NASA-TM-107021 19960003317 NASA Technical Memorandum 107021 Low-Temperature Operation of a Buck DC/DC Converter Biswajit Ray Lewis Research Center . . Cleveland, Ohio Scott S. Gerber NYMA, Inc. Brook Park, Ohio Richard L. Patterson and Ira T. Myers Lewis Research Center Cleveland, Ohio Prepared for the Applied Power Electronics Conference and Exposition cosponsored by the IEEE Power Electronics and Industry Applications Societies and the Power Sources Manufacturers Association Dallas, Texas, March 5-9, 1995 7 i i._Li,.__--,- ,-.._,,-. • :t i t;k_.'l i I ! " t - uS1 r, .... -< 1_;59 NationalAeronauticsand SpaceAdministration I r"t_q r-_,r_,,p.......... .-- LI[_N;,!iV i:".S t;,,',i,:,;TOi :,Vir,riir'i..
Transcript
NASA-TM-107021 19960003317
NASA Technical Memorandum 107021
Low-Temperature Operation of a BuckDC/DC Converter
Biswajit RayLewis Research Center
. .
Cleveland, Ohio
Scott S. GerberNYMA, Inc.Brook Park, Ohio
Richard L. Patterson and Ira T. MyersLewis Research CenterCleveland, Ohio
Prepared for theApplied Power Electronics Conference and Expositioncosponsored by the IEEE Power Electronics and Industry Applications Societiesand the Power Sources ManufacturersAssociationDallas, Texas, March 5-9, 1995
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3 1176 01422 9307 F
LOW-TEMPERATURE OPERATION OF A BUCK DC/DC CONVERTER
, BiswajitRay Scott S.Gerber RichardL.Patterson Ira T.MyersNRC/NASA Lewis Nyma, Inc. NASA Lewis Research CenterMS: 301-1 MS: 301-1 MS: 301-2
Abstract: Low-temperature(77"K)operationof a 42128V, 175 additional heat-rejection system is needed to reject theW, 50 kHz PWM Buck de/de converter designed with excess heat when the probe is near the earth orbit sincecommerciallyavailablecomponentsis reported. Overall, the RI-IUsoperate continuously. Electronics that will operateconverter losses decreased at 77"K compared to room reliably in a wide temperature range including the low-temperature operation. A full-load efficiency of 97% was temperature region will certainlyfind applicationsin futurerecordedat llquid-nltrogentemperature,comparedto 95.8%at spaceprobes.room temperature. Power MOSFET operation improvedsignificantlywhereas theoutputrectifieroperationdeterioratedat low-temperature.Theperformanceoftheoutputrdterlnductor The operation of power semiconductordevices atand capacitordid notchangesignificantlyat 77"Kcomparedto low temperatures is expectedto resultin an improvedpowerroom temperatureperformance.It is possibleto achievehigh- handling capability due to increased carrier mobility,densityandhighefficiencypowerconversionatlow-temperatures improvedreliability due to lowerjunction temperatures, anddue to improvedelectronic,electricalandthermalpropertiesof higher power density due to better thermal conductivityofmaterials. " packagingmaterialsand silicon. To investigate the overall
converter was designed, fabricated, and tested at roomLow-temperature signal level electronics in use or temperature (3000K), as well as at liquid nitrogen (LN2)
contemplated, range from single-transistor amplifiers to temperature (77°K). At the component level, the inductorcomputersystems employingmanyVLSI integratedcircuits, losses as well as the switching and conductionbehaviorof aRecent advances in high-temperature (125°K) powerMOSFETand diodewere also studied.superconductor technology have also motivated theinvestigation of power electronic devices, circuits, and PWM Buck DC/DC Convertersystems at low temperatures. Low-temperature electronicswill interface the superconductingelectronics and room- A 42 VY_20%/28V, 175 W, 50 kHz PWM bucktemperature electronics. In general, low-temperature de/de converter was designed and operated at roomelectronics (77°K) will find applications where: (1) the temperature(KT) as well as at liquid nitrogen temperatureambient includes low-temperatures such as in deep-space, (LNT). This type of converter can potentially be used in(2) a low-temperatureenvironmentis alreadypresent, such small scientific/experimentalspacecraftsuch as the proposedas in magnetic resonance imaging systems, and (3) the CLIR (Combined Lander and InstrumentedRover). Theperformance enhancement of existing room-temperature converter circuit is shown in Fig. 2, and it is designed for atechnology is of importance. In the future, low-temperature minimum output power of 35 W and a maximum outputpower electronics (LTPE)will find applicationsin MagLev voltage ripple of 0.5%.transportation,high-power motordrive systems, and power
