transmit 5-10mA over a distance of 1" using tiny surface mount inductors. Like 1210 size at most.Is this at all realistic?I've been reading up on resonant inductive power transfer and I don't really have a good grasp on it yet, but it seems like I need two coils with the same inductance and capacitors for each. And I'm guessing that for optimal power transfer I have to run the coils at their resonant frequency? So to that end I looked to see what's available on Digikey for inductors in this size range, unshielded. and with a lowish frequency that I assume will be easier to generate:https://www.digikey.com/short/3dpd01And it looks like something in the 2-10MHz range is where I might be looking at operating.I also assume the output voltage would mirror the input voltage with the coils being the same number of turns but the current would drop off as the distance increases?Another thing I'm wondering is if the two coils need to be aligned. If one is turned 90 degrees so it's perpendicular to the other will there be no power transfer at all, or will it be reduced?

author: bktemp
author_rank: Super Contributor
timestamp: May 27, 2017, 04:45:22 am
content: It is all about the coupling factor. If it is 1, you have an ideal transformer with the output voltage having the same magnitude as the input voltage.But you will never get that, it will be always <1 even for a good transformer (often >0.95).For having a good coupling factor in an air core transformer, the diameter of the coil should be larger than the distance. With two 1210 coils spaced 1" apart, your coupling factor will be very low (<0.1), so the output voltage and the transfered amount of power.You typically choose an LC tank for the transmitter, because only a tiny amount of the current flowing through the inductor gets actually transmitted to the receiver, because of the low coupling coefficient. With an LC tank the current flowing is much higher then the current you need to pump into it, because the unused energy gets recycled.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 27, 2017, 07:15:30 am
content: Just from sort of ballpark estimates (and I am certainly not an expert), this sounds impossible.Wireless power transmission is inefficient at the best of times (with two very closely spaced, tuned coils and very high efficiency regulation), but the power really, really drops off with distance - even a couple of millimeters can mean 50% losses or more. That means to compensate for the massive 1" gap, you're going to have to drive the coil on the transmitter with so much power any 1210 probably would just cook in a matter of seconds.Not with the math background to give you a proper estimate.... I think you can expect closer to 5-10 uA instead of 5-10 mA with a fairly well optimized coupling.... I really doubt you could get a full mA without cooking your transmitter (this is assuming a low logic level of like 2.5V or 3.3V, since you should be talking mW rather than mA when trying to evaluate power transfer).You'd be expecting this kind of response curve with distancehttp://www.wirelesspowerconsortium.com/data/images/1/2/figure2.jpg

author: Starlord
author_rank: Frequent Contributor
timestamp: May 27, 2017, 07:43:34 am
content: I don't understand that graph.Is Axial Distance z / D a ratio? The distance between the inductors, divided by their diameter?I suppose that might be the case. 1" / 0.1" = 10, which would appear to indicate efficiency so bad it's off the bottom right of the graph.Hm, 1210 is 3225 in metric. 1cm / 0.25 = 4... That's within the graph I think.What's this D and D2 in the top right with the colors? Could it be that you can have coils of different size? So my transmitting coil could be larger than my receiving coil, but that then affects the efficiency? And the green line represents a receiving coil 1/10th the diameter of the transmitting coil?

author: bktemp
author_rank: Super Contributor
timestamp: May 27, 2017, 08:09:19 am
content: I would interpret the graph the same way.D is the diameter of the coil (not the length!) and z the distance. D2 is probably the size of the receive coil.With a distance of 25.4mm and a diameter of about 2x2mm, let's say 2mm, that gives a ratio of 12.7Based on that your coupling efficiency is somewhere way below the graph, maybe 1E-6 or so. So you need to pump 1MW into the transmitting coil to get 1W out of the receive coil.Having a ferrite in the coil could improve the coupling, but I doubt it will be better than 1E-3 or so.If you look at the dark blue curve, if the inductor diameter gets smaller than the distance (z/D>1), it quickly becomes unusable.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 27, 2017, 08:49:00 am
content: No way you will get an inch. A few mm, definitely doable though.SMD inductors vary greatly in construction, and many are optimised to minimise the external field, which is exactly what you don't want. There are some SMD inductors designed for use as LF (125KHz) RFID antennas - these would be worth a look.As an extremely rough guide, if you imagine a sphere the diameter if the long axis of each coil, you won;t et anything useful beyond the intersection of these spherese..g if you need an inch range, your coils are going to be at least an inch long ( cored) or diameter ( open flat coil), and you'd need to keep them well aligned.Due to the inverse square law, you need to do anything you can to reduce the distance.

