The Direction of Wireless Charging Technology

Mobile devices, with always-on GPS and wireless, high-performance video and audio technology, ever-increasing applications, and near-constant use, have perceived shorter battery life in spite of improvements in battery technology. This has created a demand for more convenient and accessible ways to charge mobile gadgets. Fulfilling these needs are the wireless charging systems available on the market today. One need only place the mobile device on the charging pad to achieve charging. No more fiddling with tiny connectors that eventually exhibit wear. No more fumbling around in the dark to plug in chargers. No more helping to plug in the kids' devices for them. Yes, to fully-encased, truly waterproof devices. Yes, to plopping down at a charging table at restaurant and leaving after the meal with a charged up phone.

History

The concept of wireless power is not new. Nikola Tesla, the inventor whose work in electricity defines much of our modern lifestyle, spectacularly but ill-fatedly demonstrated the possibility of intercontinental wireless power transfer in 1891. Tesla himself was trying to illustrate the principle of electromagnetic induction discovered in 1831 by Michael Faraday, whereby electric current flowing through a wire can induce current to flow in a second wire close by. Tesla's vision was before its time, and though long distance wireless power transfer is being investigated today, it is still not practical. Short distance wireless power (otherwise known as wireless charging devices, systems, or technology), however, is a reality. Today, the most common examples of wireless charging systems are the re-chargeable electric toothbrushes and shavers found in many homes since the 1990s. Less obvious are the biomedical implants that make use of magnetic induction to transfer power safely to closed, harsh, and sensitive environments such as inside our bodies.

Qi Wireless Charging Technology

Leading the charge, pun intended, is Qi (pronounced "Chee") wireless charging technology. Intending to create an international standard for interoperability between wireless charging pads and any properly equipped mobile devices, a conglomerate of almost 200 companies including semiconductor suppliers, phone manufacturers, and wireless service providers formed the Wireless Power Consortium (WPC) in 2008 and released the Qi open standard in 2009. Since then, over 900 Qi-compliant devices have become available. Qi wireless charging pads can be purchased off the shelf, or online at the likes of Amazon or eBay. After-market receiver sleeves are available for equipping mobile devices for Qi wireless charging.

For perspective, let us consider the price of a Qi wireless charging system. A Qi single-position, guided placement charging pad (charges one mobile device that is set in particular position) might cost between $30 to $50 USD. For a little more money, one can buy a Qi single-position, free-positioning charger (where the mobile does not have to be locked in a particular position.) A Qi three-position charging pad with free-positioning can be bought for about $75 USD. A Qi charging sleeve can be had for as low as $5 USD. These prices are all comparably affordable relative to the electronics that Qi supports.

The Push for a Wireless Charging Standard

The main benefit of a wireless charging standard, from the consumer perspective, is interoperability. One need only purchase a single wireless charging pad to power the various mobile devices in the household. Sitting at the local coffee shop enjoying the free Wi-Fi, a customer could also make use of wireless charging (an emerging service) without concern about whether their device is compatible or not. Commercial proponents for standardization argue that it will mitigate consumer confusion about wireless charging technology and promote widespread adoption.

Competing Standards and Propriety Protocols

Qi is based on magnetic induction between two coils, one in the charging pad (transmitter) and the other coil in the mobile device (receiver). This imposes certain requirements and constraints. Namely, 1) there must be one transmitter for each receiver, 2) For proper operation and maximum power transfer, the maximum separation allowed between the coils is 4 cm (1.6 in), and 3) the receiver must be in a specific position relative to the transmitter, though Qi does support free-positioning of the device on the pad through the use of an array of three to eight transmitter coils. The limitations of Qi have inspired the formation of newer standards bodies, each promoting newer approaches that ostensibly address certain key issues.

Currently there are three standards organizations vying for dominance in the electromagnetic coupling-based wireless charging arena. In addition to the incumbent WPC, there is now also the Power Matters Alliance (PMA) which was founded in March 2012, and the Alliance for Wireless Power (A4WP) founded in May 2012. The PMA's Power 2.0 technology uses magnetic induction, and works very similar to Qi, with the advantage being in software that allows hotspot vendors such as Starbucks and McDonald's to monitor and control the usage of their charging spots. The A4WP is promoting WiPower technology, developed by Qualcomm, for the A4WP v1.0 standard. WiPower uses magnetic resonance, which is different from magnetic induction, and also operates at higher frequencies than Qi and Power 2.0.

