A Simplified Thermal Resistance Network Model for ... - IEEE Xplore

A Simplified Thermal Resistance Network Model for High Power LED Street Lamp Xiaobing Luo 1, 2, Wei Xiong1, Sheng Liu2,3 * School of Energy and Power Engineering, Huazhong University of Science & Technology, Wuhan, 430074, China 2 Wuhan National Lab for Optoelectronics, Huazhong University of Science & Technology, Wuhan, 430074, China 3 Institutes of Microsystems, Huazhong University of Science & Technology, Wuhan, 430074, China *Corresponding author: Sheng Liu, Telephone: 86-13871251668, Fax number: 86-27-87557074, Email: [email protected]

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Abstract Light emitting diode (LED) street lamp heavily relies on successful thermal management, which strongly affects the optical extraction and the reliability/durability of the LED lamp. In this study, a thermal resistance network model was presented to estimate the maximum heat sink temperature of the street lamp, which could be utilized to further evaluate the thermal performance of the street lamp. Two high power LED street lamps, an 114 watts and an 80 watts LED street lamp were used to evaluate the present model. Their heat sink temperatures were calculated by the model, the results showed that the maximum heat sink temperature was about 61℃ at the environment temperature of 25℃ for the 114 watts LED street lamp. For the 80 watts LED street lamp, the maximum heat sink temperature was about 42.5℃ at the environment temperature of 11℃. To prove the model feasibility, experimental investigations on the 114 watts and 80 watts LED street lamp were conducted. The results demonstrated that the heat sink temperature of the 114 watts LED street lamp remained to be stable at about 60℃ after several hours’ lighting at the room temperature of 25℃. The heat sink temperature of the 80 watts LED street lamp remained to be stable at about 42℃ at the room temperature of 11℃. Comparing the results achieved by the thermal resistance model with the experimental results, it was found that the proposed thermal resistance model could be used for temperature estimation and thermal evaluation for the high power street lamp. Introduction Theoretically, light emitting diode (LED) has many distinctive advantages such as high efficiency, good reliability, long life, variable color and low power consumption. Recently, LED has begun to play an important role in many applications [1]. Typical applications include back lighting for cell phones and other LCD displays, interior and exterior automotive lighting including headlights, large signs and displays, signals and illumination. LED will soon be used in general lighting, which consumes about 15 percent of the total energy in all over the world. An expectation about high power LED is that it will be the dominant lighting technology by 2025 [2]. Should the goal come to fruition, then up to 40 giga watts per year could be saved in the USA alone. It is generally believed that the LED can be widely used for general lighting in USA. However, in China, with the push of the government for more energy saving, the LED may be used earlier than this time. In China, the estimation by Chinese authorities is that if LED dominates generate lighting

market in 2010, one third of the present power consumption will be saved, which will greatly ameliorate the energy crisis situation in China. One typical general lighting product of LED is LED street lamp, which is emerging in market, in particular in China. For modern LED street lamps, both optical extraction and thermal management are two critical factors for their high performance. In general, most of the electronic power of street lamp is converted into heat, which greatly reduces the chips’ luminosity. In addition, the high junction temperature of LED chips in the lamp will shift the peak wavelength, which will change the color of light. Narendran and Gu [3] have experimentally demonstrated that the life of LEDs decreases with the increase of the junction temperature in an exponential manner. Therefore, a low operation temperature is essential for LED chips in the LED street lamp. Since the market demands that LED street lamp have high power and small size, there is a contradiction between the power density and the operation temperature, especially when applications require LED street lamp to operate at high power to obtain the desired brightness. In terms of thermal management of LED street lamp, to the authors’ best knowledge, there have been no reports or published papers, partially due to the fact that general lighting including street lighting is believed to be a few years away, or due to the concern that this field is highly proprietary as the market for the street lamp is huge. Although there are no reports directly related to the LED streetlamp, there have been some references to introduce the thermal management of high power LED packaging. Wilcoxon and Cornelius [4] described the thermal management approach to a light engine and presented the results of their finite element modeling. The feasibility of the modeling was proven by the experimental data and was used to assess various design aspects of the light engine to understand their effects on the overall thermal resistance. Their results out of the finite element modeling indicated that the junction temperature of the LEDs in this light engine would be close to their maximum values under the application conditions of high environment temperature. Through the use of expensive materials such as diamond/aluminum composites, the LED temperatures could be significantly reduced below the values obtained in this testing. Kim et al. [5] investigated the performance of thermal management system for LED light source in a rear projection TV. Their results showed that decreasing thermal resistance between LEDs and substrate was the most effective way to dissipate heat and the applicable limit of thermal resistance existed for various heat-dissipating conditions of LEDs. They also suggested applying the heat transport system in red, green

