EVS 28 KINTEX , Korea , May 3-6 , 2015 EV Motor Controller Target Cooling by Using Micro Thermoelectric Cooler

The motor controller is an important component in electric vehicle. Controller always generates heat and needs cooling as electric vehicle driving. Micro thermoelectric cooler (μTEC) with many advantages becomes a potential cooling device for controller recently. This study uses the commercial software ANSYS workbench 14.5 to investigate the influence of the activated number and located position of μTEC on controller chip cooling. The following simulation results are obtained: (1) μTEC activating may assist cooling between 0 second and 60 seconds. The cooling capability enhancement is pronounced as the number of activated μTEC increased. (2) μTEC activating may not assist cooling between 60 seconds and 156 seconds because the energy on the hot side of μTEC is unable to be removed effectively. “All μTEC open” method gives the poorest cooling performance in this region.


Introduction
The motor controller is an important component in electric vehicle (EV).Its developing trend is increasing output power and decreasing volume.In general, the controller contains many power electric devices such as IGBT, Diode, and Mosfet as well.During driving process, the power electric devices switch frequently and generate great amount of heat.It not only produces uneven high temperature distribution in power electric devices but also forms local hot spot inside the chip of the controller.As a result, the performance of the controller degrades significantly.
Thermoelectric cooler (TEC) consists of many semiconductor elements.It generates cold face and hot face on both side of TEC as powered by direct current.The cold side of TEC may be used to cool high temperature power electric devices.Micro thermoelectric cooler (µTEC) with the advantages of no moving parts, without noise, long operating life, small size, no refrigerants, and precise temperature control [1,2], it becomes a potential cooling device recently.At present, the researches of TEC on EV application are mostly focusing on the air conditioning system and the heating devices [2].Among those applications, µTEC may be fit into compact size of controller due to its dimension is smaller than that of power element chip.The relevant literatures are described as follows: Sullivan et al. [3] used the thin-film thermoelectric cooler (thin film TEC) to carry out the heat dissipation for the chip set of microprocessor through 3x3 matrix arrangement.It was found when 9 Thin film TEC were activated simultaneously, the maximum temperature reduction of the chip would be 11 ℃ within 0.03 seconds, but the temperature rise rapidly thereafter.Chowdhury et al. [4] studied the cooling method for chip local hot spot, and the result revealed that the temperature reduction was 7.6 ℃ by passive cooling.If the thin film TEC was also used, the maximum temperature reduction would be 14.9 ℃ under the optimal input current of 3 A. Wang et al. [5] used the thin film TEC to do parametric study of IGBT cooling.Several forced convective heat transfer coefficient (h=10,000, 20,000, and 30,000 W/[m 2 K]) are used.The comparison results between cooling with TEC and without TEC are obtained.Not only the maximum temperature could be reduced, but also the maximum temperature difference of all devices was reduced to approaching zero ( 06 .0 max = ∆T ℃).Zhang et al. [6] used a three-dimensional thermoelectric model to carry out the simulation and optimization analysis for the hetero-structure thin film TEC -Si/SiGe.The research results revealed that such µTEC could reduce the temperature of chip by 4.5 ℃ at room temperature and 7 ℃ at ambient temperature of 100 ℃.Yang et al. [7] added a mini-contact structure in thin film TEC heat dissipation module.The research results revealed that such structure could effectively reduce the heat transferred from the surrounding of hot spot to µTEC, so as to reduce the temperature of hot spot significantly.
From the above mentioned literatures, it is known that the µTEC is mostly applied to processor chip cooling nowadays.Its characteristics are also suitable for using in the heat dissipation problem of motor controller.But the application in vehicle has its uniqueness.During vehicle driving, the controller needs to deal with different driving patterns and perform different way of control, especially in the case of high power demand, such as climbing and acceleration.The controller chips will generate great amount of heat under these conditions.At this moment, the µTEC can remove chip energy and reduce temperature quickly.In this study, the commercial finite element software ANSYS workbench 14.5 is used to analyze the cooling performance of motor controller assembly with µTEC.The effect of the activated number and located position of µTEC on controller chip cooling will be discussed.

