Cree - Cree-Sic-Makes-Solar

Cree - Cree-Sic-Makes-Solar
Silicon Carbide Diodes Make Solar
Power Systems More Efficient
October 2008
Silicon Carbide Diodes Make
Solar Power Systems More Efficient
Michael O’Neill, Applications Engineering Manager, Cree Inc.
Silicon carbide (SiC) diodes have already
penetrated the quickly expanding solar
inverter market, particularly in Europe.
Cree’s 1200-V SiC Schottky diodes are
being used in place of their silicon (Si) PiN
counterparts in the boost section of the DC
link and will soon be seen in the inverter
sections of commercially available systems.
Improvements in material quality, size and
cost in the last few years have made SiC a truly
viable replacement for silicon in power devices.
With wafer size increasing (Cree is currently
using 4-inch wafers), defect densities going
down, and material costs dropping, larger
power devices are now available with SiC.
The technology has many unique properties
that make it a near-ideal material for highvoltage or high-temperature operation.
First, SiC’s thermal conductivity is several
times that of gallium arsenide and more than
three times that of silicon. This enables the
fabrication of higher-current density devices.
Also, the breakdown field of SiC is almost
ten times the breakdown field of silicon, so
an equivalent design in SiC will have ten
times the breakdown voltage rating of the
silicon part. Because of this, it is possible to
create very-high-voltage Schottky diodes.
Finally, SiC is a wide-bandgap material and,
because of this, has the ability to operate
at much higher temperatures than any Si
device. Figure 1 shows the difference in
thermal conductivity, electric field breakdown
and bandgap between SiC, GaAs and Si.
Cree uses SiC to manufacture high-voltage
Schottky diodes. By virtue of the fact that
Schottky diodes are unipolar devices, there
is no minority carrier recombination, thus
Figure 1. Materials Comparison
This document is provided for informational purposes only and is not a warranty or a specification. For product
specifications, please see the data sheets available at www.cree.com/power. For
warranty information please contact Cree Sales at [email protected]
Silicon Carbide Diodes Make Solar
Power Systems More Efficient
October 2008
leading to zero reverse recovery currents
(Figure 2). There is, however, a very small
junction-capacitance charge. The magnitude
of this charge is negligible in comparison to
the equivalent reverse recovery charge in a
Si PiN device, and it is also independent of
temperature, forward current and switching
di/dt. These Schottky diodes also have
zero forward recovery voltage and turn on
immediately. These switching characteristics
have a sometimes-overlooked benefit of greatly
energy demand is expected to increase by
19 percent over the next ten years, and in
developing countries the demand will increase
even faster. Europe has already embraced the
benefits of solar power and some countries are
providing incentives to both businesses and
individuals to switch to solar energy. Because of
the overall system efficiency improvement with
SiC Schottky diodes, many manufacturers of
solar equipment are turning to the technology.
Solar panels collect the sun’s energy and
Figure 2. Recovery of Schottky diode versus Silicon equivalent
reducing EMI. These devices eliminate diodeswitching losses in a power-conversion system,
which in turn greatly reduces turn-ON losses in
the associated switch that has to commutate
the reverse recovery currents associated
with Si PiN devices. Because of this increased
efficiency and performance, SiC Schottky diodes
are the ideal solution for solar-energy systems.
The consensus estimate from available sources
shows that electricity accounts for 39 percent
of the world’s energy use. In the United States,
convert it to a positive DC voltage. This voltage
varies with the intensity of the sun’s rays to
which the panels are exposed. That voltage is
boosted to a fixed DC voltage by means of a
boost converter switching at high frequency
(Figure 3). Cree’s SiC Schottky diodes eliminate
the boost diode switching losses and greatly
reduce the MOSFET or IGBT turn-on loss.
This significantly improves the boost section
efficiency. An inverter then converts the fixed
DC voltage to a usable AC voltage of fixed
This document is provided for informational purposes only and is not a warranty or a specification.
For product specifications, please see the data sheets available at www.cree.com/power.
