Transient thermal simulation of high power LED and its challenges

Abstract

Transient thermal analysis (TTA) is widely used to measure the transient thermal impedance (Zth) and the thermal resistance of LEDs because reliability and lifetime of LED depends critically on junction temperature. To predict up-front in the product development process the lifetime of LED modules, calibrated finite element (FE) models are used. In this paper a FE-model for a family of high power LED is developed, i.e. different number of LED dies on ceramic sub-mounts of different sizes and calibrated to the Zth measurements. Based on the CAD data for the selected LED module (two LED dies on ceramic carrier), different modern FE tools (ANSYS, Comsol and Flo-EFD) are used for transient FE simulation and benchmarked. All tools deliver appropriate results when best practice FE modeling is applied i.e. mesh quality, correct boundary condition, material data and contact resistances. To model the Zth (t) measurement correctly, the suited approach of thermal boundary condition is investigated and a temperature boundary condition is proven as correct, practical and numerical efficient approach. The specific effect of heat generated in the converter of white LEDs on the transient impedance curve is revealed and investigated. Afterwards one FE-tool is coupled with the commercial optimizer OptiSLang. Based on available material data the FE model of the 2-chip LED module is calibrated to the experimental measured transient impedance curve. The calibrated model parameters are used to simulate the Zth (t) curves of another high power LED module of this family. It was found that the simulated curves matched the experimental Zth (t) curves of the LED modules. This validates the calibrated material properties for this entire LED family.