supplies for high-speed supercomputers. An important Based on the steady-state analysis for continuousaerospace application of LTPE is deep-space exploration, conduction mode of operation [1], the following designAs shown in Fig. 1, the ambient temperatureof a space equationsare used forthe power circuitdesign:probe is on the order of 40°K in deep-space. The electronics F"o(1- Dm_)T,
" will thus have to operate reliably over a wide temperature Lf > (1)range which includes the low-temperature region. - 2Io._a_Currently, radio-isotope heating units (RHU) are used to (1-D=_)V o
• keep electronics warm for deep-spaceprobes. However, an Cf >- 8f_AV ° (2)
where,D.in -- minimum duty-ratio= Vj/_,.,.=, Ion.in= Figs.4-11 andarediscussedin the followingsection. Theminimumoutput (load)current for continuousconduction control circuitryas well as all measuringand sensingmodeofopcration,f_ = switelfingfrequency= I/T,, and AVo instrumentswere at room temperaturewhile the power= peak-to-peakoutputripplevoltage, circuitrywas in the LN2Dewar,resultingin a non-compact '
circuitlayout. The power converterwas able to restart atBased on equation (1), the requiredoutput filter 77_K.
inductorof 100 BHwas designedusing a molypermalloy Discussionof Results •powder(MPP)core. Silver-platedcopperwirewith teflonresintapeinsulationsuitablefor wide-temperatureoperation The recordedcircuitefficiencyis shownin TableI.was used for winding the inductor. The MPP core was Efficiencyimprovedby about 1.5%due to the use of aexpectedto operateat LNT with a somewhatincreasedloss snubberboth atRTas wellas LNTwhilereducingunwanted[2,3]. An output filter capacitance of 60 BF was used high frequency ringing between circuit inductance andinsteadofthe calculatedvalueof 45 IxFfrom equation(2), to MOSFETjunctioncapacitanceas can be seen in Figs. 4-7.providea designmarginfor the outputripplevoltageagainst Moreinterestingly,the circuit efficiencyimprovedby aboutthe drop in capacitorvalue at low temperatures. Standard 1.3%at LN2temperaturewith or withoutthe snubbercircuit.lowESRmetalizedpolypropylenefilm capacitorswere used An efficiencyof97%wasrecordedforLNToperationwithabecauseoftheirsuperiorlow-temperaturecharacteristics, snubbercircuit.
Powersemiconductorselection: Forlow temperatures,the TableI MeasuredEfficiencyprimarysemiconductormaterial is Si, althoughGaAsalso ofPWMBuckDC/DCConverterhas considerablepotential and the primary device is the Room Liquid-nitrogen R-C snubberfield-effect transistor in-various forms [4]. Reduced temperature temperature connectedtemperatureoperationoffers improvementsin performance operation operationthroughimprovementof materials-basedpropertiessuch as 94.2% 95.5% Noelectronic carrier mobility, thermal conductivity, and 95.8% 97.0% Yeselectrical conductivity. Substantial improvements in
reliabilityare also expectedsince degradationmechanisms At LNT,the diodelossalmostdoubledcomparedtoare thermallyactivated. Therefore,LNT operation of Si- RT operationdue to increasedforwardvoltage drop. Thebasedpowersemiconductorsis of greatinterestforachieving reversesaturationcurrentofa p-njunctionis proportionaltohigh efficiencyconverters, the squareofthe intrinsiccarder density,whichdecreasesby
approximately 30 orders of magnitude between RT andForthiswork,an IRFP250 powerMOSFET(33 A, LNT. The large reduction in saturation current is
200V, 85 m.O.,650 pF device)is usedas the primaryswitch accompaniedby an increase in the forward bias voltageand a MUR3020PTultrafastdiode (2"15 A, 200V)as the neededto reach a given level of conductionin the forwardoutputrectifier. Bothdeviceshavea TO-3Pplasticpackage, direction. However,the reverse recoverypeak current and
time of diode rectifier improved significantly at low-ExperimentalProcedure temperature.
