A New Era in Power Electronics Is Initiated

JACEK RABKOWSKI,DIMOSTHENIS PEFTITSIS,and HANS-PETER NEE D

uring recent years, silicon carbide (SiC) power electronics has gonefrom being a promising future technology to being a potent alternativeto state-of-the-art silicon (Si) technology in high-efficiency, high-frequency, and high-temperature applications. The reasons for this arethat SiC power electronics may have higher voltage ratings, lower volt-age drops, higher maximum temperatures, and higher thermal conduc-tivities. It is now a fact that several manufacturers are capable of

developing and processing high-quality transistors at cost that permit introduction ofnew products in application areas where the benefits of the SiC technology can providesignificant system advantages. The additional cost for the SiC transistors in comparison

Digital Object Identifier 10.1109/MIE.2012.2193291

Date of publication: 15 June 2012

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1932-4529/12/$31.00&2012IEEE JUNE 2012 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 17

with corresponding Si alternativesare significantly smaller today thanthe reduction in cost or increase invalue seen from a systems perspec-tive in many applications. In all thesecases, the SiC transistors are unipo-lar devices, such as the junction field-effect transistor (JFET), the metal-oxide-silicon field-effect transistor(MOSFET), and the bipolar junctiontransistor (BJT). The latter may seemto be a bipolar device, but fromexperiments, it is found that available1,200-V SiC BJTs behave as unipolardevices in the sense that there arepractically no dynamic effects associ-ated with build up or removal ofexcess charges. The reason for this isthat the doping levels of 1,200-V SiCtransistors are so high that any con-siderable carrier injection is superflu-ous for the conduction mechanism.For voltage ratings beyond 4.5 kV,true bipolar devices will probablybe necessary. In high-voltage high-power applications such as high-volt-age direct current transmission(HVdc), insulated gate bipolar tran-sistors (IGBTs), and BJTs in SiC mayseem as the ideal switch candidatesas very high numbers of series-con-nected devices would be necessaryto withstand system voltages. How-ever, since the trend in voltage

source converter (VSC)-based HVdcis to build modular converters, theneed for voltage ratings in excess of10 kV may be questionable, sincesuch a device in SiC would have avoltage drop with a built-in potentialof more than 3 V. Since the fabrica-tion of SiC IGBTs is far more complexthan, for instance, that of an SiC JFETor a BJT, it makes sense first to fullyexploit the benefits of these devices.Today, it is possible to build switch-mode inverters in the 10100 kWrange with efficiencies well above99.5%. A successful example is a 40-kVA three-phase SiC inverter with tenparallel-connected JFETs in eachswitch position. This inverter has anefficiency of approximately 99.7%(Figure 1). Truly, a new era in powerelectronics has begun.

Overview of the AvailableSiC DevicesThe dramatic quality improvement ofthe SiC material [1] in combinationwith excellent research and develop-ment efforts on the design and fabri-cation of SiC devices by severalresearch groups has recently re-sulted in a strong commercializationof SiC switch-mode devices [1], [2].Nevertheless, the SiC device marketis still in an early stage, and today,

the only available SiC switchesare the JFET [2], [3], BJT [4], andMOSFET [5], [6]. Commercially avail-able SiC devices are still not in massproduction. Thus, the market price ofthese components is significantlyhigher than their Si counterparts. Onthe other hand, because of the lowvoltage and current ratings of theseSiC devices, they are currently notsuitable for power ratings aboveseveral hundred kilowatts. In particu-lar, voltage ratings in the range of1,200 V and current ratings of few tensof amperes are available. Regardlessof the type of SiC device, the driverfor each device counts as a vital partwhen the SiC devices operate in apower electronics converter. Thedriver requirements differ among thedevices, and they should be designedin such a way that they ensure reli-able operation. Finally, it is also worthmentioning the progress of the re-search on the SiC IGBT [2]. To startwith, an overview of the currentlyavailable SiC devices is given followedby the driver aspects for each device.

