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Power Electronics Applied to Industrial Systems and Transports 5
Series Editor Bernard Multon
Measurement Circuits, Safeguards and Energy Storage
Nicolas Patin
First published 2016 in Great Britain and the United States by ISTE Press Ltd and Elsevier Ltd
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
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Notices
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The rights of Nicolas Patin to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.
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Preface
Thisvolumecouldhavebeencalled“Allwhichwasnot discussedinthefirstfourbooksbutwhichshouldnotbe overlooked!”.Indeed,thisbookcompletesthefourpreceding booksbyaddressingtopicsthat,eventhoughnotanintegral partofthepowerelectronicsfield,arecrucialtodesigning reliableandefficientconverters.Thefirstchapterdealswith current,voltage,andtemperaturemeasurements.Such measurementsarerecurringissuesinanypowerelectronic converter.Componentsandsensorsareintroducedand specifiedbymeansoftheirtechnology,keycharacteristics, andconditioningcircuitsrequiredtoprocessinformation (whetherbyanalogordigitalcircuits).Thesecondchapter dealswithshieldingcomponentsusedcommonlyinpower electronicsaswellasinotherelectronicfields(especiallyin measuringdevices).Indeed,thetaskathandhereistostudy how“fragile”componentscanbeshieldedfromdamage causedbyvoltage,orintensity,surgesthatcanoccurineither faultlessormalfunctioningcircuits.Standardssetfor electronicprotectivesetupsarealsomentioned.Additionally, partofthechapterfocusesonexplosion-proofequipment (ATEX)andonwaterproofratingsofelectricaldevices (ProtectionIndexconcept–IPcode).Thefinalchapterdeals withelectricalenergystoringelementscommonlyusedin associationwithpowerelectronicconverters.Thischapter
addressescapacitorsandsupercapacitorsfirst,thendeals withbatteriesandaccumulators.Partofthechapterfocuses onthefeaturesofthesecomponents,especiallymonitoring, recharging,andestablishingchargeequilibrium (supercapacitorsorbatteries).Theconceptofaccuracyis addressedinthefirstchapterwhichdealswith measurements.Appendix1,dedicatedtouncertainty calculations,givesfurtherinformationonhowthe performanceofameasurementchainischaracterized.Asa finalnote,ashortappendix(Appendix2)introduceshelpful informationforconvertingbetweenmetricandimperial units.
Nicolas PATIN
January2016
SensorsforPowerElectronics
1.1.Overview
Regardlessofthequantitiesthathavetobemeasured,the sensorsusedinpowerelectronicsarecharacterizedbygeneric parametersaswellasinanyotherelectronicfield:
– precision;
– bandwidth;
– differentialinput(orsingle-endedinputwhenthesignal istakenrelativetoground);
– galvanicinsulation.
Thislastparameterisparticularlyimportantinpower electronicsinorderto:
– maintainexistinggalvanicinsulationofthepower electronicconverterforthesensor(refertotheInsulated SwitchingPowerSupplysectioninVolume3[PAT15c]);
– avoidpropagatingfaults/parasiticeffectsfromthepower circuittothecontrolcircuit(especiallyinthecaseofhigh powerapplicationswithhighvoltagesandstrongcurrents).
2PowerElectronicsAppliedtoIndustrialSystemsandTransports5
Theuniquecharacteristicsofpowerelectronicsaredueto the“choppedup”natureofmeasurablequantities(oratleast ofcertainquantitiesinthesystem).Thiscancause measuringissuesintermsofthebandwidthrequiredfor sensorsthatarenotallsuitedtopicksignalsuppresenting steepwavefronts(large dV dt and/or dI dt ).Furthermore,although theoreticallythemeasuredquantitiesdonotsuffer discontinuities(currentthroughaninductance,voltage acrossacapacitor),theirreadingscanbeaffectedbyother quantitiesundergoinglargefluctuations(currentorvoltage throughatransistor).Thosefluctuationsarelikelytoinduce parasiticsignalsviacapacitiveorinductivecouplinginthe measuringcircuitconnectorsofaprintedcircuitboard,for example.
