Thermal Considerations for High-power LED Spot Lamp

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Thermal considerations for high-power LED spot lamp Xuping Zhu1, Guiying Yu1

(1.College of metrology and measurement engineering, China Jiliang University, Hangzhou, 310018, China)

Abstract: This paper reports on thermal characterization of high power LED spot lamp. Thermal steady methods are used to measure the junction temperature and calculate the steady temperature distribution. The emphasis is placed upon the investigation of junction temperature rise of LED for a limited range of boundary conditions which include optimum design effect of heat sink, convection condition and ambient temperature. A numerical study that is carried out with initial parametric models to understand the temperature distribution of spot lamp by FEM (Finite Element Method) technology was experimentally calibrated. The convection coefficient is concluded by combining experiments and simulation results. The result demonstrated that the convection coefficient of 3.5 W/m.k comforted that we need. Then, the heat sink has been optimized by using ANSYS optimization module. Simultaneously, orthogonal experiment method is used in order to further optimize the result in a small scale. It shows the proportion of various factors that influences junction temperature and weight of the spot lamp by range analysis. Also, it is found out that the measured junction temperature and the weight of spot lamp are decreased by 1.6,and 1.13Kg, respectively. Finally, a CAD model is generated from the result of factors which is optimized by orthogonal experiment method. Keywords: LED spot lamp, ANSYS, Heat sink, orthogonal experiment method, optimum design.

1. Introduction White LED is well known as a promising device for solid state lighting. It has

the advantages of long life, good endurance of heavy impact, no mercury containing and most importantlypotentially high efficiency [1]. Which make them an ideal choice for illumination, optical communications, optical storage, etc. Recently, the introduction of high-brightness white LED and monochromatic colors has led to movement towards specialty and general illumination applications. It is expected to replace incandescent and fluorescent lamps in the near future for the advantage of less energy-consumption [2] [3]. However, light output from a commercial LEDs is usually petty, which is very small if compared with traditional light sources. In order to get enough light output from LED devices for outdoor display or illumination, a big-lamp or cluster packed from dozens or hundreds of LEDs is necessary. Some problems were encountered during high-density assembly in a big-lamp. Because of the low heat conductivity of the epoxy, the more LEDs are used in the big-lamp, the poorer heat dissipation will be. Also the increased electrical currents used to drive the LED focus more attention on thermal management because the efficiency and reliability of LED strongly depend on successful thermal management [4].

LED thermal design is carried out mainly from two aspects: one way is based on the law of energy conservation to improve the light extraction efficiency and reduce

the heat flux; the other way is to improve heat loss capability through the optimum design of heat sink outside the LED device. This heat sink not only meets heat transfer requirement of general lighting systembut also reduces the cost. Therefore, the current high-power LED thermal management, especially in the LED lamps and lighting system is mainly realized by optimizing the structure of the heat sink to improve system cooling capability. Thus, the optimum design of heat sink of high-power LED application is especially critical.

It is generally known that thermal behavior of LED lighting application is affected by more factors than in the case of unit package [5]. There have been several reports on thermal characterization of LED packages level thermal design [6] [7] [8] and optimum design of heat sink [9] [10] [11]. However, no report is found on the thermal analysis of high-power LED spot lamp system, optimum design of sunflower heat sink by orthogonal design method and confirm convection coefficient by comparing experiment with simulation results so far to the best knowledge of the authors.

Both thermal analysis for high-power LED spot lamp and optimum design of heat sink with optimum module and orthogonal experiment method are discussed in this study. The Finite Element Method (FEM) is used as a function for thermal design. A typical sunflower heat sink is adopted for simulating. The temperature at the bottom of the heat sink is measured, and then compared with ANSYS simulation results, to obtain the actual convection coefficient work as two results are approximately equal.

2. Thermal modeling According to the heat transfer theory, the steady heat conduction equation with

heat generation show in Eq. (1) is used to obtain the temperature distribution until the spot lamp reached steady.

0

..

