10 IEEE Power Electronics Society NEWSLETTER Fourth Quarter 2004
INTRODUCTIONRecent breakthroughs in Silicon Carbide(SiC) material and fabrication technologyhave led to the development of High-Voltage, High-Frequency (HV-HF) powerdevices with 10-kV, 15-kHz power switchingcapability. Programs are underway todemonstrate half-bridge modules with 15-kV, 110-A, 20-kHz capability in the next fewyears. The emergence of HV-HF deviceswith such capability is expected to revolu-tionize utility and military power distributionand conversion by extending the use ofPulse Width Modulation (PWM) technologyto high voltage applications.
Wide bandgap semiconductors such asSiC have long been envisioned as the mate-rial of choice for next generation powerdevices [1]. Although wide bandgap semi-conductor materials have superior proper-ties, the realization of power device qualitysubstrates and fabrication technologiesrequired overcoming many technical chal-lenges. The rapid advances in single crystalSiC over the last decade have ushered in anew era of wide bandgap power semicon-ductor devices. In 2004, Dr. Calvin Carter ofCree Inc. received the US National Medal ofTechnology from President George W. Bushfor: “his exceptional contributions to thedevelopment of Silicon Carbide wafers,leading to new industries in wide bandgapsemiconductors and enabling other newindustries in … more efficient/compactpower supplies, and higher efficiencypower distribution/transmission systems.”
Currently, there are significant effortsunderway to accelerate the development andapplication insertion of the new HV-HF SiCdevices needed for commercial and militarypower conversion and distribution applica-tions. The goal of the ongoing DefenseAdvanced Research Projects Agency(DARPA) Wide Bandgap SemiconductorTechnology (WBST) High Power Electronics(HPE) program directed by Dr. John Zolperis to develop 15-kV class power semicon-ductor devices enabling future electric ship,more electric aircraft, and all electric combatvehicles. DARPA is particularly interested indeveloping the power electronics devicetechnology deemed necessary to enable 2.7MVA Solid State Power Substations (SSPS) forfuture Navy warships.
The benefits of HV-HF semiconductortechnology have also been identified by the
Electric Power Research Institute (EPRI)including advanced distribution automationusing solid-state distribution transformerswith significant new functional capabilitiesand power quality enhancements. In addi-tion, HV-HF power devices are an enablingtechnology for alternative energy sourcesand storage systems. The emergence of HV-HF power devices presents unique opportu-nities and challenges to the power electron-ics industry in specifying the device require-ments and establishing PWM convertertopologies for high voltage applications.
HIGH VOLTAGE POWER CON-VERSION APPLICATIONSFigure 1 shows the application ranges forthe majority of power semiconductordevices indicating shaded areas where SiC islikely to have an impact in the near future.Generally the power device market sizedecreases with increasing voltage and cur-rent requirement. Presently the market sizefor the relatively lower voltage and currentPower Supply area is several times largerthan for all other applications combinedwith device sales of approximately$5B/year. For higher voltage applicationssuch as Motor Control and Traction, thedevice current requirements typicallyincrease as the voltage requirement increas-es due to the large power requirements inthese applications. Anexception to these trends isin the power distributionarea where the HV-HFPower Conversion wouldrequire devices for a widerange of current ratings andthe market size could berelatively large. However,the HV-HF PowerConversion market has notyet developed due to the6.5 kV voltage limit andslow switching speed ofhigh voltage Silicon powerdevices.
Over the last twodecades, PWM power con-version technology, withits superior efficiency andcontrol capability, haschanged the way power isconverted in almost all low
and medium voltage power conversionsapplications from 100 V to 6.6 kV. Due tofundamental limitations of Silicon devices,the on-resistance increases and switchingspeed decreases as the blocking voltagerequirement is increased. The switchingspeeds in low voltage power supplies are ashigh as several MHz and decrease to sever-al kHz for high power traction. The higheron-resistance and slower switching speedincrease losses and limits applicability ofPWM for high power and utility applica-tions.
The developments of Silicon IGBTsover the last decade have enabled high fre-quency power conversion to be used atincreasingly higher power levels. RecentlySiC power Schottky diode products havealso been introduced that increase switch-ing speed capability by reducing diodereverse recovery loss. It is expected thatSiC power devices will continue to aid theevolution of increasing PWM frequencyand power levels in the Power Supplyand Motor Control areas as SiC Schottkydiode and MOSFET products are intro-duced with higher voltage and current rat-ings. Because SiC devices have the capa-bility to increase the voltage beyond thatof Silicon into the 10 kV through 25 kVrange with much higher switching speedfor a given blocking voltage, they providethe revolutionary potential to extend high
Emerging Silicon-Carbide Power Devices Enable Revolutionary Changes in High VoltagePower ConversionBy: Allen Hefner, Ranbir Singh, Jason Lai
Figure 1. Application ranges for the majority of power semi-conductor devices indicating shaded areas where SiC is likelyto have an impact in the near future.
Fourth Quarter 2004 IEEE Power Electronics Society NEWSLETTER 11
frequency PWM switching power conver-sion into the relatively large volume appli-cation area of utility HV-HF PowerConversion.