Broadlyspeaking,theroutingofaprintedcircuitboardfor apowerelectronicconverterrequirespayingcarefulattention tothefollowingcriticalelements:
– powercircuitconnections;
– measuringcircuitconnections(toavoidintroducing parasiticeffectsinthereadingsasmuchaspossible);
– connectionsofcontrolsignalsofelectronicswitches(to avoidoscillations/parasites,therefore,avoidunwantedswitch commutationsorattheveryleastavoidingslowcommutations leadingtoadditionallosses).
Inthisrespect,circuitminiaturizationlimitsinductive couplinginthebestwaypossible(and,toalesserextent, capacitivecoupling).Itisalwayspossible,usingfine qualitativereasoning,toevaluatewhetheragivenrouting “geometry”isadequateornot.Thisqualitativeanalysiscan evenbecarriedoutbeforeperformingaquantitativeanalysis (usuallylengthy)requiringexpensiveandcomplexequipment (suchasretrievalsoftwareforparasiticcomponentsdueto componentwiring,andsimulationsoftware).Inorderto conductthisreasoning,itisessentialtofocusonlyonthe criticalconnectionslistedpreviously,andtoneglect
non-criticalconnections(powersupplytracksforcontrol circuits,ifthelatterareproperlyuncoupled,thanksto capacitorsconnectedascloseaspossibletotheirpower supplypins).
Figure1.1. Parasiticcouplingschematics(inductiveandcapacitive)for sensorswithcurrentoutputa)andvoltageoutputb)
Moreover,knowingthenatureofthequantitiescarrying theinformationatthesensoroutputallowsustodetermine theoutputsignalcouplingvulnerabilities(capacitiveor inductive).Forexample,considerthecaseofasensor supplyinga current reflectingthemeasuredquantity(thisis thecaseforalargenumberofHalleffectsensors).Then, inducinganEMFonalongtrackseparatingthesensorfrom thedataprocessingcircuitwill,inthiscase,havelittle impactonthemeasuredsignal.Thisisillustratedin Figure1.1(a).Indeed,letusassumethesensorisanideal currentsourceandthesignalmeasuredattheacquisition circuit(forinstance,ananalog-to-digitalconverter–ADC)is obtainedbyrecordingthevoltageacrossaresistorplacedas closeaspossibletotheADC.Thenundertheseassumptions, thesensoroutputimpedanceisacurrentsourceimpedance
i.e.theoreticallyinfiniteimpedance(eventhoughonlyvery largeinpractice),thereforeitwillnotbepossiblefortheEMF inducedbyinductivecouplingonthe(possiblylong) connectingtrackstoinjectaparasiticcurrentintheloop.On theotherhand,acapacitivecouplingwithaclosedtrack undergoingstrong dV dt wouldbeabletoinjectaparasitic C dV dt typecurrent,thereforeaffectingtheacquisitioncircuitand, consequently,thereadings.
Onthecontrary,thecircuitconnectingasensordelivering a voltage (reflectingthemeasuredquantity)toanacquisition circuit(Figure1.1(b))willnotbevulnerabletocapacitive couplingaslongastheimpedanceofthiscircuitislow(which isthecaseforasensoractinglikeanidealvoltagesource). Consequently,ifthecircuitdoesnothavetoohighan impedance(especially,areactanceduetotrackinductance), the C dV dt typecurrentwillbeabsorbedbythevoltagesource, andwillnothaveanyimpactonthevoltagemeasuredbythe ADC.However,thesamecircuitwillbeveryvulnerableto inductivecoupling.Thisisbecauseinductivecouplingwill induceanEMFthatwilladdtothevoltagesuppliedbythe sensor,resultinginthevoltageperceivedbytheADC.
Itisimportanttokeepinmindthatcapacitiveand inductivecoupling,althoughdifferentinnature,areoften duetobadrouting(oftheprintedcircuitboard).Common denominatorsare:
– longparalleltrackscarryingchoppedupcurrentand/or voltageforthepowercircuit,ontheonehand,andthesignal comingfromasensorandleadingtoanacquisitioncircuit (analogordigital)ontheother;
– “forward”and“return”conductorsplacedtoofarapart thatcreateaninductiveloop(self-inductance)andintroduce avulnerabilitytoinductionfromanothercircuit(mutual inductance).
