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Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system
MR-11928; No of Pages 9 Microelectronics Reliability xxx (2016) xxx–xxx
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Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system Chang-Woo Han a,b, Seung-Boong Jeong a, Myung-Do Oh c,⁎ a b c
Hyosung Corporation, Power & Industrial Systems R&D Center, Anyang, Republic of Korea Graduate School of Mechanical and Information Engineering, University of Seoul, Seoul, Republic of Korea Department of Mechanical and Information Engineering, University of Seoul, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 3 November 2015 Received in revised form 26 January 2016 Accepted 27 January 2016 Available online xxxx Keyword: Airflow rate Insulated-gate bipolar transistor (IGBT) Junction temperature Porous media model Power conversion system (PCS) Thermo-fluid simulation model
a b s t r a c t The junction temperature of the insulated-gate bipolar transistor (IGBT) module, which belongs to power semiconductor devices, directly impacts on the system performance of the power conversion system (PCS), and therefore, the accurate prediction of the airflow rate passing the heat sink block of the IGBT module is very important at the thermal design stage. In this paper, the thermo-fluid simulation was developed with the T–Q characteristic curve to predict the junction temperature of the IGBT module and the airflow rate of the heat sink block. The porous media model was adopted in the heat sink block with fins and the filled air between fins of the heat sink block in the PCS to remove the heavily concentrated mesh problems in the heat sink block. The proposed simulation model was compared to the experimental value for the hot spot temperature on the heat sink block and the differences were within the average 4.0% margin of error in the comparison. This simulation model can be used to evaluate the suitability of the cooling design according to various operating conditions of the fan and IGBT module with benefits of the reduction in the mesh generation and the computation time. Also, this simulation model increases the flexibility of predicting the airflow rates in the PCS due to the change of the airflow passage structure in the PCS or the capacity of the fan. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Renewable energy sources such as wind power and photovoltaic (PV) have the characteristics of a non-uniform output power due to the environmental conditions and the mismatch between the time of demand and supply. To compensate for the weakness of renewable energy sources, the energy storage system (ESS), which consists of the power conversion system (PCS) and the battery as shown in Fig. 1, has been supplied to many energy management companies [1,2]. The PCS either stores idle energy to a battery or extracts energy from a battery. Power semiconductor devices such as an insulated-gate bipolar transistor (IGBT) are the key component in the design of the PCS and are used to match the electrical characteristics between end users and grid systems depending on renewable energy sources. While the electrical characteristics of an electric power are converted by power semiconductor devices, the air- or water-cooled heat sink is used to dissipate heat by electric losses such as switching and conducting losses. In order to use semiconductor devices securely, a number of researchers evaluated and improved the cooling performance of the heat sink at the component level using numerical and
⁎ Corresponding author. E-mail address: [email protected] (M.-D. Oh).
experimental methods [3–10]. It is also important to predict the temperature of the semiconductor device with different cooling types and operating conditions at the thermal design stage. Some studies predicted the junction temperature, which is the highest operating temperature of the actual semiconductor device in the electronic device and is a key factor for assessing the suitability of the cooling design, in order to check the thermal status of the power semiconductor device at the component level under various operating conditions using the computational fluid dynamics (CFD) technique and the thermal network model [11–17]. Most of the previous studies have evaluated the cooling performance at the component level but have never been carried out at the system level such as with a PCS. When power semiconductor devices are integrated into a system such as the PCS, problems such as the reduction of the flow rate of a cooling medium can occur. With regards to the air-cooled type, the common problem involves the reduction of the airflow rate in the heat sink block by the system effect. As shown in Fig. 2, the operating point of a fan is shifted from (a) to (b) by the pressure rise due to the resistance elements in the system. Even if the design criterion of the junction temperature of a power semiconductor device is satisfied at the component level, the actual performance of a power semiconductor device in the system level can fail due to the reduction of the airflow rate by the system effect. Therefore, it is very important to predict the airflow rate of the heat sink in consideration of the system effect.
http://dx.doi.org/10.1016/j.microrel.2016.01.018 0026-2714/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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2. Description of physical models 2.1. Power conversion system
Fig. 1. Configuration of the PCS with an energy storage system connecting the components.
In some studies [18–19], the numerical simulation of the system level was conducted with a small number of meshes because the system was comprised of a small number of components and simple geometry. In contrast, P.R. Parida et al. [11] had to deal with elements of about 4.7 million to simulate the flow field of only a single component model. For this case, it will require enormous meshes to analyze the cooling performance of the system which contains several components. As a result, the use of high performance workstations and heavy computation time are required to simulate the system with all the components and the availability of the simulation model is reduced. This paper addresses the numerical simulation model composed of the reasonable minimum number of meshes to predict the airflow rate of the heat sink block in consideration of the system effect and to increase the availability of the numerical simulation. The technique of the porous media model was adopted on the simulation model of the system level to reduce the number of meshes that are heavily concentrated in the IGBT stack, which includes the IGBT/diode chips, heat sink blocks, and air between fins of the heat sink block. Now, one is able to calculate the airflow rate of the heat sink block in consideration of the effect by the interior structures in the PCS. The T–Q characteristic curve was proposed to predict the junction temperature of the IGBT module and the hot spot temperature on the heat sink block using the numerical model of the previous study [20]. Experimental works were conducted to evaluate the accuracy of the numerical simulation model and were performed under various operating conditions for two different PCSs using some electrical devices.
Fig. 2. Shift of the operating point of a fan due to the resistance changes by the system effect.
The PCS investigated in this study is shown in Fig. 3 and is divided into IGBT stack, reactor, and AC grid connection parts. The IGBT stack part, which consists of several IGBT modules, air-cooled heat sinks, fans, and electric components, converts a DC power source such as a battery to an AC power source. The reactor part plays a role to remove a harmonic frequency of an AC power source rectified by IGBT modules. The AC grid connection part performs a function of connection between an AC power source and grid power system. Most of the outdoor-type PCSs have the structure that outdoor air, which contains foreign materials such as dust, fallen leaves, and small insects that have flown into the PCS through louvers on the upper side. The air heated by IGBT modules, reactors, and other electric components is exhausted through the bottom and front side louvers. Among some PCSs, the exit duct of the IGBT stack is connected to the region of the reactor part in order to increase the airflow rate toward the reactors. While the indoor-type PCS has the structure that the air induced from the front and bottom side louvers and is exhausted to the top side louvers, as the risk to inflowing foreign materials is relatively lower than the outdoor-type PCS. Even if the PCS is in the indoors, the filters are installed in the front and bottom side louvers to prevent the inflowing of indoor foreign materials. In common with both the outdoor and indoor-type PCS, the fans for the IGBT modules are directly attached on the heat sink block to improve the cooling performance and other fans for the reactors are mounted on the top of the cabinet to avoid interference with the structure such as bus bars, damping resistors, and so on.
Fig. 3. Three-dimensional physical models and airflow passage of the outdoor-type PCS (isoview).
