INOM EXAMENSARBETE ELEKTROTEKNIK,AVANCERAD NIVÅ, 30 HP

, STOCKHOLM SVERIGE 2017

Reliability Study of SiC-Based Power Electronic Devices in DC-DC Converter Used in Heavy-Duty Electric and Hybrid Vehicles

SANAZ NAMAYANTAVANA

KTHSKOLAN FÖR ELEKTROTEKNIK OCH DATAVETENSKAP

TRITA -EE 2017:179

ISSN :1653-5146

www.kth.se

Abstract

A DC-DC converter is used in electrified and hybrid vehicles to supply powerto the low voltage (ex. 24V) system including headlights, horn, air conditioningsystem, wipers, radio, etc. The converter is fed from a high voltage (ex. 650V)battery, which is available in electric/hybrid vehicles, and transfers a relativelyhigh power. SCANIA’s conventional converters, used so far, have silicon-basedswitches, i.e., Si IGBT; and there is an intention to replace the converter with theupgraded counterpart in which SiC-based transistors (SiC-MOSFET) are usedinstead. Wide band-gap silicon carbide (SiC) semiconductor material offers pos-sibilities of faster switching, high-temperature operation, and higher breakdownvoltage for power transistors. SCANIA is investigating the reliability of thesecond generation converter in which the Si IGBT transistors are replaced bySiC-MOSFET transistors. In this thesis, the reliability of a SiC-based switchesused in DC-DC converter of electrified trucks is investigated. The investigationis principally based on different reliability tests results carried out in both switchand converter levels.

To investigate the reliability of SIC MOSFET transistors, first different failuremechanisms, such as gate oxide layer degradation, high-frequency side effects,etc., are introduced, and corresponding test results are presented and discussed.On converter level, the reliability study of SCANIA’s first generation converteris considered, and the weak components in the converter are identified.

In this thesis, the test results provided by SiC-MOSFET and converter suppliersare analyzed and compared with the similar test results conducted on the Si-based converter. In additions, SCANIA performs some particular tests based onits own standardization related to different environmental working conditions,such as high ambient temperature and high vibration situation, to assure thematurity and robustness of the SiC-based converter. These test results arepresented and discussed.

By comparing investigation outcomes acquired from different suppliers and cus-tomers, it is shown that the SiC MOSFET transistor is more efficient that Si-based transistor. It is also demonstrated that SiC MOSFET is more robust andreliable in high power, high voltage, and high switching frequency applications.

The SCANIA’s second generation DC-DC converter has shown advantages overthe first generation; it is more efficient, lighter, and more compact. From thereliability point of view, the second generation has passed almost all relevanttests.

Sammanfattning

En DC-DC-omvandlare används i elektrifierade fordon för att ge ström åt desslågspänningssystem (ex. 24V) vilket kan omfatta bl.a. inklusive strålkastare,horn, luftkonditioneringssystem, vindrutetorkare, radio etc. Omvandlaren matasfrån fordonets högspänningsbatteri (ex 650VDC) och överför en relativt hög ef-fekt till lågspänningssystemet. SCANIAs befintliga omvandlare använder sigav kiselbaserade transistorer (Si IGBT), och det finns en avsikt att ersättaomvandlaren med en uppgraderad motsvarighet vid vilken kiselkarbidbaseradetransistorer (SiC MOSFET) används istället. SiC-baserade halvledarmaterialerbjuder bl.a. möjlighet till högre switch-frekvenser, högre drifttemperaturoch högre spänningstålighet. I denna avhandling utreds tillförlitligheten avSiC-baserade transistorer som används i DC-DC-omvandlare inom elektrifier-ade fordonsbranschen. Undersökningen baseras huvudsakligen på resultat frånolika tillförlitlighetstester utförda på både transistor- och omvandlarnivå.

För att undersöka och analysera tillförlitligheten hos SiC MOSFET-transistorerhar olika felmekanismer såsom nedbrytning av ”gate oxid”-skiktet, högfrekventabiverkningar, etc., presenterats och diskuterats tillsammans med motsvarandetestresultat.För jämförelse har man på omvandlarnivå, utrett tillförlitligheten av Scania’sbefintliga omvandlare och identifierat dess svaga komponenterna.

I denna studie har testresultaten, som tillhandahålls av leverantörer av SiC-MOSFET transistorer, analyserats och jämförts med liknande testresultat somhar genomförts på Si-baserade omvandlare. Utöver det utför Scania vissa speci-fika tester som är baserade på egna standardiserade prover, för att försäkrasig om omvandlarens mognad och robusthet. Dessa är relaterade till olikamiljöförhållanden, t.ex. hög omgivningstemperatur och hög vibrationsnivå.Testresultaten presenteras och diskuteras i avhandlingen.

Genom att jämföra testresultat från olika leverantörer kan man dra slutsatsenatt SiC MOSFET-transistorer är effektivare än Si-transistorer. Dessutom visadesig att SiC MOSFET är mer robust och tillförlitlig i applikationer som kräverhögre effekter, högre spänningar och högre switching-frekvenser.

Den andra generationen av Scania’s DC-DC-omvandlare har visat flera förde-lar över den första generationen; nämligen att den är mer effektiv, lättare, merkompakt och billigare. Från ett tillförlitlighetsperspektiv har den andra gener-ationen har passerat nästan alla relevanta tester.

Acknowledgment

To my great parents Bahman and Vida and my lovely sister Ainaz and dearAli: for providing me with continuous encouragement and support throughoutmy years of study and writing this thesis. This accomplishment would not have

been possible without you. Many Thanks!

I would like to express my deepest thank to my thesis supervisor Ninos Poli,senior engineer in Hybrid Systems Development at Scania AB for suggestingthe topic, his great supervision, and kind support. It was a great honor for meto work under his supervisions in my first working experience in Sweden.

I would also like to express my sincere gratitude to my thesis examiner Prof.Hans-Peter Nee, professor in department of Electrical Energy Conversion atKTH Royal Institute of Technology, for his great advice, motivation, and im-mense knowledge. His guidance helped me in all the time of my study at KTH.I could not have imagined having a better examiner than Hansi for my masterthesis.

I would like to thank Thord Nilson, Lars Lindberg, and Björn Ericson and ZsoltToth-Pal for their insightful information, comments, generous advice and sup-port. Without they precious support it would not be possible to conduct thisresearch.

My sincere thanks also goes to my kind mentor, Pär Sundbäck, expert engineerat Scania who provided me an opportunity to join Scania for my master thesiswork.

Last but by no means least, I am grateful to the university staffs at schoolof Electrical Engineering and Ph.D. students of this school especially at thedepartment of Electrical Energy Conversion for their guidance during my twoyears of study at KTH, also the NEB group members at Scania AB who helpedme in this work.

Sanaz NamayantavanaStockhom, Dec. 2017

Contents

1 Introduction 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Aim and Outcome of Thesis . . . . . . . . . . . . . . . . . . . . . 31.4 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Material, Application, Working Principles and Marketing ofSiC-Based Power Electronic Devices 42.1 Semiconductor Power Transistors . . . . . . . . . . . . . . . . . . 4

2.1.1 MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2 IGBT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Wide-Bandgap Semiconductor Materials . . . . . . . . . . . . . . 52.2.1 Benefits of Wide-Bandgap Semiconductors . . . . . . . . . 52.2.2 SiC MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.3 Comparison between SiC MOSFET ans Si IGBT . . . . . 8

2.3 Market and Application Areas of SiC Devices . . . . . . . . . . . 132.4 Suppliers of SiC Power Transistors . . . . . . . . . . . . . . . . . 13

3 DC-DC Converter Used in SCANIA’s Hybrid Vehicles 153.1 System Description . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2 DC-DC Converter Types . . . . . . . . . . . . . . . . . . . . . . . 163.3 Location of DC-DC converter in Hybrid Vehicles . . . . . . . . . 17

3.3.1 Converter Structure . . . . . . . . . . . . . . . . . . . . . 173.4 Components and Characteristics . . . . . . . . . . . . . . . . . . 203.5 Charactristics of SiC MOSFET used in DCC2 . . . . . . . . . . . 213.6 Main Features of DCC2 With SiC MOSFET . . . . . . . . . . . 24

4 SiC Transistor and Converter Reliability Tests 254.1 SiC MOSFET Reliability . . . . . . . . . . . . . . . . . . . . . . 25

4.1.1 Bond Wire failure (Packaging Reliability) . . . . . . . . . 264.1.2 Body Diode degradation . . . . . . . . . . . . . . . . . . . 274.1.3 Gate threshold voltage degradation . . . . . . . . . . . . . 284.1.4 Gate Oxide Reliability . . . . . . . . . . . . . . . . . . . . 284.1.5 Electromagnetic Compatibility (EMC) . . . . . . . . . . . 284.1.6 Ringing / Overshoot . . . . . . . . . . . . . . . . . . . . . 294.1.7 Short Circuit . . . . . . . . . . . . . . . . . . . . . . . . . 294.1.8 dV/dt Capability . . . . . . . . . . . . . . . . . . . . . . . 304.1.9 Robustness Under Avalanche Condition . . . . . . . . . . 30

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4.2 Power Electronic Device Reliability Tests . . . . . . . . . . . . . 304.2.1 JEDEC Reliability Tests: . . . . . . . . . . . . . . . . . . 31

