A Thorough Review of Cooling Concepts and Thermal Management Techniques for Automotive WBG Inverters: Topology, Technology and Integration Level

energies

Review A Thorough Review of Cooling Concepts and Thermal Management Techniques for Automotive WBG Inverters: Topology, Technology and Integration Level

Ekaterina Abramushkina 1,2,* , Assel Zhaksylyk 1,2, Thomas Geury 1,2 , Mohamed El Baghdadi 1,2 and Omar Hegazy 1,2

1 Mobility, Logistics and Automotive Technology Research Centre (MOBI), Department of Electrical Engineering and Energy Technology (ETEC), Faculty of Engineering, Vrije Universiteit Brussel (VUB), 1050 Brussel, Belgium; [email protected] (A.Z.); [email protected] (T.G.); [email protected] (M.E.B.); [email protected] (O.H.) 2 Flanders Make, 3001 Heverlee, Belgium * Correspondence: [email protected]

Abstract: The development of electric vehicles (EVs) is an important step towards clean and green cities. An electric powertrain provides power to the vehicle and consists of a charger, a battery, an inverter, and a motor as the main components. Supplied by a battery pack, the automotive inverter manages the power of the motor. EVs require a highly efficient inverter, which satisfies low cost, size, and weight requirements. One approach to meeting these requirements is to use the new wide-bandgap (WBG) semiconductors, which are being widely investigated in the industry as an alternative to silicon switches. WBG devices have superior intrinsic properties, such as high thermal Citation: Abramushkina, E.; flux, of up to 120 W/cm2 (on average); junction temperature of 175–200 ◦C; blocking voltage limit of Zhaksylyk, A.; Geury, T.; El Baghdadi, about 6.5 kV; switching frequency about 20-fold higher than that of Si; and up to 73% lower switching M.; Hegazy, O. A Thorough Review losses with a lower conduction voltage drop. This study presents a review of WBG-based inverter of Cooling Concepts and Thermal cooling systems to investigate trends in cooling techniques and changes associated with the use Management Techniques for of WBG devices. The aim is to consider suitable cooling techniques for WBG inverters at different Automotive WBG Inverters: power levels. Topology, Technology and Integration Level. Energies 2021, 14, 4981. Keywords: cooling system; wide-bandgap (WBG); silicon carbide (SiC); gallium nitride (GaN); https://doi.org/10.3390/en14164981 automotive inverter; electric vehicle

Academic Editor: Hongwen He

Received: 15 June 2021 Accepted: 4 August 2021 1. Introduction Published: 13 August 2021 The electric vehicle (EV) market has grown rapidly for several decades as an alternative to conventional internal-combustion engine (ICE) vehicles, resulting in reduced air Publisher’s Note: MDPI stays neutral pollution [1]. The trend of reducing emissions in the atmosphere is demonstrated by the with regard to jurisdictional claims in results of the voluntary agreement between the European Commission and the European published maps and institutional afﬁl- Automobile Manufacturers Association (ACEA), the Japanese Automobile Manufacturers iations. Association, and the Korean Automobile Manufacturers Association. This agreement has a target to reduce CO2 emissions by at least 37.5% compared to a 2021 baseline, which equates to the reduction of CO2 emissions to 59.4 g/km by 2030 (ﬂeet-average emissions are tested through the New European Driving Cycle) [2,3]. Moreover, the European Union Copyright: © 2021 by the authors. (EU) has set targets to reduce greenhouse emissions in the 2030 climate and energy frame- Licensee MDPI, Basel, Switzerland. work to 55% compared to 1990 [3]. According to the ACEA, the percentage of electrically This article is an open access article chargeable and hybrid cars compared to the total of all newly registered cars in Europe distributed under the terms and increased from 4.3% to 22.4% between 2017 and 2020 (Figure1)[4]. conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Energies 2021, 14, 4981. https://doi.org/10.3390/en14164981 https://www.mdpi.com/journal/energies EnergiesEnergies2021 2021, ,14 14,, 4981 x FOR PEER REVIEW 22 of 2121

FigureFigure 1.1. ACEAACEA statistics:statistics: fuelfuel typestypes ofof newnew passengerpassenger carscars inin Europe,Europe, %.%.

TheThe propulsionpropulsion systemsystem playsplays aa centralcentral rolerole inin providingproviding tractivetractive efforteffort andand energyenergy managementmanagement ofof anan electricelectric vehicle.vehicle. TheThe propulsionpropulsion systemsystem includesincludes thethe energyenergy storage,storage, powerpower converters,converters, electricelectric motors,motors, andand associatedassociated controllerscontrollers [5[5].]. EVEV propulsionpropulsion systemssystems havehave beenbeen steadilysteadily improved,improved, withwith thethe mainmain developmentdevelopment trendstrends includingincluding higherhigher powerpower density,density, andand reductionreduction inin size,size, weight,weight, andand costcost [6[6].]. TheThe increaseincrease inin powerpower densitydensity andand decreasedecrease inin size size can can lead lead to to poor poor heat heat dissipation dissipation and and thermal thermal stress, stress, which which in turnin turn leads leads to additionalto additional failures failures of powerof power converters converters [7]. [7]. Thus, Thus, to boostto boost reliability, reliability, the the main main challenge challenge of theof the propulsion’s propulsion’s thermal thermal management management is to is obtainto obtain a compromise a compromise between between power power density den- andsity size.and size. This This compromise compromise allows allows thermal thermal stress stress to to be be avoided avoided and and provides provides a a better better operatingoperating conditioncondition throughthrough aa properproper coolingcooling system system design design [ 8[8].]. Initially,Initially, coolingcooling systemssystems inin EVsEVs usedused forcedforced airair cooling,cooling, beforebefore evolvingevolving toto liquidliquid coolingcooling as a more more effective effective solution solution [9]. [9 ].Because Because air aircooling cooling is less is lessthermally thermally efficient efficient than than liquid cooling, in current automotive applications the inverter air cooling is only liquid cooling, in current automotive applications the inverter air cooling is only possible possible for low power levels. Liquid cooling of EVs is usually provided using a low- for low power levels. Liquid cooling of EVs is usually provided using a low-temperature temperature cooling loop (65 ◦C) for the power inverter [10,11], a high-temperature loop cooling loop (65 °C) for the power inverter [10,11], a high-temperature loop for the electric for the electric machine [12], and a separate battery-cooling loop (20–40 ◦C) [13–15]. This machine [12], and a separate battery-cooling loop (20–40 °C) [13–15]. This research focuses research focuses on the cooling design issues of automotive inverters with new wide- on the cooling design issues of automotive inverters with new wide-bandgap (WBG) sem- bandgap (WBG) semiconductors. Recent contributions regarding the advanced cooling of iconductors. Recent contributions regarding the advanced cooling of automotive inverters automotive inverters show two main approaches. The first approach is to eliminate the show two main approaches. The first approach is to eliminate the low temperature loop low temperature loop through high thermal performance cooling circuits, such as direct through high thermal performance cooling circuits, such as direct liquid cooling and dual- liquid cooling and dual-sided cooling. [13,16–19]. The second approach is to use air cooling sided cooling. [13,16–19]. The second approach is to use air cooling for the inverter and for the inverter and machine, instead of separate cooling loops for each. An example is machine, instead of separate cooling loops for each. An example is the integrated motor the integrated motor drive (IMD) [20]. The air-cooling loop for the IMDs requires the inverterdrive (IMD) to operate [20]. The at highair-cooling temperatures. loop for Becausethe IMDs the requires inverter the operating inverter temperaturesto operate at high are limitedtemperatures. with silicon Because (Si) semiconductors,the inverter operating using newtemperatures semiconductor are limited materials with may silicon lead (Si) to widersemiconductors, applications using of air new cooling. semiconductor WBG semiconductors materials may are lead a possible to wider substitution applications for of air Si semiconductorscooling. WBG semiconductors in automotive applications. are a possible substitution for Si semiconductors in auto- motiveThe applications. thermal performance of a device can be evaluated using different parameters. The The thermal performance of a device can be evaluated using different parameters. thermal resistance, Rth, is a parameter of power modules that indicates the heat transfer capabilityThe thermal and resistance, determines Rth the, is temperaturea parameter differenceof power modules between athat heat indicates source and the aheat case trans- or a fer capability and determines the temperature difference between a heat source and a case heat sink, if specified. Therefore, the junction to heat sink thermal resistance value, Rthj-h, is aor crucial a heat parameter sink, if specified. for heat dissipationTherefore, efficiency.the junction In addition,to heat sink the designthermal of resistance power modules value, Rthj-h, is a crucial parameter for heat dissipation efficiency. In addition, the design of power or cooling systems favors devices with a low Rth to maintain the junction temperature, Tmodulesj, within or the cooling acceptable systems range. favors The devices thermal with resistance a low valueRth to maintain is directly the proportional junction tem- to theperature, material Tj, thicknesswithin the (t) acceptable and inversely range. proportional The thermal to resistance the thermal value conductivity is directly (propor-σ) and dissipationtional to the area material (A) [21 thickness–28]. (t) and inversely proportional to the thermal conductivity (σ) andHigh dissipation thermal performance area (A) [21–28]. of a power converter can be achieved in different ways, for example,High thermal using newperformance materials of with a power interesting conver thermalter can be properties achieved for in powerdifferent module ways, manufacturing.for example, using Some new of thesematerials materials with intere can onlysting be thermal applied properties at manufacture, for power for example, module newmanufacturing. materials for Some substrates of these and materials semiconductors. can only Otherbe applied thermal at manufacture, interface materials for example, (TIMs)

