Evaluation of the thermal performance with different fin shapes of the air-cooled heat sink for power electronic applications

Journal of International Council on Electrical Engineering ISSN: (Print) 2234-8972 (Online) Journal homepage: http://www.tandfonline.com/loi/tjee20 Evaluation of the thermal performance with different fin shapes of the air-cooled heat sink for power electronic applications Chang-Woo Han & Seung-Boong Jeong To cite this article: Chang-Woo Han & Seung-Boong Jeong (2016) Evaluation of the thermal performance with different fin shapes of the air-cooled heat sink for power electronic applications, Journal of International Council on Electrical Engineering, 6:1, 17-25, DOI: 10.1080/22348972.2015.1115168 To link to this article: https://doi.org/10.1080/22348972.2015.1115168 2016 The Author(s). Published by Taylor & Francis Published online: 05 Feb 2016. Submit your article to this journal Article views: 1235 View related articles View Crossmark data Citing articles: 2 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tjee20 Download by: [37.44.206.71] Date: 21 December 2017, At: 02:57

Journal of InternatIonal CounCIl on electrical engineering, 2016 Vol. 6, no. 1, 17 25 http://dx.doi.org/10.1080/22348972.2015.1115168 OPEN ACCESS Evaluation of the thermal performance with different fin shapes of the air-cooled heat sink for power electronic applications Chang-Woo Han and Seung-Boong Jeong Power & Industrial Systems r&d Center, Hyosung Corporation, republic of Korea ABSTRACT The proper selection of the heat sink, which is attached at the insulated-gate bipolar transistor (IGBT) module to dissipate heat by electric losses of the IGBT/diode chips, is important to satisfy the design criterion of the IGBT module. Prior to the performance evaluation of the air-cooled heat sink using the numerical method, the suitability of the simulation model was validated through the experimental result of the developed product. The simulation model predicted the hotspot temperature on the heat sink within a margin of error of 5.6 percent. From the verified numerical method, the thermal performance of the heat sink was evaluated according to the shape of the fins. The heat sink with the perforated fins had an excellent thermal performance because the rate of increment of the dissipation area was greater than the rate of decrement of the convection coefficient. The selected heat sink with the perforated fins was attached at the IGBT module and the junction temperature of the IGBT module was predicted. The predicted junction temperature was 131.4 C and the result satisfied the design criterion of 140.0 C. 1. Introduction Power semiconductor devices such as an insulated-gate bipolar transistor (IGBT) module are the key component at the thermal design stage of a power electronic system such as a power conversion system (PCS), a high voltage direct current (HVDC) system, a static synchronous compensator (STATCOM), and so on. As the electric performance of semiconductor devices is improved, the power density of semiconductor devices is increased, and then the thermal problem of semiconductor devices becomes one of the issues with the thermal design. The IGBT module is a three-terminal semiconductor device primarily used as an electronic switch, and is noteworthy for combining high efficiency with fast switching capability in newer devices. The performance of the IGBT module is affected by its operation temperature; thus, it is recommended that the IGBT module should be worked at lower temperatures to maintain the fast switching speed and the low switch loss attributes. The IGBT module is consisted of the IGBT/ diode chips, the direct copper bonded (DCB) substrate with the ceramic, the solder, the base plate (Cu) and the silicone gel covering the substrate and the IGBT/diode chips as shown in Figure 1. All components of the IGBT module are wrapped up with the ARTICLE HISTORY received 10 august 2015 accepted 29 october 2015 KEYWORDS Heat sink; insulatedgate bipolar transistor (IgBt) module; junction temperature; perforated fin plastic cover. The thermal energy generated by electric losses of the IGBT/diode chip is transferred through the copper plate and ceramic layers to the base plate of the module. It is then dissipated by convection heat transfer in the cooling system such as the heat sink and the heat pipe. In general, the heat sink is divided into three types, viz., air-, water-, and refrigerant-cooled type. The air-cooled heat sink is simple and easy-to-use in configuration, but the rate of heat transfer is less than other cooling methods for the same size. The water-cooled heat sink has higher thermal capacity than the air-cooled heat sink, but it needs additional equipment such as a heat exchanger, pump, and so on. The refrigerant-cooled heat sink