Anker wireless chargering pad

Wireless Charging has had a hit a miss relationship with the mobile industry, dipping in and out of product ranges and flitting between spec sheet feature and accessory status. 2015 saw technologies mature and a major merger between A4WP and PMA, which leaves the wireless charging industry in an interesting place going forward.

As is stands, there are currently only two major groups that have shipped mobile devices containing their wireless charging technology – the Wireless Power Consortium and the AirFuel Alliance (formerly A4WP and PMA). However, there are a number of other smaller groups and businesses looking to promote their own more unique technologies as well.

Battle for the future standard

The Wireless Power Consortium’s inductive Qi standard is most likely to be familiar to you, as it powers a range of smartphones, accessories and products. The PMA standard, which is now folded into AirFuel, has also appeared in a number of smartphones and also struck deals to provide charging stations to businesses such as Starbucks. Both of these standards are based on inductive charging technology, which is typically quite short range and can be quite finicky. Rezense, the old A4WP standard was based on resonance technology, but this design has yet to appear in any smartphones.

magnetic fields

Coils of wire create magnetic fields, which can be used to generate a current flow in a separate, insulated coil. This is the basis of transformer technology and both inductive and resonance chargers.

Inductive and resonance based technologies produce quite different results from an end user perspective, despite being based on the same engineering principle that coils of wire can be used to transmit power over the air.

If you are interested in the science, inductive charging uses tightly coupled coils with a slightly “off-resonance” frequency for high transfer efficiency, resulting in an efficient use of power but at the cost of high sensitivity to coil misalignment. This is why Qi and PMA devices often use magnets to line-up devices with the receiving pad and are limited to very short charging distances, typically just 45mm.

The only real difference between Qi and PMA are the transmission frequencies and connection protocols used to communicate with devices and control power management. Resonant charging is a little different, typically operating over a larger distance of a couple of inches by tuning the frequency of the oscillation to precisely match between the receiver and transmitter. This allows for a longer transfer distance before power diminishes but with less optimal power transfer than induction technology. One of the other big benefits of a resonance design is that power can be transferred to devices regardless of their orientation in the magnetic field and can also power multiple devices from a single transmitter coil.

Qi also now has a resonance design in its 1.2 specification for longer power transfer up to 2.8cm, but due to ensuring compatibility with existing Qi transmission frequencies, Q factor and heat limitations, it is not as effective at transmitting power over long distances as a system designed specifically for that purpose.

Although Qi may have the advantages associated with early adoption, AirFuel is now attempting to push both inductive and resonant technologies into single products. This would provide the best of both worlds and has added an interesting dynamic for device manufacturers to consider.

Building products

Clearly then, there are pros and cons to consider when picking a standard to put into your gadget. Major mobile semiconductor players, including Qualcomm and MediaTek, have been working to provide integrated circuits that support at least one of the various standards. We have already seen the standard war produce products that support more than one type of technology too. For example, the Samsung Galaxy S6 was the first smartphone to support both the inductive Qi and PWA standards.

Other companies are taking the multi-mode approach much further, by supporting resonance on top of both inductive standards. NuCurrent is leading the way with components that supported Qi, PMA and A4WP, and announced the world’s first 10 watt inductive and resonant charging antenna. At CES 2016, the company unveiled a number of new charging products designed for wearables, which support Qi and AirFuel standards at low and high frequencies.

samsung galaxy s6 edge wireless charger aa 1

We are likely to see many more devices that support multi-mode wireless charging in the future.

MediaTek’s MT3188 chip also supports the three technologies and Texas Instruments has a range of receiver and transmitter controllers designed to accommodate standards from both groups. Semiconductor companies NXP, Semtech and IDT, to name just a few, also provide multi-mode coils, so there is plenty of choice out there for device manufacturers and we may see more multi-mode products in the future.

The first generation of accessories were pretty much limited to power charging stations for your home, but the wireless charging market has expended into lots of new product segments since then. Automotive is one of the growing sectors and a number of manufacturers, such as Audi and Mercedes, announced wireless charging capabilities at CES 2016. The Wireless Power Consortium is particularly interested in building its technology into public spaces and businesses, and envisions future devices with smaller batteries but with enough charging points around to easily keep them topped up.

Wireless charging is also branching out from its roots to become even more useful to consumers. The technologies have communication protocols built in to ensure the correct power transfer between devices and this is being used communication with other systems. For example, it can be used with smart home solutions, such as controlling lights or temperature, or transmitting information about your favourite radio station or even your optimal seat position to your car’s on-board computer.

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Radio transmission and the future

While the Wireless Power Consortium and AirFuel Alliance are the two largest groups looking to pushing their standards to the market, a number of smaller companies are also peddling other quite novel ideas.

At CES 2016 we went hands-on with Humavox’s various pieces of wireless charging equipped gear. Unlike induction or resonance based designs, this technology is based on near field radio frequency transmission. Humavox is looking at similarly short ranges as existing wireless standards, but rather than large metal coils, the company’s technology uses a small integrated circuit to handle the power transfer and conversion, allowing for some discrete implementations.

Energous is another company with a radio wave based technology, but unlike Humavox it is touting much longer ranges from its standard, reaching up to 15 feet. Last year the company announced a rather expensive transmitter hub that is capable of delivering 5.5 watts of power to devices 5 feet from the hub, around 3.5 watts of power at 10 feet, and 1 watt at 15 feet. The extra range really is the selling point of Energous’ technology, but at close range it is still relatively competitive with existing chargers.

You could certainly charge a number of your home gadgets with the Energeous hub, but it is also a promising design for business environments, where a small number of hubs could cover an entire office with wireless power. Unfortunately the comparatively high set-up cost and lack of consumer device support is really slowing adoption. To help get things moving, the company announced a new small form factor and lower cost RF charging IC at CES 2016, which is designed for low power wearable products. The technology still boasts 6-inches of range and can be accompanied by smaller, less expensive charging docks for your gadgets.

If that wasn’t enough choice, it turns out that you can also use ultrasound to transmit power between devices. UBeam is one such standard based on this lower frequency transmission and it boasts multiple device charging with a range of up to 4 meters with 1.5 watts of power. Unfortunately its power output and heat waste doesn’t quite match up with other ideas in the market, and the technology requires line of sight between devices in order to transmit power successfully.

Wrap Up

Clearly there’s lots of promising technology here, but there’s still one big question to be asked of wireless technology: why should consumers choose it over the convenience of a bedside USB cable? Honestly, it’s a tough one to answer, but ease of use and ubiquitous gadget support and charging stations will probably be required.

Despite being around for a number of years already, wireless charging still hasn’t become a critically important technology in the gadget market. Perhaps some of the improvements and new product ideas that are in development will push wireless charging into the mainstream.

Robert Triggs
Lead Technical Writer covering SoCs, displays, cameras, and everything in between. In his spare moments you'll find him building audio gadgets.