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Short introduction! I've recently built electric skateboard (you can look up the commercial ones) and I want to design/get a wireless charging system for it!

The problem is, it seems the current Qi chargers are not capable enough! I would need ideally 50-150w of power to the battery directly.. These little qi ones are only able to output 2.5-5w max per unit.. + There is also a limit on how many devices the charging mat can sustain it seems (+ their power ratings in general are also not great - Samsung has up to 1amp charging power, it looks)

http://www.bestbuy.com/site/samsung-mini-qi-wireless-charging-pad-black/7250019.p?skuId=7250019

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I found a company, which seems to be related to developing wireless technology exlusively, take a look at them:

Search: Proxy module 100w (the 3rd result should be the one!)

They have a product for 100w output but it is rather bulky.. plus, I believe the price could also be quite steep as it is individually produced model/product and they will probably want some hefty honorarium for developing such product with their intelectual property...

Btw - I also found such a chip called: BQ 51050B you can look it up, it is made by texas instruments, it is not possible for me to post any more links for now, so I cannot give datasheet directly.

Anyways the chip is for receiving power only and charging the batteries, so it still needs a coil, for receiving power!

It looks like it is already designed for this application (the chip), but the question still remains:

Where to get the parts (coil, especially) to build more powerful wireless transmission module / system?

Thanks for answers anyways - I hope I can come up with something using this forum / Q * A site :)

BTW the max coil diameter should be in the 150mm range / 5.905 in, as the boards are not typically more wider than 200-220mm (~8-9inch.

I umderstand that the diameter alone does not play part.. but the wire thickness and count.. but diamater still should be taken into consideration, because as I said, the receiver coil shouldnt be much wider than that!

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    \$\begingroup\$ Too lazy to read all of it. But why not dozens of QI-like systems? \$\endgroup\$
    – user76844
    Commented Jan 21, 2017 at 14:57
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    \$\begingroup\$ Since you've found a 100W one the answer is obviously yes. If you're too lazy even to include the links as plain text I'm too lazy to go searching for them. \$\endgroup\$
    – user16324
    Commented Jan 21, 2017 at 14:59
  • \$\begingroup\$ witricity.com have designs for Writicity WiT-3300 up to 3.3kW, but like RF power Amps and RF receivers with intelligent controls you can expect current prices to be starting around $10/W in production. Still interested? vs the cost of a cable and a supply @$1/W \$\endgroup\$
    – D.A.S.
    Commented Jan 21, 2017 at 15:22
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    \$\begingroup\$ Yes, the products I design a lot generally transfer power to a rotating armature fitted with monitoring electronics to measure strains and temperature. However its only in the region sub ten watts but I understand the principles. At about 100mm I got an efficiency of about 20% but there was a kot of metal around to sap the magnetic fields. My coils were made f4om litz wire just so that I could generate a tens of amps in the transmit coil to overcome the losses. You won't find many folk who have transmitted over a hundred watts though. \$\endgroup\$
    – Andy aka
    Commented Jan 21, 2017 at 21:23
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    \$\begingroup\$ Make or buy, safe or not, cheap or not, efficient or not, pick any 2. \$\endgroup\$
    – D.A.S.
    Commented Jan 21, 2017 at 22:22

2 Answers 2

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Wireless Power transmission. Is it possible to achieve 100-200w (load side) power using off-the shelf components?

Here's an answer that takes you some distance in understanding some of the physics and the problems. What you want sounds a lot like how a transformer works but without the magnetic core shared by the primary and secondary windings. For a regular transformer you can easily get power transfer efficiencies greater than 95% but without a core you start to hit problems.

The main problem is that the alternating magnetic flux produced by the driven coil only loosely couples with the receiving coil. At best, given the scenario described, this might be 30%. With only 30% of the flux received, the receiver has to have 3 times as many turns to get a 1:1 voltage transfer. That's a voltage transfer and not a power transfer.

When you have a load connected that takes significant current, that field starts to reduce - the current in the receive coil produces a counteracting field. This then reduces the net field and voltage drops. More load current means more voltage reductions.

