By Noah Imam, Inteleaf, for Mouser Electronics
Having to charge multiple devices can be frustrating. Whether it’s at an airport terminal or a coffee shop, power
outlets can be a contentious commodity. Imagine if you could charge your device simply by setting it down. No more
wires, dongles, or bulky chargers. With wireless charging technology,
likely to become commonplace.
Today, the average person may have a phone, laptop, tablet, and a smartwatch, which are all steadily losing battery
power and waiting to be recharged. According to the Radicati mobile statistics report, there will be over 9 billion
mobile devices in 2015. In the midst of it all, a tech
looms over unplugging and charging wirelessly. We’ve seen these before, first with VHS vs. Beta, USB vs. Firewire, and more recently between HD DVD and Blu-ray Disc. With
potentially billions of dollars at stake, Qi (pronounced “chee”) and Rezence (pronounced “REH-zence”) wireless power
standards are vying for the same lucrative market share. How do these technologies compare, and which of these will
emerge as the industry standard?
Figure 1: Qi-compliant Wireless Charging System (Source: Texas
Strong coupled inductive charging is analogous to how a transformer operates with a primary and secondary winding.
Sending a current through the primary winding results in a magnetic field described by the Biot-Savart Law:
Since r = distance, the magnetic flux (dB) is increased as distance is decreased. Inductive charging requires two
planar coils of similar size, orientation, and in close proximity (within 5mm) for efficient power transfer.
Although inductive charging is limited to a one-to-one relationship between the transmitter and receiver, it is a
simple and efficient implementation of wireless power.
Weakly coupled magnetic resonance, although less energy efficient, reduces the proximity requirements of inductive
charging. Nikola Tesla was famous for his experiments using the electromagnetic field (EMF) of the Earth’s
ionosphere in an attempt to distribute energy. Similarly, magnetic resonance (non EMF) relies on magnetic fields but
allows primary and secondary coils to have fewer magnetic field lines in common. This increases the system’s spatial
freedom, allows the coils to differ in shape and size, and allows multiple devices to charge from a single source,
establishing a one-to-many relationship.
Resonance is a property of an oscillating system which drives another system to oscillate with a greater amplitude
at a designated frequency. For example, if you have two tuning forks that respond to the same frequency, by playing
one fork the other tuning fork will begin to vibrate due to resonance. This same property is achieved in electrical
systems through resistor, inductor, and capacitor (RLC) circuits. Efficiency in this case is modeled by the ‘Q’
factor which represents electrical resistance:
Although this is independent of distance, for a field to propagate over a larger area requires greater power, which
is why resonant systems are less efficient compared to inductive chargers.
In 2009, Qi introduced its wireless low power specification backed by the Wireless Power Consortium (WPC). WPC is a
group of over 200 technology companies, smart phone manufacturers, and semiconductor suppliers. The technology is
tried and tested and relies on strong inductive coupling, providing 5W of power between small distances of around
5mm. The Qi spec also includes a dedicated communication protocol allowing devices to specify the exact amount of
power they need. Perfect for small devices such as phones, mp3 players, and portable Bluetooth speakers, the Qi
standard has been adopted by various OEMs including Samsung and Nokia for the Galaxy and Lumia mobile phones. Since
it’s introduction there have been over 800 Qi licensed products available on the market. Going forward, Qi continues
to be an attractive solution for medium and high power devices from 15W up to 1kW.
Figure 2: Example Qi Wireless Power Consortium Compliant RX/TX configuration
Rezence approaches wireless charging through magnetic resonance technology, providing up to 50W of power. Supported
by the Alliance for Wireless Power (AW4P), which merged with the Power Matters Alliance (PMA) on June 1st, 2015,
Rezence is supported by over 250 companies and is strongly advocated by semiconductor giant Intel. The main difference between the two wireless charging methodologies stem from their
inductor coil configuration.
Rezence implements loosely coupled magnetic resonance, which provides greater freedom for the charging device with
respect to positioning. For example, a device can still be fully charged even if there is a book between the device
and the charging mat. This added mobility comes at the cost of reduced charging efficiency, about 45% vs 65% for Qi.
Furthermore, Rezence requires the device to have Bluetooth
order to communicate its power needs and Bluetooth is common on most mobile devices. Although today’s Qi devices are
for the most part tightly coupled, the Qi standard provides flexibility to support both resonant and inductive
technologies, while Rezence remains strictly resonant and loosely coupled.
