Optimized semi-sphere lens design for high power LED package

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Abstract

In recent years, high power light emitting diodes (LEDs) have played a very important role in many fields such as LED TV back light or white LED illumination. However, most of past LED package researches have focused on the directivity of LED units; the issue of the extraction efficiency usually has been investigated limited to the chip level, but no much attention paid at the package level. However, this should be a very important topic especially for today’s high power LED output demand as we believe. In this work, utilizing a simulation tool, we show how the geometry of the package affects the efficiency of chip’s output power. An optimized semi-sphere package is proposed, which can improve the efficiency by a factor of 23% compared with the traditional cylinder package. Even considering a needed directivity (e.g. 2θ1/2=24°) for some specific applications, the semi-sphere module can still have 17% output power more than that of a domed cylinder package. These results are very valuable and can be used as a reference guide for the design of the high power LED package.

Highlights

► The optimized semi-sphere lens package for high power LED package is found. ► The simulation shows that 20% more efficiency is obtained than a cylinder package. ► This design can combine a hemi-sphere reflector to control the directivity. ► The efficiency with this new module is higher than a domed package by 17%.

Introduction

It has been acknowledged that solid state lighting with LED is the future trend as well as its increasingly wide application in large size backlighting for LCD display and automobile lighting, etc.; this popularity is mainly due to several excellent properties of LEDs such as low power consumption, good reliability, long life, short response time, variable colors and environmental friendly [1], [2].

To achieve better performance on extraction efficiency [3] at the chip level, there are many methods developed to improve it, such as truncated-inverted pyramid geometry [4], [5], [6], surface texturing [6], [7], and photonics crystal [8]. The author also suggested a method which combines omni-directional reflective base with the wavelike hemi-sphere array surface and the extraction efficiency can be improved by a factor of 40% as compared with that of the conventional LED and it agrees well with the simulation results [9].

At the package level it is known that the package material usually can improve the extract efficiency further due to its refraction index (n) mediation on the large difference between the LED chip (n  3.0) and the air (n  1.0). Therefore main topics of LED package hitherto focus on the control of the viewing angle (directivity), [10], [11] the color temperature adjustment and its uniformity in space [12]. In this work, we study another fundamental and very important issue about the high power LED package; that is, how the geometry shape and the size of package influence LED chip’s extraction efficiency. The simulation shows that with a proper design and optimized hemi-sphere package size, the extraction efficiency can be improved by more than 20% for both ordinary chip sizes and high power large ones, compared with the conventional cylindrical shapes. This result could be very useful for high power LED package and its practical applications.

Section snippets

LED model and package simulation

In order to investigate the roles which package shape and size play, the software called SPEOS is utilized; it can provide not only the directivity information but also the individual power budget of each part in a package LED module. Firstly we should build a simplified but reasonable LED model. As is well known, the real model of LED die is very complicated and most importantly is very variable for different manufactures, such as the following factors: the material used, the die size, the

Conclusion

From the above discussion, it is reasonable to infer that at the package level the semi-sphere geometry can offer the best output efficiency because any other polyhedrons or shapes suffer from more TIR than it does and thus have lower extraction efficiency; the simulation results tell that the efficiency difference can reach at least 20% for both small chip size and high power large one. Even at the module level, our semi-sphere package combining with a spherical mirror can afford better power

Acknowledgments

This study was supported by the National Chung Hsing University, Taiwan, ROC. This work was also supported by the NSC of ROC under Contract Nos. NSC 98-2221-E-005-018-MY3, NSC 98-3114-E-006-004-CC2, and NSC 100-2622-E-005-006-CC3.

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