77 K Operation of a Multi-resonant Power Converter

7/29/2019 77 K Operation of a Multi-resonant Power Converter

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77 K OPERATION OF A M ULT I-RESONANT POWER CONVERT ER

BiswajitRay' Scott S. GerberNRC/NASA Lewis Nyma, nc.21000Brookpark RoadCleveland, OH 44135MS: 301-1 MS: 301-12001Aerospace ParkwayBrookpark, OH 44142

Abstract: The liquid-nitrogen temperature (77 I C) operation ofa55W, 200 kHz, 48/28V zero-voltage switching multi-resonantdddc converter designed with commercially availablecomponents is reported Upon dipping the complete converter(power and control circuits) into liquid-nitrogen, the converterperformance improved as compared to the room-temperatureoperation. The switching fr equency, resonant frequency, andthe characteristic impedance did not change significantly.Accordingly, the zero-voltage switching was maintained fromno-load to full-load for the specified line variations. Cry&electronics can provide high density power converters,especially for high power applications.

IntroductionA sigruficant amount of work has been done onlow-temperature microelectronics including supercomputer,and microwave receivers for radio astronomy and deep-spacecommunication [11. However, recent advances in hightemperature ceramic superconductors, with a transition

temperature as high as -125 K, has generated great interestin low-temperature power electronics that will findapplications in diverse fields such as deep-space explorationprograms, cryogenic instrumentation, magnetic resonanceimaging systems, super-conducting magnetic energy storagesystems, high power motors and generators, and switchingpower conversion systems. The operation of powerelectronics at low temperatures is expected to result in amore efficient, dense, and reliable system compared to roomtemperature operation due to better thermal, electrical, andelectronic properties of materials at low temperatures.Performance improvement of several bipolar and FET powersemiconductor devices at low temperatures has been studiedin [2,3].

The objective of this work was to study thefeasibility of operating a dc/dc power converter in a widetemperature range from300K to77K. Potential applicationof this wide temperature operation includes future deep-space missions. For thispurpose, a 48 Vk20%/28 V, 55 W,200 kHz zero-voltage switching, multi-resonant buck dc/dc

RichardL.Patterson Ira T . MyersNASA LewisResearch Center21000Brookpark RoadCleveland, OH 44135MS: 01-5

converter is designed and tested at room temperature (300K )as well as liquid nitrogen temperature (77K). Experimentalresults indicate that it is possible to successfully operateresonant converters at 77 K designed with standardcommercially available devices and components. Moreover,efficiency of the power converter did improve slightly at 77K as compared to300K operation.

Zero-V oltage Switching Multi-Resonant DC/DCConverterThe circuit diagram of the constant frequency, zero-voltage switching, multi-resonant (ZVS-M R) buck dc/dcconverter is shown in Fig. 1. The multi-resonant switchingconfiguration I4-61 absorbs all major circuit parasiticsincluding junction capacitance of all semiconductor devicesand leakage inductance of the transformer winding fortransformer isolated topologies. Al so, all switchingtransitions occur at zero-voltage while full dc conversionratio range is achievable from no-load to full-load keepingthe switch current and voltage stresses atamoderate level.The constant frequency ZVS-MR topology is also bi-directional in current and it is inherently protected againstshort circuit at load terminals under certain conditions [5 ,6 ] .The converter is regulated against the line and loadvariations by controlling the duration between turn-offinstants of SI and Szr where as their tum-on instants arecircuit dependent. The ZVS-MR converter has multiplemodes of operation [6], and the specific operating modedepends on the converter line and load conditions. Eachmode consists of the following four topological stages: SI

and S2 on, S on and S2 off, S off and S2 on, and SI and SZoff. The detailed analysis of the converter operation isdiscussed in [ 5 , 6 ] . The theoretical waveforms for the fourmodes of operation are shown in Fig. 2, and the dcconversion ratio ( m=Vfl, , .) versus normalized load current(IoN=IOZdV,,where Z,, == is shown in Fig. 3 fordifferent values of control duty ratio (0,= ). Figures- tr,

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Rico-MayaguezCampus.Thi s work was performed while the first author held a National Research Council-NA SA Lewis Research Associateship, on leavefrom LheUniversityofPuerto

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2 and 3 are taken from [6] tobeused here indesigning thepower converter. Both figures are for a normalizedswitching frequency (& = switching frequency/resonantfrequency) of 0.9. The characteristic impedance of theresonant circuit is given by:

Power c ircui t desien: For the55 W, 200 kHz, 48Vf20%/28V converter, m = 0.73 and m,,,, = 0.49. From Fig. 3,ION,- is 1.24 for a normalized switching frequency of 0.9.The corresponding range of control duty ratio is 0 to 0.25.Using (l), the characteristic impedance is 24.24R. Finally,knowing fN to be 0.9, the resonant frequency is 222.2 l rHzand the resonant components are Lo = 17.4 pH and CO=29.55nF.

