A Comprehensive Review on the Core Thermal Management Improvement Concepts in Power Electronics

A heat sink is a specific type of heat exchanger integrated with heat generating devices – mostly electronics – for the sake of thermal management. In the design procedure of heat sinks, several considerations such as manufacturing cost, reliability, thermal and hydraulic performance have to be included. In the past few decades, the prevailing trend of electronics design miniaturization has led to high-power-density systems necessitating high performance cooling concepts. This paper intends to provide a comprehensive review on various employed heat transfer enhancement techniques in cooling procedures of electronics thermal management devices, with a focus on core ideas. The main motivation is to give a rapid overview on the key concepts in different high-performance cooling designs along with a quantitative comparison between the different concepts all in one reference which is missing in literature. For this, the key idea of each design is firstly categorized, and then a detailed description is provided for each case. The discussed categories consist of concepts based on channel cooling in various scales, phase transition, jet impingement, spray cooling and hybrid design. At the end, quantitative comparison is illustrated for thermal and hydraulic performance of a selection of the reviewed references covering all these different categories. Based on this comparison, an overview on thermo-hydraulic performance of the presented categories is provided, and recommendations for future studies are given based on this and the detailed review of references.


I. INTRODUCTION
In order to achieve reliable and highly-efficient operation for power electronics, especially for those cases with high requirements on compactness following the ongoing miniaturization trend, it is necessary to provide thermal management solutions capable of dealing with high-power-densities. Thermal management is a crucial topic in a variety of applications including electronics, solar collectors and furnace engineering [1]. Specifically, in electronic devices, in order to avoid damage caused by formation of local hot spots on The associate editor coordinating the review of this manuscript and approving it for publication was Lei Wang. the one hand, the maximum temperature within the device is desired to be kept in a restricted range [2]. On the other hand, in order to achieve the desired reliability, smooth cooling with uniform temperature distribution has to be achieved. This explains why efficient cooling mechanisms have become an important topic of numerous research activities with the growing trend of developing high-power-density electronic devices with compact design. For this, various thermal management techniques have been developed. As illustrated in FIGURE 1 along with corresponding references, the major methods in this reference list, that form the main categories in this paper, are channel cooling-based thermal management designs in various scales including mini-channel and FIGURE 1. Classification of the reviewed research categories (a: reprinted with permission from [35], b: reprinted with permission from [41], c: reprinted with permission from [60], d: reprinted with permission from [84], e: reprinted with permission from [76]). microchannel, phase transition-based cooling techniques, jet impingement and spray cooling designs along with hybrid designs benefitting from combination of these methods.
A wide variety of general cooling concepts is available in literature, and they are different from the viewpoint of cooling capacity, involved mechanisms and improvement potential. A primary method for electronics cooling is to attach highly-conductive extended surfaces called fins with various structures to the heat dissipating part [3]. Heat is conducted through the solid layers to the extended surfaces, and natural convection releases heat to the ambient. Obviously, thermal interface materials play a key role in such designs [4], and an area increase is the core idea. However, the advances in electronics have necessitated higher cooling capacities making this method incompatible with a majority of more recent designs [5].
In order to achieve higher cooling performance, forced convection-based air cooling concepts have emerged. Fan induced air cooling, benefiting from the forced convection mechanism, was proposed as another thermal management solution in electronics industry. Further improvement of the heat transfer coefficient in this method for instance by higher air flow velocities is accompanied by higher noise and requires high power. Ultimately, it is the low heat capacity of air which impedes the usage of forced convection-based air cooling concepts in high-power-density applications [6].
Another major thermal management method is liquid cooling. The designs in this category show higher cooling capacity making them suitable for being integrated with compact electronics and high power densities [7]. Liquid cooling methods can be categorized by various concepts including direct and indirect approaches. The wide application of liquid cooling VOLUME 8, 2020 has been emerged in different design concepts including cold plates, jet impingement-based designs and immersion cooling [8], [9]. Each of the above-mentioned methods has attracted growing attention in the past few years. In cold plate designs, channel cooling in various scales from micro to mini-channel is used. The microchannel topic has been the center of attention according to its desirable features for miniaturization. In fact, in spite of low flow rate and small hydraulic diameter in microchannels which lead to laminar flow regime, effectively a high heat transfer can be achieved by allowing many small channels in parallel, inducing a large solid-fluid interface. In addition, it benefits from design flexibilities such as diameter reduction for increasing heat transfer coefficient which is in accordance with compact design scope that has made this method attractive to thermal management designers [10]. However, the increased pressure drop can lead to restriction of further miniaturization [11]. This is why tuning the parameters in microchannel-based designs has formed a great proportion of research items in this field [12].