=+

+

+

qZ

TKzY

TKyX

TKx zzyyxx

, (1)

Where T is the temperature; x, y and z are the spatial coordinates. K xx, K yy and K zz are

the coefficient of heat conductivity in x, y and z orientation. is the heat generation.

For the maximum junction temperature of LED need to be below 125[12][13], most of heat generated from LED can be transferred to the outside environment efficiently by means of conduction.(A small of amount of energy in comparison is taken away by heat radiation.) Heat sink surface exchanged heat with air on the basis of Newtons equation show in Eq. (2).

q..

, (2) ( BS TThq = )Where q is the heat flux, h is the convection coefficient, Ts is the temperature of heat sink surface, TB is the environment temperature. Meanwhile, LED spot lamp lighting B


system also satisfies Fouriers law as shown in Eq. (3)

xTq

= , (3)

Where is conductivity, x is the distance of heat transfer. Then Eq. (1)-Eq. (3) were discredited by finite element method show in Eq. (4),

[ ][ ] { }QTK = , (4) Where [K] is conduction matrix; [T] is junction temperature matrix; {Q} is heat flow matrix. This is the basic equation of finite element method to solve steady state temperature distribution, the junction temperature matrix [T] is settled by solving conduction matrix [K] in ANSYS.

The modeled structure is depicted in Fig.1 and the structure is composed of 1010 LED chip , die attach, reflector and heat sink. The initial structure of heat sink has a dimension of 128mm54mm2.5mm, center cylinder radius 27mm and the fin number is 28 pieces. Detailed structure of LED package is simplified and the effect of gold wire is not considered in modeling. Also, the fixtures (screws) are omitted. Reflector is composed by aluminum. All the thermal parameters of LED spot lamp materials are employed in the table 1.

Table1. Thermal parameters of LED spot lamp materials

Material AL LED chip Die attach

K xx (w/m. k) 237 65 56.5

(a) (b)

Fig. 1 Schematic of LED Spot Lamp lighting system structure: (a) is side view;

(b) is overhead view

The solid model of spot lamp is imported into ANSYS which is built in CAD software (solidworks). Since the whole model is composed of many B-spline curves, a free meshing was adopted to build finite element model of spot lamp that shows in Fig.2, the software automatically generates basic grids to each component. However,


grid density of LED chip must be increased since its temperature gradient is larger than any other components. Otherwise, the simulation results will vary greatly.

For the CAE simulation, the boundary condition is set to implement convection for external surface of the lamp body and the heat sink. The ambient temperature is fixed at 13. Suppose the input power of LED chip is 100W and photoelectric conversion efficiency is 10%. Hence, the heat generation is 4.25e9W/m3. To simplify the model, the package process of LED chip and the additional contact resistance between each layer are neglected.

Fig. 2 The finite element model of LED spot lamp

3. Experiments and confirm the convection coefficient In this study, the convection coefficient is confirmed by combining experiments

and simulations. The temperature of LED spot lamp was tested by handheld infrared thermometer and the test point was set at the bottom of the heat sink, as is showed in Fig.1. The working current was 3A and the voltage was 30V. The testing time was continued for 2 hours and the data was recorded every 5 seconds.

As the experiment result shows, LED spot lamp will get thermal equilibrium after 25 minutes when the temperature of the test point was 61. Then, the steady temperature distribution of the lamp was calculated by ANSYS with the convection coefficient of 5 W/m.k, to get the result of 56. In order to make the CAE simulation results as accurately as possible, the convection coefficient was changed persistently according to the temperature acquired by experiments. Finally, the result demonstrated that the convection coefficient of 3.5 W/m.k comforted what we need.


a (b)

Fig.3 The temperature distribution of initial structure: (a) spot lamp; (b) heat sink

4 Results and discussion

4.1 Optimum design of heat sink based on ANSYS optimal module

As we know, there are several factors about sunflower heat sink to affect heat transfer effect in the spot lamp system. An effective heat sink for one factor is not likely to be optimal in other conditions. Custom heat sink size for LED junction temperature test is not possible. Simulation allows several runs of slight variation of the design at low cost and time for further insight to understand and optimize the design for the spot lamp.