Althoughproximitybetweenthepowercircuitandthe measuringcircuitcannotbeavoided(becauseofcourse,the sensormustmeasureavariableofthepowercircuit!),the problemslistedaboveshouldbeminimizedasmuchas possible.Firstly,couplingcanbeminimizedbytracingtracks asshortaspossibleonthe“powerside”andonthe “measuringside”andbyanglingthemat90° withrespectto eachother.Secondly,inthecaseofsingle-ended measurements,minimizingthedistancebetweena“forward” anda“return”conductorcanbefacilitatedbyimplementing earthplanes.Otherwise,fordifferentialmeasures,the problemcanbesolvedbyroutingthetwotracksinparallel withspacingassmallaspossible(evenmoreefficientifthese tracksareangledat90° withrespecttothepowercircuitery (orcircuit)inordertominimizedisturbances).
1.2.Currentsensors
Commoncurrentsensorscanbegroupedinthreebroad families:
– shunts(resistors);
– currenttransformers;
– Halleffectsensors.
Eachofthesesensorcategorieshavedifferent performancesintermsofaccuracyandbandwidth(ormore precisely,maximummeasurablefrequency).However,certain sensortypeshavethenoteworthyabilitytomeasure adirectcurrent;forexampleshunts,whereascurrent transformersaregrosslyincapableofperformingsucha measurement.
1.2.1. Currentmeasuringshunts
1.2.1.1. Ohm’slawandrelatedcomplications
Sinceashuntoperateslikearesistor,itsoperationcan simplybedescribedusingOhm’slaw:
Thevoltageacrosstheshuntisproportionaltothecurrent passingthroughit.Therefore,knowingtheproportionality coefficient R isenoughtobeabletoderive,fromthevoltage reading,thecurrentcirculatingthroughthebranchonwhich theshuntisconnected(inseries).Unfortunately,even assumingthatthevoltagewasmeasuredperfectly(whichis neverthecaseinpractice),notknowingthevalueof R remainsanissue.Beforediscussingthisresistance measurementandcalibration,letusrecalltherelation linkingresistance R (in Ω)tothephysical(resistivity ρ in Ω.m)andgeometric(length l andsection S inmand m2 ) propertiesoftheresistiveelementconsidered:
Inpractice,ashuntshouldallowforthecurrenttobe measuredwithoutdisturbingthepowercircuitexcessively, throughwhichthecurrentcirculates.Inparticular,this keepstheconverterefficiencyfrombeingdeterioratedtoa greaterextentaslosseswillinevitablybetriggeredbythis component.Additionally,theselossesmightimpactthe readingsthemselves.Indeed,lossesleadtoheatingand heatinghasaneffectonmaterialresistivityaswellasonthe geometricdimensionsoftheresistiveelement(thermal expansion).Asaguideline,theresistivityat 300K ofsome metalsaswellastheirassociatedtemperaturecoefficient α areexhibitedinTable1.1.The α coefficientcorrectsfor materialresistivityaccordingtothefollowinglinearfunction:
Inpractice,puremetalsarenotusedtomakeresistors (precisionresistorsininparticular)butitisimportanttobe awarethatresistivityofthematerialsusedvarieswith temperature(somewithapositivetemperaturecoefficient–PTC,whileotherswithanegativetemperaturecoefficient–NTC).Thisphenomenoniscorrelatedtodimensionvariation causedbythermalexpansion.Moreorlessstrongresistance variationsshould,therefore,beexpectedforcomponentsthat werenotdesignedspecificallytoobtaintemperaturestable resistiveshunts(extremecasesareusedastemperature sensorscalledPTCorNTC“thermistors”–referto Chapter2).