SiC JFETThe first attempts to design and fabri-cate an SiC JFET were made in theearly 1990s when the main researchissues were dealing with the designoptimization to realize high-powerand high-frequency SiC devices [7]. Itwas during these years that a fewresearch groups had started men-tioning the advantageous character-istics of the SiC material comparedwith Si [8], [9]. However, from thestructure design point of view, theearly-year SiC JFET was sufferingfrom relatively low transconductancevalues, low channel mobilities, anddifficulties in the fabrication process[8], [9]. During the last decade, theimprovement on the SiC material andthe development of 3- and 4-in wavershave both contributed to the fabrica-tion of the modern SiC JFETs [2], andit was around 2005 when the firstprototype samples of SiC JFETs werereleased to the market.

One of the modern designs of theSiC JFET is the so-called lateral chan-nel JFET (LCJFET), as shown in

FIGURE 1 A 40-kVA SiC inverter (3003 3703 100mm3) shown together with a copy ofIEEE Industrial Electronics Magazine.

18 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n JUNE 2012

Figure 2. The load current throughthe device can flow in both directionsdepending on the circuit conditions,and it is controlled by a buried p

gate and an n source p-n junction.This SiC JFET is a normally on device,and a negative gate-source voltagemust be applied to turn the deviceoff. By applying a negative gate-source voltage, the channel width isdecreased because of the creation ofa certain space-charge region, and areduction in current is obtained.There is a specific value of the nega-tive gate-source voltage, which iscalled pinch-off voltage, and underthis voltage, the device currentequals zero. The typical range of thepinch-off voltages of this device isbetween 16 and 26 V. An impor-tant feature of this structure is theantiparallel body diode, which isformed by the p source side, the n

drift region, and the n drain. How-ever, the forward voltage drop of thebody diode is higher compared withthe on-state voltage of the channel[2], [10] at rated (or lower) currentdensities. Thus, for providing theantiparallel diode function, the chan-nel should be used to minimize theon-state losses. The body diode maybe used for safety only for short-timetransitions [11]. This type of SiC JFEThas been released by SiCED (Infin-eon) a few years ago, and it is going tobe commercial in the near future.

The second commercially avail-able SiC JFET is the vertical trench(VTJFET), which was released in 2008by Semisouth Laboratories [12], [13].A cross-section schematic of itsstructure is shown in Figure 3. TheVTJFET SiC JFET can be either anormally off (enhancement-modeVTJFET-EMVTJFET) or a normally on(depletion-mode VTJFET-DMVTJFET)device, depending on the thicknessof the vertical channel and the dop-ing levels of the structure. As othernormally on JFET designs, a negativegate-source voltage is necessary tokeep it in the off state. On theother hand, a significant gate current(approximately 200 mA for a 30-Adevice) is necessary for the normallyoff JFET to keep it in the conduction

state. The pinch-off voltage for theDMVJFET equals approximately6 V,whereas the positive pinch-off volt-age for the normally off one is slightlyhigher than 1 V. Comparing this typeof SiC JFETs to the LCJFET, theabsence of the antiparallel bodydiode in the DMVTJFET makes theLCJFET design more attractive fornumerous applications. However, aSiC Schottky diode can be connectedas the antiparallel diode for theVTJFET. This diode may be used forshort-time transients in the same way

as the body diode of the LCJFET.Except during these short-timetransients, the reverse currentshould flow through the channel.The additional SiC Schottky diodeis especially attractive if severalVTJFETs are connected in parallel,and the voltage across the transis-tors is lower than the thresholdvoltage of the diode. In this case,only one diode would be necessaryfor all parallel JFETs because of theshort (

Two additional SiC JFET designshave been presented [Figure 4(a)and (b)] [14]. The buried grid JFET(BGJFET), shown in Figure 4(a),makes use of a small cell pitch, whichcontributes to the low specific on-state resistance and high saturationcurrent densities. The absence of theantiparallel body diode and the diffi-culties in the fabrication processcompared with the LCJFET count astwo basic drawbacks of this design.Figure 4(b) shows the doublegate vertical channel trench JFET(DGVTJFET), which is actually a com-bination of the LCJFET and theBGJFET designs, and it has been pro-posed by DENSO [14]. This designcombines fast switching capabilitydue to the low gate-drain capacitancewith low specific on-state resistancebecause of the small cell pitch anddouble gate control.