has an excellent thermal performance, but has a weakness in that the design and the maintenance are relatively difficult. Then, the design engineer must determine a cooling method which is suitable in the operating and environmental conditions of a power electronic system because each cooling method has its own advantages and disadvantages. The main purpose to design, simulate, test, and evaluate the heat sink is to securely use the IGBT module when it is integrated into a power electronic system. So, a number of researchers evaluated and improved the thermal performance of the heat sink using the numerical CONTACT Chang-Woo Han cwhan@hyosung.com 2016 Hyosung Corporation this is an open access article distributed under the terms of the Creative Commons attribution license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

18 C.-W. Han and S.-B. JeonG Module plastic cover NTC DCB Thermal switch Base Plate of heat sink Copper Ceramic (0.9mm) Copper Silicone gel IGBT/diode chip (0.57mm/0.52mm) Solder (0.5mm) Base Plate of the module (3mm) Thermal Grease (75µm) Air between fins * Drawing not to scale Fin of heat sink Figure 1. Cross-sectional view of the packaging associated with IgBt and diode chip. and experimental methods [1]-[8]. It is also important to predict the temperature of the semiconductor device with different cooling types and operating conditions at the cooling design stage. Some studies predicted the junction temperature, which is the highest operating temperature of the actual semiconductor device in the power electronic device and is the key factor for assessing the suitability of the cooling design, to check the thermal status of the IGBT module under various operating conditions using the computational fluid dynamics (CFD) technique and thermal network model [9]-[15]. In this paper, prior to the development of the PCS, the thermal performance of the heat sink attached at the IGBT module was evaluated with different shapes of the fin of the heat sink using the numerical simulation method, and then the heat sink was selected from the simulation results. The selected heat sink was evaluated as to whether or not it satisfied the design criterion of the junction temperature of the IGBT module. 2. Description of physical models 2.1. Problem description Every power semiconductor device uses the heat sink to dissipate heat generated by electric losses, whether in a chip or in a module package. The task of the heat sink is to transfer the thermal energy by the IGBT module to the cooling medium such as air and water. Also, the structure of the heat sink plays a role as a base of the semiconductor device. As shown in Figure 1, the heat sink is attached at the base plate of the IGBT module and the thermal interface material (TIM) is applied between the IGBT module and heat sink in order to minimize the thermal contact resistance. The IGBT/diode chip repeats the ON-OFF operation to convert DC to AC or AC to DC, and the IGBT module generates electric losses such as switching and conduction losses. Electric losses are converted to heat, and then it has an adverse effect on the IGBT module. In general, due to the vulnerable characteristics of the IGBT module to heat, the junction temperature of the IGBT/diode chip should be monitored in real-time. If the setting temperature is over on the thermal switch and negative temperature coefficient (NTC) thermistor, the operation of the IGBT module should be shut down. The IGBT module used in this study can be operated up to 150 C, but the design criterion of the IGBT module is set at 140 C, in consideration of the temperature measurement error, the instantaneous peak current, the power cycling, and unexpected environmental conditions. A power module for the PCS is chosen to convert the characteristics of power source. The selected device is Infineon Corporation PrimePACK TM 3 module and NTC, rated for 1.7 kv and 1.0 ka [16]. The size of the IGBT module is 89 mm (width) 38 mm (height) 250 mm (length). In order to dissipate thermal energy by the IGBT module, the air-cooled heat sink was used as the size of the heat sink was 280 mm (width) 115 mm (height) 400 mm (length) and the material was set to be aluminum alloy 6063 with the thermal conductivity of 217.8 Wm 1 K 1. The heat sink was attached at the base plate of the IGBT module and the TIM, Mementive Performance Material Inc. YG6111 [17], was applied between the IGBT module and the heat sink. The thermal conductivity of the TIM was 0.84 Wm 1 K 1 and its thickness was about 75 μm. Air flowing into the heat sink was assumed to be 40 C in order to consider the extreme environment condition and the condition of the ambient air around the IGBT module and the heat sink was assumed to be stationary air with a convection heat transfer coefficient of 5 Wm 2 K 1.