The typical sunflower heat sink c ommonly used for LED lighting applications can be easily represented in a CAE model using APDL program. In this study, the optimum design of a typical sunflower heat sink is influenced by five factors: fin height, fin width, fin number, fin thickness and center cylinder radius. The range of each factor listed in table.2 is constrained by reasonable design limitations for this high power LED spot lamp.

Table.2 The range of design variable Design variable

Fin width (mm)

Fin height (mm)

Fin thickness (mm)

Fin number (pieces)

Center cylinder radius(mm)

Range 40-59 100-128 1-3.5 24-28 21-30

The maximum junction temperature of lighting system and the weight of heat sink are separately set to state variable, object function. To prevent jumping out of circulation ahead of scheme, all the variable tolerances are set to 1e-5. The result of optimum design is listed in table.3. The steady temperature distribution of optimum structure is showed in Fig 4.


Table.3 The result of optimum design by ANSYS optimal module

Design variable

Fin width Fin height Fin thickness Fin number (mm) (mm) (mm) (pieces)

Center cylinder radius(mm)

Range 50.7 120.5 1.6 26 21.6

(a) (b)

Fig.4.The temperature distribution of structure optimized by ANSYS: (a) spot lamp; (b) heat sink.

It shows that the maximum junction temperature of spot lamp is 73.3 and the weight of heat sink is 1.16Kg. So, it is found out 1.6lower than the initial structure which is showed in Fig.3. Meanwhile, the heat sink weight is reduced by 0.94 kg.

4.2 Optimum design of heat sink based on orthogonal experiment method Utilizing the proposed orthogonal experiment method for heat sink optimum

design will further optimize the result in a small scale. Design of experiment analysis is performed using a four lever and five input factors test matrix. The parameters are defined as fin height, fin width, fin number, fin thickness and center cylinder radius. To understand the full-scale experiment by analyzing the part test is the basic characteristics of orthogonal experiment method and it will be a significant reduction in the number of tests. Hence, the experiment efficiency will be greatly improved. The orthogonal experiment method is implemented based on orthogonal table as is shown in table.4. Sixteen structure of heat sink and the corresponding CAE simulation results are summed up. It can summarize from table.4 that the model of lever number 13 that the steady temperature distribution is showed in Fig5 is superior to the structure optimized by ANSYS optimum module. Although the maximum junction temperature is not decreased, the weight of heat sink is further reduced by 0.19Kg.


Table.4 Orthogonal experiment table Level number Fin

number (pieces)

Fin thickness

(mm)

Fin height (mm)

Center cylinder radius (mm)

Fin width (mm)

LED Tj

Heat sink weight kg

1 24 1.1 124.5 23.6 54.7 72.15 1.08 2 24 1.6 122.5 22.6 52.7 73.18 1.20 3 24 2.1 120.5 21.6 50.7 74.82 1.31 4 24 2.6 118.5 20.6 48.7 75.76 1.40 5 25 1.1 122.5 21.6 48.7 76.29 0.93 6 25 1.6 120.5 20.6 54.7 72.54 1.15 7 25 2.1 118.5 23.6 52.7 73.55 1.45 8 25 2.6 124.5 22.6 50.7 72.55 1.64 9 26 1.1 120.5 23.6 50.7 74.37 1.04

10 26 1.6 118.5 22.6 48.7 75.57 1.16 11 26 2.1 124.5 21.6 54.7 72.55 1.45 12 26 2.6 122.5 20.6 52.7 71.86 1.62 13 27 1.1 118.5 21.6 52.7 73.30 0.97 14 27 1.6 124.5 20.6 50.7 72.49 1.19 15 27 2.1 122.5 23.6 48.7 73.22 1.66 16 27 2.6 120.5 22.6 54.7 70.10 1.78

optimize by ANSYS

26 1.6 120.5 21.6 50.7 73.30 1.16

The results of CAE simulations have been averaged per factor lever to study

their effects on the junction temperature of Spot lamp are showed in Table5-Table7 together with the weight of heat sink. Meanwhile, variation of maximum temperature and heat sink weight with each factor is vividly described in Fig6-Fig8.