The other method of liquid cooling in the subcategory of direct methods is immersion cooling. This concept benefits from the desirable feature of having the fluid close to the heat source, for instance the electronic chip [9]. In this method, natural convection and two-phase flow are involved; however, an important restriction of this method -i.e. the requirement of liquid compatible with the device -led to restrictions of its application in thermal management devices.
Liquid jet impingement has been shown to be an efficient cooling technology by achieving thin thermal boundary layers and desirably high heat transfer coefficients, which make the concept suitable for compact designs [13]. However, the more complicated design and manufacturing procedure of this type of cooling devices compared to channel cooling designs led to intense competition between these two methods.
Another efficient thermal management method is spray cooling which works by breaking the coolant into droplets and impinging them to the hot surface. The role of the thermal boundary layer acting as a thermally insulating layer in channel cooling and impingement cooling designs is minimized in this method, so it is known as an efficient concept widening its application in high capacity cooling and general compact designs [14]. However, there are important parameters restricting performance of spray-based thermal management designs, and the most important one is the high pressure drop required in the nozzle to produce spray droplets [15].
In another category of cooling devices, phase transition is utilized. Phase change material (PCM) and heat pipebased designs can be placed in this group. The main advantage of heat sinks utilizing PCM lies in the significantly higher heat absorbed in latent form compared to sensible heat absorption designs [16]. However, low thermal conductivity of conventional materials used as PCM led to a restriction of this approach, and also a great attention is attracted to the enhancement of effective thermal conductivity in these systems [17] and [18]. On the other hand, heat pipes, which can be placed in different categories from indirect liquid cooling to phase transition cooling designs, benefit from liquid-vapour phase change mechanism. In fact, a heat pipe is partially filled by liquid, and its inner wall called wick acts as capillary pump. Heat is absorbed by the working fluid in the evaporator side leading to fluid evaporation. Following this, the vapour goes to the condenser side due to a pressure gradient and is condensed there. The wick structure transports the fluid back to the evaporator. As a result, benefitting from this mechanism, the effective thermal conductivity of a heat pipe reaches several tens of thousands of Watt per meter Kelvin, which makes it an attractive technology for thermal management purposes [19], [20].
With the growing trend towards high-power-density electronics applications on the one hand, and performance restrictions of conventional thermal management solutions on the other hand, the use of high performance heat transfer enhancement concepts in this field has become more critical in the past few years [21]. Since heat sink design for being integrated with electronic devices is becoming more compact and needs to be capable of dealing with high-power-density thermal management applications, it seems necessary to provide a review on novel ideas on heat transfer enhancement methods used in such systems in one review paper. By doing this, it is intended to collect inspiring ideas in one reference to facilitate the design procedure for future studies.
The main motivation of this review paper goes back to the fact that the reviews available in literature in the field of thermal management are restricted to a specific topic, such as phase transition cooling [1] and [85], channel cooling ideas [86], nanoparticle dispersion [87] and jet impingement cooling [88]. However, in this paper, the focus is on core heat transfer improvement ideas used in various designs with different heat transfer mechanisms from phase transition, natural to forced convention and different working fluids ranging from air, liquid, refrigerants, nanofluids, and phase change materials with different physics from single phase convection to multiphase. These are grouped in a comprehensive classification ranging from channel cooling-based heat sinks, impingement designs, spray cooling, phase transition cooling to hybrid cooling systems used for thermal management of power electronics (see FIGURE 1). Moreover, a quantitative comparison is made between all these different ideas at the end in order to provide the thermal management designers with the opportunity of rapid scan of the core ideas used in different papers and a quantitative comparison of the cooling capacities. Note that all of the reviewed references may not necessarily be integrated with power electronics; however, their key concepts are reviewed here as the representative inspiring ideas that can be potentially utilized in future cooling designs integrated with power electronics. Besides, it is noteworthy that for each thermal management idea, there may be several papers, but at least one representative paper is brought to compare with other different concepts in order to provide the thermal management society with a guideline. Graphical review of papers containing high performance ideas for heat transfer enhancement in channel cooling-based heat sinks: (a) reprinted with permission from [22]; (b) reprinted with permission from [32]; (c) reprinted with permission from [34]; (d) reprinted with permission from [36]; and (e) reprinted with permission from [39].