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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2.2. IGBT module A power module for the PCS is chosen to convert the characteristics of the power source. The selected device is Infineon Corporation PrimePACK™3 module and NTC, rated for 1.7 kV and 1.0 kA [21]. The size is 89 mm (width) × 38 mm (height) × 250 mm (length). Fig. 4 shows the cross-sectional view of the IGBT module and the heat sink block. The IGBT module package consists of the aluminum oxide (Al2O3) ceramic substrate soldered on a copper (Cu) base plate, IGBT and diode chips lying on the ceramic substrate, and a silicone gel covering the substrate and IGBT/diode chips. All components of the IGBT module are wrapped up with a plastic cover. IGBT/diode chips repeat the ON–OFF operations to convert DC to AC and generate 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 temperature of the thermal switch or negative temperature coefficient (NTC) thermistor exceeds the setting temperature, 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 junction temperature is set at 135 °C in consideration of power cycling, temperature measurement errors, instantaneous peak current, and unexpected environmental conditions. 2.3. Air-cooled heat sink Every power semiconductor device uses the heat sink block to dissipate heat generated by electric losses, whether in a chip or in a module package. The task of the heat sink block is to transfer the thermal energy of the IGBT module to a cooling medium such as air, water, and refrigerant. In addition, the heat sink block plays a role as a base of a power semiconductor device. In this study, the heat sink of the air-cooled type was used to remove heat by electric losses of the IGBT/diode chips. The size of heat sink block is 280 mm (width) × 115 mm (height) × 400 mm (length) and the material is aluminum alloy Al 6063. The heat sink block consists of two parts: one base plate and a number of fins as shown in Fig. 4. In a previous model, one fin was inserted into one groove of the base frame and then the dissipation area of heat became relatively narrow. In order to increase the dissipation area of the heat sink block, the double layer fin was mounted on the base plate [20]. The heat sink block was attached to the base plate of the IGBT module and the thermal interface material (TIM), Mementive Performance Material Inc. YG6111 [22],
3
was applied on between the IGBT module and the heat sink block to minimize the thermal contact resistance. The thermal conductivity of the TIM was 0.84 Wm−1 K−1 and its thickness was controlled at approximately 75 μm. With its double inlet centrifugal fan with forward curved impellers, Ventas Ventilatorer GD 133-2J [23], was used to blow the air into the heat sink block. One can observe that the maximum airflow rate of the fan is 958 m3 hr−1 and the shutoff pressure is 606.1 Pa on the fan performance curve in Fig. 2. Fig. 2 also shows that the acceptable maximum airflow rate is about 811.3 m3 hr−1 (a), when the heat sink block used in this study is only attached to the fan. 3. Numerical simulation 3.1. Numerical simulation procedure The junction temperature of the IGBT module is the most important criterion at the thermal design stage of the PCS. The most influential factor affecting the junction temperature is the airflow rate of the heat sink block. In order to predict the junction temperature, the airflow rate of the heat sink block is to be calculated accurately. Even if the geometry of the PCS is complex as shown in Fig. 3, the simulation model of the PCS of the system level must be constructed to predict the accurate airflow rate including flow resistances by the interior structures such as louvers, filters, and electric components. For this reason, Lee et al. [18] and Breier et al. [19] constituted the simulation model of the system level and predicted the hot spot temperature of semiconductors. For their cases, the numerical meshes were heavily concentrated in semiconductors, and then it became necessary to use the high performance workstations and a lot of computation time was required for the numerical work. The dimension of the PCS is over the order of 103 mm, whereas the distance between fins of the heat sink block the thickness of the fin are the order of 100 mm. From the system level perspective, the appearance of the fins of the heat sink block is similarly seen to the porous material such as a filter. Also, as air is passing the fins of the heat sink block, the non-uniform flow is uniformly aligned and the pressure drop is taken place by the friction resistance on the surface of the fin. These flow phenomena appeared in the porous material. In terms of the appearance and the flow phenomenon, the heat sink block at the system level can be considered as porous material. Thus the porous media model was adopted in this study to calculate the pressure drop in the heat sink block inside the PCS without the configuration of the complex mesh. The porous media model has been used to calculate the effect of flow resistances by X.L. Ouyang et al. [24]. He modeled the cylindrical
Fig. 4. Cross-sectional view of the packaging associated with IGBT and diode chips of Infineon PrimePACK™3 module and NTC.
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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fin as the porous media and compared the cooling performance according to the different modeling methods. As a result, the modeling of the heat sink as the porous media seems reasonable to simulate the micro fin structure in the PCS. In the previous design method [20] as shown in Fig. 5, in Step 1 (the black dotted line), the simulation of the IGBT stack model was conducted at the component level. Here, the P–Q curve was adopted as the boundary condition of the fan. As the flow field of the IGBT stack model is calculated, the operating point of the fan can be determined in the P–Q curve by the iterative calculation, and then the junction temperature of the IGBT module is calculated through the energy field. Here, one should know that the operating point of the fan in the IGBT stack cannot match with the real operating point when the IGBT stack is integrated into the PCS of the system level. As shown in Fig. 5, the proposed design method (the red solid line) integrating the system is quite different from the previous design method when only taking the IGBT stack into consideration. In the proposed method, the boundary condition of the fan is considered as the constant inlet velocity depending on the airflow rate. The flow and energy field of the IGBT stack model of Step 1 are sequentially calculated with various airflow rate conditions in Step 2 and Step 3, respectively, as the forced convection is dominant in the computational domain. The characteristic curve of the junction and hot spot temperature can be plotted against the airflow rate as shown in Step 4. If one can construct this characteristic curve, it is convenient to predict the required airflow rate of the heat sink block with different power conditions and the design temperature according to the change of the capacity of the fan or the airflow passage structure in the PCS.
In the PCS model of Step 5, most of the electric components are modeled in to solids, and the geometry of the fan is simplified with the P–Q curve as the boundary condition. The fins of the heat sink block and air between the fins are modeled as the porous media of Fig. 4. The heat sink blocks and the filters are assigned to the fluid region in order to replace the porous media. Thereby, the problem with a great number of meshes, which are concentrated in the IGBT stack, can be relaxed without the need for high performance workstations and a lot of computation time. The porous model condition of the heat sink block with micro fins requires the permeability and the inertial resistance factor. These conditions can be extracted from the resistance curve related to the airflow rate and the pressure drop in Step 2. The flow field of the PCS model is obtained using the boundary and model conditions and the airflow rate of the heat sink block is calculated in Step 6. Finally, the junction and hot spot temperature can be predicted as the design temperature at the intersection of the curve of Step 4 by the airflow rate obtained from Step 6.
3.2. Grid systems Due to the complex geometry of the PCS, the computational domains of the system level were meshed with unstructured non-uniform tetrahedral grids as shown in Fig. 6, which were generated by the commercial code ANSYS ICEM CFD. Initial mesh of the described model used a size of the order of 2 mm. Fine meshes were generated around all solid components and normal to all boundary surfaces. At least 40.8 million cells were required in the computational domain to model the
Fig. 5. Comparison of previous and proposed design methods for the PCS model and IGBT stack model. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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Fig. 6. Interior structure of the meshed domain with fine meshing at all the solid components (rear side).