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new materials for substrates and semiconductors. Other thermal interface materials (TIMs) can be chosen at the time of device development [29]. Due to their commercial Energies 2021, 14, 4981 3 of 21 availability, power modules can be used as inverter switches [8]. Another approach is to change the basic concepts of cooling. Examples of this approach are the design of new power module packages with reduced thermal impedance, and new hybrid (air–liquid) orcan integrated be chosen (at at thethe timepower of device module development or inverter [29 level)]. Due cooling to their commercialtechniques availability,[30]. Thus, cool- power modules can be used as inverter switches [8]. Another approach is to change the ing circuit design tends to apply new materials with the lowest thermal resistance and, at basic concepts of cooling. Examples of this approach are the design of new power module thepackages same time, with reducedimplement thermal an optimal impedance, coolin andg new architecture. hybrid (air–liquid) Combining or integrated these approaches (at providesthe power the module highest or power inverter density level) cooling and th techniquese most efficient [30]. Thus, cooling cooling of the circuit device. design tendsWBG to apply semiconductors new materials have with striking the lowest intrinsic thermal properties resistance and,and at are the currently same time, widely availableimplement commercially an optimal cooling [31,32]. architecture. The power Combining electronic theseindustry approaches has adopted provides this the type of semiconductorhighest power densityduring andthe thepast most decade. efficient As coolinga result, of numerous the device. studies about WBG inverter coolingWBG have semiconductors been conducted. have This striking paper intrinsic presents properties a review and of are different currently WBG widely inverter coolingavailable approaches commercially to examine [31,32]. Thethe powerchanges electronic in cooling industry associated has adopted with WBG this type devices. of The papersemiconductor describes during cooling the techniques past decade. for As automotive a result, numerous inverters studies as a part about of WBG EVs, inverter and explores cooling have been conducted. This paper presents a review of different WBG inverter how the use of WBG semiconductors may result in more compact and effective cooling cooling approaches to examine the changes in cooling associated with WBG devices. The systems.paper describes cooling techniques for automotive inverters as a part of EVs, and explores howThe the paper use of is WBG divided semiconductors into eight maysections. result Section in more 2 compactdescribes and new effective WBG coolingsemiconduc- torssystems. with a focus on their thermal properties. Section 3 details the classification of cooling approachesThe paper for isautomotive divided into inverters. eight sections. Sections Section 4 and2 describes 5 include new examples WBG semiconductors of recent inverter air-with and a focusliquid-cooling on their thermal implementations, properties. Section respectively.3 details theSection classification 6 focuses of coolingon promising coolingapproaches techniques, for automotive which inverters.may soon Sections attract4 moandre5 includeattention, examples related of to recent WBG inverter semiconductors.air- Section and liquid-cooling 7 is dedicated implementations, to summarizing respectively. the main Section research6 focuses findings on promisingand highlighting cooling techniques, which may soon attract more attention, related to WBG semiconductors. trends in automotive inverter cooling systems. Section 8 presents the main conclusions. Section7 is dedicated to summarizing the main research findings and highlighting trends in automotive inverter cooling systems. Section8 presents the main conclusions. 2. Wide-Bandgap Semiconductors 2. Wide-BandgapWBG semiconductors Semiconductors are a promising alternative to Si semiconductors [32–37]. Be- cause WBGthe WBG semiconductors switches are are commercially a promising alternative available in to wide Si semiconductors ranges of current [32–37 and]. Be- voltage ratings,cause the this WBG technology switches areis mature commercially enough available to be inused wide in ranges industry of current [26,27,33,38,39]. and voltage Com- ratings, this technology is mature enough to be used in industry [26,27,33,38,39]. Compared pared to Si devices, WBG devices have a number of superior physical properties, such as to Si devices, WBG devices have a number of superior physical properties, such as high 2 2 high thermal flux of up to 120 W/cm2 (Si—10 W/cm2 ); high junction temperature, Tjmax, of thermal flux of up to 120 W/cm (Si—10 W/cm ); high junction temperature, Tjmax, of 175 to 200 °C◦ (Si—Tjmax = 150◦ °C); high limit of blocking voltage (Si—maximum 6.5 kV); 175 to 200 C (Si—Tjmax = 150 C); high limit of blocking voltage (Si—maximum 6.5 kV); higherhigher switching switching frequencyfrequency (about (about 20-fold 20-fold compared compared to Si); andto Si); lower and switching lower switching losses (up losses (upto 73%to 73% lower) lower) with with a lower a lower conduction conduction voltage dropvoltage [40 –drop42]. Spider [40–42]. diagrams Spider comparing diagrams com- paringthe parameters the parameters of Si with of Si those with of those SiC and of SiC GaN and are GaN shown are in shown Figure2 in[ 43Figure]. Despite 2 [43]. the Despite theadvantages advantages of WBG of WBG devices devices and their and decreasingtheir decreasi cost,ng it should cost, it be should noted thatbe noted they are that still they are stillmore more expensive expensive than than conventional conventional semiconductors semiconductors [44]. [44].

Figure 2. Parameter comparison of Si with respect to SiC and GaN, edited from [43]. Figure 2. Parameter comparison of Si with respect to SiC and GaN, edited from [43].

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The most developed and widespread WBG devices are those using gallium nitride (GaN)The [45–48] most developed and silicon and carbide widespread (SiC) WBG[32,44 devices,49–52] are semiconductors. those using gallium These nitride two types of (GaN)WBG [devices45–48] and are silicon becoming carbide increasingly (SiC) [32,44, 49popular–52] semiconductors. in the field of These EVs twodue types to their of higher WBGblocking devices voltage, are becoming frequency, increasingly and operation popular temperature in the ﬁeld [53,54]. of EVs due The to growth their higher in the market blockingrevenuevoltage, of WBG frequency, devicesand can operation be seen temperaturefrom the Yole [53,54 reports’]. The growth road inmaps the market(see Figure 3) revenue[55,56]. ofDespite WBG devices the fact can that be seenthe GaN from semicond the Yole reports’uctors road are mapsconsidered (see Figure to be3)[ suitable55,56] . for use Despitein automotive the fact that inverters, the GaN GaN semiconductors technology are do consideredes not currently to be suitable have for many use in commercially automotive inverters, GaN technology does not currently have many commercially available available high voltage modules compared to SiC [57–60]. Therefore, most of the examples high voltage modules compared to SiC [57–60]. Therefore, most of the examples in this paperin this are paper based are on based SiC inverters. on SiC inverters.

Figure 3. Road maps for WBG power devices according to Yole reports: SiC power devices and GaN Figure 3. Road maps for WBG power devices according to Yole reports: SiC power devices and GaN power devices. power devices. A previous study [61] using a WBG switch in a special high-temperature package with integratedA previous microchannel study [61] liquidusing coolinga WBG showed switch results in a special for peak high-temperature power dissipation package capabilitywith integrated up to 5 microchannel kW/cm2, i.e., significantly liquid cooling higher showed than that results of a Sifor die. peak Theoretically, power dissipation thesecapability advantages up to could 5 kW/cm lead to2, i.e., positive significantly changes in higher the cooling than of that a power of a converter,Si die. Theoretically, for instance,these advantages by reducing could the volume lead to and positive production changes costs in of the the cooling cooling system,of a power combined converter, for withinstance, an increase by reducing in power the density volume [44]. and However, produc WBGtion technologycosts of the is cooling still constrained system, by combined limitations in its thermal packaging technology because of low thermal conductivity [62,63]. with an increase in power density [44]. However, WBG technology is still constrained by To increase operation temperatures up to the device’s physical limitations (175–200 ◦C), high-temperaturelimitations in its packaging thermalshould packaging be designed. technology Examples because can be of found low in thermal the literature; conductivity for[62,63]. instance, To Luincrease et al. presented operation a high-temperaturetemperatures up packaged to the device GaN High’s physical Electron limitations Mobility (175– Transistor200 °C), high-temperature (HEMT), which showed packaging sufficient should performance be designed. operating Examples at 250 ◦C[ can64]. be In addi-found in the tion,literature; in 2020 for Suganuma instance, considered Lu et al. newpresented highly conductivea high-temperature materials forpackaged interconnections GaN High Elec- insidetron Mobility a package Transistor to provide (HEMT), optimal thermal which resistanceshowed sufficient at operating performance temperatures operating greater at 250 ◦ than°C [64]. 200 InC[ addition,65]. in 2020 Suganuma considered new highly conductive materials for interconnectionsDue to their advantages, inside a package WBG devices to provide have optimal good thermal thermal performance resistance properties at operating tem- that can justify optimization or downgrading of an application’s existing cooling system, peratures greater than 200 °C [65]. for instance, using cheaper air cooling instead of liquid cooling. Overall, this can lower the volumeDue to and their cost advantages, of EV powertrains, WBG devices which is have the main good trend thermal in the performance EV industry. As properties anthat example, can justify in 2011 optimization Zhang H. et or al. downgrading [53] performed anof an extensive application’s comparison existing of Si andcooling SiC system, inverters.for instance, As a using result, cheaper the fuel-to-wheel air cooling efficiency instead of hybridof liquid electric cooling. vehicles Overall, (HEVs) this was can lower increasedthe volume by 4.8%and cost due of to betterEV powertrains, regenerative which braking. is the Fuel main usage trend decreased in the from EV industry. 3.94 to As an 3.36example, L/100 km,in 2011 which Zhang is consistent H. et withal. [53] an equivalentperformed fuel an economy extensive improvement comparison of aboutof Si and SiC 13.8%.inverters. The efficiencyAs a result, ofplug-in the fuel-to-wheel hybrid electric effici vehiclesency (PHEVs)of hybrid increased electric byvehicles 13.8%, and(HEVs) was theincreased average by electricity 4.8% due consumption to better regenerative during the Urban braking. Dynamometer Fuel usage Driving decreased Schedule from 3.94 to (UDDS)3.36 L/100 drive km, cycle which was is reduced consistent from with 425.1 an to 308.1equivalent J/m, representing fuel economy a decrease improvement of 27.5%, of about due to the use of SiC switches. In addition, trends in EV inverters, as suggested by Zeng 13.8%. The efficiency of plug-in hybrid electric vehicles (PHEVs) increased by 13.8%, and et al. in 2020, show that WBG inverters have had a higher peak power and power density comparedthe average to Si electricity devices during consumption the past decade during [18 the]. Urban Dynamometer Driving Schedule (UDDS) drive cycle was reduced from 425.1 to 308.1 J/m, representing a decrease of 27.5%, due to the use of SiC switches. In addition, trends in EV inverters, as suggested by Zeng et al. in 2020, show that WBG inverters have had a higher peak power and power density compared to Si devices during the past decade [18].

