How Qi Wireless Charging Uses Backscatter Modulation

This article explains the technical process of how the Wireless Power Consortium (WPC) Qi standard uses backscatter modulation to enable communication between a power receiver and a power transmitter. We will explore the mechanics of inductive coupling, how the receiver alters its electrical load to send data packets, and how the transmitter demodulates these signals to safely and efficiently regulate wireless power delivery without needing a separate radio frequency transceiver.

The Basics of Inductive Coupling in Qi

The Qi wireless charging system consists of two main components: a base station (transmitter) and a mobile device (receiver). The transmitter contains a primary coil, and the receiver contains a secondary coil. When alternating current passes through the primary coil, it generates an oscillating magnetic field. When the secondary coil is placed within this field, the magnetic flux induces an alternating current in the receiver, which is then rectified into direct current to charge the device's battery.

Because the two coils are inductively coupled, any electrical change on the receiver side directly affects the magnetic field, which can be detected on the transmitter side. The Qi standard leverages this physical property for communication via backscatter modulation, also known as load modulation.

How Backscatter Modulation Works

Instead of using a dedicated communication chip like Bluetooth or Wi-Fi, the Qi receiver communicates with the transmitter using the same magnetic field that transfers power. This is called in-band communication.

To send feedback to the transmitter, the receiver intentionally varies the electrical load connected to its secondary coil. Inside the receiver, a communication modulator switches small capacitors or resistors in and out of the circuit at specific intervals.

  1. Altering the Load: When the modulator switches the extra load into the circuit, it changes the impedance of the secondary coil.
  2. Reflecting the Signal: Because the primary and secondary coils are linked by the magnetic field, this change in impedance is "reflected" back to the primary coil.
  3. Detecting the Change: On the transmitter side, the reflected impedance causes measurable fluctuations in the current or voltage flowing through the primary coil.

Encoding and Decoding the Data

The Qi standard uses a specific encoding scheme to turn these load fluctuations into readable digital data.

The receiver uses a differential bi-phase encoding method at a data rate of 2 kilobits per second (kbps). A logical "1" is encoded by making two load transitions within a single bit period, while a logical "0" is encoded with a single transition.

The transmitter continuously monitors its primary coil using a demodulation circuit. This circuit tracks the amplitude or phase of the coil’s current and voltage. When the receiver modulates its load, the transmitter detects the resulting tiny spikes and dips in the coil current, filters out the main power frequency (typically between 110 kHz and 205 kHz), and translates the transitions back into binary data packets.

What Information is Sent via Feedback?

Backscatter modulation is strictly unidirectional, flowing only from the receiver to the transmitter. This feedback loop is critical for four main phases of wireless charging:

  • Identification and Configuration: When a device is placed on a charger, it uses backscatter modulation to wake up the transmitter, identify itself as a Qi-compatible device, and communicate its maximum power requirements.
  • Control Error Packets: During active charging, the receiver constantly monitors its own voltage and current. It sends "Control Error Packets" roughly every 250 milliseconds, telling the transmitter to increase, decrease, or maintain the current power level. The transmitter adjusts its resonant frequency or duty cycle accordingly.
  • Foreign Object Detection (FOD): The receiver tells the transmitter precisely how much power it is receiving. The transmitter compares this value to the amount of power it is sending. If there is a large discrepancy, the transmitter assumes a foreign metal object (like a coin or key) is absorbing the magnetic field and shuts down to prevent overheating.
  • Charge Complete: Once the device's battery reaches 100%, the receiver sends an "End Power Transfer" packet, instructing the transmitter to shut off the magnetic field and enter standby mode.