4.3 DC-DC Converter Reliability . . . . . . . . . . . . . . . . . . . . 33

5 Test Results and Reliability Analysis 345.1 Transistor Test Results Provided by Supplier . . . . . . . . . . . 34

5.1.1 Si and SiC Power Cycling Test Implemented by Supplierof DCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.1.2 SiC MOSFET Tests Conducted by a Transistor Manufacture 375.2 DCC2 Test Results Provided by Supplier . . . . . . . . . . . . . 425.3 DCC2 Tests Results Provided by SCANIA . . . . . . . . . . . . . 43

6 Conclusion and Future Work 446.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Appendix A Test Standards 46

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List of Figures

2.1 Cross-section of an N-channel power MOSFET (in left) and IGBT(in right). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Potential applications of Si, GaN, and SiC power switching tran-sistors based on their output power and operating frequency(45). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 Fundamental differences between Si and SiC family devices (6). . 82.4 Benefits of SiC in traction equipment (17). Figure courtesy of

the Bombardier. . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.5 Loss reduction in converter and motor by using SiC MOSFET

compared with their counterparts with Si IGBT (17). Figurecourtesy of the Bombardier. . . . . . . . . . . . . . . . . . . . . . 10

2.6 Power loss reduction by 70% in SiC inverter comparing with Sicounterpart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.7 Loss comparison between Cree’s Si and SiC devices of similarratings (18). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.8 Comparison of power-loss (right) and efficiency (left) in EV trac-tion inverter with ratings of 90kW, and 8kHz when using Si IGBTand SiC MOSFET power device. . . . . . . . . . . . . . . . . . . 11

2.9 Comparing the power-loss in Si IGBT, and SiC MOSFET. . . . . 112.10 Turn-Off waveform, Current tail in Si IGBT causes switching loss

than SiC MOSFET with no current tail. . . . . . . . . . . . . . . 122.11 Geographical split of power device sales (24). . . . . . . . . . . . 14

3.3 Electric Circuit Diagram of DC-DC converter (31). . . . . . . . . . . . . . .183.4 Transistors are diagonally synchronized, i.e., Q1 with Q4, and Q2

with Q3 are switched On and Off simultaneously. . . . . . . . . . 183.5 PWM signal generating circuit . . . . . . . . . . . . . . . . . . . 193.6 Triangle signal is carrier, sinusoidal is the reference, and the puls

signal is the PWM signal generated based on comparison betweencarrier and reference signals. . . . . . . . . . . . . . . . . . . . . . 19

3.7 SiC MOSFET schematic and symbol. . . . . . . . . . . . . . . . 213.8 Normalized SiC MOSFET on-resistance (RDS(on)) versus temperature.(Manufacture-

A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.9 Switching loss comparison.(Manufacture-A) . . . . . . . . . . . . 22

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3.10 Efficiency of SiC MOSFET vs. Si IGBT @100kHz Boost converter.(Manufacture-A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.11 Efficiency of SiC MOSFET (@100 kHz) vs. Si IGBT (25 kHz).(Manufacture-A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.12 DCC1 and DCC2 efficiency as function of output power. . . . . 24

4.1 Bond wire damage: Breakage and lift-off at the marked area (Leftside) and failure due to bond wire lift-off (Right side) (36). . . . 26

4.2 A cross section view of the conventional SiC MOSFET showingbody diode, gate oxide layer and parasitic BJT (45). . . . . . . . 27

4.3 Cross Section and Equivalent Circuit of a MOSFET (8). . . . . . 294.4 Thermal variation under power cycling test. Th and TL are re-

spectively the upper and lower thermal limits (20). . . . . . . . 324.5 By repeatedly switching the current ON and OFF, the junction

temperature of DUT rises and falls (46). . . . . . . . . . . . . . 32

5.1 Power cycle test conducted on Si an IGBT chip with two bondwires (42). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.2 Forward voltage of body diode (in SiC MOSFETs) under powercycling test (42) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.3 Gate threshold voltage of SiC MOSFETs under power cyclingtest (42) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.4 On-state resistance SiC MOSFETs under power cycling test (42) 365.5 Power Cycling test comparing SiC performance vs Si (42) . . . . 365.6 Measuring gate oxide with constant current method (21). . . . . 375.7 Threshold voltages of 5 Cree SiC MOSFETs using constant cur-

rent method for Ids = 300µA at room temperature and during1000 hours.(21). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.8 Threshold voltages of 5 Rohm SiC MOSFETs using constant cur-rent method for Ids = 2mA at room temperature and during 1000hours.(21). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.9 The schematic diagram of body diode conduction test set.(21) . . 405.10 Body-Diode forward voltages of Rohm SiC MOSFET during 780

hours.(21) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.11 Body-Diode forward voltages of Rohm SiC MOSFET during 1800

hours.(21) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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List of Tables

2.1 SiC benefits in industrial application level . . . . . . . . . . . . . 9

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Chapter 1

Introduction

1.1 Motivation

To lead the pioneering role in introducing new technologies in heavy-duty hy-brid vehicles, SCANIA aims to benefit the full potential of SiC transistors tobring advantages to the converters used in electrified vehicles. The advancedoperational specifications of SiC transistors–such as higher switching frequency,higher junction temperature, and endurance against higher voltage–offer someunique benefits to the power electronic converters. Compactness, robustness,higher efficiency, and more reliability are among the possible benefits. Therefore,it can be concluded that the SiC-based converters are the best fit for electrifiedvehicles to boost the reliability of power supply system.With a close cooperation with SiC transistor manufacturers and also with theconverter suppliers, SCANIA is going to replace the Si-based converter witha SiC-based converter in its hybrid and electrified vehicles. One of the majorsteps, in this regard, is assuring the reliability of the new converter under dif-ferent operational and environmental circumstances. This project is conductedby SCANIA to investigate the reliability of this new converter.

1.2 Background

The transportation industry plays a significant role in energy consumption andenvironmental air pollution in almost all over the world. For example, in 2005the transportation system accounted for one-third of all energy use and carbonemissions in the United States (15). Reducing the CO2 emission by lowering thefossil fuel consumption in the transportation system has become an importantpart of green technologies to lighten the global warming due to human activity(19). Developing electric vehicles (EV) and electric hybrid vehicles (EHV) isone of the promising solutions to lower the CO2 emission and provide a moreenvironmentally-friendly driving experience.Besides air pollution reduction, EVs and HEVs offer some benefits such as:increase of the overall system efficiency by recapturing the braking energy and

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saving it into the vehicle’s battery; generating less noise that is an importantfactor in urban areas.

In EVs, the energy source in the vehicle is a rechargeable battery, and the mainprime mover in the power-train system is an electric motor. There exist a powerconditioning system between the battery and the motor to regulate the requiredpower and execute the control actions that the driver desires. The power condi-tioning systems are basically the power electronic converters which can convertand control the electric parameters, voltage and current, at both ends of thepower conditioning systems. In EHVs, the mechanical driving force can eitherbe provided by a combustion engine, which consumes gasoline, or by an electricmotor which is fed from a rechargeable battery through a power conditioningsystem.

In vehicles, there are some low-voltage (e.g. 24V) loads such as lights, horn,wipers, control systems, etc. In non-hybrid vehicles these loads are supplied bya DC electric generator (alternator). Alternator supplies the power when theengine is running. This limits the availability of power supply when engine is offwhich means only essential loads like starter is supplied by battery. In hybridvehicles, there is a need for 24V power supply, even when the engine is OFF,for example, during electric drive. In electric hybrid vehicles (EHVs), there isa high-voltage battery which supplies energy to the power-train. This batteryalso supplies the 24V loads by a DC/DC-converter. So there is no more needfor a DC generator. To make sure the loads are supplied under any condition,the reliability of DC-DC converter must be high.

DC-DC converter in a EVs/EHVs is a step-down converter which converts ahigh DC voltage to a low DC voltage through three steps:

• a high DC voltage to a high AC voltage by a full-bridge inverter consistsof power electronic switches (transistors).

• The high AC voltage to a low AC voltage by a transformer.

• The low AC voltage to a low DC voltage by a rectifier that consists ofdiodes.

More details about the converter will be given in chapter3.

Silicon-Carbide-based power electronic transistors have been available to en-gineers since the release of the first commercial SiC Schottky Barrier Diodes(SBD) in 2001. At present, besides SiC SBDs, other controllable devices suchas SiC MOSFETs and SiC JFETs are available in the market. There is a rapidgrowth in replacing Si IGBT with SiC MOSFET more benefits are resulting intheir applications (18) and nowadays many transistors’ suppliers such as CREE,ROHM, ST Microelectronics are introducing their generation three SiC MOS-FETS.

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1.3 Aim and Outcome of Thesis

The technology of electrified vehicles is developing rapidly by taking advantageof electrical and power electronic devices such as electric motor and semicon-ductor switches (transistors). The EV technology takes another step of ad-vancement is using silicon-carbide (SiC) power electronic devices, which haveshown more advantages over silicon (Si) counterparts. SCANIA has intendedto upgrade a DC-DC converter–used in hybrid trucks and buses–by replacing SiIGBT with SiC MOSFET.

