Original Link: https://www.anandtech.com/show/9130/wireless-charging-standards-in-mobile
The State of Wireless Charging Standards in Mobile
by Andrei Frumusanu on April 2, 2015 8:00 AM EST
Wireless charging in the mobile space has seen its debut almost 6 years ago now with the introduction of the Palm Touchstone. Back in the day, this was quite a revolutionary advancement for mobile devices and I still remember the discussion on how wireless charging would be the future. Six years later, I have yet to own a wireless charger or know somebody who uses one. Analyst prediction of quick adoption failed to materialize and the industry is still trying to consolidate a universal charging standard that would be compatible across all devices. While Palm sparked the wireless charging wars back in 2009, it took device manufacturers many more years before we reached (or have yet to reach) a level of adoption such that the average consumer would be able to confidently use the technology as a de-facto everyday way of charging their devices in the same way that microUSB has.
With the advent of Qi and PMA as opposing and incompatible charger technologies the industry saw a period of uncertainty over which standard would finally make it into the mainstream. In the end, it might be neither, but before we delve into the future, let’s have a look at how wireless charging has evolved over the last few years and how the mechanisms actually work.
A Timeline of Events and Standards
While Palm was first to introduce a wireless charger in the Palm Pre in form of the Touchstone, this was a proprietary standard which wasn’t adopted by other vendors. In fact, while Palm announced and presented the Touchstone at CES 2009, a small group of manufacturers including Logitech, Philips, Sanyo and Texas Instruments gathered around in December of 2008 to form the Wireless Power Consortium, or WPC.
The WPC released later in 2009 the first specification the Qi 1.0 which would become the first proper open wireless charging standard for low power devices. “Qi”, pronounced “chee”, is named after the same Chinese word for “life force” or “energy flow”. In the following months and years the WPC saw a lot of companies adopt the standard and join as members of the consortium. The big names such as LG, Motorola, HTC, Samsung, Sony and Nokia were on board and at this point it looked like Qi was on its way to become the de-facto standard for large scale adoption.
The Nokia Lumia 820 and 920, and more importantly the Nexus 4 were the first devices which introduced Qi charging built-in by default by the manufacturers and were available towards the end of 2012. The biggest enabler of wireless charging though was probably Samsung – starting with the Galaxy S3, Samsung integrated wireless charging capability into their PMICs and exposed on the back of the phones not only pins for the charger coils, but also power contacts which we’ll come back to later. Cheap charger coils that could be added between the battery and cover and attached to the wireless charging pins meant that users could quickly experience wireless charging without too great of an investment.
While the WPC seemed to have won the standards-race with an early start and rising adoption in 2012, the same year the Power Matters Alliance (PMA) was formed as a competing standard. While the WPC seems to have concentrated in the mobile space, the PMA aims to be a more generic standard for other use-cases. Starbuck’s June 2014 announcement of the adoption of PMA notoriously surprised consumers as PMA had seen very little adoption by the market versus Qi. PMA seems to continue to gain adoption by sheer brute force introduction of North American outlets such as McDonalds and Starbucks but faced the problem of needing special charging adapters as no device was yet compatible with the new standard.
While both the WPC and PMA make use of inductive charging technology, a third standard appeared in early 2012 in the form of “Rezence”. The standard is developed by the Alliance for Wireless Power (A4WP). Here the power transfer technology is based on magnetic resonance instead of electromagnetic induction. Rezence sees support of some big companies such as Intel, Broadcom, Qualcomm and Samsung and promises a true alternative to induction charging that solves many of the problems faced with Qi and Powermat (PMA) solutions.
In January 2015 things were shook up again as the PMA and A4WP announced a merger of the two alliances. The merger aims to consolidate the swath of charging standards. While the merger won’t be finalized until June 30th 2015 and things could still change till then, what this means in practice is that Rezence will be seeing a much faster and larger adoption than anticipated through boost of PMA members.
At the time of writing the A4WP has a published 122 members, PMA 68 and the WPC 213 companies.
While Rezence seems to be the most promising candidate technology-wise to being adopted as the “USB of wireless charging” in the distant future, equipment manufacturers will first see the introduction of dual-and tri-standard compatible devices. The Galaxy S6 was one of the first devices to include both Qi and Powermat compatible charging built-in, it still lacks Rezence charging capability. What I expect for future devices though is cross-standard compatibility with help of adoption of solutions which support both inductive and resonance charging.
