Reference Design for Interfacing Current Output Hall Sensors and CTs With Differential ADCs/MCUs

63 Pages·2015·10.34 MB·English

by  Texas Instruments, Incorporated

TI Designs
Reference Design for Interfacing Current Output Hall
Sensors and CTs With Differential ADCs/MCUs
DesignOverview DesignFeatures
Thisdesignprovidesareferencesolutionfor • OnboardCurrent-OutputHallSensortoMeasure
interfacingcurrentoutputHallsensorsandcurrent NominalCurrentupto25-ARMS
transformers(CTs)todifferentialADCs(standalone
• CurrentMeasurementAccuracyof0.5%
andintegratedintoMCU).Thedifferentialsignal
• CommonReferenceSolutionfor InterfacingBoth
conditioningcircuitisdesignedtomeasuremotor
CTandCurrentOutputHallSensorWith
currentwithanaccuracyof±0.5%acrossthe
DifferentialADCorMCU
operatingtemperaturerangefrom−25°Cto+75°C.
Theoutputcommon-modevoltageofthedifferential • SelectableOutputCommon-Mode Voltagefor
amplifier canbeselectedtoeither1.25Vor2.5V. DifferentialAmplifier
• DesignedtoEvaluateWithDelfinoF2837xControl
DesignResources
Card
• DesignedtoEvaluateWithExternalADC
TIDA-00368 DesignFolder
(ADS8354)for InterfacingWithMotorController
TPS62150 ProductFolder
TPS7A3001 ProductFolder FeaturedApplications
TPS7A4901 ProductFolder
• ACVariableSpeedIndustrialDrives
OPA322 ProductFolder
THS4531A ProductFolder • ServoMotorDrives
TLV1702 ProductFolder • UPSSystems
LP2992A-5.0 ProductFolder • SolarInverters
REF2025 ProductFolder
• AC/DCandDC/DCPowerSupplies
ASKOurE2EExperts
WEBENCH®CalculatorTools
DC BUS_P TPS62150 +6 V LP2992
+15 V TPS7A4901 +12 V
MOTOR
-15 V TPS7A3001 -12 V
DC BUS_M U V W POWER SUPPLY
+5 V REF2025 12..255 V V
REFERENCE
Hall Effect Current
Sensor from LEM
burden Tcohme pvaorltaatgoers a fcorro OssC B duertdeecnti oisn .also given as input to the
VTH (Pos) VTH (Neg)
Level shifting OPA322
Gain + Filtering THS4531A
TLV1702
Note: The SMA Overcurrent Detection
connectors are used to
connect to external ADC
EVMs
AnIMPORTANTNOTICEattheendofthisTIreferencedesignaddressesauthorizeduse,intellectualpropertymattersandother
importantdisclaimersandinformation.
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SystemDescription www.ti.com
1 System Description
Currentmeasurementisaninherentpartofanyinverter-drivenapplication.Oneimportantreason for
measuringthemotorcurrentistoperformacontrolalgorithm.Vectorcontrolanddirecttorquecontrol
requirecurrentsensingforcontrolpurposes.Informationregardingmotorparametersisalsoimportantfor
severalcontrolschemes.Thestatorcurrentmeasurementisusedtoestimatetheseparameters.The
currentmeasurementisalsousedforhardwareoverloadandearthfaultprotection.
Theinverteroutputratingmustbederatedwhileoperatingathigher temperatures.Highertemperatures
canbeto theresultofincreasedambienttemperature,afaultyfan, orobstructionsincoolingpath.In
thesescenarios,thecurrentmeasurementassistsinderatingtheinvertercurrenttokeepthepower
deviceswithinitspermissiblerangeofoperatingtemperatures.
Themotorcurrentcanbemeasuredatdifferent locationsintheinverter. Figure1 showsanoverviewof
theusualmeasurementlocations,consideringathree-phaseinverterfor amotorcontrolapplication:
A
C
M
C
3~
C
B B B
A
A) Current measurement in the DC- and DC+ link
B) Current measurement in the bottom side emitter path of each half-bridge
C) Current measurement in the output phases
Figure1.TypicalMeasuringLocationsforCurrentMeasurementinMotorDrives
Thecheapestvariant “A”ofcurrentmeasurementisoftenusedfor applicationsinthelowerpowerrange.