Experimental R esults and DiscussionThe complete converter circuit is shown in Fig. 4.The resonant and filter inductors used molypermalloypowder (MPP) cores. The input and output fil ter capacitorsare film type and the resonant capacitors are mica andpolypropylene types. Two plastic packaged IRF 640MOSFETs (18 A, 200 V, 0.18R, 430 pF) are used as powersemiconductor switches. The open-loop control circuit usesCMOS timer and driver ICs. The power as well as thecontrol circuits were placed in a LN, Dewar flask. Also, toobserve various current and voltage waveforms usinginstrumentation at room temperature resulted in a circuitlayout with long wires. Data were recorded both at roomtemperature (RT) and liquid-nitrogen temperature (LNT).The converter was run at full-load for one hour beforerecording any data either at RT orLNT. For LNT operation,the circuit was first run at RT and then dipped into theDewar flask while the power was on. In addition theconverter was kept in liquid nitrogen for an extended periodof time without power and then successfully restarted at 77

K.Majority carrier devices should exhibit improvedspeed at temperatures down to 77 K because of increased

carrier mobility due to decreased carrier scattering, increasedsaturation velocity, reduced junction capacitance andreduced line resistance. For the control circuit design, onlythe CM OS ICs are used because they are expected to givebetter overall performance in terms of higher speed and moredense electronic packages and because their performanceimproves with decreasingtemperatures [7]. The signal delay(rD)n going from the input of one device to that of the nextis composed of [8] the internal device switching delay (7w),the time it takes to charge and discharge ( q , )he total loadcapacitance (CL) ncluding the wiring capacitance, and the

time it takes to charge and discharge (Q) the junctioncapacitances (Cd. Thus,7D=r, +rL+7,where.

D,td

r,=- for E ; E,,PVDLvasr, =- for E2E,

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In the above equations, V& and VDare the supply and drainvoltages, ID, , EsQt,nd vmtare the saturation drain current,saturation channel electric field, and saturation channelcarrier velocity, and L and p are the channel length andchannel carrier mobility, respectively.

Timing parameters for the control circuit wererecorded when driving a 1000 pF load. The switchingfrequency changed by only 1%, from 200 kHz at 300 K to202 kHz at 77 K. Thi s slight increase in frequency can beattributed to the decreased value of the timing capacitor andfaster switching of the senliconductor devices. The rise andfall time of the drive signals decreased only slightly at 77K.However, the rise and fall delay time of the driver ICimproved sigmfkantly. Fig. 5 shows the improvement inrise delay time, from 170 nS at 300 K to 106 nS at 77 K, forthe high side driver. The measurements do venfy theconcept expressed in equations2-5.

The measured full-load efficiencies of the converterat RT and LNT are 93.9% and 94.8%, respectively. Theefficiency &d improve as expected although by only 0.9%.The circuit loss is 3.64 W and 3.2 W at RT and L NToperation, respectively. Relevant converter waveforms forfull-load operabon are shown in Figs. 6and 7. As can beseen, the waveforms at RT and LNT are almost identical.The multi-resonant converter topology practically eliminatesswitching loss; however, the conduction loss is still verymuch present. The conduction loss of the power MOSFETdecreases sigruficantly at L NT due to reduction in drain-to-source resistance caused by increased electron mobility.However, this 1mprovement is greatly offset by increasedconduction loss of the MOSFET bodydiode. At LNT, themeasured forward voltage drop of the bodydiode is almostdouble the RT value.

For zero-voltage turn-on, the body-&ode of thepower MOSFE'T is forced to conduct before the device57


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Fig.6 Converter waveforms at m mtemperature under full-load condition.( Vh 48.3 V, 4 *1.24 A, V 28.6 V, =1.965A)

Fig. 8 Converter waveforms at m m emperatureunder short circuit condition.( Vh = 48 V, I, 0.095 A, I 2.21 A)

Fig. 7 Converter waveforms at liquid nitrogentemperature under full-load condition.( Vh=48.3 V, 4 =1.2654 V =28.98 V, I. =1.99A)

F iS 9 Converter waveforms at liquid nitrogentemperature under short circuit condition.(V, =48 V, 4, =0.091 A, I =2.27 A)

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conducts in the forward direction. Accordingly, the totalconduction loss in the switch is composed of the MOSFETconduction loss and the bodydiode conduction loss. Asdiscussed in the previous paragraph, the MOSFETconduction loss decreases with decreasing temperatureswhere as the bodydiode conduction loss increases. Theimprovement in switch conduction loss can be sigmficant ifthe MOSFET is used only in the forward conduction modewithout using its bodydiode. This leads to the fact that thezero-voltage switching may not be the optimal resonanttopology if the improvement in switch conduction lossat lowtemperatures is critical. However, the half-wave zero-current switching resonant topology [9] could be used toexploit the low temperature characteristic of MOSFETswithout using the bodydiode. The zero-voltage switching ispreferred over zerocurrent switching at RT because of lowerconduction loss and lower peak current stress. If zero-current switching resonant topology is used for lowtemperature operation, neither the conduction loss nor thepeak current stress is a problem. At low temperatures,conduction loss reduces sigmficantly due to increased carriermobility, and also a given device can cany much highercurrent compared to RT due to increased mobility andincreased substrate and metal thermal conductivities.

