| With performance contiue to increase and cost continue to drop, high power LED has been more and more widely used in the lighting field. In order to complete replace the traditional light sources, there are still some problems in the production of LED need to resolve. First, for manufacturing process limit, GaN-based materials are only permit heterogeneous substrate growth. Si substrate is gaining more and more interest as a better selection for its good thermal conductivity, low cost, easy to obtain large size, and becoming research hotspot in recent years. Large lattice mismatch and thermal expansion coefficient difference between Si and GaN easily lead to GaN epilayers subject to large tensile stress and affect the performance of epitaxial wafer. Need to carefully design buffer layers and interlayer structures. Second, LED has low external quantum efficiency and most of the rays are total reflection back to chip and convert into heat. The luminous efficiency of LED is very sensitive to temperature. So need to improve the light extraction efficiency of LED and optimize the structure of heat dissipation. At the same time, in order to improve the light efficiency and illumination uniformity, LED need secondary light distribution design. Finally, LED chip has manufacture defect. In harsh environments, these defacts will cause reliability problems and lead to degradation to LED. In this thesis, the main contents are as follows:(1) The growth mode and growth mechanism of GaN epitaxial thin films were discussed, and the formula for the thermal stress caused by thermal mismatch between the substrate and the epitaxial layer was analyzed. Finite element model based on coupled field was established. Theoretical simulation results showed that HT-A1N buffer with double LT-AlN interlayers can evident relax the tensile stress induced by the thermal mismatch between Si and GaN. Changed the thickness of the LT-AlN, the optimum value was between 20-27nm. Experiment observation results were corresponding to calculation results. Change LT-AlN interlayses to step gradient AlxGa1-xN interlayses, the maxium thermal stress of GaN film can reduced to 0.101 GPa.(2) Based on the edge ray theory and law of extended conservation of light, combing with bidirectional iteration algorithm and equivalent grid dividing, realizes one-to-one correspondence between LED emergent ray space and specific spot on the road. A freeform surface lens was generated, and it can convert light distribution of the LED to rectangle. Simulation results showed that LED’s light energy was almost limited in a rectangular area of 30m×13m, the whole light efficiency was 90.4%. The horizontal illumination uniformity and vertical illumination uniformity can reach 83.2% and 71.7% respectively. When multiple LEDs were arranged in 30m spacing, the facula width on the pavement was about 14m. At this time, vertical illumination uniformity increased to 77.2%. The lens has a high tolerance redundancy. Experiment results show that the light distribution curve of LED street light is bat-wing distribution. The horizontal and vertical average illumination was 31.171x and 25.721x, and energy was limited in ±62° and ±40°, respectively. It can meet requirements of national standard for lighting design of urban road.(3) The influence of the micro lens array on the light extraction efficiency was analyzed. No gap square, triangular, hexagonal, hyperbolic micro lens array were designed. Compare to traditional micro lens array, no gap micro lens array have higher light extraction efficiency and diffusion angle. The corresponding values were improved 10-20% and 7-45° respectively. Combine freeform surface lens and micro lens array with optical collimator, an optical engine was achieved. This optical engine need not concerning the initial radiation distribution of LED and can obtain highly illumination uniformity on target surface.(4) In order to reduce the manufacturing cost of high power LED street light and ensure the heat dissipation ability, use orthogonal experiment method to optimized heat sink parameters. Fin depth, fin spacing, fin thickness and fin length were considered, and simulation software was used to model and simulate the LED junction temperature. The finial optimization results of these 4 parameters were 14mm,16mm,4mm,450mm respectively. Simulation and experimental results showed that the heat sink can effecively transfer heat. In enclosed sapce without wind, LED junction temperature rise of 47.9℃.(5) Temperature conduction control equation and moisture diffusion control equation were discussed. The temperature field and moisture field of LED bulb were simulated. LED bulb reach the thermal equilibrium need only about 5 minutes, while the moisture diffusion is 70h. Based on above simulation results, thermal stress and hygroscopic stress were analyzed. Thermal stress was much higher than hygroscopic stress, so the former is the main cause of deformation and cracking. However, moisture can reduce the adhesion between packaging material and also lead to accelerate aging of lamp.(6) To research the environment reliability of LED lamp, temperature aging test and temperature/humidity aging test were carried out. The failure mode and failure mechanism under different aging test were discussed. The common failure phenomena include LED chip light-emitting efficiency decreased, package silicone ture to yellow, fluorescent powder conversion efficiency decreased and material cracking, these would led to lower luminous flux and higher color temperature. Test data showed that LED bulb life accord with Weibull distribution. Arrhenius life model and Eyring life model were established for single temperature stress and double temperature/humidity stress respectively. Under 25℃ and 40℃/50%RH, we calculated the failure time of LED bulb were about 23000h and 17000h. |
PDF Full Text Request