高功率發光二極體之亮度特性

李佩璇

doi:10.6844/NCKU.2011.00595

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高功率發光二極體之亮度特性

Luminance Characteristics of High Power Light Emitting Diodes

李佩璇 , Ph.D　　Advisor：周榮華　　

英文 DOI： 10.6844/NCKU.2011.00595

發光二極體；接面溫度；熱阻；電性實驗法；順向偏壓法；光通量衰減；脈衝偏壓；數值模擬； ANSYS ； Light Emitting Diode (LED) ； Junction Temperature ； Thermal Resistance ； Electrical Test Method (ETM) ； Forward Voltage Method ； Luminous Decay ； Pulsed Bias ； Numerical Simulation ； ANSYS

Abstract │ Reference (124) │ Times Cited (2) │ Altmetrics

Abstract 〈TOP〉

隨著高功率發光二極體(Light-emitting diode, LED)元件發光效率與可靠度的提高，對環境汙染較小的發光二極體大量被使用在日常生活中，取代傳統的白熾燈泡及日光燈勢必是未來的趨勢。然而，發光二極體的光電轉換效率仍然過低，大部份的輸入能量皆以熱能的形式逸散，造成晶片接面溫度上升。發光二極體的接面溫度(Junction temperature)直接且深遠地影響其輸出特性與元件壽命。因此，準確的量測及評估發光二極體之接面溫度是必須且非常重要的。
在本論文研究中，透過積分球的光通量實驗，比較量測所得到的光通量，結果發現發光二極體的熱阻值並非影響光通量衰減的主要原因，光通量衰減幅度主要取決於發光二極體晶片隨溫度衰退的外部量子效率。使用有限單元分析軟體ANSYS 12.0建立三維數值模型進行暫態熱傳分析，並且以順向偏壓法分別量測四種不同顏色的LED晶片接面溫度。從暫態模擬可以看到發光二極體隨著點亮時間而溫升，模擬結果與實驗所量測到的溫升曲線具有十分接近的趨勢，其中由模擬所得到以氮化鋁為基板材料模型(紅色與琥珀色)與以藍寶石為基板材料模型(白色與綠色)的熱阻值分別為5.04°C/W與6.88°C/W。另外以電性測試法(又稱為順向偏壓法)對四個不同顏色(紅、綠、白和琥珀色)各二十顆的發光二極體進行接面溫度的量測。由實驗結果可以得知紅、綠、白和琥珀色的熱阻值範圍分別為：3.39~9.57°C/W、6.45~7.94°C/W、6.33~7.97°C/W與3.98~ 8.20°C/W。磷化鋁鎵銦族的發光二極體(紅光與琥珀光)的熱阻值變異性高於氮化銦鎵族的發光二極體(綠光與白光)。
另外，脈衝偏壓實驗的部分，在操作頻率1k Hz輸入電流工作週期(duty cycle)為80%下，分別對白光與紅光發光二極體施加445.7mA 和 418.4mA的順向電流，發現可以在達到與正常操作相同照度的前提下，分別降低白光與紅光發光二極體元件的溫度1.4°C與2.0°C。因此，白光發光二極體的使用壽命可以增加292個小時，衰減的光通量亦減少0.32%；紅光發光二極體的使用壽命可以增加658個小時，衰減的光通量則可以減少1.00%。本研究發展的方法可用於量測不同顏色發光二極體的熱阻值與光衰，有助於進行先進的產品設計和模組可靠度分析，以提升發光二極體的發光強度與延長使用壽命。所提出的非破壞性量測方式不侷限於特定的元件封裝型式，亦可廣泛地應用在其他的發光二極體模組。

Parallel Abstract 〈TOP〉

With the advancement of emitting efficiency and reliability of the high power light-emitting-diode (LED) module, which leads to less environmental pollution, the LED has been applied to a large amount of daily-life illumination and its replacement of traditional incandescent bulbs and fluorescent lamp is believed to be the future trend. However, the electrical-to-optical conversion efficiency of LED is still too low, and most of input power has been dissipated as heat, which results in the increase of junction temperature within the LED chip. The junction temperature of the LED directly and greatly affects its output characteristics and lifetime. Therefore, it is necessary and extremely important that the junction temperature of the LED be measured and evaluated accurately.
In this study, via the luminous flux experiment in the integrating sphere, the comparison of measurement results shows that the thermal resistance of LED is not the major determinant of its luminous flux decay. Obviously, the luminous flux decay of LED is dependent on the external quantum efficiency of chip material being used. A 3D numerical simulation, based on commercial software ANSYS 12.0, was applied to carry on transient thermal-conduction analysis, and the measurement of diode forward voltage was implemented to estimate the junction temperature of four different LED samples. The simulated temperature for LED model was found to increase with time, while the slug temperature also showed the same trend in simulation and experiment. The simulated thermal resistance were 5.04°C/W and 6.88°C/W for AlN substrate model (red and amber LED) and sapphire substrate model (white and green LED), respectively. The Electrical Test Method (ETM) (also called the forward voltage method) was used to measure the K factor and junction temperature of LEDs with four different colors (red, green, white and amber). Twenty samples for each color. The experimental results show that the ranges of the estimated thermal resistance for red, green, white, and amber LED samples lie between 3.39 to 9.57°C/W, 6.45 to 7.94°C/W, 6.33 to 7.97°C/W, and 3.98 to 8.20°C/W, respectively. The thermal resistance of an AlGaInP-material-based LED (red and amber) reveals higher variability than that of an InGaN-material-based LED (green and white). 
In the pulsed bias experiment on the basis of 80% duty cycle of input current and 1k Hz operation frequency, the input current of 445.7mA and 418.4mA is respectively required in the white and red LED samples to provide the equivalent illuminance as in normal operation. Moreover, the values of the LED slug temperature appeared to be lower 1.4°C and 2.0°C as compared to the results in the normal steady operation. It is found that a white LED which is activated by the form of duty cycle having 292 hours longer life time than by the normal operation, and the decay of luminous flux would be reduced 0.32%. The similar condition has been found in red LED, and 658 hours longer life time can be achieved under duty cycle than normal operation, and the decay of luminous flux would be reduced 1.00%.
The method developed in this study can be used to measure the thermal resistance and luminous decay of LEDs with different colors, and it is helpful in advanced product designs and module reliability analysis to enhance the light intensity and prolong the lifetime of LEDs. The proposed non-destructive measurement method is not restricted to a particular packaging form, and it can be widely applied to other LED modules as well.

Reference ( 124 ) 〈TOP〉

[7] U.S. Department of Energy, “Lifetime of White LEDs”,
連結：
[8] X. A. Cao, S. F. L. Boeuf, L. B. Rowland, C. H. Yan and H. Liu, “Temperature- Dependent Emission Intensity and Energy Shift in InGaN/ GaN Multiple- Quantum-Well Light-Emitting Diodes”, Applied Physics Letters, Volume 82, Issue 21, pp.3614-3616, 2003.
連結：
[11]G. Murtaza and J. M. Senior, “Method for Extracting Thermally Stable Optical Singals from a CaAlAs LED Source”, IEEE Photonics Technology Letters, Volume 7, No. 5, pp. 479-481, 1995.
連結：
[15]A. A. Bergh, “Bulk Degradation of GaP Red LEDs”, IEEE Transactions on Electron Devices, Volume 18, Issue 3, pp.166-170, 1971.
連結：
[16]R. A. Sigsbee, “Electromigration and Metalization Lifetimes”, Journal of Applied Physics, Volume 44, Issue 6, pp. 2533-2540, 1973.
連結：

Times Cited (2) 〈TOP〉

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