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
Jul 3, 2020
[From the last episode: We looked at CNNs for vision as well as other neural networks for other applications.] We'€™re going to take a quick detour into math today. For those of you that have done advanced math, this may be a review, or it might even seem to be talking down...
Jul 2, 2020
Using the bitwise operators in general, and employing them to perform masking operations in particular, can be extremely efficacious....
Jul 2, 2020
In June, we continued to upgrade several key pieces of content across the website, including more interactive product explorers on several pages and a homepage refresh. We also made a significant update to our product pages which allows logged-in users to see customer-specifi...

Featured Video

Product Update: New DesignWare® IOs

Sponsored by Synopsys

Join Faisal Goriawalla for an update on Synopsys’ DesignWare GPIO and Specialty IO IP, including LVDS, I2C and I3C. The IO portfolio is silicon-proven across a range of foundries and process nodes, and is ready for your next SoC design.

Click here for more information about DesignWare Embedded Memories, Logic Libraries and Test Videos

Featured Paper

Cryptography: A Closer Look at the Algorithms

Sponsored by Maxim Integrated

Get more details about how cryptographic algorithms are implemented and how an asymmetric key algorithm can be used to exchange a shared private key.

Click here to download the whitepaper

Featured Chalk Talk

High-Performance Motor Control Solutions Through Integration

Sponsored by Mouser Electronics and Qorvo

Brushless motors have taken over the market for a huge number of applications these days. But, it’s easy to blow up your BOM cost with all the motor control and power management components required. In this episode of Chalk Talk, Amelia Dalton chats with Marc Sousa of Qorvo about the Power Application Controller (PAC) that can lower your BOM, trim down your component list, and give you several other benefits as well.

Click here for more information about Qorvo Power Application Controllers®