This thesis aims to investigate the reliability of the new SiC-based DC-DCconverter. The entire procedure of the converter reliability study, in this thesis,is based on different test results implemented on both switch and converter level.There are numerous types of reliability tests targeting different possible failuremechanisms in transistor and converter level. Some tests are performed basedon operational extremes such as converter overloading, a short circuit in outputterminal, etc., and some other tests are based on the environmental extremessuch as high ambient temperature, high vibrations, fall down, etc.

Some of the tests are performed by the transistor manufacturer(s), some byconverter supplier(s), and some other by SCANIA. The results of these testsare presented in this thesis, and when possible, the results of the same testperformed both on Si- and SiC-based converters are compared with each otherto obtain an intuitive understanding of the reliability enhancement/degradationin SiC converters.

1.4 Thesis Outline

The report has been outlined as follows. In Chapter 1 the aim of the projectis presented. In the first section of Chapter 2, the general aspects of SiC semi-conductor are given, and a brief comparison between SiC and Si switches arepresented. The chapter follows with SiC-based switch applications in industry,and in the last part; introduction to different SiC-device suppliers besides sta-tistical data of marketing and application areas of SiC Devices are given. InChapter 3 the DC-DC converter used in SCANIA’s hybrid trucks and buses isoverviewed, and detailed comparison between Si- and SiC-based DC-DC con-verter (DCC1 and DCC2) is presented, the characteristics of the SiC MOSFETbeing used at (DCC2) is given and finally the features of SiC-based converterhas been listed. In Chapter 4, the reliability analysis on SiC MOSFET andDC-DC converter has been discussed by introducing their conventional failuremechanisms and different reliability tests . Relatively, the results and explana-tions of the reliability tests are given in Chapter 5 and the reliability analysisis performed. The conclusion and future work are brought in Chapter 6. TheAppendix includes relevant standards of the reliability tests.

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Chapter 2

Material, Application,Working Principles andMarketing of SiC-BasedPower Electronic Devices

In this chapter, first, power semiconductor transistors, i.e., IGBT and MOS-FET are briefly introduced. Then the wide bandgap semiconductor materialsand their role in power transistors are mentioned. The efficiency and reliabilityoriented comparison between IGBT and SiC MOSFET is made based on infor-mation provided by transistor manufacturers and also information found in theliterature. Market, application, and suppliers of SiC devices are brought at theend of the chapter.

2.1 Semiconductor Power Transistors

2.1.1 MOSFET

The Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) is a voltagecontrolled device, and is used for amplifying or switching electronic signals. Asshown in Fig. 2.1 the body of a MOSFET is usually connected to the sourceterminal which makes it a three-terminal device similar to other Field EffectTransistors (FET). MOSFET, as a high speed switch, is widely used in computerand communications technology (1).

MOSFET is a unipolar device which means that only one sort of current carrier(electron or hole) is involved in the flow of electric current. For instance, forN-channel MOSFETs, carriers are only electrons. The charge carriers do notoccupy the mid-zone (shown as N- in Fig. 2.1). Therefore, there is neither astorage tank which must be cleared when switches off nor any tail current (seeSection 2.2.3). This is why the switching time in MOSFETs is very short; in

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other words the switching frequency is high. The switching times–in the rangeof a few tens of nanoseconds to a few hundred nanoseconds (33)–suitable forhigh frequency applications (e.g., tens of MHz).

Figure 2.1: Cross-section of an N-channel power MOSFET (in left) and IGBT(in right).

2.1.2 IGBT

A silicon-based insulated-gate bipolar transistor (Si IGBT) is a high-power semi-conductor device, commercialized in 1980s (39), which inherits the advantagesof both Power MOSFET and BJT (bipolar junction transistor). IGBT is con-structed from four alternating layers (P-N-P-N) and is controlled by a metal-oxide-semiconductor (MOS) gate structure shown in Fig. 2.1. Like the MOS-FET, the IGBT benefits from a high-impedance gate which requires only a smallamount of energy to switch the device (33). Similar to BJT, the IGBT has ahigh current capability because of low conducting resistance.

The switching time ( turn-on and turn-off times) of an IGBT is on the order offew µs, which makes it ideal as a switching device. The IGBT power modulesare available to ratings as large as 6.5 kV and 1200 A (4).

Currently, the IGBT has been widely used as a key component in medium- andhigh-power applications such as motor drives in vehicle and traction transporta-tion, electric power systems, etc. In the next sections, operational characteristicsand application areas of IGBT are compared against those of SiC MOSFET.

2.2 Wide-Bandgap Semiconductor Materials

2.2.1 Benefits of Wide-Bandgap Semiconductors

Semiconductor materials–such as silicon (Si), gallium nitride (GaN), and siliconcarbide (SiC)–are used to construct the semiconductor switches such as thediode and transistor. These switches are the fundamental devices in electronicand power electronics applications.

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The conventional semiconductor materials (non-wide-band-gap), such as Si,have a band-gap (difference in energy level) around 1 to 1.5 electron-volt (eV),while this gap increases to the order of 2 to 4 eV in wide-band-gap (WBG)semiconductors (21; 16; 41). Wider gap in the WBG semiconductor material,such as SiC, boosts the electronic switches to be able to operate at the highertemperature (in the order of 300), higher voltage, and the higher frequency.The improvements in characteristics of WBG devices can be listed as follow

• Higher bandgap energy (e.g., SiC has three times bigger than Si )

• Higher thermal conductivity / Operating temperature (e.g., thermal con-ductivity of SiC is three time higher than Si)

• Shorter time to be On and Off / Higher switching frequency

• Higher blocking voltage (e.g., SiC has ten times higher dielectric-breakdownfield strength compared to Si (44))

• Lower on-state resistance

The enhancement in WBG semiconductors results in economic (trade-off be-tween higher price of WBG material and their higher beneficial results shallbe regarded) and technical advantages in the device application level. For in-stance, the reduction of power losses helps to reduce volume, weight, and cost ofpower-converter components such as enclosure, filters, and cooling system. Asanother example, by increasing the switching frequency of a device the switch-ing harmonics shifts to higher frequencies which can reduce losses and, hence,the size of the load and/or filter components (41). The benefits of using WBGsemiconductor materials in power electronics can be listed as

• High efficiency devices

• High power density (small size and light weight module)

• Low filtering requirements

• Smaller cooling system

Depending on frequency and power level, the application areas of Si, SiC andGaN differs as illustrated in Fig. 2.2. As seen, SiC is suitable for high powerapplications–in power range of 10 KVA to 10 MVA–such as wind turbine, rail-way, PV and industrial macro inverters, etc.

GaN is suitable for high speed devices operating within frequency range of 10kHzto 10MHz where the power is low such as PC, micro inverters, servers, etc.

Si-based switching devices have been used traditionally in the whole range ofpower and frequency. However, with the advent of SiC and GaN, the Si switchesare replaced by SiC counterparts in high power applications, and by GaN coun-terparts in high frequency (with low power) applications (24).

As the main focus of this report is on the SiC–among all wide bandgap materials–hereafter only the SiC and its related subjects will be addressed.

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Figure 2.2: Potential applications of Si, GaN, and SiC power switching transis-tors based on their output power and operating frequency(45) .

2.2.2 SiC MOSFET

From the SiC device family, the SiC MOSFET is the latest product, which wascommercialized by Cree Inc. in 2010. As a voltage-controlled device, the SiCMOSFET is the most attractive candidate for future power electronic applica-tions (21).

In the first generation of SiC MOSFETs, the reliability of the SiC body diodewas an issue hindering its rapid growth (21). However, MOSFET manufacturerssuch as ROHM Co. and Cree Inc. could ease this problem to an acceptableextend, and nowadays the ROHM’s second and Cree’s third generation SiCMOSFETs show significant improvement. SiC MOSFET has become so maturepower-electronic device that many manufacturers such as ST Microelectronic,Mitsubishi, Infineon, General Electric (GE), etc. are following the productionof this futuristic semiconductor device.

The devices available today are the SiC DMOSFET (Figure 2.1) and the SiCtrench MOSFET recently introduced by ROHM Co., Ltd. Finally, power mod-ules based on several parallel-connected SiC DMOSFET dies (with or withoutSchottky diodes in parallel) are available from both manufacturers. Other man-ufacturers such as Mitsubishi, Infineon, and General Electric (GE) are alsodeveloping SiC MOSFET devices. However, most of this work has been per-formed utilizing devices from Wolfspeed and ROHM Co., Ltd. covering allgenerations(21).

Fundamental Differences between SiC MOSFET and Si Devices

Si-based devices have relatively higher on-state resistance. To decrease thisresistance in IGBT, minority carriers are injected into the drift region (thisphenomenon is called conductivity modulation). The conductivity modulation

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in IGBT generates tail current when the device turns off, resulting in a signifi-cant amount of switching loss. Therefore, IGBT cannot be as fast as MOSFETin switching, but it has higher voltage and power rates.

Figure 2.3: Fundamental differences between Si and SiC family devices (6).