The commercially available SiCJFETs are mainly rated at 1,200 V,while 1,700 V devices are also avail-able on themarket. The current ratingof normally on JFETs is up to 48 A, anddevices having on-state resistances of100, 85, and 45 mX at room tempera-ture can be found. As for the normallyon JFETs, the normally off counter-parts (EMVTJFETs) are available atcurrent ratings up to 30 A and haveon-state resistances of 100 and 63mX.

SiC BJTThe SiC BJT is a bipolar normallyoff device, which combines both alow on-state voltage drop (0.32 V at100 A/cm2) [4] and a quite fast-switching performance. It has beendeeply investigated, designed, andfabricated by TranSiC. A cross sec-tion of this device is shown in Fig-ure 5, where it is obvious that this is

an NPN BJT. The low on-state voltagedrop is obtained because of thecancellation of the base-emitter andbase-collector junction voltages. TheSiC BJT is a current-driven device,which means that a substantialcontinuous base current is requiredas long as it conducts a collector cur-rent. The available SiC BJTs have avoltage rating of 1.2 kV and currentratings in the range of 640 A, whilecurrent gains of more than 70 havebeen reported at room temperaturefor a 6-A device [15]. The fabricationof the SiC BJTs with improved sur-face passivation leads to current rat-ings of 50 A at 100 C and currentgains higher than 100. However, thecurrent gain is strongly temperaturedependent, and in particular, it dropsbymore than 50% at 250 C comparedwith room temperature. The develop-ment of SiC BJTs has been successful,and in spite of the need for the basecurrent, SiC BJTs having competitiveperformance in the kilovolt range areexpected in the future.

SiC MOSFETSeveral years have been spent on theresearch and development of the SiCMOSFET. Specifically, the fabricationand stability of the oxide layer hasbeen challenging. A cross section ofa typical SiC MOSFET structure isshown in Figure 6. The normally offbehavior of the SiC MOSFET makes itattractive to the designers of powerelectronic converters. Unfortunately,

Gate Gate

T-Gate

P+ Top Gate

n-epi

n+n+ n+

p+ Buried Gatep+ Buried Gate p+ Buried Gate

n- Drift Regionn- Drift Region

n+ Substrate n+ Substrate

Drain(a) (b)

Drain

B-Gate

Source Source Source

FIGURE 4 (a) Cross section of BGJFET and (b) SiC DGTJFET.

Collector Contact

N+ Substrate JTE Implant

Base ImplantN Collector

P Base

N Emitter

Emitter Contact

Surface Passivation

BaseContact

FIGURE 5 Cross section of the SiC BJT.

20 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n JUNE 2012

the low channel mobilities causeadditional on-state resistance of thedevice and thus increased on-statepower losses. Additionally, the reli-ability and the stability of the gateoxide layer, especially over long timeperiods and at elevated tempera-tures, have not been confirmed yet.Fabrication issues also contribute tothe deceleration of SiC MOSFETdevelopment. However, the currentlypresented results regarding the SiCMOSFETs are quite promising, and itis believed that such devices will beavailable for mass production withina few years. The SiC MOSFET is themost recent SiC switch, which wasreleased during the end of 2010 fromCree, while other manufacturers(e.g., ST Microelectronics) are veryclose to releasing their own MOSFETin SiC [1], [5]. At present, 1.2-kV SiCMOSFETs with current ratings of1020 A and on-state resistances of80 and 160 mX are available on themarket. Furthermore, SiC MOSFETchips rated at 10 A and 10 kV havealso been investigated by Cree as apart of a 120-A half-bridge module[43]. When compared with the state-of-the-art 6.5-kV Si IGBT, the 10-kV SiCMOSFETs have a better performance.However, the commercialization ofsuch a unipolar SiC device is not fore-seen in the near future.

