While wireless charging (WC) sounds like a useful technology in theory, its full potential has been limited by design-level barriers.
The inductive charging process. Image used courtesy of Proxi and Computer World
Namely, the limited range of WCs has barred the full potential of Qi technology: sure, the technology is technically “wireless,” but if the phone has to be directly on the stand to work, does it really afford users that much freedom? At least a user can use their phone with some range (the length of the charging cable) while using wired charging.
While there are many companies working on WC at a distance, Xiaomi, one of the world’s top smartphone makers, may actually have resources and commercialization rapport to make truly wireless charging a reality.
Xiaomi Announces Mi Air Charge Technology
Xiaomi’s recent claim of a long-range wireless solution is not the company’s first brush with wireless technology in general—an example being its Mi 10 Ultra smartphone, which is capable of 120 W wired charging and 50 W wireless. Compare that with the standard 5 W USB charging that most phones (iPhone included) offer, and you can see why this feature is being called an engineering feat.
The company has now continued adding to its résumé of charging technologies with its new Mi Air Charge technology.
Mi Air Charge includes a beacon antenna, which sends position information without high power consumption. Image used courtesy of Xiaomi
Xiaomi calls its Mi Air Charge a “revolution” in wireless charging, offering WC of multiple devices at once at a radius of several meters. The technology can provide up to 5 W of power for each device from a central beacon and is said to charge through solid material without loss of efficiency.
While a standard USB wired charger will produce 5 W (5 V at 1 A), Xiaomi is matching these numbers at a true distance. Many of the technical details are still unreleased, but we’ll cover what we do know.
Charging at a Distance: How the Transmitter Works
Mi Air charging technology, like Qi technology, consists of a transmitting device (dubbed by Xiaomi the “charge pile”), and a receiving device (the smartphone). The key to the technology is spatial positioning and efficient energy transfer.
The charge pile consists of a five-phase interference antenna array and a 144 antenna phase control array. The five-phase array detects signals from the smartphone homing beacon to accurately localize the smartphone. Then, the phase control array transmits millimeter waves to the phone through beamforming.
Graphic showing the concept of phased array antenna beamforming. Image used courtesy of Radar Tutorial
We can assume that the charge pile’s phased array antenna arrays work on the principle of interference, where the superposition of several radiation sources can help the array localize a transmitter or direct its own transmissions.
By observing the interference patterns from the smartphone’s homing beacon, the antenna array can determine the smartphone’s location. In the same way, the charge pile can then transmit the millimeter waves to the smartphone via its antenna array.
The signal can be directed to its intended target at a distance when each antenna transmits different phase-shifted signals to constructively interfere in the direction of the receiver. This widely-used technique allows for purely electronic detection and steering of signals.
On the Receiver Side
On the receiver side (i.e the smartphone) the technology consists of a beacon antenna array and a receiving antenna array.
Using the methods described above, the beacon antenna transmits signals to the charge pile so the pile can determine the position of the smartphone. While this requires continuous transmission, Xiaomi claims this transmission requires very low power, so battery life should not be affected.
The receiving antenna array consists of 14 antennas that convert the millimeter waves into useful DC energy through its rectifying circuit. Because AC/DC conversion can be a large source of inefficiency, Xiaomi likely devoted plenty of attention to detail to the power efficiency of this rectifying circuit.
A MOSFET rectifier circuit with power factor correction. Image used courtesy of Texas Instruments
Academic research has proposed many ways in which AC/DC conversion can be made more efficient. One of the main sources of inefficiency in the standard bridge rectifier is the forward drop of the diode bridge.
A solution that’s been widely accepted has been a digitally-controlled MOSFET bridge that behaves like a rectifier, but without the forward voltage drop that the diodes introduce. Other techniques include power factor correction, which attempts to force the voltage and current waveforms in phase.
Xiaomi’s Air Charge presents some exciting possibilities in the electrical engineering world since the company claims this is the first long-range wireless charging technology that’s been integrated into a major smartphone brand.
What design challenges remain in the way of widespread adoption of true wireless charging? Share your thoughts in the comments below.