For warranty information please contact Cree Sales at [email protected]
Silicon Carbide Diodes Make Solar
Power Systems More Efficient
October 2008
frequency (typically 220 V, 50 Hz for Europe
and 110 V, 60 Hz for North America). The SiC
Schottky diodes eliminate diode-switching
losses in the freewheeling diodes of this section
along with reducing IGBT turn-ON losses.
Inverter efficiency is significantly improved.
Silicon-based inverters typically operate at
close to 96-percent average efficiency. With
batteries provide input power to the inverter
that in turn supplies power to the end-user.
Typically
a
system
today
will
cost
about $10 per watt, so a 3-kW system should
cost approximately $30,000. Obviously, a
more-efficient system will mean a faster
payback to the consumer. Again, global
energy concerns are driving the adoption
Figure 3. Boost Converter simplicity with SiC diode
a more efficient system, more of the energy
delivered by the solar panels gets converted
to usable electricity. With SiC devices, the
inverter’s average efficiency can be boosted
up to 97.5%. This represents up to 25%
reduction in inverter losses. Considering that
these solar systems are designed to operate
for at least 30 years, this represents a
considerable improvement in energy savings,
and by virtue of reduced temperatures,
the system will have higher reliability.
Solar-power systems available today are
generally divided into two categories, grid-tied
and off-grid. The grid-tied system, as its name
implies, is tied to the utility grid. Power for
the end-user originates either from the solar
panels of the system or the grid, depending
on load demands, time-of-day, etc. These
systems have a metering capability whereby
power from the solar system can be delivered
back to the grid during low-demand periods.
Off-grid systems are standalone and contain
batteries and sometimes a back-up generator
as well. The solar panels charge up a bank of
batteries through a charge controller, and the
of alternative energy sources and silicon
carbide provides an additional benefit.
The largest solar market in the world right
now is in Germany, in part because its utilities
offer the incentive of buying excess power
back from customers at three times the rate
at which it charges for delivering it. When
a customer generates power from a solar
system, if he’s not using that power himself, the
utility’s metering system allows the customer
to generate that power back on to the grid.
The utility will pay the customer for the power
he’s given them. Because of this, in Germany,
the payback period for a solar system is
considerably shorter than in other countries.
Although the largest market for solar power
right now is in Europe, it is expanding worldwide.
Many developing countries are turning to solar
as a viable source for energy. With increased
environmental regulations and rising energy
costs, solar energy may see widespread
adoption in the United States, sooner rather
than later. California, Arizona and Nevada
already offer incentives for converting to solar
power, and these three states stand to gain
This document is provided for informational purposes only and is not a warranty or a specification.
For product specifications, please see the data sheets available at www.cree.com/power.
For warranty information please contact Cree Sales at [email protected]
Silicon Carbide Diodes Make Solar
Power Systems More Efficient
October 2008
significantly from such programs due to the
amount of sunshine they enjoy. Cree has been
providing high-voltage SiC Schottky diodes to
this and other markets for about four years,
and we are working to expand the line of
offered devices to address future needs as well.
Solar-panel makers are continually striving
to design a more-efficient product, as they
typically run at 15 to 20% efficiency. Given
that statistic, it is the boost converter and the
inverter that have the greatest impact presently
on the system’s overall energy efficiency, which
is why SiC diodes play such an important role.
Research into development of more-efficient
materials for photovoltaic panels is ongoing.
With more-efficient panels, much higher
power ratings will be delivered for a much
smaller area of paneling. At that point, SiC
diode manufacturers should see a shift in
their customers’ needs, from the 10-amp to
20-amp ratings of today to 50-amp, 100-amp
and higher current-rated devices. In fact, Cree
has a commercially available 1200-V 50-amp
device today. With further advances in SiC
material technology and the move to largerdiameter wafer, Cree is well positioned to be
a leading provider of these future devices.
This document is provided for informational purposes only and is not a warranty or a specification.
For product specifications, please see the data sheets available at www.cree.com/power.
For warranty information please contact Cree Sales at [email protected]
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