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3. Classification of Existing Cooling Techniques 3. Classiﬁcation of Existing Cooling Techniques The cooling of the inverter aims to provide an acceptable junction temperature of the powerThe semiconductors cooling of the during inverter operation. aims to provide Operating an acceptable at acceptable junction temperatures temperature increases of the thepower reliability semiconductors and efficiency during ofoperation. the device. Operating The cooling at acceptable systems temperaturesfor automotive increases inverters the reliability and efﬁciency of the device. The cooling systems for automotive inverters have adopted numerous different architectures and solutions (Figure 4). Research on gas- have adopted numerous different architectures and solutions (Figure4). Research on cooledgas-cooled inverters, inverters, for example, for example, can canbe found be found in Chintamani’s in Chintamani’s paper paper [66]. [66 Nevertheless,]. Nevertheless, two fundamentallytwo fundamentally different different approaches approaches can be can considered be considered in inverter in inverter cooling: cooling: air and air and liquid cooling.liquid cooling.

FigureFigure 4. 4.ClassificationClassification of of cooling cooling techniquestechniques forfor automotive automotive inverters. inverters. Air cooling can be divided into forced and non-forced. The main advantages of air coolingAir cooling are the implementation can be divided simplicity into forced and and low non-forced. cost. Until recently, The main there advantages has been little of air coolinginterest are in the air coolingimplementation for automotive simplicity inverters and becauselow cost. the Until overall recently, efficacy there of this has form been of little interestcooling in is air lower cooling than for that automotive of liquid cooling. inverters Thus, because liquid the cooling overall is aefficacy more popular of this form and of coolingwidespread is lower solution. than However,that of liquid the emergence cooling. Thus, of WBG liquid semiconductors cooling is a with more the popular ability to and widespreadoperate at higher solution. temperatures However, has the increased emergence the interestof WBG in semiconductors air cooling. Forced with air coolingthe ability toprovides operate theat higher principal temperatures operation efficiency has increased with cheaper the interest andeasier in air coolingcooling. (see ForcedSection air4 cool-). ingNon-forced provides the air coolingprincipal is notoperation used in efficiency automotive with applications cheaper and because easier of cooling the low (see heat Sec- tiondissipation 4). Non-forced capability. air cooling is not used in automotive applications because of the low heat dissipationLiquid cooling capability. can be classified on basis of water-based or non-water-based coolant liquids.Liquid Water cooling cooling can be is theclassified simplest on andbasis most of water-based popular approach or non-water-based for automotive coolant inverters, and non-water cooling usually implies that the driving motor in EVs uses oil liquids. Water cooling is the simplest and most popular approach for automotive invert- cooling [67–70]. With regard to automotive inverter cooling, non-water cooling involves ers, and non-water cooling usually implies that the driving motor in EVs uses oil cooling the use of water composites and has a special application in liquid cooling. The most [67–70].popular With water regard composite to automotive coolant is 50% inverter water cooling, and 50% non-water ethylene glycol cooling [71] (seeinvolves Section the5). use of waterRegarding composites the type and of has contact a special with theapplication coolant, liquid in liquid cooling cooling. can be categorizedThe most popular into waterdirect composite and indirect coolant cooling. is 50% Indirect water cooling and 50% is an ethylene approach glycol in which [71] (see the powerSection module 5). hasRegarding no direct contactthe type with of contact the liquid. with Asthe cancoolant, be seen liquid from cooling Figure can5a, be a conventionalcategorized into directpackaging and indirect scheme cooling. for indirect Indirect cooling coolin includesg is an a approach power device in which mounted the throughpower module the hassolder no direct over contact a ceramic with substrate. the liquid. A commonlyAs can be seen used from substrate Figure is direct5a, a conventional bonded copper packaging(DBC). scheme The substrate, for indirect in turn, cooling is placed includes through a power DBC solderdevice on mounted the base platethrough (Cu, the AlSiC, solder overetc.). a ceramic Electrical substrate. interconnections A commonly are provided used su viabstrate wire is bonds. direct The bonded entire copper construction (DBC). is The substrate,located on in theturn, heat is sinkplaced or coldthrough plate DBC through solder TIMs on to the ensure base heatplate dissipation (Cu, AlSiC, [8 ].etc.). All ofElec- tricalthese interconnections material and layer are configurations provided via affectwire thebonds. final The thermal entire resistance. construction The differenceis located on thebetween heat sink the or direct cold and plate indirect through liquid TIMs cooling to en issure show heat in Figure dissipation5. In direct [8]. All liquid of these cooling, mate- the base plate, TIM, and heat sink are replaced with only a base plate, thus reducing the rial and layer configurations affect the final thermal resistance. The difference between number of layers inside the module (Figure5b); as a result, the thermal resistance decreases. theFor direct instance, and indirect Mcpherson liquid et al.cooling (2017) is compared show in Figure the thermal 5. In direct resistance liquid of cooling, three module the base plate,configurations: TIM, and heat (a) indirect sink are cooling replaced with with a conventional only a base flat plate, cold thus plate; reducing (b) direct the cooling number ofwith layers a soldered inside the pin-fin module cold plate;(Figure and 5b); (c) as direct a result, cooling the with thermal a silver resistance sintered pin-findecreases. cold For instance,plate. The Mcpherson latter configuration et al. (2017) almost compared halved the thermalthermal resistance resistance compared of three withmodule indirect configurations:cooling [(a)72]. indirect In the case cooling of indirect with cooling,a conventional the thermal flat conductivitycold plate; (b) of thedirect base cooling plate, TIM, with a solderedand heat pin-fin sink play cold a plate; key role and in (c) the direct overall coo heatling dissipation with a silver capability sintered [8 pin-fin,21,35,73 cold]. The plate. Thedirect latter cooling configuration approach involvesalmost eliminatinghalved the thethermal cold plate resistance or heat sink,compared in addition with toindirect the cooling [72]. In the case of indirect cooling, the thermal conductivity of the base plate, TIM, and heat sink play a key role in the overall heat dissipation capability [8,21,35,73]. The

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Energies 2021 14 , , 4981 direct cooling approach involves eliminating the cold plate or heat sink, in addition6 of 21 to the TIMs, and placing the power module in direct contact with water through the base plate direct cooling approach involves eliminating the cold plate or heat sink, in addition to the only [72,74]. TIMs,TIMs, and and placing placing the the power power module in in direct direct contact contact with with water water through through the base the plate base plate onlyonly [72,74]. [72,74 ].

(a) (b)

Figure 5. Schematic module representation(a) with liquid cooling and pin fins: (a) indirect(b) cooling; (b) direct cooling.

Figure 5.Figure Schematic 5. Schematic module module representationFrom representation the construction wi withth liquid liquid cooling coolingperspective,and and pin pin fins:there fins: (a) ( indirectarea) indirect single-sided cooling; cooling; (b) direct (Figure(b) cooling.direct 6a) cooling. and double- sided (Figure 6d) liquid cooling systems [54]. In single-sided cooling, the power module FromFrom the the construction construction perspective,perspective, there there are single-sided are single-sided (Figure6 a)(Figure and double-sided 6a) and double- is cooled(Figure only6d) liquid from cooling one systemsside. The [ 54 standard]. In single-sided single-sided cooling, thecooling power modulemethod is uses cooled a power sided (Figure 6d) liquid cooling systems [54]. In single-sided cooling, the power module moduleonly with from a one cold side. plate The attached standard to single-sided the base plate cooling trough method the usesthermal a power interface module materials is cooled only from one side. The standard single-sided cooling method uses a power [75,76].with Double-sided a cold plate attached cooling to the involves base plate changing trough the the thermal module interface construction materials in [75 such,76]. a man- module with a cold plate attached to the base plate trough the thermal interface materials ner thatDouble-sided two substrates cooling are involves assembled changing one the ab moduleove the construction other with in the such power a manner devices that embed- [75,76].two Double-sided substrates are assembledcooling involves one above changing the other the with module the power construction devices embedded in such a man- ded between them [77]. The double-sided design allows the cooling efficiency to be inner thatbetween two themsubstrates [77]. The are double-sided assembled design one ab allowsove the the other cooling with efficiency the power to be increaseddevices embed- creased because twice as much heat can be dissipated [28,78]. In addition, the bonding ded becausebetween twice them as much[77]. The heat double-sided can be dissipated design [28,78 allows]. In addition, the cooling the bonding efficiency wires to be in- wiresinside inside the the module module can be can eliminated, be eliminated, which improves which improves reliability [reliability76]. The double-sided [76]. The double- creased because twice as much heat can be dissipated [28,78]. In addition, the bonding sidedtechnique technique can can be an be efficient an efficient solution solution because because the dissipation the dissipation of power fromof power both offrom the both of wires inside the module can be eliminated, which improves reliability [76]. The double- the semiconductor’ssemiconductor’s surfaces surfaces leads leads to a to reduction a reduction in thermal in thermal impedance impedance of up to 30% of up [79 ].to 30% [79]. sided technique can be an efficient solution because the dissipation of power from both of the semiconductor’s surfaces leads to a reduction in thermal impedance of up to 30% [79].

(a) (b)

Figure 6.Figure Single-sided 6. Single-sided cold coldplates:(c plates:) (a) (flat;a) ﬂat; (b ()b microchannel;) microchannel; ((cc)) pin pin ﬁn; fin; (d )(d power) power module module(d) for double-sided for double-sided cooling. cooling.

