The Wireless Power Consortium established Qi, an open interface standard that defines wireless power transfer via inductive charging over distances of up to 4 cm (1.6 inches). The system works with a charging pad and a suitable gadget that sits on top of it, charging via resonant inductive coupling.
Apple, Asus, Google, Huawei, LG Electronics, Samsung, Xiaomi, and Sony are among the 488 companies that support the standard as of February 2020.
The Qi standard, which was first introduced in 2008, had been used by more than 200 smartphones, tablets, and other devices by 2017.
Electromagnetic induction between planar coils is used by devices that follow the Qi standard. The Base Station, which is connected to a power source and provides inductive power, and Mobile Devices, which consume inductive power, make up a Qi system.
A power transmitter in the Base Station consists of a transmitting coil that generates an oscillating magnetic field, whereas a power receiver in the Mobile Device consists of a receiving coil. Faraday’s law of induction causes the magnetic field to create an alternating current in the receiving coil. The inductive power transfer is efficient thanks to the close spacing of the two coils and shielding on their surfaces.
The A2 reference Qi low-power transmitter has a coil of 20 turns (in two layers) in a flat coil, wound on a form with a 19 mm inner diameter and a 40 mm outer diameter, with a below-coil shield of soft iron at least 4 mm larger in diameter, giving an inductance of 241 microhenries, according to the 2017 version 1.2.2 of the Qi specification (referenced above). When coupled to the receiver coil, this coil is placed in a series resonant circuit with a 200 nF capacitor, yielding a resonant circuit with a natural resonance at 140 kHz.
How The Qi Receiver Work
The Qi power receiver hardware reference design 1 starts with a rectangular coil of wire 44 mm x 30 mm outer size, 14 turns of wire, and an above-coil magnetic shield, which is also from version 1.2.2 of the Qi specification. This coil is connected to a pair of capacitors in a parallel resonant circuit (of 127 nanofarads in series and 1.6 nanofarads in parallel). The 1.6 nanofarad capacitor is used to distribute the power output.
A resonance modulator consisting of a pair of 22 nanofarad capacitors and a 10 k resistor in a T configuration can be switched across the 1.6 nanofarad capacitor to provide a digital communications channel back to the power transmitter. When the T network is switched across the 1.6 nanofarad capacitor, the coupled system’s resonant frequency changes significantly, which is sensed by the power transmitter as a change in supplied power.
Features And Specifications
In August 2009, the WPC released the Qi low-power specification. After registering, you may download the Qi specification for free. According to the Qi specification, “low power” inductive transfers use inductive coupling between two planar coils to deliver power below 5 W. These coils are normally 5 mm apart, but they can be as much as 40 mm apart and possibly more. The Qi Baseline Power Profile has replaced the Qi low-power specification (BPP).
A digital control loop regulates the output voltage by allowing the power receiver to communicate with the power transmitter and request more or less power. Backscatter modulation is used to communicate unidirectionally from the power receiver to the power transmitter. The power-receiver coil is loaded in backscatter modulation, adjusting the current demand at the power transmitter. These current changes are tracked and decoded into the data needed for the two devices to communicate.
The Wireless Power Consortium began expanding the Qi specification to include medium power in 2011. The Medium Power standard currently produces 30 to 65 W as of 2019. It is expected to sustain up to 200 W in the future (typically used for portable power tools, robotic vacuum cleaners, drones and e-bikes).
The WPC also demonstrated a high-power specification “Ki” in 2015, which can deliver up to 1 kW and can be used to power kitchen appliances and other high-power utilities.
The Qi Extended Power Profile (EPP) specification, which supports up to 15 W, was introduced by WPC in 2015. EPP is commonly used to charge mobile devices, such as BPP. LG Electronics, Sony, Xiaomi, and Sharp Corporation are among the phone companies that support EPP.
WPC created Proprietary Power Delivery Extension (PPDE) to allow phone OEMs to offer more power beyond the 5 W or 15 W of the Baseline or Extended Power Profiles. Only Samsung has released their compliance test so far. Apple, Huawei, and Google are among the other phone firms that use proprietary standards for fast wireless charging.
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