Wireless Charging Overview
Qi (pronounced "Chee") is the global wireless charging standard developed and licensed by the largest technology alliance in the wireless charging industry, the Wireless Power Consortium (WPC). An open platform with the support of nearly 150 WPC member companies, including major mobile phone manufacturers, wireless service providers, and semiconductor companies, and with well over 300 Qi-certified products introduced into the market since the specification was first introduced in 2009, Qi is helping to bring wireless charging of mobile devices into the mainstream. Products that carry the Qi logo on their packaging are interoperable, allowing consumers the freedom to charge any Qi-compliant device, given any Qi charger.
The Qi system consists of a flat charging pad, and a mobile device equipped with a compatible receiver. When the mobile is placed on top of the charging pad, the device is charged via electromagnetic induction. Essentially, an alternating current passed through a coil in the charging pad generates a magnetic field that induces a voltage in a coil in the receiver, which can then be used to power the mobile directly or charge the battery.
Qi provides for electrical power transfer up to 4 cm (1.6 inches), with typical efficiency around 70 percent, with 80 to 85 percent efficiency possible with careful design, better shielding, and newer techniques such as the use of ultra thin coils. The low-power specification that exists today delivers up to 5 watts to receivers, enough for smart phones and small mobile gadgets.
A 10 to 15 watt extension is in the works to enable rapid phone charging and charging of consumer tablets. In development is a medium-power specification that will deliver up to 120 watts for charging larger devices such as laptop computers and power tools.
For applications requiring greater separation between the charger and receiver, such as through a desk, there is magnetic resonance technology. Though magnetic induction and magnetic resonance are both based on similar principles involving coils, AC currents, and magnetic fields, a magnetic resonance implementation offers several advantages including, 1) a range of several inches or more, 2) charging is possible through an obstruction between the charger and receiver, such as a magazine, 3) multiple devices on a charging pad can charge at the same time, and 4) flexibility in orientation and positioning of the receiving devices on the pad (Qi can achieve expanded free positioning of the receiver using a three coil transmit array). The Alliance for Wireless Power, or A4WP, champions WiPower™, the first wireless charging standard that is based on magnetic resonance. WiPower was approved in January, 2013, and devices using this standard are projected to be available in late 2014. The WPC is also working on Qi-compliant magnetic resonance technology to enable longer range charging.
Wireless power technology is still evolving, and it remains to be seen which direction the technology will go, what influences proprietary charging approaches will exert, whether standards will merge, whether manufacturers will offer multi-mode solutions that support more than one standard, and ultimately which solutions will be embraced by consumers.