The experimentalsetup is shownin Fig. 3. The The conductionloss of the power MOSFETcontrol circuitrywas kept at room temperaturewhile the decreasedsignificantlyat LNTdue to reductionin drain-to-power circuitrywas placed in a Dewar flask. Data were sourceresistance. Theprimaryreasonfor this improvementrecordedfor full-load (175 W of output power)operation is the increased mobility of carriers due to reducedboth at room and LN2 temperatureusing the same circuit scattering. Carrier freezeout is not a problem for thelayout. For LNT data, the Dewarflask wasfirst filledwith enhancement mode MOSFETs since the source-drainLN2 and thenthe power circuitboardwas dippedwhile the regionsare heavilydoped. The turn-on loss of the switchconverterwas powered on. The converterwas operated (0.5CV2)also reducedat LNTdue to decreasedvalueof the ,continuouslyfor one hour beforerecordingeach set of data. drain-to-sourcejunction capacitance[5] as can be seen inAn R-C snubberacross the powerMOSFETwas used to Figs. 8 and9. There is a significantturn-offloss (0.5LI2)reducethe switchingtransients. The snubberconsistedof a dueto resonancebetweenthe circuit layoutinductanceand ,50_ resistorin serieswith a 0.0021xFcapacitor.Datawere MOSFETjunctioncapacitanceas can be seenin Figs.4 andrecordedwithandwithoutthe snubberto studythe switching 5. However,LN2operationdid reducethe turn-offloss aperformance of the semiconductor devices at low little. As canbe seenin Figs. 10 and 11, the frequencyoftemperature.Someof the recordedwaveformsare shownin oscillationat turn-offincreasedat LNTbecauseof slightly
2
reducedvalueof MOSFETjunctioncapacitance.This BatureWorksituation can be improved significantlyby having a compactcircuitlayout which was not possible in this casebecausethe In this work, only the power components werepower circuit was dipped in LN: whereas the control dipped in LN2, whereas the control circuit was at roomcircuitry was at room temperature. Also, all the temperature. However, performanceof the convertercan bemeasurementswere taken outsidethe Dewar. furtherimprovedffthe control circuit canalso be operatedin
, LN2,making it possible to design compact powerconverters.The best way to attack this problem is to use a Currently, the design of power control circuits using
resonant(soft) switching technique instead ofa PWM (hard) commercially available CMOS and BiCMOS integratedswitching topology. Such a circuit employing zero-voltage circuits is being carried out. The operation of integratedswitching forboth switches is shown in Fig. 12. Ultra-high circuits at cryogenic temperatures will result in increasedefficiencycan be achieved from this type of circuit at LNT speed, reduced latch-up susceptibility, reduced leakagebecausethe switching loss will be practicallyeliminated, and current, and reduced thermal noise. The variation ofthe conduction loss, which is significant at room switching frequency and its effect on the power convertertemperature,is reduced dramatically due to very low drain- performance will have to be investigated.m-sourceon-resistanceatLNT [6,7].
ConclusionsThe conductionloss of MOSFETdecreaseswhereas
that of the diode rectifier increases at low-temperature, This workdemonstratesthat it is possible to designmaking synchronousrectificationhighly attractivefor low- and operate power convertersefficiently at low-temperaturestemperaturepower converters as shown in Fig. 13. Another with commerciallyavailable components. The PWM buckinteresting finding was that-the power MOSFETs rise and converter efficiency improved by 1.2% at liquid-nitrogenfall times improved marginally at LNT, unlike in signal temperature compared to room temperature operation.MOSFETs,where the speedof operation improvedby almost Performanceof power MOSFETimproves significantlyanda factor of two [4]. This finding supports the results that of rectifier detoriates at low temperatures, makingreported in[8], synchronous rectification highly attractive. Passive
components loss did not change significantly withThe MPP inductor loss was expected to increase decreasing temperatures. Ultra-high efficiency power
