Parameter Sensing in PowerElectronic ModulesModern inverter
applications and electrical drive control algorithms depend on
accurate measurementsregarding key parameters. For the application
and the inverter two parameters are of special interest. Theline
current driving the application and the power module’s temperature
provide the necessary informationon the state of operation. New
power modules like Infineon’s MIPAQ family integrate proper
sensors, aswell as adapted electronics, to provide highly accurate
readings and ease the design and improvement ofcompact high
performance drives. Dr. Martin Schulz, Dr. Ulrich Schwarzer,
Infineon Technologies,Warstein, Germany
As a voltage across a resistor isproportional to the current
flowing,utilising resistors as current sensors isamong the most
basic ideas to measureelectric currents. Despite the losses
thatinherently appear, the method has acertain appeal as shunts as
a single partare reliable, stable across a widetemperature range,
cost- efficient androbust. Shunts do not suffer fromovercurrents
and have neither hysteresisnor offset effects. Additionally, shunts
canbe mounted using well- establishedprocesses. Positioning the
shunt closer tothe heatsink and into the power electronicmodule
provides an excellent thermalinterface, and even allows for
largecurrents to be handled. Infineon’s MIPAQbase series features
specially designedshunts to precisely measure theapplication
current. Figure 1 displays theoutstanding linearity of these
sensors onthe example of moduleIFS150B12N3T4_B31.
Though other methods of capturing the
current exist, dimensions, EMI andtemperature development inside
thedemanding environment of a powermodule make shunts the
predestinedsolution for the integration. It was shownthat other
methods like on-chip currentsensing using specially designed IGBT
chips
cannot provide an equally sophisticatedmeasurement [1].
Current measurementIt seems to be a small step from current
sensing to current measurement. In detailhowever, current and
temperature range as
Figure 1: Measured voltage across a 1mΩ Shunt with in the MIPAQ
base
Figure 2: MIPAQ sense featuring a 100A sixpack, shunts and
integrated Σ/Δ-Converter
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well as accuracy demands and the needfor galvanic isolation are
challengingtargets. To minimise part numbers andspace requirements,
Infineon hasexpanded the coreless transformertechnology (CLT) to
form an analog todigital converter based on the wellestablished
Sigma/Delta (�/�) method.This converter achieves an accuratereading
and, at the same time, providesgalvanic isolation. Built into the
MIPAQsense as displayed in Figure 2, this devicein conjunction with
the also integratedshunts can be used to form a highlyaccurate
measurement system. As the �/�-converter forms an
integrating method, a decimator is neededto get the information
on the instantaneouscurrent. If a common sinc³-decimator
withvarying oversampling rate (OSR) isprogrammed into a FPGA, the
trade-offbetween speed and accuracy can bedemonstrated [2].
Comparing this costefficient solution with a Pearson currentprobe
reveals the potential of this set-up asdepicted in Figure 3.With an
oversampling rate of OSR = 16,
a result with 14bit of resolution is achieved,showing only
marginal differencescompared to the Pearson probe. As theresult of
the decimator already is digitalinformation, no further conversion
isnecessary to apply it to a microcontroller forcontrol
purpose.
Temperature measurementThe baseplate’s temperature as a
further
parameter is sensed in a variety of powerelectronic modules.
Materials with welldefined thermal dependencies are widelyused as a
temperature sensor. Modernpower modules contain a resistor with
negative temperature coefficient (NTC) tocapture the baseplate’s
temperature.As the NTC is a passive component,
additional electronics is needed totransform the NTC’s
temperaturedepending resistance into a signal that canbe used by a
microcontroller. One methodto do so would be to apply a
constantcurrent to the NTC and capture thevoltage across the
device. As theresistance is a function of temperature,the voltage
at constant current resemblesthe same characteristics. As for the
currentmeasurement, digital information wouldbe the preferred
solution. Theimplementation within the MIPAQ servemodules therefore
transforms thetemperature dependency RNTC(T) into atemperature
dependent frequency f(T).Here too, the isolation barrier formed
byCLT is used to provide a signal that isgalvanically separated
from the powerelectronic section. Simply counting pulsesfor a
predetermined time is sufficient to
get an accurate reading of the baseplate’stemperature. Due to
the large thermalcapacitances involved, the time taken forthe
conversion is of secondaryimportance. Counting pulses for 50ms
oreven 100ms leads to proper information,as displayed in Figure
4.As a consequence of the NTC’s
characteristics, the relationship betweenpulses and temperature
is not perfectlylinear. Nevertheless, an approximationeither
piecewise linear polygonal or ofhigher order will provide a
temperatureinformation with an accuracy of ±1K.
The trend in developmentToday, power electronics modules
already contain basic sensors like shuntsor NTC-resistors. New
developmentssupport designers in coping with theongoing demands of
higher powerdensities by adding necessaryfunctionalities into the
power electronicssection, saving space, time anddevelopment effort.
This integration isconsidered to be an ongoing trend, sofuture
products are expected to combineeven more powerful sensors
ormeasurement technology.
Literature[1] Domes, Daniel; Schwarzer Ulrich:
IGBT-Module integrated Current andTemperature Sense Features
based onSigma-Delta Converter, PCIM 2009,Nürnberg, Germany[2]
Hogenauer, E. B.: An Economial
Class of Digital Filters for Decimationand Interpolation, IEEE
Transactions onAcoustics, Speech and SignalProcessing, Volume 29,
Issue 2, Apr1981 pp. 155 – 162
18 POWER MODULES www.infineon.com
Issue 7 2009 Power Electronics Europe
Figure 3: Current measured with Pearson probe and
Σ/Δ-Converter
Figure 4: Relationship of temperature and pulses counted
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