Magnetic resonance describes the transfer of electrical energy between two coils that are tuned to resonate at the same frequency. When the transmitter and receiver side both oscillate at the same frequency, the receiver obtains power from an electromagnetic field generated by the transmitter that it converts into electrical current to power or charge a mobile device. Advantages of magnetic resonance charging include 1) a range of several inches or more, even through obstructions or surfaces 2) multiple devices on a charging pad can charge at the same time; and 3) flexibility in orientation and positioning of the receiving devices on the pad. Advocates of the A4WP call these advantages "spatial freedom."

Some companies are proponents of proprietary protocols (sometimes in addition to standards), possibly for licensing rights. Intel, Apple, and WiTricity (backed by auto-industry giants Toyota, Mitsubishi, and Delphi) are such companies, and each is large enough to exert influence in the market. Table 1 helps clarify the differences between the three main electromagnetic coupling-based wireless charging standards.

Table 1. Comparison of competing standards for wireless charging technology

New Developments:

Standards Unification and Mixed-Mode Solutions

Watch for unification efforts with the standards in the near future. In a recent bid to start these conversations, Qualcomm, who helped found the A4WP last year, surprisingly joined both the WPC (September 20, 2013) and the PMA (October 1, 2013.) Qualcomm's communicated intention is to enable the WPC and PMA to leverage the A4WP's work in magnetic resonance technology. At the PMA, Qualcomm will co-chair a working group with WiTricity, whose proprietary technology is magnetic-resonance based, to define a "dual-mode" specification that seamlessly supports both magnetic induction and magnetic resonance technology. The WPC, already working on its own form of magnetic resonance to enable longer range between transmitters and receivers, will also likely be receptive to gaining access to the expertise.

Silicon suppliers are already keen to the idea of mixed-mode solutions. Integrated Device Technology (IDT) offers the IDTP9030 wireless power transmitter IC and the IDTP9020 wireless power receiver, both of which are capable of "multi-mode" operation. The IDT devices support both the Qi standard as well as proprietary formats for added features, improved safety, and increased power output capability of up to 7.5 watts. Dynamic switching provides a seamless transition between Qi and the proprietary mode.

Qi Specification Expansion

Currently, only the Qi low-power specification is available, providing up to 5 watts to support mobile phones and other smaller devices. The WPC is working to release the Qi medium-power specification that delivers up to 120 watts of power to support larger devices such as tablets, laptops, and portable drills.

Improving Efficiency

Electromagnetic, coupling-based wireless charging technology - in general - has a broader, more fundamental issue at hand to address, namely inherent lower efficiency than power achieved by plugging into the wall or USB, due to losses when transferring power over the air gap between transmitters and receivers. Typical efficiency of these systems is around 70 percent. With careful design, better shielding, high-quality drive components, and new techniques such as the use of ultra-thin coils to reduce transfer loss, it is possible to increase efficiency to between 80 and 85 percent.

The reduced efficiency of wireless power compared to wired power is relatively acceptable for low-power mobile gadget charging applications. But lost power is released as heat, and this becomes a safety issue with higher power applications. The power loss also means wasted energy, which is not a friend to environmentalists or our wallets. Therefore wireless powering of larger electronics, such as TVs and refrigerators, is not really feasible at present.

Near Field Communication (NFC)

NFC is starting to take a role in the world of wireless power. NXP Semiconductor has demonstrated the ability to provide wireless charging for two standards in one pad, and NFC can be used to trigger the pad and communicate which standard to support.

There are also efforts underway to combine wireless power transmission technology with NFC to power small devices. In this vision, NFC devices would be able to receive power when placed on an enabled laptop computer. There is still technology development work required, including optimizing the current NFC antenna design for efficient wireless power transfer. In addition, the NFC standard needs revision to support wireless power.

The Future

It's hard to speculate on the future of wireless charging technology, as it is not simply performance driven. Marketing and financial (intellectual property rights and licensing) motivations steer the direction as well. Also, many of the companies that have a hand in the game are industry giants, capable of swaying the direction at any time.

There are also alternative wireless power technologies being developed, such as ultrasound or solar power, which diverge from electromagnetic approaches.

What we do know is that wireless power has a convenience factor that will keep everyone plugging away... in search of newer, better, smaller, faster, and more cost-effective wireless power solutions.

Landa writes from Dallas, TX. She holds a BSEE, and has 18 years experience in the electronics industry, from tech startups to Fortune 500 companies.