author: tatus1969
author_rank: Super Contributor
timestamp: May 27, 2017, 09:43:25 am
content: What is it that you are trying to achieve? Maybe I can help (10y+ RFID experience).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2017, 09:57:24 am
content: An inch distance? Maybe with inch-sized coils.If you need isolation rather than distance, windings on opposite sides of PCB material can do very nicely. These can be made much wider than the PCB thickness, so the coupling can be quite good (> 0.5).It also doesn't work well if it has to allow for some change in distance. Like if the actual range is 0.25 to 1.0 inch. 0.25" is too close and the system needs to be re-tuned, or the design needs to use that much more reactive power (about double, in this case) to guard-band the operating range.It's also not something that can deliver on-demand power very well. A typical transformer does this nicely, because of its high coupling; but coupled inductors have a constant impedance (in a sense, the impedance varies with distance, hence the above), meaning you can't throttle the power output while maintaining a constant output voltage. (You can do some nonlinear tricks with rectification, and you can always add a regulator or converter to handle the extra range.) If you need to change the average power level, you're probably better off PWMing the whole thing -- but mind the rise/fall time is proportional to Q (which is inversely proportional to k), so to have meaningful PWM, you need minimum pulse widths (that is, tone bursts / gaps) longer than that, so your modulation can end up in the audio range very easily (which also means more work filtering that ripple).Tim

author: Someone
author_rank: Super Contributor
timestamp: May 27, 2017, 12:13:05 pm
content: Quote from: Starlord on May 27, 2017, 03:02:10 amYou'll need inductors with a sympathetic (really poorly shielded) design for power transfer to work, and the alignment will be very important. The size constraint will be your biggest hurdle as its possible to practically do with larger inductors and if you step up to 3x3mm footprints it should be trivial (albeit it very poorly coupled). For frequency stick to an ISM band unless you can shield it very well.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 27, 2017, 12:28:58 pm
content: Quote from: Starlord on May 27, 2017, 03:02:10 amSimply, no, it is not at all realistic.Perhaps if we knew WHY you think you need 1 inch separation, we can brain-storm a more realistic solution to your undisclosed problem.

author: Marco
author_rank: Super Contributor
timestamp: May 27, 2017, 02:47:25 pm
content: Quote from: T3sl4co1l on May 27, 2017, 09:57:24 amI had a thought about receivers which I wonder if could work.Could you use an receiving inductor with a self resonance frequency substantially higher than the transmitting frequency. Then use a bidirectional switch to short the receiver coil during one half of the transmitting cycle from peak to peak. Detect when the derivative of the current in the coil becomes zero to detect the peak, then open the short at which point the current can only go through a rectifier and all gets dumped into a relatively small capacitor (lets say it will dump the energy in 1/10th of the cycle time).Repeat for each half cycle and you get almost all the energy with none of the annoying tuning, only the transmitter would be constructed for sine resonance to minimize losses and interference with modulation of the rails of the driver to adapt the power transfer. Something semi-resonant like class E would still be efficient for the transmitter, but would have a much broader frequency range. The semi-resonance of the receiver will still cause some extra interference, but it should have much less effect (most of the energy keeps sloshing around in the air and the transmitter resonant tank)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2017, 06:28:38 pm
content: You can't do resonant transfer without resonance (as amazing as that sounds) -- not without severely compromising the transfer impedance you get from a given k.Dropping resonance from one is similar to squaring k. (I'm not sure if it's actually this, but it is much worse.)You could clamp it, letting it ring up, then open the switch at an opportune time, though.Note that it's not just SRF, but SRF-when-loaded-by-switch that matters here.Auto-tune would be interesting too, say by switching capacitors in (this works the same as old fashioned S-correction in hi-def CRTs).Tim

author: nctnico
author_rank: Super Contributor
timestamp: May 27, 2017, 07:46:44 pm
content: Quote from: Starlord on May 27, 2017, 03:02:10 am1" isn't a problem (been there, done that) and the current is rather low so no problem either. What you'll need are inductors which are intended for wireless power transfer though because these are 'open' and the ferrite material helps to concentrate and shield the magnetic field. Alignment will be important as well as having a resonant tank at both the transmitter and receiver. It will help a lot if one of the inductors can be a lot bigger so alignment isn't such an issue. You'd have to experiment a bit to see where you can get.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 27, 2017, 07:55:11 pm
content: Quote from: nctnico on May 27, 2017, 07:46:44 pmIt is with a 1210 size component.