2008 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP 2008) 978-1-4244-2740-6/08/$25.00 © 2008 IEEE

and blue LED light system to ensure the product quality. Liu’s group [6, 7, 8] studied a micro jet array cooling system for the thermal management of a high power LED lighting source. Experimental and numerical investigations on such an active cooling system were conducted. An infrared thermometer and thermocouples were used to conduct on-line temperature measurement and evaluate the cooling performance of the proposed system respectively. The experimental and numerical results demonstrated that the microjet-based cooling system has super cooling performance. Petroski [9] developed an LED-based spot module heat sink in a free convective cooling system. A cylindrical tube longitudinal fin (CTLF) heat sink was used to solve the orientation problem of LEDs. Chen et al. [10] presented a silicon-based thermoelectric (TE) for cooling of high power LEDs. The test results showed that their TE device could effectively reduce the operation temperature of high power LEDs. Acikalin et al. [11] used piezoelectric fans to cool LEDs. Their results showed that the fans could reduce the heat source temperature by as much as 37.4℃. Piezoelectric fans have been shown to be a viable solution to the thermal management of electronic components and LEDs. Treurniet and Lammens [12] presented a thermal design method of a multi-chip LED module that was able to handle an increasing thermal load up to 20 Watts. In addition, they proposed a compact model to estimate the junction temperature of the different dice at an arbitrary load. Arik and Weaver [13] carried out a numerical study to understand the chip temperature profile due to bump defects. Finite element techniques were utilized to evaluate the effects of localized hot spots at the active layer of chip. Tan, Liuxi et al. [14] studied the effects of various defects in terms of voids, cracks, delaminations on the thermal and optical performance of LEDs subjected to both powering and moisture loadings. For the high power LED street lamp, the present authors [15] already conducted a thermal analysis. A numerical model for an 80 watts LED street lamp was built, the comparison of the simulation with the experiment demonstrated that the numerical model was feasible. Although numerical model can effectively predict the temperature distribution, it is still not convinent for engineering applications in the early design stage. It is necessary to find some closed-form or semiempirical equations to estimate the temperature. In this research, a thermal resistance network model was proposed to estimate the maximum heat sink temperature of the street lamp. To prove the model feasibility, experimental studies on an 114 watts street lamp and an 80 watts LED street lamp were conducted. Through the comparison between the experimental results and the thermal resistance modeling results, it was found that the present thermal resistance model would be helpful for the temperature estimation on the high power LED street lamp. Typical Structure of High Power LED Street Lamp Figure 1 and Figure 2 are the schematic diagrams of 80 watts and 114 watts LED street lamps. They represent one typical structure of high power LED street lamps to make full use of design freedom of LEDs. The kind of lamp is mainly

composed of three parts: high power LED modules, a mechanical frame for both the heat dissipation and support of the LED modules, and four slim PCBs for the power input of LEDs. The lamp frame consists of aluminum base and fins, which are made as one integrated design for saving fabrication cost and decreasing thermal resistance.

3 W LEDs and Lens

5 W LEDs and Lens PCB for power input

Aluminum base Fins

Lamp Frame including aluminum base and fins

Figure 1. Schematic diagram of the 80-W LED street lamp.

Figure 2. Schematic diagram of the 114-W LED street lamp. As for the 80 watts LED street lamp shown in Figure 1, twenty high power LED modules are directly bonded on the aluminum base for reducing thermal resistance. They are distributed on the aluminum base in four rows. The modules in the central two rows are 3 watts LED packages, each of which includes three 1 watt power chips. The other two rows consist of 5 watts LED modules, each of the 5 watts packages includes four chips, and they are supplied with 5 watts power. Four slim PCBs are located on the aluminum base and used for providing power for the four rows of LEDs. For the 114 watts LED street lamp shown in Figure 2, ninety six high power LED modules are bonded onto the heat sink. They are distributed on the heat sink base in eight rows. The ninety six LED modules are the same, their default input powers are 1 watt, however, here they are supplied with 1.188 watts power, therefore, the total input power for this lamp is about 114 watts. For the above two lamps, when the electronic power is supplied, LEDs generate light and also heat. The heat is