Journal of InTernaTIonal CounCIl on electrical engineering 19 Figure 2. Schematic of (a) the physical model, (b) the single section of the heat sink, and (c) the simulation domain. 2.2. Geometry configuration The full configuration of the heat sink, which is attached at the IGBT module, is shown in Figure 2(a). The entrance and exit plane should be adequately distant from the heat sink so that the results become independent of the boundary positions, and then the extended inlet and outlet region, L e,in and L e,out, were considered as shown in Figure 2(c). To prevent the sudden extension of the height between the heat sink and the frame of the fan, the fan region L f and H e were considered. The induced air enters the heat sink with an inlet area of 144 280 mm 2, and the heated air is exhausted to the outlet area of 100 280 mm 2. The area of pressure rise by the fan is 100 280 mm 2. In this study, the fins of the three different geometries, viz., the plate, perforation, and protuberance fin, were considered as shown in Figure 3. By default, three fins are possible to manufacture and the number of fins, which are mounted on the base plate of the heat sink, can differ according to the manufacturing situation. The plate fin (Figure 2(b) and Figure 3(a)) has uniform thickness (t = 0.8 mm), and the channel width (w c = 3.2 mm) between the plate fins is also uniform. Thin plate fins are fixed on both the bottom and top side of the two base plates (h b = 12 mm), taking the structural safety into account. The perforated fin (Figure 3(b)) has the four perforations with a long rectangular cross section (2.03 22.4 mm 2 ). These perforations act as the channel and the direction of fluid flow is perpendicular to the cross-section of the channel. The protuberance fin (Figure 3(c)) has a number of protuberances (1.5 0.5 mm 2 ) on both sides of the plate fin. By increasing the dissipation area of the fin, the rate of the convection heat transfer can be increased. The thickness of the flat plate of the protuberance fin, (t = 3.0 mm), is thicker than the plate fin due to the structural and manufacturing problem. Due to the uniform size of each channel, the simulation domain was modeled with a single section of the channel as highlighted by the dotted lines in Figure 2(b) and (c). Then, there are some benefits of the reduction in the mesh generation and computation. The section width, w s, of the plate, perforation, and protuberance fin is 4 mm, 6.85 mm and 10.3 mm, respectively. In order to simplify the complex electric loss distribution of the IGBT module as shown in Figure 2(a), the heat flux modeled as electric losses was uniformly and constantly generated in the region, L heat, as shown in Figure 2(c).

20 C.-W. Han and S.-B. JeonG Figure 3. Schematic of (a) the plate fin, (b) the perforated fin, and (c) the protuberance fin. Figure 4. Mesh configuration of the heat sink with the plate fin for the one-pitch model. Table 1. result of the grid independency study. Grid size airflow rate [m 3 hr 1 ] Mean nusselt number 369,468 6.005 2.287 526,812 5.969 2.255 862,008 5.931 2.228 2,462,688 5.911 2.208 3. Numerical simulation 3.1. Grid systems The numerical solution must be independent to the grid size. Then, several grid systems were studied in order to reach the grid independency. The hexahedral meshes were employed to compute the thermal and fluid flow field in the domain generated by the commercial code ANSYS ICEM CFD as shown in Figure 4. Fine meshes were generated around the fin and concentrated in the region such as the entrance and exit of the heat sink or in the fan region that the air flow was expected to be changed suddenly. The airflow rate of the heat sink and mean Nusselt number of the fin were compared with different grid sizes under the same fan operating condition. To specify the heat sink in this study, at least 369,468 cells were required in the computational domain. From the results on Table 1, the grid size of 369,468 had shown a negligible difference of the airflow rate and mean Nusselt number as compared with a higher grid size of 2,462,688. Consequently, the final mesh system with 369,468 cells was adopted for the numerical simulation model. 3.2. Governing equations The incompressible steady continuity, momentum, and energy equation were solved together with realizable k-ε turbulence model in Eq. (1) [18] to simulate the velocity and temperature field using the commercial CFD solver ANSYS Fluent, where the buoyancy and radiation effects were neglected. The flow field was considered to be a laminar and turbulent mixing region because the Reynolds number was larger than 2,300 in the heat sink region but was smaller than 2,300 in the extended inlet and outlet region. The pressure-based segregated solver was used as the solution algorithm, where the governing equations were sequentially solved. The pressure-velocity coupling term was obtained by the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm. No-slip boundary conditions were used at the solid surfaces. ( ) φ ρu (1) x i φ k Γ k k = S i x φk i where, ρ is the air density (kgm 3 ), Γ Φ is the diffusion coefficient, u i is the air velocity vectors (ms 1 ), S ϕ,k is the source term of the general flow, and heat generation, ϕ is any one of the components shown in Table 2. Further information on the transport equations is provided in the ANSYS Fluent theory guide [18]. The boundary conditions for the present study were illustrated in Figure 2(c). The boundary conditions can be expressed as follows: At x = 0 and x = W s, both the surfaces are set as symmetrical boundaries. At y = 0 (z = L heat ), uniform heat flux q is applied on the bottom wall.