This work should enable to have a quick scan on the previous works and also to inspire from high performance ideas as a starting point to come up with new exclusive thermal management ideas. For each category in the proposed classification, an introductory description, a table and a schematic of ideas are provided in the next sections. Finally, a quantitative analysis is done to provide fair comparison between the various reviewed methods from thermo-hydraulic performance point of view. Moreover, at the end of this paper, the gaps that can be potentially filled later are listed in the conclusions.

II. HIGH PERFORMANCE IDEAS IMPLEMENTED IN CHANNEL COOLING -BASED HEAT SINK DESIGNS A. INTRODUCTION TO CHANNEL COOLING
Channel and mini/micro channel cooling-based thermal management designs are reviewed in this section. According to the advances in manufacturing methods, channel cooling in all scales has attracted considerable attention as thermal management solution [89]. In contrast, there are still undeniable challenges involved in channel cooling, including pressure drop/pumping power penalty and restricted heat transfer due to the formation of a boundary layer, and non-uniform cooling due to undesirable temperature rise in the coolant. To tackle these issues, several ideas have been implemented in order to reduce the thermal boundary layer and to achieve uniform cooling at minimal pressure loss. In FIGURE 2, a schematic of some channel cooling designs is provided. As illustrated in this figure, the facilitated manufacturability made channel configuration ideas one of the most widely-investigated methods in this class. In addition, inserting different solid bodies such as pin fin, lattice structures and various types of vortex generators form another main concept for heat transfer enhancement in this category. Similarly, flow agitation by simple structures such as reeds is a simple and efficient method, used to cause strong vortices in the flow to disturb boundary layer and enhance heat transfer. Another part of studies in this context is skin design such as nature-inspired shark skin structure as illustrated in the mentioned figure. Finally, enhancing thermal performance of a heat sink by improving the coolant is the other field reviewed here. In addition to single phase channel cooling, another possibility is to use a boiling fluid as a coolant with higher cooling capacity compared to its single-phase counterpart due to utilization of latent heat of vaporisation in the heat transfer process. Besides, uniform temperature distribution should be in principle easier achieved in flow boiling heat transfer since it occurs at the fluid saturation temperature. However, there are restricting challenges associated with this approach. Flow instabilities such as flow reversal causing strong severe pressure drop fluctuations and wall temperature instabilities are the examples. Accordingly, benefitting from positive aspects of multiphase channel cooling along with tackling the challenges has become a hot topic in this field [90]. The characteristic parameters in this category of thermal management designs include flow rate, Reynolds number (Re), Prandtl number (Pr), Strouhal number (St), hydraulic diameter (D h ) and geometrical parameters associated with each case. Besides, the response functions considered in the design procedure are the approximated total thermal resistance (R th ), thermal efficiency index (η i ), thermal performance factor (η), heat transfer coefficient in terms of Nusselt number (Nu) or convective coefficient (h), and hydraulic loss in terms of pressure drop ( P), friction factor (f ), and pumping power (P.P). In addition, the junction temperature (the highest operating temperature of the active region within a semiconductor device), peak temperature, maximum temperature difference and critical heat flux (in two phase cooling) are also used to describe thermal performance. The equations describing the abovementioned parameters are as follows [27]: p , where A and p represent flow cross section and wetted perimeter respectively.
• Re = ρUD µ , where ρ, U , D and µ represent fluid density, flow characteristic velocity, characteristic length and dynamic viscosity, respectively, where c p , µ and k are specific heat, dynamic viscosity and thermal conductivity respectively.
U , ν is vortex shedding frequency, L is characteristic length and U is flow velocity where P t , T h and T c are total heat load generated by the device, hot spot temperature and inflow bulk temperature, respectively. Note that there is another version of total thermal resistance where P t represents the input heat flux on the heating surface (W/m 2 ). In this case, thermal resistance unit is Km 2 /W instead of K/W.
where q, T w and T b are heat flux, channel wall temperature, and average coolant bulk temperature respectively.
• Nu = hD k , where h, D and k represent convective coefficient, characteristic length, and fluid thermal conductivity, respectively.
