Under the Qi specification, "low power" for inductive transfer means a draw of 0 to 5 W. Systems that fall within the scope of this standard are those that use inductive coupling between two planar coils to transfer power from the power transmitter to the power receiver. The distance between the two coils is typically 5 mm. It is possible to extend that range to at least 40mm.[1] Regulation of the output voltage is provided by a digital control loop where the power receiver communicates with the power transmitter and requests more or less power. Communication is unidirectional from the power receiver to the power transmitter via backscatter modulation. In backscatter modulation, the power-receiver coil is loaded, changing the current draw at the power transmitter. These current changes are monitored and demodulated into the information required for the two devices to work together.[2]
The WPC published the Qi low power specification in August 2009.[5] The Qi specification is available as free public download.[6] In 2011, the Wireless Power Consortium began to extend the Qi specification to medium power.[citation needed] The low-power specification delivers up to 5 watts; the medium-power specification will deliver up to 120 watts