SiC IGBTThe Si-based IGBT has shown anexcellent performance for a widerange of voltage and current ratingsduring the last two decades. The fabri-cation of a Si n-type IGBT started on ap-type substrate. Such substrates arealso available in SiC, but their resistiv-ity is unacceptably high and preventsthese components from being used inpower electronics applications. Fur-thermore, the performance of the gateoxide layer is also poor, resulting inhigh channel resistivities. Theseissues have already been investigatedby many highly qualified scientists,and it is believed that such SiC devi-ces will not be commercialized withinthe next ten years [2], [16]. On thecontrary, even if high-voltage SiCIGBTs will be commercially available

in the future, it is not obvious thatthey will give low-power losses as ahigh-voltage SiC JFET (e.g., 3.3-kV SiCJFET) if modularized circuits such asmodular multilevel converter (M2C)are used for high-voltage high-powerapplications [38].

Gate and Base Driversfor SiC DevicesTo use the advantageous perform-ance of SiC devices compared withthe Si counterparts, special driverdesigns are required. Such gate andbase drivers should be able to pro-vide rapid switching for the SiC devi-ces but should also have the lowestpossible power consumption. Addi-tionally, high-temperature operationis also preferable for these driversbecause of the high-temperature ca-pability of the SiC devices. Variousdrivers for SiC JFETs and SiC BJTshave been proposed since these devi-ces started to be available on the mar-ket. Drivers for SiC MOSFETs havebeen omitted in this presentation as

they are essentially the same asthose for Si MOSFETs, except that ahigher gate voltage (more than 20 V)is required in the on state.

Normally On SiC JFET Gate DriverThe normally on SiC JFET is a voltage-controlled device. Thus, a negativegate-source voltage, which should belower than the pinch-off voltage, isrequired to keep this device in the offstate. The most widely used gatedriver for the SiC JFET was proposeda few years ago [17] (Figure 7). Themain part of the driver is a parallel-connected network consisting of adiodeD, a capacitor C , and high-valueresistor Rp, while a gate resistor Rg isconnected in series with the gate.

During the on state of the SiC JFET,the output of the buffer, Vg, equals0 V, and the device is conducting thecurrent. When the JFET is turned off,the buffer voltage, Vg, is switchedfrom 0 V to Vs. At this time, a cur-rent peak is supplied to the gate-source junction of the JFET through

Rp

D

C

CdgRg

Vs

Cgs

VgNormally On

SiC JFET

0

0

FIGURE 7 Gate driver of the normally on SiC JFET.

Drain

n+ Substrate

p Well p WellN+ N+

Source SourceGate

n-Drift

Gate Oxide Layer

FIGURE 6 Cross section of the SiC MOSFET.

JUNE 2012 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 21

the gate resistor Rg, and the capaci-tor C , as shown with the dashed linein Figure 7. Hence, the parasiticcapacitance of the gate-source junc-tion, Cgs, is charged, and the voltagedrop across the capacitor C equalsthe voltage difference between Vsand the breakdown voltage of thegate. During the steady-state opera-tion in the off-state, a low current isonly required to keep the JFET off.This current is supplied through theresistor Rp, as shown with the solidline in Figure 7. It is worth mentioningthat the value of Rp should be chosencarefully to avoid breakdown of thegate-source junction. Regardless ofthe choice of Rp, it is possible toadjust the switching performance toany desired speed by selecting anadequate value of the gate resistor

Rg. The switching performance of thisgate driver, when it is used with a SiCLCJFET, is shown in Figure 8.

Despite the overwhelming advan-tages of the normally on SiC JFET, themain problem with this device is thata possibly destructive shoot through isobtained in case the power supply forthe gate driver is lost. It is, therefore,more thanessential todesign smart gatedrivers to overcome suchproblems.

