Hongyu Tang

Design of vertical fin arrays with heat pipes used for high-power light-emitting diodes

Abstract As Light-Emitting Diodes (LEDs) are negatively affected by high temperature, the thermal design for them is critical for better light quality, reliability and lifetime. In this work, a thermal design of vertical fin arrays with heat pipes as passive cooling was applied. The heat pipes can supply high thermal conductivity with much less weight and volume compared to copper or aluminum base and consequently less obstruction to air flow with enhanced natural convection. As the natural convection and radiation dominate heat transfer in this case, the optimum vertical fin spacing was calculated by the most used empirical correlations. Then, the design was numerical investigated by Computational Fluid Dynamics (CFD) to obtain best thermal performance. As the fin spacing was both optimized by correlations and modelling, the optimum thermal design achieved. Finally, we manufactured and tested the design experimentally which consistently approved the thermal design compared to correlations and simulation.

Junction temperature measurement to optimize thermal design of LED arrays

The drive to increase electrical currents input, and to achieve high lumen output for High Brightness Light Emitting Diode (HB LED) has led to a series of thermal problems. Hence, thermal management is one of key design parameters as the temperature directly affecting the maximum light output, quality, reliability and life time. For LED arrays, the arrangement of chips will affect the thermal performance and junction temperature is very important in the design. However, because direct junction temperature determination is hard to detect due to the encapsulation, indirect methods are more preferred. Among them, the diode forward-voltage method is the most accurate one. Therefore, we used the diode forward-voltage method to optimize thermal design of LED arrays in this work.

Thermal analysis and optimization of IGBT power electronic module based on layout model

In this work, one layout model of insulated gate bipolar transistor (IGBT) module are built by using a general analytical solution, which is used to analyze the effect of thermal spreading resistance on the whole temperature distribution of a rectangular board with multiple eccentric heat sources. IGBT power electronic module is optimized by changing the arrangement of IGBT and FRD chips on the cooling system with consideration of junction temperature (Tj), temperature uniformity. According to the thermal analysis and optimization on the layout models, quantitative designs of IGBT power electronic module for chip arrangement are achieved.

Cause analysis on highly depreciated indoor LED product in CSA020

With application of accelerated luminous depreciation test method of CSA020 which can reduce the lifetime test from 6,000 hours to less than 2,000 hours, LED lighting products with highly lumen depreciation or short lifetime can be easily screened out. Many potential factors can lead to that highly depreciation issue. This paper attempted to perform highly depreciation cause analysis by investigation on the activation energy Ea and junction temperature Tj of the highly depreciation of indoor LED products. Results were shown Ea of Product 4 is 0.29eV, even 0.09eV lower than the reference good product. Junction temperature Tj of Product 4 is 109deg.C, around 30deg.C higher than the reference good product, which means the junction temperature Tj is a key factor which caused highly depreciation. During lifetime design, both Ea and Tj are important and should be considered equally.

Influences of viscoelasticity of polybutylene terephthalate (PBT) on the thermal interface contact of LED spotlight module

The influence of viscoelastic behavior of polybutylene terephthalate (PBT) on reliability of the thermal contact of a spotlight module is evaluated. The contact between a module and the heat sink is evaluated using non-linear finite element model. The viscoelastic behavior of PBT was obtained under multiple frequencies from single cantilever DMA experiments. The relevant viscoelastic parameters (i.e. Relaxation time τi, Shear relaxation coefficient Gn and volume relaxation coefficient Kn) were obtained by using the linear regression. The mechanical behavior of the contact interface under high temperature storage was simulated. The simulation results agree with the experimental measurement. The proposed simulation is an efficient tool to depict the effect of PBT viscoelastic behavior on the reliability of LED spotlight module.

A novel design of heatsink-less LED base fluorescent lamp retrofit

A new “heatsink-less” LED based T8 retrofit design is proposed in this study. The lamp consisted of light emitting elements directly attached on a flexible substrate. The substrate is mount on to the inner wall of a full cylindrical glass tube. The design is intended to simplify the lamp construction. However, since the metal-based heat sink is eliminated, the thermal design is a concern. Finite element analysis was performed to optimize the thermal design by varying metal line routing on the flexible substrate, and the glass tube structure. The optimal result shows that the junction temperatures of the LED inside the lamp is about 52°C driven under nominal current. The simulation result is comparable with the experimental measurement. In addition, Darveauxs fatigue prediction model was implemented to calculate the thermal cycles for crack initiation and propagation, based on the inelastic strain energy density accumulated per cycle during thermal cycling. The details of the design and the optimization strategy is discussed.

Effect of material creeping on the reliability of thermal interface of LED spotlight module

In this study, the reliability of LED spotlight module with consideration of the contact between a module made of polybutylene terephthalate (PBT) and the aluminum heat sink is investigated by non-linear finite element modeling. The mechanical behavior of the contact interface under high temperature storage was simulated. The force applied on the thermal interface (FTIM) was obtained from the simulation and experiments. The simulation results agree with the experimental measurement. The proposed simulation is an efficient tool to depict the effect of the creeping of PBT on the reliability of thermal interface of LED spotlight module.

Thermal behavior of flip chip LED packages using electrical conductive adhesive and soldering methods

This paper investigated thermal behavior of three kinds of FC packages connected with AuSn, SAC305, and electrical conductive adhesive using 1-D thermal resistance circuit method, and finite element method as well as experimental measurements. Simulation results indicated that the low thermal conductivity of electrical conductive adhesive led to high thermal resistance at this layer and decreased the thermal dissipation of the package as a bottleneck when the heat power was 0.74W in this study. The junction temperature and Rjc of adhesive package was much higher than the other two and led to package failure. SAC soldering package showed similar heat dissipation ability with the AuSn one. Additionally, the results of FE analysis show better agreement with experimental observation compared with 1-D thermal resistance circuit method.

Thermal analysis and optimization design of LED streetlight module

In this paper analyzes the thermal characteristics of light emitting diode (LED) streetlight module which has serious problems of heat dissipation. Firstly, Finite Element Method (FEM) combined with the heat transfer theory was utilized to calculate the thermal performance of an LED streetlight module by the ANSYS. Then, the thermal behavior of LED streetlight module is investigated by experimental measurements which are used for verifying the feasibility of the simulation. Finally, the structure of the heat sink module was optimized by the Taguchi experimental design method, and some suggestions were proposed.