Typically, thecurrentmeasurementisdoneontheDCminusbus.Becausethislocationmaybeusedas
thereferencepotentialoftheMCU,isolatingthesignalisnotnecessary.Variant “C”isanotherlocationfor
currentmeasurementandislocatedprimarilyinthelow-to-mediumpower range.Inthecaseofthis
design,the currentismeasuredattheemitter ofthebottom insulated-gatebipolartransistor (IGBT)of
eacharminathree-phaseinverter.Thethirdcurrent measurementmaynotbenecessary,asthe usercan
derivethisvaluefromcalculationsbasedonthemeasuredtwocurrentsignals.Theadvantageofvariant
"B"issimilartothatofvariant"A",inthatthenegativesectionoftheDC-buscanbetakenasthe common
referencepotential.However,adisadvantageofmeasuringvariant"B"istheincreasedstrayinductance.
Inhighdynamicdrivesandhigh-powerapplications,current isusuallymeasuredintheoutputphasesof
theinverter(variant"C" inFigure1).Thethirdcurrent sensorisnotnecessaryinthiscase,either.
TheTIDA-00368designisintendedforcurrent measurementusingcurrent-outputHall-effectsensorsin
theACandDCdrives(variantC).Thegoalofthisdesignistoprovideareferencesolutionfor a
differentialsignalconditioningcircuittomeasuremotorcurrent usingcurrent-outputHalleffectsensors
andcurrenttransformers(CTs).CurrentoutputHalleffectsensorsaretypicallyavailablefromcompanies
likeLEMTechnologieswww.lem.comandVACUUMSCHMELZEorVACwww.vacuumschmelze.com.
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www.ti.com SystemDescription
Oneverycommonmethodtomeasurecurrent istousesingle-endedADCsintegratedintothecontroller.
Single-endedmeasurementsaremorepronetonoiseinalargerdrivesystem,whichcanleadto
inaccuraciesinthemeasurement.Differentialmeasurementhelpstoovercomethenoiseissues.The
TexasInstrumentsDelfino™seriesofmicrocontrollers(consistingofdifferentialinput ADCs),makeit
possibleto measurethedifferentialsignalsovercomingthenoiseinterference.
ThesignalconditioningcircuitforHall-effect sensorsisrequiredfor thefollowingreasons:
• CurrentoutputHallsensorsaretypicallypoweredfrom ±12Vor ±15Vleadingtoabipolaroutput
swing.InterfacingthisoutputswingtoanADCwitha3.3-Vreferencerequiresanamplifierandlevel
shifting.
• Withindustrial motordrives,usingthesensoruntil200%to250%ofitsratingisfairlycommon.
Detectingtheoverloadconditionandprotectingthedriveisimportant.
• Higherinputoffsetvoltage,temperaturedrift, noise, andlinearityleadstoerror inmotorcurrent
measurements,whichalsoaffectscontrolloopperformance.Theproperimplementationofasignal
conditioningcircuitreducestheerrorsandnoisefromthesensors.
• IfusingadifferentialADCinsideaTexasInstrumentsDelfino™MCUfromtheF2837xseries, the
common-modevoltagemustbeshiftedfrom2.5Vto1.25V.
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BlockDiagram www.ti.com
2 Block Diagram
Figure2 showsthesystemblockdiagramfor theTIDA-00368design. Thedesignusesanonboard
current-outputHalleffectsensortomeasurethemotorcurrent.Aprovisionexiststoconnectanexternal
CTtothesignalconditioningcircuitforthepurposesofcurrent measurement.Theburdenresistoris
availableontheboardforbothHalleffectsensorsandCTs.
Thevoltageacrosstheburdenresistorissignal-conditionedusingtheOPA322andTHS4531Adevices.