• P.P = Q P, where Q and P are volumetric flow rate and pressure drop, respectively.
where u and L refer to average flow velocity and length of the channel, respectively. Besides, P, D h and ρ are flow pressure drop, hydraulic diameter of flow passage and fluid density, respectively.
where Nu and f are Nusselt number and friction factor, respectively.
where Nu and f are Nusselt number and friction factor, respectively. Note that in this equation, the subscript p refers to plain channel in the absence of thermal enhancement. It is noteworthy that the two presented forms of thermal enhancement factor (η) show the same concept of considering both thermal and hydraulic performance; however, in the second version, less emphasis is placed on hydraulic loss. The detailed review of channel cooling thermal management concepts is presented in TABLE 1. the reviewed concepts include channel configuration modification, conductive insert immersion, obstruction introduction, flow vibration, use of structured surface, and coolant property enhancement. As discussed in this table, in the channel cooling concepts, the applied approaches are mainly targeting on disturbing boundary layer for the sake of thermal mixing enhancement in terms of temperature uniformity, heat transfer coefficient improvement and junction temperature decrease at minimized hydraulic cost in terms of pressure drop or pumping power. In the majority of the reviewed references, the enhancement methods lead to a rise of pressure drop. However, there are ideas with negligible pressure drop increase, and there is even an idea reducing pressure drop, which is the channel skin design. Finally, two-phase channel cooling which normally achieves higher thermal performance compared to single phase shows promising opportunities for further improvements.

III. HIGH PERFORMANCE IDEAS IMPLEMENTED IN PHASE TRANSITION -BASED HEAT SINK DESIGNS A. INTRODUCTION TO PHASE TRANSITION COOLING
Solid-liquid and liquid-gas phase changes are the most widely-used phase transition-based designs in power electronics thermal management. For this, two main approaches are the utilization of phase change materials (PCM) and heat pipe assisted heat sinks. Thermal management by utilizing PCMs has been considered as a promising technique due to numerous desirable features including high heat storage density as a consequence of high latent heat of fusion for common PCMs, and being able to withstand a large number of cycles at relatively constant melting temperature. However, the main drawback of PCM-based thermal management is the low thermal conductivity of PCM materials. The other phase transition-based group of devices which benefit from the high effective thermal conductivity of heat pipes face some challenges in terms of heat transfer performance from heat pipe to device and regarding design compactness, which should be also complemented by innovative ideas. The main idea of heat pipe was proposed by Grover et al. [91]. Cotter [92] developed a basic theory that has been used since then as a basis for heat pipe design. Heat pipes make use of a liquid-gaseous phase change of a working fluid to transport heat from an evaporator to a condenser. Since the phase change takes place at a constant temperature, the device is capable of transferring heat for long distances with small temperature gradients. This makes them act as a ''thermal superconductor'' and an efficient alternative for thermal management of high-power-density electronic components. However, conventional heat pipes suffer from restrictions in thermal performance that are tried to be eradicated by the ideas discussed in this section. According to the mentioned plus and minus points for the abovementioned phase transition-based heat sink designs, heat transfer enhancement has become a hot topic in this field. In this section, the related innovative ideas employed to enhance these systems are reviewed. An overview is provided in FIGURE 3.
The characteristic parameters in PCM based thermal management devices are phase change time, operation time of electronic devices without exceeding critical temperature, melting fraction, heat source temperature, solid insert efficiency, input power, and the stored energy. On the other hand, filling ratio is one important parameter in heat pipe assisted devices, and it is defined as the ratio of working  fluid volume to the total volume of the miniature-loop heat pipe. In addition, condenser/evaporator temperature and their temperature difference along with evaporation, condensation and total thermal resistance form the other important parameters in heat pipe-assisted thermal management devices. These thermal resistance values are calculated as follows [48]: and R th t = R th e + R th c , Where P h , T a , T e and T c are heat load and temperature at adiabatic section, evaporator and condenser, respectively.
Finally, the heat transferred by the heat pipe for calculation of heat transfer capacity by Q transferred Q input is the other characteristic parameter.
