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LED light module and driver power design
It is known that each LED has a little different drive voltage. A well designed, effectively compensated driver eliminates / reduces non-uniform currents appearing on each LED. LED light suppliers proclaim their power supply units as constant current output, but in-depth studies / analysis often reveal contradictory results. (See Table 2.)
Table 2. Uniformity
Higher the junction temperature lower the LED life span. All LED products form Edisonís portfolio guarantee a 50,000 hours life-time, by ensuring that the LED junction temperature is controlled below 75įC and heat sink temperature variation is maintained below 2įC. Edison and its vendors employ the DOE methods to analyze the effects of the fin pitch, thickness, height of heat sink to evolve best solutions. Our heat sink design and size ensures a below 75įC junction temperature and a heat sink temperature variation of under 2įC.
Batwing reflector design provides the most uniform lighting distribution and needs to be supplemented with cut-off light pattern for glare limitation. The electrical components cover the LED driver and the LED module which eventually determine the quality and life of the ultimate product. It is a well known fact that each LED has a different drive voltage and it is no guarantee that all LEDs of a module driving at the same voltage be built into a street light. To overcome this handicap, one needs to ensure a very good driver design that compensates for this inherent electrical variation of individual LEDís and ensures the assured life of the LEDís and thus the product.
LEDís characteristics are entirely different from traditional lamps where raising heat would mean raising efficiency, whereas with LED, raising heat would mean destroying the LED, - not just going down on efficiency. Table 1 above illustrates a general comparison of various light sources. Design and manufacture of LED light fixtures employs careful consideration of thermal, mechanical, electrical, optical and control factors for a value for money product.
For the past few years, LED street lights have been widely talked about for the mere fact that there exists a huge market and an LEDís power saving performance is evident particularly in replacing the 250 W or 400W mercury lamps. In order to build an LED street light one needs to ensure the minimum life span of five years for the LED, the light pattern should meet the roadway regulation speculations, dust and water protection to IP65, and sufficient mechanical strength to ensure longevity which has to be borne in mind. Based on these requests, one needs to consider the thermal, optical, power and mechanism to design LED street light. LED life depends on LED junction temperature. Figure 1. Data shows the relationship of life versus light maintenance for different junction temperatures. In order to guarantee a 50,000 hours life, the LED junction temperature has to be controlled to less than 75 Degree C.
Comparison of LED and traditional lamp
Over the past few years, LED light-out efficiency has largely risen from 35 lm/W (2006) to 100 lm/W (Q1í2009). With the increase of LED efficiency LED products that replace traditional lighting have been deeply anticipated, especially in high power consumption products. However, LED product quality is an issue which has to be taken seriously as fly by night manufacturers are destroying and tarnishing the real potential of this very beautiful light source
LED street light design technology
Table 3. Current of different LEDs
Generally, the LED light module is designed by series - parallel combined circuit; (Fig. 4). But, since every LED drive voltage is different, so each LED gets a different current even though a constant current design was employed. In such a scenario, if one LED fails, it quickly extend to others, through a chain reaction due to other LEDís passing more current.
Edisonís LED fixtures employ the state of art multi-channel circuit design on both the power and LED light module (Fig. 5); single channel power output constant current to the single series LEDs to ensure each LED gets the constant current. The measured result shown in Table 3, where a very uniform current passes to each channel. The constant current of using multi-channel design is not influenced by LED voltage variation.
Current of multi-channel design
A well designed LED fixture is a combination of thermal, optical, mechanism and power components and reflects on their life and efficiency