Table1.1. Resistivityandtemperaturecoefficientsofsomemetals
1.2.1.2. Seebeckeffect
Anotherphenomenon,knownastheSeebeckeffect,isat workinvoltagemeasurementsacrossashunt.Thiseffectis usuallynotcriticalinpowerelectronicapplications.The SeebeckeffectisathermoelectriceffectthatgeneratesEMF attheinterfacebetweentwometalswhenthisinterfaceis heateduptoacertaintemperature.Sincetheshuntis solderedtocables,ormoreprobablytoaprintedcircuit board,twointerfacescanbeidentified(inreality,fourdueto
solderseamsinvolvinganadditionalmetal)andasmany EMFs.Thetemperaturecomponentnormallybeinguniform, voltagescanbeexpectedtocompensateeachother.In practice,thisisnotalwaysthecase,duetodimensionsof tracksinparticular.Intheseconditions,atemperature differencecanappearand,asaconsequence,aresidualEMF remains(thatis,theoperatingprincipleofthermocouples thatarecommonlyusedformeasuringtemperaturein theindustry).However,theorderofmagnitudeof thermoelectricallygeneratedvoltagesissosmallthatthese voltageswillnormallybeovershadowedbyvoltagesinduced effectivelybycurrentflow.Itisoftendesirabletohavea minimumofseveraldozensifnothundredsofmillivolts acrossashuntinordertogetreliableandaccuratecurrent measurement,whereasthermoelectriceffectgenerates voltagesproportionaltothetemperatureinKelvinswith (positiveornegative)coefficientsoftheorderofafew µV/K.A thermalimbalanceoftheorderofahundredKelvinsina properlymanufacturedpowerelectronicconverterisnot realistic.Itis,therefore,impossibletointroducea“parasitic” voltageviathisphenomenon,notevenofthemillivoltorder. Incontrast,thisphenomenonisnon-negligiblewhile measuringveryweaksignals(voltageorcurrent)andneeds tobetakenintoaccountinthedesignofprecisioncircuits (refertopublication[KEI10]forfurtherinformationonthis topic).Inordertoovercomethisproblem,veryweakshunts (forexample 0, 0001Ω foraVishayWSMS2908shunt1)come equippedwithconnectionsusinglowthermalEMFalloys (< 3 µV/°C).Potentialsareestablishedforeachmaterialand alistcanbefoundonWikipedia.Ontheotherhand,with regardstoconnectionswithcopperconductors(whicharethe mostcommononprintedcircuitboards),theEMForderof magnitudeofafewcouplesareenoughtoevaluatethe parasiticvoltagesintroducedbythisphenomenon(see 1Thoseshuntsarespecializedfor“wattmeter”typeapplications.
Table1.2).Whenitcomestosolderingseams,theCd70 Sn30 alloycanserveasareferencebecauseitsSeebeckcoefficient withregardstocopperisclosetozero.Thisisnotthecaseof commonalloyssuchasthetraditionalstandardleadSn60 Pb40 (1 3 µV/°C)soldering,whereasanalloywithveryhighlead contentalsoperformsexcellently(Pb90 Sn10 ).Itshould, however,benotedthattheiruseisproblematicregarding regulationsinforcebecauseofthepresenceofcadmiumor lead,althoughexemptionsfromRoHSstandardsexist (lead-acidbatteriesforinstance).Inconclusion,theSeebeck effecthasaweakimpactonrelativelyhighvoltage measurementsanditshouldnotbeconsideredamajor problemforcurrentmeasurements,assuch,(evenifthe voltageacrossshuntsisratherweak).TheSeebeck phenomenonisevenlessofanissueforvoltage measurements,unlessdealingwithveryspecificapplications, forwhich,“manipulated”levelsareweakandwhere measurementperformances(intermsofaccuracy)arehigh. Itshould,nevertheless,benotedthatthistypeofeffectcan becompensatedforintemperaturemeasuringcircuitswith Pt100typefour-wire-connectionsprobeswhereextreme accuracyissometimessoughtafter(seesection1.4.3).
Metalcouples Seebeckcoefficient QAB (in µV/°C)
Copper–Copper ≤ 0 2 µV/°C
Copper–Silver 0 3 µV/°C
Copper–Gold 0.3 µV/°C
Copper–Lead/Tin 1to3 µV/°C
Copper–Silicon 400 µV/°C
Copper–Kovar ∼ 40to70 µV/°C
Copper–Copperoxide ∼ 1000 µV/°C
Table1.2. Seebeckcoefficientswithrespecttocopper forsomematerials(source:[KEI10])
R EMARK 1.1.– Componentpins(passiveorintegrated circuits)areconventionallymanufacturedinKovar.Kovaris aniron-basedalloycomprising(inmass)of 29 %nickel, 17 %
10PowerElectronicsAppliedtoIndustrialSystemsandTransports5
cobaltand,insmallerquantities,carbon(< 0, 01 %),silicon (0, 2 %)andmanganese(0, 3 %).