Recent EPRI reports (1001698, 1002159 –see www.epri.com for abstracts) concludedthat a solid-state distribution transformer,referred to as the Intelligent UniversalTransformer (IUT), would add significantnew functional capabilities and power qual-ity enhancements to those available fromconventional copper and iron transformers.The IUT is expected to be a cornerstonedevice in advanced distribution automation(ADA). A more recent EPRI report (1009516)identified SiC power devices as the solutionfor the HV-HF semiconductor devices need-ed for the IUT and estimated that HV-HFPower Conversion could represent a rela-tively large segment of the power semicon-ductor market.
A major driving force spearheading thedevelopment of HV-HF power devices isthe ongoing DARPA WBST HPE programfocused on developing the technologydeemed necessary to enable a Solid StatePower Substations (SSPS) for future Navywarships. Current distribution approachesbeing considered for the next generationof aircraft carriers and destroyers employa 13.8 kV AC power distribution that isstepped down to 450 V AC by using large(6 ton and 10 m3) 2.7 MVA transformers.Substantial benefits in power qualityenhancement, advanced functionality,size, and weight are anticipated byreplacing this transformer with an allsolid state design.
Figure 2 shows an example three level[2] solid state transformer indicating varioussecondary output options (EPRI reports1001698). The transformer consists of, fromleft to right, a high voltage active front end(AFE) rectifier stage, a three level dc link, ahigh voltage inverter, a high frequencyhigh voltage transformer, low voltage recti-fiers, and various output modules such as aDC/DC converter, 400 Hz AC inverter, andvarious voltage level 60 Hz AC inverter out-puts. The AFE rectifier stage provides aflexible utility interface with power factorcorrection. The high voltage inverter pro-vides high frequency AC required toreduce transformer size and providespower quality voltage regulation functions.Both the AFE rectifier and high voltageinverter require HV-HF semiconductors; forexample a single phase 13.8 kV, 15 kVA res-idential distribution transformer with 8 kVline to neutral requires semiconductorsdevices to switch at 15 kV, 3 A and thethree phase 2.7 MVA SSPS requires devicesto switch at 15 kV, 160 A.
RECENT PROGRESS INHV-HF POWER DEVICESTable 1 compares the basic material prop-erties of Si and SiC. The wider bandgap of4H-SiC results in higher operating temper-ature capability and better tolerance toheating during fault conditions. The pri-marily advantage of 4H-SiC for powerdevices is that it has an order of magnitudehigher breakdown electric field. For agiven blocking voltage requirement, thehigher breakdown electric field allows thedesign of SiC power devices with thinner(0.1 times that of Silicon devices) andmore highly doped (more than 10 timeshigher) voltage-blocking layers. For major-ity carrier power devices, such as powerSchottky diodes or MOSFETs, the combi-nation of 0.1 times the blocking layerthickness with 10 times the doping con-centration can result in a factor of 100
advantage in on-resistance. For conductiv-ity modulated devices such as PiN diodesor IGBTs, SiC results in a factor of 100faster switching speed due to the lowerlifetime required to conductivity modulatethe thinner blocking layer.
Because the SiC material provides amuch lower on-resistance than Silicon, con-ductivity modulated Silicon devices can alsobe replaced by majority carrier SiC deviceswith faster switching speed [3]. For exam-ple, new SiC Schottky diode commercialproducts have recently been introduced[4,5] to replace slower conductivity modu-lated Silicon PiN diodes. Although thesefirst SiC power device product offeringshave been low voltage (300 V to 1200 V)Schottky diodes, the HV-HF devices dis-cussed below break the Silicon voltagecapability limit and will be a key enablingtechnology of the future.
Figure 2. Example three level solid state transformer indicating various secondary outputoptions.
Figure 3. SiC 10 kV power MOSFET: (a) current voltage characteristics and (b) switchingwaveforms.
12 IEEE Power Electronics Society NEWSLETTER Fourth Quarter 2004
Figure 3a shows the output current volt-age characteristics of a 0.04 cm2 10 kV 4H-SiC power MOSFET [6]. The continuous cur-rent capability of 1.5 A indicated on the fig-ure is determined using the 200 W/cm2power dissipation capability of typicalpower device packages. The 4.6 kV bus, 1.3A switching waveforms for this deviceshown in Figure 3b indicate a 100 ns switch-ing time for the gate resistance of 50 Ohmresulting in a 0.3 A peak gate current. Themeasured turn-on and turn-off switchingenergy for this conditions is approximately0.15mJ resulting in a switching power loss of112 W/cm2 at 15 kHz. The static power lossfor 50% duty cycle and the 1.3 A is approx-imately 75 W/cm2. Thus this device canoperate with a 4.6 kV bus, 50% duty cycle,and 15 kHz in a typical 200 W/cm2 powerpackage. The on-state voltage and switchingparameters indicate that the devices are wellmatched for paralleling or large area die.