Journal of InTernaTIonal CounCIl on electrical engineering 21 Table 2. notation of the governing equation (1). equation Φ Γ Φ,eff S Φ Continuity 1 0 0 Momentum u i μ eff p/ x i + S V turbulence kinetic energy k μ eff /σ k P k ρε + G k turbulence kinetic energy dissipation rate ε μ eff /σ ε ε(c 1 P k C 2 ε)/k + C 3 G k ε/k energy T k S T At y = 0 (except for z = L heat ), the bottom surface is set as convection. At y = H, the top surface is set as convection. At z = 0 (Flow entrance), the inlet is set as pressure inlet and inlet temperature T = T in. At z = L f (Fan), the interior wall is set as pressure jump. At z = L (Flow exit), the outlet is set as pressure outlet. Defining an average convection coefficient h (Wm-2K- 1) for the fin of the heat sink, the total heat transfer rate may be expressed as Eq. (2). Q = ha ( ) T S T (2) where, Q is the heat loss of the IGBT module (W), A is the heat dissipation area of the fin (m 2 ), T S is the surface temperature of the fin (K), T is the ambient air temperature (K). The mean Nusselt number is able to express as Eq. (3). where, L ch is the characteristic length of the channel of the fin (m) and k air is the thermal conductivity of air (Wm 1 K 1 ). In this study, the channel width was relatively smaller than the fin height, and then the channel width was used as the characteristic length when the mean Nusselt number is calculated. 4. Experiments 4.1. Experimental system Nu = hl ch k air (3) In order to compare the thermal performance of the fins, the numerical simulation model was verified to the hotspot temperature on the heat sink that was the developed product. Figure 5 shows the experimental rig. The IGBT module was mounted on the heat sink and electric losses were uniformly and constantly generated in each IGBT module with 1,300W (IGBT: 936W, Diode: 364W). To blow the air into the heat sink, the fan (ebm-papst, DV6224) was mounted on the top side of the heat sink [19]. The experimental device is capable of supplying a constant power source to the IGBT module during one set of experiments with a constant electric loss to measure the temperature on the heat sink. 4.2. Temperature measurements The measurement of the junction temperature of the IGBT/diode chips is required to evaluate the numerical simulation results. For the case of the direct measurement method, inserting thermocouples to measure the junction temperature of the IGBT/diode chip has a risk of short-circuiting when the contact between the temperature sensor and IGBT/diode chip occurs. In the experiment of this study the indirect measurement method was then selected to get the junction temperature of the IGBT module. First, the surface temperature of the heat sink, which is called the hotspot temperature, T hot spot, in the following discussion, was measured, and then the junction temperature of the IGBT module, T junc., was predicted by adding the temperature difference which is equal to the amount of heat (kw) transferred from the junction (IGBT or diode chip) to case (heat sink block) multiplied by the junctionto-case thermal resistance (K/kW), R th, chip, from Eq. (4). T junc. = T hot spot + R th,chip Q loss (4) where, Q loss is the electric loss of the IGBT or diode chip. The thermal resistance is basically given by the maker of the IGBT module. The junction-to-case thermal resistance of the IGBT and diode chip used in the experiment is 33 K/kW and 66 K/kW, respectively [18]. The measurement point should be chosen to the spot of the maximum temperature on the heat sink, and thus this point was chosen through the numerical results. The K-type thermocouple was used as the temperature sensor, and several calibrated thermocouples were attached at the measurement point that was located between the IGBT module and heat sink. Thermocouples were connected to the data acquisition (DAQ) system, and measured data was stored every one second. 5. Results and discussions 5.1. Verification of the numerical simulation model Temperature rise test of the IGBT module was conducted at a constant temperature of 40 C in the constant temperature chamber. The hotspot temperature on the heat sink was reached at 100 C and the temperature difference between R, S, and T phase was within 1 C.