Figure 3: Alliance for Wireless Power (A4WP) compliant hardware overview of
Rezence Wireless Power demonstrating loosely coupled resonator and wireless data communication.
Technology alone won’t determine the winner. If we recall the Blu-ray and HD DVD showdown, Blu-ray ultimately won
because its hardware was more accessible via the Playstation 3 and Blu-ray content dominated the retail outlets and
distribution channels. Microsoft attempted to even the playing field with an HD DVD add-on for the Xbox 360, but
there was little incentive for the average user to purchase additional hardware. However, unlike the rigid divide
between Blu-ray Disc and HD DVD, the companies backing Qi and Rezence have not taken a firm position on one standard
over the other. Both Qi and Rezence have shared backers suggesting that manufacturers haven’t yet reached an
inflection point that would indicate which technology they will choose to support in the long term. IC manufacturers
such as Texas Instruments and Renesas / IDT want to supply to
largest market possible. OEMs like Samsung, Toshiba, and Lenovo want to avoid obsolescence. Their goal is to
maintain compatibility with future versions of wireless charging platforms for as long as possible. Neither IC
manufacturers nor OEMs currently benefit by alienating themselves from one technology or siding with the other.
Qi undoubtedly has a lead in market share among mobile phone manufacturers and other wireless electronics, with
products such as the Google Nexus 6 and Samsung Galaxy S6 topping the list. Nonetheless, the adoption rate of
wireless charging has been slow. According to the IHS Consumer Awareness of Wireless Charging report, only 20% of
mobile phone users are charging wirelessly. However, in 2015, knowledge and awareness of wireless charging more than
doubled to 76% of US consumers. More importantly, the same report states that the market for wearables that charge
wirelessly is expected to rise from just over 400 million USD in 2015 to 1.1 billion USD by 2019. Adoption of
wearables may give Qi an insurmountable advantage. Publicly, Apple has remained passive between the Qi and Rezence
struggle. Nonetheless, the Apple Watch charger, while not officially a Qi licensed product, is Qi compliant and
leads the wearables market accounting for over 70% of total revenue. With such a dominant presence, Apple may cause
other manufacturers to follow suit in the same way they have with previous technologies such as touch screens and
Rezence and the A4WP specification are newer and manufacturers haven’t had a chance to release products that
implement this technology. Despite Qi dominating existing wireless charging products, mass public awareness of
Rezence could quickly change that. Select locations of Starbucks, Coffee Bean & Tea Leaf, and McDonald's are now
providing Rezence Powermats. More importantly, Intel is pushing for next generation laptops, Intel Atom powered tablets, keyboards, mice, and other PC peripherals
to adopt Rezence wireless charging. Since Intel heavily influences marketing and advertising for OEMs such as Dell,
HP, and Lenovo, it’s very possible that we may see a rapid shift to Rezence wireless chargers.
Medical device and automotive
markets, which are more risk averse and focused on reliability, are less likely to standardize on a technology until
one becomes predominant. However, should one standard evolve to focus in on a specific industry, they could
certainly solidify their position in the market as a standard for many years to come.
Ultimately, the wireless power standard format war will be largely determined by three factors:
Consumers don’t care about how a solution is implemented as long as it works flawlessly with their devices.
Furthermore, in order for wireless charging to become common, Qi and Rezence will have to push device manufacturers
to bundle wireless chargers as a standard accessory instead of a costly additional purchase. Chargers that require
additional applications, manual pairing, or external charging cases will be less popular than devices that
seamlessly integrate wireless charging. Many already know the abysmal user experience of wall chargers, with tangled
wires and outlet hunting. Qi has demonstrated an easy-to-use solution that is already available in numerous devices
whereas Rezence currently offers faster charging rates. In the future, the success of either technology will depend
on how fast industry heavyweights such as Intel and Apple influence and integrate wireless charging solutions widely
into their end products.
Noah Imam is a Software Engineering Consultant to Inteleaf with experience in designing
software and expertise in Linux,
embedded kernel development, and ARM architecture. He has a B.S. in Electrical & Computer Engineering from the
University of Texas at Austin. Inteleaf is an Austin Texas based hi-tech, B2B product management and custom software
development consulting firm, solving the most critical business, strategic, customer and financial challenges faced
in developing, designing, and bringing new products to market.
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