What is magnetic resonance coupling?

Resonant magnetic coupling is a wireless charging technology discovered at an American university in 2007. The research team then demonstrated the possibility of efficiently transfer 60W of power over a distance of a few meters while maintaining good efficiencies. This technology works by the use of transmitters and receivers having the same resonant frequency and large quality factors. To obtain resonators with high quality factors, it is important to minimize various losses (conductive losses, radiative losses and others) and select a higher resonant frequency. The resonant systems with high quality factors make it possible to maintain a high transfer efficiency even if only a small portion of the magnetic flux emitted passes through the receiver, for example when there is a misalignment or a large distance between the resonators. On the contrary, magnetic induction uses transmitters and receivers having a low quality factor. For energy transfer to be possible, the emitter and the receiver must be in very close proximity and have the same dimensions so that almost all of the magnetic field emitted is picked up.

Wireless charging based on magnetic resonant coupling has the potential to permanently change the way we charge just as Wi-Fi has revolutionized the way we exchange information. These revolutions are so large that they require the collaboration of multiple companies and the creation of standards to ensure that everyone of them follows the same guidelines and that all products work with each other. For wireless charging, the two standards developed are Qi (2008), based on magnetic induction and integrated in most cell phones, and AirFuel Alliance (2015), based on resonant magnetic coupling. GPHY believes that AirFuel is the standard that will truly meet the needs of consumers and permanently change the way we charge our devices on a daily basis.

Qi – First generation wireless charging

The Qi standard is based on magnetic induction technology. This standard, born in 2008, was first integrated into cell phones from Samsung 2015. Since that time, the standard has grown and has been integrated into many devices from several major brands. This standard uses lower frequencies than AirFuel, which reduces the complexity of electronic design and facilitates its integration in devices. However, induction chargers contain many limitations for the customer experience that unfortunately can never be resolved.

Here are the main issues with the Qi standard and magnetic induction:

    - Alignment between the charger and the devices is very critical. Magnetic induction is only effective if the receiver (in the cell phone) captures almost all of the field emitted by the transmitter (in the induction charger). For this reason, charging cell phones with slightly thicker than average cases often does not work with induction. It is therefore impossible for these chargers to transfer power through surfaces (tables, desks, nightstands, etc.), unless significantly altering the furniture by removing almost the entire thickness of the material locally. Even with inductive chargers placed on surfaces, users often find their cell phone uncharged if the cell phone has not been placed precisely enough on the charger or if a vibration has caused it to move while charging.

    Figure 1 : Example of a Qi receiver inside a cell phone
    Exemple de chargeur Qi (induction magnétique)
    Figure 2 :Example of a Qi charger

    - The transmitter and receiver must be the same size to enable charging. It is therefore impossible to have a charger that can both charge a cell phone and a smart watch since a cell phone induction charger is much larger than a watch. It also represents a real challenge for manufacturers of electronic devices who are imposed on fixed and often restrictive dimensions to add induction charging to their products.

    - The frequencies used with Qi heat the nearby metal. The metal in cell phones can heat up while charging, which can affect the life of batteries and cell phones. If the cell phone is partially misaligned during charging, the temperature rise is even more critical.

A study by the University of Warwick on Qi chargers illustrates these problems well. The image below shows that the cell phones external temperature remains high for a very long time even during perfectly aligned charging. When the cell phone is misaligned of only a few millimeters, it creates even greater thermal issues (see photos below to see the difference). The temperature inside the cell phone definitely increases to much more than 31 degrees Celsius and this can cause permanent damage to the battery.

Figure 3

We see that using an induction wireless charger is not as simple as it seems. You have to align the phone well each time it is charged and hope that no vibration will cause it to move otherwise the charging may stop or the internal temperature of the device may rise and permanently reduce the quality of the battery.