Figure 6. Single-sided cold plates:TheThe (colda) flat; cold plate ( plateb) microchannel; configuration, configuration, (c when) pinwhen applicable,fin; applicable, (d) power can affectmodule can the affect for heat double-sided dissipationthe heat dissipation capability cooling. capa- of a power module. The dissipation capacity is related to the heat dissipation surface, bility of a power module. The dissipation capacity is related to the heat dissipation sur- whichThe cold is usually plate determined configuration, by the coldwhen plate applicable, surface area. can The affect more commonthe heat structures dissipation of capa- face, which is usually determined by the cold plate surface area. The more common struc- bilitycold of platesa power are flat, module. multiple The channel, dissipation microchannel, capacity and is pin related fin (Figure to 6the). Regarding heat dissipation direct sur- turesliquid of cold cooling, plates when are the flat, cold multiple plate is eliminated, channel, themicrochannel, base plate can and fulfill pin the functionsfin (Figure of 6). Re- face, which is usually determined by the cold plate surface area. The more common struc- gardingthe cold direct plate liquid and can cooling, also be implementedwhen the cold in different plate is configurations. eliminated, the base plate can fulfill tures of cold plates are flat, multiple channel, microchannel, and pin fin (Figure 6). Re- the functions of the cold plate and can also be implemented in different configurations. garding direct liquid cooling, when the cold plate is eliminated, the base plate can fulfill In addition to standard air and liquid cooling, several other cooling techniques can the functions of the cold plate and can also be implemented in different configurations. be used for WBG inverters (see Section 6). These inverters have a smaller chip size but In addition to standard air and liquid cooling, several other cooling techniques can operate at higher temperatures. From the cooling system design perspective, this implies be used for WBG inverters (see Section 6). These inverters have a smaller chip size but the use of hot and small areas to dissipate heat. Hoque et al. (2020) presented a modular operate at higher temperatures. From the cooling system design perspective, this implies heat sink for enhanced thermal performance of GaN switches [80]. Another approach to the use of hot and small areas to dissipate heat. Hoque et al. (2020) presented a modular heat sink for enhanced thermal performance of GaN switches [80]. Another approach to

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Energies 2021, 14, x FOR PEER REVIEW In addition to standard air and liquid cooling, several other cooling techniques7 of can 21

be used for WBG inverters (see Section6). These inverters have a smaller chip size but operate at higher temperatures. From the cooling system design perspective, this implies theincreasing use of hot the and thermal small performance areas to dissipate of a small heat. chip Hoque is to et use al. (2020)an electric presented field technology; a modular heatan example sink for for enhanced GaN transistors thermal performancecan be found of in GaN [81]. switchesHeat pipes [80 may]. Another be a solution approach when to increasingsome area therequire thermal a higher performance heat dissipation of a small capability. chip is to useA jet an impingement electric ﬁeld technique technology; is an example for GaN transistors can be found in [81]. Heat pipes may be a solution when also considered to be an effective cooling approach for automotive inverter applications some area require a higher heat dissipation capability. A jet impingement technique is also [82]. Moreover, immersion cooling allows high heat transfer coefficients (h > 2 kW/(m2·K)) considered to be an effective cooling approach for automotive inverter applications [82]. to be achieved for the entire cooling system [83]. The basic principle of an immersion cool- Moreover, immersion cooling allows high heat transfer coefﬁcients (h > 2 kW/(m2·K)) to ing technique is the submergence of the power modules in a thermally and not electrically be achieved for the entire cooling system [83]. The basic principle of an immersion cooling conductive liquid [84]. In addition, integrated motor drive (IMD) topologies are currently technique is the submergence of the power modules in a thermally and not electrically gaining attention. Because of its proximity to the motor, the integrated inverter should conductive liquid [84]. In addition, integrated motor drive (IMD) topologies are currently work at higher temperatures. Therefore, WBG inverters, which have higher operating gaining attention. Because of its proximity to the motor, the integrated inverter should temperatures, are suitable for integration with the electrical machine. However, the work at higher temperatures. Therefore, WBG inverters, which have higher operating proper cooling design of the integrated system remains important, as shown in recent temperatures, are suitable for integration with the electrical machine. However, the proper coolingstudies design[85]. of the integrated system remains important, as shown in recent studies [85].

4.4. AirAir CoolingCooling AirAir coolingcooling isis thethe simplestsimplest andand cheapestcheapest coolingcooling approach.approach. TheThe mainmain problemproblem withwith airair coolingcooling isis aa lowlow heatheat flux.flux. AsAs aa result,result, thisthis techniquetechnique isis onlyonly suitablesuitable forfor lowlow powerpower applications.applications. However,However, thethe highhigh ratedrated temperaturetemperature andand thermalthermal fluxflux dissipationdissipation capabilitycapability ofof WBGWBG semiconductorssemiconductors cancan compensatecompensate forfor thethe lowlow fluxflux ofof airair cooling. cooling. AlthoughAlthough thethe powerpower densitydensity of the liquid-cooled SiC SiC inverters inverters is is higher higher than than that that of of the the Si Si inverters, inverters, a aliquid-cooling liquid-cooling system system implies implies additional additional comp components,onents, which which increase increase the the volume volume and and re- reduceduce the the overall overall power power density. density. Thus, Thus, the air-cooling the air-cooling system, system, which which is the issimplest the simplest imple- implementation,mentation, can increase can increase the power the power density density of the of inverter the inverter [18]. [Hensler18]. Hensler et al. et (2017) al. (2017) pro- proposedposed an anunusual unusual air-cooled air-cooled inverter inverter configuration, configuration, in in which which the the heat heat sink has thethe shapeshape ofof aa hexagon, and the inner inner volume volume of of the the heat heat sink sink is is separated separated by by a plastic a plastic tube tube as asan anair airpath, path, resulting resulting in a in very a very compact compact solution. solution. The The principle principle of the of thehexagon hexagon heat heat sink sink is de- is depictedpicted in inFigure Figure 7a7 [86].a [ 86 Anot]. Anotherher example example was wasdeveloped developed by Liu by et Liu al. etusing al. using3D air-cooled 3D air- cooledSiC packaging SiC packaging for a 20 for kW a 20 inverter kW inverter (see the (see co thencept concept in Figure in Figure 7b) [87].7b) [The87]. research The research of Li ofet Lial. et(2020) al. (2020) showed showed an air-cooled an air-cooled 500 kW 500 modular kW modular inverter inverter with witha conventional a conventional heat heatsink, sink,in which in which the efficiency the efficiency of the of inverter the inverter was about was about 98.74% 98.74% [88]. [88].

(a) (b)

FigureFigure 7.7.Concepts Concepts ofof volumetricvolumetric heatsinksheatsinks forfor airair cooling:cooling: ((aa)) hexagonalhexagonal heatheat sinksink forfor anan inverter;inverter; ((bb)) 3D3D heatheat sinksink forfor aa half-bridgehalf-bridge powerpower module.module.

ZengZeng etet al.al. proposedproposed aa designdesign ofof aa lightlight andand compactcompact EVEV inverterinverter withoutwithout aa liquidliquid coolingcooling system.system. TheThe highesthighest temperaturetemperature reachedreached byby thethe SiCSiC MOSFETMOSFET waswas 110110◦ °C,C, whichwhich isis thethe ratedrated temperaturetemperature ofof WBGWBG semiconductors.semiconductors. ItIt waswas alsoalso provedproved thatthat thethe forcedforced air-air- cooling technique provides a thermal flux of 200 W/cm2, and the SiC MOSFET module has its own heat flux of 120 W/cm2, which is 10-fold higher than that of the Si alternative. The high heat flux of the WBG module means that heat can be more easily dissipated from the chip surface. Thus, the smaller heat flux of air cooling, compared to liquid cooling, is

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cooling technique provides a thermal flux of 200 W/cm2, and the SiC MOSFET module Energies 2021, 14, x FOR PEER REVIEW 2 8 of 21 has its own heat flux of 120 W/cm , which is 10-fold higher than that of the Si alternative. The high heat flux of the WBG module means that heat can be more easily dissipated from the chip surface. Thus, the smaller heat flux of air cooling, compared to liquid cooling, is sufficientsufficient for for WBG WBG power power converters. converters. Moreover, Moreover, the the optimized optimized design design of of the the Al Al heat heat sink sink reducesreduces the the thermal thermal resistance resistance of of the the heatsink heatsink from from 0.44 0.44 to to 0.35 0.35 K/W K/W [ 18[18].]. AnotherAnother air-cooled air-cooled SiC SiC inverter inverter was was introduced introduced by by Wang Wang et et al. al. (2019), (2019), in in which which the the powerpower module module was was cooled cooled with with a a thermally thermally optimized optimized heat heat sink sink (the (the principle principle is is shown shown FigureFigure8). 8). The The optimized optimized heat heat sink sink was was generated generated using using a MATLABa MATLAB Genetic Genetic Algorithm Algorithm (GA)(GA) and and manufactured manufactured usingusing 3D3D printing te technology.chnology. This This comprehensive comprehensive heat heat sink sink al- allowslows sufficient sufficient heat heat to to be be dissipated dissipated from from the the semiconductors semiconductors to enable to enable the theuse useof air of cool- air coolinging rather rather than than liquid liquid cooling. cooling. Moreover, Moreover, two two modules modules in one phasephase werewere split split into into two two submodules,submodules, i.e., i.e., a a top-side top-side module module and and a a bottom-side bottom-side module, module, attached attached to to two two separate separate heatheat sinks, sinks, which which were were placed placed one one above above the the other other for for better better heat heat dissipation. dissipation. Thermal Thermal optimization resulted in a reduction in the volume of the heat sink of 27% and reduced the optimization resulted in a reduction in ◦the volume of the heat sink of 27% and reduced junctionthe junction temperature temperature from from 108.3 108.3 to 102.1 to 102.1C[89 °C]. [89].

FigureFigure 8. 8.Principle Principle of of a a GA-optimized GA-optimized heat heat sink sink for for air air cooling cooling of of a a double-sided double-sided power power module. module.

