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How to
Design a 10kW Three-Phase AC/DC Interface Step by Step PART
II: Semiconductor
Loss Calculation Demystified Dr. Uwe Drofenik Gecko-Research GmbH ETH Zentrum, ETL H13 CH-8092 Zurich, Switzerland Phone +41-44-632 4267 Fax +41-44-632
1212 Email May 28,
2009 PART
II: Semiconductor
Loss Calculation Demystified Semiconductor
losses are composed of conduction losses plus switching losses. Both are temperature-dependent.
For simplification, here our first assumption is that during operation the
junction temperature of all semiconductors is constant (e.g. 125°C). If our
thermal design can guarantee this maximum temperature, this is a safe
assumption. The junction temperatures might actually be lower during
operation which means lower temperature-dependent losses which means lower
temperature as originally assumed. In Part III of the report we will
compare this simplified approach with a simulation taking the temperature
dependence of the losses into account. Conduction and Switching Losses: Measurement &
Simulation
Fig. 1: IGBT Turn-On at High Junction Temperature. Figure 1
shows measured IGBT turn-on at high junction temperature. IGBT voltage uCE
(uT) needs some nanoseconds to fall from blocking state (350V) to
zero. In this time the collector current (iT) rises from zero to
full conduction state (here: 50A). Voltage multiplied with current gives loss
pT. Therefore, you get a loss peak during each switching action.
The energy content of pT during switching is typically given in
the data sheet of the power semiconductor (EON and EOFF).
Turn-on at
high temperature represents a worst case concerning losses. The reverse
recovery current from the diode causes loss in the transistor. Therefore,
switching losses are always characteristic for a combination transistor –
diode. Generally,
the switching losses are dependent on ·
Combination transistor – diode ·
Junction temperature ·
Gate driver of the transistor ·
Parasitic wiring inductance (PCB
layout) ·
Current and blocking voltage
Fig. 2: IGBT Turn-Off at Low Junction Temperature. Figure 2
shows the turn-off measurement at low temperature for the same IGBT (a
typical temporary operating state after turning on a device). At
room-temperature (25°C) switching losses are comparably small but it’s an
important worst case concerning overvoltage peaks. We want to mention it
here, although it has no direct impact on the thermal design. Conduction
loss is simply on-voltage multiplied by the current, and integrated over the
conduction period. In the following we will show how to model this
efficiently. First, Let’s Find the Final Parameters of the Two
Topologies Before we
select semiconductors and calculate the losses, we have to find our final
circuit parameters. In Part I, the
average frequency of both converters, the Bidirectional 3-Phase AC/DC PWM
Converter with Impressed Output Voltage (‘VSR’ in the following) and the
Vienna Rectifier, was well below 15kHz. This might cause acoustic noise
problems. Therefore,
we need to find parameters that give higher switching frequencies. After some
trial and error employing GeckoCIRCUITS (get a free trial version at www.gecko-research.com)
we chose VSR: L = 0.35mH; h =
1.0 ŕ fP,VSR,avg = 23kHz How to Select Semiconductors from a Datasheet? First, we
select the power semiconductors for the VSR. We need a blocking voltage of at
least 700V (the converter DC-side voltage), but it is essential to provide
some safety margin (remember the 30% overvoltage in figure 2!). Blocking
voltage capability of at least 1000V would be fine. But keep in mind: The
higher the blocking voltage, the higher the losses. In the
datasheet a typical current is often given as DC at a certain temperature,
sometimes even room temperature. It means that the chip can handle the
current if you guarantee that your cooling system keeps the junction
temperature at this value. This is often unrealistic, so we have to be very
careful here. Every
semiconductor manufacturer provides products and data sheets a little bit
different. For demonstration we will select our semiconductors for the VSR
from www.infineon.com.
Follow the link “Power Modules” from “Product Categories” at their homepage.
Then select “IGBT Modules”. At the product page select “IGBT Modules up to
1200V SixPACK” from the drop-down list “Get Technical Specifications” to get
a list of semiconductors. Here, you can make a preliminary selection quickly.
To start
with, we chose the six-pack power module FS25R12YT3 with 1200V blocking
voltage and 25A continuous current at 80°C junction temperature. If we are
not satisfied with the losses in the following, we will have to select an
alternative component.
Fig. 3: Output Characteristic IGBT-Inverter. What Data Do We Need for a Simplified Analysis? The
datasheet provides a lot of data. What do we need right now for our
simplified analysis? The graph
“Output characteristic IGBT-inverter (typical) at VGE=15V” from
the datasheet, shown in figure 3, tells us about the conduction loss of one
IGBT for two different junction temperatures (25°C and 125°C). We want to
operate the power chips at junction temperatures somewhere around 100°C in
order to get the most out of the silicon and to avoid reliability problems.
