Wireless Power Market Heats Up

Vendors fight it out over which is the best technology as demand for more convenient charging grows.


The wireless power market is in flux as established technologies meet newer approaches. Old standards battles have simmered somewhat, but competing messages remain.

What the public ends up using will depend heavily on public charging infrastructure, but the stakes are significant. The market for battery chargers is forecast to reach $25B by 2022. Most of those chargers plug into the wall, but as wireless charging becomes increasingly available it could take a larger chunk of the charging market over time – as much as $5B over the next couple years. The prospect of no longer having to fiddle with wired chargers promises greater convenience both for consumers and for other industrial markets.

“The wireless power industry is new, relatively speaking, and kind of confusing right now because there are many different methods of transferring power wirelessly,” said Mike Harmon, director of marketing at NuCurrent.

There currently are two predominant approaches to wireless charging. One is called Qi (pronounced chee), operating between roughly 100 and 300kHz. The other is Airfuel, operating at 6.78 MHz. They competed for the consumer market several years ago, but Apple settled the argument by going with Qi for iPhone wireless charging. Still, Airfuel is positioning itself as the heir apparent, even as other charging technologies are developed quietly in the background.

Qi is governed by the Wireless Power Consortium (WPC). Airfuel has a more complicated history. There were two “resonant” standards in the past — the Power Matters Alliance (PMA), with its PowerMat standard, and the Alliance for Wireless Power (A4WP), with its Rezence standard. The battles raged, and, eventually, PMA and A4WP got together. The result was Airfuel, and the governing body became the Airfuel Alliance.

Inductive coupling and resonance
Today’s well-established wireless chargers use magnetic inductance to transfer power over the air. An oscillating current running through a coil in the charger (the “transmitter”) sets up a magnetic field that can be detected by another coil in the device being charged (the “receiver”). That transferred magnetic energy can become electric current that’s then sent to the battery.

There are two important aspects to this. One is the ability to couple to the magnetic field. The second is the ability to efficiently turn that magnetic energy into electrical energy. That last part is easy to discount, but it’s where much of the challenge has been.

Early wireless charging approaches relied exclusively on the ability of one coil to couple magnetically to another coil. This is called “inductive charging.” It has sometimes been positioned in contrast to another approach, referred to as “resonant charging.” But, in fact, the two can work together.

Resonance refers to the ability of two objects to exchange energy by oscillating at a natural resonant frequency. Energy transfer can be more effective near such a frequency, but it depends on the distance between the charger and the target device.

That distance gets to the notion of “magnetic coupling.” Both the charger and the target have wire coils whose magnetic fields can interact strongly with each other. For closely coupled systems, the two coils are in close proximity such that both coils feel the magnetic field strongly.

Fig. 1: Closely coupled inductive charging. Source: NuCurrent

In this arrangement, resonance can still play a part. “The receiver needs to have a defined resonant frequency,” said Menno Treffers, chairman and co-founder of the WPC. “If you bring the transmitter and the receiver close together, the optimum operation for efficiency and power transfer is not at the resonant frequency, but slightly off. And the farther you move away, the closer you have to go to the resonant frequency to make the transfer optimal. It’s impossible to operate at the exact resonance frequency of the receiver because they start to fight each other.”

As the coils get yet farther apart, we enter the realm of what is generally called “resonant” charging, where the resonant element dominates. The fields look similar to the inductive version, but fewer of the magnetic field lines intersect both coils.

Fig. 2: Loosely coupled resonant charging. Source: NuCurrent

The names “inductive” and “resonant” are often used to describe two different kinds of systems – closely and loosely coupled. In reality, both systems rely on resonance to a degree, so that terminology isn’t completely accurate.

When close coupling is the main mechanism, the two coils must be the same size. Because many of the devices being charged are small, such as an electric toothbrush, the charger coils also must be small. With smaller coils, the field is more restricted in extent, limiting how far apart the two coils can be and still transfer power effectively. Greater distance is possible, but the coils need to be larger for that to work. At present, closely coupled coils must be within a few millimeters of each other.

Fig. 3: Closely coupled coils, having the same size and a small “z” distance apart. Source: WPC

With so-called resonant charging, coupling is no longer the main mechanism, and so the charging coil can be larger than the receiving coil. This allows greater spatial flexibility when placing a phone or other device near the charger. One implication of this is the desire to place transmitters under a counter or desktop, with receivers simply being placed on the counter above it for charging.