22 C.-W. Han and S.-B. JeonG Figure 5. Photograph of the experimental rig to verify the numerical simulation model. Flow direction Figure 6. temperature distribution on the heat sink of the developed product with heat sources of 1,300W (unit: C). In order to verify the suitability of the simulation model, the simulation domain was extended to the entire heat sink as shown in Figure 2(a). The IGBT module was modeled including the IGBT/diode chip, substrate, solder, and TIM. In Figure 6, the temperature of the IGBT chip was higher than the diode chip due to the difference of the power density, and the surface temperature was highly distributed as the airflow was directed toward to the downstream. The temperature at the measurement point was predicted to be 94.4 C that was lower than the experimental result. The main cause is estimated as the thickness difference of the TIM. The thickness of the TIM, which was recommended by the maker of the IGBT module, was considered in the simulation model. It is very difficult to control the thickness of the TIM in the developing prototype even though there is the thickness difference of the TIM between the simulation model and prototype. If the thickness of the TIM is thicker than the present condition, the thermal resistance in the TIM is increased and then the hotspot temperature on the heat sink is raised. As the thickness of the TIM is controlled as the same design condition during the assembly process, the errors will be expected to be reduced. The simulation model predicted the hotspot temperature on the heat sink within a margin of error of 5.6 percent. The magnitude of this error range is an acceptable level to compare the thermal performance of the fin with different geometries. 5.2. Comparison of the thermal performance The most important factor for selecting the heat sink was considered as the hotspot temperature in this study. When considering the cost, the secondary factor was chosen as the weight of the heat sink. Table 3 shows that the heat sink with the perforated fins is excellent with respect to the thermal performance and the weight. Compared with the plate and protuberance fin, the rate of increment of the dissipation area was greater than the rate of decrement of the convection coefficient. Consequently, the relative hotspot temperature of the perforated fin was the lowest among three fins. As for the plate fin, the weight of the heat sink with plate fins was relatively heavy because the fins were fixed on both the bottom and top side by the two base plates. In Table 3, the dissipation area of the protuberance fin was greater than the plate and perforated fin but the convection coefficient of the protuberance fin was much less than the fin of the two types. That is the reason why the effective velocity near the fin surface of the protuberance is slower than the plate fin as shown in Figure 7. As the protuberances are concentrated on the surface of the plate fin in the vertical direction, the flow resistance is increased near the solid surface, and then the fast velocity was formed in the area that is located between the plate fins. If the channel width of the protuberance fin is decreased and the number of the fins is increased in the horizontal direction, the effective velocity near the protuberance becomes fast, and it is then expected to improve the thermal performance. Table 3. Comparison of the thermal performance and the weight with different shapes of the fin. Type Surface temperature of the fin [ C] Convection coefficient [Wm 2 K 1 ] dissipation area [m 2 ] Weight [kg] Plate fin 71.1 25.9 5.388 15.2 Perforated fin 68.1 24.1 7.152 14.4 Protuberance fin 82.6 16.0 6.027 17.1

