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  • Switching Monitoring High Voltage DC Power Supplies up to … · 2018. 3. 20. · M140, EN Switching & Monitoring High Voltage DC Power Supplies up to 1000V By Pinkesh Sachdev Product
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M140, EN Switching & Monitoring High Voltage DC Power Supplies up to 1000V By Pinkesh Sachdev Product Marketing Engineer, Mixed Signal Products Linear Technology Corp. Introduction DC power supplies in the hundreds of volts are not as uncommon as one might think. An application that may first come to mind is electric vehicles where Li-Ion battery stack voltages range up to 400V. But some lesser known high voltage applications are in modern fighter aircraft, such as the F-22 Raptor and the F-35 Lightning II, which are primarily powered from 270V DC for faster and precise performance. Large solar arrays can output 600V or higher, while rectifying AC voltages in industrial motor drives yield DC voltages ranging from 170V to 680V. And for many years, there has been research and development to move power distribution within a data center from AC to high voltage DC (380V or ±190V), lowering power conversion steps, facility footprint and operational costs while easing integration with renewable energy such as solar. Distributing power at higher voltages lowers current levels, reducing resistive losses (I 2 •R), which can be used to reduce cabling weight. All of these high voltage supplies need to be switched on or off and soft-started into loads. For energy monitoring and optimization, it is essential to digitally monitor the voltage and current flowing on these high voltage buses. Any circuit controlling these supplies needs to be galvanically isolated for operator safety and to protect the low voltage electronics from the dangerously high voltage. Methods to Control Inrush Current & Monitor Power When designing high voltage supplies, an important goal is to safely control the startup inrush current into the capacitive load, e.g., the DC bus capacitors following a typical bridge rectifier. A simple method to lower inrush current is by using negative temperature coefficient (NTC) thermistors, also known as inrush current limiters (ICL, Figure 1a). These thermistors start with a high resistance (e.g., few ohms) at room temperature before the supply or load is turned on; the high resistance limits the inrush current at turn-on. As current flows, the thermistor heats up and its resistance drops by one to two orders of magnitude (by 10x–100x to below an ohm). These thermistors cost anywhere from $0.13 to $7 each, depending on current and resistance ratings. While simple to use, one problem is that a quick power cycling (on-off-on) may not limit inrush on the second power-up if the thermistor didn’t have sufficient time to cool down to the high resistance state. NTC thermistors suffer from wide tolerances (±25%), and since the inrush current is tied to the steady state current through the resistance drop ratio, the inrush cannot be flexibly adjusted to arbitrarily low levels. ICLs find applications in vacuum cleaners, fluorescent lamps, and switched-mode power supplies, reducing inrush to the bridge rectifier’s DC bus capacitors.

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Page 1: Switching Monitoring High Voltage DC Power Supplies up to … · 2018. 3. 20. · M140, EN Switching & Monitoring High Voltage DC Power Supplies up to 1000V By Pinkesh Sachdev Product