2008 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP 2008)

dissipated out into the environment through the aluminum base and fins on the base. Thermal Resistance Model and Its Calculations Good optical and thermal performances are the design keys to a successful LED street lamp. The optical design of the street lamp is to achieve good light quality and make the road bright and comfortable to the passengers. However, the optical characteristics of the present LED street lamp will not be discussed in this paper, its temperature distribution and thermal performance will be the main concern. For the street lamps shown in Figure 1 and Figure 2, low thermal resistance is highly appreciated for achieving good thermal and optical performance. The thermal resistance for every LED module in suck kind of street lamp includes four parts, which are demonstrated in Figure 3. The first part is the packaging thermal resistance ( Rcp ) of the high power LED, which is related to the chip packaging technology. The second part is the bonding thermal resistance ( Rbd ) between LEDs substrate and the aluminum base with fins, which is mainly determined by the bonding material and thickness. The third part is the thermal spreading resistance ( Rsp ) between LEDs and the aluminum base with fins, which is affected by many geometrical sizes such as chip substrate size, the size and material of the aluminum base and so on. The last part is the thermal convection resistance ( Rconv ) between the fins and the environment, which is influenced by many factors such as fin structure, area and environment wind speed. In Figure 3, T j is the maximum junction temperature of the LED chips,

Tc −c is the maximum temperature in the aluminum base, and Ta is the ambient temperature.

Actually, the bulk material resistance ( Rbk ) exists in the thermal resistance network, which is usually included in the calculation of the thermal spreading resistance( Rsp ). For the present high power LED lamps, the bulk material resistance is very small so that it can be neglected [15]. In the four thermal resistances, thermal spreading resistance is the most important and difficult to be calculated. For the high power LED street lamps discussed in this paper, many LED modules are distributed on the heat sink and each LED module creates heat, so there are many heat sources distributed on the heat sink. The heat produced by each LED module is transferred to the heat sink and finally is dissipated into the environment. During the heat transfer process from each LED module to the heat sink, one thermal spreading resistance exists because heat transfers from small area such as LED module to the larger area such as heat sink. Therefore, as shown in Figure 3, for one high power LED street lamp, there are many thermal spreading resistances since it consists of many LED modules. These thermal spreading resistances( RSM − sp ) are connected in parallel. For the high power LED street lamps shown in Figure 1 and Figure 2, since the size of each LED chip module is the same, also their sizes are very small compared with the heat sink size, although the position for every heat source is different in the base plate, the thermal spreading resistances ( RSM − sp ) for each LED module in Figure 3 still can be regarded as the same. Therefore, it is very easy to simplify and calculate the final thermal spreading resistance ( Rsp ) of the LED street lamp, which can be expressed as,

Rsp =

RSM − sp n

(1)

where n is the LED module number for the high power LED street lamp, as shown in Figure 3. For the 80 watts street lamp shown in Figure 1, n is 20. For the 114 watts street lamp shown in Figure 2, n is 96. As to the thermal spreading resistance of the single LED module RSM − sp , its maximum value is defined as:

Rmax − SM − sp = where

Tmax − Tb Q

(2)

Tmax is the maximum temperature in the contacting

area between the LED module and heat sink, Tb is the average temperatures over the LED module and base areas. Q is the heat transfer rate of each LED module. About this thermal resistance, a closed-form equation has been proposed by the Lee et al [16,17]. Based on these references,

Rmax − SM − sp = Figure 3. Thermal resistance model of LED module in the high power LED street lamp.

ψ max k ⋅a⋅ π

(3)

where a is the equivalent radius of the LED module, k is the thermal conductivity of the heat sink material.