1.2.1.3. Aging
Whenaresistorisfunctioning,itheatsup.However,when heatingup,thecomponentstartsageing(asdoesanyother electroniccomponent).Beforereachinganadvancedstateof decay,theresistanceparameterisslowlyaltered.Ageingis inevitableandneedstobetakenintoaccountwhiledesigning ameasuringcircuit.Twodifferentsituationsmightarise:
– case1:accuracyrequirementsareweakenoughthatthey willnotbeaffectedbythedecayingoftheshuntresistor throughouttheentirelifespanofthedevice;
– case2:overtime,theunmanagedresistancedecayleads toanunacceptabledeteriorationofmeasurementaccuracy.
Thesecondcase(notcommoninpowerelectronicsbut frequentformeasuringdevices)willrequiretheuserto conductrecalibrationsofthemeasuringcircuitonaregular basisinordertocompensateforthedecay.Inthefirstcaseon theotherhand,thiswillnotbenecessary,orattheveryleast, notoncethemanufacturingprocessoftheitemhasbeen completed(itispossibletoperformaninitialcalibrationin casethetoleranceonthecomponentvalueisinsufficientfor theaccuracyexpected).
Asaguideline,theVishayWSMS2908shuntdatasheet givesthefollowinginformationoncomponentdecay throughoutitslifespan:LoadLife(1000hat+70°C, 1, 5 h “ON”, 0, 5 h“OFF”): ± 1, 0 % ∆R.
1.2.1.4. Resistortechnologiesandtheconceptofnoise
Fromthepointofviewofresistortechnologies,letusfocus, firstofall,onpowerresistors:theseareusuallywoundintoa coilonaceramicmount.Theyaredesignedtosustain importanttemperaturerisesanddissipateonlyafewwatts.
Theyshouldnotusuallybeusedforcurrentmeasuring applicationsinanelectronicconverter.Thereasonforthisis thattheyhaveaninductivebehaviorthatcancauseproblems iftheshunthastobeplacedinserieswithaswitching component(however,theycouldbeusediftheshunthadto beplacedinserieswithaninductorbecause,inacasesuchas this,thecurrentwouldbeacontinuousfunctionoftime). Lowerpowerresistors(< 1 W)arebuiltbylayeringcarbonon aceramicrod:thosearethemostcommonlyused through-holeresistorsinanalogelectronics(especiallyduring practicalsessionsonbreadboards).
Amatter,whichhasnotyetbeenraised,concernsnoisein resistors:freeelectronsarealwaysinBrownianmotion (erraticmotion)aslongastemperatureisdifferentfrom absolutezero(0 Kor 273.15°C)2.Thisleadstoanoisesignal withintheresistorsthatcanbemodeledbyarandomly distributedvoltagesourcewithvariance
where kB istheBoltzmannconstant(1 3806 × 10 23 J K 1 ), T isthetemperatureinK, R istheresistancein Ω and B isthe bandwidthconcernedinHz.Itshouldbenotedthatthis expressionofthermalnoise(orJohnsonnoise)iswell-verified formetallicresistors,butturnsouttobetooweaktoaccount fornoiseinresistorswithcarbonlayers:inthistypeof resistors,anadditional 1/f noiseaddstothewhitenoise (noisewhosepowerspectraldensityisconstantforall frequencies)definedbyequation[1.4].Theseresistorswith carbonlayersharethischaracteristicwithSMDtype resistors(SMDstandsforSurfaceMountedDevice)whose technologyusesthickfilms(thick-filmresistors).Those thick-filmresistorsaremanufacturedusingapaste
2Whichisnotalwaysthecaseinpractice!
12PowerElectronicsAppliedtoIndustrialSystemsandTransports5
containingamixtureofinsulatingelementsandconducting elements(rutheniumoxide)depositedontothesurfaceofan aluminainsulatingsubstratetoathicknessoftheorderof severalhundredsof µm.Thistechniquemakesitpossibleto obtainresistorswithastandardaccuracyofaround5to 1 % (atbest 0.5 %).Thesealsohavealimitedtemperature stabilityofseveralhundredppm/°C(ppm=partspermillion) oratbestseveraldozenppm/°C.Thick-filmcomponentscan eventuallybeusedaslow-costshuntsforcurrent measurementsbutwillnotgivehighlyaccurateresults.
