【Original】The battle of wireless charging standards: Qi vs Rezence

Nowadays, everyone has a mobile phone, laptop, tablet, smart watch, etc., and the use process will cause the power to drop, and all of them may need to be charged. According to the Radicati mobile data report, there were more than 9 billion mobile devices on the market in 2015. Of course, the competition for wireless charging standards is becoming more and more obvious. We have experienced many similar standard battles before, such as VHS vs Beta, USB vs Firewire, and the recent HD DVD vs Blu-ray Disc. Behind these are billions of dollars in the market, and the competition for wireless charging standards between Qi (pronounced "chee") and Rezence (pronounced "REH-zence") is also competing for market share. How do these technologies compare, and which ones will become industry benchmarks?

Figure 1: Qi standard wireless charging system (Source: TI)

Strong coupling inductive charging is similar to the conversion between the main winding and the secondary winding of a transformer. When a certain current passes through the main winding, a certain magnetic field will be excited in the surrounding area, which is called the Biot-Savart Law:

From the above formula, we can see that r represents the distance, and the magnetic flux (dB) will increase as the distance decreases. Inductive charging requires two planar coils of similar size and direction kept close to each other (within 5mm) to achieve power transmission. Although inductive charging is limited to a one-to-one relationship (similar to a transmitter corresponding to a receiver), it is a very simple and efficient wireless charging mode.

Although the weak coupling magnetic resonance method has low energy efficiency, it reduces the requirement for close distance of inductive charging. Nikola Tesla once conducted a famous experiment in which he used the electromagnetic field (EMF) of the earth's ionosphere to try to distribute energy. Similarly, magnetic resonance (non-electromagnetic field) relies on the magnetic field, but allows the primary coil and the secondary coil to have less overlapping magnetic field areas, which is equivalent to improving the spatial freedom of the system. The shape and size of the coil can also be different, allowing multiple devices to be charged through a "power supply" and establish a one-to-many relationship.

Resonance is the property of an oscillating system that causes another system to oscillate at a given frequency with a greater amplitude. For example, if you have two tuning forks, they both respond to the same frequency because one of the forks causes the other to vibrate due to resonance. The same phenomenon also occurs in electronic systems, using resistor, inductor and capacitor (RLC) circuits. The efficiency of this model is expressed by the Q factor:

Although the Q factor is independent of distance, more energy is required to propagate over a larger range, which is why resonant systems are less efficient than inductive charging systems.

Qi introduced its wireless low-power specification in 2009 and is supported by the Wireless Power Consortium (WPC), an organization of more than 200 technology companies, smartphone manufacturers, and semiconductor suppliers. The technology has been tested to rely on strong inductive coupling and can provide 5W of charging power at short distances greater than 5mm. Qi is also designed with a dedicated communication protocol that allows devices to define the required charging amount. This is very suitable for small devices such as mobile phones, MP3 players, and portable Bluetooth speakers. The Qi standard has been adopted by many OEM manufacturers, such as the Samsung Galaxy and Nokia Lumia series of smartphones. Since its launch, there have been more than 800 Qi-licensed products on the market. In the future, Qi will still be an attractive solution for medium and large-scale devices with power consumption of 15W to 1KW.

Figure 2: Qi wireless charging standard compatible RX/TX mechanism

Rezence uses magnetic resonance technology to achieve wireless charging, which can provide 50W of power and has been supported by AW4P (Wireless Power Alliance, which merged with PMA organization on June 1, 2015). Currently, more than 250 companies use this technology, and semiconductor spoiler Intel also uses this technology. The main difference between the above two wireless charging methods lies in the design of the induction coil.

Rezence implements loosely coupled magnetic resonance, which allows for more freedom in placement of charging devices. For example, a full charge can be achieved even with a book between the device and the charging mat. This undoubtedly increases flexibility, but reduces charging efficiency by about 45%, while the Qi standard has a charging efficiency of 65%. In addition, Rezence requires devices to have Bluetooth support for communication, which is common on most mobile devices. Although Qi-standard devices are now tightly coupled, the Qi standard also has a certain degree of flexibility, supporting both magnetic resonance and induction, while Rezence is still strictly magnetic resonance and loosely coupled.