Theoutputofthesignalconditioningcircuitisconnectedto:
1. 180-pinconnectortointerfacewiththeinternalADCoftheDelfino™F2837xseriesofMCUs
2. SMA connectorstoevaluatetheperformanceofthesignalconditioningcircuit withexternalADCslike
ADS8354
TheHalleffectsensorispoweredusingapower supplyof ±12V,whichisgeneratedfrom±15Vusing
TPS7A4901andTPS7A3001linearregulators. Thesignalconditioningcircuit ispoweredusing5-VDC
generatedfromthe+15-Vsupply.The+5Visusedtopower theOPA322andTHS4531Aoperational
amplifiers (op-amps).FortheTHS4531A,theoutputcommon-modevoltage(VOCM)canbesetat +2.5 V
whenusedwithexternal5-V ADCsor+1.25VwhenusedwiththeinternalADCofaDelfinoF2837x
controller.TheREF2025referenceisusedtogeneratethereferencevoltageof+1.25Vand+2.5V.
Toprovideprotectionagainstovercurrent faultconditions,feedthesensedcurrent totwocomparators,
whichareavailableinasingledevice,theTLV1702.Onecomparatorisfor thedetectionofovercurrentin
apositivehalf-cycleandtheotherisforthedetectionofovercurrentinanegativehalf-cycle.Both
protectioncircuitshavearesponsetime oflessthan500ns.
DC BUS_P +6 V
TPS62150 LP2992
+15 V TPS7A4901 +12 V
MOTOR
-15 V TPS7A3001 -12 V
U V W
POWER SUPPLY
DC BUS_M
2.5 V
+5 V REF2025
1.25 V
REFERENCE
Hall Effect Current
Sensor from LEM
The voltage across Burden is also given as input to the
burden comparators for OC detection.
VTH (Pos) VTH (Neg)
Level shifting OPA322
Gain + Filtering THS4531A
TLV1702
Overcurrent Detection
Note: The SMA
connectors are used to
connect to external ADC
EVMs
Figure2.TIDA-00368SystemBlockDiagram
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www.ti.com BlockDiagram
2.1 Highlighted Products
TheTIDA-00368referencedesignfeaturesthefollowingdevicesfromTexasInstruments:
• TPS62150– Astep-down(buck)regulator witha3-to17-Vinput,0.9- to6.0-Voutput, −40°Cto 85°C,
16-pinQFN(RGT),andagreen(RoHSandnoSb/Br)plannedeco-friendlyclassification.
• TPS7A3001 –Asingle-output,high-powersupplyrejectionratio(PSRR) low-dropoutregulator(LDO)
witha200-mA,adjustable −1.18-to−33-Voutputand −3-to−36-Vinput.Thedevicefeaturesultra-low
noise,an8-pinMSOP(DGN), –40to125°Coperatingtemperature,andagreen(RoHSandno Sb/Br)
plannedeco-friendlyclassification.
• TPS7A4901 –Asingle-output,HighPSRRLDO witha150-mA,adjustable1.2-to33-Voutput,
3-to36-Vinput.Thedevicefeaturesultra-lownoise, an8-pinMSOP(DGN), −40°Cto125°Coperating
temperature,andagreen(RoHS &noSb/Br)plannedeco-friendlyclassification.
• OPA322 –A20-MHz,low-noise,rail-to-railinput andoutput(RRIO),CMOSoperationalamplifierwitha
1.8-to5.5-Vrange, −40°Cto125°Coperatingtemperature,8-pinSOIC(D0008A),andagreen (RoHS
andnoSb/Br)plannedeco-friendlyclassification.
• THS4531A –Anultra-lowpower,rail-to-railoutput (RRO),fully-differentialamplifier.
• TLV1702– Adual,2.2-to36-V micropowercomparator.
• LP2992AILD-5.0/NOPB – Amicro-powered,250-mA,low-noiseultra-low-dropoutregulatorina6-pin
LLP(Pb-free).
• REF2025– Adual-outputvoltagereferenceDBV0005Awitha2.5-Vloadcurrent.