1.2.1.5. SMDthin-filmresistors
Forcurrentmeasurementapplications,SMDthin-film resistorsturnouttoperformconsiderablybetter(butarealso moreexpensive).Theseresistorsstillrelyonanalumina substrate,howevertheirresistiveelementisobtainedby depositingathinnichrome(NiCr)3 film(withathicknessof aroundonehundrednanometers).Sinceitisbasedon metallicfilm,noiseislimitedsolelytoJohnsonnoise(white noise).Furthermore,theseresistorscanbelaser-trimmed efficiently(seeFigure1.2)inordertoobtainaccuratevalues (thebestaccuracyofoff-the-shelfcomponentsisofthe orderof0.01%).Additionally,theyaresubstantially temperature-insensitive(thereareresistorsthathaveaTCR4 lowerthan 1 ppm/°C).
Last,butnotleast,importantissueconcerningSMD resistors(boththinandthick-film)istheirfrequency response:theabsenceofconnectionpinslowersthe possibilityofparasiticinductanceofthesecomponents significantly.Fromthere,currentmeasurementsobtained withsurface-mountedshuntscanbevalidforawider frequencyrangethanwiththrough-holecomponents.This canproveusefultodesignconverterswhoseswitching
3Asindicatedbyitsname,itisanickelandchromiumalloy.
4TemperatureCoefficientofResistance
frequenciescanexceedamegahertz(forresonanceconverters inparticular).
Figure1.2. Lasertrimmingmotifstoadjustthin-film precisionresistors(source:GSILumonicsinc.)
1.2.1.6. Four-terminalshunt
Connectionsbetweenashuntandtheprintedcircuit board,onwhichitisplaced,canintroduceresistancesthat willimpactandlowerthemeasurementaccuracy. Furthermore,thesecharacteristicscandispersefromone boardtoanother.Intheseconditions,theaccuracyofweak resistorscanbecompromised,therebyrenderingthem unusable.However,aswasdiscussedpreviously,shunts shouldhaveaminimalimpactonthecircuitstowhichthey areconnected.Therefore,theuseoflowvalueresistorsis crucial.
Inordertogetridofthislimitation,precisionresistors (generallythosethathavealowresistancevalue)havenot twobutfourterminals.Thiskindofpackagingisexplained bythefactthattwooftheterminalsarededicatedtothe currentflow(“power”terminals)whereastheothertwoare usedtopickupthevoltageacrosstheintegratedresistive element.Sincethereisnocurrentflowingthroughthe voltagemeasurementcircuit(orattheveryleast,averylow currentbecauseoftheveryhighinputimpedanceofthe
14PowerElectronicsAppliedtoIndustrialSystemsandTransports5 measurementcircuit),noohmicvoltagedropcanbe triggered.Asaconsequence,eventhoughvoltagedropsare effectivelyobservableinthepowercircuit(atthesoldering seams),theywillnotbe“seen”bythevoltagemeasurement circuit(seeFigure1.3).
Figure1.3. Operatingprincipleofafour-terminalshunt
Thesetypesofcomponentscanbefoundin“through-hole” versionsbutalsoasSMDsasshowninFigure1.4.Inorderto obtainthebestaccuracypossible,thevoltageandcurrent terminalsshouldnotbeconnecteddirectlytosharedpadson aprintedcircuitboard:thisisespeciallytrueforSMDshunts forwhichthe“smallprotrudingtabs”can,sometimes,go unnoticed.Thesolderpadblueprintshowninthefigureis veryclear:fourpads(andnottwo!)havetobesolderedonto theprintedcircuitboardfortheshunttoworkproperly.The largerpadsarededicatedtocurrentflow(ofthepower circuit)whereasthetwosmallerpadsshouldbeconnectedto thevoltagemeasurementcircuit(differential–seenext section).