author: Marco
author_rank: Super Contributor
timestamp: May 27, 2017, 08:24:01 pm
content: Quote from: T3sl4co1l on May 27, 2017, 06:28:38 pmIt's resonating at DC (short = infinite capacitor). There's a resonance mismatch, but because you take the energy out of the "secondary" each half cycle it won't oppose the primary's field unlike with normal mismatch. I intuit it's this opposing feedback which reduces energy transfer when there is a mismatch, but my EM knowledge is so poor I don't trust it much.I guess I'll ask spice whether a half cycle of a cosine current for a low coupling factor mH:mH transformer puts more energy in the secondary starting from zero current, or a secondary made resonant with a capacitor can put more energy in a resistor load in a half cycle in steady state.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2017, 08:41:23 pm
content: "Resonating at DC" is a meaningless statement; or, perhaps even better: okay, fine, sure, whatever -- but at DC, you have zero power transfer, so, so what?Time domain and pulsed analyses inevitably fail on this sort of thing. In my first real job, I was tasked by the company president to design the "fastest possible control" for induction heating. He thought it could be done every 1/4 cycle. What's so special about 1/4, versus other fractions of a cycle, I don't quite get, but he clearly didn't get the continuity of a resonant circuit: its behavior depends on a long history of past inputs, namely, the last (Q / f_0) seconds, give or take. (It's a weighted average over time, tapering off exponentially towards zero with that time constant. There are no hard cutoffs in this kind of system.)I eventually convinced him to be satisfied with the system I designed, a digital PID controlled PLL, updated each cycle, with a one-cycle pipeline (so, the timing for the next power cycle is calculated during one cycle, using the sampled measurements from the previous cycle). That made the RMS calculations tricky (measuring output V/I/P) since the average was done over a variable number of ADC samples (typically a few hundred).That was still quite a bit excessive, as the PID constants we chose ended up quite small.Anyway, even with perfect tuning (a frequency-agile induction heater is a rather special case, as it has to sweep to find its operating point, which isn't usually at resonance anyway, but another set limit instead), you're limited on startup time because of the Q factor.Another way to put it: you're storing so-and-so much resonant energy, but only delivering (or extracting) so much every cycle.Note that Vrms*Irms == VA, but if it's all VARs, this has units of energy -- in the same way that torque (N*m) is a unit of force, a "sideways" (rotating) force.Tim

author: Marco
author_rank: Super Contributor
timestamp: May 27, 2017, 10:14:41 pm
content: Quote from: T3sl4co1l on May 27, 2017, 08:41:23 pmCurrent induced in the secondary coil is power transferred, the second half cycle would immediately wipe the energy out again of course, but that's why you'd want to extract it beforehand.QuoteSure you can, you can "just" massively increase the rail voltage (maybe with a separate driver with it's own series inductor) to quickly add energy to the system, or you can "just" disconnect the tank and dump the inductor energy into a smaller capacitor or some water cooled resistance wire to stop on a dime. Because of thermal time constants I don't see how it would ever be justified, but still possible.It's only a steady state resonant system when you let it be.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2017, 11:00:43 pm
content: But if you have the spare supply voltage, and transistor amperage, to direct drive it, why not do that?That's also something we did: a direct drive induction heater. The size of a small walk-in refrigerator, and all that work just for 150kW. And yeah, 150kW, it can heat up a hunk of steel in a hurry (or the outside, anyway), but there was >1MVA of IGBTs in the sucker.And yeah, economy of scale is against you when your inverter modules are a few kilobucks a pop, and it's less of a big deal when you're comparing a $1 transistor to a $0.20 capacitor. But then again, if you're making only 10,000 of them, that's a kilobuck of savings to be had...Tim

author: Starlord
author_rank: Frequent Contributor
timestamp: May 28, 2017, 02:20:27 am
content: What I was hoping to achieve was to solder an SMD inductor to an SMD LED back to back with maybe a capacitor sandwiched in there, and then create a larger PCB surface with a lot of transmitters. The larger PCB could then be placed under a table and these grains of rice LEDs would begin to glow dimly within say an inch of the surface and brightly within around a quarter of an inch. The further away they'd glow the better because I don't have any control over how thick the table might be. Of course if that's simply too great a distance then I suppose one could create a pad to sit on the table instead so you could get them within a quarter of an inch of the inductors.I know lighting LEDs with wireless power is possible since there's plenty of tutorials out there on it, but the coils used for most of them are fairly large in relation to the LED. I wanted to know if I was possible to miniaturize it.I did come across this example of something some researchers at Stanford have done to use light to fire neurons in mice, but the technology they used is way over the top and requires a special cage which transmits in the ghz range and it's somehow tuned to the mouse's body so it acts as a resonator? Really advanced stuff. But despite being really advanced the LEDs are quite dim so are unsuitable for my needs. I was hoping to get around 5mA into the LED.http://healthtechinsider.com/2015/09/02/wireless-power-activates-led-implants-in-mice/

author: Marco
author_rank: Super Contributor
timestamp: May 28, 2017, 05:06:10 am
content: Why do the transmitting coils have to be so small then?

