LED Street Light Design Technology

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 emperature 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.

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. 

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.