Wireless Power Standards: The Wireless Power Consortium (Qi) vs. the AirFuel Alliance

When it comes to wireless power standards, there are three key players: the Wireless Power Consortium (Qi), the Power Matters Alliance (PMA), and the Alliance for Wireless Power (A for WP, or Rezence). To compete with Qi, the latter two have formed the AirFuel Alliance.

In this blog post, learn the main differences between these compliance standards, and see how the AirFuel Alliance stacks up to Qi.

Comparison of Wireless Power Standards

Qi, also known as the Wireless Power Consortium, is arguably the most versatile alliance in wireless power transfer. There are many companies in many different markets driving Qi’s technology advancements. Essentially, Qi is trying to touch everywhere they can with wireless power.

There are over 300 member companies and more than 1,000 certified products associated with Qi. Its power class is pretty impressive, reaching anywhere from 0W to 2.4kW of power. (It’s important to note that 2.4kW isn’t available yet; there are working solutions, but those are considered prototypes and are not yet certified to Qi’s own standard). Qi uses inductive charging technology for its products.

In order to compete with the much larger alliance of Qi, Rezence and PMA joined forces to create what they call the AirFuel Alliance. Together, they have 150 member companies.

Rezence, also known as the Alliance for Wireless Power (A for WP), is driven primarily by one or two large companies and have a more limited range of certified products. Rezence uses resonant technology and has only one certified product type, which is in the phone and tablet market. Thus, Rezence has a fairly limited scope in what it’s looking to support.

The PMA, or Power Matters Alliance, is driven primarily by Duracell-Powermat. With its later entry into the market, the PMA has 29 certified products in the consumer market (which are almost all phones and tablets). Like Qi, the PMA uses inductive technology for its products.

Standard Wireless Power Technologies: Inductive vs. Resonant

Just what do we mean by “inductive” and “resonant” technologies? Here are the main differences between the two.

Inductive technology, which is a closely coupled solution, is the type of compliance used by Qi. This technology transfers power using low-frequency resonant tanks (100-205kHz) over very short distances (mostly anything under 10mm).

In 2009, the first standard for Qi had a 5W power requirement (“Low Power”). In 2015, that was increased to 15W capability (“Medium Power”). This year, Qi is hoping for over 100W (“High Power”). Those are currently in testing and should be rolled out later this year.

The other wireless power technology, resonant, is considered a loosely coupled solution. Primarily championed by the AirFuel Alliance, this technology uses a high-frequency resonant tank (6.78MHz) to transmit power over long distances (multitudes of feet). Resonant technology offers the ability to charge multiple devices at the same time, with a capability of up to 22W for upcoming systems.

On the radio spectrum, Qi is a very low-frequency solution, while Rezence fits somewhere between AM and FM. The way each of these frequencies are picked by the standards isn’t so much a technology limitation as what was available for them to use. They then optimized their circuits to that frequency.

So while it’s possible to wirelessly charge at any number of frequencies, for compliance to each standard, we need to follow these basic guidelines.

Qi Compliance Standards

The companies supporting Qi aren’t just small companies. Brands like Nokia, HTC, Energizer, Sony, Panasonic, and others are backing it as well. There are many companies in many markets interested in what Qi can do.

So when we say “Qi standard,” what are we talking about? Basically, Qi power transfer specifications have three main parts: interface definition, performance requirements, and compliance testing.

Part one, the interface definition, includes your transmitter and receiver design requirements, system control, and communications interface. In this requirement, foreign object detection must be enabled. The coils need to be able to recognize they’re in the presence of another Qi-compliant coil so they’re not transmitting power into the world and draining energy. Part one also dictates the operating frequency of the ICs (100-205kHz), defines the resonant tank circuit, and defines coil construction with both mechanical and electrical parameters.

Part two, the performance requirements, is listed on the Qi website as available only to paying members. Essentially, it’s along the lines of 70% efficiency at 1cm. If efficiency drops below 70%, the controller will shut off power and will not transmit until efficiency reaches 70% again.

Part three, the compliance testing, simply has to do with where you can be tested to become Qi authorized. There are four testing locations around the world: one in the U.S., one in Germany, and two in Asia.

Qi standard has three different voltage classes for transmit coils:

  • 5V: USB applications
  • 12V: Automotive applications
  • 19V: Laptop power supplies

Each voltage class has a different chipset and coil requirement. The primary difference in coils between voltage classes is their inductance.

To learn more about wireless power transfer, check out our wireless power solutions from Wurth Electronics, and stay tuned to our blog!