Qualcomm Rezence can wirelessly charge through metal

No tricks, just careful tuning and detail work

Qualcomm Snapdragon logoLast week Qualcomm announced their Rezence wireless charger works through metal, but what are the details. SemiAccurate dug into the tech and the answer is about what you would expect if you understand the underlying science.

Traditionally wireless charging of devices needed one or two things, a non-metal cover for the charging coils and possibly a very carefully aligned set of devices. The former is true for both the resonance based charging schemes like Rezence and tightly coupled ones like PMA and Qi, the latter isn’t. For tightly coupled charging systems you need a close and tightly aligned set of coils but luckily that doesn’t affect Qualcomm and their Rezence standard.

In case you haven’t been following wireless charging standards bodies lately, there were three standards, A4WP which is Qualcomm and others using resonant charging tech. Earlier this year that organization merged with rival PMA, a deal that was finalized in June. PMA uses tightly coupled charging tech, as does the third Qi standard which is not part of the merger. A4WP and PMA are now going to be ‘one standard’ with two radically different underlying physical layer technologies.

How does this work? Easy if you understand networking stacks and layers. The physical layer, that would be resonant or tightly coupled technology, is and will remain incompatible between them. Above that on the stack, all comms, signaling, and so on layers will be common between the PMA and A4WP standards. This should simplify chips quite a bit as there are already dual and tri-mode charge controllers on the market. Qi will still be an outlier but given the names behind the A4WP and PMA team, I know who I would bet on.

So back to the technology of working through metal. As you might have noticed over the last few years, more and more devices are coming with metal backs. Why? The moronic rush to the point of too thin and well beyond. You need rigidity and resistance to cracking, glass is very rigid up to a point… Dropping a metal case is usually a lot less problematic than an all glass one too.

With stiffness achieved via metal, all you need to do is pass radio signals through it, and that too is a bit of a challenge. Just for fun don’t pass just one radio signal through it, pass many GSM, LTE, Wi-Fi, Bluetooth, FM, GPS, and lots of other signals though the metal. Can you say lots of antennas many of which have quite differing requirements? It isn’t easy and making all but the screen out of a substance that blocks those signals quite effectively is not conducive to a device that needs good reception.

What is an RF engineer to do when an OCD boss with a penchant for smooth, uninterrupted metal surfaces gives her a fashionable design that kills any signal going through it? Quit, say something that will get the entire team fired, fake it, or cut slits in the case to place the antennas in. In case you are wondering, most companies take the third or fourth option depending on their quality standards.

If you have ever seen a solid metal surface that has an LED display light up behind when you touch it, it works the same way. Take a big chunk of metal and grind the back until it is wafer thin, then you put a very bright LED behind it. If you mount the LEDs right the surface should be strong enough to not dent under a light breeze either. For antennas you do much the same, cut a slit in the metal back, side, or whatever, and mount your electronics behind it. Thin metal attenuates the signal much less than thick metal so a cleverly designed antenna will work through a ‘solid metal’ back at the cost of a little stiffness.

“Aha” you say, so that’s how they make charging work through metal, cut slits so it is so thin the backing is effectively not there. Simple, elegant, and wrong. Qualcomm does nothing of the sort, Rezence works through a ‘thick’ metal back, and by thick we mean the usual thickness of a non-ground or slit backing.

The first caveat is quite visible in the video here, the backing is aluminum. I asked Qualcomm about this and they confirmed that the wireless charging indeed works through aluminum but not ferrous metals. This was qualified by the statement that structural metal phone backs are almost all aluminum so steel isn’t an issue. Qualcomm focused on tuning the system through that metal for obvious reasons.

So how did they do it? Essentially they tuned the case to resonate with the antenna and they can pick up eddy currents from it too. The case becomes a part of the antenna plus you can wire parts of the case or backing to work in series. In the end you can pick up about an amp of current (Note: Not sure about the voltage but we presume it is 5V) through/with an aluminum back panel on your phone.

This is a tricky exercise though, you need to tune the case and antenna individually for things like camera openings, buttons, Wi-Fi, various antenna slits, and all of that. It works but it is not as simple as spot-welding a few leads to the case and hoping it all ends well. There is a lot of engineering needed, something we assume will become easier and more commonplace as niche specific CAD/CAE tools evolve. The point of the Qualcomm announcement and demo was that it could be done, and that is exactly what they did.

Another little gem in the press release was that Qualcomm is working to put the Rezence circuitry on the motherboard itself. With that in mind the obvious question is how much board area does it take up? The answer would be <100mm^2 or so, 80 or so may be achievable with a lot of hard work. That isn’t much unless you are talking about a cell phone board where it becomes tight but doable. If any adventurous company decides to leave out conventional charging features, it could be close to a wash.

If you look at the video above, you might notice that the phones in question can charge on both faces, up or down. That is because some of the antenna is in the side of the device. It loses a little efficiency when charged screen down but in a public place that may be a very worthwhile tradeoff for some individuals.

In the end the tech is fairly straightforward as far as the science goes, take a chunk of aluminum and tune it to resonate at the right frequencies as the charging circuit needs. This is also possible with the PMA and Qi tightly coupled chargers but given the frequencies they operate on, heat will be much more of an issue, much more. The Qualcomm Rezence solution is here now, works now, and probably is available as a reference design if you smile and ask sweetly. Given how tech comes to market, we expect to see it somewhere by CES if not before.S|A

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Charlie Demerjian

Roving engine of chaos and snide remarks at SemiAccurate
Charlie Demerjian is the founder of Stone Arch Networking Services and SemiAccurate.com. SemiAccurate.com is a technology news site; addressing hardware design, software selection, customization, securing and maintenance, with over one million views per month. He is a technologist and analyst specializing in semiconductors, system and network architecture. As head writer of SemiAccurate.com, he regularly advises writers, analysts, and industry executives on technical matters and long lead industry trends. Charlie is also a council member with Gerson Lehman Group. FullyAccurate