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

www.infineon.com POWER MODULES 17

Power Electronics Europe Issue 7 2009

p17-18 Feature Infineon.qxd_Layout 1 14/10/2009 10:50 Page 17

http://www.infineon.com

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

INDUCTORS / CHOKESFROM POWDER COMPOSITE MATERIA LS

+49 (0 )7122 / 8 2 5 98 -0 www.HKRweb .de

p17-18 Feature Infineon.qxd_Layout 1 14/10/2009 10:51 Page 18

http://www.infineon.comhttp://www.HKRweb.de