5.5. Liquid Liquid Cooling Cooling LiquidLiquid cooling cooling is is currently currently the the best best option option for for EVs EVs because because of of higher higher thermal thermal transfer transfer capabilitycapability compared compared to to air. air. Moreover, Moreover, for for power power levels levels applied applied to to automotive automotive inverters, inverters, liquidliquid cooling cooling has has proven proven effectiveness, effectiveness, which which justifies justifies the costthe cost and challengesand challenges in implemen- in imple- tationmentation [74]. Indirect [74]. Indirect liquid liquid cooling cooling with flat with cold flat plate cold isplate a mature is a mature technology technology and can and now can ≥ benow found be infound the literaturein the literature as a “conventional” as a “conventional” or “standard” or “standard” structure structure (with Rth (with0.8 R K/W).th ≥ 0.8 DirectK/W). and Direct indirect and indirect liquid cooling liquid cooling structures structures with complex with complex cold plates cold are plates more are advanced more ad- topologiesvanced topologies [73]. Nevertheless, [73]. Nevertheless, the use of the the use direct of liquidthe direct cooling liquid approach cooling has approach increased has becauseincreased it shows because the it best shows thermal the best performance. thermal performance. By eliminating By eliminating some layers some of the layers power of modulethe power solution, module such solution, as the TIMsuch layeras the and TIM the layer cold and plate, the thecold direct plate, cooling the direct approach cooling allowsapproach the thermal allows the resistance thermal of resistance the power of module the power to be module reduced to by be up reduced to 30% [by90]. up Among to 30% the wide range of direct liquid cooling approaches, jet impingement (0.44–0.48 K/W), [90]. Among the wide range of direct liquid cooling approaches, jet impingement (0.44– turbulator (0.2–0.55 K/W), and microchannel (0.13–0.24 K/W) technologies can be distin- 0.48 K/W), turbulator (0.2–0.55 K/W), and microchannel (0.13–0.24 K/W) technologies can guished as major groups [91]. The direct liquid approach shows better thermal properties, be distinguished as major groups [91]. The direct liquid approach shows better thermal such as a halving in the thermal resistance compared to indirect liquid cooling [92]. How- properties, such as a halving in the thermal resistance compared to indirect liquid cooling ever, direct liquid cooling has a more complex implementation process, which results [92]. However, direct liquid cooling has a more complex implementation process, which in higher costs. Thus, the application of direct liquid cooling is justified only to achieve results in higher costs. Thus, the application of direct liquid cooling is justified only to higher thermal performance. It is worth noting that there can be a power module scale for achieve higher thermal performance. It is worth noting that there can be a power module power electronic devices, called integrated power module cooling, and a power converter scale for power electronic devices, called integrated power module cooling, and a power converter scale to evaluate the efficiency and thermal performance. In the following subsections, single-sided and double-sided cooling approaches are discussed for direct and indirect cooling implementation.

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Energies 2021, 14, x FOR PEER REVIEWscale to evaluate the efﬁciency and thermal performance. In the following subsections,9 of 21

single-sided and double-sided cooling approaches are discussed for direct and indirect cooling implementation.

5.1.5.1. Single-SidedSingle-Sided CoolingCooling Single-sidedSingle-sided liquid liquid cooling cooling is is still still thethe mostmost widespreadwidespread and and cost-effectivecost-effective solution solution for for automotiveautomotive inverters.inverters. AtAt present,present, indirectindirect liquidliquid coolingcooling developmentdevelopment isis representedrepresented byby newnew structures structures of of cold cold plates plates and and heat heat sinks, sinks, where where applicable. applicable. For For instance, instance, Mademlis Mademlis et al. et (2021)al. (2021) presented presented a heat a heat sink sink with with rectangular rectangular pin finspin andfins optimizedand optimized cooling cooling channels channels [93]. The[93]. most The commonmost common cold plate cold type plate for type WBG for inverters WBG inverters in recent in contributionsrecent contributions has a pin has fin a structurepin fin structure [94]. [94]. QiQi etet al.al. (2019)(2019) investigatedinvestigated aa 3030 kVAkVA three-phase three-phase SiC SiC inverter inverter with with a a liquid-cooled liquid-cooled coldcold plate plate for for high-temperaturehigh-temperature operation. operation. The The inverter inverter continuously continuously operated operated for for several several hourshours inin aa high-temperature high-temperature thermalthermal chamber,chamber, where where an an ambient ambient temperature temperature of of 180 180◦ C°C waswas achieved. achieved. TheThe maximummaximum temperaturetemperature ofof thethecommercial commercial SiC SiC modules, modules, in in the the special special high-temperaturehigh-temperature packaging,packaging, reachedreached 150150 ◦°C,C, whereaswhereas thethe temperaturetemperature ofof thethe cold cold plate plate andand coolant coolant were were 85 85 and and 50 50◦C, °C, respectively respectively [95]. [95]. In [ 96In], [96], a 100 a kW 100 SiC kW MOSFET SiC MOSFET three-phase three- inverterphase inverter (34 kW/L) (34 kW/L) with a flatwith cold a flat plate cold was plate tested was at tested 105 ◦ Cat ambient 105 °C ambient and 65 ◦ Cand coolant 65 °C temperatures.coolant temperatures. The thermal The resistancethermal resistance from the junctionfrom the to junction the coolant to the was coolant 0.16 K/W, was and0.16 theK/W, maximum and the die maximum temperature die temperature was 113 ◦C. A was two-level 113 °C. optimized A two-level cold optimized plate was presentedcold plate inwas Gurpinar presented et al. in (2018)Gurpinar for aet liquid-cooled al. (2018) for 125a liquid-cooled kW inverter, 125 and kW the inverter, results showed and the aresults high thermalshowed performancea high thermal at performance a maximum operatingat a maximum capability operating of SiC-MOSFETs capability of [SiC-MOSFETs97]. The two- level[97]. coldThe platetwo-level cools cold a power plate modulecools a power and a DC module link capacitor and a DC bank link fromcapacitor different bank sides; from thisdifferent concept sides; is depicted this concept in Figure is depicted9. Another in Fi examplegure 9. Another of cold plate example thermal of cold optimization plate thermal can beoptimization found in Huber can be et found al. (2018), in Huber in which et al. six (2018), different in which SiC powersix different modules SiC werepower presented modules withwere a presented minimal thermal with a minimal resistance thermal of 0.331 resistance K/W [98 ].of 0.331 K/W [98].

FigureFigure 9. 9.The The principleprinciple ofof thethe two-sidedtwo-sided cold cold plate plate for for a a module module and and DC DC link link capacitors capacitors cooling. cooling.

BeckerBecker etet al.al. carriedcarried outout thermalthermal optimizationoptimization ofof aathree-phase three-phaseSiC SiCMOSFET MOSFET1200 1200V V 150150 A A automotive automotive inverter inverter with with indirect indirect liquid liquid 50/50% 50/50% water–glycolwater–glycol coolingcooling and and a a copper copper heatheat sink sink with with round round pins. pins. A coolingA cooling liquid liquid circulates circulates inside inside an aluminum an aluminum case with case a liquidwith a pressureliquid pressure drop of 25drop kPa of at 25 the kPa pin at area. the Thepin SiCarea. power The SiC module power with module high conductivity with high conduc- Si3N4 (90tivity W/(m Si3N·K))4 (90 substrate W/(m·K)) was substrate pressure was contacted pressure with contacted a pin ﬁn with heat a sink.pin fin The heat experiment sink. The includedexperiment changing included the changing number the of parallelnumber modulesof parallel and modules the distance and the between distance them. between It wasthem. found It was that found if the that distance if the between distance the between chips wasthe reducedchips was from reduced 8 to 1 from mm, only8 to 1 three mm, parallelonly three SiC parallel devices SiC could devices be used could rather be used than 4.rather The simulationthan 4. The ofsimula the completetion of the assembled complete inverterassembled showed inverter that showed the junction that the temperature junction temperature of the hottest of the chip hottest was reduced chip was from reduced 226 ◦ ◦ tofrom 154 226C withto 154 small °C with temperature small temperature differences differences of ∆T = 5.1 ofC ∆ betweenT = 5.1 °C the between chips. Although the chips. ◦ 154AlthoughC is still 154 high, °C is the still maximum high, the Tmaximumj of the SiC Tj semiconductorof the SiC semiconductor used in this used research in this was re- ◦ aroundsearch 170was C.around Moreover, 170 °C. the hotMoreover, spot temperature the hot spot can betemperature optimized bycan changing be optimized the inlet by liquidchanging temperature the inlet liquid and ﬂow. temperature As a result, and the flow. design As alloweda result, the thermaldesign allowed resistance the to ther- be reducedmal resistance by 32.1% to be [99 reduced]. by 32.1% [99].