Normally Off SiC JFETGate DriverEven though the normally off SiCEMVTJFET seems to be a voltage-controlled device, a substantial gatecurrent is required during the con-duction state to obtain a reasonableon-state resistance [18]. In additionto this, high-current peaks should be

provided to the gate so that therecharge of the gate-source capaci-tance of the device is faster. A two-stage gate driver with resistors hasbeen proposed for a 1,200-V/30-Anormally off JFET (Figure 9). Thisdriver consists of two stages: thedynamic one with a standard driverand a resistor RB2, which provideshigh voltage, and thus high-currentpeaks, during a short time period toturn the JFET on and off rapidly. Thesecond stage is the static one with adc/dc step-down converter, a BJT,and a resistor RB1. The auxiliary BJTis turned on when the dynamic stageis completed. The static stage is ableto supply a gate current of approxi-mately 200 mA during the on state ofthe JFET. An important advantage ofthis driver is the absence of thespeed-up capacitor, whichmight causeduty-ratio limitations because of theassociated charging and dischargingtimes. Finally, another gate driver fornormally off SiC JFETs has beenpresented in [19] to use the fastswitching performance of this device.

SiC BJT Base DriverAs already mentioned, the SiC BJT isa current-driven device, whichrequires a substantial base currentduring the on state. The simplestbase-drive unit for SiC BJTs consistsof a series-connected resistor withthe base, which is supplied by a volt-age source. However, the switchingperformance of such a driver is poorwhen it is optimized for low powerconsumption. Considering this basedriver, the switching performancecan be improved by connecting aspeed-up capacitor, CB, in parallel tothe resistor [4] (Figure 10). Hence,the switching performance might beimproved, but this improvementdepends on the supply voltage, Vcc.The higher the supply voltage, thefaster the switching transients, but atthe same time, the power consump-tion is increased. Therefore, it seemsthat the switching performance andthe power consumption in this caseis a tradeoff.

To combine fast switching per-formance and low power consumption,

VCC

RDRVRB2

RB1

dc

dc

VEE

Normally OffSiC JFET

FIGURE 9 Two-stage gate-drive unit fornormally off SiC JFETs.

VCC

RDRVRB B

E

C

CB

FIGURE 10 Base-drive unit with speed-upcapacitor for a SiC BJT.

2 35.00 A100 V BW BW 20.0 ns 2.50GS/s

13.20 V

2222222222222222222222 3333333333333333333 111111111111

FIGURE 8 Turn-off process of the normally on SiC JFET (VDS: drainsource voltage100 V/divand ID: drain current5 A/div, time base 20 ns/div).

22 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n JUNE 2012

a base driver consisting of two voltagesources can be employed [20]. A turn-on process using this driver is shownin Figure 11. The high-voltage supplycontributes to the high switchingspeed, while the power consumptionof the driver is optimized by using thelow-voltage supply connected to acarefully chosen base resistor. Whenthe turn-on process starts, high-cur-rent peaks are provided to the baseso that the SiC BJT is turned onrapidly. During the conduction state,the base current is determined by thecurrent gain. Since this continuousbase current is supplied from a low-voltage source, the losses during theon-state can be kept fairly low.

Parallel Connection ofSiC DevicesIn several power electronics applica-tions, higher current ratings arerequired than those of the availableSiC transistors. It is, therefore, neces-sary to build eithermultichipmodulesor to parallel-connect several single-chip components. In both cases, it isessential to keep track of both steady-state and transient current sharing ofparallel-connected chips. Unless thetransient current is not equally sharedamong the devices, the switchinglosses caused in the device that con-ducts the highest current will behigher, resulting in an increasedtemperature. Similarly, a nonuniformsteady-state current sharing due tothe spread of the on-state voltagedrops or resistances will also resultin unequal temperature distribution.

The problems that are faced whennormally off SiC JFETs are connectedin parallel have been shown in [21]. Asimilar investigation regarding SiCBJTs has also been presented by thesame authors. In particular, it has beenshown that the transient currents ofthese devices might suffer from mis-matches, especially at high switchingspeeds. But, on the contrary, duringsteady-state operation, they haveshown excellent current sharing.

