Optical Simulation Analysis of High Power LED Package Structure

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Optical simulation analysis of high power LED package structure

Yinong Liu a,b,c Yiping Wu a,b Bing An a,b,* a Huazhong University of Science & Technology, Wuhan 430074, China

b Wuhan National Laboratory for Optoelectronics, Wuhan 430074, China c Shenzhen Research Institute of Huazhong University of Science & Technology, Shenzhen 511816, China

*corresponding author at: College of materials science & engineering, Huazhong University Science & Technology, Wuhan 430074, China. Tel.: +86 27 87544454; Fax: +86 27 87792402; E-mail address: [email protected].

Abstract The single LED package structure determines

the light intensity distribution and the well-designed package structure will be conductive to light emitting and improve its external quantum efficiency. This paper based on the optical package structure of high power LED, in order to analyze the influence of the structure of LED package on optical performance and the feasibility of the packaging structure design, the simplified high power LED optical models were established in Tracepro software. The light distribution curve of LED was obtained and the difference between measured data and simulation result was compared. By changing the LED optical model parameters in the experiment, such as: the shape of reflector, lens design and the position of phosphors to get the light distribution curves under different parameters of package structure and then we analyze the effect of various packaging structures on optical performance to find the package structure optimization, so that it could be used in actual production getting a higher available luminous flux and the light extraction. What’s more, designing a package structure which can be achieving the specific light intensity distribution meets the requirements of the LED light source in different areas.

1 Introduction

Since the first red LED(Light Emitting Diode) that was invented by Holonyak and Bevacqua in 1962[1], LED has a wide application in illumination markets due to its advantages of high efficiency, low power consumption, environmental friendliness, long life, and small size. The market for high power LEDs is growing rapidly in various applications such as large size flat panel

backlighting, street lighting, vehicle forward lamp, museum illumination and residential illumination [2]. With the rapid development of compound semiconductor technology, its luminous efficacy far exceeds the commonly used incandescent light bulb, fluorescent lamp and HID lamp. It has been widely accepted that LED solid-state lighting will be the fourth illumination source to substitute those lamps.

As a new type of light source, its potential value is receiving more and more attention. The main function of LED packaging is to protect the LED chip, enhance the light extraction and provide a path for dissipating the generated heat [3]. Through the Secondary Optics Design to optimize the light distribution of LED, the LED emitted light is more reasonable to meet the requirements of all kinds of applications. In the optical design for the packaging, we should take how to achieve the high luminous efficacy into consideration. However, in LEDs, photons travel in random direction and there are too many photon trajectories to consider making the quantitative analysis extremely difficult. In this paper, we used Tracepro software based on Monte Carlo method to trace the photons, from generation to coupling out of the lamp, statistically using random numbers [4].

2 Optical model

The basic composition structure of the high power LED include: LED chip, pins, gold wire, reflecting cup, lens, phosphor, and substrate. A cross-sectional structure of a Luxeon package is shown in Fig.1.

2011 International Symposium on Advanced Packaging Materials (APM 2011)978-1-4673-0149-7/11/$26.00 ©2011 IEEE

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Fig. 1. A cross-sectional structure of a Luxeon package

In this paper, we mainly study the influence of LED package structure on the light extracting rate, whether the model structure of LED is in accordance with the output optical light distribution requirements. From this aspect, we could use a simplified model ignoring the base, pins, metal wire, substrate and focus on the position of reflecting cup, lens type and size, phosphor. The schematics of an LED lamp and simplified model established in Tracepro software is shown in Fig. 2.

a

b Fig. 2 Schematics of an LED lamp (a) and a simplified model (b)

We plan to define the material property of epoxy resin (refractive index 1.5), surface source of LED chip (light emitting surface of Lambertian type, luminous flux of 1lm) and the surface property of inner surface of the reflector (80% reflectivity, 20% absorptivity). Using the Tracepro software to ray tracing and get the emitted light

intensity distribution curve based on this LED model.

3 Simulation results and Discussions 3.1 Reflecting Cup

A significant number of the photons coupled out of the chip would be directed laterally and they would be mostly reflected from the reflecting cup [4]. Consequently the shape of the reflecting cup has a closely relationship with beam angle. The reflecting cup has the shape of a truncated cone, then we keep the top surface radius and height in constant and change the bottom surface radius, that means changing the cup slanting angles, simulating the LED light extraction rate and light emitting space angle. The simulation results are shown in Fig. 3.

��variable radius parameters for simulation�Bottom surface radius

(mm) Initial luminous flux

(lm) Emitted luminous flux

(lm) 0.5 1 0.54 0.7 1 0.56 0.9 1 0.61

a

b

c Fig. 3 Simulation of rectangular candela distribution plot

(a) radius 0.5mm (b) 0.7mm (c) 0.9mm�


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It can be seen from the figure that with the bottom surface radius of LED reflecting cup decreasing, the cup slanting angle becomes lager and the LED light extraction rate increases. Because along with the increase of the cup slanting angle, the light emitted from the reflecting cup will meet less reflections and the energy loss is also substantially reduced, leading to improve the LED light extraction rate. Meanwhile, the beam angle increased at first and then decreased with the reduction of bottom surface radius. The reason is that with the further increase of the cup slanting, the light emitting out of the chip reaches to the top surface of the reflecting cup and total reflection may occur in the top surface of the reflecting cup when light propagate to air (optically thinner medium) from epoxy resin (optically denser medium), thus the beam angle will be decrease [5].