Unlike IGBT, SiC MOSFET do not need conductivity modulation to decreasethe on-state resistance because it has much lower drift-layer resistance than Sidevices. In principle, MOSFETs do not generate tail current, and consequently,they have much less switching loss than IGBT. As a result, the SiC MOSFEThas small on-state resistance plus small switching loss. Therefore, it is an idealdevice for high frequency and high power applications. Compared to high-voltage (600V-900V) silicon MOSFET, a SiC MOSFET has smaller chip area(packaging can be more compact) and very small recovery loss of body diode(6). Figure 2.3 shows the fundamental differences between Si and SiC-baseddevices.

2.2.3 Comparison between SiC MOSFET ans Si IGBT

The SiC MOSFET application areas are mainly where the conventional Si IGBThas been used. Based on significant improvements in operational characteristicsof SiC MOSFET, it is foreseen that this device surpasses its predecessor insome applications, primarily where the power density, efficiency, and reliabilitymatters; for example in electric vehicles. Therefore, it is essential to understandthe differences between these two transistors.

In this section, the performance and properties of Si IGBT and SiC MOS-FET are compared comprehensively. The comparison is particularly focusedon overall efficiency of power-electronic apparatus (e.g., converter used in EVtraction system). To do so, the conduction losses as well as switching losses ofSiC MOSFET are investigated. Moreover, power loss variation under differenttemperatures is taken into account.

The SiC MOSFET manufacturers and costumers have profiled many studies

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and investigations proving that SiC MOSFET has superiority over Si IGBTs interms of the power-loss reduction as shown in Table 2.1.

Table 2.1: SiC benefits in industrial application level

Company Demonstration of SiC Benefits Figure

Bombardier Traction system application Fig. 2.4 & 2.5Mitsubishi Power loss reduction in UPS Inverter Fig. 2.6Cree Inc. SiC MOSFET power loss vs. IGBT Fig. 2.7ST Micro. Power-loss reduction in traction inverter Fig. 2.8ROHM Ltd. SiC MOSFET power loss vs. IGBT Fig. 2.9

Figure 2.4: Benefits of SiC in traction equipment (17). Figure courtesy of theBombardier.

As seen in Fig. 2.4, three advantages– energy efficiency, reduced cooling, andincreased power density–have been mentioned in traction application when SiCdevices are used instead of Si devices. Besides lower power loss in SiC device, thehigher switching frequency causes the cancellation of the low-order harmonicswhich leads to power loss reduction in transformer and motor. As a result,the overall traction system efficiency is much higher than that of Si device.Moreover, the harmonic cancellation makes less noise in the traction motor.

Lowering losses, thanks to SiC devices, in converter, transformer, and motorresults in simpler cooling equipment. For instance, an active cooling system canbe replaced by passive one which neither requires auxiliary power supply norgenerates noises. The faster switching capability of the SiC devices brings abouta more harmonic cancellation, and consequently, smaller filtering componentsare needed.

By using smaller and lighter cooling and filtering equipment, the whole weightof converter and peripheral equipment is decreased and occupy less space. Thesefeatures which result in a high power density in electrical systems are important

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Figure 2.5: Loss reduction in converter and motor by using SiC MOSFET com-pared with their counterparts with Si IGBT (17). Figure courtesy of the Bom-bardier.

in applications like EV traction systems.

Figure 2.6: Power loss reduction by 70% in SiC inverter comparing with Sicounterpart.

Besides power loss reduction in switch and converter level, the SiC-based drivecauses lower losses in an electric motor as shown in Fig. 2.5. This is becauseof the higher switching frequency of the SiC MOSFET which cancels the low

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Figure 2.7: Loss comparison between Cree’s Si and SiC devices of similar ratings(18).

Figure 2.8: Comparison of power-loss (right) and efficiency (left) in EV tractioninverter with ratings of 90kW, and 8kHz when using Si IGBT and SiC MOSFETpower device.

Figure 2.9: Comparing the power-loss in Si IGBT, and SiC MOSFET.

order harmonics. Therefore, the portion of the power loss which is caused byharmonics in electric motor is reduced significantly.

As seen in Fig. 2.6 the switching power loss in SiC MOSFET is significantlylower when comparing with that of IGBT. This achievement is also demon-strated by Cree in Fig. 2.7.

An interesting power loss characteristic is depicted in Fig. 2.8 where ST hasshown that in case of IGBT inverter the power efficiency depends of the inverter’sloading, i.e., efficiency is smaller in trivial loading, whereas the efficiency profileis almost flat in SiC MOSFET inverter.

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In Fig. 2.9, ROHM has provided further details of power loss reduction mech-anism in SiC MOSFET with respect to IGBT. It has shown that the turn-offswitching loss is more than turn-on switching loss in IGBT, however, in SiCMOSFET it is opposite. The turn-off switching loss has been reduced more,and it is less than turn-on switching loss. This phenomena occurs because ofthe cancellation of the current tail when switch turns off (Fig.2.10).

Figure 2.10: Turn-Off waveform, Current tail in Si IGBT causes switching lossthan SiC MOSFET with no current tail.

Beside research and investigations conducted in industry, many studies basedon simulation and experimental results are available in the literature comparingthe performance, efficiency, and reliability of Si IGBT and SiC MOSFET (23;13; 38; 34; 27; 35; 39; 30; 22).

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2.3 Market and Application Areas of SiC Devices

Trends in use of SiC in power electronics in the last two decades is a result of itspotential benefit when efficient solutions and high power densities are desired.

The market of SiC devices (diodes and transistors) is forecasted to rise frommore than $200m in 2015 to over $550m in 2021. From 2010 the SiC powermarket have experienced 26% of growth per year (29). It is forecasted in 2015to see more growth–39% per year–from 2015 to 2020.

SiC diodes dominate the SiC market with 85% of the market share. It is fore-casted that this leading position will not change for several years (12). Inparallel, SiC transistors are increasing in the market and expected to reach 27%market share in 2021. From application type point of view, the power factorcorrection (PFC) power supply has been leading in the market with almost 50%share until 2014. However, this market share has been declined after 2014.Photovoltaics (PV) inverters are dominating the market today, as many PVinverter manufacturers use SiC diodes and MOSFETs in their products (12).

In SiC application areas, it is foreseen that the automotive sector will have thehigher ramp-up because of EV and HEV growth. The solar power, rail traction,and AC motor drive are expected to boost the SiC device market(12).

A price comparison of a 60kW DC-AC HEV inverter with three different switchtypes: Si, SiC and GaN transistors done in 2016. The inverter consists of 12diode and 12 transistor of 650V and 100 Amp. In year 2013, the inverter pricewith SiC or GaN is much higher than that of Si (SiC is 60% and GaN is 40%higher). It should be noticed that by using SiC or GaN the inverter loss hasbeen decreased so that the water cooling system has been eliminated, but theoverall price is still higher. It has been foreseen that in 2020 the inverter costwill breakdown such that the SiC price in 2020 is almost the same as the priceof Si inverter in 2012. Even though the SiC device prices will decrease, the SiCinverter will still be 19% more costly than Si inverter. The GaN inverter willbe cheaper than the Si inverter by -6% (11).

Based on current development status, both SiC and GaN are used in WBGmarkets in the medium voltage range between 600-900V (battle field) while themost market of GaN is in the low-voltage application and the market of SiCcovers both the medium-voltage application with the voltage range higher than900V up to 1.7 kV and the high-voltage application. Yole Developpement visionfor 2020 (24) expects that SiC covers the application of high voltage up to 3.3kV in 2020.

Electric and hybrid vehicles automotive industry are also among the medium-voltage applications of SiC WBG material in their power electronic devices.

2.4 Suppliers of SiC Power Transistors

According to the Yole Developpement Institute, China plays an important roleon power device integration, by 39% of power device sales by region in 2016

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(24). Japan and Europe are also strong among the high power device andapplication assemblies such as automobile, grid, motor drives, PV inverters, etcwith respectively 17% and 18% of the region sales.Geographical splitting of thepower device sales are shown in Fig. 2.11 (24).

Figure 2.11: Geographical split of power device sales (24).

There are several suppliers of SiC power device such as Infinion, Wolfspeed(Cree), Rohm, ST Microelectronic, GeneSiC, Fuji Electric, Mitsubishi Electric,Fairchild semiconductor, etc. Development of SiC power device started with aSiC diode in 2001 and the first SiC transistor was built in 2006 while the SiCMOSFET development started from 2011.

Based on applications, 1200 V is the popular voltage range being produced bymanufacturers such as ST Microelectronics, ROHM Semiconductor, Wolfspeed,and Microsemi Power Products Group in 2016.

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Chapter 3

DC-DC Converter Used inSCANIA’s Hybrid Vehicles

In this chapter the DC-DC converters that are used in SCANIA’s hybrid vehicles are investigated. The topology, technical specifications, importance, the devel-opment of this converter are discussed in this chapter. The second generation of the DC-DC converter which is benefiting from SiC technology is introduced. Finally, the features of this converter and its SiC MOSFET transistors are de-tailed.

3.1 System Description

The main components of a typical hybrid vehicle consist of an electrical ma-chine, a hybrid battery pack, a DC-AC voltage inverter and a DC-DC voltage converter. The electrical machine is normally an AC machine which is fed by the inverter. There are two operating modes for the machine: driving mode and braking (or energy recapturing) mode. In the first mode, the electrical machine operates as a motor and drives the vehicle. The machine power is provided by the inverter, which converts the DC high voltage (500 - 750V) from the hybrid battery pack (hybrid system) to an AC voltage that drives the electrical machine. In the second case, when the vehicle is decelerating the electrical machine is used to harvest the braking energy by acting as a generator. In this mode, the inverter converts the AC voltage from the electrical machine to a DC voltage and power is fed back to the hybrid battery pack.