In terms of passive components, the measured MPPinductor loss at LNT remained practically the same as that atRT. The eddy-current loss is expected to increase at lowtemperatures due to decreased core resistivity [lo] and thewinding loss is expected to decrease due to decreasedresistance The film and mica capacitors are usable at lowtemperatures as they show avery small drop in capacitanceand are relatively stable [111. The overall low-temperatureeffect of inductors and capacitors on th e converterperformance was found to be relatively small . The resonantfrequency and the characteristic impedance of the converterremained practically unchanged at LNT, and accordingly didnot alter the available load rangeand zero-voltage switchingwas maintained for both switches from no-load to full-load.This was possible to achieve because the switchmg frequencyof the converter increased by only I%, from 200 kHz at 300K to 202 kHz at 77K . This change in switching frequencyis highly dependent on the manufacturer and type of CMOSICs used in designing the control circuit. In general, theswitching frequency may vary unpredictably and therefore,constant frequency resonant topologes may not be able toachieve resonant switching under full load and linevariations. However, the variable frequency resonanttopologies might beable to achieve resonant switching evenat low temperatures.

The ZVS-MR converter is capable of sustaining ashort circuit at the load terminals for a limited range ofcontrol duty ratio as can be seen in Fig. 3. This

characteristic isexperimentally verifiedbothatRT and LNT,and the corresponding experimental waveforms are shown inFigs. 8and9, respectively. For higher values of control dutyratio, the converter can sustain a short circuit condition.However, Si no longer switches at zero-voltage andaccordingly the switching lossincreases sigruficantly.

ConclusionsIt 1s possible to design and operate power electronicconverters at low temperatures using commercially availablecomponents. Overall , the converter performed satishctorilyboth atRT and LNT. Slight improvement in perf ormancewas observed at LNT with no visible degradation. Theconverter restarted successfully at 77 K . The thermalcycling and reliability issues were not addressed in thi swork. However, for future space or industrial application of

low temperature electronics, the thermally activatedelectronic, electrical, and mechanical issues must beinvestigated.References

1. R. K. Kirschman, Zow-temperature semiconductorelectronics,Roc. Symp. Low Temperature Electronic DeviceOperation, Vol. 91-14, pp. 1-12, The Electrochemical Society,1991.2. J . L. Hudgins, S. Menhart, and W. M. Portnoy, The lowtemperature switching performance of thyristors and MOSFETs,IEEE-PESC Rec., pp. 429434, June 1990.3. 0.M. Mueller and K. G. Herd, Ultra-high efficiency powerconversion using cryogenic MOSFETs and HT-superconductors,IEEE-PESC Rec., pp. 772-778, June 1993.4. W. A. Ta l k , M. M. Jovanovic, and F. C. Lee, High-frequencymulti-resonant converter technology and its applications, IEECod., July 1990.5. D Maksimovic and S. Cuk, Constant frequency control ofquasi-resonantconverters, IEEE Trans. PowerElectronics, Vol.6. Z. R. Martinez and B. Ray, Bidirectional dc/dc powerconversion using constant frequency multi-resonant topology,IEEE-APEC Rec.,pp. 991-997, Feb. 1994.7. M. J. Deen, Low temperature microelectronics: opportunitiesand challenges,Proc. Symp. L o w Temperature Electronic DeviceOperation, Vol. 91-14, pp. 25-38, The Electrochemical Society,1991.8. S.M. Szt:, Physicsof semiconductor devices, page454,2nd.ed.,Wiley-Interscience, 1981.9. F. C. Lee, High-frequency quasi-resonant and multi-resonantconverter technologies, Proc. IEEE Int. Cod. Ind. Electronics, pp.10. F.W. Ackermann andW A. Klawitter,Magnetic propertiesof commercial soft magnetic alloys at cryogenic temperatures,Adv. Cryogenic Engr., Vol. 16,pp. 46-50,Plenum Press, 1971.11. R. Blanchard andR. Severns, Designing switch-mode powerconvertersfor very low temperatureoperation, Proc.Powercon 10,pp,D-2.1-D-2.11,1983.

6,No.1,pp. 141-150, J an. 1991.

509-521, Oct. 1988.

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77 K Operation of a Multi-resonant Power Converter

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