How Qi wireless power transfer works

The Qi wireless power transmission system uses magnetic induction to transmit power to a power receiver subsystem included in the mobile device when placed over a power transmitter.

The subsystems for power transmitters and receivers contain coils and circuits that manage the communication and energy transfer between them.

The fundamental physical principle that governs the functionality defined in the Qi specification for wireless power transmission is magnetic induction: the phenomenon that a time-varying magnetic field generates an electromotive force in an adequately positioned inductor. In a Qi wireless power transmission system, this electromotive force creates a voltage across a coil-shaped inductor. It is used to drive the electronics of a suitable load to which it is connected. Conventional transformers use the same effect to achieve an inductive power transmission between a primary coil and a secondary coil, which are tightly coupled through a magnetic core.

Although a Qi-based system is similar to a conventional transformer in that energy is transferred from a first coil to a second coil, it is also very different due to the much weaker magnetic coupling between these coils. A conventional transformer has a magnetic coupling coefficient close to one, while a Qi-based system generally has a magnetic coupling coefficient in the range of 0.5 or less.

In the Qi-based system, energy is transmitted from the power transmitter contained in the Qi charger to a power receiver included in the Qi smartphone. Before the charging process begins, the transmitter and receivers communicate with each other to determine whether the mobile device can actually be charged, whether it should be charged, the amount of energy required, etc. In short, communication ensures adequate power transfer from the power transmitter to the power receiver. The communication channel can also be used to activate location-based services by providing an SSID, a Bluetooth connection or a unique ID.

At the start of the charging process, the power transmitter conducts an alternating electric current through its coils, which generates an alternating magnetic field according to Faraday’s law. This magnetic field is in turn picked up by the power receiver coil and converted to alternating current by a converter, which can be used to charge the battery.

An essential characteristic of the magnetic field is that it can be transmitted through all non-ferrous non-metallic materials, such as plastics, glass, water, wood and air. In other words, no cable or plug is required between the power transmitter and the power receiver.