Wireless charging is evolving to become a feature that users of smartphones, tablets, notebooks, and wearables are coming to expect as a basic product capability. One remaining challenge is that, while these devices don’t all have the same power requirements, users want to limit the number of wireless charging stations they need — ideally, to just one. And among the factors to be considered in developing a single-station solution are the various standards that have emerged. This article looks at these competing standards and the optimum combination of standard and technology.
For years, mobile device users have been surfing the web, talking on the phone, and creating peer-to-peer connections — all without any physical connection. Such wireless convenience has generated new applications and market sectors. Yet most wireless users are only partially “wireless.” When it comes to charging, the vast majority continue to connect devices to a wall outlet — with a cable.
The growth of wireless charging
According to research firm HIS, the market is substantial and will show rapid revenue and unit growth until it reaches maturity and saturation toward the middle of next decade.
Fig. 1: World market for wireless power indicates significant growth for the rest of this decade.
IHS user research indicates that, in 2015, about 90% of consumers would choose to have wireless-charging functionality in their mobile devices. Approximately one-fifth of these consumers have already adopted the technology, while most have not used it yet but are keen to do so — a huge potential market in which the consumer is convinced of the need and the technology needs to deliver.
Consumer desire is clear: to be able to charge devices as quickly and fully as possible, allowing at least a day’s usage between charges.
Wired charging has evolved; chargers have become smaller, more efficient, and smarter. Mobile consumers can carry a portable battery to allow “on-the-go” charging during periods of heavy use, although charging the battery pack and, subsequently, the device remains predominantly wired. Batteries are improving, yet weight remains significant — often more than the device itself.
Wireless charging represents the logical next step of the charging evolution. While still in the “early adopter” phase, over the past two years, 221 million devices capable of being charged wirelessly have shipped. In parallel, 83 million charging stations (mostly standalone charging pads) have shipped.
Many of today’s chargers are in public spaces such as cafés and airports; indeed, most consumers who tried wireless charging experienced the technology somewhere other than at their home or office.
Standards and alliances
As with many emerging technologies, especially those with huge revenue potential, multiple (often incompatible) standards are being developed. While these serve to drive the technology, the lack of a truly universal solution might also stifle adoption.
The Wireless Power Consortium (WPC) was founded in 2008 and is comprised of about 230 members from 20 countries, including consumer electronics, semiconductors, and wireless operators. WPC members support the Qi (pronounced “chee”) standard.
The WPC verifies product compliance through a network of WPC-authorized labs. There are more than 1,200 Qi-certified products on the market, including more than 300 transmitter/charging devices and more than 90 Qi-enabled smartphones. The installed base is estimated at more than 150 million devices and ABI Research forecasts that this will exceed 700 million devices by 2020.
Through partnerships, Qi is being installed in cars, restaurants, hotels, airports, and corporate offices — there are in excess of 4,000 public Qi charging locations already deployed.
From a technical perspective, Qi is an inductive standard that supports tightly coupled charging. Under the WPC banner, PowerbyProxi is a resonant solution that offers full spatial freedom and delivers up to 5 W in a one-to-one or one-to-many charging scenario. The technology fits inside smartphones due to a proprietary wireless receiver, while a built-in Foreign Object Detector ensures that metallic objects do not interfere or overheat.
The Power Matters Alliance (PMA) and the Alliance for Wireless Power (A4WP) were formed as separate organizations in 2012 from various telecom, consumer device, automotive, furniture, and other companies. PMA was primarily focused on tightly coupled inductive solutions, whereas A4WP was working on loosely coupled resonant technology.
In June 2015, the two organizations were formally merged and, later that year, rebranded as the AirFuel Alliance (see Fig. 2). The merger reduced the number of organizations (and competing standards) from three to two and was praised for bringing the goal of a single, interoperable standard one step closer. ON Semiconductor numbers itself among approximately 160 post-merger members.
The AirFuel Alliance has a broad technology platform encompassing inductive, resonant, and uncoupled technologies.
Inductive technology is relatively mature and is deployed in millions of devices worldwide. This close-coupled technology offers efficiencies up to 80% with scalable charging to suit devices with different power requirements.
AirFuel’s resonant technology allows for a “drop-and-go” charging experience through any surface and was branded “Rezence” in 2013. Rezence offers scalable charging and is not affected by the presence of metallic objects such as keys or coins.
Looking to the future, the AirFuel Alliance is also working on uncoupled technologies that will transmit power up to several meters. There are multiple technologies/transmission media being considered, including RF, ultrasonic, and laser. The initial specification supports power up to 5 W over distances of 15 feet (5 meters); this is expected to increase to 15 W in the second release in 2017.
Alongside the technology aspects, the AirFuel Alliance is developing infrastructure standards for wireless charging. These include definitions of interfaces, command and reporting structures, and network protocols — all of which will add sophistication and user benefit.
Fig. 2: After the 2015 merger of PMA and A4WP, the AirFuel Alliance is the leading driver of wireless power technology.
While there is still some way to go, the ultimate goal is to deliver a single, fully interoperable system that is capable of charging multiple devices with different power needs at the same time without interference. Once this is achieved, then we will have arrived at the true “drop-and-go” charging experience that will drive universal adoption by consumers.
The role of semiconductor technologies
There are two fundamental aspects to wireless charging: power and magnetics. In many ways, the technology shares much of its heritage with modern switch-mode power supplies (SMPS). These typically consist of a primary and secondary conversion stage coupled by a transformer and often use resonance as a means of increasing efficiency. As such, much of the R&D being carried out by leading semiconductor firms into improving the efficiency of SMPS is applicable to building better wireless solutions.
As Fig. 3 shows, a wireless charging system consists of a transmitter and receiver, which are very close in function to the primary and secondary of the SMPS. The difference is that with wireless power, the “transformer” is split into two coils — one in the charger and one in the device. Even though the device is placed on the charging pad (and therefore “touching”), it is correct to use the term “wireless charging” because the charging surface and device case are non-conductive and the charging coils are separated by an insulating gap of several millimeters or more.
Fig. 3: Block diagram of a typical resonant wireless charging solution.
Semiconductor vendors are now developing complete wireless charging solutions, including power management, bridge rectifiers, power conversion stages, power switches (MOSFETs), and battery chargers. R&D research continues to focus on reducing size and cost while driving efficiency ever higher. In addition, many companies also now offer advanced development tools and reference designs that make it easier for design engineers to bring wireless charging systems to market quickly.
Clearly, there is some time needed before a truly universal “drop-and-go” standard is adopted globally, but with resonant technologies maturing and enabling loosely coupled charging, hundreds of major manufacturers backing standards, millions of devices shipping, and 90% of consumers wanting a solution, the future for wireless charging as the primary charging method of the future is assured.
BY AJ ELJALLAD, Senior Manager, Wireless Segment, ON Semiconductor, www.onsemi.com