| 摘要: | 近年來，白光發光二極體(White LED)在固態照明中扮演著非常重要的地位，而LED封裝體內光與熱的影響必須了解其機制。本研究以有限元素法及蒙地卡羅統計方法再配合自行推導的螢光粉光與熱轉換方程式建立一套從晶片到螢光粉整個白光LED封裝體光電熱數值模擬模型，利用實際發光層的電流密度當作LED出光光源，此數值模型可以計算及分析封裝螢光粉後整體的溫度分布型態、出光功率、色溫、演色性和色彩均勻性的變化，利用實驗量測接面溫度、光功率、色溫、演色性和色彩均勻性與模擬驗證結果有一致性的分佈。
本研究以藍光激發黃色YAG螢光粉作為實驗用白光LED，從白光與藍光LED暫態溫度實驗量測結果推論加入螢光粉會增加螢光膠的熱容量而降低熱擴散率，即使白光LED接面溫度在同功率下比藍光LED高，但隨著電功率的增加接面溫度上升幅度又不像藍光LED這麼明顯。而由實驗與模擬結果了解白光LED在低於電功率0.8W(約140°C)時黃光出光效率會較藍光好，而超過0.8W後黃光出光效率會低於藍光甚至出現光衰退行為，主要是晶片與螢光粉材料性質不同，在低溫時晶片衰退速率較快，而高溫時則是螢光粉衰退速率較快所導致。
最後我們利用數值分析模擬常見的三種封裝體：填充式(Filling)、鑄模式(Molding)和遠離式(Remote)三種不同功率下光、熱與色的差異性。結果顯示遠離式封裝在不同功率下都有較佳的光輸出功率和最低的色溫飄移現象，較適用於照明上；而其餘兩種封裝形式都有與晶片或銅基板接觸的情形，熱交互作用較強，鑄模式在不同功率下雖有較佳的色彩均勻性，但因其體積小溫度高，高功率下色溫飄移太過劇烈且光功率衰退較快，而填充式色溫飄移幅度沒有鑄模式劇烈，但其相對色彩均勻性較差，已目前常見的LED看板或是背光元件來說，高功率下需要較好的光輸出功率及色彩均勻性，因此我們分別利用上述兩者優點提出幾個可以同時提升光輸出功率與色彩均勻性的方法，即為1.螢光粉體積不能太小。2.必須接觸晶片及銅基板。3.螢光粉必須為低濃度。這幾個方法可以分別改善鑄模式與填充式的缺點，並設計較佳的螢光粉封裝體。
In recent years, white light-emitting diode (White LED) playing an important role in solid-state lighting, so that we must understand the effect mechanism of light and heat inside the LED package. This study uses the finite element method (FEM), Monte Carlo statistics method and phosphor transform equation of light and heat derivate by ourselves to build a couple of light, electrical and thermal numerical simulation model. It includes entire white LED package from chip to phosphor. We use the realistic current density to be the light source of active layer in simulation. Under numerical simulation, we calculate and analyze overall LED temperature distribution, light output power, color temperature, color rendering index and angular color uniformity. Through reality experiment to measure junction temperature, light output power, color temperature, color rendering index and angular color uniformity, this experimental result get good agreement with the simulation.
The study uses yellow phosphor excited by blue light as the experimental white LED. From the result of transient temperature measure experiment, between blue and white LED, we conclude that phosphor increases the heat capacity and decreases thermal diffusivity of phosphor encapsulate. The white LED junction temperature is higher than blue LED under the same electric power. But when electric power increased the rise of white LDE junction temperature is not obviously. Through the white LED simulation and experiment results, we know yellow light efficiency is higher than blue light below electrical power 0.8W (about 140°C), and when electrical power exceed 0.8W, the yellow light efficiency may lower than the blue light even appear decay behavior. Because of the different material property between phosphor and chip, the chip decay rate is faster at low temperature and at higher temperature the phosphor decay rate is faster.
Finally, we simulate three common packages: Filling, Molding and Remote, analyze the difference of light, heat and color at different power. The result shows Remote package has better light output power and lowest color temperature shift, it can apply to lighting. The other packages connect with chip and copper slug have stronger heat exchange. Molding package has uniform angular color distribution at different power, but the temperature is higher for its small volume, so color temperature shift drastically and light output power decay faster. Filling package color temperature shifting is more stable than Molding package, but it has poor angular color uniformity. For the common LED screen or backlight components must have better light output power and angular color distribution at high power. We use the advantages of Molding and Filling packages to propose improvements: the volume of phosphor encapsulates cannot too small, it should contact with chip and copper slug and should be low concentration. These ways can improve Molding and Filling packages disadvantages, and design better phosphor packages. |