5.2. Double-Sided Cooling Double-sided liquid cooling is considered for motor driving applications. For instance, in 2020 Liu et al. [100] presented double-sided cooling of the SiC power module, and Möller et al. [92] suggested a double-sided cooling concept for an automotive

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5.2. Double-Sided Cooling Double-sided liquid cooling is considered for motor driving applications. For instance, inapplication. 2020 Liu et Moreover, al. [100] presented a double-sided double-sided liquid cooling cooling of themodule SiC power with jet module, impingement and was Möllerreported et al. in [ 92Tang] suggested et al. [101]. a double-sided cooling concept for an automotive application. Moreover, a double-sided liquid cooling module with jet impingement was reported in Catalano et al. [28] carried out an investigation of SiC-based power modules arranged Tang et al. [101]. in a Catalanohalf-bridge et al. to [28 prove] carried the out efficiency an investigation of double-sided of SiC-based cooling power (the modules concept arranged of the double- insided a half-bridge module tois proveshown the in efficiency Figure 10). of double-sided To compare cooling the thermal (the concept performance of the double- of single- and sideddouble-sided module is cooling, shown in the Figure thermal 10). To resistance compare thewas thermal calculated performance for a wide of single-range andof boundary double-sidedconditions. cooling,The heat the transfer thermal coefficient resistance was was calculated used as fora figure a wide of range merit of boundaryfor the boundary conditions.conditions; The thus, heat h transfer= 101 W/(m coefficient2·K) means was used natural as a figureconvection of merit and for hthe = 109 boundary W/(m2·K) indi- 2 2 conditions;cates contact thus, with h = an 101 ideal W/(m heat·K) meanssink. Mo naturalreover, convection two direct-bonded and h = 109 W/(mcopper· K)substrates indicates contact with an ideal heat sink. Moreover, two direct-bonded copper substrates based on Si3N4 and AlN were compared. According to the research results, double-sided based on Si N and AlN were compared. According to the research results, double-sided cooling shows3 4 lower thermal resistance only when a high heat transfer coefficient is cooling shows lower thermal resistance only when a high heat transfer coefficient is 2 achievedachieved (h (h > 106> 106 W/(m W/(m2·K)).·K)). In theIn the case case of the ofhigh the high heattransfer heat transfer coefficient coefficient and the AlNand the AlN typetype of of substrate, substrate, the the reduction reduction of thermal of thermal resistance resistance was up was to 70%.up to It 70%. should It beshould noted be noted thatthat the the cost cost of of the the AlN AlN substrate substrate is high; is high; thus, thus, using using this substrate this substrate is justified is justified only if highly only if highly efficientefficient cooling cooling is needed.is needed.

FigureFigure 10. 10.Schematic Schematic double-sided double-sided module module representation. representation.

In 2018, Hitachi’s Research & Development Group developed a full-bridge SiC auto- In 2018, Hitachi’s Research & Development Group developed a full-bridge SiC automotive inverter with an improved thermal design. According to the research result, the double-sidedmotive inverter liquid with cooling an applicationimproved ensuredthermal a 35%design. reduction According in thermal to the resistance research with result, the adouble-sided final value of aboutliquid 0.175 cooling K/W. application Moreover, the ensured double-sided a 35% structure reduction enabled in thermal a solution resistance towith be derived a final forvalue the issueof about that occurs0.175 K/W. when Moreover, the parallel the connections double-sided of semiconductors structure enabled a aresolution required to tobe share derived a large for outputthe issue current. that Inoccurs common when structures, the parallel in which connections all intercon- of semicon- nectionsductors areare implemented required to viashare Al wires,a large the output imbalance current. in the In current common can bestructures, considerable, in which all especiallyinterconnections when the are switching implemented frequency via is Al high. wire As, chip-connecting the imbalance structurein the current was built can be con- tosiderable, provide anespecially isometric when current the pattern, switching leading frequency to a reduction is high. in theA chip-connecting current deviation structure between each chip from 10% to 2%. As a result, the overall design was able to reduce power was built to provide an isometric current pattern, leading to a reduction in the current losses by 60% in comparison to the conventional single-side cooled silicon modules [76]. deviation between each chip from 10% to 2%. As a result, the overall design was able to 6.reduce Other Promisingpower losses Cooling by 60% Approaches in comparison to the conventional single-side cooled silicon modulesThe tendency [76]. toward higher powers and smaller sizes requires more efficient cooling with high heat flux. Although WBG devices differ from Si devices in terms of a large number6. Other of Promising parameters, Cooling the most Approaches significant in terms of cooling are the smaller chip size and highThe heat tendency flux due toward to the higher intrinsic powers material and properties. smaller sizes When requires combined, more these efficient two cooling properties are challenging for a cooling system, because it should dissipate a greater amount with high heat flux. Although WBG devices differ from Si devices in terms of a large num- of heat from a smaller size. To ensure uniform temperature distribution, a cooling system shouldber of haveparameters, high dissipation the most capability. significant WBG in semiconductors terms of cooling become are “hot the points”smaller during chip size and unbalancedhigh heat flux temperature due to the distribution, intrinsic whichmaterial lowers properties. the reliability When ofthe combined, entire device. these Thus, two proper- theties WBG are challenging inverters’ cooling for a requires cooling a highsystem, cooling because capacity it should and homogenous dissipate temperaturea greater amount of maintenanceheat from a of smaller the power size. modules’ To ensure chips. unifor Thism section temperature presents coolingdistribution, techniques a cooling and system approachesshould have that high provide dissipation a high thermal capability. performance WBG semiconductors and may be successfully become applied “hot points” to dur- emerginging unbalanced WBG power temperature electronics distribution, systems. which lowers the reliability of the entire device. Thus, the WBG inverters’ cooling requires a high cooling capacity and homogenous temperature maintenance of the power modules’ chips. This section presents cooling techniques and approaches that provide a high thermal performance and may be successfully applied to emerging WBG power electronics systems. New materials could be used to enhance the heat dissipating ability and provide a balanced temperature distribution. The use of metal foam for a heat sink, for instance, was presented by Lee et al. with Samsung Electronics Co in 2020. The authors investigated two

Energies 2021, 14, x FOR PEER REVIEW 11 of 21

Energies 2021, 14, 4981 heat sink structures: a metal foam (Type 1), and a metal foam with11 ofa 21pin fin baseplate (Type 2) for high power IGBTs. The concept of a metal foam layout in a power module is Energies 2021, 14, x FOR PEER REVIEW 11 of 21 shown for Type 1 and Type 2 in Figure 11a and 11b, respectively. Porous heat sinks pro- videNew better materials heat couldtransfer be used with to a enhance low-pressure the heat dissipatingdrop in comparison ability and provide to a microchannel a tech- balanced temperature distribution. The use of metal foam for a heat sink, for instance, was nology, which usually requires pressure drops of 30–100 kPa [102]. The presented liquid heatpresented sink structures: by Lee et al.a metal with Samsungfoam (Type Electronics 1), and Co a inmetal 2020. foam The authorswith a investigatedpin fin baseplate cooling design provides a low Rth = 0.185 K/W and a low inlet pressure Pin = 5–15 kPa. (Typetwo 2) heat for sink high structures: power IGBTs. a metal The foam concept (Type 1), of and a metal a metal foam foam layout with a in pin a ﬁnpower baseplate module is (Type 2) for high power IGBTs. The concept of a metal foam layout in a power module is shownAlthough for Type the 1 andresults Type were 2 in presentedFigure 11a andfor IGBTs, 11b, respectively. the high thermal Porous heatflux sinkscapability pro- of porous shownheat forsinks Type makes 1 and Type this 2technology in Figure 11a andsuitable 11b, respectively. for WBGs. Porous For instance, heat sinks provideTakai et al. (2017) sug- videbetter better heat heat transfer transfer with with a low-pressure a low-pressure drop in drop comparison in comparison to a microchannel to a microchannel technology, technology,whichgested which usually the usuallyuse requires of arequires pressureporous pressure dropsheat ofsink 30–100drop fors a kPaof GaN-based 30–100 [102]. The kPa presented inverter[102]. The liquid [103]. presented cooling liquid coolingdesign design provides provides a low Rth a= low 0.185 R K/Wth = 0.185 and aK/W low inletand pressurea low inlet Pin = pressure 5–15 kPa. P Althoughin = 5–15 kPa. Althoughthe results the were results presented were presented for IGBTs, the for high IGBTs, thermal the high ﬂux capabilitythermal flux of porous capability heat sinks of porous heatmakes sinks this makes technology this technology suitable for suitable WBGs. Forfor instance,WBGs. For Takai instance, et al. (2017) Takai suggested et al. (2017) the sug- gesteduse of the a porous use of heata porous sink for heat a GaN-based sink for a inverterGaN-based [103]. inverter [103].

(a) (b) Figure 11. Concept of a metal foam layout in a power module: (a) a Type 1 structure; (b) a Type 2 structure.

(a) The optimal structure of a cold plate can(b )also increase the heat transfer capability of

FigureFigure 11. Concept 11. Concept of a of metal athe metal device. foam foam layout layout Cold in in aplates a power power with module:module: direct ( a()a) a a Type single-sidedType 1 structure;1 structure; (bliquid) a(b Type) a Typecooling 2 structure. 2 structure. have a higher thermal performance and simpler implementation process compared, for example, to double-sided modules.TheThe optimal optimal Thus, structure structure to design of of a a cold coldhighly plate effective cancan alsoalso coolingincrease increase thefor the heata heatWBG transfer transfer inverter, capability capability complex of cold plate theof device.structures the device. Cold can Cold plates be plates used.with with direct The direct simplest single-sided single-sided approach liquid liquid coolingto cooling enhance have have the a higher effectiveness thermalthermal per- of a cold plate performance and simpler implementation process compared, for example, to double-sided formance and simpler implementation process compared, for example, to double-sided modules.or base Thus,plate to is design to develop highly effectivea structure cooling with for aa WBG larger inverter, surface complex for heat cold removal. plate In 2018, for modules.structuresinstance, Thus, can Infineon beto used.design TheTechnologies highly simplest effective approach presented cooling to enhance forthe a liquid WBG the effectiveness inverter,direct cooled complex of a cold Hybrid-PACKTM platecold plate with structuresordifferent base plate can basebe is to used. develop plate The structures. a simplest structure approach with The a largertherma to enhance surfacel experiment for the heat effectiveness removal. included In of 2018, athe cold for measurement plate of or base plate is to develop a structure with a larger surface for heat removal. In 2018, for instance,three cold Infineon plate Technologies types: ribbon-bonded presented the liquid (this direct conc cooledept is Hybrid-PACKTMdepicted in Figure with 12), pin fin, and instance,different Infineon base plate Technologies structures. The presented thermal experiment the liquid includeddirect cooled the measurement Hybrid-PACKTM of three with flat. At the flow rate of 10 l/min, the result for the flat plate was Rth = 0.153 K/W, for the differentcold plate base types: plate ribbon-bonded structures. (thisThe concept therma isl depictedexperiment in Figure included 12), pin the fin, measurement and flat. At of threetheribbon cold flow rateplate bonded of types: 10 l/min, plate ribbon-bonded the Rth result = 0.132 for the(this K/W, flat conc plateandept wasfor is depicted Rtheth = pin 0.153 infin K/W, Figure structure for 12), the ribbonpin Rth fin,= 0.125 and K/W [73]. bonded plate R = 0.132 K/W, and for the pin fin structure R = 0.125 K/W [73]. Despite flat.Despite At the flow the thratefact ofthat 10 thel/min, thermal the result resistance for the offlat the plate thpin was fin Rcoldth = 0.153 plate K/W, was forlower, the the differ- the fact that the thermal resistance of the pin fin cold plate was lower, the difference was ribbonence bonded was not plate large Rth =(about 0.132 K/W,5.6%), and and for the the mapinnufacture fin structure of aR thribbon-bonded = 0.125 K/W [73]. cold plate is not large (about 5.6%), and the manufacture of a ribbon-bonded cold plate is simpler and simpler and cheaper [90]. Despitecheaper the [90 fact]. that the thermal resistance of the pin fin cold plate was lower, the difference was not large (about 5.6%), and the manufacture of a ribbon-bonded cold plate is simpler and cheaper [90].

