Impact of 1.2kV SiC-MOSFET EV TractionInverter On Urban Driving

Hyeokjin Kim, Hua Chen, Jianglin Zhu, Dragan Maksimovic and Robert EricksonDepartment of Electrical, Computer and Energy Engineering

University of Colorado BoulderBoulder, Colorado, 80309

Email: hyeokjin.kim, hua.chen, jianglin.zhu, maksimov, rwe@colorado.edu

Abstract—Replacement of an electric vehicle conventional Si-IGBT traction inverter with a SiC-MOSFET inverter can achievereductions in urban driving cycle average loss by a factor of four,reduction in peak loss by a factor of three, and reduction in semi-conductor die area by a factor of two. An 80 kW EV powertrainbased on the Nissan LEAF is modeled in MATLAB/Simulink,and EPA standard driving cycles such as UDDS, HWFET, andUS06 are simulated. Scenarios of a 600V Si-IGBT inverter basedon the Nissan LEAF, a 1200V Si-IGBT inverter based on theToyota Prius, and a 1200V SiC-MOSFET inverter are designedusing currently available devices. A comprehensive loss modelincluding switching and conduction loss is developed and the totalloss of the SiC-MOSFET traction inverter over EPA standarddriving cycles shows a reduction in urban driving cycle averageloss by a factor of four and peak loss by a factor of three, aswell as semiconductor die area by a factor of two, relative to theSi-IGBT traction inverter.

I. IntroductionThe traction inverter of an electric vehicle (EV) or hybrid

electric vehicle (HEV) plays an important role as its powerconversion unit (PCU), but its loss significantly impacts the ve-hicle MPGe (Mile-per-Gallon equivalent). The traction invertersemiconductors significantly impact efficiency, power density,and cooling requirements. Improvements in efficiency of theEV or HEV traction inverter translate directly into increasedMPGe as measured by improved CAFE (corporate averagefuel economy) efficiency, and also translate into reduction ofcooling capacity size and weight. The silicon insulated-gatebipolar transistor (Si-IGBT) is widely employed in commercialtraction inverters such as the Nissan LEAF [1] or ToyotaPrius [2]. However, low efficiency is observed at the lowmotor RPM operating points that dominate the urban drivingcycle; this can be ascribed to inferior Si-IGBT switchingcharacteristics. The SiC-MOSFET has been researched asan alternative semiconductor device for traction inverters,owing to low switching energy loss [3] and high thermalreliability [4]. In this paper, the SiC-MOSFET traction inverteris designed with a commercial device, Cree 1200V/25mΩ,to evaluate the power density gain of SiC-MOSFET tractioninverter and loss reduction over EPA standard driving cyclessuch as UDDS, HWFET, or US06, relative to conventionalSi-IGBT traction inverters. For comparison, two conventionalSi-IGBT inverters that employ 600V Si-IGBTs and 1200VSi-IGBTs are designed with commercial devices and theirresulting performances are compared. An EV powertrain based

on the Nissan LEAF is modeled in Matlab Simulink andis described in Section II. A semiconductor loss model thatincludes switching and conduction loss is discussed in SectionIII. The resulting efficiencies and losses over EPA standarddriving cycles show significant improvement in urban drivingcycle through SiC-MOSFET traction inverter. Further resultsare discussed in section IV.

II. Electric vehicle powertrain simulation model

To evaluate the operating points of the traction inverter overstandard EPA driving cycles, the EV powertrain is modeled inMATLAB/Simulink. A simplified top-level block diagram isshown in Fig. 1. The vehicle model tracks a speed commandusing a closed-loop controller represented as the driver ofdiagram. The torque command output of the driver is fed intothe current regulator which drives a traction motor. Vehicleparameters are imported from the Nissan LEAF [5] and arelisted in Table I. Permanent magnet traction AC (PMAC)machine parameters are modeled based on the EV tractionmotor. The PMAC machine is controlled by the conventionalconstant angle with field weakening control scheme. With thiscomplete vehicle powertrain model and control scheme, EPAstandard driving cycles such as UDDS, HWFET, or US06, aresimulated based on the following assumptions:

• The space vector control scheme is applied for motorcontrol.

• The mechanical brake is disabled so that deceleration isperformed by regenerative braking.

• Parasitic circuit inductance and capacitances, oscillationduring switching transitions, and dead-time of PWMcontrol are neglected.

• Motor stator inductance and resistance are constant.• The battery is modeled as an ideal (constant) voltage

source.

