Many of the latest smartphone handsets are shipping with wireless charging capabilities, and various new car models can charge such items. Coffee shops around the world have tested out the technology, and there are many evaluation platforms - for all sorts of different wirelessly-based charging systems.
Based on all this, what is holding up the widespread use of wireless charging? The main challenge is competing standards. The Qi specification from the Wireless Power Consortium (WPC) and the various specifications that make up the AirFuel Alliance have been at loggerheads over the years with different technologies, making it hard for equipment makers to be confident that consumers can easily charge their smartphones, smartwatches, tablets or laptops wherever they are.
There have also been some engineering challenges to factor in too. Qi systems require the charging coils on the transmitter and receiver to be a few millimetres apart and reasonably well aligned to get the fastest charging. This is something that doesn't just happen when a user puts their phone handset down on a table to charge.
The unit being charged and the underlying charger also have to communicate, and there are many different ways for this to happen. There are various fundamental techniques involved too - from inductive and capacitive charging to magnetic resonance. Each will have its particular engineering trade-offs. At the same time, users also want faster charging, which means higher power, and this can create safety issues for wireless topologies, as metal getting in the way can heat up and cause injuries. It is essential not to forget, that in addition to all of these considerations, equipment makers want the lowest overall cost.
Introducing The WPC
Set up in 2008, the WPC is an open, collaborative standards development group of more than 600 member companies from around the globe (although it should be noted that many companies are involved with both competing organisations in the wireless charging domain). It launched the Qi standard (pronounced ‘chee') soon after, so this has been the most well-established technology for smartphones and other portable mobile devices powered at 5W to 15W. To a certain extent though this has held the standard back as any innovation must be backwards compatible with systems designed a decade ago.
Qi uses a low frequency closely coupled inductive charging mechanism - which represents the lowest cost approach. However, the WPC has also branched out with a cordless kitchen standard, for domestic appliances (covering 200W to 2.2kW), and a medium power standard, for power tools, robotic vacuum cleaners, e-bikes and other battery-powered devices (at 30W to 65W). Both of these are still under development and use different technologies to avoid the backward compatibility challenge.
Figure 1: Texas Instruments’ BQ51222 single-chip receiver for wireless charging.
The Qi standard uses low frequency (112kHz to 250kHz), low data rate in-band communication. This limits the charging separation to 3mm to 5mm and has a clear impact on placement flexibility.With in-band communications, the data being sent is typically piggy-backed on the wireless power transmission. The trouble is that this is uni-directional from the receiver to the transmitter, without feedback from the transmitter to the receiver, and only works when a single receiver is associated with a single transmitter. There are ways around this, by using multiple separate coils or an array and different communication protocols - but these suffer from problems with backwards compatibility, as well as higher cost. There are many single chip Qi controllers in production, from suppliers such as Texas Instruments and NXP, that are helping to drive down costs of simple wireless connections.
Figure 2: The NXQ1TXH5 Qi-based wireless charging transmitter from NXP.
The AirFuel Alliance
The merger of Alliance for Wireless Power (A4WP) and Power Matters Alliance (PMA) in 2015 to create the AirFuel Alliance managed to consolidate two rival