The two common commercialized charging solutions in the majority of mobile devices are USB Power Delivery and Qi wireless charging. Previously we discussed the standardization of USB Power Delivery and how wired charging has evolved over the past few years. We now focus on how Qi wireless charging became the de-facto wireless charging technology in consumer electronics, mainly smartphones and tablets. Even though wireless charging has been around since 2008, it has recently gained popularity in mobile devices, especially since Apple adopted this technology with the iPhone 8 and newer generation iPhones.
Wireless Charging Standards
There are three wireless charging standards for consumer devices – Qi, PMA (Power Matters Alliance) and A4WP (Alliance for Wireless Power). All three wireless power standards are based on Faraday’s law of induced voltage and utilize inductive coils for wireless power transmission, but are defined to function at different frequencies with different control schemes. As such, each wireless power standard offers unique benefits in technology, with different levels of industry support and market share. The most widely adopted wireless charging standard by the majority of mobile devices is Qi (pronounced “chee”), which is the industry standard created by the Wireless Power Consortium (WPC).
Whereas the Qi standard works over the approximate frequency range of 100-205 kHz, the PMA standard delivers up to 5W over almost twice that frequency (6.8MHz). Both the PMA and Qi standards are actually quite similar and are based on magnetic induction principles in which a current through one coil generates a magnetic field that in turn will induce voltage in the second coil. Both PMA and Qi use different protocols between the wireless power receiver and transmitter to support wireless charging.
Basics of Qi Wireless Power Transfer
As stated previously, wireless power transfer is based on Faraday’s law of magnetic induction. A depiction of wireless charging implementation in mobile devices is shown below (Figure 1).
The wireless charging transmitter acts as a power source that has a wireless charging integrated circuit (IC), typically a MCU, DC-DC converter and a switched transistor bridge to drive the transmitter coil. A series capacitor is placed to set the resonant frequency. The transmitter may have more than one coil for better alignment with the receiver coil. The current flowing through the transmitter coil will induce a voltage in the receiver coil. The receiver circuit will rectify the induced voltage, filter it and then applies it to the battery that needs to be charged. The battery inside the portable device receives the power and charges up. The receiver can command the transmitter to adjust the charging current or voltage, and also to stop transmitting power completely when end of charge is indicated.
Evolution of Qi Standards
The first Qi specification version v1.0 was released in 2010 with maximum power transfer capability of 5W with a limited choice of transmitter and receiver designs. Version 1.1 improved and increased design freedom of transmitters as well as increased sensitivity of foreign object detection (FOD). Version 1.2 increased the power transfer up to 15W and included improved thermal test requirements for the transmitters.
The latest WPC specification standard is version v1.3, which was released in December 2020 and can deliver up to 15W with improved FOD and thermal safety. The latest released version requires authentication between the transmitter and the receiver to take advantage of 15W without any safety or thermal concerns.
The WPC has already started working on the new specification to increase the power level up to 30W to 45W to charge laptops and tablets (Figure 2).
Recently the WPC has formed another standardization to charge kitchen appliances wirelessly at a power level of 200W. Similarly, this standard can be adopted to charge other numerous high-powered devices.
With the increase in the power levels of wireless charging, the applications are boundless – from not only consumer devices perspective, but also from high power applications such as laptops, kitchen appliances, robots and 5G routers. For military applications, higher power wireless charging will help the Army’s plans of adopting wearable batteries with increased capacity by allowing them to recharge faster while eliminating the wear and tear associated with cabled charging due to erosion from austere environments or aggressive use.