slightly at LNT due to increased fiux-demity and decreased conversion is possible using resonant circuits at low-core resistivity [9]. However, measurements indicate that temperatures. Research must continue to exploit the low-the loss practically remained unchanged. Further study of temperature characteristics of signal and power level activelow-temperature magnetics is needed to understand the and passive components in improving the overall powertemperature dependence of hysteresis and eddy current systemporformance.losses, and other magnetic properties. The output filtercapacitor used is film type and its value is expected to drop Referencesslightly atLNT [6],however, it was not studied in this work.The increased thermal conductivity of the capacitor body t.N. Mohan, T. M. Undeland, and W. P. Robbius, "Power electronics:
will in effect increase its current capability as will the eonverte_applieationsanddesigrt,'pp. 66-74, JolmWiley, 1989.2. "rape woundcores - designmanual,"TWC-400, Magnetic,Inc., 1992.voltage capability of the insulating film. 3. F. W. Aekemaann,eLal. "Magneticpropertiesofcommerdal softmagnetic
alloys at cryogenictemperatures,"Adv. CryogenicEngrg., Vol. 16, pp.46-50,
In terms of reliability, many degrading mechanisms PlenumPress,1971.4. ILK. Kirsdmaan,"Cold electronics:an overview,"Cryogenics,Vol.25, No.are thermally activated and they follow an Arrhenius-like 3,pp.l15-122,Mar.1985.equation for the mean-time-between-failure(MTBP) or s.R. Singhand B. J. Baliga, "Analysis and optimizationof power MOSFETs
for eryogeniooperation," Solid Stale Electronic, Vol. 36, No. 8, pp. 1203-lifetime given by, 1211,1993.MTBF oce E+,/_r (3) _.P_m_.a_ and_. Scv_, "D_ig_g _h_d-mod_ power,_,.v_,+._where, Ea is an activation energy for electromigration, forveryiow_ operation,"Prec. Powercon 10, pp. I)-2.1-I)-2.11,• 1983.interdiffusion, chemical reaction, or corrosionand is in the 7. o. M. Mueller and K. G. Herd, 'qdltra-high efficiencypower conversionrange of 0.4 to 1.0 eV for these processes, k is Boltzmarm's usingcryogenicMOSFETsand HT-superconductors,"IEEE PESCrec., pp.
constant, and T is temperature in °K. Therefore, as the 772-77g,june1993.• temperatureis reduced,thereis an exponentialincreasein s.j. L Hudglns, S. Menhart,and W. M. Portnoy, "The low tempexalm_switchingperformanceof thyristorsand MOSFETs,"IEEE PESC Rec., pp.
lifetime. However, this theoreticaljustificationhas not been 429-434,Junet990.yet supported experimentally. Thermal cycling is another 9. J. J.c,nlew_ andP..t. Powen,"r_ depe,denceof magnetimportantunexploredarea. losses,,, Adv. Cryogenic Engrg., Vol. 7, pp. 303-310, Plenum Press, 1962.
51_s/div
1k 50V/div _ _
- r -_ I r!
500 W/div L
Pswitch L
Fig. 1 Solar intensity and space probe temperature.Fig. 4 Switch waveforms at roomtemperature.
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I August 1995 Technical Memorandum4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
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Low-TemperatureOperationof a BuckDC/DCConverter
s. AUTNOR(S) WU-233-02--0C •
BiswajitRay, Scott S. Gerber, Richard L. Patterson, and Ira T. Myers
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATIONREPORT NUMBER
National Aeronautics and SpaceAdministrationLewisResearchCenter E-9343-1Cleveland,Ohio 44135-3191
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORINGAGENCY REPORT NUMBER
Preparedfor the Applied Power Electronics Conference and Expositioncosponsored by the IEEE Power Electronics and IndustryApplicationsSocieties and the PowerSourcesManufacturers Association, Dallas,Texas, March 5-9, 1995. Biswajit Ray,NationalResearchCouncil--NASA ResearchAssociate at Lewis Research Center;,Scott S. Gerber,NYMA, Inc., 2001Aerospace Parkway,Brook Park, Ohio 44142; Richard L. Patterson and Ira T. Myers, NASA Lewis Research Center. Responsible person, Richard L.Patterson, organization code 5430, (216) 433-8166.
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SubjectCategories20and33This publication is available from the NASA Center for Aerospace Information, (301) 621--0390.
13. ABSTRACT (Maximum 200 words)
Low-temperature (77°K) operation of a 42/28V, 175 W,50 kHz PWM Buck dc/dc converter designed with commerciallyavailablecomponentsis reported.Overall,theconverterlossesdecreasedat 77°Kcomparedto roomtemperatureopera-tion.Afull-loadefficiencyof 97%wasrecordedat liquid-nitrogentemperature,comparedto 95.8%atroomtemperature.Power MOSFET operation improved significantly where as the output rectifier operation deteriorated at low-temperature.The performance of the output filter inductor and capacitor did not change significantlyat 77°K compared to roomtemperatureperformance.It ispossibleto achievehigh-densityandhighefficiencypowerconversionatlow-temperaturesdue to improved electronic, electrical and thermal properties of materials.