In the following we will use the data taken at 125°C junction temperature for
loss calculation. This will guarantee some safety margin for our thermal
design. This simplifies the problem significantly without reducing accuracy
too much. From figure 3 we can estimate the following parameters: With the
two parameters above, conduction losses at TJ = 125°C can be
easily described employing GeckoCIRCUITS. The switching losses are a little
bit more difficult to describe. Switching transients
like those shown in figures 1 and 2 are strongly dependent on many parameters
that may not be easily available for the system design engineer. Also, a
simulation of the switching transient would force the simulator to reduce the
numerical step width significantly. This would slow down the simulation at
each switching action and would be a permanent source of numerical
instability. That’s why a circuit simulator like SPICE is generally not well
suited for power electronics. Simulation
speed and numerical stability are the reasons why GeckoCIRCUITS performs the
simulation of an ideal switch within one single numerical time-step. You can
test the free online version of GeckoCIRCUITS at www.gecko-research.com/applet-mode/geckocircuits_demo.html
where some of the examples shown here are available, or ask The
switching energy is modelled in form of a single pulse at each switching
action which contains exactly the energy as given in the datasheet, although
the shape of this pulse is dependent on the numerical step width and might incorrect
in reality. This
approach is very fast, extremely accurate and numerically stable. Because
such a switching loss calculation is done “in parallel” to the circuit
simulation, the lost energy is not lost within the power circuit. Therefore,
the system input currents might be higher in reality than in the simulation,
because additional input energy is needed for the switching losses. Higher
input current might impact the converter behaviour. This is especially a
problem if switching losses are high and/or dominating. Therefore,
it is important to basically understand how GeckoCIRCUITS (and other circuit
simulators) calculate switching losses.
Fig. 4: IGBT Switching Losses Dependent on Current. The
switching losses are shown in figure 4 as loss energy in dependency of
current. The curve in the datasheet of the FS25R12YT3 is labelled “switching
losses IGBT-inverter (typical)”. Voltage at measurement was 600V and junction
temperature was 125°C.
Fig. 5: Output Characteristic Module’s Diodes. The
switching loss description can be simplified by a characteristic slope:
These two
values plus the blocking voltage at measurement, VCE = 600V,
complete the data we need for loss calculation of the IGBTs. Finally, the
losses of FS25R12YT3 ‘s internal diodes are described by the characteristic
shown in figure 5. Switching losses of the diodes are neglected here. Set Up the Simulation of the Semiconductor Losses The next
step is to characterize the power switches in the simulator according to the
datasheet values. Double-click the IGBT-symbols in the power circuit of
GeckoCIRCUITS. Set the parameters for loss calculation with the values
derived from the datasheet as shown in figure 6. Do the
same for the power diodes. Note that we omit the switching losses for the
diodes.
Fig. 6: Setting Conduction and Switching Losses of
the IGBT. You see in
figure 6 that it would have been possible to build a more detailed
semiconductor loss model which would also be temperature dependent. Here we
employ a simple model where all junction temperatures are assumed to be 125°C
which give a safety margin in the thermal design.
Fig. 7: Transient Semiconductor Losses Detected in GeckoCIRCUITS.
Figure 7
shows how the loss blocks (“Loss”, red) of GeckoCIRCUITS are employed. After
setting the IGBT characteristics (figure 6), double-click the loss-block to
select a semiconductor. Figure 7 also displays an open dialog of Loss-block Pv.7,
where IGBT.2 is being selected from a drop-down list of all available power
semiconductors. Employ the
Flow-block from the green Measure-tab to access the transient power flow of
each Loss-block. The output of the green Flow-blocks can now be visualized
and analysed in a SCOPE. If you run
the simulation, you will get a result like the one shown in figure 8 for the
transient losses of the two IGBTs and the two diodes in the first bridge leg.
Fig. 8: Transient Semiconductor Losses, VSR. The diodes
display only conduction losses, the IGBTs also show switching losses in form
of Dirac-pulses where the pulse-height is defined by the numerical
step-width. Only the energy-content of the pulse has meaning. In figure
9, we zoom into the time-behaviour of the transient loss of figure 8 (thermal
power in [W]). We can clearly distinguish between conduction loss, turn-on
and turn-off loss.