Apple has already tried to develop Airpower, a Qi wireless charger with some spatial freedom. Their idea was to partially overlap multiple Qi transmitters so that a transmitter is always fairly well paired with the cell phone receiver regardless of its position on the charger. The goal was to offer a single device able to charge the whole product range of Apple (iPhone, AirPods, Apple watch). After years of development, they abandoned the project because they failed to design a product that met their quality standards. Other companies have also embarked on the development of similar products but eventually they all encounter the same problems: the charging area has zones where no transmitter is well aligned with the receiver. In these zones, devices charge at low speed and at low efficiency while undergoing prolonged adverse thermal effects. Following the abandonment of this project, Apple still addressed the critical alignment issue with the MagSafe product which uses a system of magnets to ensure alignment between transmitter and receiver. However, the charger remains magnetized to the cell phone when the user takes the cell phone in their hands and must then pull on the charger to unhook it. It sure looks a lot like the good old days when cell phones were charged with a standard charging cable ... It undoubtedly shows the limitations of the Qi standard and why this standard is sooner or later doomed to failure. At GPHY, we strongly believe that wireless inductive charging does not meet the needs of consumers and that this technology's inherent limitations will always slow the progress of the Qi standard.

Figure 4 : AirPower

AirFuel Alliance - Wireless Charging 2.0

AirFuel Alliance is a standard born in 2015 which has learned from the problems of the Qi standard and which has been designed to answer these problems. It is more complex to design charging systems with resonant magnetic coupling, but the user experience is on a whole new level. Here are the main advantages of AirFuel and resonant magnetic coupling:

    - Daily electronic devices can be charged at higher power and more efficiently without the need for critical alignment between transmitter and receiver. Airfuel offers much larger charging areas, with heights ranging from 3mm (chargers on surfaces) to 50mm (chargers under surfaces). For example, GPHY’s first products offer areas of approximately 250mm by 200mm from a charger placed under surfaces. No matter where the receiver is placed in this area on the surface, the device will charge.

    - Thanks to the high quality factors, even if only a fraction of the emitted field passes through the receiver, the transfer efficiency remains high. It is therefore possible to use large transmitters with small receivers, which is impossible with Qi inductive charging. Therefore, a single transmitter can charge all types of devices ranging from a laptop to a smartwatch without compromising on the charging speed of each. Electronics manufacturers could thus design receivers of multiple shapes and adapt them more easily to the other mechanical constraints of their various products.

    - The frequency requested by the AirFuel standard is higher than Qi, which heats up the nearby metals much less and therefore affects electronic devices less.

To sum up, a wireless charger respecting the AirFuel standard is much easier to use and will really change the way we charge our devices.

The images below show how quickly transfer efficiency drops with a Qi charger and misalignments of just a few millimeters. It is also shown the difference between the "spatial freedom" of a Qi compared to the products that GPHY is developing with Airfuel.

Figure 5 : Efficiency of a Qi wireless charger as a function of misalignement (%)
Figure 6 :Scale view of the charging zone of a Qi (color) compared with the zone of GPHY's product ELIA (grey)


Why don't we see more AirFuel compatible products ?

As mentioned earlier, developing a product with Airfuel is more complex than developing a product with Qi, but technological advances are making it more and more accessible. Among these technological advances, there is in particular the development and significant cost reduction of electronic components using gallium nitride (GaN) semiconductors. Also, the increasing accessibility to computers with sufficient performance to perform complex and precise simulations helps a lot with the design of resonators and circuits.

Finally, it is not untypical that simpler technologies dominate the market first. However, when too many problems persist with a technology, it is inevitable that a better technology will eventually replace it to really meet the needs of consumers. We only have to think of the success of audio and video cassettes several years ago and the evolution that followed to CDs and now to online platforms. GPHY therefore believes that it is inevitable that a better standard than Qi will seize the market in the next few years and it is AirFuel that is destined to take this place. We want to be at the source of this revolution by offering the best possible product to charge your daily electronic devices.

If you would like to know more about wireless charging technologies, click here!



Hubert Audet,

CTO, GPHY