M140,EN

Switching&MonitoringHighVoltage

DCPowerSuppliesupto1000V

ByPinkeshSachdevProductMarketingEngineer,MixedSignalProducts

LinearTechnologyCorp. IntroductionDCpowersuppliesinthehundredsofvoltsarenotasuncommonasonemightthink.AnapplicationthatmayfirstcometomindiselectricvehicleswhereLi-Ionbatterystackvoltagesrangeupto400V.Butsomelesserknownhighvoltageapplicationsareinmodernfighteraircraft,suchastheF-22RaptorandtheF-35LightningII,whichareprimarilypoweredfrom270VDCforfasterandpreciseperformance.Largesolararrayscanoutput600Vorhigher,whilerectifyingACvoltagesinindustrialmotordrivesyieldDCvoltagesrangingfrom170Vto680V.Andformanyyears,therehasbeenresearchanddevelopmenttomovepowerdistributionwithinadatacenterfromACtohighvoltageDC(380Vor±190V),loweringpowerconversionsteps,facilityfootprintandoperationalcostswhileeasingintegrationwithrenewableenergysuchassolar.Distributingpowerathighervoltageslowerscurrentlevels,reducingresistivelosses(I2•R),whichcanbeusedtoreducecablingweight.Allofthesehighvoltagesuppliesneedtobeswitchedonoroffandsoft-startedintoloads.Forenergymonitoringandoptimization,itisessentialtodigitallymonitorthevoltageandcurrentflowingonthesehighvoltagebuses.Anycircuitcontrollingthesesuppliesneedstobegalvanicallyisolatedforoperatorsafetyandtoprotectthelowvoltageelectronicsfromthedangerouslyhighvoltage.MethodstoControlInrushCurrent&MonitorPowerWhendesigninghighvoltagesupplies,animportantgoalistosafelycontrolthestartupinrushcurrentintothecapacitiveload,e.g.,theDCbuscapacitorsfollowingatypicalbridgerectifier.Asimplemethodtolowerinrushcurrentisbyusingnegativetemperaturecoefficient(NTC)thermistors,alsoknownasinrushcurrentlimiters(ICL,Figure1a).Thesethermistorsstartwithahighresistance(e.g.,fewohms)atroomtemperaturebeforethesupplyorloadisturnedon;thehighresistancelimitstheinrushcurrentatturn-on.Ascurrentflows,thethermistorheatsupanditsresistancedropsbyonetotwoordersofmagnitude(by10x–100xtobelowanohm).Thesethermistorscostanywherefrom$0.13to$7each,dependingoncurrentandresistanceratings.Whilesimpletouse,oneproblemisthataquickpowercycling(on-off-on)maynotlimitinrushonthesecondpower-upifthethermistordidn’thavesufficienttimetocooldowntothehighresistancestate.NTCthermistorssufferfromwidetolerances(±25%),andsincetheinrushcurrentistiedtothesteadystatecurrentthroughtheresistancedropratio,theinrushcannotbeflexiblyadjustedtoarbitrarilylowlevels.ICLsfindapplicationsinvacuumcleaners,fluorescentlamps,andswitched-modepowersupplies,reducinginrushtothebridgerectifier’sDCbuscapacitors.

Page 2: Switching Monitoring High Voltage DC Power Supplies up to … · 2018. 3. 20. · M140, EN Switching & Monitoring High Voltage DC Power Supplies up to 1000V By Pinkesh Sachdev Product

ToovercometheNTCthermistordownsideofnoinrushlimitingonaquickrestart,ashortingrelayisusedinparallelwiththeresistor.Thisisknownasastepstartrelay(Figure1b).Atturn-on,theparallelrelayisopenandinrushislimitedbytheresistor.Atimerisalsostarted;whenitexpires,therelayisshortedacrosstheresistor.Loadcurrentnowflowsthroughtherelay.Onaquickrestart,thestep-startrelayisabletoprovideinrushlimiting.Thistechniquerequirestheadditionofashortingrelayandatimertocontrolitsturnon.Theincreasedcomplexityraisesthesolutioncostintothe$20–$30range.

Otherinrushcurrentcontroltechniquesincludezerocrossingtriacs,activepowerfactorcontrolcircuits,andinductiveinputfilteringwithdamping.Mostofthesearecomplicated,bulky,expensive,andapplicableonlytoACinputs.Onemethodforisolatedcurrentmonitoringisbyusinganisolationamplifieracrossacurrentsenseresistorandadifferential-to-singleendedconversionamplifierfeedinganADC.Anothermethodistouseanisolateddelta-sigma(ΔΣ)modulatorwithanexternaldigitalfilter.Asseen,controlling,protecting,andmonitoringhighvoltageDCsuppliesrequirescobblingmanycomponentstogetherandmakingthemworksafelyandseamlessly.Thisisnotatrivialtask.Thesediscretesolutionsarelarge,component-intensive,expensive,andlacksafetycertifications.Anintegratedandcertifiedsolutionisneededtoshortendesigntimeandcertificationeffortfrommanymonthsintoafewweeks. IntegratedSolutionforHighVoltagePowerControl&TelemetryTheLTM9100µModule®(micromodule)ICisacompactall-in-onesolutionforcontrolling,protecting,andmonitoringhighvoltagepowersuppliesupto1000VDC(Figure2a).A5kVRMSisolationbarrierseparatesthelogicanddigitalinterfacefromtheswitchcontrollerdrivinganexternalN-channelMOSFETorIGBTswitch.Theisolationisneededforcontrolcircuitprotection,operatorsafety,andbreakinggroundpaths.Theloadissoft-started(Figure2b)andthesupplyisprotectedfromoverloadwithacurrent-limitedcircuitbreaker.Isolated10-bitADCmeasurementsofloadcurrentandtwovoltageinputsareaccessedviatheI2C/SMBusinterface,enablingpower,energyandthermalmonitoringofthehighvoltagebus.