author: Starlord
author_rank: Frequent Contributor
timestamp: May 28, 2017, 05:21:33 am
content: They don't. Unless they do?I assumed they needed to be the same size to transmit energy efficiently, and that there would be no benefit to making the transmitting coil larger if I needed to match the inductance and such anyway.I also didn't want to use expensive thick inductors, and I don't know if it would be possible to make coils with PCB traces or how to go about this. If they have to match the inductance of the receiving coil that seems especially difficult.There's also this graph which was posted:http://www.wirelesspowerconsortium.com/data/images/1/2/figure2.jpgAnd 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.So is there some benefit to using larger transmitting coils while keeping the receiving coils small? And is this something I could implement with PCB traces rather than winding coils of wire?

author: bktemp
author_rank: Super Contributor
timestamp: May 28, 2017, 07:45:36 am
content: Quote from: Starlord on May 28, 2017, 05:21:33 amYes, it is always better it the coils are larger:The efficiency goes down if one coil is smaller than the other one, but it will go down even more with both coils being small.One example:Both coils have the same size, and the same distance, so z/D=1 and we look at the dark blue curve and get an efficiency of 0.4.If we now make one coil 1/10 the size, the efficiency will be 0.002 (dark green curve).But if me make both coils 1/10 size gives a z/D=10. The dark blue curve is outside the graph, the efficiency is somewhere around 1E-6 or so.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2017, 10:28:36 am
content: A huge transmit coil gets you the advantage that, although you're wasting a ton of power making a huge field that mostly doesn't go anywhere -- you get the best coupling possible to the receiver coil, given that distance.You'll want to do this at low frequencies, maybe 100kHz, to avoid emitting RFI. And yes, you'll definitely need capacitors to resonate at that frequency. But sure, lighting up LED "throwies" on a table is easily possible.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 28, 2017, 11:10:38 am
content: I've played around with LEDs powered with wireless transfer like this. You want a large coil diameter 100-200mm. Range depends on the size of the rx coil -with a 5mm drum inductor, 50-100mm range is doable with a white LED,which will produce a decent output at 1mA or so. You want the highest inductor value you can get, typically 220-470uh for smaller coils. With a ~1-2nf cap your frequency is in the 1-200khz range, which is a good compromise for efficiency.To drive the tx coil, a simple solution is a half-bridge MOSFET driver (e.g. tc442x), driving a series resonant cct. Supply voltage as high as you can -12-24v is good.Note the voltage at the LC junction can hit a few hundred volts.at resonance, so you need a cap with a suitable rating.

author: bktemp
author_rank: Super Contributor
timestamp: May 28, 2017, 11:21:21 am
content: Quote from: mikeselectricstuff on May 28, 2017, 11:10:38 amIs series resonance a good idea?I see this quite often, but I don't like it. Without any load there is a huge current in the LC tank, but as soon as you put a load on it, the power gets greatly reduces. That is exactly the opposite as you need it.My prefered method of driving coils with a low coupling factor is a royer type oscillator: It provides a constant output voltage over a wide load range. The peak voltage is 3.14x the supply voltage, so you don't need any protection against high voltages in case of no load. And you can easily adjust the field strength by adjusting the LC ratio.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 28, 2017, 12:25:09 pm
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: May 28, 2017, 01:17:59 pm
content: Quote from: Starlord on May 28, 2017, 05:21:33 amTaken 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 trythisfor the transmitting coil (if you have some litz wire you could make your own, but the ferrite film behind it helps a bit) andthisas 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2017, 05:19:41 pm
content: 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).Quote from: mikeselectricstuff on May 28, 2017, 12:25:09 pmAnd 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

author: Starlord
author_rank: Frequent Contributor
timestamp: May 28, 2017, 06:26:32 pm
content: 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-inductorIs 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2017, 06:32:36 pm
content: Quote from: Marco on May 28, 2017, 01:17:59 pmA 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.QuoteNow, 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.pngFor 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

author: Marco
author_rank: Super Contributor
timestamp: May 28, 2017, 07:38:59 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 29, 2017, 11:09:54 am
content: 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.cfmof 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