Figure 4a shows the forward conductioncharacteristics at different temperatures for a0.5 cm2, 10 kV SiC PiN diode indicating acontinuous current rating of approximately30 A (60 A/cm2) [7]. The SiC PiN diode for-ward conduction characteristics only haveslight temperature dependence where thecurrent increases with temperature in thelower current range and decreases with tem-perature in the high current range. Thus the
device should share current well when par-alleled in a high current module. Reverserecovery characteristics for the 10 kV, 0.5cm2 SiC PiN diode are shown in Fig. 4b. Thereverse recovery time is 200 ns. This 10 kVSiC diode has a better on-state voltage toreverse-recovery time tradeoff than commer-cial Si 5 kV diodes and there are no Sidiodes with 10 kV blocking capability. Thus,these devices represent a revolution in recti-fier performance and capability for highvoltage, high frequency power conversionapplications.
CONCLUSIONSThe emergence of HV-HF devices is expect-ed to revolutionize utility and military powerdistribution and conversion by extendingthe use of PWM technology to high voltageapplications. SiC power MOSFETs with 10kV, 1.5 A, 15 kHz switching capability andPiN diodes with 10 kV, 30 A, 200 ns reverserecovery time have already been demon-strated. The DARPA WBST HPE program isexpected to develop half -bridge moduleswith 110 A, 15 kV, 20 kHz capability in thenext few years. The emergence of HV-HFpower devices presents unique opportuni-ties and challenges to the power electronicsindustry in specifying the device require-ments and establishing PWM convertertopologies for high voltage applications.
ACKNOWLEDGEMENTSContribution of the National Institute ofStandards and Technology; not subject tocopyright. The device results presented herewere developed by Cree Inc. with supportfrom Dr. John Zolper at DARPA under ONRcontract N00014-02-C-0302 monitored by Dr.Harry Dietrich.
REFERENCES[1] K. Shenai, R. S. Scott, B. J. Baliga,
[2] J. Rodriguez, J-S. Lai, F. Z. Peng,“Multilevel Inverters – A survey oftopologies, controls and applications,”IEEE Transactions on IndustrialElectronics, vol. 49, no. 4, (2002) pp. 724–738.
[3] A. R. Hefner, R. Singh, J. S. Lai, D. W.Berning, S. Bouche, and C. Chapuy “SiCPower Diodes Provide BreakthroughPerformance for a Wide Range ofApplications,” in IEEE Transactions onPower Electronics, vol. 16, pp. 273 – 280(2001).
[4] R. Singh, J. A. Cooper, M. R. Melloch, T.P. Chow, J. W. Palmour, “SiC PowerSchottky and PiN Diodes,” IEEETransactions on Electron Devices, vol.49, no. 4, (2002) pp. 665 –672; also seewww.cree.com for product information.
[5] D. Stephani, “Status, prospects and com-mercialization of SiC power devices,”Proc. 59th Device Research Conference,pp. 14, June 25-27, 2001. Notre Dame,USA; also see www.infineon.com forproduct information.
[6] S. Ryu, S. Krishnaswami, M .O’Loughlin,J. Richmond, A. Agarwal, J. Palmour,and A. R. Hefner, “10-kV, 123 mW-cm24H-SiC Power DMOSFETs,” IEEEElectron Device Letters, Vol. 25, No. 8,pp 556.
[7] Mrinal K.Das, et. al., Drift-Free, 50 A, 10kV 4H-SiC PiN Diodes with ImprovedDevice Yields, to appear in ECSCRM 2004.
Figure 4. SiC 10 kV PiN diode: (a) current votlage characteristics and (b) reverse recoverycharacteristics.
Allen R. Hefner Jr.(IEEE Fellow) receivedthe B.S., M.S., and Ph.D.degrees in electricalengineering from theUniversity of Maryland in1983, 1985, and 1987respectively. He joinedthe Semiconductor
Electronics division of the National Institute ofStandards and Technology in 1983. Currently,he is the Project Leader for Power Devicesand Thermal Measurements Project and theEmbedded Sensor System-on-a-Chip Project.His research interests include characterization,modeling, and circuit utilization of powersemiconductor devices and MEMS-based inte-grated sensor System-on-a-Chip technologies.
Ranbir Singh (IEEEMember) received the B.Tech degree from IndianInstitute of Technology, N.Delhi, India in 1990 andMS & Ph.D. degrees fromNorth Carolina StateUniversity in 1992 and1997 respectively, all in
Electrical Engg. He has been conductingresearch on SiC power devices first at Cree Inc.in Durham, NC from 1995 to 2003, and sincethen at the National Institute of Standards andTechnology, Gaithersburg, MD. His interestsinclude the development and understanding ofa wide range of SiC power devices includingMOSFETs, IGBTs, field controlled thyristors,JBS, PiN and Schottky diodes.
Jih-Sheng (Jason) Lai(IEEE Senior Member)received the Ph.D.degree in electrical engi-neering from theUniversity of Tennessee,Knoxville. He joinedVirginia PolytechnicInstitute and State
University in 1996. Currently, he is aProfessor and the Director of the FutureEnergy Electronics Center. His main researchareas are energy related power electronicsand utility power electronics interface andapplication issues.
