42
WWW.POWERSYSTEMSDESIGN.COM
SPECIAL REPORT : ALTERNATIVE ENERGY
lifetime leads to higher power
densities along with less material
used per kW installed. The diagram
in Figure 2 summarizes recent and
ongoing developments in power
semiconductor technologies.
Technological change
Figure 2 also hints to the fact,
that from a certain point on, a
technological change is needed
to overcome the drawbacks of
an existing technology. In case
of power semiconductors, wide
band gap materials like Silicon
Carbide (SiC) or Gallium Nitride
(GaN) are promising candidates
to further improve efficiency. Two
options arise from using these
new materials.
First, a change from IGBTs being
bipolar transistors towards field
effect based devices overcomes
the PN-junction
dilemma.
Paralleling IGBTs
still leads to a
forward voltage
across a PN-
junction and thus
limits the benefit
in regards of
efficiency. Field
effect based
devices however
feature a channel
resistance and
paralleling n
devices results in
an improvement
of the overall
resistance by
a factor n-1.
Efficiency becomes
a question of how many devices
are integrated, immediately
correlating it to money spent.
A second approach leads to hybrid
devices, combining silicon IGBTs
with SiC Schottky barrier diodes
as depicted in Figure 3. SiC diodes
allow higher turn-on speed for the
IGBT, reducing the turn-on losses;
the absence of a recovery charge
eliminates the diode’s recovery
losses.
System development
Today, the most widely used
topology in power electronics
includes a three-phase inverter
based on a 2-level half-bridge
as a basic building block.
Depending on the application,
a change in topology may lead
to benefits regarding efficiency.
Recently, solar inverters have
seen a transition from two-
level to three-level designs.
The change was driven by the
efficiency gain that results from
using 650V semiconductors
instead of 1200V components.
Among others, the inherently
lower switching losses contribute
to the gain in efficiency.
In an approach to minimize
material content while
maximizing efficiency, Infineon
has successfully cooperated with
the University of Nottingham to
combine new technologies in a
different topology. The outcome
was a matrix converter that
was built using silicon carbide
JFETs. This 4-quadrant converter
achieved 97% efficiency at full
load and even higher values at
partial load (see Figure 3).
Good enough?
Efficiency in modern energy
conversion has massively grown
throughout the last decades.
Nevertheless, growing energy
demand along with harvesting
and storing renewable energies
makes further improvements in
this field a necessity. More and
more, electricity has to pass
semiconductors on its way from
generation to consumption,
making highly efficient
semiconductors a true gateway to
saving energy. Engineers will have
to strive to achieve even higher
efficiencies in future with a clear
target ahead. Less than “1” is
never good enough.
www.infineon.com
Figure 3: Built in efficiency, 20kVA converter with
SiC-JFETs measuring 12.2cm x 6.2cm x 11.7cm and
weighing 1.7kg
The IoT needs wireless charging
& energy harvesting
By: Tony Armstrong, Linear Technology
Traditionally devices were connected by wires to their power sources
T
he “Internet of
Things” (IoT), refers
to a growing trend
to connect not only
people and computers, but all
sorts of “things” to the Internet,
and therefore, each other. By way
of example, consider if you will
applications such as industrial
plants or large infrastructure
projects where connecting more
sensors (or actuators) in more
places can increase efficiency,
improve safety, and enable entirely
new business models.
Traditionally wires connected
devices and sensors to their power
sources. Now, rather than the
challenge and expense of running
cables all around a facility, it is
now possible to install reliable,
industrial-strength wireless
sensors that can operate for
years on a small battery, or even
harvest energy from sources such
as light, vibration or temperature
gradients. Furthermore, it is also
possible to use a combination of a
rechargeable battery and multiple
ambient energy sources too.
Moreover, due to intrinsic safety
concerns, some rechargeable
batteries cannot be charged by
wires but require being charged via
wireless power transfer techniques.
Energy harvesting & wireless
power
State-of-the-art and off-the-
shelf energy harvesting (EH)
technologies, for example in
vibration energy harvesting and
indoor photovoltaic cells, yield
power levels on the order of
milliwatts under typical operating
conditions. While such power
levels may appear restrictive, the
operation of harvesting elements
over a number of years can
mean that the technologies are
broadly comparable to long-life
primary batteries, both in terms of
energy provision and the cost per
energy unit provided. Moreover,
systems incorporating energy
harvesting will typically be capable
of recharging after depletion,
something that systems powered
by primary batteries cannot do.
Nevertheless, most
implementations will use an
ambient energy source as the
primary power source, but will
supplement it with a battery that
can be switched in if the ambient
energy source goes away or is
disrupted. This battery can be
either be rechargeable or not and
this choice is usually driven by
the end application itself. So it
follows that if the end deployment
Figure 1: LTC3331 Energy Harvester & Battery Life Extender
SPECIAL REPORT : ALTERNATIVE ENERGY
43
WWW.POWERSYSTEMSDESIGN.COM