ψ max =

ε ⋅τ 1 + ⋅ (1 − ε ) ⋅ Φ c π π

area. heff is an equivalent natural convection efficient when

(4)

the heat sink of the street lamp is regarded as a plate without its fins. Obviously, heff is much larger than h . The relation

In equation (4),

Φc =

tanh(λc ⋅τ ) +

λc Bi

λ 1 + c ⋅ tanh(λc ⋅τ ) Bi

between h and heff can be described as,

Ah = heff Abase

(5)

In equations (10) and (11), A is the total heat sink area that is exposed to the environment. Abase is the heat sink base

where

λc = π +

1

(6)

π ⋅ε

In the above several equations, b is the equivalent radius of the heat sink of the street lamp, t is averaging thickness of the heat sink. Rconv is the thermal convection resistance of the

heat sink. ε is dimensionless heat source radius, it can be expressed as,

ε= as,

τ

a b

(7)

is dimensionless heat sink thickness, which is defined

τ=

t b

(11)

(8)

Bi is effective Biot Number, it can be expressed as, h ⋅b Bi = eff (9) k where heff is the equivalent natural convection efficient

area, and it does not include the fin area. The value of h can be calculated by using the methods proposed by reference [18]. Prof.Bar-Cohen in reference [18] gave the detailed analysis and discussions about the methods. It should be noted that the orientation of the fin heat sink of the present LED street lamp is horizontal, therefore, the heat transfer equations for both heat sink base and fins are a little different from those in reference [18]. In this calculation, computation iterations should be used for obtaining the results because of the coupled parameters in the equations. For the bonding thermal resistance Rbd and module packaging thermal resistance Rcp , they are easy to be obtained since they are usually provided by the thermal adhesive vendors and LED module vendors. According to the above discussions, it is noted that all the thermal resistances shown in Figure 3 can be achieved, therefore, the temperatures such as Tc −c , T j in the model can be calculated and estimated. Thermal Resistance and Temperature Calculations on the 80-W and 114-W LED Street Lamp

for the heat sink of the LED street lamp. Combining equations (3) to (9) into equation (2), the maximum thermal spreading resistance of the single LED module Rmax − SM − sp can be calculated. Since this value is acquired, according to equation (2), the maximum temperature in the LED module area Tmax can be obtained, which is nearly same as the value of Tc − c . Based on equation (1), finally, the total thermal spreading resistance for the LED street lamp can be obtained. For the other thermal resistance in the model shown in Figure (3), Rconv can be expressed as,

1 Rconv = (10) hA where h is averaging natural convection coefficient in all heat sink area, it is different from heff . In the original references [15,16], the heat sink connecting with the heat source is a plate, however, for the heat sink of the LED street lamp, it has many fins, to apply the situation into the equations, the heat sink with many fins should be equal to a plate in heat transfer area. Therefore, h is the real heat transfer coefficient based on the heat sink area including fin

Figure 4. Details of heat sink used in the 80-W lamp. (All dimensions are in mm.) For the 80 watts and the 114 watts LED street lamps shown in Figure 1 and Figure 2, the sizes of the heat sinks are shown in Figures 4 and 5 in details. The lengths of both heat sinks are 600mm and 550mm respectively. The length directions in Figures 4 and 5 are vertical to the paper. Based on the above mentioned model, by using equations (1) to (11), and supplying as inputs the parameters such as power, ambient temperature, sizes and physical properties of air and fin, the different values in the models for 80 watts and 114 watts LED street lamps can be obtained and are listed in the following Table 1.


To prove the feasibility of the above thermal resistance network model, the temperature distributions of the aluminum base and fins of the 80 watts and 114 watts LED street lamp were measured by thermocouples in the experiments.

Figure 5. Details of heat sink used in the 114-W lamp. (All dimensions are in mm.) Table 1. Calculation results for 80 watts and 114 watts LED street lamps Parameters 80 watts 114 LED Lamp type watts Averaging heat transfer coefficients. of 1.5 fins and base ( h ) (W/m.K) 1.87 Thermal spreading 0.1 0.008 resistance of the LED street lamp ( Rsp )(K/W) Thermal resistance of thermal interface material ( Rbd )(K/W)

2.5

Thermal resistances of chip packaging ( Rcp )(K/W)

3-W module 5-W module 0.863

Thermal convection resistance ( Rconv )(K/W) Averaging temperature of fins and base (℃) Maximum temperature of fins and base (℃) Averaging junction temperature (℃)

