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.
Since the introduction of the Wireless Power Consortium’s Qi standard in 2009, a number of integrated circuit solutions for Qi-compliant wireless charging have become available. Today’s Qi semiconductor offerings integrate all of the necessary intelligence, control, power management, and communication functionalities into tiny micro-packages, enabling end equipment designers to achieve high performance and efficiency goals, while meeting competitive cost, small-size, and fast time-to-market targets.
The Qi wireless power system consists of a charging pad housing a power transmitter, and a mobile device with a power receiver. When the mobile device rests on the charging pad, the receiver communicates to the transmitter, requesting the appropriate amount of power desired. The transmitter transfers power to the receiver via coupled inductors, with the primary coil in the transmitter, and the secondary coil in the receiver. The receiver sends feedback to the transmitter requesting more or less power, and the transmitter monitors and acts on this information as needed in the closed digital control loop. The charging pad powers down for most of the time to save energy, waking occasionally to check for the presence of a receiver. After a receiver is authenticated, the charging pad stays on.
Silicon is naturally available in two categories: transmitter solutions, and receiver solutions.
Look for Qi-compliant wireless power transmitter ICs that maximize power transfer efficiency while offering features such as:
Available silicon for the receiver side includes:
Given that multiple wireless charging protocols exist today, it may be advantageous to consider multi-mode devices that support both Qi and other formats, such as proprietary protocols, for even broader application.
Fundamentally important to achieving a Qi-compliant wireless charging system, is proper implementation of the magnetics. To help achieve this end, the Qi standard outlines the physical requirements for the transmitter and receiver coils, as well as their alignment and shielding. The standard also provides information on tuning the coils to resonance.
The Qi specification lays out the different types of transmitter design implementations that are allowed. For each transmitter design, there is a specification section that strictly describes the respective coil, including shape, dimensions, materials, number of turns, and number of layers. Given the fixed parameters, certain catalog coils have been tested and approved for compatibility with particular power transmitter control ICs, simplifying the charging pad design process.
Give careful consideration to the matching capacitor, as it is critical to achieving the desired resonance frequency and proper system operation. The total resonant tank capacitance value is defined by the Qi specification and is a requirement, not a guideline. Choose capacitors of high quality dielectric and sufficient voltage rating in order to pass Qi-compliance certification.
The reason for the limited design freedom on the power transmitter side is to ensure interoperability of the charging pad with the largest number of mobile devices.
Due to greater variance in the design requirements for mobile devices (for example, a smart phone is very different from a wireless headset), the Qi standard provides minimal guidelines for the receiver parameters to enable greatest design flexibility. Designers can choose from catalog products that meet the design criteria, or create a custom receiver coil. Application notes such as Texas Instruments’ SLYT479 “Designing a Qi-compliant receiver coil for wireless power systems” help guide designers through the design process.
Close alignment of the transmitter and receiver coils is critical to achieving efficient power transfer. The closer, the better, however the Qi standard allows a maximum separation of 40 millimeters.
Adequate shielding at the bottom face of the transmitter coil and the top face of the secondary coil helps to direct the magnetic field to the coupled zone to maximize efficiency. Shielding also contains the fields to avoid interfering with the device or adjacent systems. It also limits the exposure of users to the magnetic field, although at Qi’s low operating frequencies (100 to 205 kHz range) the magnetic waves are not harmful to humans. The two basic methods for shielding at low frequencies are diversion of the magnetic flux with high-permeability materials such as ferrite or copper, and counteracting the magnetic flux by generating opposing flux according to Faraday’s law. A combination of materials and techniques can be used to achieve the desired shielding effectiveness. The Qi specification establishes the dimension, placement, and materials of the shielding on the power transmitter side. Shielding is recommended, but optional on the receiver side.
Cost and thickness are key drivers when selecting the appropriate electromagnetic shield.
The most efficient way to develop end applications for Qi wireless charging is to start with available evaluation kits (EVK) or evaluation modules (EVM). These tools allow designers to easily and quickly demonstrate the features and performance of prospective devices, speeding the development effort. Some tools may even have received Qi certification by independent testing facilities, and therefore can be used as Qi-compliant reference designs.
Mouser stocks both Qi wireless power transmitter and Qi wireless power receiver evaluation modules. These are complete transmitter-side and receiver-side solutions, including all the necessary components, such as the coils to allow immediate evaluation. The receiver evaluation modules do need to be placed on a Qi-compliant wireless charging pad for testing, and likewise, the transmitter evaluation modules need to be paired with a Qi-compliant receiving device.