Journal of InTernaTIonal CounCIl on electrical engineering 23 Figure 7. Velocity distribution of the channel section in the exit of (a) the plate fin, (b) perforated fin, and (c) the protuberance fin. 5.3. Suitability of the selected heat sink The heat sink with the perforated fins was selected from the numerical simulation results with respect to thermal performance and cost. When the IGBT module was mounted on the selected heat sink, the junction temperature of the IGBT module was predicted to evaluate the suitability of the selected heat sink. Figure 8 shows the configuration of one IGBT module and half of the heat sink with the perforated fins which is the developing prototype. Ambient air of 40 C is induced from top side by the fan, Ventas Ventilator GD133-2J [20], and the heated air by IGBT module is exhausted to the bottom side through the air-path guide. Electric losses are constantly generated in one IGBT module with 1,640W (IGBT: 1,177W, Diode: 457W). Figure 9 shows the temperature distribution on the heat sink to the developing prototype. Similar to the result of Figure 6, the relatively higher temperature was distributed at the IGBT/diode chip that was located in the downstream direction. The surface temperature of the monitoring point and the thermal switch was 103.0 C and 77.0 C, respectively. The maximum hotspot temperature on the heat sink was reached at 105.7 C and the spot was Figure 8. Configuration of the IgBt module attached at the heat sink with perforated fins. Figure 9. temperature distribution on the heat sink with the perforated fins of the developing prototype (unit: C). located in 190 mm away from the entrance of the heat sink. The hotspot temperature was located below the IGBT chip, and, therefore, the thermal resistance of the junctionto-case was used as 33 K/kW. The predicted junction temperature of the IGBT module, which was considered with a numerical margin of error of 5.6 percent, was 131.4 C, and the result satisfied the design criterion of the IGBT module.

24 C.-W. Han and S.-B. JeonG 6. Conclusion The junction temperature of the IGBT module, which is a part of a power semiconductor device, directly affects system performance such as a PCS, thus making the proper selection of the heat sink to be very important at the thermal design stage. In this paper, the thermal performance of the heat sink was evaluated according to the shape of the fins, and the heat sink was selected. The selected heat sink was evaluated as to whether or not it satisfied the design criterion of the junction temperature of the IGBT module. Prior to the performance evaluation of the air-cooled heat sink using the numerical method, the suitability of the simulation model was validated through the experimental result of the developed product. The simulation model predicted the hotspot temperature on the heat sink within a margin of error of 5.6 percent. The magnitude of this error range is an acceptable level to compare the thermal performance of the fin with different shapes. The heat sink with the perforated fins exhibited an excellent thermal performance because the rate of increment of the dissipation area was greater than the rate of decrement of the convection coefficient. The selected heat sink with the perforated fins was attached at the IGBT module, and the junction temperature of the IGBT module was predicted. The predicted junction temperature of the IGBT module was 131.4 C, and the result satisfied the design criterion of the IGBT module. Disclosure statement No potential conflict of interest was reported by the authors. Notes on contributor Chang-Woo Han He received the B.Sc. and M.Sc. degree in mechanical engineering from the University of Seoul, Seoul, Republic of Korea, in 2004 and 2006, respectively. He joined Hyosung Corporation since 2007 and is currently a principal researcher for Hyosung Corporation. Also he is currently working toward the Ph.D. degree in mechanical engineering at the University of Seoul. His research interests include thermo-fluid simulation and cooling design in the power electronic systems such as a PCS, HVDC, STATCOM. Seung-Boong Jeong He received the B.Sc. and M.Sc. degree in mechanical engineering from the Seoul National University, Seoul, Republic of Korea, in 1999 and 2001, respectively. He is currently a chief researcher for Hyosung Corporation. He is currently the technology leader of the thermo-fluid technology group of the Power and Industrial Systems R&D Center. His research interests include thermo-fluid simulation and design in electrical machines and power electronics systems. Mr. Jeong is a member of the ASME. ORCID Chang-Woo Han Seung-Boong Jeong References http://orcid.org/0000-0003-0550-2993 http://orcid.org/0000-0003-2733-7933 [1] Chang YW, Chang CC, Ke MT, Chen SL. Thermoelectric air-cooling module for electronic devices. 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