Permittable Radiated Energy from WC

[nataddrho]

Greetings.

I've designed and manufactured about 20 units of a small wireless charger for a product I am working on as a side project from my day job. The charger is very simple: 5VDC from USB-C source, boost converter to 12VDC, and then a low power H-Bridge (UCC37321 / UCC37322 opposite each other) driving a Wurth charge coil and an appropriate capacitance in series to resonate at 6.78 MHz. I use a precision clock for the frequency source. It works fine and the capacitor voltage approaches 50V. The receiving device has a 6.78 MHz tank (capacitor in parallel with smaller coil) attached to an energy harvester, which has an efficient recitifier and boost electronics for charging an LiPO.

The wireless charger draws about 0.1A, and I receive ~3mA at a range of about 1/4" on average, which a about 1.5 coil widths distance for the receiving device. The Q is about 1300 theoreticaly, but there is a little more bandwidth experimentally. If I use NP0 low tolerance parts then I should be within something acceptable without needing to auto tune. There is a lot of room for optimizing but good enough to showcase the functionality of the product to a company.

The question I have is:

I have not yet gone through the process for FCC testing for "intentional" radiating devices. I've only done "un-intentional". I chose 6.78 MHz because I don't yet have a good artistic feel for how much power is acceptable. It may be that the wireless charger will never ciolate standards at any frequency, and I would get better efficiency at lower frequencies that I think are forbidden.

For ISM, I tried to figure out what the radiated energy is, but this is the first time doing this. Can you check my calculations? I am assuming the transmitting coil has 0.4ARMS. The transmit and receive coil both have ferrite pads on opposite sides to help with flux concentration, but that is not included in the calculation. (SEE ATTACHED PDF)

Nate

 

[vfone]

According to the regulation, theoretically you don't have to follow the 25uV/m at 300m requirements IF you stay within +/-15kHz tolerance limits.
https://www.law.cornell.edu/cfr/text/47/18.301
I think that if use a low ppm TCXO, you don't have to follow the above rule.

Using lower frequencies (e.g. 100kHz) for WPT, have advantages including less emissions and better System Efficiency.
Documents using expensive GaN transistors for WPT at 6.78MHz shows 99% drain efficiency, but never mention the System Efficiency.
Using 100kHz and cheap MOSFET transistors can reach more than 95% System Efficiency, and when using HF 6.78MHz the System Efficiency is in 70% range.

 

[FvM]

As far as I see, 6.78 MHz isn't generally licensed for ISM applications in the FCC domain. 13.56 MHz e.g. is. But apart from this point, I wonder why you stick to the MHz range for wireless charging. Why not < 150 kHz as Qi does?

 

[nataddrho]

I have no feel for how much power what I made is transmitting into the air, and I don't know how to measure it. I don't know what frequency is best to use.

 

[FvM]

Radiated power of basic coupler configurations (e.g. planar coil) can be estimated according to EM theory.

 

[nataddrho]

Also I wanted to point out that I must use resonant charging and not inductive, since my coil size must be tiny and the distance must be at least one coil width. Qi uses inductive charging which is not a technology I can use. Resonant frequencies are commonly at 6.78 MHz. I did do EM analysis in the pdf I attached originally and was asking for a test expert to check my work.