Qi Wireless Charging Standard

- details and specifics about the Qi Wireless Charging Standard developed and promoted by the Wireless Power Consortium..

Qi, pronounced chee, has become the accepted standard for wireless charging.

In order to provide interoperability, a key requirement for wireless charging, it was necessary to develop a standard that could be adopted by the major manufacturers. In this way the Qi charging standard enables one charger to be used for devices from many different manufactures and in this way become an accepted approach. If different chargers were required, this would require many different chargers for different devices and lead to confusion in the marketplace.

Accordingly the Qi wireless charging standard has become the accepted solution.

The name Qi comes from Asian philosophy where it means "vital energy" which relates to wireless charging which can be seen as an intangible flow of power.

Wireless Power Consortium

In order to be able to provide a standard for wireless battery charging technology, an industry group called the Wireless Power Consortium was set up.

The Wireless Power Consortium held its first meeting in Hong Kong in December 2008. Since its inception its membership has grown from the eight initial companies to over 100.

In August 2009, the Wireless Power Consortium published the Qi low power specification, and then just a month later the first product was certified.

Under the auspices of the Wireless Power Consortium, the importance of wireless power has grown, with more companies taking the technology seriously. The first products using wireless power for their battery charging required the addition of an additional sleeve. However the first product with integrated Qi receivers for wireless battery charging were announced in March 2011.

Qi wireless charging standard basics.

The Qi standard has been developed by the Wireless Power Consortium and is applicable for electrical power transfer over distances of up to 40 millimetres (1.6 inches).

The Qi wireless charging standard uses a power transmission pad that is akin to the charger that would be used in a normal wired system, and a compatible wireless power receiver that is located in the device to be charged. The power transfer then takes place using magnetic induction.

The way in which the wireless power charging is achieved conforms to the standard and allows a single Qi wireless charger from any supplier to be used to charge any Qi compatible device. As most wireless charging systems now conform to this standard, it means that only one charger pad is required to charge all devices.

There are two categories for Qi wireless chargers:

  • Low power:   The low power category covers chargers that can deliver power in the range 0 - 5 watts. This will cover most of the domestic portable devices like mobile phones, music players, Bluetooth earpieces, etc..
  • Medium power:   Chargers in the medium power category can deliver power up to 120 watts.

Within any Qi wireless charging system, there are two main elements:

  • Base Stations:   The Qi base station is the device that provides inductive power for the wireless transmission. As such it contains a power transmitter of which a major element is the transmitting coil. The overall Qi charger typically has a flat surface. This is normally referred to as the Interface Surface. The mobile device or devices may be placed on top of this surface.
  • Mobile Devices:   The Qi mobile devices are those which consume the wirelessly transmitted power. This is typically used to charge the battery contained within the mobile device.

To keep the efficiency as high as possible, it is necessary to ensure that he coupling between the transmit and receive coils is as high as possible. To achieve this the mobile device must be placed in the position where this can be achieved. The Qi standard defines two ways of achieving this:

  • Guided positioning:   This form of placement on a Qi charger involves the use of means to guide the user to place the mobile in the correct place on the Qi base station for charging.
  • Placement anywhere:   The second form of Qi charger placement does not require the user to accurately place the mobile device on a particular area of the charging surface. Instead a wider area is usable. This can be achieved by using more than one transmitter coil.

The frequency used for Qi chargers is located between about 110 and 205 kHz for the low power Qi chargers up to 5 watts and 80-300 kHz for the medium power Qi chargers.

Qi data protocol

The Qi wireless power standard includes the capability for the base station and mobile device using a limited data transmission system. This enables the charge and base station to communicate to provide information about state of charge and the like. In this way, the mobile device can inform the base station when it is charged, etc.. In this way the base station can adapt its output to meet the needs of the mobile device and removing power transmission the mobile device is fully charged.

In operation, initial detection occurs as a result of the deection of a change in capacitance or resonance of the transmitter coil. Once an object has been detected the base station validates the device placed on it for Qi charging compatibility. The base station sends an 8 bit data string. The receiving mobile device has to respond and provide the signal strength. The base station then sends multiple digital pings to provide information about the optimum positioning of the mobile device. Only when the device has been validated and the information been passed will charging commence. During the Qi charging process, the mobile device sends control data packets to adjust the power level and finally terminate charging.

The Qi charger data communication protocol is defined to ensure that all devices can communicate properly.

Qi Standard Data Format
Qi Capability Details
Data speed 2 kbps
Bit encoding Bi-phase
Byte encoding Start bit; 8 bit data; parity bit; stop bit
Packet structure • Preamble (>=11 bit)
• Header (1 Byte)
• Message ( 1 to 27 Byte)
    One complete message per packet
    Payload for control
• Checksum (1 Byte)

Qi foreign object detection

One of the issues that could arise with a wireless charging system is that a foreign metallic object is placed on or near the Qi charger. When this occurs, the charger can induce currents to flow in the object, dependent upon its placement and the magnetic coupling, and as a result the object can become exceedingly hot. This could present a danger, especially in a domestic environment.

To prevent this occurring, the base station or charger is able to sense the presence of these objects and cut the charging action to ensure that only proper Qi mobile devices are coupled to the magnetic induction field and receive energy.

Qi specification versions

In order to meet the latest requirements, the Qi standard has been updated to cover new areas and to improve on others. The table below gives a summary of some of the highlights of the various Qi standard versions.

Qi Standard Versions
Qi Version Details
Version 1.0 • Qi transmitter power level 5 Watts.
• Transmitter designs include: single coil transmitter, coil array transmitter, and moving coil transmitter.
• Variety of Qi receivers
• Limited flexibility in Qi transmitter designs
Version 1.1 • Increased Qi transmitter design flexibility: 12 different transmitter specifications available.
• Improved "Foreign Object Detection". This is an important feature because it prevents heating of metal objects in the neighbourhood of an active transmitter.
• Possibility of a Qi transmitter powered through a USB port.

By Ian Poole

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