October 17
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Hips
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omponEnts
Wireless charging: advanced
technology delivers real benefits
By Stephan Schächer and Milko Paolucci,
Infineon
Power needs to catch up with data
and become wireless to truly empow-
er the latest generation of mobile de-
vices. In this technical article, Infineon
reviews the design challenges and
standards that drive this new sector
before looking at technologies that
make this vital step possible.
Power is essential to every electronic device
- that will never change. However the cur-
rent approach of charging from a wall out-
let is becoming obsolete. Charging needs to
become more convenient for the user, they
do not want to plug in the device but simply
place it on a surface to charge. More than this,
consumers want to be able to do this in pub-
lic places such as airports, hotels, event venues,
fast food chains and coffee shops, meaning
that standardization needs to mature to drive
universal compatibility. Somehow, designers
have to find a way to re-invent current char-
gers to be able to deliver power through ‘thin
air’. The current switch mode power supply
(SMPS) approach relies on a magnetically
coupled transformer to change the voltage
level and transfer power from the primary side
to the secondary. In wireless, this transformer
is split between the charger and the device. As
the windings are separated (by the case thick-
ness and air), the coupling is looser than with
a normal SMPS. However, power can be trans-
ferred with unexpected efficiency with the
correct magnetic design as the coils and cou-
pling have a far greater impact on the overall
performance than with an SMPS. Other than
the magnetics, issues including efficiency,
mechanical packaging, electromagnetic inter-
ference (EMI), thermal management and
metallic foreign objects (such as coins and
keys) create further challenges for designers.
As with many emerging technologies, mul-
tiple incompatible standards develop which
stifle progress until a universal solution
emerges. Wireless charging has two indus-
try alliances and two standards. The Wireless
Power Consortium (WPC) supports the Qi
inductive standard that supports tightly cou-
pled charging. Qi has become the mainstream
standard, covering over 80% of all wireless
charging receivers. The Power Matters Alli-
ance (PMA) and the Alliance for Wireless
Power (A4WP) were formed as separate orga-
nizations. PMA focused on tightly coupled
inductive solutions whereas A4WP worked
on loosely coupled resonant technology. PMA
and A4WP merged and rebranded as the Air-
Fuel Alliance (AFA).
Currently, there are three topologies for wire-
less charging, offering different advantages.
Single-coil inductive is the simplest and most
prevalent solution. Supported by Qi and Air-
Fuel, this employs a single transmitter coil and
requires exact and close positioning of the
device and the transmitter, which precludes
charging through surfaces. This approach can
only charge a single device. Multi-coil enables
intelligent systems that detect the coil closest
to the device and direct the power accordingly.
The broader charging field allows you more
freedom in placing the device to be charged.
AFA supports a resonant approach that relies
on resonance between the transmitter and
receiver to transfer energy far more efficiently.
This approach charges multiple devices from
a single coil and allows for a greater distance
(up to 50mm) between the transmitter and
receiver. This flexibility in positioning of the
device gives a ‘drop and go’ experience with
efficiencies up to 80%. Although an inductive
solution can deliver more power in a precisely
defined and controlled scenario, the resonant
approach delivers an efficient energy transfer
with higher placement freedom.
The resonant approach permits higher power
ratings, allowing laptops or power tools to be
charged wirelessly. The three key elements of
a wireless charging solution are the adapter/
charger, the transmitter and the receiver. The
adapter is often separate and connected via a
cable to the transmitter, although they could
equally be combined. It powers the transmit-
ter from the mains, usually with a regulated
5-20V DC. The transmitter contains a MOS-
FET-based bridge topology inverter to convert
the DC power into an alternating magnetic
field. A microcontroller and driver compo-
nents provide flexibility and functionality.
There are two primary topologies used for
resonant (AirFuel) applications, Class D and
Class E. Class D offers an almost flat efficiency
curve over a wide load range and is therefore