UDDS, HWFET, and US06 driving cycles vs. time andspeed are shown in Fig. 2 and the resulting motor operatingpoints are shown in Fig. 3. The urban dynamometer drivingcycle (UDDS) is centered in the low RPM region, while thehighway fuel economy driving cycle (HWFET) is centeredin the medium RPM region. The US06 driving cycle includes

SpeedrefDriver

Currentregulator

1Mv

1SPMAC

iaibiciabc

idq

id iq

WheelTorque Vq

Vd

TorqueForcevehicle

Forceresistance

Speed

Air+androlling+resistance

Speed

+_

Fig. 1. Block-diagram of a simplified electric vehicle powertrain simulation model

0 200 400 600 800 1000 1200 1400Time [s]

0

10

20

30

40

50

60

70

80

90

MP

H [m

ile/h

our]

US06UDDSHWFET

Fig. 2. Driving schedule of UDDS (blue), HWFET(red), and US06(green)

both urban driving and highway driving, with more aggressiveacceleration and higher speed.

TABLE IEV powertrain simulation parameters

VehicleMv (Vehicle weight) [kg] 1493 + 250Cd (Aerodynamic drag coefficient) 0.28Av (Veh. front cross-section area) [m2] 2.29Wheel radius [m] 0.316Maximum speed 95 mphGear ratio 7.94

EnvironmentCr (Rolling Ω coefficient) 0.01ρ (air density) [kg/m3] 1.204

III. Si-IGBT and SiC-MOSFET traction inverter

Based on the EV powertrain simulation results discussedin section II, three traction inverters, 600V Si-IGBT, 1200VSi-IGBT, and 1200V SiC-MOSFET, are designed with thespecifications listed in Table II. A low voltage motor isemployed for the 600V traction inverter, while a high voltage

0 2000 4000 6000 8000 10000Speed [RPM]

-250

-200

-150

-100

-50

0

50

100

150

200

250

Tor

que

[Nm

]

US06UDDSHWFET

Fig. 3. Motor operating points over UDDS (blue), HWFET(red), andUS06(green) driving cycles

motor is utilized for the 1200V traction inverter. Since themechanical load and delivered power to the wheel are identicalfor the low voltage and high voltage motors, the low voltagemotor requires high peak current while lower peak currentis necessary for the high voltage motor. Also, owing to thesuperior current density characteristics of the SiC-MOSFET[6] compared to the Si-IGBT, the current density of the 1200VSiC-MOSFET is similar to the current density of the lower-voltage 600V Si-IGBT. This advantage leads to a reductionin semiconductor die area by a factor of two by means ofutilizing high voltage motor and SiC-MOSFET.

To evaluate the semiconductor drive-cycle energy loss, acomprehensive loss model including switching loss and con-duction loss is developed. The switching loss model of Si-IGBT is modeled as

ES W = Ksw × Ica × Vce

b (1)

and the coefficients are listed in Table III. Comparisonsof switching energy loss of loss model and manufacturer’sdatasheet are shown in Fig. 4 and 5. The predicted switchingenergy loss of Eq. 1 shows good agreements with the measuredloss of manufacturer’s datasheet. Based on this loss model,

TABLE IITraction inverter design parameters and PMAC motor parameters

600V Si-IGBT 1200V Si-IGBT 1200V SiC-MOSFET

Device Infineon Infineon Cree C2M0025120B andIKW75N60T IKW40T120 Cree CPW5-1200-Z050B

Vbat 350 V 800 V 800 VSemiconductor die area [mm2] 3900 3464 1801Current rating per switch 825 A 360 A 360 ACurrent power density [A/mm2] 1.27 0.62 1.20

PMAC motorPoles 6 6Ls [mH] 0.183 0.452φ f [Wb] 0.09 0.11Peak current 683 A 278 A

a loss comparison of the low voltage Si-IGBT and high-voltage Si-IGBT under identical power is shown in Fig.6. Assuming identical power is delivered by both devices,the 600V Si-IGBT delivers higher current to the load than1200V Si-IGBT and shows more loss than the 1200V Si-IGBT owing to inferior conduction loss characteristics. Thisincreases noticeably peak loss of the 600V Si-IGBT inverter,compared to the 1200V Si-IGBT inverter. Detailed results arediscussed in the following section.

