1.Introduction

In recent years, LED lighting efficiency has largely increased from 35lm/W (2006) to 100lm/W (Q1’2009). Judging from the increasing LED efficiency, LED products are expected to replace traditional lighting, especially in high power consumption products. Due to the extensive lighting market, there are various new market entrants in the recent years. However, the quality of the LED products became a huge concern, especially among the new companieshad not been the expert in the lighting field before. On the other hand, LED’s characteristic is obviously different from that of the traditional lamp. For example, increase of halogen lamp efficiency is depended on increasing the heat, but overheating is a problem for LEDs. Directivity of LEDs are very different from that of traditional lamps. Differences between LEDs and traditional lamps are showed in Table I. Due to the existing differences, the thermal, optical, power, mechanism, and control of LEDs becomes crucial in their design as lighting products.
 
In the recent two years, the topic LED streetlights have been widely discussed due to the scale of its  market and its power-saving performance, especially for replacing the 250W or 400W mercury lamp. To build LED streetlights, the key issue is to reach and meet a specific specification. For example, it needs to guarantee a lifespan of 5 years (though LED lifespan is approximately 50,000hours); meet the specific light pattern requirement to comply with roadway regulations, reach dust and water-resistance at IP65, provide enough mechanism strength, etc. Based on these requirements, the design of LED streetlights need to take into consideration the thermal, optical, power, and mechanism. LED lifespan depends on the LED junction temperature, (see Figure 1). Cree has published data to show the relationship of lifespan versus light maintenance for different junction temperature. Therefore, to guarantee a 50,000 hours life, the LED junction temperature has to be controlled under 75C. As for the illumination uniformity of roadway, the batwing light pattern is inevitable. The cut-off light pattern for glare limitation is also required. The other important subject is the driver (power) design and LED light module design. As there is minor difference in drive voltage of each LED,  uniform drive voltage of LEDs built into the streetlights cannot be guaranteed. Therefore, an imperfect circuit design will induce a non-uniform current to drive LED which will impact its lifespan. AOP understands that issues related to its designwill directly affect the quality of LED streetlights, hence, AOP has combined thermal, optical, power, mechanism in its LED design to ensure the quality of the streetlights.

Table 1: Comparison of LED and tradition lamp

Figure 1: LED light decay vs life-time (Source: Cree)

2.LED street light design and analysis
 
An excellent LED lighting product has to take thermal, optical, power and mechanism (industry design) into consideration.

2.1 LED streetlight thermal design

Poor LED junction temperature control affects the lifetime of LEDs. To guarantee 50,000hours life-time, AOP has designed streetlights which LED junction temperature controlled under 75°C, and heat sink temperature variation constrained below 2°C to ensure all LEDs applied on the streetlights reach the same lifetime. AOP has found the optimal solution using a DOE method to analyze the effect of the fin pitch, thickness, height of heat sink, (see Figure 2). A reasonable heat sink size to reach designed target is found: LED junction below 75°C and heat sink temperature variation under 2°C.

Figure 2. LED Optimal heat sink design

2.2 LED street light optical pattern design

As mentioned above, to apply the streetlights on the roadway, it needs to meet roadway illumination specifications, i.e. the optical pattern has to meet the uniformity (>0.33) of roadway, as well as glare rating. In addition to meeting the roadway regulation specification, it is also crucial to ensure that the light output projects over a large area. Since if the streetlight only projects over a distance of 3 times its mounted height, it would mean that at 6m tall, the light output is only distributed over 18m of roadway. Therefore, as there is a specific length roadway needs to install street light, then you need more street lights to apply to meet the roadway specification and avoid the zebra pattern appeared. Hence, to avoid increasing the quantities of street light, AOP designed a batwing optical light pattern (patent pending, the pattern has measured by ITRI, see Figure 3), which can reach 4 times mounting height (max. cd located on 60 degree) and illumination uniformity is large than 0.33 required, see Figure 4 and 5. This is a case study for AOP street light installed on 8m pole height, and projected on 32m roadway length. The illuminance distribution is showed in Figure 4. This light optical pattern can largely reduce the street light installation quantities as on a specific roadway length. By the way, the optical pattern is also met the cut-off requirement. AOP also provided .IES file for designer to design the different roadway project.

Figure 3. LED light pattern            Figure 4 Illuminance on roadway
        (Patent Pending)
    
Table 2. Uniformity

2.3 Mechanism and Industry Design (Patent Pending)

In AOP’s street light, we also considered the moving and installation convenience, see Figure 5 and 6. In the power space, there has installed a socket, so the operator can easily install the city cable into the socket and its process is 100% same as traditional lamp installation. By the way, the bow shape design in the front of luminaire can let the operator easily moved. This luminaire is a dust- and water resistant and rating reaches IP 65.             

Figure 5 LED Bow shape design       Figure 6. Power Cover and Socket

2.4 LED light module and driver power design

As we known, each LED existed a little different drive voltage, so how to avoid the non-uniform current appeared on each LED, it became a very important key design. Although, every street light supplier says their power supply is used constant current output, actually after measured the LED current and found there existed a very large variation, see Table 2. That was due to the LED light module is designed by series and parallel combined circuit, see Figure 7. So, if each LED drive voltage is not the same then LED will get different current even though you used the constant current design. Hence, if one of LEDs failed, then it will be quickly extended to others which was due to the other LED will pass more current. AOP’s street light is different from other suppliers design, we used the multi-channel circuit design (patent pending) on both of the power and LED light module, see Figure 8, per channel power output constant current to the single series LEDs to ensure each LED passed the constant current. The measured result, see Table 3. it showed a very uniform current pass to each channel. The constant current of using multi-channel design will not be influence even though the varied LED voltage.                

Figure 7 LED module series and parallel circuit


Table 4: Current of multi-channel design

4.Conclusion:

AOP’s street light design is combined thermal, optical, mechanism and power, hence, all of the possible effects on the LED life are all considered. That is why AOP is very confident about its street light product. Additionally, based on ITRI measurement data, AOP's street light luminaire’s efficacy already reaches 72.8 lm/w, its performance is hard to rival with at this time.

Tsung-Chieh, Chen, P.h. D
System Integration Development Division
Alliance Optotek Corp.