FigureFigure 12. 12.Concept Concept of a ribbon-bonded of a ribbon-bonded cold plate. cold plate. Figure 12. Concept of a ribbon-bonded cold plate. Heat pipes are passive two-phase heat transfer devices. This cooling approach in- Heat pipes are passive two-phase heat transfer devices. This cooling approach includes an evaporator, a condenser, a vapor line, a liquid line, and a vapor-removal channel cludes an evaporator, a condenser, a vapor line, a liquid line, and a vapor-removal channel [61]. It should be noted, however, that two-phase cooling systems, in general, are more [61]. It should be noted, however, that two-phase cooling systems, in general, are more effectiveeffective than thansingle single phase phase systems systems [104]. Th[104].e operational The operational principle principleof a heat pipe,of a heatas pipe, as shownshown in Figure in Figure 13, is 13,to dissipate is to dissipate the heat the from heat small from hot small sources hot to sources larger areas to larger through areas through evaporationevaporation and condensationand condensation [105,106]. [105,106]. This circulation This circulation of liquid ofleads liquid to an leads increased to an increased heatheat transfer transfer and, and,most mostimportantly, importantly, provides provides higher thermal higher uniformitythermal uniformity [107]. The [107].heat The heat

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Heat pipes are passive two-phase heat transfer devices. This cooling approach includes antransfer evaporator, capability a condenser, a vaporof heat line, apipes liquid line, is anddetermined a vapor-removal only channel by [61 the]. It cooling capacity of the refrig- shoulderant be [79]. noted, Moreover, however, that two-phase adding cooling heat systems, pipes in allows general, arethe more same effective heat transfer level to be obtained than single phase systems [104]. The operational principle of a heat pipe, as shown in Figurefrom 13 smaller, is to dissipate devices the heat [108]. from small Therefore, hot sources toheat larger pipes areas through in cooling evapo- systems are particularly suit- rationable and for condensation WBG devices [105,106 because]. This circulation they ofprovide liquid leads high to aner increased heat dissipation heat from small areas, lead- transfer and, most importantly, provides higher thermal uniformity [107]. The heat transfer capabilitying to ofa heatbetter pipes temperature is determined only distribution by the cooling capacity among of the power refrigerant devices. [79]. For instance, Gupta et al. Moreover, adding heat pipes allows the same heat transfer level to be obtained from smaller devices(2018) [108 presented]. Therefore, heat cold pipes plates in cooling with systems embedded are particularly heat suitable pipes for WBG that reduced the maximum tem- devicesperature because of they the provide device higher by heat 37 dissipation °C and from improved small areas, temperature leading to a better distribution [108]. In addition, temperature distribution among power devices. For instance, Gupta et al. (2018) presented coldChang plates et with al. embedded proposed heat pipesa heat that pipe reduced integrated the maximum heat temperature sink, of which the decreased the temperature deviceof the by 37heatsink◦C and improved by 6 °C temperature and the distribution thermal [108 resistance]. In addition, between Chang et al. the coolant and chip junction proposed a heat pipe integrated heat sink, which decreased the temperature of the heatsink by10-fold 6 ◦C and the[109]. thermal resistance between the coolant and chip junction 10-fold [109].

FigureFigure 13. Working 13. Working principle of principle a heat pipe. of a heat pipe. Immersion cooling is one of the promising alternatives to conventional liquid cooling. This methodImmersion consists of placingcooling heat is sources one with of directthe promising contact in circulating alternatives liquid to to conventional liquid cool- dissipate the generated heat (the principle is shown in Figure 14). The most important requirementing. This for method immersion coolingconsists systems of is placing to use an electrically heat sources non-conductive with working direct contact in circulating liquid fluid,to dissipate which allows the homogenous generated cooling. heat Equal (the thermal principle distribution is is shown crucial in termsin Figure 14). The most important of increasing the reliability for high power density power devices [104]. Potential types ofrequirement fluids suitable for for immersion immersion cooling are cooling de-ionized systems water, mineral is to oil, use fluorocarbon- an electrically non-conductive work- based fluids, and synthetic oil [83]. Immersion cooling, in general, is either single-phase oring two-phase. fluid, Thewhich former allows includes homogenous using coolants only cooling. in one state Equal of aggregation, thermal and distribution is crucial in terms theof latter increasing implies physical the statereliability changes. Thisfor cooling high approachpower has density a higher heatpower transfer devices [104]. Potential types of coefficient, which is important for increasing electrical power requirements, and can be implementedfluids suitable as one cooling for loopimmersion for the inverter cooling and motor are in EVs, de-ionized thus eliminating water, the mineral oil, fluorocarbon- mainbased motor fluids, 105 ◦C coolant and loopsynthetic [68]. Yuki oil et al. [83]. (2020) presentedImmersion an on-board cooling, two-phase in general, is either single-phase immersion-cooled SiC inverter with a high heat flux of 500 W/cm2, and showed that such aor cooling two-phase. technique can The meet allformer of the requirements includes for using automotive coolants cooling because only itin is one state of aggregation, and simple,the latter compact, implies and highly physical effective [110 state]. changes. This cooling approach has a higher heat transfer The integrated motor drive (IMD) has emerged on the automotive powertrain market, andcoefficient, is rapidly evolving which to becomeis important a key technology for increasing for compact and electrical efficient motor power requirements, and can be drivesimplemented [111]. Cooling technologiesas one cooling are being developedloop for with the integrated invert motorer and drives motor [85]. in EVs, thus eliminating the The recent literature has shown interest in WBG inverters for integrated solutions [112–115], inmain which airmotor cooling 105 can be°C applied coolant for low-power loop [68]. motor Yuki drives. et For al. instance, (2020) Dai presented et al. an on-board two-phase sharedimmersion-cooled an air-cooled IMD concept SiC inverter in which the with operation a high procedure heat of flux the air-cooling of 500 W/cm2, and showed that such system is as follows: (1) inlet air is first drawn radially across the heat sink, which conducts heata cooling out of the powertechnique modules; (2)can the meet air is then all pulled of the axially requirements through the motor’s for airgap automotive cooling because it is tosimple, cool the machine. compact, A thermal and finite highly element effective analysis showed [110]. the predicted temperature

Figure 14. Immersion cooling principle.

The integrated motor drive (IMD) has emerged on the automotive powertrain market, and is rapidly evolving to become a key technology for compact and efficient motor

Energies 2021, 14, x FOR PEER REVIEW 12 of 21

transfer capability of heat pipes is determined only by the cooling capacity of the refrigerant [79]. Moreover, adding heat pipes allows the same heat transfer level to be obtained from smaller devices [108]. Therefore, heat pipes in cooling systems are particularly suitable for WBG devices because they provide higher heat dissipation from small areas, leading to a better temperature distribution among power devices. For instance, Gupta et al. (2018) presented cold plates with embedded heat pipes that reduced the maximum temperature of the device by 37 °C and improved temperature distribution [108]. In addition, Chang et al. proposed a heat pipe integrated heat sink, which decreased the temperature of the heatsink by 6 °C and the thermal resistance between the coolant and chip junction 10-fold [109].

Figure 13. Working principle of a heat pipe.

Immersion cooling is one of the promising alternatives to conventional liquid cool- Energies 2021, 14, 4981 ing. This method consists of placing heat sources with direct contact 13in of circulating 21 liquid to dissipate the generated heat (the principle is shown in Figure 14). The most important requirement for immersion cooling systems is to use an electrically non-conductive work- distributioning fluid, which in the IMDallows with homogenous a current-source cooling. inverter Equal (CSI) for thermal steady-state distribution operation is at crucial in terms a rated load (3 kW) with 25 ◦C ambient air and an assumed air flow of 1.03×10−2 m3/s. Theof increasing maximum efficiency the reliability of the inverter for high was power 97.7% withdensity a peak power temperature devices of 107[104].◦C forPotential types of thefluids WBG suitable devices, andfor 96%immersion for the machine cooling with are an inductor’s de-ionized peak water, temperature mineral of 101 oil,◦C. fluorocarbon- Thesebased results fluids, show and the synthetic feasibility oil of [83]. using Immersion air cooling for cooling, low-power in general, IMD WBG-based is either single-phase solutionsor two-phase. [13]. Another The former example includes of IMD cooling using was coolants presented only in Lillo in etone al. instate 2018. of This aggregation, and isthe a liquid-cooledlatter implies IMD physical in which state each motorchanges. winding This is cooling connected approach to one power has modulea higher heat transfer through an innovative multilayer PCB structure, which is designed to minimize the space requiredcoefficient, for thewhich end windings. is important The coolant for increasing first passes electrical through the power power requirements, section, and and can be theimplemented stator windings as one are then cooling immersed loop infor the the same invert coolant.er and This motor solution in isEVs, based thus on aeliminating the directmain substratemotor 105 cooling °C methodcoolant with loop targeted [68]. Yuki impingement et al. (2020) cooling, presented which can providean on-board an two-phase extremelyimmersion-cooled low specific thermalSiC inverter resistance. with It cana high be concluded heat flux that of some 500 techniques,W/cm2, and which showed that such are not widely used and rarely applied alone, may improve the efficiency of matured liquid cooling.a cooling In Lillo’stechnique study, thecan combination meet all of of the spray requirements cooling and immersion for automotive cooling provides cooling because it is effectivesimple, liquidcompact, cooling and in anhighly IMD [effective14]. [110].

