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
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
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
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
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
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.
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