Thermoelectric
Cooling Principle and Mathematical Model

Thermoelectric Cooling Principle
The thermoelectric cooling principle is described as follows [8]: If without external magnetic field exist, there will be five effects in the thermoelectric cooling process, those are: Seebeck effect, Peltier effect, Thomson effect, Joule effect, and Fourier effect.As shown in Figure 1, when a direct current is applied to one or more pairs of n-p semiconductor thermoelectric device, the electrons and holes at one side of thermoelectric couple moves from p-type material to n-type material by jumping from low energy level to higher energy level.In this process, the thermal energy is absorbed, so the temperature of cold side is reduced.At the other side of thermoelectric device, the thermal energy is released.This is the Peltier effect.When the temperature difference is appeared on both sides of p-n pairs, a Seebeck voltage will be generated.When a current passes through a thermoelectric device with temperature difference on both sides, an energy exchange will be occurred between the device and the environment due to the Thomson effect.This effect is proportional to the current and temperature gradient.When there is a temperature difference between the hot side and cold side of thermoelectric device, a heat conduction effect will be generated, which is the Fourier effect.When a current passes through a thermoelectric couple, an irreversible Joule heat will also be generated.Considering an ideal thermoelectric cooler, when the heat moves from the cold side to the hot side, the heat at the cold side and the hot side can be represented as: Where, Q h and Q c are the energy (W) at the hot side and the cold side, respectively, n α and p α are the Seebeck coefficient (V/K) of n-type and ptype thermoelectric material, respectively, k is the thermal conductivity (W/[mK]) of thermoelectric material, A is the area (m 2 ) of thermoelectric material, L is the height (m) of thermoelectric material, T h and T c are the temperatures (K) at the hot side and the cold side, respectively, I is the input current (A) of thermoelectric cooler, and e ρ is the resistivity (Ωm).In the equations ( 1) and ( 2

=
(3) In equation ( 3), the parameters influencing ZT are the Seebeck coefficient, thermal conductivity, and resistivity.The coefficient of performance (COP) for TEC is defined as: Where, e P is the input power (W).

ANSYS Thermoelectric Couple Governing Equation
This study uses the commercial software ANSYS to carry out the simulation analysis with Thermal-Electric module on Workbench 14.5.In the thermoelectric couple governing equation used in that module, the energy equation is: Where, ρ is the density (kg/m 3 ), p C is the specific heat (J/[kgK]), T is the absolute temperature (K).Also, q r is the heat flux (W/m 2 ), which can be represented by the Seebeck coefficient and the thermal conductivity as , and q & is the heat generated per unit volume (W/m 3 ).The charge continuity equation can be represented as: Where, J r is the current intensity (A/m 2 ), which can be represented by When the steady-state analysis is carried out, equations ( 5) and ( 6) can be simplified as: Where, N is the number of thermoelectric element pairs.According to the material parameters provided by the manufacturer of thermoelectric device used in this study associated with equation ( 9) to equation (11), the coefficient of performance for µTEC can be obtained as shown in Table 1.This parameter will be used for device material setting in ANSYS software.Before conducting the heat transfer simulation, the coefficient of performance (COP) for µTEC should be confirmed first.At pre-processing, the hexahedron 20 nodes couple element (SOLID 226) is adopted to set up the mesh.Every part has at least four layers of mesh at thickness direction.Total mesh of the whole equivalent model is 2000 as shown in Figure 2. The corresponding COP of temperature difference at hot side temperature of 25 ℃ is shown in Figure 3, and the maximum value is 13.5.

Local Model Analysis of Controller Chip Set
The real model of motor controller and Mosfet chip local model are shown in Figure 4.The main purpose of this study is to understand the heat transfer behaviour between micro thermoelectric cooler and Mosfet chip set.In order to catch the characteristics of µTEC more easily and reduce the simulation time also, the original controller model with 24 heating chips is simplified into a local model with single heating chip (hereinafter called local model).The local model contains the following elements: Mosfet chip, equivalent printed circuit board (PCB eq ), aluminum base, and heat sink.A rectangular space in aluminum base right beneath the heating chip is arranged for install 9 µTEC.The dimension of each µTEC is 3.6 mm width, 3.6 mm length, and 0.06 mm height.The space between two µTEC is 0.5 mm.The computational mesh of the local model is designed according to the following method: hexahedral 20 nodes -SOLID 90 type mesh is used in heating chip, hexahedral 20 nodes couple element -SOLID 226 type mesh is used in µTEC, and tetrahedral 10 nodes hot element -SOLID 87 type mesh is used for other components.Total number of mesh is 68,115.The whole finite element model is shown in Figure 5, and the detailed parameters are shown in Table 2.This study adopts the transient analysis.The total analysis time is 156 seconds.In order to simulate the driving condition of the vehicle, the ambient temperature is keeping at 40 ℃, the convective heat transfer coefficient on the boundary is setting at 5 W/[m 2 K], the heating power of the chip is setting at 8.333 W, and the heating time of the chip is arranged between 1.1 seconds and 156 seconds.The maximum input current of µTEC is 0.5 A at 1.1 seconds from the beginning of the simulation time, then the current value drops gradually in linear way subsequently.When the time reaches 156 seconds, the current is the lowest value of 0.1 A. In order to investigate the optimal number of activating µTEC, eight different activating methods are designed as shown in Fig. 6.Those are: (1) 9 µTEC are all deactivated, indicated by "Open 0", (2) center µTEC activated and 8 other µTEC are all deactivated, indicated by "Open 1", (3) 4 µTEC near center one activated and 5 other µTEC are all deactivated, indicated by "Open 4 Inside", (4) 4 µTEC near outer corner activated and 5 other µTEC are all deactivated, indicated by "Open 4 Outside", (5) 5 µTEC near center and include that one activated and 4 other µTEC are all deactivated, indicated by "Open 5 Inside", ( 6) 5 µTEC at center and outer corner activated and 4 other µTEC are all deactivated, indicated by "Open 5 Outside", (7) 8 µTEC exclude center one activated and center one deactivated, indicated by "Open 8", and (8) 9 µTEC are all activated, indicated by "Open 9".The U, V, and W alternating output power with different arms arrangement in associate with 3D heat source and heat sink geometry in EV application may produce uneven or hot spot temperature distribution on controller.The function of the above eight activating methods on chip cooling serves as target cooling of EV controller.