Fig. 4: Loosely coupled coils, both with different coil sizes (left) and greater “z” distance apart. Source: WPC

One way of expanding the flexibility of placement – even for close coupling – is to have several coils in an array or even overlapping. The charging systems then figure out which coil is closest to the target device to energize that coil.

“The better transmitters also use coil arrays,” said Treffers. “Automotive chargers use coil arrays. And arrays give you an area that is almost infinite, depending on how many coils you put in.”

Fig. 5: An example system with multiple overlapping coils. Source: WPC

It’s possible to charge multiple devices on such a setup, although it’s more of a challenge for inductive systems because the multiple energized coils can couple with each other as well. But that is changing. “There are chargers that can charge multiple phones,” said Jim Crnkovic, vice president of engineering at NuCurrent. “They are essentially multiple Qi transmitters built into one device. Some of those products have multiple Qi certifications.”

Kitchens and hearing aids
The WPC also is working on a new standard for household frequency with higher power called Ki, enabling what they call the Wireless Kitchen. “Ki is under development for a whole ecosystem of induction-based cooktops and appliances that can receive up to 2.2 kW of power,” said Harmon.

This uses the same types of inductive coils as are used in induction cooktops. “It can be added as an upgrade for existing induction cooktops,” said Treffers. “Because of the larger coil, the magnetic field can travel a larger distance, making possible zones on a kitchen countertop where appliances could be powered.”

This is more about powering a device than charging a battery. Appliances like blenders and coffee makers could be built with a matching Ki coil instead of a plug, and they could then be powered by placing them above the transmitting coil.

Another standards process being initiated is called WattUp, and it’s driven by Energous (prounounced enerjuss) through the Airfuel Alliance. The effort has just been announced, and the Airfuel Alliance is soliciting participation by other companies in the field. This approach provides a third way of doing power transfer, by using RF energy.

Fig. 6: Receiving power as a function of transmission distance. The zones refer to semiconductor technologies used, details of which were not available. Source: Energous

Energous already has products in this space, and it chose 915 MHz (an ISM band) as the charging frequency. Those products can operate close in, so-called near-field, and farther away — mid-field or far-field.

Energous claims to be able to charge at higher power levels at near-field, up to around 40W, compared to a maximum of 15W so far for Qi. Because it doesn’t involve coupled coils, there’s no need for a flat surface or for any specific shape. This makes it particularly attractive for irregularly shaped items like earbuds or hearing aids.

With a more general approach that can be applied to a variety of devices, Energous claims it can reduce the number of part numbers for different shapes and models of device. It also should provide a single approach to charging anywhere from contact chargers — which use pogo pins, meaning they’re not wireless — to far-field.

Such shapes might create a concern for designers when creating these systems, but RF simulators can accommodate any shape. “The beauty of the finite-element approach is that it can handle these arbitrarily shaped objects,” said Tianze Kan, application engineer at Ansys.

Further benefits of RF charging include the ability to charge multiple receivers easily with one transmitter, as well as the smaller size of the antenna needed to pick up the higher frequencies.

Fig. 7: RF charging. Source: NuCurrent

Health concerns
The caution with RF charging is safety. There are strict regulations that must be met around the world for any radiative type of device like this, and so-called Part 15 of the FCC regulations in the United States limits power to 4W of effective isotropic radiated power (EIRP) with a maximum conducted transmit power of 1W. That being too limiting, Energous looked to work with Part 18, which covers the ISM band.

“We had to find a definition within the existing rules that the FCC would agree was acceptable,” said Stephen Rizzone, CEO of Energous.

The focus is on power absorbed by a human – the “specific absorption rate,” or SAR. The limit is 1.6 W/kG at 1 meter. The kG in the SAR measurement relates to the material that’s absorbing the power, which in the case of people is their skin. “We were able to agree that the FCC would accept a transmission where the power was confined within the equivalent of one wavelength,” or roughly 0.3m, said Rizzone.

That has evolved slightly. “For our first Part 18-approved transmitter, the FCC used [one wavelength] as a reference distance … for the energy roll-off around a receiver with a maximum distance from the transmitter to the receiver of 1 m,” said Cesar Johnston, executive vice president of engineering at Energous. “Currently, the FCC is approving wireless power transfer at up to 1 m under Part 18.”

