A Study On The Thermal Design Of High-power LED

The present thesis puts forward a novel packaging structure for high-power white LEDs, which is tested through thermal analysis using ANSYS software and enhanced according to the analysis results. With the limitations of cost and size taken into consideration, a thermal structural optimization is performed to the LED packaging, which is specified as follows:The structural model of LED is constructed, and through simulation with ANSYS, the steady-state and transient-state temperature fields of LED are obtained. At steady-state, the temperature of the LED chip is 396K, and that of LED lamp-chimney is 320K. The temperature of the LED chip measured and calculated is 394.5K, which corresponds to that of the simulation.Several optimization designs adopting different substrate materials and different Al heat-sink LED sizes are proposed, and their simulation results are compared. It is shown that the optimal result is achieved when both Cu-Mo-Cu composite substrate and Al heat-sink are used, and the temperature of the LED chip is decreased to 63℃, with the difference in temperatures reduced to 11℃. The consumption of Al reaches the minimal level when Al heat-sink is used only. Optimization designs of this type or similar ones strike a balance between cost and effect, and therefore demonstrate higher feasibility.Temperature-accelerated aging experiments and current-accelerated aging experiments on copper-substrated high-power white LEDs are conducted, with lifetime analysis carried out on the basis of the results of the former type. According to the Arrhenius relationship, the lifetime of LEDs is estimated to be 74,700 hours under the normal temperature of 25℃, and 10,900 hours under the high temperature of 60℃.Before and after aging experiments, the current-voltage curves of white LEDs are measured. It is found that the reversal leakage current shows an identical increase to tunnel current after aging, due to the increased defect density in the active area of the chip. Although the junction temperature is lower in current-accelerated experiments, the chip undergoes a quicker degradation, presumably under the joint effect of high temperature and high current density. The luminous flux of LEDs shows similar declines in both types of aging experiments, probably as a result of higher junction temperature in temperature aging experiments, which leads to the degradation of the phosphor and packaging resin cover of the chip, eventually resulting in the decline of light output efficiency.