Results and Discussions
The analysis is conducted in a transient condition and results are shown in Figure 7.The temperature distributions can be divided into two regions.First region is located between 0 second and 60 seconds, second region is located between 60 seconds and 156 seconds.In the first region, chip temperatures are maintained at constant value of 40 ℃ from 0 second to 1.1 seconds, those curves vary significantly from 1.1 seconds to 4.4037 seconds.Chip temperature on first activating method "Open 0" increases from 40 ℃ to 50.414 ℃ sharply, the temperature increases with low constant slope hereafter.Chip temperature on second activating method "Open 1" increases from 40 ℃ to 47.632 ℃ sharply, and then increases with a constant slope little higher than that of the curve "Open 0" hereafter.Chip temperature on forth activating method "Open 4 Outside" increases sharply first then decreases to 43.47 ℃, and then increases with a constant slope little higher than that of the curve "Open 1" hereafter.Chip temperature on third activating method "Open 4 Inside" increases sharply first then decreases to 42.149 ℃, and then increases with a constant slope little higher than that of the curve "Open 4 Outside" hereafter.Chip temperature on sixth activating method "Open 5 Outside" increases sharply first then decreases to 41.022 ℃, and then increases with a constant slope little higher than that of the curve "Open 4 Inside" hereafter.Chip temperature on fifth activating method "Open 5 Inside" increases sharply first then decreases to 39.721 ℃, and then increases with a constant slope little higher than that of the curve "Open 5 Outside" hereafter.Chip temperature on seventh activating method "Open 8" increases sharply first then decreases to 35.798 ℃, and then increases with a constant slope little higher than that of the curve "Open 5 Inside" hereafter.Chip temperature on eighth activating method "Open 9" increases sharply first then decreases to 33.652 ℃, and then increases with a constant slope little higher than that of the curve "Open 8" hereafter.Roughly speaking, the controller cooling performance may be divided into the following two conditions: First condition: µTEC may assist cooling on first region between 0 second and 60 seconds.The cooling capability enhancement is pronounced as the number of activated µTEC increased.In this region, the poorest cooling performance for "Open 0" method may be observed due to the highest temperature among eight methods.It is reasonable because there is no TEC assist cooling adopted in such design.On the other way, the best cooling performance for "Open 9" method may be observed due to the lowest temperature among eight methods.It is acceptable because all TEC are activated to assist cooling in such design.Second condition: µTEC may not assist cooling on second region between 60 seconds and 156 seconds.In this region, the poorest cooling performance for "Open 9" method may be observed due to the highest temperature among eight methods.On the other way, the best cooling performance for "Open 0" method may be observed due to the lowest temperature among eight methods.The reason why µTEC may not assist cooling in this region is because the energy on the hot side of µTEC is unable to be removed effectively.Such energy will accumulate on the heat sink and bring back to the controller chip to induce reheating phenomena and increase chip temperature as well when the activation time exceeds a certain range.In present study, the margin of the exceeding time is around 60 seconds.This poor heat dissipation effect is required to be improved.The first feasible method is to extend the effective thermoelectric cooling time of µTEC, and the second feasible method is to propose a fast way of heat removing method from hot side of µTEC.
The detail data are shown in Table 3.First column shows seven µTEC activating methods.
Second column shows the time of sharp temperature change ending.Third column shows chip temperature at the time shown in second column.Forth column shows chip temperature without activated µTEC.And fifth column shows temperature difference.It is noticed from the table that the temperature of the controller chip is keeping at maximum value of 50.414 when ℃ nine µTEC are all closed.In this case, the chip always maintain at maximum temperature without external cooling device assistant.As µTEC open and activates, the temperature of controller power element reduces effectively in short period of time.In the table, temperature difference is defined as the deviation between the temperature of the chip as µTEC is closed and the temperature of the chip as µTEC is open.When one µTEC is open as apply "Open 0" method, the external assisting cooling power is the least amount, and the minimum temperature difference is obtained.As the number of activating µTEC increases, the temperature difference increases too.
), three items at the right hand side of the equations are caused by the Peltier effect, Fourier effect, and Joule effect, respectively.The performance of TEC can be represented by the thermoelectric figure of merit ZT.
electric field (V/m), and D r is the intensity of electric flux (C/m 2 ).

