wireless power transfer frequency selection

[eeefreakians]

why we are choosing a frequency in the range of khz.

most literature survey uses 100khz frequency. pls explain wat is the freq selection criteria8-O

 

[treez]

you mean like the isotera system?, or you are talking the splashpower type system?...but anyway, i think its MHz plus if its splash power , otherwise its too difficult to direct to where you want it to go.

 

[Warpspeed]

There may also be a problem with external RF radiation and interference to other services.
You cannot just build a high power an RF generator for any frequency, it will need to be on one of the ISM bands (industrial, scientific, medical).

There you can kick up as much noise as you like.

 

[kthackst]

In general, you want to design a wireless power link to be as efficient as possible. Picking lower frequencies like 100 kHz, as Warspeed suggests, means you don't have to deal with the headache of picking an ISM band.

But if you have serious design constraints affecting the size and form factor of your coils, you might want to pick a frequency where they operate more efficiently (i.e. have high Q factors). Q factor is a function of frequency. Generally, large coils have higher Q at lower frequencies and smaller coils have higher Q at higher at higher frequencies. Popular RF frequencies are 6.78 and 13.56 MHz.

 

[bhl777]

In general, you want to design a wireless power link to be as efficient as possible. Picking lower frequencies like 100 kHz, as Warspeed suggests, means you don't have to deal with the headache of picking an ISM band.

But if you have serious design constraints affecting the size and form factor of your coils, you might want to pick a frequency where they operate more efficiently (i.e. have high Q factors). Q factor is a function of frequency. Generally, large coils have higher Q at lower frequencies and smaller coils have higher Q at higher at higher frequencies. Popular RF frequencies are 6.78 and 13.56 MHz.

Can I ask several similar frequency selection questions? My application is for implantable electronics devices using wireless power transfer systems. In my application, I will have two coils: one power coils operating in either 6.78 or 13.56MHz, another one is the data coil. I want the data coil to operate within a frequency range. My questions are
(1) For example, my frequency range of the data coil is 1MHz to 5MHz, does this range meet ISM band requirement? I do not care about the amplitude of the generated signal at the secondary data coil, but I do want the frequency not get distorted.
(2) Is there any practical issues in using this data coil together with the power coil? such as interference.
(3) If I can use 1MHz to 5MHz for my data coil, is there any difference in using 6.78Mhz power coil or using 13.56MHz?
(4) If I have other choices of the frequency range for the data coil, such as 20MHz to 25MHz. If I do not care about the size of the data coil, is there any other differences in the practice between these two frequency ranges? (1-4MHz and 20-25MHz).

Thank you!

 

[Warpspeed]

For practical reasons, you might want to place the power transmission frequency above the data transmission frequency, the further apart the better.
Any harmonics or broadband noise generated from the power transmission will then be well out of the way of your data link.

 

[bhl777]

For practical reasons, you might want to place the power transmission frequency above the data transmission frequency, the further apart the better.
Any harmonics or broadband noise generated from the power transmission will then be well out of the way of your data link.

Thank you Warpspeed. According to your input, it seems power coil with 6.78MHz and data coil with above 20MHz will be the best choice. Would you tell me what are the detailed problems in using this selection, compared to a case with 1-5MHz in the data coil and 13.56MHz in power coil?

 

[Warpspeed]

I would run the data channel at 6.78 Mhz and the power circuit at 13.56 Mhz.

These bands are harmonically related for a very good reason.
Any interference generated at 6.78 Mhz is bound to be mostly harmonics.
Knowing that, the powers that be, very wisely made the ISM bands harmonically related so any resultant interference is only to other ISM users.

So its best to run the power at the higher of the two frequencies to lessen any interference to your own system..

 

[bhl777]

I would run the data channel at 6.78 Mhz and the power circuit at 13.56 Mhz.

These bands are harmonically related for a very good reason.
Any interference generated at 6.78 Mhz is bound to be mostly harmonics.
Knowing that, the powers that be, very wisely made the ISM bands harmonically related so any resultant interference is only to other ISM users.

So its best to run the power at the higher of the two frequencies to lessen any interference to your own system..

Thank you Warpspeed. If my power coil is 13.56MHZ, and my data coil needs to accomodate a range of frequencies, instead of a fixed 6.78MHz.
(1) Ideally, should I use 6.78 as a central frequcy, then define my range such as 4.78 to 8.78MHz?
(2) If I still use 1-5MHz in my data coil, and 13.56MHz in my power coil, do you think will it still functional in getting the target frequency at the secondary data coil?

 

[Warpspeed]

What exactly is this 1-5 Mhz signal ?

 

[bhl777]

What exactly is this 1-5 Mhz signal ?

They are the sinusoidal/square signal to drive the data coil. This signal is a fixed frequency signal, but its range can be adjusted within 1MHz to 5MHz. For example, when I input a 2MHz signal at the primary data coil, as far as I can generate a 2MHz sinusoidal signal at the secondaary data coil, that meets my target.
I just want to make sure if I use 13.56MHz power coil, it will not impact the 2MHz signal I received at the secondary data coil. Will it?
Thank you!

 

[Warpspeed]

I just want to make sure if I use 13.56MHz power coil, it will not impact the 2MHz signal I received at the secondary data coil. Will it?

The 2Mhz receiving antenna will pick up a lot of 13.56 Mhz energy for sure, but that can be fairly readily filtered out leaving the 2Mhz signal unmolested.

Low received 2 Mhz data amplitude is less of a problem, because it can be amplified up after reception, unlike the received raw power received at 13.56 Mhz.

If you are transmitting at 13.56 Mhz most of the energy will be there, plus some wideband phase noise and harmonics. There should be nothing below that.

 

[bhl777]

The 2Mhz receiving antenna will pick up a lot of 13.56 Mhz energy for sure, but that can be fairly readily filtered out leaving the 2Mhz signal unmolested.

Low received 2 Mhz data amplitude is less of a problem, because it can be amplified up after reception, unlike the received raw power received at 13.56 Mhz.

If you are transmitting at 13.56 Mhz most of the energy will be there, plus some wideband phase noise and harmonics. There should be nothing below that.

Thank you Warpspeed. As I understand, you think that should not be a problem in tranmitting the signal at the data coil at a frequency between 1MHz to 5MHz, while the frequency at the power coil is 13.56MHz. Is it correct?

In addition, if I need to purchase a set of eval kits to do this experiment, do I need to buy the boards for power coil and data coil seperateley? Is there any eval kit that includes both coils and allow the input from a function generator, so I can use to tune the frequency of the data coil between the specific range (such as 2MHz)?

 

[Warpspeed]

I doubt if you can just buy some off the shelf circuit boards and string it all together and expect it to work first time.

Everything really needs to be properly designed and tested to suit your specific purpose, of which I have absolutely no idea.

 

[bhl777]

I doubt if you can just buy some off the shelf circuit boards and string it all together and expect it to work first time.

Everything really needs to be properly designed and tested to suit your specific purpose, of which I have absolutely no idea.

I am able to replace the components by myselfs. But I am not sure where to find the eval kit with function generator (or any other tunable) input. If there is no such tunable input board exsit, do you know how the frequencies are fixed on the eval boards?