Formoreinformationoneachofthesedevices, seetherespectiveproductfoldersatwww.TI.com orclick
onthelinksfortheproductfoldersonthefirstpageofthisreferencedesignunderDesignResources.
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3 Hall Sensors
3.1 Closed-Loop Hall Effect Sensor
TheconductorthatcarriesacurrentIP(seeFigure3)generatesamagneticfield, whichisconcentrated in
amagneticcircuit.Thisfieldcanbemeasuredinanair gapbyusingaHallelement.TheHallelementhas
thepropertyofconvertingthemagneticfluxintoavoltage,whentheelementissuppliedwithaconstant
currentIC.Whenapplyingtheclosed-loopprinciple,theHallvoltageisonlyusedfor balancingtheprimary
andthesecondaryflux.Theadditional secondarycoil(with2,000turns,for example),carriesacurrent, IS
(whichequals1/2000thoftheprimarycurrent),topreciselycompensatefor thefieldoftheprimary
conductor.Thetotalfluxthenequalszero. OperatingtheHallgeneratorinazerofluxconditioneliminates
thedriftofgainwithtemperature.Whenthemagneticfluxisfullycompensated(zero),themagnetic
potential(ampere-turns)ofthetwocoilsisidentical;so,Equation1 canbecalculatedas:
N ´I =N ´I whichcan alsobewrittenasI =I ´N /N
P P S S S P P S (1)
Consequently,thesecondarycurrent,I ,istheexactimageoftheprimarycurrent,I ,beingmeasured.
S P
Insertinga “measurementresistor”,R ,inserieswiththesecondarycoilcreatesanoutputvoltage thatis
M
anexactimageofthemeasuredcurrent. Togiveanorderofmagnitude,thetypicalnumberofsecondary
turnsisN =1000to5000andthesecondarycurrentisusuallybetweenI =25mAto300mA,although
S S
itcanbeashighas2A.
Figure3.OperatingPrincipleofClosed-LoopHallTransducer
3.2 Advantages and Limitations
Closed-looptransducersarecapableof measuringDC,AC,andcomplexcurrent waveformswhile
ensuringgalvanicisolation.Theadvantagesofclosed-loopsensorsincludeverygoodaccuracyand
linearity, lowgaindrift,widebandwidth, andfast responsetime.Anotheradvantageisthat theoutput
currentsignaliseasilyscalableandwell-suitedtohighnoiseenvironments; nevertheless,closed-loop
transducersareavailableinvoltageoutputconfigurations.Again,aswithmost magnetic-based
measurementtechniques,insertionlossesareverylow.Themainlimitationsoftheclosed-looptechnology
arethehighcurrentconsumptionfromthesecondarysupply(whichmust providethecompensationas
wellasbiascurrent),thelargerdimensions(morenoticeableonhighcurrent transducers),amore
expensiveconstructioncomparedwiththesimpleropen-loopdesigns, andalimitedoutputvoltage dueto
theinternal voltagedropsacrosstheoutputstageandsecondarycoilresistance.Again,depending on the
applicationrequirements,theadvantagesoftenoutweighthelimitationsandtheaccuracyandresponse of
aclosed-loopsolutionisdesirableoverotheralternatives.
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3.3 Details of Hall Sensor “LAH 25-NP” from LEM Technologies
Selectingtherighttransducerisoftenatradeoffbetweenseveralparameters:operatingcurrent range,
outputsignaltype,accuracy,frequencyresponse,dv/dt,temperaturerange,weight,size, costs, and so
forth.TheLAH25-NPisaclosed-loop(compensated),multi-rangecurrenttransducer fromLEM
TechnologiesthatusestheHalleffect.Thistransduceristypicallyusedfor themeasurementofcurrent
(DC,AC, orpulsed)withgalvanicseparationbetweentheprimaryandsecondarycircuit.Figure4 shows
theinternal structureoftheLAH25-NPdevice.
Figure4.Internal StructureofLAH25-NP
TheLAH25-NP transducerusesadualpower supplyandprovidesthecurrent output frompin"M".The
output(orsecondary)currentisproportionaltotheprimarycurrent andturnsratio.