2.5

4.1

10

2.42 0.37

40.33

60.92

45.41

61.7

3W module 5W module

68.13

72.93

Experimental Study 1. System and temperature measurement

74.332

Figure 6. Experimental setup. Figure 6 shows the experimental setup. The orientation of the heat sink and the system were set up as the application conditions. The fin base was placed blow, the fin tips were on the top. The tests were conducted at a natural environment. For 80 watts and 114 watts LED street lamp tests, the ambient temperature was about 11℃ and 25 ℃ respectively. Several thermocouples were placed at different positions of the aluminum base and fins. The temperature data obtained by the thermocouples was transferred to the data acquisition system and displayed on the PC monitor. The model of the data acquisition system in the experiment was Keithley 2700 multimeter and control unit 7700. 2. Accuracy analysis In the experiments, the temperature was the main parameter for system evaluation, and it was directly measured by thermocouples. Since there were no other indirectly measured parameters, the errors associated with this experiment mainly included measurement error of the thermocouples and reading error of the digital multimeter. Standard T-type thermocouples (Cu-CuNi) were used in the experiments. During the temperature range from -30℃ to 150℃, their measurement error was about 0.2℃. The data acquisition system had a reading error of 1℃ since the cold junctions of the thermocouples used the default setup supplied by the system, not the ice bath with constant 0℃. Therefore, the total error of the temperature measurement for the experiments was about 1.2℃. Experimental Results and Analysis Figure 7 shows the variation of the heat sink temperature with the operation time for the 80 watts LED street lamp. In the experiments, as described above, the room temperature was about 11℃ and there were sixteen thermocouples to measure the temperatures at sixteen different positions, much data was obtained and it was difficult to display them in one figure synchronously. Thus the temperatures obtained by four thermocouples numbered as 2, 3, 11 and 13 were used for description. In Figure 6, it can be seen that the fin temperature increased as time extended, initially, the fin temperature was nearly the same as the room temperature. After the lamp was


activated, its temperature increased. Several hours later, it reached steady situation and the temperature remained to be stable to be nearly 42℃. It is also noted from Figure 6 that the temperatures achieved by all thermocouples showed the same change trend. The temperature differences among the thermocouples were very small, considering the reality that the measurement error was about 1.2℃, it cannot differentiate the temperature difference demonstrated in Figure 6. 44 42

Comparison between Experimental Results and Modeling Results As discussed in the above sections, the modeling results demonstrate the temperature distribution in different parts of the street lamp, including the junction temperature. However, in the experimental results, the junction temperature could not be achieved because of the measurement difficulty, the surface temperature of heat sink was measured, therefore, in the comparison, the heat sink temperature will be the only parameter to evaluate the modeling result. The comparison results are shown the following table.

40 38

No. 2 thermocouple No.11 thermocouple No. 3 thermocouple No.13 thermocouple

Temperature(OC)

36 34 32 30 28 26 24 22 20 18 16 0

2000

4000

6000

8000

10000

12000

Time (S)

Figure 7. Variation of the heat sink temperature with the operation time for 80-W LED street lamp

60

Temperature

(OC)

50 No.12 thermocouple No.5 thermocouple

40

No.3 thermocouple

Str Ambient Experim Thermal eet temperature ental result resistance lamp modeling result type 114 25℃ 61℃ 60.92℃ watts LED 80 11℃ 42℃ 40.33℃ watts LED According to Table 1, it is clear that the modeling results are close to the experimental ones, the temperature difference achieved by the two methods for 114 watts LED street lamp was just 0.08℃. For 80 watts LED, it was 1.67℃. The comparison proves the feasibility of the model. Summary In this paper, a simplified thermal resistance network model for LED street lamp was proposed and model was validated by experiments for two prototype street lamps. It is expected that the simplified model can be used for rapid design of thermal management for LED street lamps and other similar high power LED applications.

No.14 thermocouple No.11 thermocouple

30

Acknowledgments We acknowledge the financial support from Key Technology R&D Program of Hubei Province, China. (2006AA103A04) and GuangDong RealFaith Optoelectronics Inc., GuangDong, China.

No.9 thermocouple No.7 thermocouple

20

10 0

5000

10000

15000

Time (second)

20000

25000

Figure 8. Variation of the heat sink temperature with the operation time for 114-W street lamp. Figure 8 shows the variation of the heat sink temperature with the operation time for the 114 watts LED street lamp. In the experiments, the room temperature was about 25℃. In Figure 8, it can be seen that the heat sink temperature increased as time extended, initially, the fin temperature was nearly the same as the room temperature. After the lamp was activated, its temperature increased. Several hours later, it reached steady situation and the maximum temperature remained to be stable to be nearly 61℃.

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