TheLTM9100utilizesLinearTechnology’sisolatorμModuletechnology(Figure3)totranslatesignalsandpoweracrossanisolationbarrier.SignalsareencodedintopulsesandpassedacrosstheisolationboundaryusingcorelesstransformersformedintheµModulesubstrate,resultinginanextremelyrobustbidirectionalcommunicationscheme.Uninterruptedcommunicationisguaranteedforcommonmodetransientsof50kV/µs.TheisolatedsideispoweredbyafullyintegratedDC/DCconverter,includingthetransformer,eliminatingtheneedforexternalcomponents.Toguaranteearobustisolationbarrier,eachLTM9100controllerisproduction-testedto6kVRMS.TheLTM9100willberecognizedbytheUL1577standard,savingmonthsofcertificationtimefortheendequipmentmanufacturer.Highdistancethrough-insulationtranslatestoahigh±20kVESDlevelacrossthebarrier.TheμModulepackageintegratesseveralcomponentsandtechnologiestodeliveracost-effective,advancedsolutionthatminimizesboardspaceandimproveselectricalandthermalperformance.

Duetoitsisolatednature,theLTM9100iseasilyconfiguredforhighside,lowside(groundreturn)andfloatingapplications(Figure4).TheLTM9100isversatileenoughtocontrol

Page 3: Switching Monitoring High Voltage DC Power Supplies up to … · 2018. 3. 20. · M140, EN Switching & Monitoring High Voltage DC Power Supplies up to 1000V By Pinkesh Sachdev Product

inrushcurrentnotjustinhot-swappablecards,butalsoinACtransformers,motordrives,andinductiveloads.Adjustableundervoltageandovervoltagelockoutthresholdsensurethattheloadoperatesonlywhentheinputsupplyisinitsvalidrange.The22mmx9mmx5.16mmBGApackageprovides14.6mmofcreepagedistancebetweenthelogicsideandtheisolatedside. ConclusionTraditionally,highvoltageDCsuppliesarefoundinindustrialenvironments,butotherelectronicsystemsarealsomovingtowardshighervoltagestolowercostsandraisetheefficiencyofpowerdistribution,especiallyinpower-hungrysystems.Thesesuppliesneedasimplewaytocontrolinrushcurrent,protectthemselvesandtomonitortheirusage.Thefirst-of-its-kindLTM9100deliversthissimpleandcompactsolution,beatingdiscreteandrelaybasedcircuitsbyprovidingacertifiedready-to-gosolution,savingboardareaandmultiplemonthsofdesigntimeandcertificationeffort.Alltheneededfunctionality,includingdigitaltelemetryandisolatedpower,iswrappedinacompact,surfacemount,andlow-profileBGApackage. FIGURES

Figure1.InrushControlLimiters.(a)NegativeTemperatureCoefficient(NTC)Thermistor(b)StepStartRelay

Figure2.(a)LTM9100AnysideTMHighVoltageIsolatedSwitchControllerwithTelemetry(b)LTM9100soft-starting,a270VLoadwithControlled200mAInrushCurrent

Page 4: Switching Monitoring High Voltage DC Power Supplies up to … · 2018. 3. 20. · M140, EN Switching & Monitoring High Voltage DC Power Supplies up to 1000V By Pinkesh Sachdev Product

Figure3.InsidePeekatLinearTechnology’sIsolatorµModule®Technology

Figure4.LTM9100inaHighSideorLowSideSwitchingApplication


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