specific interior structures such as the IGBT stacks, reactors, and electric components being used under these conditions. To check the grid dependency of the model, more refined meshes was tested with a mesh size of the order of 1.5 mm and then around 78.3 million cells were reached for the entire computation domain. The numerical results obtained by two mesh systems, viz., 40.8 and 78.3 million cells, were comparable within a 0.2% margin of error in the airflow rate of the heat sink block and the testing was accepted to satisfy the grid independent criteria with a 1% margin of error. As a result, the final mesh system with 40.8 million cells was adopted for the numerical simulation model of the system level. 3.3. Governing equations The flow phenomenon inside the PCS was predicted as the threedimensional complex turbulent flow. So, the steady Navier–Stokes, energy and continuity equation were solved together with realizable k-ε turbulence model in Eq. (1) [25] to simulate the flow and energy fields using the commercial CFD solver ANSYS Fluent, where the buoyancy and radiation effects were neglected. The region where y+ was below 15 was occurred around some solid surfaces and then the scalable wall function [25] was adopted to avoid the deterioration of the standard wall function under grid refinement. Regarding the scalable wall function, the purpose is to force the usage of the log low in conjunction with the standard wall functions approach. The pressure-based segregated solver was used as a solution algorithm where the governing equations were solved sequentially. The pressure–velocity coupling term was obtained by the SIMPLE (Semi-Implicit Method for PressureLinked Equations) algorithm [25]. No-slip boundary conditions were used on the solid surfaces. ∂ ∂Φk ρui Φk −Γ k ¼ SΦk ∂xi ∂xi
ð1Þ
where, ρ is the air density (kgm−3), ΓΦ is the diffusion coefficient, ui is the air velocity vectors (ms−1), SΦ,k is the source term of the general flow and heat generation, Φ is the velocity u, turbulence kinetic energy k (m2 s−2), turbulence kinetic energy dissipation rate ε (m2 s−3), and temperature T (K). The porous media model incorporates an empirically determined flow resistance in a region of the model defined as “porous” [25]. Porous media is modeled by the addition of a momentum source term to the standard fluid flow equation of Eq. (1). The source term is composed
of two parts: a viscous loss term and an inertial loss term. This momentum sink contributes to the pressure gradient in the porous cell, creating a pressure drop that is proportional to the fluid velocity in the cell. To recover the case of simple homogeneous porous media μ 1 Si ¼ − vi þ C 2 ρvjvj α 2
ð2Þ
where, α is the permeability and C2 is the inertial resistance factor. Simulation data available in the form of pressure drop against velocity through the porous media can be extrapolated to determine the coefficients for the porous media. The pressure drop, Δp (Pa), against velocity can be plotted to create a trend line through several points yielding the in Eq. (3) from Step 2 in Fig. 5. Δp ¼ av2 þ bv
ð3Þ
where, a and b are coefficients of the quadratic function. A simplified version of the momentum equation, relating the pressure drop to the source term, can be expressed as Δp ¼ −Si Δn
ð4Þ
where, Δn is the porous media thickness (m). From Eqs. (2) and (4), the coefficients of C2 and 1/α are 1.932 and 1,967,489, respectively, when air temperature is 50 °C. 4. Experiment 4.1. Experimental system In this study, the experimental devices were constructed in consideration of the operating condition of the PCS to evaluate the numerical simulation model for the local temperature on the heat sink block of the IGBT stack. As the voltage drop occurred in a real battery during operation, a battery of the real system as shown in Fig. 1 was substituted by the power source simulation device in the experimental system to supply the setting DC power for constant operating conditions as shown in Fig. 7. DC voltage of 1050 VDC to 750 VDC is supplied to the PCS through the power source simulation device and is converted to an AC power source of 440 VAC by the PCS. Here, the voltage of 1050 VDC is the fully charged condition of the battery and the voltage of 750 VDC is the lower limit of the battery operation. AC power of
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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Fig. 7. Configuration of the experimental system with the power source simulation device.
380 VDC is supplied to the experimental system and the converter AC power of 380 VDC is produced from the experimental system. Fig. 7 shows the configuration of the experimental system which consists of the power source simulation device, PCS, and power transformer. As the PCS used in this study is the component for converting DC power, DC power must be supplied to the PCS. So, we constructed a device such as a real battery that can convert AC power to DC power. The experimental device should be able to control the quantities of electric losses of the IGBT module in order to simulate various operating conditions. To fulfill the characteristics of the power source, the power source simulation device was composed of the induction voltage regulator (IVR), parallel power transformers with different winding connection methods, and the rectifier. The experimental device is capable of supplying a constant DC power to the PCS during one set of experiments with constant electric loss to measure the temperature on the heat sink block of the IGBT stack. 4.2. Temperature measurements The measurement of the junction temperature of IGBT/diode chips in the PCS is required to evaluate the numerical simulation results. With regard to the direct measurement method, inserting thermocouples to measure the junction temperature of IGBT/diode chips has a risk of short-circuit when the contact between the temperature sensor and IGBT/diode chip occurs. In the experiment of this study, the indirect measurement method then was selected to obtain the junction temperature in the IGBT module. First, the surface temperature of the heat sink block, which is called the hot spot temperature, THot spot, in the following discussion, was measured and then the junction temperature of the IGBT module, TJunc, 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 junction-to-case thermal resistance (K/kW), Rth,chip, from Eq. (5) [21]. T Junc ¼ T Hot spot þ Rth;chip Q Loss
4.3. Experimental conditions The airflow rate of the heat sink block inside the PCS is mainly determined by the type and the internal layout of the PCS. For this reason, the experiments were performed on both the outdoor and indoor-type PCS. Also, the damper condition of the PCS was considered as the experimental parameter whether the damper is open or not. The damper was used to control the airflow rate of the PCS. Damper closed is the operating condition of the rated airflow rate, while damper open is the operating condition of the increased airflow rate. Accuracy and applicability of the numerical simulation model of the system level were investigated by the experiments for different operating conditions of the PCS. Prior to the main experiment, in order to evaluate the reproducibility of the measurement, the temperature measurement was repeatedly carried out for five times under Case 1–3 in Table 1. Repeated experiments were performed to assess the reproducibility of the measurement method. From the result of five times experiments, the average hot spot temperature on the heat sink block was 111.6 °C and its deviation was 1.1 °C. The measurement method was enough to reproduce the experimental results because the temperature deviation was below 1% level of the average temperature. In order to verify the temperature prediction by the numerical model at the system level, several experiments were conducted under various load conditions using the measurement method previously verified. The electric losses listed in Table 1 were generated in one IGBT module and were separated to the IGBT and the diode losses. The damper condition is the parameter varying the airflow rate of the heat sink block, and the electric condition is the parameter affecting the loss of
ð5Þ
where, QLoss is the electric loss of the IGBT or diode chip (kW). Thermal resistance is basically given by the catalog of the IGBT module. The junction-to-case thermal resistances of the IGBT and diode chip used in the experiment were 33 K/kW and 66 K/kW, respectively [21]. The measurement point should be chosen according to the spot of the maximum temperature on the heat sink block, and then this point was chosen through the numerical results of the IGBT stack model as shown in Fig. 8 [20]. The K-type thermocouple was used as the temperature sensor and twelve calibrated thermocouples were attached at the measurement points between the base of the heat sink block and the IGBT module as shown in Fig. 9. Thermocouples were connected to the data acquisition (DAQ) system and measured data was stored every one second.
Fig. 8. Temperature rise distribution on the heat sink block obtained through the results of the previous study.
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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Fig. 9. Measurement point of the heat sink block with the IGBT module (a) front view and (b) side view.
the IGBT module. Ambient air temperature of the PCS was assumed to be 50 °C in order to take the extreme environment condition into consideration. Detailed operating conditions are listed in Table 1. 5. Results and discussion The simulation results were compared with the experimental results to verify the accuracy of the numerical model. Two types of the PCS with different operating conditions were considered as described in Table 1. Fig. 10 displays comparison results of the hot spot temperatures of the experiment and the numerical simulation. The experimental results were obtained from taking an average of the hot spot temperatures at twelve measurement points. Fig. 10 shows that the differences are within a 3.5 and 4.6% margin of error, between all the experiments and the simulation calculations of the outdoor and indoortype PCS. Also tendency of the temperature corresponds to the experimental results with different operating conditions. The temperature differences between experimental and numerical results were from 0.7 °C to 7.2 °C. The magnitude of this error is of a sufficient enough level to accept in order to lay out the structure of the PCS and determine the operating range of the IGBT module at the design stage. Fig. 10 demonstrates that all the simulation temperatures are lower than the experimental results. This consistency indicates the consistent causes that were involved in the experiment or in the simulation such as the thickness of the TIM and the flow resistances of the filter. The thickness of the TIM recommended by the manufacturer of the IGBT module was considered in the calculation [23]. It was very difficult to measure the thickness of the TIM in a prototype even though there was the thickness difference of the TIM for each IGBT stack. If the thickness of the TIM is thicker, the thermal resistance is increased and then the hot spot
temperature on the heat sink block is raised. As the thickness of the TIM is controlled as the same design condition during the assembly process and the real characteristics of flow resistances of the filter from the measured data are considered, the errors are expected to be reduced. The proposed design model estimated the experimental results in less than the average 4.0% margin of error with respect to the averaged hot spot temperature. As it is used for the proposed design method and simulation model for the thermal design, the junction temperature of the IGBT module can be predicted and confirmed for various operating conditions. Additionally, it can be used not only for selecting the fan and the filter but also for designing the airflow passage structure of the PCS. The numerical simulation model at the system level was used to predict the airflow rate of the heat sink block. The operating conditions of the PCS are described in Table 1 and the simulation results of the airflow rate passing the heat sink block are summarized in Table 2. The averaged airflow rate in the table is the value that is taken as an average for all heat sink blocks within the PCS. The deviation of the airflow rate represents the standard deviation to the averaged value of the airflow rate for each heat sink block and the required airflow rate means the minimum airflow rate satisfying the design criterion of the junction temperature of the IGBT module. If the averaged airflow rate becomes larger than the required airflow rate, we can say that the junction temperature satisfies the design criterion. For Case 1–1 and 1–2, the airflow rate passing the heat sink block was different as 482.7 m3 hr−1 and 718.0 m3 hr−1, respectively, due to the different damper conditions, but the required airflow rate was the same as 115.4 m3 hr−1 because of the same electric condition. On the contrary, the airflow rate of the heat sink block for Case 1–2 and 1–3 was equal to 718.0 m3 hr−1 due to the same operating condition
Table 1 Operating conditions of the outdoor and indoor-type PCS. Case
1–1 1–2 1–3 2–1 2–2
Type
Outdoor Outdoor Outdoor Indoor Indoor
Damper Condition
Closed Open Open Closed Closed
Electric condition
Electric Losses
Ambient temperature
Voltage
Current
IGBT
Diode
950 VDC 950 VDC 950 VDC 1050 VDC 1050 VDC
1300 AAC 1300 AAC 1600 AAC 1400 AAC 1500 AAC
845 W 835 W 1101 W 989 W 1115 W
295 W 287 W 368 W 370 W 402 W
50 °C 50 °C 50 °C 50 °C 50 °C
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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Fig. 10. Comparison of the hot spot temperatures of simulations and experiment results with different operating conditions (a) the outdoor-type PCS and (b) the indoor-type PCS.