An investigation of parallel-con-nected normally on SiC JFETs has beenpresented in [22]. The twomost criticalparameters when parallel-connecting

SiC JFETs are the pinch-off voltageand the reverse breakdown voltageof the gate. It was found that dif-ferences of approximately 25% inswitching losses could result from adifference in the pinch-off voltage of0.5 V. Additionally, the transient cur-rent may not be equally sharedbecause of differences in the statictransfer characteristics.

One solution to the parallel con-nection of SiC devices could besorting with respect to the mostcritical parameters, which affect theirswitching performance. However, asit was shown in [22], this is notalways sufficient. Thus, development

of new drivers is essential to handleparallel connection reliably.

A successful example of the paral-lel connection of ten JFETs in eachswitch position is the 40-kVA inverter[23] shown in Figure 1. Despite thehigh number of parallel-connecteddevices, the total semiconductorcost is approximately US$37.5/kW ofrated power, which is higher com-pared with the corresponding costwhen Si IGBTs are employed. How-ever, the outstanding switching per-formance of SiC JFETs (Figure 12) andthe outstandingly high efficiency (morethan 99.5%) constitute the two drivingfactors to invest in such a converter.

Vce

Ic

3.94 VStop20.00 / 1ns21.002.00A /100V/ ns1 2 3 4

24

FIGURE 11 Turn-on process of the 6-A SiC BJT with dual-source base driver (Vce: collec-toremitter voltage100 V/div and Ic: collector current2 A/div, time base 20 ns/div).

IJFET3

2

Idiode

Vds

6.50 VStop20.00 / 1ns33.172.00 V/20.0A / 20.0A / ns1 2 3 4

FIGURE 12 Turn-off process of 10 parallel SiC JFETs (Vds: drainsource voltage200 V/div and IJFET: drain current20 A/div of one single JFET) with antiparallel SiC Schottkydiode (Idiode: diode current20 A/div, time base 20 ns/div).

JUNE 2012 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 23

PossibilitiesBased on the brief analysis of theavailable SiC power devices pre-sented earlier, it is clearly shown thattheir features are moving powerelectronics into new areas. When itcomes to the design stage of powerelectronic converters with SiC devi-ces instead of classic Si counterparts,three different design directions maybe chosen (Figure 13).

Since switching times in the rangeof nanoseconds have been reported,the most obvious direction is theincrease of the switching frequencyup to a few hundred kilohertz. In thecase of the available Si devices forvoltage ratings above 600 V, suchhigh switching frequencies can beonly reached when soft switching isemployed [24]. From a systemperspective, the design advantagesare reduction of size and weight ofpassive elements (e.g., inductors,capacitors) [25]. This results in bet-ter compactness of the converter,especially in dc/dc converters andinverters with passive filters. Lastbut not least, as the switching speedsare higher and the harmonics areshifted up to higher frequencies, thesize of electromagnetic interference(EMI) filters also will be reduced [26].

Both short switching times andlow voltage drops across SiC devicesresult in significant reductions inpower losses. Therefore, in applica-tions where the increase of switching

frequency is not crucial, one of theother two possible design directionsmight be chosen. Since the reductionof the power losses is equivalent toan efficiency increase, a direct benefitis the reduction of size and weight ofthe cooling equipment [23]. Further-more, water cooling or air-forcedcooling systems could be unneces-sary as the amount of dissipated heatis a few times lower. High efficiency isalso important in power delivery anddistribution systems as well as pho-tovoltaic applications because thelower power loss is directly recalcu-lated into profit.