TABLE IIISi-IGBT loss model parameters

600V Si-IGBT, IKW75N60TKsw−on 484.3 × 10−12

aon 1.6bon 1.468

Ksw−o f f 88.6 × 10−9

ao f f 1.18bo f f 0.9

1200V Si-IGBT, IKW40T120Ksw−on 1.637 × 10−9

aon 1.536bon 1.45

Ksw−o f f 1.687 × 10−6

ao f f 0.9693bo f f 0.7

The resulting efficiency contours in the speed-torque planefor the 600V Si-IGBT and 1200V Si-IGBT are shown in Figs.7 and 8, respectively. Efficiencies of the 1200V Si-IGBT and600V Si-IGBT inverters show a consistent tendency whichexhibits high efficiency at high RPM regions and low efficiencyin the region of low RPM with high torque. This characteristicleads to high average efficiency for highway driving but lowaverage efficiency for urban driving. EPA standard driving

300 400 500 600 700 800 900Vce [V]

1

2

3

4

5

6

7

8

9

10

Elo

ss [m

J]

1200V-Eon loss model,Ic=40A1200V-Eon,Ic=40A1200V-Eoff loss model,Ic=40A1200V-Eoff,Ic=40A600V-Eon loss model,Ic=75A600V-Eon,Ic=75A600V-Eoff loss model,Ic=75A600V-Eoff,Ic=75A

Fig. 4. 600V and 1200V Si-IGBT switching energy loss of loss model anddatasheet as function of collector-emitter voltage, 1200V device is tested under40A of Ic and 600V device is tested under 75A of Ic

trajectories are superimposed on 600V Si-IGBT efficiencycontours in Fig. 9.

A switching loss model of the SiC-MOSFET has beendeveloped based on the piecewise linear function model [7].A comparison of the predicted switching energy loss and themanufacturer’s published datasheet values is shown in Fig.10. 1200V SiC-MOSFET inverter efficiency contours in thespeed-torque plane are shown in Fig. 11. In comparison withthe 600V Si-IGBT and 1200V Si-IGBT inverters, superiorefficiency over wide operating region, especially at low RPMand high torque region, is observed in the SiC-MOSFETinverter. This superior characteristic results in a substantialgain in average efficiency for urban driving.

TABLE IV650V Si-IGBT, 1200V Si-IGBT, and 1200V SiC-MOSFET traction inverter average efficiency, quality factor Q, and peak loss for UDDS, HWFET, and

US06 driving cycles

600V 1200V 1200VSi-IGBT Si-IGBT SiC-MOSFET

Switching frequency 5kHz 5kHz 5kHzUDDS avg. efficiency 97.51% 98.14% 99.39%HWFET avg. efficiency 99.06% 99.30% 99.72%US06 avg. efficiency 98.60% 98.86% 99.65%CAFE avg. efficiency 98.21% 98.66% 99.54%CAFE Q factor 54.79 73.74 215.69Peak loss 2.2 kW 1.5 kW 0.8 kW

0 20 40 60 80 100 120 140Ic [A]

0

5

10

15

Elo

ss [m

J]

1200V-Eon loss model,Vce=600V1200V-Eon,Vce=600V1200V-Eoff loss model,Vce=600V1200V-Eoff,Vce=600V600V-Eon loss model,Vce=400V600V-Eon,Vce=400V600V-Eoff loss model,Vce=400V600V-Eoff,Vce=400V

Fig. 5. 600V and 1200V Si-IGBT switching energy loss of loss model anddatasheet as function of collector current, 1200V device is tested under 600Vof Vce and 600V device is tested under 400V of Vce

0 5 10 15 20 25 30V

CE # I

C [kW]

0

20

40

60

80

100

120

140

160

180

200

Loss

[W]

1200V Si-IGBT net loss1200V sw. loss1200V cond. loss600V Si-IGBT net loss600V sw. loss600V cond. loss

Fig. 6. Loss comparison of 600V Si-IGBT under 350Vce (red) and 1200VSi-IGBT under 800Vce (blue) as function of power: solid line:net loss,star:switching loss, square:conduction loss

Fig. 7. Efficiency contours of 600V Si-IGBT inverter at 350Vbus in speed-torque plane

Fig. 8. Efficiency contours of 1200V Si-IGBT inverter at 800Vbus in speed-torque plane

Fig. 9. UDDS driving trajectory (blue solid line), HWFET driving cycletrajectory (green solid line), US06 driving cycle trajectory (yellow solid line)and efficiency contours of 600V Si-IGBT inverter at 350Vbus in speed-torqueplane

0 10 20 30 40 50 60Ids [A]

0

0.5

1

1.5

2

2.5

Elo

ss [m

J]