of the damper, but the required airflow rate was increased from 115.4 m3 hr−1 to 382.5 m3 hr−1 because the electric losses of the IGBT module were increased by higher electric current from 1300A to 1600A. As the output current was raised for Case 2–1 and 2–2 similar to Case 1–2 and 1–3, the required airflow rate was also increased from 270.3 m3 hr−1 to 380.2 m3 hr−1. The averaged airflow rate of the heat sink block of the indoor-type PCS was increased by 154.9 m3 hr−1 more than of the outdoor-type PCS under the closed damper condition. If the airflow rate of the outdoor-type PCS can be increased, than the current model the IGBT module can be operated in more severe operating conditions. This suggests that the designer of the PCS should make an effort for the improvement of the airflow passage structure in the outdoor-type PCS. Even though the same fan is attached to the inlet of the heat sink block with the same capacity, the airflow rates of the heat sink block can be varied by the differences of the ventilation area of the louvers, the layout of the electric components, and the thickness of the filter. In Fig. 11, when the airflow rate is increased up to 637.6 m3 hr−1 in Case 1–1, the junction temperature is predicted as 129.3 °C for 950 VDC and 1600AAC, which satisfies the design criterion. If the damper is closed in Case 1–3, the airflow rate is reduced to 482.7 m3 hr−1 and then there is little design margin of the junction temperature. On the other hand, the indoor-type PCS is difficult to expect a considerably large effect by improving the airflow rate and reducing the junction temperature because the gap is relatively small between the averaged and the acceptable maximum airflow rate. In all operating conditions, the deviation of the airflow rate appeared to be small enough to ignore, thus in the following discussion, the average airflow rates were used to calculate the junction temperature of the IGBT module and the hot spot temperatures of the heat sink block. The T–Q characteristic curves proposed in this study were numerically obtained for each case. Case 2–2 with the greatest loss condition was selected for discussion, because the different operating conditions
revealed a similar tendency. Fig. 12 represents the T–Q characteristic curve for Case 2–2. This characteristic curve indicates the relationship between the junction and hot spot temperatures with respect to the airflow rate passing the heat sink block. The circular symbols represent the junction temperature of the IGBT chip and the triangle symbols stand for the hot spot temperature on the heat sink block. The black symbols are the calculated temperatures by the previous design method, the red symbols are the calculated temperatures by the proposed design method, and the blue symbols are the design criteria airflow, respectively, in the T–Q characteristic curve. The blue dash and the dot line indicate the upper limit of the junction and hot spot temperature, respectively. Fig. 12 clearly demonstrates the gradual reduction effect of the temperature of the IGBT module as the airflow rate is increased. Minimum airflow rate to satisfy the design criterion of the junction temperature can be calculated by using the T–Q characteristic curve. While the junction and hot spot temperatures are able to be predicted under the condition of the IGBT modules integrated into the PCS. In Fig. 12, the junction temperature of the IGBT module was calculated from the intersection of the characteristic curve and the vertical line of the airflow rate with 637.6 m3 hr−1 for Case 2–2. The hot spot temperature on the heat sink block was also calculated from the similar method with the junction temperature. The calculated junction and hot spot
Table 2 Numerical simulation results of the airflow rates of the heat sink block and the temperature of the IGBT module with different operating conditions of the outdoor and indoortype PCS. Case
1–1 1–2 1–3 2–1 2–2
Airflow rate [m3 hr−1]
Temperature [°C]
Averaged
Deviation
Required
Hot spot
Junction
482.7 718.0 718.0 637.6 637.6
3.6 1.6 1.6 3.9 3.9
152.9 152.9 470.0 329.5 465.2
99.8 95.2 109.0 105.3 110.6
114.1 109.3 127.6 122.1 129.0
Fig. 11. Numerical simulation results of the junction temperature of the IGBT module with different operating conditions of the outdoor and indoor-type PCS.
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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benefits resulting in the reduction in the mesh generation and the computation time. Also, this simulation model improves the flexibility of predicting the airflow rate by changing the airflow passage structure in the PCS or the capacity of the fan. Lastly, one can use the T–Q characteristic curve proposed in this study for the thermal design by the prediction of the junction and the hot spot temperature with different airflow rates and electric operating conditions. Acknowledgments This work was supported by the 2015 Research Fund of the University of Seoul, and the authors would thank the Power & Industrial Systems R&D Center of Hyosung Corporation for the realization of the prototype and for the thermal test. References Fig. 12. Numerical simulation results of the junction temperature of the IGBT module and the hot spot temperature on the heat sink block with different airflow rates for Case 2–2. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
temperatures were 129.0 °C and 110.6 °C, respectively (red dots). Regarding the airflow rate using the previous design method, the calculated junction temperature is 125.9 °C and the hot spot is 107.1 °C, respectively (black dots). The difference of the airflow rate is 137.7 m3 hr−1 and the junction temperature is 3.1 °C between the previous and proposed design methods. One can observe that the airflow rate is less sensitive to the control temperatures of IGBT module. Figs. 11 and 12 also indicate that the airflow rates can be reduced further with less safety margin from the current operation through the proposed design method. The other results are represented in Table 2. 6. Conclusions The junction temperature of the IGBT module, which belongs to power semiconductor device, directly impacts on the system performance of the PCS, and the accurate prediction of the airflow rate passing the heat sink block of the IGBT module is also very important in thermal design stage. In this paper, the thermal design method of the PCS was proposed to predict the reduced airflow rate when the power semiconductor devices are integrated into the PCS and the suitability of the predictions in the design method was evaluated by the experimental works. The prediction method of the junction temperature of the IGBT module and the airflow rate of the heat sink block was developed using the model of the system and component level. The porous media model was adopted in the numerical simulation model of the heat sink block with fins and filled air in order to remove the heavily concentrated mesh problems in the block. The simulation model of the PCS can predict the accurate airflow rate including the system effect of flow resistances by the interior structures such as louvers, filter, and electric components. Also, the T–Q characteristic curve was proposed to predict the junction temperature of the IGBT module and the hot spot temperature on the heat sink block. The proposed simulation model was compared to the experimental value for the hot spot temperature on the heat sink block and the differences were within the average 4.0% margin of error in the comparison. The magnitude of this error range is at an acceptable enough level in order to lay out the structure of the PCS and to determine the operating range of the IGBT module at the design stage. As a result, the simulation model was developed to predict the junction temperature of the IGBT module at the thermal design stage with
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Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system Chang-Woo Han a,b, Seung-Boong Jeong a, Myung-Do Oh c,⁎ a b c
Hyosung Corporation, Power & Industrial Systems R&D Center, Anyang, Republic of Korea Graduate School of Mechanical and Information Engineering, University of Seoul, Seoul, Republic of Korea Department of Mechanical and Information Engineering, University of Seoul, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 3 November 2015 Received in revised form 26 January 2016 Accepted 27 January 2016 Available online xxxx Keyword: Airflow rate Insulated-gate bipolar transistor (IGBT) Junction temperature Porous media model Power conversion system (PCS) Thermo-fluid simulation model