Traditionally, an alternator is used in a vehicle to supply the 24 V electrical system (24V_sys) and charge the 24 V lead-acid batteries. The main problem with this system is that the power supply from the alternator is available only when the engine is running. This problem is avoided in hybrid vehicles by using a DC-DC converter (DCC).

The DCC supplies the VCA system with sufficient power, independent of the operational state of the combustion engine. This is obtained by transforming

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power from the hybrid system to the 24V_sys. The advantage of this approach is that the DCC can be active even when the engine is not operating. Moreover, the efficiency of a voltage converter is higher than that of an alternator.

– More information and details of this section are omitted from the public report due to the supplier’s confidential policy.–

3.2 DC-DC Converter Types

The first generation of the DCC, which is called DCC1 and is used in SCANIA’s current hybrid vehicles, is based on Si IGBT transistors. The DCC has been upgraded to newer version, called DCC2, by using SiC MOSFET transistors instead of IGBTs. As discussed in Chapter 2, the SiC MOSFETS has introduced more technical advantages comparing to the Si IGBTs. Comparing with DCC1, the DCC2 is approximately 30% lighter in weight and 50% smaller in size. Moreover, the DCC2 is more efficient (5%) with higher power density (-50%loss) which is the better fit for hybrid vehicles where saving space and lowering the weight are highly concerned.

– More information and details of this section are omitted from the public report due to the supplier’s confidential policy.–

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3.3 Location of DC-DC converter in Hybrid Ve-hicles

The placement of the DCC varies depending on the type of vehicle (bus or truck). It can be either on the side or top of the vehicle.

–more details of this section are removed from public report due to SCANIAS’s confidential policy.–

3.3.1 Converter Structure

There are several types of DC-DC converters such as Buck, Boost, Fly back, etc. used in industry (33). The one adopted in DCC is a full-bridge phase shift zero voltage transition (ZVT) converter (40), also known as unidirectional DC transformer. This converter has two main advantages over the other types of DC-DC converter. Firstly, it can convert a DC voltage to another DC voltage which is largely different in magnitude, e.g., 600V to 24V. Secondly, it can transfer a huge power ( it can reach to several MW) from one side to another.

The voltage conversion–from high to low level–is fulfilled in DCC through three stages. In the first stage, the DC high-voltage is converted to AC high-voltage by a full-bridge inverter with PWM (pulse width modulation) switching scheme. In the second stage, the AC high-voltage is applied to the primary side of a transformer (see Fig. 3.4), and an AC low-voltage is induced on the secondary side. In the last stage, a diode-based rectifier converts the AC low-voltage to DC low-voltage. The rectifier can be either as a full bridge or three-leads diode rectifier. The latter form is adopted in DCC2 as shown in Fig. 3.4.

At both ends of the converter there are low-pass passive filters (i.e., inductance and capacitors) to eliminate the high-frequency switching harmonics and provide permissible quality for the DC voltage at the DCC output. There are also two EMC filters on both sides of the converter. Note that control part of DCC is not shown in the figure.

The transmission power is almost the same on both sides of the converter. This means that the electric current on the high-voltage side is low, and on the low-voltage side is high. Since the power-loss heavily depends on the current, Ploss = RI2, on the low-voltage side of the converter where the diodes and polarity protection transistors are connected, there is much more power-loss, and consequently, the generated heat. From reliability analysis point of view, these high-current components are more prone to failure. In the converter reliability investigation section 4.3, this problem is discussed in details.

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Figure 3.3: Electric Circuit Diagram of DC-DC converter (31).

Operating Principle of the ZVT converter in DCC

The full-bridge inverter in the high-voltage side of the DCC (or in ZVT con-verter) uses controllable power-semiconductor transistors (IGBT in DCC1, and SiC MOSFET in DCC2), and regulates the output voltage under different load-ing conditions. The switching frequency as well as the transformer operating frequency is determined by this bridge.

The simplified s chematic d iagram o f t his i nverter i s s hown i n F ig. 3 .4. As seen, there are four transistors, Q1 to Q4, and four anti-parallel diodes in the inverter side. Transistors are diagonally synchronized, i.e., Q1 with Q4, and Q2 with Q3 are switched On and Off simultaneously. The Switching frequency of these transistors in DCC2 is high enough to eliminated a significant amount of harmonics. Principally, in a PWM-based inverter, when the ratio of switching frequency to the fundamental AC voltage frequency increases, more harmonic orders are eliminated. A a result, the converters loss decreases and filtering components become smaller.

Figure 3.4: Transistors are diagonally synchronized, i.e., Q1 with Q4, and Q2with Q3 are switched On and Off simultaneously.

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A PWM switching is based on a comparison between a carrier signal (usuallytriangular form) and a sinusoidal reference signal as shown in Fig. 3.5. Theoutput of this comparison are pulses that are used for switching the transistors.For instance, the generated signal from the circuit shown in Fig. 3.5 is fed toQ1 and Q4, and another signal which is shifted in phase by 180 is fed to Q2and Q3.

Figure 3.5: PWM signal generating circuit

The PWM signal generation is shown in Fig. 3.6 for better understanding. Thegenerated AC voltage at the output of the inverter (input of the transformer)is analogous to the PWM signal, i.e., puls form with peak value equals to DCvoltage magnitude. However, when this signal is processed with Fourier Series,the fundamental component (sinusoidal with the frequency equal to that of thereference signal) is significant.

Figure 3.6: Triangle signal is carrier, sinusoidal is the reference, and the pulssignal is the PWM signal generated based on comparison between carrier andreference signals.

As shown in Fig. 3.4, when Q1 and Q2 are switched On, the induced voltageat the secondary winding of the transformer is such that the upper diode in therectifier side conducts, and this voltage is applied across the load (via filter). Asimilar function is performed when Q2 and Q3 are switched On and lower diodein rectifier side starts conducting.

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3.4 Components and Characteristics

As a high efficiency and fast semiconductor switch, the SiC MOSFET has impacton the DCC2 components. The size of components in DCC2 has been generallysmaller in size. As an example the DC-link capacitor in DCC1 is nearly twotimes larger than the same capacitor in DCC2.

– More information and details of this section are omitted from the public reportdue to the supplier’s confidential policy.–

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3.5 Charactristics of SiC MOSFET used in DCC2

The SiC MOSFET is used in the DCC2 converter is a Silicon carbide PowerMOSFET Fig.3.7 shows the schematic and symbol of this transistor.

Figure 3.7: SiC MOSFET schematic and symbol.

Some of the key futures about this transistor are given below. As can seen in Fig.3.8 Manufacture-A SiC MOSFET on-resistance has a very low variation againsttemperature comparing to the other manufacturer similar device. For exampleif the temperature increases from 25C to 200C, the resistance increases justby 20%.

Figure 3.8: Normalized SiC MOSFET on-resistance (RDS(on)) versustemperature.(Manufacture-A)

This feature makes manufacture-A SiC MOSFET suitable for using in the hightemperature applications when with a low Ron in the device, conduction lossis relatively low. This can improve the thermal design of the power electron-ics, improve the system efficiency, and reduce cooling systems. Normalized SiCMOSFET on-resistance (RDS(on)) versus temperature is shown and comparedwith SiC MOSFET counterpart of other two competitor companies (Manufac-ture B and Manufacture C)in this Figure.

Table shown in Fig.3.9 compares the switching losses between Manufacture-ASiC MOSFET and Si IGBT (similar voltage rating and RON ). As seen from the

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Figure 3.9: Switching loss comparison.(Manufacture-A)

figure the switching losses of the SiC MOSFET are significantly lower than theSi IGBT also the (Eon) and (Eoff ) increases less when the temperature varies(increases).

For instance, the switching losses (Eoff ) of the SiC MOSFET increase by 25%when increasing temperature from 25C to 170C, while the switching loss(Eoff ) at Si IGBT increases by 75% when increasing the temperature in thesame range (10).

Figure 3.10: Efficiency of SiC MOSFET vs. Si IGBT @100kHz Boostconverter.(Manufacture-A)

This feature enables the system to operates at high switching frequency thusreducing the design of passive components being used in filters, and make thetotal size and volume of the system (converter here) smaller.

Based on another applicable test, the efficiency of a 1200V SiC MOSFET at100kHz has similar value with the efficiency of a 1200V Si IGBT at 25kHz

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(Fig.3.10).

Figure 3.11: Efficiency of SiC MOSFET (@100 kHz) vs. Si IGBT (25kHz).(Manufacture-A)

In other word, the efficiency of the SiC MOSFET is comparable with the effi-ciency of the Si IGBT but at 4 time higher switching frequency (Fig.3.11). Byapplying SiC MOSFET transistor in the system we can have a smaller, lighter,and more cost effective device with a higher efficiency and lower losses.

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3.6 Main Features of DCC2 With SiC MOSFET

The main features of DCC2 are listed bellow:

• Lower cost

• Higher efficiency (energy saving equals to approximate 400 liters of diesel/year)

• Smaller size.

• Lower weight.