At MWC 2015 we’ve seen the demonstration of a slew of such solutions by for example MediaTek, Broadcomm and others. It is clear that manufacturers are going for multi-standard compliant end-devices in the future, and it’s the ecosystem around the charging stations which will decide on how things will evolve in the future.
Inductive Coupling - The Basics
With the history of wireless charging out of the way, the remaining is question is how does it all work? To start, we need to understand the basics of inductive coupling.
Power transfer of inductive coupling happens by creating an alternating magnetic field on the transmitter coil, that magnetic flux is then converted into an electrical current in the receiver coil. The generated electrical current depends on the amount of flux generated by the transmitter coil, and how much of a percentage the receiver coil is able to capture. The distance, size, and positioning of the receiver coil relative to the transmitter coil decides the “coupling factor” of the two coils.
A higher coupling factor means a more efficient power transfer through loss reduction and less heating. In general, we differentiate between tightly coupled and loosely coupled systems.
Source: WPC
In tightly coupled systems, the transmitter and receiver coils both are of the same diameter size and aligned with each other with a minimal distance in-between them. This setup ensures most of the magnetic flux is captured by the receiver coil to be able to be converted into an electrical current.
Loosely coupled systems are defined by setups which do not capture all of the transmitter’s flux, either because the receiver coil distance is too big, or because the coil diameters don’t match and the receiver coil is smaller than the transmitter coil.
Source: WPC
While we mention loosely coupled systems and we’ll get back to them short when investigating Rezence charging, both WPC and PMA standards based on tightly coupled systems.
Optimal alignment of the transmitter to the receiver coil is crucial for a high coupling factor and high efficiency power transmission; because of this, there is a problem of practicality to actually achieve this alignment. To alleviate the issue, Qi transmitters allow for multiple transmitter coils that may overlap.
Source: WPC
By employing multiple transmitter coils over each other, we have a greater area of “optimal coupling” over which a device can be placed on. The control IC of the charging system will then detect which coil is best suited for the power transmission and then use that one for generating the magnetic flux. It is also possible to power on multiple coils simultaneously to achieve a higher power transfer if the receiver device is designed for it.
WPC & PMA Power Control
In Qi and PMA based systems, control over how the coils are powered is done via data communication between the transmitter and the receivers. This is done via data modulation (Either amplitude or frequency based) on the power carrier frequency. The in-band communication requires advanced signal processing and this is where manufacturers’ solution ICs come into play.
WPC wireless power functional diagram, Source: Texas Instruments
This communication is also the main difference and source of incompatibility between the Qi and PMA standards, as without a reliable and interoperable signal the charging process will not start. Powermat differs to Qi by using RFID handshake instead of modulation on the power signal.
The direction of power transfer is always from power transmitter to power receiver. The transmitter idles with in a low power state, which depending on vendor is in the 60mW range. When the power receiver detects a transmitter, it identifies itself to the transmitter as a compliant device and sends configuration information such as the desired charging power. Once power transfer is initiated, the receiver can send error packets requesting more or less power. The power transmission stops upon either receiving an “End Power” message or a period of non-communication of 1.25s.
The coils of a WPC-compliant device operate as resonant half-bridge on a 50% duty cycle with a 19V(±1V) DC power input. Depending on power demand, the frequency in the coil is modulated between 110 and 205 kHz. Powermat on the other hand works in the 277-357 kHz range.
On the receiver side, WPC gives the manufacturers much more liberty in terms of coil-design. While the transmitter is required to have a certain size and provide certain power for standard’s sake, the receiver is allowed to vary in size to adapt to the device form factor the system is implemented into.
Qi in its first low-power specification allows up to 5W of power transfer, with a revision on newer charging systems allowing up to 10W.
Unfortunately PMA does not publicize the standard’s specifications to non-members, so we don’t have any finer details on how it compares to Qi. It’s nevertheless safe to assume that the basic principles of the two standards work in similar fashion.
Rezence Resonance Charging
Earlier we saw the distinction between tightly coupled (TC) and loosely coupled (LC) induction charging systems. While A4WP’s Rezence system is called a magnetic resonance charging system, the basic principle is still based on magnetic induction. The critical difference between Qi and Powermat standards is that it is a Rezence is based on a loosely coupled coil system.
Inductive power frequencies and use-cases (Source)
A4WP systems work on a much higher frequency compared to tightly coupled systems such as defined by PMA and WPC. Rezence’s operating frequency is defined on a strict 6.78MHz (±15kHz) band compared to the 110-357kHz range that Qi and Powermat operate in.