a b s t r a c t The junction temperature of the insulated-gate bipolar transistor (IGBT) module, which belongs to power semiconductor devices, directly impacts on the system performance of the power conversion system (PCS), and therefore, the accurate prediction of the airflow rate passing the heat sink block of the IGBT module is very important at the thermal design stage. In this paper, the thermo-fluid simulation was developed with the T–Q characteristic curve to predict the junction temperature of the IGBT module and the airflow rate of the heat sink block. The porous media model was adopted in the heat sink block with fins and the filled air between fins of the heat sink block in the PCS to remove the heavily concentrated mesh problems in the heat sink block. The proposed simulation model was compared to the experimental value for the hot spot temperature on the heat sink block and the differences were within the average 4.0% margin of error in the comparison. This simulation model can be used to evaluate the suitability of the cooling design according to various operating conditions of the fan and IGBT module with benefits of the reduction in the mesh generation and the computation time. Also, this simulation model increases the flexibility of predicting the airflow rates in the PCS due to the change of the airflow passage structure in the PCS or the capacity of the fan. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Renewable energy sources such as wind power and photovoltaic (PV) have the characteristics of a non-uniform output power due to the environmental conditions and the mismatch between the time of demand and supply. To compensate for the weakness of renewable energy sources, the energy storage system (ESS), which consists of the power conversion system (PCS) and the battery as shown in Fig. 1, has been supplied to many energy management companies [1,2]. The PCS either stores idle energy to a battery or extracts energy from a battery. Power semiconductor devices such as an insulated-gate bipolar transistor (IGBT) are the key component in the design of the PCS and are used to match the electrical characteristics between end users and grid systems depending on renewable energy sources. While the electrical characteristics of an electric power are converted by power semiconductor devices, the air- or water-cooled heat sink is used to dissipate heat by electric losses such as switching and conducting losses. In order to use semiconductor devices securely, a number of researchers evaluated and improved the cooling performance of the heat sink at the component level using numerical and
⁎ Corresponding author. E-mail address: [email protected] (M.-D. Oh).
experimental methods [3–10]. It is also important to predict the temperature of the semiconductor device with different cooling types and operating conditions at the thermal design stage. Some studies predicted the junction temperature, which is the highest operating temperature of the actual semiconductor device in the electronic device and is a key factor for assessing the suitability of the cooling design, in order to check the thermal status of the power semiconductor device at the component level under various operating conditions using the computational fluid dynamics (CFD) technique and the thermal network model [11–17]. Most of the previous studies have evaluated the cooling performance at the component level but have never been carried out at the system level such as with a PCS. When power semiconductor devices are integrated into a system such as the PCS, problems such as the reduction of the flow rate of a cooling medium can occur. With regards to the air-cooled type, the common problem involves the reduction of the airflow rate in the heat sink block by the system effect. As shown in Fig. 2, the operating point of a fan is shifted from (a) to (b) by the pressure rise due to the resistance elements in the system. Even if the design criterion of the junction temperature of a power semiconductor device is satisfied at the component level, the actual performance of a power semiconductor device in the system level can fail due to the reduction of the airflow rate by the system effect. Therefore, it is very important to predict the airflow rate of the heat sink in consideration of the system effect.
http://dx.doi.org/10.1016/j.microrel.2016.01.018 0026-2714/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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2. Description of physical models 2.1. Power conversion system
Fig. 1. Configuration of the PCS with an energy storage system connecting the components.
In some studies [18–19], the numerical simulation of the system level was conducted with a small number of meshes because the system was comprised of a small number of components and simple geometry. In contrast, P.R. Parida et al. [11] had to deal with elements of about 4.7 million to simulate the flow field of only a single component model. For this case, it will require enormous meshes to analyze the cooling performance of the system which contains several components. As a result, the use of high performance workstations and heavy computation time are required to simulate the system with all the components and the availability of the simulation model is reduced. This paper addresses the numerical simulation model composed of the reasonable minimum number of meshes to predict the airflow rate of the heat sink block in consideration of the system effect and to increase the availability of the numerical simulation. The technique of the porous media model was adopted on the simulation model of the system level to reduce the number of meshes that are heavily concentrated in the IGBT stack, which includes the IGBT/diode chips, heat sink blocks, and air between fins of the heat sink block. Now, one is able to calculate the airflow rate of the heat sink block in consideration of the effect by the interior structures in the PCS. The T–Q characteristic curve was proposed to predict the junction temperature of the IGBT module and the hot spot temperature on the heat sink block using the numerical model of the previous study [20]. Experimental works were conducted to evaluate the accuracy of the numerical simulation model and were performed under various operating conditions for two different PCSs using some electrical devices.
Fig. 2. Shift of the operating point of a fan due to the resistance changes by the system effect.
The PCS investigated in this study is shown in Fig. 3 and is divided into IGBT stack, reactor, and AC grid connection parts. The IGBT stack part, which consists of several IGBT modules, air-cooled heat sinks, fans, and electric components, converts a DC power source such as a battery to an AC power source. The reactor part plays a role to remove a harmonic frequency of an AC power source rectified by IGBT modules. The AC grid connection part performs a function of connection between an AC power source and grid power system. Most of the outdoor-type PCSs have the structure that outdoor air, which contains foreign materials such as dust, fallen leaves, and small insects that have flown into the PCS through louvers on the upper side. The air heated by IGBT modules, reactors, and other electric components is exhausted through the bottom and front side louvers. Among some PCSs, the exit duct of the IGBT stack is connected to the region of the reactor part in order to increase the airflow rate toward the reactors. While the indoor-type PCS has the structure that the air induced from the front and bottom side louvers and is exhausted to the top side louvers, as the risk to inflowing foreign materials is relatively lower than the outdoor-type PCS. Even if the PCS is in the indoors, the filters are installed in the front and bottom side louvers to prevent the inflowing of indoor foreign materials. In common with both the outdoor and indoor-type PCS, the fans for the IGBT modules are directly attached on the heat sink block to improve the cooling performance and other fans for the reactors are mounted on the top of the cabinet to avoid interference with the structure such as bus bars, damping resistors, and so on.
Fig. 3. Three-dimensional physical models and airflow passage of the outdoor-type PCS (isoview).