RF charging also is closely related to using RF for energy harvesting. The main difference appears to be the fact that RF charging uses a specific frequency in order to guarantee charging, and the receiver can be focused narrowly on that frequency. Harvesting usually implies scavenging ambient signals for whatever can be gleaned from them. That may require a wider-band receiver to access more frequencies, and that can hurt efficiency. Energous refers to the intentional RF-charging approach as “active,” while energy harvesting is “passive.”

Frequency and efficiency
For higher-frequency approaches, the main challenge has been the efficiency of converting the transferred energy to electrical energy. Historically, much of that energy was lost in silicon-based electronics. “The power electronics have to be efficient at those frequencies,” said Sanjay Gupta, president of the Airfuel Alliance. “You can have good transfer over the air, but if you burn it up as heat in your semiconductors, it’s no good.” He added that, as a result, “The state of the art with power semiconductors for consumer electronics five to seven years ago was kilohertz.”

According to the Airfuel Alliance, the advent of gallium nitride (GaN), which is a wide-bandgap semiconductor material, has made possible the efficient processing of the transferred power. Past technology favored the lower-frequency approaches. GaN opens up higher frequencies.

Energous looks to GaAs as an important technology in addition to GaN. While GaN has a place in the transmitter, its value is still unclear for rectification on the receiving side, where GaAs is used. “GaN is still in its early days, and we have to find the best way to fit it into what we do,” said Johnston. “From a power amplifier point of view, it’s well-suited, but as a rectification technology, it still has some challenges. GaAs technology has better diodes.”

Opinions differ as to which frequency is most efficient. Both the WPC and the Airfuel Alliance can point to papers that give their respective technologies good marks. The WPC funded a paper that gave Qi the efficiency edge. According to the paper’s abstract, “This method estimates energy transfer efficiency over a charge-cycle for the 110–205kHz system to be 59.2%, whereas a 6.78MHz system operating in resonance is 39.8%.” Meanwhile, the Airfuel Alliance wrote a report with Utah State University that had efficiencies reported on some equipment as high as 70% for Airfuel.

When measuring efficiency, all sides agree it’s important to measure the entire system, not just the coil-to-coil efficiency. Design efforts must consider both the fields and the electronics. “We work on both sides, the electromagnetics as well as the power electronics,” said Mark Solveson, application engineering manager at Ansys. “We feel it’s really important to be able to simulate both of those systems together.”

More efficient electronics generate heat, but they don’t eliminate it. Kan noted that heat must be accounted for during design, particularly at higher frequencies. “The higher switching frequencies can get us into a higher power density, physically reducing the size, which then has the potential problem of removing the heat as the power density increases and the footprint decreases,” he said.

In general, it’s easier to achieve higher efficiency at lower frequencies. “The resistance of conductors in the system increases as frequency goes up, increasing losses,” said Crnkovic. He places much of the blame for increasing resistance on the “skin effect.” As the frequency goes up, current concentrates towards the surface of the wire. In a twisted wire, it can mean that the inner strands may conduct little to no current.

This can be helped by using “Litz” wire, which insulates each of the strands to keep the current from migrating outside the strand. The skin effect still holds for the individual strands, but not the bundle as a whole. “Litz wire is normally good to about 2 MHz or so, so it is beneficial for Qi frequencies — and not usable at AirFuel frequencies — or NFC,” said Crnkovic.

Fig. 8: The skin effect as seen on a cross-section of a twisted wire, where darker blue indicates more current. On the left, current migrates to the edge of the bundle. On the right, Litz wire keeps current within each strand. Source: Bryon Moyer/Semiconductor Engineering

This can make it easier to implement low-frequency high-power devices, but that’s not a rule. Energous claims to deliver 40W at 915MHz as compared to Qi’s 15W in the 100kHz range.

There were also rumors about interference from Qi. Any such charger will create overtones, and in theory, Qi could create interference in radio or TV bands. NuCurrent, which is relatively neutral in the standards battles, was unaware of this particular issue. But the company said there had been situations where it interfered with the passive key-entry feature on some automobiles, which operates at the same frequency, and could result in doors being inadvertently unlocked.

Because Airfuel is in the ISM band, many of the harmonics also end up within that band. Notably, the first harmonic is at 13. 76MHz, where NFC operates. While the ISM band has less regulation for EMI, devices are still required to play nicely with other devices using that part of the spectrum, so interference can’t be ignored.

“There are rules one must follow to operate within the ISM bands, and all products that operate in these bands must conform to these,” said Airfuel’s Gupta. “These are not as stringent as those in the licensed bands. But devices that operate in these frequencies can be subject to interference from others operating in these bands.”