Figure 3: Alliance for Wireless Power (A4WP) hardware-compatible Rezence wireless charging standard: loose coupling and wireless data communication

Technology itself does not have a winner. For example, if we recall the competition between Blu-ray and HD DVD, Blu-ray eventually won because the hardware it used was more accessible, and Blu-ray products occupied the retail and distribution channels. Microsoft tried to provide HD DVD add-on features on Xbox 360, but users were not very enthusiastic about purchasing additional hardware. However, unlike the strict division between Blu-ray discs and HD DVDs, the companies supporting Qi and Rezence do not have a clear position on the standard. Qi and Rezence have common supporters, which shows that major manufacturers have not yet reached a critical point to determine which technology to support in the long term. IC manufacturers such as TI and IDT want to occupy the largest market share, and OEM suppliers such as Samsung, Toshiba and Lenovo want to avoid obsolescence of their own technology. Their goal is to maintain compatibility with future wireless charging platforms for as long as possible. Neither IC manufacturers nor OEM manufacturers can be separated from the latest technology.

There is no doubt that the Qi standard has a leading position in the mobile phone and other wireless electronic products market, with some products such as Google Nexus 6 and Samsung Galaxy S6 at the top of the list. Despite this, the adoption rate of wireless charging is still very low. According to the IHS Consumer Wireless Charging Awareness Report, only 20% of mobile phone users adopt wireless charging. However, in 2015, the knowledge and awareness of wireless charging among consumers in the United States more than doubled to 76%. More importantly, the report also pointed out that the wireless charging market for wearable devices will grow from $400 million in 2015 to $1.1 billion in 2019. The adoption of wearable devices may bring a very big advantage to Qi. Apple has been playing passively between Qi and Rezence in public. However, although the Apple Watch is not a Qi licensed product, it is compatible with the Qi standard and has led to the wearable market accounting for more than 70% of total revenue. In this dominant situation, other manufacturers may also follow the previous technology, such as touch screen and fingerprint recognition.

Rezence and A4WP specifications are the latest to be introduced, and manufacturers have not yet released products that use the technology. Although Qi dominates the current wireless charging products, the public believes that Rezence may soon change this. You can choose to install Rezence charging facilities in Starbucks, coffee shops or tea houses, and McDonalds. More importantly, Intel is pushing the next generation of laptops, Intel Atom tablets, keyboards, mice, and other PC peripherals to adopt the Rezence wireless charging standard. Because Intel will seriously affect the marketing and advertising business of OEM manufacturers such as Dell, HP, and Lenovo, we are likely to see the rapid rise of Rezence wireless charging.

Medical device and automotive technology markets are risk-averse and reliability-focused, and are unlikely to standardize technologies until one becomes dominant. However, if a standard becomes an industry standard, it is certain to continue to consolidate its position in the market for many years to come.

The final battle for wireless charging standards comes down to three factors:

1. Charging time
2. Ease of operation
3. Practicality

Consumers only care about whether their devices can be used normally, they don't care how it is achieved. In addition, in order to make wireless charging more common, Qi and Rezence will have to push device manufacturers to bundle wireless charging as a standard accessory without requiring users to purchase it separately. Chargers also require additional applications and manual pairing. Traditional charging methods will become less and less popular. Devices with integrated seamless wireless charging functions will become more common. Many people are already aware of the poor user experience of wall chargers, complicated interfaces and wires. Qi has demonstrated an easy-to-use solution that has been adopted in many devices, while the Rezence standard can provide faster charging speeds. In the future, the victory of these two technologies will depend on the influence of technology companies like Intel and Apple and the widespread integration of wireless charging solutions into their products.

About the author:

Noah Imam is a software engineering consultant with extensive experience and expertise in designing mobile software, Linux, embedded kernel development, and ARM architecture. He received his bachelor's degree in electrical and computer engineering from the University of Texas at Austin. Intelleaf is an Austin-based high-tech company that provides B2B product management and custom software development consulting services, solving key business, strategic, customer, and financial challenges in the development, design, and promotion of new products.

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