Qi Wireless Charging: Technology and Protocol Overview

Wireless power is becoming increasingly popular for charging portable mobile devices. The Wireless Power Consortium (WPC) has developed a standard, called Qi (pronounced “chee”), to facilitate wireless electric power transfer using inductive coupling over distances up to 40mm. This system consists of a power transmission pad and a compliant receiver, typically found in portable mobile devices.

To charge a device, you simply place it on the Qi-enabled pad. The pad then wirelessly charges the device using inductive coupling. The Qi standard currently supports power delivery of up to 5 Watts, making it suitable for low-power devices. The WPC is also developing a high-power specification that aims to deliver up to 1 Kilowatt of power.

The WPC boasts a diverse membership, including companies like HTC, LG, Huawei, Motorola Mobility, NTS, Nokia, Philips, Sony, and Samsung. Founded in 2008, the consortium includes approximately 80 member companies from Asia, Europe, and America.

How Qi Wireless Charging Works

Let’s delve into the workings of wireless charging. The system comprises three essential elements:

1. Power Transmitter
2. Power Receiver
3. Communication protocol between them

The device to be charged is placed on the pad, which supports wireless power transmission.

The diagram below illustrates how power is transferred using layers of packets. These continuous waves are absorbed by the device being charged. Inductive power transfer is achieved through the coupling of magnetic fields from the primary coil (in the transmitter) to the secondary coil (in the receiver).

Figure-2 depicts typical components in a wireless charging system. The transmitter includes an AC-to-DC module that converts AC power to DC power. This DC power is then supplied to the pad, which transmits it wirelessly using the primary coil.

The receiver’s secondary coil captures the power and rectifies it to provide unregulated DC power. A voltage conditioning module then converts this unregulated DC to regulated DC power, which is supplied to the device being charged to prevent damage.

Qi Communication Protocol

The communication protocol in Qi wireless charging works as follows:

The power receiver acts as the “talker.” Here’s a breakdown of the process:

• When a device is placed on the pad, the power transmitter (the pad) detects its presence using an analog ping.
• A digital ping from the power transmitter signals the power receiver to respond with a signal strength packet. If the signal strength is valid, the transmitter activates the primary coil.
• The power receiver then sends a packet containing setup and configuration information to the power transmitter.
• The power receiver now dictates whether it needs power. Based on the received packets, the power transmitter will increase or decrease power, or enter a low-power state. This is why the power receiver is considered the “talker” in the system.
• The power receiver transmits control error packets every 250 ms during normal operation, or every 32 ms when larger signal variations are needed. In response, the power transmitter sends power packets every 5 seconds under normal circumstances.
• When the power receiver no longer requires power from the transmitter, it sends an “End Power” message, OR it ceases transmission for approximately 1.25 seconds. In either scenario, the power transmitter enters a low-power state.