The reviewed phase transition based thermal management concepts are provided in detail in TABLE 2. In the PCM based concepts, the conducted review shows that the main challenge is to deal with the weak thermal conductivity of phase change materials. This explains why a majority of works in this field are focused on the use of conductive inserts or other methods in order to improve the functionality of PCM based designs in minimizing and stabilizing maximum temperature of the target system. The heat pipe assisted thermal management devices reviewed in this article benefit from various ideas from modification of evaporator and condenser to modification of wick in order to enhance the thermal performance of heat pipes in terms of quick response, minimized thermal resistance and reliable operation.

IV. HIGH PERFORMANCE IDEAS IMPLEMENTED IN JET IMPINGEMENT -BASED HEAT SINK DESIGNS A. INTRODUCTION TO JET IMPINGEMENT COOLING
The jet impingement technique has attracted the attention of thermal management designers in the past decade due to desirable features including higher local thermal absorption, potential of achieving more temperature uniformity, and suitability for hot-spot-targeted cooling design. However, impingement-based cooling designs suffer from various restrictions. Interferences between adjacent jets or interactions due to collision of surface flows, large size/ weight of fluid delivery system, and the required pumping apparatus can be named as a few. In this section, the recent research items containing novel ideas for improving impingement-based heat sink designs for being integrated with power electronics are reviewed. The different enhancement techniques are mainly categorized in six groups including impingement configurations, immersed jet array, impinged area design, impingement manifold design, utilization of multiphase impingement cooling and additively manufactured impingement nozzles (see FIGURE 4).
The important parameters and response functions used for performance evaluation and design of impingement-based systems are jet dimensions, jet-to-wall distance, critical heat flux (two phase impingement cooling), surface superheat defined as the surface and saturation temperature difference (two phase cooling), steady state temperature, outlet temperature, maximum -minimum temperature difference, heat flux, coolant flow rate, Reynolds number, heat transfer coefficient (Nu and h), pressure drop, and pumping power.
Besides, pressure coefficient in the following form is also used as another response function in this section among the reviewed references [57]: where P x , P atm , ρ and u represent local static pressure on target surface, atmosphere pressure, coolant density and impingement velocity. Meanwhile, another performance evaluation criterion used in this field is overall performance as a balance between thermal and hydraulic considerations in the form of the area under the curve of thermal resistance as a function of pumping power [60].
A wide variety of methods are used as impingement design modifications in impingement based cooling concepts as reviewed in TABLE 3. The major ideas are those using various jet configurations for strong and uniform cooling performance, surface modification and coolant properties enhancement. The review shows that in general, jet impingement based cooling with potentially high number of cooling units is an efficient thermal management solution with a potentially low pressure drop. Meanwhile, the reviewed articles in this table also show that the manufacturing challenges of fluid delivery system for such designs are being tackled   by the new trends in manufacturing technologies especially additive manufacturing which may facilitate novel impingement designs.

V. HIGH PERFORMANCE IDEAS IMPLEMENTED IN SPRAY COOLING -BASED HEAT SINK DESIGNS A. INTRODUCTION TO SPRAY COOLING
Spray cooling is recognized as higher cooling capacity method compared to natural/forced convection cooling methods discussed earlier [84]. In this method, a liquid coolant is emitted from a pressurized nozzle and breaks up into droplets impinging the hot target surface. As a result of this impingement, droplets are turned into a thin liquid film which flows radially along the surface. Obviously, compared to the normal/tangential flow impingement-induced heat sink designs where coolant generates a thermally insulating liquid boundary layer and experiences undesirable temperature rise, in spray cooling the hot surface permanently receives fresh coolant droplets, leading to high cooling performance. In a relatively different spray cooling mechanism entitled electrospray cooling, electric potential is applied between liquid and target surface to produce the spray for evaporation heat removal. This technology benefits from several advantages over conventional spray cooling including lower required power to generate the spray compared to mechanical spray systems with high friction losses, and avoidance of droplet deceleration by drag force. This is achieved by droplet acceleration by an electrostatic force, leading to droplets impinging on the surface with higher velocities.
Investigation of spray cooling is categorized by approaches on the spray and droplet level. The former examines spray cooling performance from flow behaviour, surface conditions, fluid properties, nozzle array and similar viewpoints. However, in the droplet level analysis, the impact of droplet flow (single droplet or droplet burst) on the film flow (dry surface, stationary film, and flowing film) is analysed. Obviously, in the spray level examination approach, deep understanding of detailed heat transfer mechanisms (convection between hot surface and film flow, nucleate boiling on the hot surface, conduction in film flow, and interfacial evaporation from liquid film to surroundings) is missing unlike the droplet level [15]. The categories of enhancement ideas exploited in this field are illustrated in FIGURE 5 and a detailed description is provided in TABLE 4.