Figure 1 :
Figure 1: Operation principle of thermoelectric cooler [8] 3 Finite Element Analysis 3.1 Equivalent Model of Micro Thermoelectric Cooler In the ANSYS software, a three-dimensional thermoelectric model is created first for the followup simulation analysis.Because of the dimensions of the parts in the model are different widely, the mesh quality always poor by using regular meshing technique.So the equivalent model of µTEC is created for the simulation.Its cooling parameters are derived from the energy balance equation.The equivalent Seebeck coefficient eq α , (V/K), equivalent thermal conductivity eq k , (W/[mK]), and equivalent resistivity eq ρ , (Ωm) can be obtained from the following equations:.α α N eq 2 =

Figure 2 :Figure 3 :
Figure 2: Finite element model of equivalent micro thermoelectric cooler

Figure 4 :Figure 6 :
Figure 4: Motor controller and local model Figure 7(a) shows the temperature distributions of the controller chip versus time under eight activating methods.Total simulation time is 156 seconds.It is noticed that eight temperature curves merge at a cross point near 40 seconds.Enlarge view of the results between 0 and 60 second is shown in Figure 7(b) for easily reading purpose.
For example, temperature difference equals 6.944 ℃ as four outside µTEC are all open, temperature difference equals 10.693 ℃ as five inside µTEC are all open, and temperature difference equals 16.762 ℃ as nine µTEC are all open.It is seen from the table that the application of µTEC can reduce the temperature of controller actually.According to the data in table 3, the position of the µTEC will influence the µTEC cooling capability obviously.For instance, compare the temperature difference of the following two cases: 「4 inside µTEC open 」 and 「 4 outside µTEC open 」 , temperature difference equals 8.265 ℃ for case「4 inside µTEC open」 and temperature difference equals 6.944 ℃ for case「4 outside µTEC open」.With the same amount of activating µTEC, temperature difference for case of µTEC located at outside position is 1.321 ℃ smaller than that of µTEC located at inside position.One reason why different position of µTEC reveals different temperature difference is because of the different heat transfer characteristics appears on different conduction path.The other reason is the setting of the chip power source is defined as the total power of the chip volume.The energy of the power element is coming from the center of the chip and then conducted to the exterior boundary of the element.When µTEC is located near the center area of the chip, it shows better cooling performance.The same result may be obtained for the following two cases: 「5 inside µTEC open」 and 「 5 outside µTEC open 」 .The optimum design of EV motor controller cooling by using µTEC may be obtained by conducting the following further analysis: (1) different size (include width, length, and height) of µTEC.(2) different number of µTEC.(3) different µTEC located position with respect to chip.

Figure 7 : 5 Conclusions
Transient analysis results of local model Table 3: Occur time and temperature of maximum temperature difference The application of µTEC on EV motor controller chip cooling has been analysed in this study by using a simplified local model of the controller.According to the simulation results, the following conclusions are obtained: (1) µTEC may assist cooling between 0 second and 60 seconds.The cooling capability enhancement is pronounced as the number of activated µTEC increased.Temperature difference between "0 µTEC open" and "1 µTEC open" is 2.782 ℃, and temperature difference between "0 µTEC open" and "All µTEC open" is 16.762 ℃. (2) µTEC may not assist cooling between 60 seconds and 156 seconds because the energy on the hot side of µTEC is unable to be removed effectively."All µTEC open" method gives the poorest cooling performance in this region.

Table 1 :
Parameters for equivalent model of micro thermoelectric cooler