• Galvanic insulation between high and low voltages

• WEG cooled (water ethylene glycol)

• Possible to be connected in parallel (to increase the power)

• CAN communication, J1939, CANopen

Figure 3.12: DCC1 and DCC2 efficiency as function of output power.

As can be seen in Fig.3.12, the efficiency DCC1 and DCC2 has been measuredbased on their output power at 28V output voltage with input voltages from450V to 800V. It is clear that the efficiency of DCC1 is generally lower thanDCC2. It is also noticeable that the efficiency of DCC1 decreases when the loadincreases, whereas, the efficiency of DCC2 is generally stable.

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Chapter 4

SiC Transistor and ConverterReliability Tests

In this chapter, the most common failure modes that occur on the transistor andconverter level are presented first, and then the tests required to identify thefailures are explained. The chapter follows with results of the tests performedon SiC MOSFET and DCC. Finally, the reliability analysis is discussed based onthe results of the reliability tests that are conducted by SCANIA, DCC supplierand transistor suppliers, including Cree, Rohm, and ST.

4.1 SiC MOSFET Reliability

In this section, the common failure modes of SiC MOSFETs are reviewed, andthe indicators which can show the degradation and failure in the device arediscussed.

The reliability of a SiC-based transistor can be affected by the following issuesresulting in some failure modes:

• Thermo-mechanical stress and mismatching CTE (coefficient of thermalexpansion):

– Bond wire lift-off (also referred as packaging reliability)

– Delamination and cracking

– Solder fatigue

– Burnout failures

• Electrical stress:

– Body diode degradation (reverse current stress)

– Gate threshold voltage degradation

– Avalanche breakdown

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• High frequency switching side effects:

– Electromagnetic compatibility (EMC)

– Ringing / Overshoot

– Low tolerant against short circuit

– dV/dt event

4.1.1 Bond Wire failure (Packaging Reliability)

A bond wire connects a power semiconductor chip face to connecting elementsor other chips. The bond wires are generally composed of aluminum, copper ,or gold. In the power modules with rated currents more than 10 Amps per chip,usually several bond wires are connected in parallel–to distribute the currentamong wires–as a single strand of bond wire is limited in its current capability.

An aluminum bond wire, in comparison with copper and silicon, has higherthermal expansion coefficient. Under high temperature operation, the bondwire length changes and causes damages on the welded connection of the bondfeet. In worse condition the bond wire lifts off and disconnects the circuit.Beside high temperature, a mechanical stress can also cause the same effect. Amechanical tension can be a result of a high surge current. When such currentflows throw a bunch of wires, the resulting magnetic force causes the wires beforcefully attracted toward each other, and those in sides could be lifted off.

After losing some bond wires, the remaining wires must carry extra currentwhich results in even more heat in remaining bond feet. Under this condition,the aging process of the device is more accelerated. Where there is only onebond wire left, the current density will be so dense that the wire starts meltingand an internal arc occurs and finally, the chip is destroyed (9).

Figure 4.1: Bond wire damage: Breakage and lift-off at the marked area (Leftside) and failure due to bond wire lift-off (Right side) (36).

The bond wire lift-off failure mode is evaluated by power cycling test which isexplained in detail in Section 5.1.1.

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4.1.2 Body Diode degradation

As shown in Fig.4.2, MOSFET structure contains a PN-junction which acts asan anti-parallel diode–known as the body diode–and is usually used as a pathfor reverse current flow in electric circuits.

Figure 4.2: A cross section view of the conventional SiC MOSFET showingbody diode, gate oxide layer and parasitic BJT (45).

In (45; 37; 26), it has been shown that the body diode can be degraded undercontinuous current and results in higher voltage drop and, consequently, higherpower loss.

The typical solution for this problem is the attachment of a SiC Schottky Bar-rier Diode (SBD) to the MOSFET. The additional diode has a lower on-stateresistance–hence lower forward voltage–and faster recovery time. By adding theSBD, an alternative path for flyback current is provided which slows down thedegradation of the body diode. However, additional SBD increases the devicecost.

As another solution for body diode degradation, in (45) it was claimed thatby adding a highly doped n-type channel layer an alternate current path forreverse currents can be provided. This solution is based on the change in thegate threshold voltage. The additional cost of this solution is lower than that ofthe extra SBD solution since the n-type channel layer forms part of the design ofthe MOSFET and does not require additional SiC substrate (18). In applicationphase, it should be considered how often and how much reverse current will flowthrough the SiC MOSFET. If the reverse current is only a small portion of thenominal current, the solutions mentioned above can be ignored for the sake ofeconomic saving.

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4.1.3 Gate threshold voltage degradation

The threshold gate voltage of SiC MOSFET can be changed if either positiveor negative voltage is applied to the gate for a long period. This problemhas been resolved in the second generation of SiC MOSFETs but a change ofapproximately 0.25V can be expected (18). The cause of this problem is the thinlayer of the gate oxide and high gate voltage, typically -5V and +20V. It has beenshown in the literature (18) that the threshold voltage change is reversed whenan opposite voltage is applied. This implies that for the switching applicationswhere the gate voltage regularly switches between negative and positive values,a small variation in the threshold voltage will not result in a significant issue.

As a solution to this problem, it has been shown in (45) that adding a highlydoped n-type channel to MOSFET minimizes the threshold voltage variation.This proposed solution, which is known DioMOSFET. Also more sophisticatedgate oxide processing steps with added NO gas has been introduced minimizingthe surface trap density between gate oxide and the SiC christal.

Furthermore, it has been confirmed in (43) that the reliability of current gateoxide technology under high-temperature, high-stress (high voltage applied tothe gate) can be acceptable if a gate drive circuit is properly designed.

4.1.4 Gate Oxide Reliability

The gate oxide layer in SiC MOSFETs, shown in Fig.4.2, is designed with smallerthickness and higher electric field compared with Si MOSFETs. Therefore, thereis higher possibility of reliability issue (current leakage) in oxide layer under anabnormal condition such as a short circuit in the device (44).

Under short circuit condition or high current switching condition, the DC busvoltage is almost applied across the device and creates a high electric field overthe oxide layer. As a result, a leakage current flows through the oxide layer.This current can be intensified if the device temperature rises and lead to SiCMOSFET destruction.

4.1.5 Electromagnetic Compatibility (EMC)

As mentioned in previous chapters, one of the advantages of using SiC MOSFETsis the high-frequency switching which leads to the reduction in the size of passivecomponents required for filtering. For high-speed switching a high-speed circuitis needed to drive the gate by charging the gate capacitance in shortest possibletime.

The high-speed switching results in an increased Electromagnetic Interference(EMI) due to the high rate of current change (di/dt) when the gate capacitance ischarging and discharging. The EMI is also the cause of the high-speed switchingof the load current (18). The higher the load current, the higher is the EMIeffect.

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An increased EMI can create noise, instability and unexpected behavior for thecontrol and communication system circuits. Therefore, an appropriate protec-tion system is needed to prevent EMI detrimental effect.

4.1.6 Ringing / Overshoot

The ringing effect is an overshoot of the voltage across the SiC MOSFET ( thisphenomenon occurs for any high-speed switching device). The voltage overshootis caused by the high-speed switching transient of SiC MOSFET when there areparasitic inductance and capacitance in the circuit (18). Under this condition, aresonant interaction can occur, and the resulting oscillating voltage can exceedthe voltage rating of the switch (or other adjacent device) and cause damageand failure.

There are two types of solutions for this problem: first, optimum circuit designby which the parasitic elements are reduced (e.g., the current path is shortened);second, adding snubber circuit to provide a dissipation path for excess energyand damp the voltage oscillations.

4.1.7 Short Circuit

Because of high-frequency switching, SiC MOSFETs are prone to experienceshort-circuiting events. The resulting damage can be significant as the on-stateresistance of SiC MOSFET is very low.The rate of change of current is very high in SiC MOSFET; e.g., current canrise to 10 times of the nominal value within 10 to 20 S (18).

Figure 4.3: Cross Section and Equivalent Circuit of a MOSFET (8).

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The fault detection time and response time of protection mechanism must bevery small (2 − 5S ).

4.1.8 dV/dt Capability

A sharply change in the drain-source voltage of a MOSFET results in a displace-ment current flowing through the drain-source PN junction capacitance (C) asshown in Fig. 4.3. This current, which can be calculated as i = C(dVc/dt),causes a voltage drop of i.Rb due to the resistance Rb of the p-layer. The re-sistance is in parallel with base-emitter terminals of a parasitic NPN transistor(see Fig. 4.3). If the voltage drop exceeds the base-emitter threshold voltage,the transistor will be on (8) which leads to transistor breakdown.

According to ROHM investigation (6), dV/dt phenomenon is not a serious issuewith SiC-MOSFETs, because the current gain of the parasitic transistor is small.

4.1.9 Robustness Under Avalanche Condition

An avalanche event occurs when a voltage spike exceeds the breakdown voltageof a semiconductor device, and cause a high current to flow through the deviceand break it down. The avalanche event can result in triggering the parasiticBJT, which was explained in previous section.

In high voltage applications, semiconductor devices are connected in series ob-tain the desired blocking voltage. In such arrangement, an unequal voltagesharing among devices can drive one or more of them into avalanche condition,eventually causing the breakdown of the entire group of devices (25).