Eon loss model,Vds=600Eon,Vds=600Eoff loss model,Vds=600Eoff,Vds=600Eon loss model,Vds=800Eon,Vds=800Eoff loss model,Vds=800Eoff,Vds=800

Fig. 10. 1200V SiC-MOSFET switching energy loss predicted by loss modelvs. datasheet values, as a function of current at Vds = 800V (red), and atVds = 600V (blue)

IV. Efficiency simulation of Si-IGBT and SiC-MOSFETtraction inverters

The computed inverter average efficiencies for UDDS,HWFET, and US06 drive cycles, and CAFE average, arelisted in Table IV. The inverter Q factor is defined asQ =

∫‖Pout‖/

∫Ploss; higher Q leads either to larger output

power without modification of cooling capacity or to reduc-tion in cooling requirements without degradation of outputpower. Loss contours for the 600V Si-IGBT inverter and the1200V SiC-MOSFET inverter are shown in Fig. 12 and 13,respectively. Peak loss is observed in the peak torque region,while efficiency is proportionally increased as power is higher.

Fig. 11. Efficiency contours of 1200V SiC-MOSFET at Vbus = 800V inspeed-torque plane

Since the inverter DC input voltage is constant, higher currentleads to higher switching and conduction loss. For identicalinverter control schemes, the 600V Si-IGBT and 1200V Si-IGBT inverters show high efficiencies for the highway drivingcycle, owing to small current, but the urban driving efficiencyis observed to be low. However, the 1200V Si-IGBT invertershows higher urban driving cycle average efficiency than the600V Si-IGBT inverter owing to the reduced current at highervoltage. This leads to smaller peak loss and average loss forthe urban driving cycle in the 1200V Si-IGBT with highvoltage PMAC machine without upgrading to SiC. Furtherimprovement is observed with use of SiC-MOSFETs, owingto superior device performance. A reduction in peak loss by afactor of 3 is observed in the SiC-MOSFET traction inverter,relative to the 600V Si-IGBT inverter. Also, a reductionof a factor of 4 in average loss is achieved for the urbandriving cycle. Since loss translates directly into cooling systemcapacity, a significant reduction in cooling system size andweight may be possible. As a result, less energy loss withhigh current power density of SiC-MOSFET inverter enablesa significant reduction in inverter hardware size associatedwith manufacturing cost, compared to conventional Si-IGBTinverters.

V. Conclusions

Reductions in average urban driving loss by a factor of four,peak loss by a factor of three, and semiconductor die area by afactor of two are achievable by use of a 1200V SiC-MOSFETinverter with 800V PMAC traction motor, compared to theconventional Si-IGBT inverters. To evaluate the performanceof the 1200V SiC-MOSFET traction inverter, an 80 kWEV powertrain based on the Nissan LEAF is modeled inMATLAB/Simulink with the UDDS, HWFET, and US06 EPA

Fig. 12. Loss contours of 600V SiC-MOSFET inverter at Vbus = 350V inspeed-torque plane

Fig. 13. Loss contour of 1200V SiC-MOSFET inverter at Vbus = 800/rmVin speed-torque plane

standard driving cycles. A 600V Si-IGBT inverter based onthe Nissan LEAF, a 1200V Si-IGBT inverter based on theToyota Prius, and a 1200V SiC-MOSFET traction inverter aredesigned using commercial semiconductor devices. Throughthe high current density of SiC-MOSFET devices, and throughutilization of a high-voltage PMAC motor, the required semi-conductor die area for the SiC-traction inverter is reducedby half compared to the 600V or 1200V Si-IGBT tractioninverter. A comprehensive loss model including switching lossand conduction loss is developed whose predictions show goodagreement with manufacturer’s datasheets. Based on this lossmodel, reductions in peak loss and urban driving averageloss are observed by means of high voltage inverter andhigh voltage PMAC machine. Furthermore, inverter losses are

analyzed with resulting EV powertrain simulation data overUDDS, HWFET, US06, or CAFE and show a reduction inurban driving loss by means of high voltage motor and inverterthrough 1200V Si-IGBT. Even more significant efficiencyimprovement is observed in the 1200V SiC-MOSFET tractioninverter. Peak loss is reduced by a factor of three and averageloss for the urban driving cycle is reduced by four, comparedto the 600V Si-IGBT inverter. The significant loss reductionmay enable a reduction in cooling system size and weight, aswell as an improvement in EV range and MPGe.

Acknowledgement

The information, data, or work presented herein was fundedin part by the Office of Energy Efficiency and RenewableEnergy (EERE), U.S. Department of Energy, under AwardNumber DE-EE0006921.

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