editor's blog
Subscribe Now

Wireless Power and Heating

We covered wireless power before, and one of the points of differentiation was that of inadvertent heating of nearby items. With systems using the lower 200-kHz frequency range, nearby largish metal items like coins and keys can heat up. The systems themselves are designed to detect this and shut the charger down, which addresses the safety issue. It’s just a bother if you think your phone is being charged when in fact it isn’t due to something else around there.

But then it was pointed out that heating can theoretically be an issue for any frequency; it’s just a matter of the thickness of the material and the frequency used. Higher frequencies would create heating in thinner objects; lower frequencies would heat thicker objects. Which means that the 6.78-MHz range of charging can also cause heating for some thinner range of metallic items.

So in the MHz range, keys and coins are fine; is there anything else that might accidentally come in range? Turns out there is one thing: CDs, which have a very thin foil in them (standard kitchen-grade aluminum foil is too thick). And, confirming with WiTricity, yes, they can actually heat up. (And system designers can detect the issue and shut down, just as the lower-frequency systems do. Which means the phone-didn’t-charge bother could happen there too). It could probably be argued that it’s less likely for a CD to be in the way (and, one might ask, who still uses CDs, anyway?) But, they were eager to point out, in cases where there was heating, they had never seen an instance of a CD actually losing any data.

That’s all well and good, at least until I started extrapolating the Cota technology (which we covered today), which uses RF at 2.4 GHz. If the MHz range affects CDs, it’s pretty much impossible to imagine something so thin that the GHz system would affect it. Just following that line of thought, I then realized that integrated circuits have extremely thin films of metals in them. Could this be a problem?

I asked this of Ossia, and they reminded me that the signal power being delivered to charge a phone is no higher than what the phone itself transmits. So if the phone isn’t heating its own metal, then the charging shouldn’t either.

Bear in mind that neither of us did the calculation to see if those IC thin films fall into a range that would even theoretically be affected; the power argument makes it an academic calculation. It also occurs to me on further hindsight that this isn’t resonant charging; it is, as PowerByProxi pointed out, more like harvesting RF for energy. So that might change the entire scenario.

So, in summary:

  • 200-kHZ resonant systems can heat objects like keys and coins; systems can detect and shut down for safety
  • 6.78-MHz resonant systems can heat CDs; systems can detect and shut down for safety
  • 2.4 GHz RF systems should have no heating issues.

Leave a Reply

featured blogs
Jan 27, 2023
Wow, it's already the last Friday in January, so time for one of my monthly update posts where I cover anything that doesn't justify its own full post or which is an update to something I wrote about earlier. Automotive Security I have written about automotive secur...
Jan 26, 2023
By Slava Zhuchenya Software migration can be a dreaded endeavor, especially for electronic design automation (EDA) tools that design companies… ...
Jan 24, 2023
We explain embedded magnetoresistive random access memory (eMRAM) and its low-power SoC design applications as a non-volatile memory alternative to SRAM & Flash. The post Why Embedded MRAMs Are the Future for Advanced-Node SoCs appeared first on From Silicon To Software...
Jan 19, 2023
Are you having problems adjusting your watch strap or swapping out your watch battery? If so, I am the bearer of glad tidings....

featured video

Synopsys 224G & 112G Ethernet PHY IP OIF Interop at ECOC 2022

Sponsored by Synopsys

This Featured Video shows four demonstrations of the Synopsys 224G and 112G Ethernet PHY IP long and medium reach performance, interoperating with third-party channels and SerDes.

Learn More

featured chalk talk

The Composite Power Inductance Story

Sponsored by Mouser Electronics and Vishay

Power inductor technology has made a huge difference in the evolution of our electronic system designs. In this episode of Chalk Talk, Amelia Dalton chats with Tim Shafer from Vishay about the history of power inductor technology, how Vishay developed the most compact and efficient power inductor on the market today and why Vishay’s extensive portfolio of composite power inductors might be the best solution for your next embedded system design.

Click here for more information about Vishay Inductors