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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2.2. IGBT module A power module for the PCS is chosen to convert the characteristics of the power source. The selected device is Infineon Corporation PrimePACK™3 module and NTC, rated for 1.7 kV and 1.0 kA [21]. The size is 89 mm (width) × 38 mm (height) × 250 mm (length). Fig. 4 shows the cross-sectional view of the IGBT module and the heat sink block. The IGBT module package consists of the aluminum oxide (Al2O3) ceramic substrate soldered on a copper (Cu) base plate, IGBT and diode chips lying on the ceramic substrate, and a silicone gel covering the substrate and IGBT/diode chips. All components of the IGBT module are wrapped up with a plastic cover. IGBT/diode chips repeat the ON–OFF operations to convert DC to AC and generate 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 temperature of the thermal switch or negative temperature coefficient (NTC) thermistor exceeds the setting temperature, 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 junction temperature is set at 135 °C in consideration of power cycling, temperature measurement errors, instantaneous peak current, and unexpected environmental conditions. 2.3. Air-cooled heat sink Every power semiconductor device uses the heat sink block to dissipate heat generated by electric losses, whether in a chip or in a module package. The task of the heat sink block is to transfer the thermal energy of the IGBT module to a cooling medium such as air, water, and refrigerant. In addition, the heat sink block plays a role as a base of a power semiconductor device. In this study, the heat sink of the air-cooled type was used to remove heat by electric losses of the IGBT/diode chips. The size of heat sink block is 280 mm (width) × 115 mm (height) × 400 mm (length) and the material is aluminum alloy Al 6063. The heat sink block consists of two parts: one base plate and a number of fins as shown in Fig. 4. In a previous model, one fin was inserted into one groove of the base frame and then the dissipation area of heat became relatively narrow. In order to increase the dissipation area of the heat sink block, the double layer fin was mounted on the base plate [20]. The heat sink block was attached to the base plate of the IGBT module and the thermal interface material (TIM), Mementive Performance Material Inc. YG6111 [22],
3
was applied on between the IGBT module and the heat sink block to minimize the thermal contact resistance. The thermal conductivity of the TIM was 0.84 Wm−1 K−1 and its thickness was controlled at approximately 75 μm. With its double inlet centrifugal fan with forward curved impellers, Ventas Ventilatorer GD 133-2J [23], was used to blow the air into the heat sink block. One can observe that the maximum airflow rate of the fan is 958 m3 hr−1 and the shutoff pressure is 606.1 Pa on the fan performance curve in Fig. 2. Fig. 2 also shows that the acceptable maximum airflow rate is about 811.3 m3 hr−1 (a), when the heat sink block used in this study is only attached to the fan. 3. Numerical simulation 3.1. Numerical simulation procedure The junction temperature of the IGBT module is the most important criterion at the thermal design stage of the PCS. The most influential factor affecting the junction temperature is the airflow rate of the heat sink block. In order to predict the junction temperature, the airflow rate of the heat sink block is to be calculated accurately. Even if the geometry of the PCS is complex as shown in Fig. 3, the simulation model of the PCS of the system level must be constructed to predict the accurate airflow rate including flow resistances by the interior structures such as louvers, filters, and electric components. For this reason, Lee et al. [18] and Breier et al. [19] constituted the simulation model of the system level and predicted the hot spot temperature of semiconductors. For their cases, the numerical meshes were heavily concentrated in semiconductors, and then it became necessary to use the high performance workstations and a lot of computation time was required for the numerical work. The dimension of the PCS is over the order of 103 mm, whereas the distance between fins of the heat sink block the thickness of the fin are the order of 100 mm. From the system level perspective, the appearance of the fins of the heat sink block is similarly seen to the porous material such as a filter. Also, as air is passing the fins of the heat sink block, the non-uniform flow is uniformly aligned and the pressure drop is taken place by the friction resistance on the surface of the fin. These flow phenomena appeared in the porous material. In terms of the appearance and the flow phenomenon, the heat sink block at the system level can be considered as porous material. Thus the porous media model was adopted in this study to calculate the pressure drop in the heat sink block inside the PCS without the configuration of the complex mesh. The porous media model has been used to calculate the effect of flow resistances by X.L. Ouyang et al. [24]. He modeled the cylindrical
Fig. 4. Cross-sectional view of the packaging associated with IGBT and diode chips of Infineon PrimePACK™3 module and NTC.
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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fin as the porous media and compared the cooling performance according to the different modeling methods. As a result, the modeling of the heat sink as the porous media seems reasonable to simulate the micro fin structure in the PCS. In the previous design method [20] as shown in Fig. 5, in Step 1 (the black dotted line), the simulation of the IGBT stack model was conducted at the component level. Here, the P–Q curve was adopted as the boundary condition of the fan. As the flow field of the IGBT stack model is calculated, the operating point of the fan can be determined in the P–Q curve by the iterative calculation, and then the junction temperature of the IGBT module is calculated through the energy field. Here, one should know that the operating point of the fan in the IGBT stack cannot match with the real operating point when the IGBT stack is integrated into the PCS of the system level. As shown in Fig. 5, the proposed design method (the red solid line) integrating the system is quite different from the previous design method when only taking the IGBT stack into consideration. In the proposed method, the boundary condition of the fan is considered as the constant inlet velocity depending on the airflow rate. The flow and energy field of the IGBT stack model of Step 1 are sequentially calculated with various airflow rate conditions in Step 2 and Step 3, respectively, as the forced convection is dominant in the computational domain. The characteristic curve of the junction and hot spot temperature can be plotted against the airflow rate as shown in Step 4. If one can construct this characteristic curve, it is convenient to predict the required airflow rate of the heat sink block with different power conditions and the design temperature according to the change of the capacity of the fan or the airflow passage structure in the PCS.
In the PCS model of Step 5, most of the electric components are modeled in to solids, and the geometry of the fan is simplified with the P–Q curve as the boundary condition. The fins of the heat sink block and air between the fins are modeled as the porous media of Fig. 4. The heat sink blocks and the filters are assigned to the fluid region in order to replace the porous media. Thereby, the problem with a great number of meshes, which are concentrated in the IGBT stack, can be relaxed without the need for high performance workstations and a lot of computation time. The porous model condition of the heat sink block with micro fins requires the permeability and the inertial resistance factor. These conditions can be extracted from the resistance curve related to the airflow rate and the pressure drop in Step 2. The flow field of the PCS model is obtained using the boundary and model conditions and the airflow rate of the heat sink block is calculated in Step 6. Finally, the junction and hot spot temperature can be predicted as the design temperature at the intersection of the curve of Step 4 by the airflow rate obtained from Step 6.
3.2. Grid systems Due to the complex geometry of the PCS, the computational domains of the system level were meshed with unstructured non-uniform tetrahedral grids as shown in Fig. 6, which were generated by the commercial code ANSYS ICEM CFD. Initial mesh of the described model used a size of the order of 2 mm. Fine meshes were generated around all solid components and normal to all boundary surfaces. At least 40.8 million cells were required in the computational domain to model the
Fig. 5. Comparison of previous and proposed design methods for the PCS model and IGBT stack model. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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Fig. 6. Interior structure of the meshed domain with fine meshing at all the solid components (rear side).
specific interior structures such as the IGBT stacks, reactors, and electric components being used under these conditions. To check the grid dependency of the model, more refined meshes was tested with a mesh size of the order of 1.5 mm and then around 78.3 million cells were reached for the entire computation domain. The numerical results obtained by two mesh systems, viz., 40.8 and 78.3 million cells, were comparable within a 0.2% margin of error in the airflow rate of the heat sink block and the testing was accepted to satisfy the grid independent criteria with a 1% margin of error. As a result, the final mesh system with 40.8 million cells was adopted for the numerical simulation model of the system level. 3.3. Governing equations The flow phenomenon inside the PCS was predicted as the threedimensional complex turbulent flow. So, the steady Navier–Stokes, energy and continuity equation were solved together with realizable k-ε turbulence model in Eq. (1) [25] to simulate the flow and energy fields using the commercial CFD solver ANSYS Fluent, where the buoyancy and radiation effects were neglected. The region where y+ was below 15 was occurred around some solid surfaces and then the scalable wall function [25] was adopted to avoid the deterioration of the standard wall function under grid refinement. Regarding the scalable wall function, the purpose is to force the usage of the log low in conjunction with the standard wall functions approach. The pressure-based segregated solver was used as a solution algorithm where the governing equations were solved sequentially. The pressure–velocity coupling term was obtained by the SIMPLE (Semi-Implicit Method for PressureLinked Equations) algorithm [25]. No-slip boundary conditions were used on the solid surfaces. ∂ ∂Φk ρui Φk −Γ k ¼ SΦk ∂xi ∂xi
ð1Þ
where, ρ is the air density (kgm−3), ΓΦ is the diffusion coefficient, ui is the air velocity vectors (ms−1), SΦ,k is the source term of the general flow and heat generation, Φ is the velocity u, turbulence kinetic energy k (m2 s−2), turbulence kinetic energy dissipation rate ε (m2 s−3), and temperature T (K). The porous media model incorporates an empirically determined flow resistance in a region of the model defined as “porous” [25]. Porous media is modeled by the addition of a momentum source term to the standard fluid flow equation of Eq. (1). The source term is composed
of two parts: a viscous loss term and an inertial loss term. This momentum sink contributes to the pressure gradient in the porous cell, creating a pressure drop that is proportional to the fluid velocity in the cell. To recover the case of simple homogeneous porous media μ 1 Si ¼ − vi þ C 2 ρvjvj α 2
ð2Þ
where, α is the permeability and C2 is the inertial resistance factor. Simulation data available in the form of pressure drop against velocity through the porous media can be extrapolated to determine the coefficients for the porous media. The pressure drop, Δp (Pa), against velocity can be plotted to create a trend line through several points yielding the in Eq. (3) from Step 2 in Fig. 5. Δp ¼ av2 þ bv
ð3Þ
where, a and b are coefficients of the quadratic function. A simplified version of the momentum equation, relating the pressure drop to the source term, can be expressed as Δp ¼ −Si Δn
ð4Þ
where, Δn is the porous media thickness (m). From Eqs. (2) and (4), the coefficients of C2 and 1/α are 1.932 and 1,967,489, respectively, when air temperature is 50 °C. 4. Experiment 4.1. Experimental system In this study, the experimental devices were constructed in consideration of the operating condition of the PCS to evaluate the numerical simulation model for the local temperature on the heat sink block of the IGBT stack. As the voltage drop occurred in a real battery during operation, a battery of the real system as shown in Fig. 1 was substituted by the power source simulation device in the experimental system to supply the setting DC power for constant operating conditions as shown in Fig. 7. DC voltage of 1050 VDC to 750 VDC is supplied to the PCS through the power source simulation device and is converted to an AC power source of 440 VAC by the PCS. Here, the voltage of 1050 VDC is the fully charged condition of the battery and the voltage of 750 VDC is the lower limit of the battery operation. AC power of
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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C.-W. Han et al. / Microelectronics Reliability xxx (2016) xxx–xxx
Fig. 7. Configuration of the experimental system with the power source simulation device.
