Outlining inductive wireless power’s technical problem with heating metal objects, and explaining how AirFuel Resonant delivers a better solution.
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.
You’ll notice from the measurements that wireless power operation at 6.78 MHz generally yields lower heating of objects than at 115 kHz (a nominal frequency for another WPT standard) with the exception of the CD case. In order to understand this better, there are 4 principles that are useful to understand regarding eddy currents and object heating:
- Conductivity of the metal.
- General effect of frequency and field level when eddy current is uniform throughout a cross-section (i.e., very thin metal relative to skin depth of frequency).
- Effect of skin depth with respect to frequency of field.
- General WPT design principle: Ampere + Faraday Laws.
Let’s discuss each of these at a high level:
Conductivity of the metal
The conductivity of the metal affects the level of the eddy current in the metal as well as the amount of loss due to heating in the metal caused by the eddy current. Higher conductivity metals can sometimes have high eddy currents when exposed to a field but because the conductivity is high, the resultant loss is low.
Sometimes this means that in many cases copper (high conductivity) can heat up much less than steel (lower conductivity). This principle is useful in understanding why different metals heat up differently even if they are otherwise identical.
Effect of frequency and field level when eddy current is uniform
When considering the general affect of power dissipation caused by eddy currents, where skin effect is neglected, this Wikipedia explanation of eddy currents can be useful. Care should be taken to note the assumptions!
In this case, you can see that as the frequency increases or the field increases, the eddy current increases proportionally and this means the power dissipation increases by the square. This is only true when neglecting skin effect; however skin effect cannot be neglected when it dominates the effect as is the case in most objects. The thickness must be below 1/10th or less the skin depth to even consider the above power dissipation equation.
Frequency of field and skin depth
Skin effect is very important for most objects – especially for wireless power transfer frequencies (probably not as much for very low frequencies such as 50 or 60 Hz, etc.). The penetration depth can dominate the effects of power dissipation due to eddy currents – especially at higher frequencies.
For wireless power transfer several kilohertz to several megahertz, the skin effect is the primary factor for roughly determining metal object heating between a wireless power transmitter and receiver – all else being equal.
Ampere & Faraday Laws
The reason Ampere’s Law and Faraday’s Law are important principles to consider with respect to metal object heating is because these directly affect the field level (along with some optimization considerations not discussed here). In simplest terms, Faraday’s law says that the induced voltage on a receiving coil is proportional to the field level, the field frequency, and the magnetic area of that coil (including number of turns).
If you consider that for a given WPT system, at a given induced receiving coil voltage you can optimally draw a fixed power (proportional to the square of the induced voltage or field level) then if the frequency increases, the field can decrease proportionally. This means that ultimately for a given WPT system, when the frequency of operation increases, the field decreases proportionally.
Now consider the 4 principles above together. First, we can generally state (principle 4 above) that for a given WPT system whenever the frequency of operation increases (and the WPT is redesigned for that frequency) then the field level will proportionally decrease. This means (for principle 2 above) that when skin depth is neglected, there should be no overall change in power dissipation in a metal object caused by the WPT system. However, there is a significant decrease in most cases because of the skin effect (principle 3); and in this case, the depth and hence power dissipation decreases with the square root of the frequency increase. The only exception to this is for a very thin metal (microns thick) such as those found on CD’s. Note that metal foils are not thin enough for operation at 6.78 MHz – only specific metallic coatings.
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:
- Multiple devices to charge simultaneously
- Ability to charge as fast as a wire
- 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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