In some other applications, the semiconductor devices may switch an inductivecircuit whose high rate of change of current (di/dt) may create a voltage sparkacross the device and cause an avalanche event (18).

The avalanche tests have been conducted on SiC MOSFETs in (25). The testresults shows that SiC MOSFETs from Cree are relatively more stable undersingle pulse avalanche tests compared to SiC MOSFETs from RHOM.

4.2 Power Electronic Device Reliability Tests

The reliability tests aim to accelerate the aging and destructive agent (e.g.,temperature, switching time, voltage stress, etc.) of a device to analyze theaforementioned reliability issues and also to estimate the effective lifetime ofthe device. The reliability tests applicable for semiconductor switches are basedon JEDEC Standards (2) and are explained in following. Before implementingthe reliability testings, some basic observations and checks are done. In thisreport they called qualification tests which are listed as:

• Pre-Conditioning (PC): Prior to TC, H3TRB, and IOL tests, the SMD(surface-mount device) parts are checked out.

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• External Visual (EV): The device is carefully checked (e.g., visible inspec-tion and/or powering it up) to ensure that there is no obvious problemand it works normally.

• Parametric Verification (PV): Some parameters (e.g., VDS , IDS , IG inMOSFETs) are tested over rated temperature range prior to reliabilitytesting to ensure the compliance with rated datasheet information.

4.2.1 JEDEC Reliability Tests:

High Temperature Reverse Bias (HTRB):

The threshold voltage degradation issue in SiC MOSFET is investigated bythis test. The device under test (DUT) is connected with reverse bias undermaximum rated temperature (e.g., 150C). The test lasts for hundreds of hoursand the threshold voltage as well as gate leakage current are measured andchecked to see if they are within the allowable limits.

Temperature Cycling (TC):

This test is also known as thermal cycling which tests the reliability of DUTpackaging (mainly wire bonds). Under this test, the device exposed to frequentambient temperature change (e.g, ∆T = 100C). In some case the cycle oftemperature change continues (e.g., more than 1000 times) until DUT breaksdown.

Unbiased Highly Accelerated Stress Test (UHAST):

This tests investigates the robustness of a device (specially its packaging) againstextreme environments. The DUT is subjected to an extreme environment withhigh pressure, high moisture, and high temperature. Under this test the DUTis not biased.

High Humidity and High Temperature Bias (H3TRB):

This test is conducted with high temperature and high humidity. The device iselectrically connected and its bias is reverse. The test lasts for several hundredsof hours.

Intermittent Operating Life (IOL):

This test is also known as power cycling test and is the most important oneamong the power semiconductor device reliability tests.

Under power cycling test the DUT is heated up by applying a voltage to thedevice and loading it with nominal (or even more) current. The power loss, RI2,generated inside the device, creates heat. Within a period when the heat reaches

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to the chip’s maximum junction temperature, (Tj), the power is disconnectedand DUT is cooled down to a specific temperature, and one cycle is completed.See figure 4.5 The heat sink temperature is measured to estimate the junctiontemperature. The test will be repeated until the DUT fails due to the thermalstress caused in the junctions (20; 14).

Figure 4.4: Thermal variation under power cycling test. Th and TL are respec-tively the upper and lower thermal limits (20).

Figure 4.5: By repeatedly switching the current ON and OFF, the junctiontemperature of DUT rises and falls (46).

Wire-Bond Integrity (WBI):

To ensure the current handling capacity and also ensure a secure packaging, theDTU is tested under maximum temperature stress. Under this test, the DUTis biased and the temperature is raised externally.

Electro Static Discharge Characterization (ESD):

To ensure the durability of a power semiconductor switch against ESD events,the device is subjected to ESD charge.(5).

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The aforementioned testings are generally adopted for almost all power semicon-ductor devices. For SiC MOSFET, however, manufacturers and users conductmore specific reliability tests. The JEDEC organization has set a new com-mittee, in September 2017, to profile specific standard documentation for widebandgap semiconductors (3)

4.3 DC-DC Converter Reliability

The reliability of the first generation of DCC (DCC1), which is basen on SiIGBT has been investigated by the converter supplier (28). In the study, it hasbeen assumed that the converter could be active under power cycling test withhigh temperature for ten years (87,600h). However, the converter lifetime in areal application was estimated to be half of this assumption, i.e., about 40,000h.

As a rule of thumb, the life (without failure) of a product equals to the life ofthe weakest component in that product. Thereby, the most vulnerable elementsof DCC1 were identified by supplier (28) and their lifetime and failure modeswere discussed.

Based on Supplier’s investigation (28), the most critical components affectingthe DCC1 lifetime are the 5 polarity protection transistors and the 20 rectifierdiodes. It has been concluded that the hottest transistor will have a failurerate of about 0,07% over a ten year lifetime, and all the transistors together willhave a failure rate of 0.2% and all the diodes together will have 0.4% failure rateover ten years. Although being weak components, the electrolytic capacitors,opto-couplers and IGBTs will not affect the DCC Lifetime.

– More details of this section and are omitted from the public report due to thesupplier’s confidential policy.–

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Chapter 5

Test Results and ReliabilityAnalysis

In this chapter, the results of the tests performed on SiC transistors and DCCare presented and the reliability analysis is provided. Some of the DCC tests areconducted by SCANIA, and some by DCC supplier. The transistor suppliers,including Cree, Rohm, and ST, also provide some reliability test results for SiCMOSFETs.

The comparison between the reliability of DCC1 and DCC2 are given in thischapter.

5.1 Transistor Test Results Provided by Supplier

5.1.1 Si and SiC Power Cycling Test Implemented by Sup-plier of DCC

Figure 5.1: Power cycle test conducted on Si an IGBT chip with two bond wires(42).

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The DCC supplier has conducted power cycle tests on Si IGBT and SiC MOS-FETS (planar and trench). The tested IGBT is a 6x7 mm Si-chip with twobond wires, and temperature difference is T = 60C. As shown in Fig. 5.1 thefirst wire fails at about 940 000 cycles, and second wire fails at about 965 000cycles.

Figure 5.2: Forward voltage of body diode (in SiC MOSFETs) under powercycling test (42)

Figure 5.3: Gate threshold voltage of SiC MOSFETs under power cycling test(42)

For SiC devices, the power cycling tests are implemented on 2 Planar (chip areais 10mm2) and 2 Trench MOSFETS (chip area is 6mm2). For this tests thetemperature difference is T = 100C and heating time 4.5s and cooling timeis 20s. In the first tests, the body diode has been used to heat up the chip.According to (42), no noticeable degradation in body diode has been observedin either device type. Figure 5.2 shows body diode forward voltage under powercycling test with diode current of 10Am. When devices start failing (bond wiresloosens from chip), there is an increase in the voltage and a hotspot is formed(42). The test has been stopped when VG > 0.5V

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Figure 5.4: On-state resistance SiC MOSFETs under power cycling test (42)

Figure 5.5: Power Cycling test comparing SiC performance vs Si (42)

Figure 5.3 shows threshold voltage of the SiC MOSFETs under power cyclingtest. The gate current for all devices is 1 mA. The reason for sudden dropsin threshold voltage is because when bond wires loosened and lift off, the gateoxide is destroyed.

Figure 5.4 illustrates the on-state resistance RDS−on (resistance between Drainand Source) under power cycling test. The resistance is constant until devicesstart failing.

Finally, power cycling test performance of SiC MOSFETs are compared withthat of IGBTs and results shown in Fig. 5.5. The black curve in Fig. 5.5 is thecycle estimation of a device, which is depicted from 1011/∆T where ∆T is thetemperature deviation created during the test.

– More information and test results are omitted from the public report due tothe supplier’s confidential policy.–

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5.1.2 SiCMOSFET Tests Conducted by a Transistor Man-ufacture

In this part, the SiC MOSFET reliability tests conducted by a transistor man-ufacture (manufacture-x) are discussed.

Threshold Voltage Stability

To investigate the threshold voltage stability, manufacture-x has implementedrelated tests on SiC MOSFET with breakdown voltage 1200V and forward re-sistance 160 mΩ (7). Tests are repeated for both positive and negative bias gatevoltage. Each test implemented for 1000 hours with VDS = 10V , ID = 1mA.For positive bias test, the gate voltage is VGS = +20V and temperature is175C. For the negative bias test, the gate voltage is VGS = −15V and temper-ature is 150C. The average shift under positive bias is ∆VTH = 0.06V , and thedeviation of forward resistance is ∆RDS−ON = 0.1mΩ. For the negative biastest, the average shift in threshold voltage is ∆VTH = 0.01V , and the deviationof forward resistance is ∆RDS−ON = 3.2mΩ.

It has been concluded in (7) the threshold voltage is extremely stable underpositive and negative bias.

Similar test has been done in (21) on 20 SiC MOSFET from manufactures Cree(C2M0025120D) and Rohm (SCT280KE). The Gate of transistors are biasedby -10 V and -6 V respectively for Cree and Rohm devices. To check thethreshold voltage deviation resulted from oxide layer defection or gate-oxideionic contamination, the threshold voltage is measured every one hour usingconstant current method (21) which is shown in Fig. 5.6.

Figure 5.6: Measuring gate oxide with constant current method (21).