Closed-loopHallsensorsprovideanexcellent accuracyat25°C(generallybelow1%ofthenominal
range),andareducederroroverthespecifiedtemperaturerange(−40°Cto+105°C).Table1showsthe
accuracydatafortheLAH25-NPtransducer (takenfromthedatasheet).
Table1.Accuracy—DynamicPerformanceData
From LAH25-NP
PARAMETER SPECIFICATION VALUE UNIT
X AccuracyatIPN,TA=25°C ±0.3
%
Ɛ Linearityerror <0.2
L
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4 Component Selection
4.1 Calculation of Burden Resistor for LAH 25-NP
Theburdeniscalculatedaccordingtoproceduregiveninthe“TechnicalManual”fromLEMTechnologies
(4).Themeasuringvoltagegeneratedattheterminalsoftheburdenresistor(V )dependsonthe
M
amplitudeoftheprimarycurrent,I ;theturnsratioofthetransducer,K ;andthemeasuring(or burden)
P N
resistor,R .Thenominalcurrent,I ,determinesthetypeoftransduceranditsturnsratio,K .Thevoltage
M PN N
measuredatagivenprimarypeakcurrent I isdeterminedbythechoiceoftheresistorR .
P M
• Nominal currenttobemeasured:8A
• Peakcurrent →Overloadcondition:225%ofthenominal,that is8 ×2.25=18A
• Ambienttemperature:70°C
• Transducersupplyvoltage:±12V
• ReferringtothedatasheetofLAH25-NP, thePCBconnectionmust beasFigure5 showsinthethird,
encircledrow.Forthiscondition,thevalueofK isequalto3:1000.
N
Figure5.Recommended PCBConnectionsandCorrespondingParameters
4.1.1 CalculationofR for5-V ADCSystem
M
Fora5-VADC,themid-voltagevalueis2.5V.Keepingamarginof0.25V,therangeofinput voltagesfor
a5-VADCis0.25Vto4.75V.Thisrangeindicatesthat thevoltageacrosstheburdenmust be2.25V
whentheprimarycurrentisatitspeakvalue.
ForapositivepeakcurrentthroughtheprimaryoftheHallsensor→2.5V+2.25V=4.75V
ForanegativepeakcurrentthroughtheprimaryoftheHallsensor→2.5V −2.25V=0.25V
ThefollowingEquation2showsthecalculationofR for a5-Vsystem.
M
V
M
R =
M I ´K
P N (2)
UseEquation2forthefollowingvalues:
• V =2.25V
M
• I =18A
P
• K =3:1000
N
• R =41.67Ω
M
TheselectedvalueofR is42.2Ω forthepurposesoftesting.
M
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4.1.2 CalculationofR for3.3-VDifferentialADCSystem
M
Fora3.3-VdifferentialADC,theV =2.5V,sothemid-voltagevalueis1.25V.Keepingamargin of
REF
0.25V,therangeofinputvoltagesfortheADCis0.25Vto2.25V.Thisrangeindicatesthat thevoltage
acrosstheburdenmustbe1Vwhentheprimarycurrent isatitspeakvalue.
ForapositivepeakcurrentthroughtheprimaryoftheHallsensor→1.25V+1V=2.25V
ForanegativepeakcurrentthroughtheprimaryoftheHallsensor→1.25V –1V=0.25V
UseEquation2forthefollowingvalues:
• V =1V
M
• I =18A
P
• K =3:1000
N
• R =18.52Ω
M
TheselectedvalueofR is18.2Ω forthepurposesoftesting.
M
4.2 Selection of Power Supply Components
Thecurrentsensor(LAH25-NP)requiresbipolarsupplyrailsof ±12V,sotheboardisdesigned to be
poweredfrom±15V(max).
Froma±15-Vinput,mainlytwodifferent voltagesarederived:
1. ±15Vto±12VforaHallsensoraswellascomparator.
2. 15Vto 5Vforsupplyingaunipolar-supplyop-amp, onboardvoltagereferencedevice, orDelfino
controlCARD.
