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Documentation

Validation Case: LED Thermal Management – Chip on Plate (COP)

This case validates the thermal management inside an LED against the CFD and experimental results obtained in the conference paper, “Thermal Analyses and Measurements of Low-Cost COP Package for High-Power LED”\(^1\), using the Conjugate Heat Transfer v2.0 analysis type in SimScale.

Geometry

The geometry consists of a cylindrical LED module constructed using the images from the conference paper\(^1\) with certain thickness assumptions for the Silicone encapsulant and the Aluminum Pkg substrate.

led module cad geometry thermal management
Figure 1: Detailed LED module CAD geometry with associated parts used for the validation case

A cubic domain, with seven times the size of the LED module, is also constructed to model the air region surrounding the module in each direction.

Analysis Type and Mesh

Tool Type: OpenFOAM®

Analysis Type: Incompressible, steady state Conjugate Heat Transfer v2.0,

Turbulence Model: Laminar flow

Mesh and Element Types: The mesh is generated using the standard meshing algorithm in SimScale.

MeshNumber of cellsElement type
F0 (Coarse)153000Tetrahedral and hexahedral
F5 (Moderate)658000Tetrahedral and hexahedral
F8 (Fine)3500000Tetrahedral and hexahedral
Table 1: Standard mesh characteristics for three different fineness levels: 0, 5, and 8
led module standard meshing
Figure 2: LED module meshed with a fineness of 5 with tet and hex elements using the standard algorithm

Simulation Setup

Material

Solid: The materials constituting the LED are tabulated with their properties as follows:

MaterialsThermal conductivity \([W/m\ °C]\)Mass density \([kg/m^3]\)Specific heat \([J/kg\ °C]\)
Silicone (Encapsulant)0.312002000
LED Chip422330712
Sapphire (Chip substrate)353980761
Ag epoxy (die attach)82300671
Aluminum (Pkg substrate)2202702910
Thermal Grease3.611801044
Copper (Heat sink)3808800380
Table 2: Material properties for the solid parts in the LED module

Fluid: Air

  • Viscosity model: Newtonian
  • Kinematic viscosity \((\nu)\): 1.469e-5 \(m²/s\)
  • Density \((\rho)\): 1 \(kg/m^3\)

Boundary Conditions

As in the reference paper, all the faces of the LED module are modeled as no-slip walls with the temperature being zero-gradient. All the faces of the air domain are also modeled as no-slip walls with a fixed temperature value of 24 \(°C\) except for the top face which is open for natural convection.

wall boundary conditions for led thermal management
Figure 3: Mostly the wall boundary conditions are used for the solid and the fluid domain. The top face of the fluid domain is open for natural convection.

The LED module is placed in the air domain such that the Silicone encapsulant faces the direction in which gravity is modeled (+z axis).

Power Source

The experiments mentioned in the conference paper\(^1\) were performed at three different input powers: 0.96 \(W\), 1.22 \(W\), and 1.47 \(W\). The paper assumes that only 78% of the input power is transferred into heat, which is uniformly distributed over the entire chip volume. Thus, the LED Chip is modeled as an absolute power source with the values of 0.749 \(W\), 0.952 \(W\), and 1.147 \(W\) respective to the power inputs used in the experiment\(^1\).

You can read more about power sources in SimScale in the following documentation:

LED Thermal Management: Result Comparison

The simulation results from SimScale are compared against the CFD results and experimental results from [1]. The results for the temperature of the surface-junction between LED Chip and Chip Substrate in \(°C\), for various power inputs, are tabulated as follows:

Power input \([W]\)SimScale \([°C]\)CFDesign 8.0 \([°C]\)Experiment \([°C]\)% Deviation SimScale% Deviation CFDesign 8.0
0.9652.125255.15.415.63
1.2258.485761.44.767.17
1.4764.396269.37.0910.53
Table 3: Result comparison for the temperature of the surface-junction between LED Chip and Chip Substrate
led junction temperature result comparison simscale cfdesign 8.0 experiment
Figure 4: Result comparison for the temperature of the surface-junction between LED Chip and Chip Substrate

Mesh Sensitivity Study

For the power input of 1.47 \(W\), a mesh sensitivity study was also performed for meshes with a fineness of 0 (F0), 5 (F5), and 8 (F8). This study is plotted below:

mesh senstivity
Figure 5: Mesh sensitivity study performed for the power input of 1.47 \(W\) for three different fineness 0, 5, and 8 shows that the results are reliable

LED Thermal Management: Conclusion

The discrepancy is likely due to the difference between simulation setup and physical testing environment. Unfortunately, full experimental details were not provided in the conference paper.

The following conclusion can be made from this LED performance thermal management validation case:

  • Reasonable agreement with experimental trends
  • Good agreement with alternate CFD software CFDesign 8.0
  • Outperformed alternate CFD in % result deviation
led temperature distribution in simscale post-processor
Figure 6: Temperature distribution around the LED module as viewed in SimScale’s online post-processor

Last updated: May 5th, 2021

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