In this method gate-drain terminals are short circuited and threshold voltage(voltage between Gate-source terminals) is measured every one hour while aconstant current is fed to the the drain-source channel (21).Test results shown in Fig. 5.7and 5.8 reveals that threshold voltage experiencea shift at the beginning hours of the test (30 hours for Cree and 200 hours for

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Figure 5.7: Threshold voltages of 5 Cree SiC MOSFETs using constant currentmethod for Ids = 300µA at room temperature and during 1000 hours.(21).

Rohm). The voltage drift is about 15-30 mV in Cree and about 50-100 mV inRohm SiC MOSFET (21).

Figure 5.8: Threshold voltages of 5 Rohm SiC MOSFETs using constant currentmethod for Ids = 2mA at room temperature and during 1000 hours.(21).

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Breakdown Performance and Lifetime Testing

For breakdown performance, a voltage higher than nominal rating of the deviceis applied to Drain-Source in reverse bias for many hours until the device underthe test breaks down. Under this test there is no load current, therefore novoltage has been applied to the gate, i.e., VG = 0V .manufacture-x has performed this test on a SiC MOSFET with VDS = 1200V ,RDS−on =80mΩ. The temperature is increased to T = 150C in order to accelerate thebreakdown time.

To estimate the lifetime–or mean time to failure (MTTF)–of the device, thebreakdown test is repeated for some times (e.g., three times). The results ofthese tests are used to extrapolate and obtain the working hours (lifetime) of thedevice under desired voltage. An extrapolation has been made over the resultsof three breakdown tests, and the lifetime of the device for the voltage of 800Vhas been estimated to be 3 million hours.

Gate Oxide Reliability Test

A similar to previous test, a breakdown performance and lifetime estimation testis implemented for the gate dielectric (oxide). A voltage, more than nominalrating, is applied to the gate-source for many hours under high temperature.The test runs until the device breaks down. The similar tests are repeated andan extrapolation is made out of the the test results to estimate the oxide lifetimeor MTTF.

Manufacture-x has conducted this test on SiC MOSFET under temperature150C with three different voltages: 38V, 40V, and 42V. The extrapolatedMTTF for gate oxide is

8 million hours for gate nominal voltage, 20V. For maximum allowable voltagethe MTTF is lower.

Wire Bond Fatigue Testing

To test the wire bond reliability, manufacture-x has performed power cyclingtest on SiC MOSFET with two different temperature deviations: ∆Tj = 80C,∆Tj = 100C. For each test three devices are used. Wire bond failure occursin lower cycles when the temperature is higher. Results suggest that the wirebond reliability performance exceeds industry standards, which are 100,000 and20,000 cycles–respectively for ∆Tj = 80C, ∆Tj = 100C– of comparable siliconpower devices. In short, SiC MOSFETs wire bond reliability is higher than theirSi counterparts.

SiC MOSFET Body Diode Stability

To investigate the body diode stability, manufacture-x has conducted a relevanttest on 20 SiC MOSFET. Test has been implemented within two phases: first,devices are connected in reverse bias with Idiode = 10A, VGS = −5V ; second,

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devices are connected in direct bias with IDS = 20A, VGS = 20V . In the firstphase the voltage across the devices, which is body diode forward voltage VF ismeasured. In the second phase the on-state voltage across drian-source VDS,ON

is measured. The junction temperature for this test is 150C.Average voltage shift in VDS,ON is less than 0.2%, and in VF is less than 0.3%,which suggests that the body diode is extremely stable after 1000 hours.

Similar test has been done in (21) on 40 SiC MOSFET from 2 manufacturesCree (C2M0025120D) and Rohm (SCT280KE). A continues forward current isfed to the device while the gate-source terminals are shorted to prevent channelconducting. The schematic diagram of the test is shown in Fig. 5.9

Figure 5.9: The schematic diagram of body diode conduction test set.(21)

Test results are shown in 5.10 and 5.11 indicating that except one device whichis in ON-state, bipolar degradation is not significant in the rest of the devicesand the body diode degradation is negligible.

Figure 5.10: Body-Diode forward voltages of Rohm SiC MOSFET during 780hours.(21)

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Figure 5.11: Body-Diode forward voltages of Rohm SiC MOSFET during 1800hours.(21)

SiC MOSFET dV/dt Ruggedness

The SiC MOSFET stability under immediate change of voltage has been testedby manufacture-x. Under this test the device switches on to feed a load whichis a resistance with RLOAD = 47Ω. The drain is connected to a source withmagnitude of VDD = 1kV .

The voltage plummets in very short time, i.e., it falls from 90% to 10% within2.65 nsec. The calculated current after the device switched on is ID = 21.3A.

The rate of change of voltage, dV/dt, measured to 394kV/µsec with no failures.The test confirms that SiC MOSFET shown a significant ruggedness under asharp change of voltage.

SiC MOSFET Avalanche Testing

To investigate SiC MOSFET avalanche robustness, manufacture-x has tested adevice with avalanche energy of 3.5 Joules. The drain current has been increasedto 50A and switched off immediately and current plummeted sharply. The rateof change of current is so high that a huge transient voltage has been induced inthe drain side. The drain voltage has reached to 2070V, but the device showsno failure and holds its robustness under this condition.

The avalanche energy considered in this test is far exceeds the one used in SiIGBT tests, which is within the range of 10 - 100mJ . This means that the SiCMOSFET is more robust under avalanche problem than IGBT.

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5.2 DCC2 Test Results Provided by Supplier

The DCC supplier has carried out some reliability tests on the converter, whoseresults are presented and discussed in this section. The list of test has beenconducted by the supplier on the DCC2 are given below. Almost all the testshave been passed.

i. Vibration and temperature-cycling

ii. Climate tests

iii. Electromagnetic compatibility- EMC

iv. Electrical Insulation- EI safety

v. Performance test

vi. Function test

vii. Flammability Test

High frequency switching in SiC MOSFET has resulted in some significant ben-efits. However, some side effects are also resulted as well. For example elec-tromagnetic interference (EMI), caused by fast switching, can affect the controland communication systems. As another example, the short circuit possibilityincreases as the switching frequency goes higher.

Several EMC tests have been conducted on DCC2 by supplier. All of the testshave been passed successfully based on general standards. EMC test shouldfulfill the Scania Standards as well.

– Details of this section and test results are omitted from the public report dueto the supplier’s confidential policy.–

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5.3 DCC2 Tests Results Provided by SCANIA

The environmental tests consist of reliability tests conducted for power elec-tronic packages and modules.These tests are carried out into two major groups:environmental test and electrical tests according to Scania standards . Thesestandards are equivalent to the international IEC, JEDEC and MIL-STD-883standards. Almost all the test have been passed.

– Details of this section and test results are omitted from the public report dueto the SCANIAS’s confidential policy.–

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Chapter 6

Conclusion and Future Work

6.1 Conclusion

In this project, the reliability of SiC MOSFET transistors used in EV’s DC-DCconverters was investigated by presenting, comparing, analyzing, and discussingthe reliability test results provided by suppliers.

The aim of using SiC MOSFET transistors is upgrading the DC-DC converter tobe more efficient, compact, lighter, and more reliable. The investigation imple-mented in this project shows how close Scania is to its milestone for reliability ofDC-DC converter. Based on different efficiency, reliability, and durability testsimplemented on both transistor and converter levels, it was confirmed that theSiC-based converter has satisfied the Scania’s expectation to an acceptable ex-tent.

Fundamental differences between Si- and SiC-based transistors were over-viewedin terms of material, structure, applications, and operational characteristics.The advantages of SiC-based transistors were outlined as higher efficiency, higherbreakdown voltage, better thermal conductivity, and higher power density.

To investigate the SiC-MOSFET reliability, first, its failure modes were iden-tified and explained, and the relevant tests were introduced. Then, the testresults presented and associated interpretations and discussions were given toestimate the reliability of the device.

On the converter level, the reliability tests and studies conducted both on firstand second generations of DC-DC converter (i.e. DCC1 and DCC2) were pre-sented, and their weakness was pointed out. For DCC1, the weak componentsare recognized and the converter lifetime was estimated.

In this project, it is concluded that:

• Efficiency of SiC MOSFET is higher than Si IGBT which in DCC2 willresults in

– Higher efficiency

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– Lower power losses

– Smaller cooling system

– Higher power density

• Switching frequency of SiC MOSFET is higher than Si IGBT which inDCC2 will results in

– Reduction in harmonics

– Smaller filtering components

– Less power losses in filtering components

• SiC MOSFET is more robust than Si IGBT

6.2 Future Work

• The dominant reliability issue in power transistors is the bond wire lift-off or packaging reliability which is under ongoing research and recentlynew packaging technology has been introduced. A future work could bedifferent analysis on the new packaging, for instance the EMI problem.

• Optimization analysis on electric circuit boards to reduce EMI emissionfrom SiC MOSFET switching.

• Running parallel reliability tests on Si and SiC transistors (componentlevel).

• Development of reliability model for power electronic devices, speciallySiC MOSFET.

• Reliability analysis on Scania’s new generation converter to identify weakcomponents and entire converter lifetime.

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Appendix A

Test Standards

– Details of standards are omitted from the public report due to the SCANIA’sconfidential policy.–

46

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