380 VDC is supplied to the experimental system and the converter AC power of 380 VDC is produced from the experimental system. Fig. 7 shows the configuration of the experimental system which consists of the power source simulation device, PCS, and power transformer. As the PCS used in this study is the component for converting DC power, DC power must be supplied to the PCS. So, we constructed a device such as a real battery that can convert AC power to DC power. The experimental device should be able to control the quantities of electric losses of the IGBT module in order to simulate various operating conditions. To fulfill the characteristics of the power source, the power source simulation device was composed of the induction voltage regulator (IVR), parallel power transformers with different winding connection methods, and the rectifier. The experimental device is capable of supplying a constant DC power to the PCS during one set of experiments with constant electric loss to measure the temperature on the heat sink block of the IGBT stack. 4.2. Temperature measurements The measurement of the junction temperature of IGBT/diode chips in the PCS is required to evaluate the numerical simulation results. With regard to the direct measurement method, inserting thermocouples to measure the junction temperature of IGBT/diode chips has a risk of short-circuit when the contact between the temperature sensor and IGBT/diode chip occurs. In the experiment of this study, the indirect measurement method then was selected to obtain the junction temperature in the IGBT module. First, the surface temperature of the heat sink block, which is called the hot spot temperature, THot spot, in the following discussion, was measured and then the junction temperature of the IGBT module, TJunc, 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 junction-to-case thermal resistance (K/kW), Rth,chip, from Eq. (5) [21]. T Junc ¼ T Hot spot þ Rth;chip Q Loss
4.3. Experimental conditions The airflow rate of the heat sink block inside the PCS is mainly determined by the type and the internal layout of the PCS. For this reason, the experiments were performed on both the outdoor and indoor-type PCS. Also, the damper condition of the PCS was considered as the experimental parameter whether the damper is open or not. The damper was used to control the airflow rate of the PCS. Damper closed is the operating condition of the rated airflow rate, while damper open is the operating condition of the increased airflow rate. Accuracy and applicability of the numerical simulation model of the system level were investigated by the experiments for different operating conditions of the PCS. Prior to the main experiment, in order to evaluate the reproducibility of the measurement, the temperature measurement was repeatedly carried out for five times under Case 1–3 in Table 1. Repeated experiments were performed to assess the reproducibility of the measurement method. From the result of five times experiments, the average hot spot temperature on the heat sink block was 111.6 °C and its deviation was 1.1 °C. The measurement method was enough to reproduce the experimental results because the temperature deviation was below 1% level of the average temperature. In order to verify the temperature prediction by the numerical model at the system level, several experiments were conducted under various load conditions using the measurement method previously verified. The electric losses listed in Table 1 were generated in one IGBT module and were separated to the IGBT and the diode losses. The damper condition is the parameter varying the airflow rate of the heat sink block, and the electric condition is the parameter affecting the loss of
ð5Þ
where, QLoss is the electric loss of the IGBT or diode chip (kW). Thermal resistance is basically given by the catalog of the IGBT module. The junction-to-case thermal resistances of the IGBT and diode chip used in the experiment were 33 K/kW and 66 K/kW, respectively [21]. The measurement point should be chosen according to the spot of the maximum temperature on the heat sink block, and then this point was chosen through the numerical results of the IGBT stack model as shown in Fig. 8 [20]. The K-type thermocouple was used as the temperature sensor and twelve calibrated thermocouples were attached at the measurement points between the base of the heat sink block and the IGBT module as shown in Fig. 9. Thermocouples were connected to the data acquisition (DAQ) system and measured data was stored every one second.
Fig. 8. Temperature rise distribution on the heat sink block obtained through the results of the previous study.
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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Fig. 9. Measurement point of the heat sink block with the IGBT module (a) front view and (b) side view.
the IGBT module. Ambient air temperature of the PCS was assumed to be 50 °C in order to take the extreme environment condition into consideration. Detailed operating conditions are listed in Table 1. 5. Results and discussion The simulation results were compared with the experimental results to verify the accuracy of the numerical model. Two types of the PCS with different operating conditions were considered as described in Table 1. Fig. 10 displays comparison results of the hot spot temperatures of the experiment and the numerical simulation. The experimental results were obtained from taking an average of the hot spot temperatures at twelve measurement points. Fig. 10 shows that the differences are within a 3.5 and 4.6% margin of error, between all the experiments and the simulation calculations of the outdoor and indoortype PCS. Also tendency of the temperature corresponds to the experimental results with different operating conditions. The temperature differences between experimental and numerical results were from 0.7 °C to 7.2 °C. The magnitude of this error is of a sufficient enough level to accept in order to lay out the structure of the PCS and determine the operating range of the IGBT module at the design stage. Fig. 10 demonstrates that all the simulation temperatures are lower than the experimental results. This consistency indicates the consistent causes that were involved in the experiment or in the simulation such as the thickness of the TIM and the flow resistances of the filter. The thickness of the TIM recommended by the manufacturer of the IGBT module was considered in the calculation [23]. It was very difficult to measure the thickness of the TIM in a prototype even though there was the thickness difference of the TIM for each IGBT stack. If the thickness of the TIM is thicker, the thermal resistance is increased and then the hot spot
temperature on the heat sink block is raised. As the thickness of the TIM is controlled as the same design condition during the assembly process and the real characteristics of flow resistances of the filter from the measured data are considered, the errors are expected to be reduced. The proposed design model estimated the experimental results in less than the average 4.0% margin of error with respect to the averaged hot spot temperature. As it is used for the proposed design method and simulation model for the thermal design, the junction temperature of the IGBT module can be predicted and confirmed for various operating conditions. Additionally, it can be used not only for selecting the fan and the filter but also for designing the airflow passage structure of the PCS. The numerical simulation model at the system level was used to predict the airflow rate of the heat sink block. The operating conditions of the PCS are described in Table 1 and the simulation results of the airflow rate passing the heat sink block are summarized in Table 2. The averaged airflow rate in the table is the value that is taken as an average for all heat sink blocks within the PCS. The deviation of the airflow rate represents the standard deviation to the averaged value of the airflow rate for each heat sink block and the required airflow rate means the minimum airflow rate satisfying the design criterion of the junction temperature of the IGBT module. If the averaged airflow rate becomes larger than the required airflow rate, we can say that the junction temperature satisfies the design criterion. For Case 1–1 and 1–2, the airflow rate passing the heat sink block was different as 482.7 m3 hr−1 and 718.0 m3 hr−1, respectively, due to the different damper conditions, but the required airflow rate was the same as 115.4 m3 hr−1 because of the same electric condition. On the contrary, the airflow rate of the heat sink block for Case 1–2 and 1–3 was equal to 718.0 m3 hr−1 due to the same operating condition
Table 1 Operating conditions of the outdoor and indoor-type PCS. Case
1–1 1–2 1–3 2–1 2–2
Type
Outdoor Outdoor Outdoor Indoor Indoor
Damper Condition
Closed Open Open Closed Closed
Electric condition
Electric Losses
Ambient temperature
Voltage
Current
IGBT
Diode
950 VDC 950 VDC 950 VDC 1050 VDC 1050 VDC
1300 AAC 1300 AAC 1600 AAC 1400 AAC 1500 AAC
845 W 835 W 1101 W 989 W 1115 W
295 W 287 W 368 W 370 W 402 W
50 °C 50 °C 50 °C 50 °C 50 °C
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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Fig. 10. Comparison of the hot spot temperatures of simulations and experiment results with different operating conditions (a) the outdoor-type PCS and (b) the indoor-type PCS.