The third design direction is high-temperature applications, as SiC de-vices are capable of being operatedat junction temperatures above450 C. This is a totally new area ofpower electronics aiming for automo-tive, space, or drill-hole applications.However, the main deceleration fac-tor at elevated temperatures is notthe device itself, but rather a lack ofsuitable packaging technology, high-temperature passive components,and control electronics. Before realhigh-temperature converters will bereleased, small steps could be donewith the existing SiC transistors. Forinstance, with junction temperaturesin the range of 200250 C, the thermalresistance of the heat sink could behigher, which leads to a reduction involume and weight. During recentyears, promising results regarding

the technology of high-temperaturepower modules packaging have beenpresented [27]. The SiC devices wereoperating at 250 C while the casetemperature was below 200 C, butreliability and long-term stability alsoneed to be investigated. The develop-ment of high-temperature gate driv-ers has been accelerated by theintroduction of the silicon-on-insula-tor technology [28], [29] but thedevelopment of SiC control electron-ics is also promising [30].

Overview of ApplicationsIt cannot be denied that SiC powerdevice technology is currently inbloom, not to say in an explosionstage as new things such as new devi-ces, driver concepts, or applicationexamples appear continuously.Undoubtedly, only a fraction of allnew developments is published. Prob-ably some of the most interesting ex-amples are not revealed. This meansthat it is hard to give a true overviewof the current SiC technology, but it isstill possible to give an overview ofwhat has been presented previously.

In contrast to the well-establishedand mass-produced SiC Schottkydiodes, the real application of SiCtransistors in the field is still in theearly stage. However, using the datafor the four available transistortypes, it is possible to make reasona-ble forecasts about the performanceof future SiC converters. That is whya number of laboratory prototypesand demonstrators have been builtand experimentally tested by severalresearch groups around the world.

PhotovoltaicsThe advantageous features of the SiCtransistor perfectly match to meetthe two basic requirements of thephotovoltaic industry: increase inefficiency and integration of theinverter with the photovoltaic panel[31]. In the power range of a singlekilowatt, efficiencies more than 99%can be reached by replacing Si withSiC components, and even if thedevice cost is higher, the overall sys-tem benefits are significant. The SiCtransistors can also be used in future

Efficiency

Temperature

Frequencycy

FIGURE 13 Threemain directions in the design of power electronics converterswith SiC devices.

24 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n JUNE 2012

integration of small inverters on thebackside of photovoltaic panels. Here,harsh environmental conditions arefaced, and it is not possible to meethigh reliability and lifetime require-ments with Si technology. However,again the main argument against SiCdevices is the cost, but the overall sys-tem benefits as well as the expectedreduction of the device prices shouldsolve this problem in the future.

AC DrivesAs the state-of-the-art Si devicesalmost fulfill the requirements forinverter-fed ac drives [32] and thecost of SiC device cost will always behigher, there are very small chancesfor mass introduction of SiC electron-ics in this area. Nevertheless, it isvery likely that features of the SiCdevices could be utilized in manyniches, especially when high efficien-cies and high power densities arerequired [24][26] or when a highswitching frequency is necessary tofeed a high-speedmotor.

Hybrid Electric VehiclesThe gains when employing SiC tran-sistors in inverters for hybrid electricvehicles are extremely high [33][36]. The current trend deals with theintegration of power electronic con-verters with the combustion engineusing the same coolant at a tempera-ture above 100 C. It is not likely thatthe associated requirements can befulfilled by Si electronics. Thus, SiCelectronics withmuch higher temper-ature limits is the best choice, and atthe same time, very high efficienciesare possible. However, the lack of reli-able high-temperature packaging stillcounts as a serious problem. Finally,the auxiliary components, such asgate drivers and capacitors, have towithstand high temperature opera-tions, which is a great challenge.