(the peak average heat flux that is met once nucleation sites cover the heated surface completely), surface temperature and its non-uniformity, junction temperature, surface superheat as the difference between surface-temperature and saturation-temperature, heat transfer coefficient, averaged surface temperature, and thermal resistance. Besides, the difference between inlet pressure and back pressure of the spray nozzle is used for hydraulic cost description is some references. Finally, spray angle, droplet diameter, droplet velocity, and volume fraction along with surface tension can be named as the other important parameters in this class. In addition, a dimensionless number used in this category is known as Bond (Bo) number to evaluate the contribution of the capillary effect, which physically reflects the ratio of the gravitational force to the capillary force as Bo = R √ γ (ρ l − ρ V ) g, where R, γ , ρ l , ρ V , and g represent the radii of microcavities, the surface tension of coolant, the density of liquid coolant, the density of coolant gas and the gravitational constant, respectively.    Aside from all of the plus points in thermal performance of spray cooling-based thermal management devices, there are undeniable challenges restricting overall performance of these systems including significant pumping power required to provide large pressure drops for the nozzles to produce the desired fine spray. Clearly, thermal performance enhancement methods are required to deal with the challenges to enhance the overall performance of such systems and reduce  total cost in terms of pumping power. For this, several methods are used. TABLE 4 contains the core concepts and major results of spray cooling designs for thermal management. This technology is generally known for its high cooling performance due to the mentioned features which have made it suitable for high power density applications. As illustrated in this table, the major design modifications in this category are the use of structured surfaces, nozzle array modifications and coolant property enhancements. However, the motivation for adopting such approaches in spray cooling is different from the single phase cooling designs since these modifications can also alter wettability and nucleation sites in addition to a surface area increase for the sake of thermal transport enhancement. The review shows that spray cooling is an attractive technology for high performance cooling, but still there are crucial challenges to be addressed for ensuring reliable operation.

VI. HIGH PERFORMANCE IDEAS IMPLEMENTED IN HYBRID HEAT SINK DESIGNS A. INTRODUCTION TO HYBRID COOLING DESIGNS
In this section, the designs benefiting from two or more heat transfer improvement techniques known as hybrid designs are reviewed. The research items in this section utilize combinations and synergies of jet impingement, solid insert, mini-microchannel array, surface modifications, phase transition and thermoelectric coolers (see FIGURE 6  and TABLE 5). The characteristic parameters in the reviewed hybrid designs are jet dimensions, jet-to-wall distance (for impingement contained designs), volume fraction of nanoparticles (for nanofluid contained systems), thermoelectric cold and hot side temperatures, fin dimensions, flow rate, Reynolds number, heat transfer coefficients, pressure drop and pumping power, hot spot temperature, maximum temperature rise (difference between maximum and FIGURE 6. Graphical review of papers containing high performance ideas for heat transfer enhancement in hybrid designs: (a) reprinted with permission from [75]; (b) reprinted with permission from [78]; (c) reprinted with permission from [79]; (d) reprinted with permission from [81]; (e) reprinted with permission from [82]; (f) reprinted with permission from [83]. minimum substrate temperature). Besides, overall performance is defined as (h P) and thermal conductance is defined as (q (T s − T inflow )) where q,T s and T inflow are input heat flux, averaged surface temperature and coolant inflow temperature respectively.
The hybrid thermal management ideas along with the key ideas and major results are discussed in detail in TABLE 5. The table contains the designs combining at least two concepts among jet impingement, fin insertion, microchannel, structured surface, thermally enhanced coolant, foam insertion, phase transition and thermoelectric cooler. The review of the hybrid designs indicates that in the majority of the proposed designs, the combination of ideas leads to a higher thermal performance in comparison with either of the individual ideas. However, in some designs, this combination is shown to require further considerations in order to avoid side effects caused by the interactions between different individual concepts.

VII. QUANTITATIVE COMPARISON BETWEEN THE THERMAL MANAGEMENT CATEGORIES
In order to provide a fair comparison between the cooling performance of the reviewed ideas in this paper including channel cooling, spray cooling, impingement cooling, hybrid designs and phase transition based designs, the effective thermal conductance is adopted as the quantitative criterion as follows [77]: , Where q and T represent heat flux and temperature difference.