of the damper, but the required airflow rate was increased from 115.4 m3 hr−1 to 382.5 m3 hr−1 because the electric losses of the IGBT module were increased by higher electric current from 1300A to 1600A. As the output current was raised for Case 2–1 and 2–2 similar to Case 1–2 and 1–3, the required airflow rate was also increased from 270.3 m3 hr−1 to 380.2 m3 hr−1. The averaged airflow rate of the heat sink block of the indoor-type PCS was increased by 154.9 m3 hr−1 more than of the outdoor-type PCS under the closed damper condition. If the airflow rate of the outdoor-type PCS can be increased, than the current model the IGBT module can be operated in more severe operating conditions. This suggests that the designer of the PCS should make an effort for the improvement of the airflow passage structure in the outdoor-type PCS. Even though the same fan is attached to the inlet of the heat sink block with the same capacity, the airflow rates of the heat sink block can be varied by the differences of the ventilation area of the louvers, the layout of the electric components, and the thickness of the filter. In Fig. 11, when the airflow rate is increased up to 637.6 m3 hr−1 in Case 1–1, the junction temperature is predicted as 129.3 °C for 950 VDC and 1600AAC, which satisfies the design criterion. If the damper is closed in Case 1–3, the airflow rate is reduced to 482.7 m3 hr−1 and then there is little design margin of the junction temperature. On the other hand, the indoor-type PCS is difficult to expect a considerably large effect by improving the airflow rate and reducing the junction temperature because the gap is relatively small between the averaged and the acceptable maximum airflow rate. In all operating conditions, the deviation of the airflow rate appeared to be small enough to ignore, thus in the following discussion, the average airflow rates were used to calculate the junction temperature of the IGBT module and the hot spot temperatures of the heat sink block. The T–Q characteristic curves proposed in this study were numerically obtained for each case. Case 2–2 with the greatest loss condition was selected for discussion, because the different operating conditions
revealed a similar tendency. Fig. 12 represents the T–Q characteristic curve for Case 2–2. This characteristic curve indicates the relationship between the junction and hot spot temperatures with respect to the airflow rate passing the heat sink block. The circular symbols represent the junction temperature of the IGBT chip and the triangle symbols stand for the hot spot temperature on the heat sink block. The black symbols are the calculated temperatures by the previous design method, the red symbols are the calculated temperatures by the proposed design method, and the blue symbols are the design criteria airflow, respectively, in the T–Q characteristic curve. The blue dash and the dot line indicate the upper limit of the junction and hot spot temperature, respectively. Fig. 12 clearly demonstrates the gradual reduction effect of the temperature of the IGBT module as the airflow rate is increased. Minimum airflow rate to satisfy the design criterion of the junction temperature can be calculated by using the T–Q characteristic curve. While the junction and hot spot temperatures are able to be predicted under the condition of the IGBT modules integrated into the PCS. In Fig. 12, the junction temperature of the IGBT module was calculated from the intersection of the characteristic curve and the vertical line of the airflow rate with 637.6 m3 hr−1 for Case 2–2. The hot spot temperature on the heat sink block was also calculated from the similar method with the junction temperature. The calculated junction and hot spot
Table 2 Numerical simulation results of the airflow rates of the heat sink block and the temperature of the IGBT module with different operating conditions of the outdoor and indoortype PCS. Case
1–1 1–2 1–3 2–1 2–2
Airflow rate [m3 hr−1]
Temperature [°C]
Averaged
Deviation
Required
Hot spot
Junction
482.7 718.0 718.0 637.6 637.6
3.6 1.6 1.6 3.9 3.9
152.9 152.9 470.0 329.5 465.2
99.8 95.2 109.0 105.3 110.6
114.1 109.3 127.6 122.1 129.0
Fig. 11. Numerical simulation results of the junction temperature of the IGBT module with different operating conditions of the outdoor and indoor-type PCS.
Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018
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benefits resulting in the reduction in the mesh generation and the computation time. Also, this simulation model improves the flexibility of predicting the airflow rate by changing the airflow passage structure in the PCS or the capacity of the fan. Lastly, one can use the T–Q characteristic curve proposed in this study for the thermal design by the prediction of the junction and the hot spot temperature with different airflow rates and electric operating conditions. Acknowledgments This work was supported by the 2015 Research Fund of the University of Seoul, and the authors would thank the Power & Industrial Systems R&D Center of Hyosung Corporation for the realization of the prototype and for the thermal test. References Fig. 12. Numerical simulation results of the junction temperature of the IGBT module and the hot spot temperature on the heat sink block with different airflow rates for Case 2–2. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
temperatures were 129.0 °C and 110.6 °C, respectively (red dots). Regarding the airflow rate using the previous design method, the calculated junction temperature is 125.9 °C and the hot spot is 107.1 °C, respectively (black dots). The difference of the airflow rate is 137.7 m3 hr−1 and the junction temperature is 3.1 °C between the previous and proposed design methods. One can observe that the airflow rate is less sensitive to the control temperatures of IGBT module. Figs. 11 and 12 also indicate that the airflow rates can be reduced further with less safety margin from the current operation through the proposed design method. The other results are represented in Table 2. 6. Conclusions The junction temperature of the IGBT module, which belongs to power semiconductor device, directly impacts on the system performance of the PCS, and the accurate prediction of the airflow rate passing the heat sink block of the IGBT module is also very important in thermal design stage. In this paper, the thermal design method of the PCS was proposed to predict the reduced airflow rate when the power semiconductor devices are integrated into the PCS and the suitability of the predictions in the design method was evaluated by the experimental works. The prediction method of the junction temperature of the IGBT module and the airflow rate of the heat sink block was developed using the model of the system and component level. The porous media model was adopted in the numerical simulation model of the heat sink block with fins and filled air in order to remove the heavily concentrated mesh problems in the block. The simulation model of the PCS can predict the accurate airflow rate including the system effect of flow resistances by the interior structures such as louvers, filter, and electric components. Also, the T–Q characteristic curve was proposed to predict the junction temperature of the IGBT module and the hot spot temperature on the heat sink block. The proposed simulation model was compared to the experimental value for the hot spot temperature on the heat sink block and the differences were within the average 4.0% margin of error in the comparison. The magnitude of this error range is at an acceptable enough level in order to lay out the structure of the PCS and to determine the operating range of the IGBT module at the design stage. As a result, the simulation model was developed to predict the junction temperature of the IGBT module at the thermal design stage with
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Please cite this article as: C.-W. Han, et al., Thermo-fluid simulation for the thermal design of the IGBT module in the power conversion system, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.01.018