Next Generation wireless power technologies that do not heat common metal objects

Outlining inductive wireless power’s technical problem with heating metal objects, and explaining how AirFuel Resonant delivers a better solution.

Wirelessly charged devices and common metal objects.

Inductive’s metal problem

The first generation wireless power technology on the market today has a dirty secret. Inductive wireless power has a tendency to heat common metal objects, such as coins and car keys.

To get around this hurdle, inductive is often programmed to stop the flow of power when it detects metal in the vicinity. The result is a technology that is fighting an uphill battle to downplay its technical shortcomings, and the market is starting to recognize that maybe inductive wireless power technology cannot deliver the wireless power experience that customers envision. One of the most notable cases of this was in April, when Apple abandoned their inductive-based AirPower initiative.

We’ve already explored how the wireless power market will move past AirPower, but what is the path forward to overcome the issue of heating coins, keys, and other common metal items that our tech will encounter?

Looking for answers in physics fundamentals

If you took high school physics, you learned that magnetic fields do not typically traverse through metal. Ever wonder what happens to a magnetic field when it encounters a metal object? Well, as one would expect, some of the magnetic field is absorbed as lost energy by the object and some of it is reflected back as an opposing magnetic field. The energy that is absorbed by the metal object manifests itself as heat which causes the temperature of the metal object to rise.

The amount of energy absorbed by a metal object due to the magnetic field is dependent on field strength, metal composition, metal thickness, and the frequency of the magnetic field. As the frequency increases, the amount of energy absorbed by a given metal object decreases with the square root of the increase in frequency.

So, how do we apply this to wireless power transfer?

Raising the frequency with magnetic resonance

The AirFuel Resonant standard utilizes an operating frequency of 6.78MHz (more than an order of magnitude higher than the current inductive systems) and therefore plays nicely with common household metal objects, thereby simplifying product design for engineers.

Lab tested, metal object approved

Given that the frequency used for AirFuel Resonant wireless power systems is more than 45 times higher than the first generation inductive systems, the second generation systems based on AirFuel Resonant heat significantly less and therefore see much lower temperature rise in common metal objects. In fact, AirFuel Resonant systems can continue to operate normally even in the presence of common metal objects, such as coins, keys, etc.

AirFuel Alliance member companies have tested a number of different metal objects to confirm the lack of power absorption in metals at 6.78 MHz. Results of the 115 kHz and 6.78 MHz systems testing are shown in the figure below.

Graph showing the difference in metal heating between inductive and magnetic resonance.

Metal Object Heating in a 115 kHz inductive wireless charging system versus a 6.78 MHz resonant wireless charging system.

AirFuel Resonant delivers a solution

AirFuel Resonant wireless charging systems have a number of features designed to maximize the user experience in the home, car, office and public areas. In addition to inherently tolerating the presence of common metal objects (e.g., keys, coins, paper clips, stickers, and more), AirFuel Resonance has more benefits including:

  1. Multiple devices to charge simultaneously
  2. Ability to charge as fast as a wire
  3. Ability for devices to charge wirelessly without precise alignment

It is through technology advancements like AirFuel Resonant that we will accelerate the adoption of wireless power and achieve a world where we can power up without plugging in.

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