In channel cooling, impingement and hybrid cooling, the temperature difference between inflow and hot spot is used. In spray cooling, it is calculated as the difference between surface temperature and saturation temperature. Finally, in heat pipe thermal management devices as the candidate of phase transition-based cooling, it is defined as the temperature difference between evaporator and condenser. The comparison is provided in FIGURE 7. The diameter of the circles in this figure is proportional to the required pumping power except for the phase transition-based cases (No. 11 and 12), where no pumping power is required.
As illustrated this figure, the operating range of channel cooling and impingement cooling ideas shows the possibility of achieving desired performance with comparably low pumping power. However, the effective thermal performance is lower for these cases compared to other categories VOLUME 8, 2020   especially spray cooling. This can be mainly attributed to the nature of single-phase channel cooling and impingement cooling benefitting only from sensible heat absorption unlike the spray cooling or two-phase channel cooling designs.
Another reason is that the majority of channel cooling ideas available in literature are based on the laminar flow regime, and only a minority of research items has provided analysis on turbulence enhanced designs to benefit from increased momentum/energy exchange due to turbulence nature. On the other hand, the impressive effective thermal performance of spray cooling ideas is normally accompanied by high pumping power. In fact, it is known that in spray cooling cases, the crucial challenge is to keep the system working safely and reliably under high pressure differences in the order of MPa between the inlet pressure and back pressure for a long time. In addition, it is inferred that a combination of different cooling ideas and wise layout of hybrid designs can fulfil high thermal performance at affordable pumping power as achieved in case No. 13 and 15. Finally, the impingement and phase transition-based concepts indicate an overall thermal performance of the same order but at different power demands. Therefore, choosing among these methods should be based on the specific priorities of the target system.

VIII. CONCLUSION AND RECOMMENDATIONS
A review on high performance concepts used for improving heat transfer mechanisms involved in power electronics thermal management was provided in this paper. The main motivation was to give a comprehensive overview on the available thermal management ideas and potentially inspiring new ones and combinations of them. For this, a categorization covering the commonly-used heat sink designs was presented. In the categories, channel cooling in various scales, impingement cooling, phase transition-based designs, spray cooling and hybrid designs were presented. Following this, a detailed description of the core of each idea was provided. Finally, a quantitative comparison was given for a representative sample of the reviewed concepts from the viewpoint of thermal and hydraulic overall performance.
Based on the conducted study, the following items are proposed as future research recommendations.
• Turbulence-enhanced channel cooling forms a tiny minority of designs in this category. However, the upcoming manufacturing technologies enabling complicated structures for novel mixing devices, and allowing a balance between enhanced thermal performance and increased hydraulic loss are promising opportunities for adopting this approach.
• The employed heat transfer enhancement methods in spray cooling such as surface modification indicate considerable heat transfer enhancement and a great potential for power electronics thermal management, but there are still crucial challenges such as safe and reliable operation under high pressure difference between the high and low pressure sides of the nozzles. Meanwhile, in spite of the high cooling capacity at the maximum power operation point, keeping the hot spot temperature stable at real transient working conditions has to be considered in spray analysis. More works on miniaturization attempts, for instance driven by piezo actuation-based pumping, are needed in order to enable ultra-compact designs also with spray cooling concepts.
• In heat pipe-assisted cooling devices, the effective thermal conductivity and thermal performance are desirably high. But still some ideas seem to have crucial shortcomings that necessitate further studies. For instance, ultra-thin flattened heat pipes indicate impressive thermal performance, but there are still drawbacks such as decreased thermal performance under miniaturization constraints which necessitate a decrease of thickness.
More research is to be done by reconsidering the inner structure of this type of heat pipes to address such serious issues while preserving the positive thermal aspects.
• Impingement cooling with low pressure drop and hot-spot targeted cooling ability was shown to be promising for achieving compact designs by adopting approaches such as channel-confined jets. However, the generation of the impingement jets in such configurations by common methods such as orifice plate obstructions can be costly from the hydraulic performance viewpoint. Fulfilling compactness at minimized hydraulic loss is recommended as a topic of further study.
• According to the illustrated high potential of hybrid designs in achieving high thermal performance at acceptable hydraulic loss, it seems promising to find an optimum combination of the individual cooling concepts by considering the synergies between individual methods.