From the above analysis, we can see that the reflecting cup slanting angle has an impact on the light extraction rate and beam angle, selecting the appropriate one is very important.

3.2 Encapsulating Lens

The encapsulating lens is normally used to change the light distribution by the refraction of materials and reflection on the interface; different size and type of lens have a significant impact on the candela distribution plot.

3.2.1 Lens Size The sidewall cylinder radius of reflecting cup

is equal to the lens radius and we change the lens radius without changing other parameters of LED during the simulation. The simulation model and results are shown in Fig.4, Fig. 5, respectively.

a b c �

�� Simplified optical model (a) radius 1mm (b) .4mm (c) 1.8mm

a b c

Fig. 5 Simulation of polar candela distribution plot (a) radius 1mm (b) 1.4mm (c) 1.8mm

The figure shows that with the increasing of the lens radius, the normal light intensity decreases and the light intensity is evenly distributed. What’s more, the beam angle is gradually increased.

3.2.2 Lens Type

Next, we simulated three kinds of lens types, the luminescence intensity space distribution are Lambertian, Side Emitting and Batwing, respectively. The simulation results are shown in Fig. 6.


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a

b

c �� Simulation of rectangular candela distribution plot

(a) Lambertian (b) Side Emitting (c) Batwing�The figure indicates that the emergent light

through different light emitting types of lens have distinct center intensity and beam angle. Therefore we should choose the appropriate lens according to different application requirements.

3.3 phosphors Nowadays there are three general approaches

to obtaining white light LEDs. One is the mixing light from monochromatic RGB LEDs. For the “green gap” problem and the efficiencies of red, green, and blue LEDs vary over time at different rates, the use of RGB LED has been limited. Another is PC LEDs, which means using a blue LED to pump visible light-emitting phosphors integrated into the LED package. It generates the white light by mixing the blue light from LED chip with the broadband yellow light excited by

phosphor [6]. The third method is based on UV LEDs.

The most commercially available white LEDs are single-chip white LEDs--PC LEDs. In this package structure, the phosphor is dispersed within an epoxy resin that surrounds the LED die. However, a significant portion of the light is backscattered by the phosphor and lost within the LED due to absorption and has a negatively impacts on the overall efficacy of white LED. In this case, U.S. N. Narendran Professor proposes a new package method named scattered photon extraction (SPE). The schematic of SPE is shown in Fig. 7.

�� Schematic of the SPE white LED package�

In the SPE package, the phosphor is placed at a remote location from the die. The geometry of the optic element plays an important role: it efficiently transfers the light exiting the GaN die to the phosphor layer and allows most of the backscattered light from the phosphor layer to escape the optic [7]. The new SPE method enables higher luminous efficacy and shows over 60 percent improvement in light output and efficacy compared to similar commercial white LEDs [8].

Conclusions

In this paper, we built a simplified high power LED optical models in the Tracepro software. By changing the LED optical model parameters in the experiment, such as: the shape of reflector, lens design and the position of phosphors to get a clear exposition of the general law about these factors affecting on the LED light intensity distribution and light extraction rate. These laws have a practical guiding value to LED packaging manufacturing


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process and production of specific LED intensity distribution.

Acknowledgments

I would like to thank my professors for their support and instruction. I also wish to thank Shenzhen Research Institute of Huazhong University of Science and Technology for its experiment condition.

References

1. Holonyak, J. N. and Bevacqua, S. F., "Coherent (visible) light emission from Ga(As1-XPx) junctions", Applied Physics Letters, Vol. 1, No. 4 (1962), pp. 82-83.

2. Craford, M. G., "LEDs for solid state lighting and other emerging applications: Status, trends, and challenges," 5th International Conference on Solid State Lighting, San Diego ,CA, August.2005, pp. 594101-594110.

3. Zongyuan.Liu, Sheng.Liu, Kai.Wang, Xiaobing,Luo, “Analysis of Factors Affecting Color Distribution of White LEDs,” International Conference on Electronic Packaging Technology &High Density Packaging, Shanghai, July.2008, pp.1-8.

4. SongJae, Lee, “Light-Emitting Diode Lamp Design by Monte Carlo Photon Simulation,” the International Society for Optical Engineering, Vol. 4278, (2001), pp. 99-108.

5. Ning, Lei, Shi, Yongsheng, Shi, Yaohua, Chen, Yangyang, “Influence of Package structure on LED Light Extraction”, Chinese Journal of Liquid Crystals and Displays, Vol.25, No.6(2010), pp.823-825.(in Chinese)

6. Schlotter, P., Schmidt, R. and Schneider, J, "Luminescence conversion of blue light emitting diodes," Applied Physics A: Materials Science & Processing, Vol. 64, No. 4 (1997), pp. 417-418.

7. Narendran, N, Gu,Y, Freyssinier-Nova,J.P. Zhu,Y. “Extracting phosphor-scattered photons to improve white LED efficiency,” Physica Status Solidi (A), Applied Research, Vol. 202, No. 6 (2005) , pp. 60-62.

8. Narendran, N, “Improved Performance White LED”, 5th International Conference on Solid

State Lighting, San Diego, CA, August.2005, pp.594145-594150.


Optical Simulation Analysis of High Power LED Package Structure

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