Interference is one of the many aspects that must be accounted for during system design. “We are capable of handling the EMI and EMC analysis, both conductive and radiative,” noted Kan.

Wireless charging also requires a communication channel for control of the process. This allows the transmitter and receiver to communicate using handshakes to start and end the process and for any conditions that might necessitate adjusting the charging process, such as overheating.

Qi uses an in-band scheme for this communication, modulating the charging waveform. Airfuel and Energous both use Bluetooth Low-Energy, while Ki uses near-field communication (NFC). NuCurrent has a proprietary implementation that communicates in-band, which at first glance might seem to be a problem. The 6.78MHz operating frequency for Airfuel was chosen because it’s the lowest ISM-band frequency. Taken literally, that would mean that modulating it with a signal would add a lower sideband that would be outside the ISM range. But, as explained by NuCurrent, it’s a complete band 30kHz wide, leaving enough room to run a signal.

An example of how the communication works is illustrated by Jason Luzinski, senior field applications engineer at NuCurrent, in reference to Ki. “You can place your kitchen appliance onto your charger,” he said. “We start with NFC communication to ensure proper alignment and authentication of the high-power device. Then we start actual power transfer in the kilowatt range. And then on the zero crossings, we’re delivering all the power information via NFC to control the device.”

Public infrastructure
One of the main scenarios touted for wireless charging is the ability to go to a coffee shop, put your phone down on a table or counter, and have it charge while you’re drinking coffee. Qi already has some installations. “You can go to the McDonald’s in Chicago, and there’s 50 Qi chargers there,” said Harmon.

While Qi used to rely on cradles, newer Qi implementations allow mat-like charging. “In practice, nobody has issues with placing the phone correctly on the on the charger and getting it to work,” said Treffers.

Still, Treffers said coffee shops aren’t the real goal. “People work with their phones when they’re having a cup of coffee,” he said. “So it’s not a primary application for infrastructure. It’s bedside, but also in the living room and the office.”


Fig. 9: Qi charging stations at an airport. Source: WPC

Other wireless charging approaches
While inductive, resonant, and RF are the dominant approaches to wireless charging, other charging mechanisms are being explored. They include the following:

  • Capacitive. This involves electric rather than magnetic fields, and it needs two plates to create a complete circuit. The challenge here is that very high voltages are needed, making it hard to implement for casual use. Power up to 250W has been achieved across 10 inches.

Fig. 10: Capacitive charging. Source: NuCurrent

  • Electro-mechanical, or ultrasound. This literally involves sending sound that creates a vibration that can be converted into electrical energy. Receivers must be precisely aligned to efficiently convert a beam-formed acoustic signal. It has achieved 1.5W across 4m.

Fig. 11: Ultrasound charging. Source: NuCurrent

  • Laser. This has the benefit that it’s focused, so the energy doesn’t dissipate over distance the way radiated RF signals do. The challenge is that as little as 1mW can cause injury. 2W across 16 feet has been reported, although at low efficiency.

Fig. 12: Laser charging. Source: NuCurrent

  • Electric vehicles. These are moving to a wireless approach, as well. The intent is to provide better refueling options for electric vehicles as compared to gasoline engines. A new standard, SAE J2954, is due out this year for this application. It is an inductive resonant approach that operates at 85 kHz. The main goal is stationary charging, although there is discussion of having infrastructure that could operate dynamically, charging the vehicle as it drives.

The state of the industry today is that Qi dominates based on its early start and established base. Airfuel is largely being used in proprietary installations. “The 6.78MHz designs help with funky form factors that you can’t charge on a Qi pad — shoes, curved objects — and usually these companies tend to want to control their own ecosystem,” said Luzinski.

The standards wars aren’t completely over. Airfuel is positioned as the heir apparent to Qi, but the WPC doesn’t see it that way. RF is joining the fray, as well, although it’s too early to gauge its potential success. And all of the other approaches are still a long way from challenging any of the main players today.

But however the standards and approaches shake out, wireless charging appears to be on the upswing. “During the past five years, I’ve seen a lot of wireless charging products appearing in the industry,” said Kan. “And there will probably be more and more in the future.”


koen weijand says:

6.78 in-band comms is not allowed in argentina, and a few other countries.

Jerry Emerson says:

Thank you Bryon. Neat article that even this layman understood – somewhat.

I’m interested primarily in RF transfer, and think ultimately this will be used globally, once science figures it out. Your article was a good summary on where we stand.

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