Why are wireless chargers are inefficient?

shane88

Joined May 22, 2019
12
In the current market, the chargers are only 40-50% efficient.

• How much is the conversion and flux losses during transmission?
• Is there any way to improve the overall efficiency?
• Can a ferrite core help to reduce the losses?
• What if I use ferrite core in the receiver coil just like the transformer. Can it solve the efficiency problem?
Thank you.

bertus

Joined Apr 5, 2008
20,773
Hello,

The main problem with wireless chargers is the airgap.
The efficiency will get worse if the airgap gets larger.

Bertus

shane88

Joined May 22, 2019
12
Why don't we use the transformer concept instead of the air gap? We can use the core in the receiver to improve efficiency. Why the companies are not using this method. Kindly advise.

Alec_t

Joined Sep 17, 2013
11,650
Why the companies are not using this method. Kindly advise.
They've been using the method for ages. Consider a rechargable electric toothbrush, for example. Both the charger and the brush have coils which are magnetically coupled by a ferromagnetic slug in one or other coil.

nsaspook

Joined Aug 27, 2009
7,594
https://www.infineon.com/dgdl/Infineon-Whitepaper_WirelessCharging_Wireless_Resonant_Power_Transfer-WP-v01_00-EN.pdf?fileId=5546d462636cc8fb0163aaf282900eef
Why don't we use the transformer concept instead of the air gap? We can use the core in the receiver to improve efficiency. Why the companies are not using this method. Kindly advise.
The transformer concept still operates on basic field electromagnetics (EM).

These are near field devices where the fields decrease with the cube of distance (air-gap). Power is proportional to the square of the field strength so once you are past the direct induction region (reactive near field) where most of flux passes between the two coils, doubling the coil distance causes power to drop by a factor of 64. Resonant coupling theory allows you use use the near-field region to the edge of the radiation region but it's still limited to near-field magnetic field coupling drop-off.

Wired power uses EM fields to transfer power too but there the coupling is almost always very near-field reactive unless we want to create an antenna.

shane88

Joined May 22, 2019
12

shane88

Joined May 22, 2019
12
https://www.infineon.com/dgdl/Infineon-Whitepaper_WirelessCharging_Wireless_Resonant_Power_Transfer-WP-v01_00-EN.pdf?fileId=5546d462636cc8fb0163aaf282900eef

The transformer concept still operates on basic field electromagnetics (EM).

These are near field devices where the fields decrease with the cube of distance (air-gap). Power is proportional to the square of the field strength so once you are past the direct induction region (reactive near field) where most of flux passes between the two coils, doubling the coil distance causes power to drop by a factor of 64. Resonant coupling theory allows you use use the near-field region to the edge of the radiation region but it's still limited to near-field magnetic field coupling drop-off.

Wired power uses EM fields to transfer power too but there the coupling is almost always very near-field reactive unless we want to create an antenna.
Thank you for answering. I was thinking what if we use a bigger receiver and small transmitter, Won't the receiver suck all the flux from the transmitter since it's bigger in size. Most wireless charging concept work through mutual induction with similar size TX and RX. What happens if the receiver is bigger than the transmitter. Any advise would help with my research. Thank you

BobTPH

Joined Jun 5, 2013
2,541
The strongest field is close to the coil. Using a larger receiver coil would place the coils farther apart and thus less magnetic coupling.

Bob

nsaspook

Joined Aug 27, 2009
7,594
Thank you for answering. I was thinking what if we use a bigger receiver and small transmitter, Won't the receiver suck all the flux from the transmitter since it's bigger in size. Most wireless charging concept work through mutual induction with similar size TX and RX. What happens if the receiver is bigger than the transmitter. Any advise would help with my research. Thank you
Flux is just a mathematical model of something. Electric field, Magnetic field, etc ... crossing a boundary that might not even physically exist. More source flux don't mean much if the orientation of the field(s) and receiver surface are not aligned (are parallel) at the receiver.

https://www.wired.com/2014/09/the-physics-of-wireless-charging/

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crutschow

Joined Mar 14, 2008
25,438
Won't the receiver suck all the flux from the transmitter since it's bigger in size.
That should help, but it won't "suck" all of it.
Remember that any air gap allows some magnetic flux to escape being coupled to the receiver, which reduces efficiency.

shane88

Joined May 22, 2019
12
That should help, but it won't "suck" all of it.
Remember that any air gap allows some magnetic flux to escape being coupled to the receiver, which reduces efficiency.
Kindly advise what are the advantages and disadvantages of using ferrite core in the receiver coil. Are there any safety issues? Can it interrupt or disturb the phone function? I'm thinking to design a phone a cover with a receiver coil and ferrite core to improve the receiving efficiency.

Thanks.

shane88

Joined May 22, 2019
12
Flux is just a mathematical model of something. Electric field, Magnetic field, etc ... crossing a boundary that might not even physically exist. More source flux don't mean much if the orientation of the field(s) and receiver surface are not aligned (are parallel) at the receiver.

https://www.wired.com/2014/09/the-physics-of-wireless-charging/

Very useful article. It gave me a good understanding of the flux. What if we use a bigger receiver and maintain low frequency, the flux also will be bigger enough to go through all areas of the receiver. Just thinking.

BR-549

Joined Sep 22, 2013
4,938
Truly wireless would be ... being able to charge phone from anywhere. And that is not possible. At least not without Tesla's dream. OR gamma frequencies. One would need very heavy sunscreen.

So...all we have is close proximity wireless.

And if close wireless is all we have........why not just plug it in? Or dock it.

BobTPH

Joined Jun 5, 2013
2,541
And if close wireless is all we have........why not just plug it in? Or dock it.
Which is what 99% of us do.

Bob

nsaspook

Joined Aug 27, 2009
7,594
Ultimately room area wireless charging is a dog walking on two legs. We are amazed that it works, not that it works well.

dl324

Joined Mar 30, 2015
11,543
In the current market, the chargers are only 40-50% efficient.
Physics - inverse square law.

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nsaspook

Joined Aug 27, 2009
7,594
If it was just a matter of being clever companies like Apple would have cracked the wireless 'nut' long ago. Instead they discovered:

https://liesandstartuppr.blogspot.com/2019/04/wireless-charging-in-2019.html
There must have been tremendous pressure within the company to make this work, having made a major public announcement. Apple even spent over \$100 million to buy PowerbyProxi, who had claimed to have solved all these issues. Apparently not, and it would be interesting to see if PbP could actually do what they claimed when bought. It's likely heads will roll over something like this, it's a major screw up and not something to be taken lightly.