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A Qi wireless charger: Resonant as well as Inductive

All Qi receivers (phones, sleeves, backdoors, and charging cards) that have shipped in the last 3 years and continue to ship, can be charged in inductive mode as well as resonant mode. It is the transmitter (wireless charger) that defines the operation mode.

This video shows a wireless charger that can power all standard Qi phones and tablets at any distance between 0 and 30 mm.

In the video you see the wireless charger operating in resonant mode as well as in inductive mode. 

Wireless charging in inductive mode

Most Qi transmitters use tight coupling between coils. In that configuration, the best results are achieved by operating the transmitter at a frequency that is slightly different from the resonant frequency of the Qi receiver. Off-resonant operation gets you the highest amount of power at the best efficiency. This operating mode is called "inductive".

The transmit and receive coils are tightly coupled when (a) the coils have the same size, and (b) the distance between the coils is much less than the diameter of the coils.

Inductive charging of Nexus 5 Inductive charging of Nokia 920 Inductive charging of Nexus 7

Wireless charging in resonant mode

When the distance between receiver and transmitter increases, the magnetic coupling between the coils decreases. Systems with a low coupling factor have to operate at the resonant frequency of the receiver. This mode is called "resonant".

Resonant charging of Nexus 5​ Resonant charging of Nokia 920​ Resonant charging of Nexus 7

The tradeoff between resonant and inductive mode

Loosely coupled systems trade-off larger distance at the cost of lower power transfer efficiency and higher electromagnetic emissions. This may be suitable choice in applications where tightly aligned coils is impractical, but less suitable for applications with tight EMI or EMF or efficiency requirements.

Tightly coupled systems, because of their higher efficiency, tend to produce less heat which is an advantage is products with tight thermal budgets such as modern smartphones.

More information about the tradeoff between resonant and inductive mode is available in the article "Magnetic Resonance and Magnetic Induction - What is the best choice for my application?"


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