Inverting Energy Concerns Through Efficient Designs
By Martin Schiel, Future Electronics Germany
There is increasing conflict between our future
energy demands and the need to limit emissions
of greenhouse gases that contribute to global
warming. We know we need more power – in
Europe alone we will need an extra 250-300GW
of capacity by 2020 – but we cannot afford to
generate it by burning yet more fossil fuels.
Engineers and scientists believe the solution
is to supplement our traditional sources of energy
with clean, renewable energy sources
such as wind, waves and solar energy. Of these,
solar power offers the most promise. Solar
panel technology is mature and the politicians are
doing their part to encourage its uptake. Germany
in particular is encouraging homeowners to
install solar panels to supplement the public power
supply. Homeowners can even sell any excess
electricity generated back to the German domestic
energy market (at a government-specified
premium rate) by feeding it directly to the local
grid.
But there is plenty of work still to be done. The
main challenge at present is energy conversion
efficiency. Engineers are continually improving
solar panels and efficiency figures are now at 20%
under laboratory conditions. The overall efficiency
figure, however, hinges not only on the efficiency
of the solar panels but also on how well the DC
output is converted to a practical AC supply by the
inverter circuit. The right components and clever
design can significantly improve efficiency.
Figure 1: The types of inverter arrangement for solar panel arrays
Converting the Sun’s Power
An inverter’s job is to transform the DC derived
from the solar panels into AC, so that it can be
consumed by household appliances or fed into
the grid.
There are several types of inverter
arrangements used for
solar panels: central, string,
multi-string and AC modules (see
Figure 1). In the central inverter
arrangement, the solar panels
are connected in series to form
“strings.” These strings are connected
in parallel and fed to a
central inverter. The disadvantage
with this arrangement is
that the large currents flowing
in the strings demand a bulky
inverter. In addition, each string
requires a blocking diode to prevent complete
shutdown should the string fail, and the panel
must be made up of identical modules.
In the string arrangement, each string is connected
to a separate photovoltaic (PV) inverter.
The disadvantage is that several inverters are
required, pushing up costs.
The most common topology today is the
multi-string inverter. It uses an arrangement
whereby each string is connected to a separate
DC-to-DC boost converter in the inverter and
tracked separately with Maximum Power Point
Trackers (MPPT). This arrangement allows the
strings to be made up of different modules and
configurations. The DC output of each converter
is then fed into one inverter to convert it to an AC
voltage, which is then fed into the grid.
Finally, in the AC module arrangement, each solar
module is attached to its own small inverter. This
solution is both flexible and requires only very
cheap inverters. As such, it looks set to become a
popular alternative.
Choosing a Topology
While there are many inverter topologies, a few
simple examples can illustrate the important design
elements. The most common topology is an
inverter employing a 50Hz transformer. The use
of a high frequency transformer reduces its size
(and allows the use of smaller inductors), but adds
more complexity and cost to the semiconductor
side of the inverter.
Transformers are relatively heavy and expensive,
and add about 2% to the system’s power losses,
but are thought to improve safety. Nonetheless,
while most contemporary designs still employ
transformers, some newer designs do not. (Transformer-less
designs are allowed in Germany and
other countries, but not the US due to a different
grounding approach.) These types employ a booster
to raise the input voltage from the solar panel.
The power side of the inverter circuit has a major
impact on the conversion efficiency of the inverter.
Modern power devices have been designed specifically
to minimize losses and include devices such
as Punch-Through (PT) and Non-Punch-Through
(NPT) Insulated Gate Bipolar Transistors (IGBT)
featuring reduced conduction and switching losses.
Trench IGBTs have also come into vogue. These
feature an insulated trench in the transistor’s pregion
and boast ultralow collector emitter saturation
voltage.
While single-phase designs normally use 600V
components, higher output 3-phase designs need
1,200V devices. 3-phase designs also boast systematically
lower ripple currents through the DC
link capacitor, allowing a smaller device to be
used. A designer should pay special attention to
the breakdown voltage of the switching devices,
particularly at low temperatures.
Figure 2: Performance comparison – PT, NPT and Trench IGBT devices from International Rectifier
Minimizing Losses
All inverters employ microcontrollers to supervise
modulation and communication, of which
modulation control is especially important. This
determines how the power devices are driven
by a pulse width modulation signal to shape the
output current of the inverter according to the
mains voltage.
Freescale’s MCF523x microcontroller, for instance,
integrates both modulation and communication
functions into a single chip. The MCF523x family
features an eTPU module that performs the
modulation and a host of communication channels
are available (including Ethernet, UART, SPI, I2C
and CAN). Within the MCF523x family there are
numerous derivatives to meet specific system
requirements depending on the number of eTPU
channels needed and which communication
protocols are required. Alternatively, a
combination of Freescale’s inexpensive digital
signal controller, the 56F801x series (for
modulation) and a derivative from the low cost
ColdFire MCF5223x family (for communications)
could be implemented as a less expensive
two-chip solution.
Each of the topologies described above use a
different driver circuit. For example,
Avago Technologies produces IGBT drivers with
internal optical isolation, such as the HCPL-316J.
This device features the drive capability of IGBTs
with IC up to 150A, 1,200V collector/emitter
voltage, optical isolation, fault status feedback,
500ns switching speeds and integrated fail-safe
IGBT protection. It is suitable for use with a single
microcontroller.
If the inverter topology employs dual controllers,
the safety isolation can be implemented using
comparatively inexpensive optocouplers at the
interface between the two controllers. It is
possible to link the input side of the inverter with
the power circuits using functional isolation.
Recently, IGBT technology has evolved from PT
technology to NPT and then to Trench devices.
While NPT IGBTs have low conduction and
switching losses, the trench types are better
still. In addition, Trench IGBTs have lower VCE(ON)
and E(OFF) than PT and NPT IGBTs; this reduces
losses considerably across a range of switching
frequencies from 4kHz to 20kHz (see Figures 2
and 3). Trench devices also deliver higher RMS
current per mm2 than PT and NPT IGBTs. The
combination of reduced losses and higher current
density means that Trench IGBTs enhance inverter
efficiency and power density.
Trench devices are also easy to design with. No
major changes to the PT or NPT IGBT gate drive
is necessary because Trench IGBTs have a 20V
gate rating, the threshold voltage is similar to
that of PT and NPT IGBTs and the devices feature
shorter TD(ON)+TRISE and
TD(OFF)+TFALL timings. In
addition, no modification to dead time and minimum
pulse width settings are needed. Finally,
trench IGBTs are immune to high dV/dt
induced spurious turn-on.
Some good examples of popular devices used
in solar inverters are the Fairchild NPT IGBT
FGL40N120AND in a TO264 housing and the 600V
Fairchild SuperFet MOSFETs.
Figure 3: Trench IGBTs produce higher current output per volt than PT or NPT types
A Bright Market Ahead
Environmental pressures, combined with high
fossil fuel costs and government encouragement,
are finally moving solar power into the mainstream.
This has served to encourage the commercial
development of solar energy, which is becoming
a lucrative market. For example, worldwide solar
power in 2004 equated to about 1GW capacity, by
2020 this is expected to grow 1,000% to 10GW.
This equates to an average annual market growth
rate of around 30%.
The emphasis now is on improving efficiency.
While the efficiency of the panels themselves will
gradually improve, considerable gains can be made
from good design of the inverters that convert the
panel DC into usable AC voltage.
Future Electronics’ applications engineers are able
to offer advice on solar inverter design while balancing
efficiency against cost, and the company
is able to supply a range of controller and power
semiconductors from leading semiconductor firms
to help the designer optimize their circuits.
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