High imput impedance circuit
Figure1.4. Four-terminalshuntcasings(source:Vishay)
Additionally,thistypeofshuntisnotusedexclusivelyfor integratedmeasurementsinanelectronicdevice;itisalso usedinmetrology.Figure1.5showsapictureofsuchashunt fromthebrandLEMforwhichthemeasuredvoltageis pickedupbyacoaxialcable(thereisaBNCmetersocketat thebottomoftheshuntbaseplate).Itisaveryexpensive
16PowerElectronicsAppliedtoIndustrialSystemsandTransports5
componentbecauseofitsaccuracyandnominalrange. Indeed,thisshuntmakesitpossibletomeasurecurrentupto 500 Awithaaccurateandstable 0.1 %resistance.Asaside note,thisshunthasapurelyresistivebehaviorona frequencyrangerelativelyhighgivenitssize(height:around 22 cm),withacurrent/voltagephaseshiftlowerorequalthan 0.1° upto 20 kHz(andaresistancethatdoesnotfluctuateby morethan ±0.1 %onthisfrequencyrange).
1.2.1.7. Integrationintoaconverter:differentialorsingle-ended input
Measuringacurrentusingashuntthathasoneofits terminalsconnectedtothecircuitground,istheeasiest,most comfortablesituationtopickup,amplify,andeventually, filterthevoltageacrossitinordertoprocessthedatabya controlormonitoringdeviceofapowerelectronicconverter. Amplifyingthevoltageacrosstheshuntbyagainsuitedfor thedownstreamcircuitisthemostobvioussolutiontocarry thismeasurementout.Otherwise,thevoltagepickedup canbesentdirectlytoadedicatedterminalofthe control/monitoringcircuitoftheconverter.Figure1.6shows anexampleofacircuitboardlayoutimplementedforsucha solution.
Figure1.5. PowershuntfromthebrandLEM
Figure1.6. Ground-referencedshuntusedbyaMOSFETdriver (source:datasheetInternationalrectifierIR2121)
Inthecasewhereanop-amp(operationalamplifier)is usedtoadaptthevoltageacrosstheshunt(usuallyweak)to theoptimalprocessingrangeofthedownstreamcircuit5,care shouldbetakeninordertomaintaintheexpectedcurrent measurementaccuracy.Thus,componentsshouldbechosen carefullytoguaranteethesamemeasurementqualityasthe shuntintroducedinthepowercircuit.Constraintsdifferonly intermsoftemperaturedrift,providedthatitismadesure thatthetemperatureriseofthispartofthecircuit,causedby theshunt,willnotspreadtootherpartsofthedevice. Questionsmighteventuallyberaisedconcerningtherequired toleranceforresistorsplacedinaninvertingora non-invertingamplifier:measurementaccuracyshouldbeat leastasgoodastheshuntaccuracyifthedeviceisintended tobeoperationalintheabsenceofacalibrationstep.Onthe onehand,resistorscanbeofweakaccuracyifacalibrationis expectedattheendofproductionline.Ontheotherhand, theseresistorsmustmaintainagoodstabilityovertime. Additionally,itiswithoutdoubtpreferablefortheseresistors tohaveagoodtemperature-dependentstabilityinorderto guaranteealargefunctioningtemperaturerangewithout lossofmeasurementaccuracy.Eventhoughtheseresistors
5Forexample,ananalog-to-digitalconverterinput.
18PowerElectronicsAppliedtoIndustrialSystemsandTransports5
arenotexposedtotemperaturesasahighastheshuntis (70°Cforinstancefortheshunt),theywillstillbeexposedto awarmenvironmentofsubstantiallyfluctuatingtemperature duringoperations(atcoldstartorafteralongintensive operation).Addingtothat,theywill,sometimes,beexposed tovaryinglytemperedenvironments(indoororoutdoor residentialuse,sometimesinvehicles).Inthecaseofa systemundergoingcalibration,theonlythingtoverifyis that,atworst,theassemblygaindoesnotleadthe acquisitionstage(anADCforexample)tosaturationunder maximumcurrentintensity.
Concerningtheotherelementsoftheanalogcircuit, selectioncriteriadonotdifferfromconventionalissuesthat arisewhendesigninganop-ampcircuitassembly:
– offsetvoltage(“auto-zero”typeop-amp);
– symmetricalorasymmetricalpowersupply;
– supplyvoltagelevel;
– rail-to-railoutputstage;
– bandwidthandslew-rate.
