what is Qi wireless charging

This page covers Qi wireless charging and mentions how does Qi wireless charging work.

As we know usage of wireless power is convenient solution for users of portable mobile devices. WPC group of companies have developed standard which supports wireless electric power transfer using inductive coupling over distnace of upto 40mm.

The Qi wireless charging system comprised of power transmission pad and compliant receiver (e.g. portable mobile device). To charge the device, it is placed on pad which charges the device using inductive coupling.

As per Qi standard power up to 5 Watt can be delivered wirelessly. This standard is developed to support low power devices. WPC is developing high power specification which will help deliver power upto about 1 KWatt.

WPC (Wireless Power Consortium) member companies include HTC, LG, Huawei, Motorola Mobility, NTS, Nokia, Philips, Sony, Samsung etc. There are about 80 member companies from Asia, Europe and America. WPC has been founded in 2008.

Specifications	Qi wireless charging support
Distance coverage	5 mm to 40 mm
Power	Delivers power upto 5 Watt
Coupling type	Inductive coupling
Qi standard specifications	Version 1.1.2 available on WPC website

How does Qi wireless charging work

Let us understand how wireless charging works. There are three basic elements in this system.
1. Power Transmitter 2. Power receiver 3. communication protocol between them.

The device which need to be charged is placed on the device(e.g. pad) which supports wireless power transmission. The figure-1 depicts how power is transferred using layers of packets. Thess continuous waves are absorbed by charging device. Inductive power transfer will work by coupling of magnetic fields from primary coil to secondary coils.

Figure-2 depicts typical components in wireless charging system. As mentioned transmitter consists of AC to DC module which converts AC power to the DC power. This DC power is provided to the pad which transmits the same wirelessly using primary coil. The receiver's secondary coil receives the power and apply rectification to provide unregulated DC. Voltage conditioning module converts unregulated DC to regulated DC power. This regulated DC is provided to the device to be charged in order to avoid any damage.

Qi Communication Protocol

Wireless charging communication protocol works as follows. The power receiver is considered to be "talker". Let us understand the steps as mentioned below.
• As soon as device to be charged is placed on the pad, power trasmitter(i.e. pad) detects its presense using analog ping.
• Digital ping from power transmitter provides time to power receiver to reply with signal strength packet. If the signal strength is valid, transmitter puts power on primary coil.
• Now power receiver sends packet which will provide setup and configuration information to the power transmitter. Now it is the job of power receiver to inform transmitter whether it need power or not. Based on the information (i.e. packets) power receiver will increase or decrease power or enters into low power state. Hence power receiver is known as "talker" in wireless charging system.
• Power Receiver transmits control error packets at every 250 ms (during normal operation) or every 32 ms (during larger signal variation needs). In response, power transmitter transmits power packets every 5 seconds normally.
• When power receiver do not need any more power from the transmitter then it sends out "End Power" message to the transmitter OR power receiver does not transmit anything for period of about 1.25 seconds. In either of these situations, power transmitter enters into low power state.