(a) (b)

Fig.5.The temperature distribution of model optimized by orthogonal experiment method: (a) spot lamp; (b) heat sink.


Table.5 Range analysis of (a) Fin number; (b) Fin thickness. (a) (b)

Level number

Fin thicknessmm

LED Tj

Heat sink weightkg

1 1.1 74.05 1.01 2 1.6 73.45 1.18 3 2.1 73.21 1.47 4 2.6 72.56 1.61

Range analysis

1.49

0.6

Level number

Fin number

pieces

LED Tj

Heat sink weightkg

1 24 73.98 1.25 2 25 73.73 1.29 3 26 73.59 1.32 4 27 72.30 1.40

Range analysis

1.68

0.15

(a) (b)

Fig.6 Variation of Spot lamp maximum temperature and heat sink weight with: (a) fin number ;(b) fin thickness

Table.6 Range analysis of (a) Fin height; (b) center cylinder radius. (a) (b)

Level number

Fin height

mm

LED Tj

Heat sink weightkg

1 118.5 74.57 1.25 2 120.5 73.96 1.32 3 122.5 73.28 1.35 4 124.5 72.44 1.38

Range analysis

2.13

0.13

Level number

Center radius

mm

LED Tj

Heat sink weightkg

1 20.6 73.16 1.34 2 21.6 73.06 1.45 3 22.6 72.85 1.50 4 23.6 72.32 1.58

Range analysis

0.84

0.24


(a) (b) Fig.7 Variation of Spot lamp maximum temperature and heat sink weight with (a) fin height (b) center cylinder radius

Table.7 Range analysis of Fin width. Level number Fin widthmm LED Tj Heat sink weightkg

1 48.7 75.21 1.28 2 50.7 73.55 1.29 3 52.7 72.99 1.31 4 54.7 71.84 1.37

Range analysis 3.37 0.11

Fig.8 Variation of Spot lamp maximum temperature and heat sink weight with fin width As in the lever of simulation, the range analysis tables (table5-table7)

demonstrate that the proportion of various factors influenced on LED junction temperature is sorted in a descending order, fin width, fin height, fin number, fin thickness and center cylinder radius. However, these factors impacted on the heat sink weight are sorted ascending as follows, fin width, fin height, fin number, center cylinder radius and fin thickness. So we can conclude that the proportion of each factor affected on the junction temperature is different from that on the weight of heat sink.

As are shown in Figure6-Figure8, the weight of heat sink increases with the lever of each factor. On the contrary, the LED junction temperature drops down while the lever of each factor rises. In summary, fin width of the heat sink should be increased and fin thickness decreased in an appropriate range as much as possible. Finally, not



only the maximum junction temperature of LED is decreased, but also the weight of heat sink is reduced. So, the investigation on optimum design of the heat sink will be a good significant for the future production of LED lamps.

5 Summary and Conclusions A study to understand temperature distribution and thermal management of the

spot lamp lighting system are completed. In this paper, nature convection coefficient is confirmed by combining experiments and simulation results. Thermal consideration is focused on optimum design of sunflower heat sink which is fixed at bottom of this application. Several boundary conditions, including ambition temperature and nature convection coefficient are applied for thorough evaluation of LED spot lamp with various sizes of heat sink. The optimum module of ANSYS and orthogonal experiment method are used for optimizing heat sink and several parameters including fin width, fin height, fin number, center cylinder radius and fin thickness are considered.

It is demonstrated that thermal design for LED application should not only based on the maximum temperature, but also the weight of heat sink. As the result shows, the weight of heat sink is 0.97Kg which is reduced by 1.13Kg and the maximum temperature was 73.3 that was decreased by 1.6 as opposed to the original model. So it was proved to be a good solution for controlling junction temperature and weight of high power LED systems.

Acknowledgements Authors would like to acknowledge the finial support for this work provided by

the Nature Science foundation (Y 104436), Science and Technology projects of Zhejiang province (2008C21158). They would also like to thank .of the China Jiliang University for helping to prepare this manuscript.


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Thermal Considerations for High-power LED Spot Lamp

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