Forapplicationsrequiringpositiveaswellasnegativehigh-performancerails(topower precisionsignal
chaincomponents),theTPS7A49XXandTPS7A30XXdevicesaremost suitable.
Witha15-to5-Vgeneration,usingalinearregulatorisnotagoodoption,asitconsumesmore power.A
5-Vrailisusedtopowertheop-amps,whichareimportant elementsofthesignalconditioningprocess.
Therailmustbecleanandfreeofnoise. Duetotheabovereasons,thisdesignallowstheuseone
switchingbuckconverterfollowedbyalinearregulator.
TheTIDA-00368designusesthefollowingcomponents:
• 15-to6-VconversionusingaTPS62150converter
• 6-to5-VconversionusingaLP2992voltageregulator
4.3 Selection of Differential Amplifier
Thefully-differential amplifier(FDA)isacriticalpieceoftheanalogsignalchainandcanoftenhavea
dramaticimpactontheperformanceoftheentiresignalchain. TheprimaryfunctionsofanFDAusedin
thisdesignare:
1. Toprovideamplificationofthelow-levelinput signalscomingfromthesensor
2. Toprovidethedesiredcommon-mode voltageattheoutput
3. ToprovideproperfilteringforthesignalconnectedtotheADC
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Table2providesacomprehensivelistof thefactorsthat must beconsideredwhenselectingthe
operationalamplifier.
Table2.SelectionConsiderationsforFDA
REQUIREMENT BENEFITS
Highinputimpedance Minimizingthisreducesinputloadingonsensor,minimizesinputcurrentoffsetsoninputresistors
Minimizingthisreducestheamountofcurrentnoisethatconvertstovoltagenoiseoninput
Inputcurrentnoise
resistors
Voltagenoise Minimizingthisimprovestheoverallsignal-to-noiseratio(SNR)
Maximizingthisreducestheamountofinputoffsetchangesdueto50-or60-Hzcommonnoise
CMRRversusfrequency
couplingcommonnoisecouplingontheinputs
Resistivegainmatching Maximizingthisimprovesthetotalunadjustedsystemerror
Minimizingthisreducestheamountthatthetotalunadjustederrorchangesattheoutputofthe
Voltageoffsetdrift
amplifier
Designingasinglesupplyamplifiersimplifiesthesystemsupplyrequirements;thisusually
Singlesupplyoperation
correlateswithalowerpowerarchitecture
Lowpower Enablesuseinpower-sensitiveorbatterymonitoringapplications
Inputtype Usingdifferentialinputstructurecanimprovecommon-modenoiserejection
Usingdifferentialoutputstructurecanimprovecommon-modenoiserejectionattheADCinputsas
Outputtype
wellaspotentiallyreduceorrelaxthesignalconditioningcircuitry
ThisreferencedesignusestheTHS4531Adeviceasafront endamplifierfor theADC.TheTHS4531Ais
afullydifferentialop-ampandcanbeusedtoamplifydifferentialinput signalstodifferentialoutputsignals.
Figure6 showsabasicblockdiagramofthecircuit (VOCMandPDinputsarenotshown).Thegainof the
circuitissetbyRFdividedbyRG.
Figure6.DifferentialInputDifferentialOutput Amplifier
Theoutputcommon-modelvoltagefortheTHS4531AdeviceissetbythevoltageattheVOCM pin and
theinternal circuitworkstomaintaintheoutputcommon-modevoltageascloseaspossibletothisvoltage.
Ifleftunconnected,theoutputcommon-mode issettomid-supplybyinternalcircuitry,whichmaybe
overdrivenfromanexternalreferencesource.
4.4 Selection of Voltage Reference
Whenexternal ADCs(whicharepoweredusinga5-Vsinglesupply) areusedfor capturingthedata,the
referencevoltagemustbesetto2.5V.WheninternalADCs(of aDelfinocontroller)areusedin
differentialmode,thereferencepinmust bepoweredfroma2.5-Vreference(Delfinoconverts2.5Vinto a
1.25-Vreference,internally).Tworeferencevoltagesarerequiredtofulfillalltheseconditions(2.5 Vand
1.25V).
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