Fig. 9: Zoom: Transient Semiconductor Losses, VSR. Now we can
calculate the average losses. Simply open
the SCOPE’s Analysis >> Characteristic menu as shown in figure 10, and
press the ‘Calculate’-button to get the average values of the transient
losses. The
average (conduction) loss of each diode is about 8W, and the average
(conduction plus switching) loss of each transistor is approximately 54W.
Fig. 10: Average Semiconductor Losses, VSR. So the
total semiconductor loss of the VSR is 372W. The converter efficiency
is 96.4% if we neglect all other losses. Can We Get Higher Efficiency with the The much
lower average switching frequency indicates so (see end of Part I). But if
there is an improvement, is it significant? GeckoCIRCUITS makes it easy to
find out. This time
we look into the online-tables of www.ixys.com to find power semiconductors. The Vienna
Rectifier’s neutral point at the DC side splits the blocking voltage into
half so that each semiconductor only has to deal with 350V. Generally, we
want to select an IGBT with minimum 500V blocking voltage and current in the
range of 25-35A. Go “Product Portfolio” > “Power Devices” and study the
components available. The device
IXGP 30N60B2 can handle 600V and advertises a current capability of 70A if
you can make sure that the junction temperature is never higher than 70°C
(30A in case of 110°C). From the
datasheet of this IGBT we extract from the “Output characteristics @ 125 Deg.
Celsius” the following parameter for GeckoCIRCUITS:
Fig. 11: Transient Semiconductor Losses, The table
at the second page of the datasheet gives the switching losses for junction
temperature 125°C, voltage 400V and inductive load with
For all
diodes of the Vienna Rectifier we chose the device DHG30I600HA from IXYS
(single diode in TO 247, 600V/30A). For the conduction loss calculation we
get directly from the table at the first page of the datasheet threshold
voltage and slope resistance at 150°C as Figure 11
shows how to integrate loss measurement into the Vienna Rectifier model. When
we parameterize the power diodes and IGBTs of the Vienna Rectifier with these
values, we can simulate transient losses as shown in figure 12.
Fig. 12: Transient Semiconductor Losses, From the
Analysis >> Characteristic menu in SCOPE we can directly calculate the
average losses.
Fig. 13: Average Semiconductor Losses, In figure 13
you see that the average IGBT (conduction plus switching) loss is
approximately 21W, while the loss of the three different diodes of the upper
bridge leg is 12W, 7W and 3W. Total
semiconductor losses of the Vienna Rectifier in this report are 197W
resulting in a converter efficiency of 98.1% if we neglect all
non-semiconductor losses. So while
the average switching frequency of the VSR is approximately 60% higher than
for the Vienna Rectifier, the semiconductor losses are even 90% higher. This
is because semiconductors with lower blocking ability can be employed in the
Vienna Rectifier. So How Fair Was this Comparison? Obviously,
the choice of the semiconductors has a major impact on resulting losses. If
you employ oversized semiconductors the losses will be reduced significantly.
This is true for both topologies. Here we tried to find semiconductors for
both topologies of comparable voltage- and current-capabilities. For the
voltage- and power-levels in this example, you could employ a CoolMOS for the
Vienna Rectifier to reduce conduction losses and SiC-diodes to reduce
switching losses (contact PES/ETH
Zurich for more information). Furthermore, you could employ different
diodes (mains-side / neutral-point-side / free-wheeling) for optimization. We saw
that different manufacturers provide different datasheet information of their
power semiconductors. Even datasheet information for different semiconductors
of the same manufacturer might be inconsistent. It is a
main task of the system engineer to extract the semiconductor data he needs
from the data sheet. As we have seen, it comes down to a very small number of
values to characterize switching- and conduction losses in GeckoCIRCUITS with
high accuracy. What’s next? The open
questions in order to finish our converter comparison concentrate on the
temperature. You will find in Part III
of the report: ·
How do perform a ‘quick and dirty’
calculation of junction temperatures? ·
What about transient junction
temperatures? Can you guarantee junction temperatures in case of short-term
overload? ·
How to select a heat sink? What
about thermal grease? ·
How can I model a heat sink for
the circuit simulation? ·
Does it make a difference if
temperature-dependency of the losses is considered? Further Information For more
information on the Vienna Rectifier visit http://www.ipes.ethz.ch/ipes/adv_index.html
and http://www.ipes.ethz.ch/ipes/2002Vienna1/vr1overview.html Part III of this
report will be published on www.gecko-research.com GeckoCIRCUITS
was designed especially to solve such problems quickly and with minimum learning
effort. You can
test the free online version of GeckoCIRCUITS at www.gecko-research.com/applet-mode/geckocircuits_demo.html
where some of the examples shown here are available, or ask VSR: sixsw_therm.ipes
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