Figure 14. 14.Immersion Immersion cooling cooling principle. principle. 7. Comparison of Cooling Techniques WBGThe invertersintegrated operate motor at higher drive switching (IMD) has frequencies, emerged power on the levels, automotive and tempera- powertrain mar- tures.ket, and The propertiesis rapidly of evolving WBG devices to become comply with a key increasing technology power levelsfor compact and switching and efficient motor frequency requirements for power electronics devices. As shown by the literature review in this paper, the main advantages of WBG devices in cooling design are: • The higher operating temperature of WBG devices leads to the design and use of simpler and cheaper cooling techniques; • The higher switching frequencies and lower losses increase the overall efficiency of the device; • The smaller size and higher heat flux of the WBG die is suitable for compact devices, although this is challenging for cooling system design because more heat should be dissipated from a smaller area. A summary of cooling techniques and their application to WBG converters is shown in Table1. Overall, both air- and liquid-cooled WBG inverters are presented with different optimized configurations of cold plates or heat sinks, with the purpose of increasing the heat dissipation capability of the device. Energies 2021, 14, 4981 14 of 21

Table 1. Summary of reviewed cooling systems for WBG inverters.

Ambient T, ◦C Ref. Power, kW /f , Heat Sink orCold Power Density, Cooling WBG Type s /In. Coolant T, ◦C T , ◦CR , KW Key Conclusions No kHz Plate jmax th kW/L /In. Speed, l/min The forced air cooling is feasible for the low power [18] Forced-air 1.2 kV/50 A SiC 10/50 Finned heat sink 25/-/- 140 j-amb. 0.35 13 SiC inverter. The air-cooled hexagonal construction with a fan 1.2 kV/75 A SiC Flat hexagonal [86] Forced-air 2 × 27/100 23/-/- 105 j-h.s. 1.0 17.2 inside is a low inductivity, compact, and high-power MOSFET heat sink density solution. 3D, 3-sided The 3D SiC inverter with an integrated heat sink. The [87] Forced-air 1.2 kV/120 A SiC 20/20 heatsink with air 25/-/- 106 - 18.6 heat sink optimized through distribution and path pattern diameter of holes in an air path pattern. Five 100 kW inverters paralleled to achieve a high [88] Forced-air 1.2 kV/300 A SiC 5 × 100/20 Finned heat sink 40/-/- 133 j-amb. 0.071 1.25 efﬁciency 500 kW inverter. The GA-optimized heat sink allows an increase in the 1.7 kV SiC [89] Forced-air 50/20 Finned heat sink -/75/- 103 - 1.8 heat dissipation capability and decrease in the MOSFET junction temperature. Short time overloading with 55% heat increase allows Cold plate, 105/65/10 [93] Liquid 1.2 kV/450 A SiC 4/10 117 j-case 0.026 - the heat sink temperature to be maintained beneath rectangular pins water/ethylene the limits. The manifold microchannel (MMC) heat sink is made Micro-channel 105/65/10 of a high thermal conductivity material and exploited [97] Liquid - SiC MOSFET 125/30 110 - - heat sink water/ethylene in the dual-sided heat sink design. The second side cools the capacitors. The inverter is designed for operating at a high 1.2 kV/450 A SiC 140/105/10 ambient temperature. The coolant temperature is [94] Liquid 30/20 Pin-ﬁn cold plate - - 17 MOSFET water/ethylene considered to be 105 ◦C to use a single cooling loop for the inverter and motor. The inverter showed satisfying operating 1200 V/300 A SiC Commercial cold 180/50/- performance at the 180 ◦C ambient temperature [95] Liquid 30/10 150 j-case 0.2 - MOSFET plate water/ethylene because the high-temperature SiC packaging was used. 62 mm wide, A high-power density liquid-cooled inverter is [96] Liquid - SiC MOSFET 100/40 007-MXQ-01 cold 105/65/10 113 j-case 0.16 34 presented. plate The double-sided cold plate with the direct liquid Double-sided cold [76] Liquid - SiC MOSFET -/12 - 150 j-cool. 0.17 - approach allows the thermal resistance from the plate junction to coolant to be decreased by 35%. Energies 2021, 14, 4981 15 of 21

Air cooling is a cheaper and simpler solution compared to liquid cooling. Liquid cooling requires additional auxiliaries (pumps, radiator, compressor, and chiller), which increase the weight and volume of the system. Because of higher working temperatures and heat flux of WBG devices, air cooling can be applied to a wider power range of inverters compared to Si devices. As can be seen from recent studies, introducing WBG devices on the market transformed the trend from liquid to forced air cooling for low-power (up to 50 kW) [89] and low power density (up to 18 kW/L) [87] applications. In addition, some studies are devoted to volume hexagonal or cubic air-cooled inverters. Such construction increases the power density of the inverter; for example, the standard construction with a common flat and finned heat sink achieves 13 kW/L [18] and both of the presented examples of volume inverters achieve 17.2 and 18.6 kW/L [86,87]. Another approach of applying air cooling for the power inverter is a modular inverter structure. Modular air-cooled inverters from [87,88] are solutions with high power levels. Due to the use of separated modules, each module-inverter has a low power level and can be air cooled. Nevertheless, liquid cooling is still the most widespread solution for power electronics applications in EVs, and indirect liquid cooling is still feasible. Single-sided indirect cooling is developing via improvement of the cold plate, as can be seen in [93,94]. Moreover, the single-sided direct and indirect cooling for WBG inverters has been optimized in two ways: by changing the distance between the chips and changing the number of power switches [96–99]. Because WBGs have a smaller chip size, additional power switches can be used without an increase in the volume of the device, to provide better current sharing in one inverter leg as more parallel chips are used. From the thermal optimization perspective, in contrast, the reduced number of chips enables a larger distance between them. When chips have a larger distance between them, applied cooling has better heat dissipation capability and temperature distribution. The main drawback of this optimization is the increase in the operating current for each power switch. However, in the case of WBG semiconductors, this problem can be compensated for by using higher rated currents of WBGs. The double-sided cooling approach shows good thermal properties; for example, reduced thermal resistance up to 35% [28]. Such cooling is suitable for high thermal performance and high temperature solutions. Although the heat dissipation capability of the cooling system is low, double-sided cooling does not justify the complexity of its implementation. In addition, power modules with double-sided cooling have higher reliability due to the replacement of the bonding wires by more reliable interconnectors. Furthermore, the double-sided power module can be designed for an optimized current path between parallel modules [76].

8. Conclusions This paper provides a technical assessment and review analysis of the cooling technologies for automotive WBG inverters. The WBG semiconductors, classiﬁcation of cooling techniques, and details of each cooling technique are reviewed in detail. These semiconductors have different intrinsic properties that can affect the thermal properties of the overall system. WBG technology is a highly promising solution for the next generation of power electronics and automotive powertrains due to their superior intrinsic characteristics, such as high thermal ﬂux, available junction temperature, limit of the blocking voltage, and switching frequency, and low switching losses and conduction voltage drop. The commercial availability of WBG devices enables the use of cheaper and simpler air-cooling approaches for low-power motor drives up to 50 kW, rather than liquid cooling. Two main approaches aimed at increasing the heat dissipation capability of air cooling can be highlighted: the volume and the modular design of power inverter. Whereas the former provides a compact inverter with a larger surface to dissipate heat, the latter modular design provides all of the required power of the device divided between separate modules, in which each module has lower power and can be cooled using the forced-air approach. Energies 2021, 14, 4981 16 of 21

Liquid cooling remains the most cost-effective solution for a wide range of power levels. Numerous studies have been conducted on the different optimization approaches, which include new shapes of cold plates or heat sinks, the use of new materials, and the combination of two or more cooling techniques. Single-sided indirect liquid cooling remains highly popular and shows good thermal performance, particularly when new WBGs with better thermal properties are applied. Because WBGs have a smaller chip size, smaller losses, and a higher rated current and voltage, some studies suggest thermal optimization can be achieved by changing the number of semiconductors in one inverter leg and by changing the distance between the power switches. Direct liquid cooling has better thermal performance than indirect cooling, but adds complexity to the implementation. Such a cooling approach can be useful for high performance and high temperature solutions. Indirect and direct double-sided cooling are applicable for devices with a high heat transfer coefficient, because only in this case is their implementation complexity justified. According to the trend of increasing rated power levels of power inverters, high heat flux systems, such as immersion cooling, may soon become widespread. Moreover, WBG power modules, in the case of high power levels of the device, need to be cooled with special cooling approaches, which are capable of effectively dissipating heat from a small hot area. A porous heat sink, heat pipes, and special thermal packaging may be a solution. In addition, high operating temperatures of WBGs also provide an opportunity for the use of IMDs with a shared motor and inverter cooling. Due to the growing demand for increasing power levels in motor drives, cooling technology is an important part of the system. The results of this paper could be used for many future studies on the topic.

Author Contributions: Conceptualization, E.A.; resources, A.Z., T.G., M.E.B.; writing—original draft preparation, E.A.; writing—review and editing, A.Z., T.G., M.E.B.; visualization, E.A.; supervision, O.H. All authors have read and agreed to the published version of the manuscript. Funding: Explorative research, internal funding. Acknowledgments: Authors acknowledge Flanders Make for the support provided to our research group. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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