Instead of a tight coil on the transmitter and receivers, we see the usage of a specifically shaped resonator designs. An A4WP transmitter has typically larger footprint than Qi or Powermat designs, with various classes of designs available depending on power requirement. Currently the widely adopted one is the Class 3 transmitter which has a size of 204x146mm and meant for smartphone device use-cases.
A4WP Class 3 spiral type 235-135 power transmit resonator (Source 1, Source 2)
The class system allows A4WP for a variety of size power transmitter unit (PTUs) and power receiving unit (PRUs) resonator coil systems depending on the number of devices and power you are targeting to charge. The class 3 PTU supports up to 16W transmit power.
| PTU Class | Maximum transmission in power |
Minimum PRU Support |
| Class 1 | TBD | >1 x Category 1 |
| Class 2 | 10W | 1 x Category 1, 2, or 3 |
| Class 3 | 16W | 2 x Category 1,2, or 3, or 1 x Category 4 |
| Class 4 | 22W | 3 x Category 1, 2, or 3, or 1 x Category 4 |
| Class 5 | TBD | TDB |
| PRU Category | Maximum reception out power |
Example Applcations |
| Category 1 | TBD | BT headset |
| Category 2 | 3.5W | Feature Phone |
| Category 3 | 6.5W | Smart Phone |
| Category 4 | TBD | Tablet |
| Category 5 | TBD | Laptop |
The current PRU categories are mainly defined for feature phone and smartphone devices with output powers of respectively 3.5 and 6.5W. We should be seeing class 3 devices as the most prevalent category implemented in smartphones in the near future. A class 3 PTU can charge at least two class 3 PRUs at full power, which should be sufficient for most use-cases.
A4WP WPT multi-device charging architecture (Source)
As mentioned before, the LC design of the Rezence charger allows for multiple receivers for a single transmitter, enabling a "star network" of PRUs for each PTU.
While Qi and Powermat respectively use either modulation on the power signal and RFID for communication between the transmitter ans receiver devices, Rezence work with a Bluetooth Low Energy (BLE) connection between all the different actors. There is no device pairing involved as communication is done over GATT (Generic Attribute Profile), a BLE feature.
At MWC2015 multiple vendors were demonstrating their A4WP implementations and chargers, and it was much more impressive to see them in action compared to the TC systems such as Qi and Powermat. Mediatek had a vertically standing Rezence charger which was demonstrated on a Galaxy S4 with custom backplate integrating Mediatek's tri-standard charging solution and powering the device via the dedicated power pins.
Practically, Rezence also seems to be the better solution as one doesn't have to deal with alignment of the device on the charging surface. Charging surfaces can vary in size and have an advantage of reduced complexity as it still remains a single resonator coil, where as Qi requires multiple overlapped coils to achieve the same effect and Powermat doesn't allow such flexibility in the first place.
Closing Thoughts
Having shed a bit more light on the difference and characteristics of the three main charging standard today, the question is on how the future of wireless charging will look. There is an undisputable battle of politics going on between the different standards associations, with each wanting to have their say and their own solutions to the wireless power transfer problem. Though right now the odd man out in all of this is the PMA, as it's hard to argue that Powermat brings any tangible advantages over Qi, not to mention it's a much less widespread standard.
Rezence from the A4WP on the other hand seems to be promising. The A4WP sees tightly coupled systems as first-generation technology that will in the future be replaced by loosely coupled charging implementations such as Rezence. I don't think the lesser charging efficiency will be something that people will mind as the practicality improvements clearly outweigh the disadvantages. The PMA-A4WP merger is seen as a technology swap as the A4WP gains access to a tightly coupled standard and the PMA gets access to a loosely coupled standard.
Meanwhile the WPC is working on its own resonance based loosely coupled system to compete with Rezence, but it's still in the development and we don't yet have much information on it.
MediaTek's tri-standard charging solution demonstration board
IC designers are going forward with tri-standard compatible solutions for wireless charging and that's what I envison will be the the aim for most device manufacturers who will want to include wireless charging into their products. This solves the device compatibility issue, but doesn't solve the problem completely as not all devices will be able to adopt it due to differing form factors. Rezence here again offers the best offering for form factor diversity, power transfer and charger interoperability. Though with the number of factions involved in this battle, it's not necessarily a given that this will be a battle won by technology as opposed to politics.
Ultimately the wireless charging ecosystem has changed a lot over the last few years, and things could again change drastically over the next few years, so nobody knows for sure in which direction the market will shift. We'll be sure to keep an eye on WPT in the future and cover any emerging news on the topic.
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