Alltheseconventionalissuesremaintrueforvoltage measurementacrossafloatingshuntbutinthiscase,the majorissueiscommonmoderejection.Beforediscussingthis problemanditsexistingsolutions,itisworthdiscussingthe reasonwhyafloatingshuntisveryuseful(seeFigure1.7).
Themostnoteworthyadvantageofusingsuchafloating shuntconfigurationisthatitallowsustomaintainground continuitybetweentheinputandoutputofapowerelectronic converter.Indeed,ifanelectroniccardincorporatesashunt thatintroducesapotentialdifferencebetween“inputground” and“outputground”,itisstrictlyforbiddentoconnectthese groundstogetherwithinthesystemwherethecardis integrated.Ifsuchaconnectionwasindeedperformed,the
shuntwouldbeshort-circuitedandallfeaturesassociated withcurrentmeasuring(regulation,limitation/protection) wouldthenbelost.Inthecasewheretheshuntisplacedon the“positivepowerline”,groundcontinuityissecured.Input andoutputterminalscanthenbeconnectedtogether(evenif thisisseldomused,itprovestobeagoodidea!).Therefore, thiscurrentmeasurementmethodcanbeconsideredmore reliablewhenconsideringa“systemapproach”ofsaid electronicfeature.
Figure1.7. Comparingdifferentshuntconfigurationsinapower electronicconverter:a)shuntconnnectedtoground,b)floatingshunt
Nowthattheimportanceofthisconfigurationtypehas beendemonstrated,letusstudythedifficultiesof implementingitinpractice.Forthis,letusrecallthegoal thatwasinitiallydefined:tomeasurecurrent Ishunt flowing throughtheshuntbyobservingvoltage Vshunt acrossit.Then, itbecomesobviousthatinordertocarryoutthis
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20PowerElectronicsAppliedtoIndustrialSystemsandTransports5
measurementusingaground-referencedcircuitassembly, twopotentialshavetobesubtracted:
Vshunt = VA VB
Inordertoachievethisresult,itisobviousthatanop-amp subtractorcircuitisthesolution(seeFigure1.8).Thefollowing equationisobtainedbyapplyingthetraditionalapproachfor linearop-ampcircuits(V + = V ):
Figure1.8. Op-ampsubtractorcircuitschematic
Intheory,let R1 = R4 = RA and R2 = R3 = RB ,whichleads to:
s = RB R
and,intheparticularcasewhere RA = RB ,thissimplygives
Vs = V2 V1 .Unfortunately,inpractice,itisimpossibleto obtainstrictlyidenticalresistors.Itisthereforenecessaryto analyzetheactualresultdeteriorationwithregardstothe theoreticalcase.Inordertodothis,letusassumethefour
resistorshaveapproximatelythesameresistancevalue (nominalvaluewhichwillbereferredtoas R0 ).Then,the followingsimpleequationsareobtained:
Eachexpressionoftheresistances Ri (where i ∈ {1, 2, 3, 4}),hasbeenrewrittentorevealparameter ∆Ri R0 which,fromnowon,willbedenoted αi .Thefour αi parametersaredifferentbutagiventolerancerange normallyimposesanupperboundonthem:forexample,for 0.1 %resistorsitisknownthat |αi | ≤ 0, 001.Afterreplacing eachofthe Ri bytheir R0 .(1+ αi ) expressioninequation [1.6],thefollowingequationisobtained:
s = (1+ α3 ) . (2+ α1 + α2 ) (1+ α1 ) . (2+ α3 + α4 ) · V2 1+ α2 1+ α1 · V1
Next,tocontinuewiththisstudy,itispossibletoconsider (reasonably)thatforall i, |αi | 1 andthus,thatallproducts oftheform αp .αq arenegligiblewithrespecttoothertermsof thedevelopedequation[1.9].Theequationscanbesimplified as:
Thisequationisnotverypracticalconsideringthatthe coefficientsof V1 and V2 areasymmetrical.Inordertoobtain amorepracticalequation,theconceptsofcommonmode