High-Power ApplicationsThe high blocking voltage capabilityof the SiC devices makes them attrac-tive in the area of high-power con-verters for grid applications [37]such as HVdc. According to [14], uni-polar devices such as JFETs will be

the best choice in the voltage rangeup to 4.5 kV, while for higher vol-tages, bipolar devices (BJT, IGBTs)may be superior. However, the avail-ability of SiC devices with high volt-age ratings is currently limited, evenif a few interesting examples could befound in the literature. The case of a300-MVA modular multilevel con-verter for 300 kV HVdc transmissionis discussed in [38]. As the switchingfrequency is only 150 Hz, the switch-ing losses are very low. Togetherwith the low on-state resistance ofthe SiC JFETs, it is possible to achievean efficiency increase by 0.3%(900 kW) with respect to the Si IGBTscase [39]. The benefit is not only thelower amount of heat that needs tobe dissipated but also the lowerenergy cost. The possible applicationof 20-kV SiC gate turn-off thyristors(GTOs) in a 120-kV/1-kA HVdc inter-face has been discussed in [40].These devices are compared with the43 5 kV Si counterparts by using ananalytical device model and by per-forming system simulations. Employ-ing the 20-kV SiC GTOs, a significantefficiency increase is expected athigher junction temperatures.Another interesting example of thehigh voltage capability of SiC JFETs isshown in [24], where six 1.2-kV tran-sistors connected in a supercascodeconfiguration were tested up to 5 kVin a dc/dc converter. With 6.5-kVJFETs, this converter would be ableto operate at a voltage level above 20kV in a distribution system. More-over, the 10-kV SiC MOSFET is oftenconsidered for high-voltage applica-tions such as solid-state transform-ers [41]. In this case, although thepower losses are a few times lowerthan the 6.5-kV IGBTs, the switchingfrequency of such a converter isincreased from 1 to 20 kHz to reducethe size of high-voltage transformers.The 15-kV n-channel SiC IGBT hasbeen considered in a study regardingsmart grid applications [42]. Despitethe promising simulation results ofthe n-channel SiC IGBT (low on-statevoltage and switching energies lowerthan the 6.5 kV Si counterpart),experimental verification has only

been done with a 12-kV/10-A SiC IGBTin the laboratory [43].

ConclusionsA new era in power electronics isentered as new SiC transistors areintroduced in high-efficiency, high-frequency, and high-temperature ap-plications. Efficiencies well above99.5% are possible in the 10100 kWpower range. High-quality samples of1,200 V JFETs, BJTs, and MOSFETsare available, and within a few years,mass-produced products using thesenew devices will be on the market inseveral application areas. The bene-fits of the new devices are so over-whelming that it cannot be affordedfrom systems perspective to neglectthe use of these devices.

BiographiesJacek Rabkowski received hisM.Sc. and Ph.D. degrees in electricalengineering from the WarsawUniversity of Technology, Poland, in2000 and 2005, respectively. Hejoined Institute of Control andIndustrial Electronics of this univer-sity as an assistant professor in2005. Since 2010, he has been at theLaboratory of Electrical Energy Con-version, KTH Royal Institute ofTechnology, Sweden, as a guestresearcher. His research interestsinclude novel topologies of powerconverters, PWM techniques, driveunits, and converters with SiC devi-ces. He has been taking part in afew projects regarding applicationof SiC JFETs and BJTs. He is a Mem-ber of the IEEE.

Dimosthenis Peftitsis receivedhis diploma degree in electrical andcomputer engineering from theDemocritus University of Thrace,Greece, in 2008. In the same year, hespent a semester at ABB CorporateResearch in Vasters, Sweden, work-ing on his diploma thesis. Since2008, he has been working towardhis Ph.D. degree in the Laboratoryof Electrical Energy Conversion,KTH Royal Institute of Technology,Stockholm, Sweden. His researchinterests include gate and base

JUNE 2012 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 25

driver design for SiC JFETs and BJTs.He is a Student Member of the IEEE.

Hans-Peter Nee studied electricalengineering at KTH Royal Institute ofTechnology, Stockholm, Sweden, andreceived his masters, Lic. Tech., anddoctors degrees in 1987, 1992, and1996, respectively. In 1999, he wasselected to be the chair of powerelectronics at the same university.His interests are power electronicconverters for various applications,control of power electronic convert-ers and systems, and power semi-conductor components, especiallynew components such as JFETs andBJTs in SiC and their drive circuits.He is a Senior Member of the IEEE.

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