To counter this, designers use resonance. Resonating the primary transmit winding with a capacitor means that the nature of the current flowing in the primary is massively increased (maybe ten or twenty fold). Because there is more current there is more magnetic flux proportionately.

To get 100 watts across a gap probably means a current in the realm of several tens of amps RMS and this starts to mean you need to use Litz wire (easy to use and very neat but rather expensive).

It also means you need a fairly stable high frequency power oscillator that is capable of shoving out 100 watts plus all the power lost. This might be in the realm of 200 to 300 watts. Without a load on the receiver that's not a big deal but you have to be wary of producing emissions that can effect other local pieces of electronics.

All in all it's a problematic job for someone experienced in electronics and magnetics. I could probably crack it after a few months (and I have patents in this sort of AC magnetic technology) but it might take you a year or so or even longer.

Here's a simulation: -

enter image description here

I've chosen a primary inductance of 1uH, a coupling of 50% and a secondary inductance of 4 uH (twice the number of turns as primary). There is a 1 ohm load connected to the secondary and that load is series tuned with 8.4 nF to maximize power transfer. The oscillator runs at 1 MHz

When you do the math to calculate the capacitor value, you find that the effective inductance after all the partial coupling is 3 uH.

With Rval (primary coil resistance) of 0.1 ohms, it will be twice this in the secondary because there are twice the number of turns. To get 100 watts out requires 28.8 Vp-p across the 1 ohm load.

This requires 37.2 Vp-p at the input and an input current of about 10 amps RMS. My simulator tells me that the input power is about 131 watts so that's not a bad inefficiency but remember that coil coupling is 50% and overly optimistic.

So now, the coupling has dropped to 30% and immediately we have lost resonant tuning because the effective value of the inductance has changed. With the previous input being the same, the output power is a measly 5.4 Vp-p. To restore the output to being 100 watts now needs an input power of about 150 watts but now the input voltage has to be massively increased from 37.2 Vp-p to 194 Vp-p to overcome all the de-tuning effect of moving the coupling (this is the same as moving the two coils apart a little bit).

Do you see the logistics of the problem? You can optimize the tuning to give a decent power efficiency at a certain distance then, when you move the coils apart you get massive de-tuning and you then have to force a lot of reactive power into the primary winding to get what you want in terms of output power.

The simulation is a very simple circuit - there is no power oscillator and there is no receiver rectification and regulation - it's just a sine wave in and sine wave out across a 1 ohm load.

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  • \$\begingroup\$ Thanks for the lenghty description! Yes, when researching this further, there looks like to be many problems associated with high power transfer wirelessly.. it works kind of okay for low power devices but gets a lot more complicated for high power devices.. I received a reply from a company which delevops such systems and the price was in the 500-900 range for 100w model (receiver + transmitter), so basically it is a rather expensive ''luxury'' to be able to charge devices at such high power :) Non the less, will still keep looking into this from time to time \$\endgroup\$
    – Austris Miklavs
    Commented Jan 26, 2017 at 23:26
  • \$\begingroup\$ Will probably look into some easy to connect DC jacks / contacts for now, which can ''simulate'' the ease of use of wireless charging.. It looks like such solutions (high power wireless power transmission) is more suitable for applications where physical wires are really an obstacle or are hard to implement - like the forestry fields with moving parts etc where wires could be easly damaged or the object moves too aggresively for any wires to withstand such motion for long periods of time.. \$\endgroup\$
    – Austris Miklavs
    Commented Jan 26, 2017 at 23:28
  • \$\begingroup\$ I did. Still new to this forum. Thanks again for the extensive answer! I assume you answered the question why we cannot see more widesspread use of high powe wireless power transmission devices.. it just boils down to complexity or low efficiency.. \$\endgroup\$
    – Austris Miklavs
    Commented Jan 27, 2017 at 18:04
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Stumbled on this question while looking for an answer regarding a charger I bought that actually fits your needs very well.

Wurth Electronics Wireless Power 200W Development Kit You can actually download the BOM, schematics, and firmware here, if you really want to build your own or need to make changes to the design.

Mouser Article

Mouser Purchase Page ($387.50)

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