View This Issue - March 2009

TECH NOTE: LED street light design technology

Fig. 1
Fig. 1
Over the past few years, LED light-out efficiency has largely risen from 35 lm/W (2006) to 100 lm/W (Q1’2009). From the viewpoint of LED efficiency increasing, LED products that replace traditional lighting have been deeply anticipated, especially in high power consumption products. Due to the lighting market business being very large, there are many new companies joining in recent years. However, LED product quality has become a very big issue, especially in new companies’ products since these new companies are not experts in the lighting field.


Table 1. Comparison of LED and traditional lamp

CCT (K) Response Light-out theoryLight direction LifeReliabilityVibration resistantDisadvantages
LED 2600~10,000<1 secSemicondutorSpecific direction50,000 hoursHighHighHigh price
FCL 4000~8000<60 secHg vaporFull direction6000 hoursMiddleLowHg pollution, broken
Halogen 2500~3000<150 secHeatingFull direction1000 hoursLowLowPower exhaust, short life, fragile


On the other hand, LED’s characteristics are obviously different from the traditional lamp, ex. Halogen lamp efficiency raising depends on the heat increasing, but LED is very afraid of heat: worse LED temperature, worse light output. LED light direction is specific, not same as traditional lamp full direction. All of the differences between LED and traditional lamps are shown in Table 1. Since there exists obvious differences, the thermal, optical, power, mechanism and control become very important as one designs LED lighting products.

Fig. 2
Fig. 2
In the past two years, LED street lights have been widely discussed since there exists a very large market and the LED’s power saving performance is obvious especially in replacing the 250 W or 400W mercury lamp. To build an LED street light, the key issue is needed to reach and meet a specific specification, for example, it needs to guarantee 5 years (since everyone knows LED life is 50,000 hours), the light pattern has a specific request to meet the roadway regulation specification, dust and water-resistant has to reach IP65, mechanism strength needs to be enough, etc. Based on these requests, to design the LED street light, one needs to consider the thermal, optical, power and mechanism. LED life depends on the LED junction temperature (Figure 1). Cree has announced data to show the relationship of life versus light maintenance for different junction temperatures. Therefore, to guarantee 50,000 hours life, the LED junction temperature has to be controlled under 75C.

Fig. 3
Fig. 3 (patent pending)
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 about both the driver (power) design and LED light module design: we know that each LED has a little different drive voltage and we can not guarantee all of the same drive voltage LEDs have been built into the street light. So, a worse circuit design will induce a non-uniform current to drive the LED, which will impact each LED’s life. AOP knows all of the design issues that will directly affect the LED street light product quality, hence, AOP combined thermal/optical/power/mechanism to do a overall design to make sure of street light quality.
Fig. 4
Fig. 4

LED street light design and analysis
An excellent LED lighting product has to consider: thermal, optical, power and mechanism (industry design).

1. Thermal design
As we know, worse LED junction temperature, worse LED life-time. To guarantee 50,000 hours life-time, AOP has designed a street light in which the LED junction temperature is controlled under 75°C and heat sink temperature variation is constrained below 2°C to ensure all of LEDs applied on the street light can reach the same life. AOP has used a DOE method to analyze the effects of the fin pitch, thickness, height of heat sink and found a best solution; see Fig. 2. We found a reasonable heat sink size to reach designed target, LED junction below 75°C and heat sink temperature variation under 2°C.

Fig. 5
Fig. 5
2. LED street light optical pattern design
As the above mentions, if the street light wants to apply on the roadway, then it needs to meet roadway illumination specification, i.e., the optical pattern has to meet the uniformity (>0.33) of roadway, and glare rating. Except to meet the roadway regulation specification, how to let the light output project on the large roadway area is also important course. Since if your street light only can project on 3 times mounting height region, that means as mounting height is 6m, then the light output only distributed 18 m roadway length. So, as there existed 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 appearing.

Fig. 6
Fig. 6
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 Fig. 3), which can reach 4 times mounting height (max. cd located on 60 degree) and illumination uniformity is larger than 0.33 required; see Figs. 4 and 5. This is a case study for an AOP street light installed on 8 m pole height, and projected on 32 m roadway length. The illuminance distribution is shown 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 also met the cut-off requirement. AOP also provided an .IES file for the designer to design the different roadway project.


Table 2. Uniformity

96 Watt, 7156 Lm, 5200K
Max.29 Lx
Avg.13 Lx
Min.4.5 Lx
Uniformity (Min./Avg.)0.35


3. Mechanism and industry design (patent pending)
In AOP’s street light, we also considered the moving and installation convenience; see Figs. 5 and 6. In the power space, we have 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 dust- and water-resistant and its rating reaches IP 65.

Fig. 7
Fig. 7
4. LED light module and driver power design
As we know, each LED has a little different drive voltage, so avoiding the non-uniform current appearing on each LED becomes a very important key design. Although, every street light supplier says their power supply is using constant current output, actually after we measured the LED current, we found there existed a very large variation; see Table 2.


Table 3. Current of different LEDs

LED123456
ILED(mA)514.0348.0378.0235.0207.0273.0

That was due to the LED light module is designed by series and parallel combined circuit; see Fig. 7. So, if each LED drive voltage is not the same, then the LED will get different current even though you used the constant current design. Hence, if one of then LEDs failed, then it will be quickly extended to others, which was due to the other LED passing more current.

Fig. 8
Fig. 8 (patent pending)
AOP’s street light is different from other suppliers’ design; we used the multi-channel circuit design (patent pending) on both the power and LED light module (see Fig. 8); per channel power output constant current to the single series LEDs to ensure each LED passed the constant current. The measured result is shown in Table 3, where a very uniform current passes to each channel. The constant current of using multi-channel design will not be influenced even though the varied LED voltage.

Table 4. Current of multi-channel design

Channel12345678
ILED (mA)301.1300.8298.2301.5302.8298.4297.7298.1


Conclusion
AOP’s street light design is combined of thermal, optical, mechanism and power, hence, all of the possible effects on the LED life are considered. That is why we are very confident our street light product. By the way, based on ITRI measurement data, AOP street light luminaire’s efficacy already reaches 72.8 lm/W; its performance almost cannot be found in the world at this time.

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