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Of high-power white LED street lighting

In Electronic Infomation Category: R | on December   1, 2010

LED development so far, nearly 30 years, made considerable progress in white LEDs made more than ten years experience honed, developed rapidly. In the energy shortages, pollution threat posed by LED came into being, much attention. To ease the energy crisis, the mid-90s of last century, the Japanese government take the lead, following the U.S. government financial support from the national technology and DAC7744E datasheet and industry to promote the development of solid-state lighting. Chinas government (led by Ministry of Science, including the Department of Energy, Economic and DAC7744E price and Trade Commission, Ministry of Information Industry, as well as local governments) is also strongly supporting and DAC7744E suppliers and funding the development of LED, and set up the National League of the semiconductor industry to promote Chinas solid-state lighting (China termed for semiconductor lighting) plan, and is expected in 2010 white LED light efficiency can be increased to 100 lm / W, a large number of alternative light effects and can be used about 90 lm / W fluorescent lamps are widely used in general lighting, [1] [2] . In such a situation

a large number of enterprises in China, investors, researchers and technical staff have involved large-scale investment in R & D and production of LED development has so far estimated to total several billion yuan has been invested in less than a decade time have made outstanding achievements, forming a large-scale industrial chain.

The world, especially white LED on the LED so has its inherent emphasis on reason, the current huge amount of energy consumption and resulting energy shortages, rising prices, particularly caused by the environment have resulted in energy conservation become a very urgent task. Countries a large part of the energy consumed for lighting. China and the developed countries, the total lighting electricity generation accounts for about 12 to 13%, this is a very substantial energy loss. At present, the light source around the world still using inefficient incandescent light source-based classes, from tapping the potential is promising.

LED area has a large number of talents, achieved brilliant results, some researchers announced that they had developed the LED sector under certain conditions, to achieve a 120lm / W or 150lm / W in luminous efficiency. Most of the enterprises which have also claimed that their products have reached the light efficiency 70lm / W, 90lm / W or higher, because LED light radiation is directed, and its light efficiency than the radiation output of the light source alone to the uniform general directional lighting efficiency of light reflection is higher, so identified the best features of the current LED lighting and lighting in all areas of vigorously promoted a few years ago has been declared in 2010 will be largely replaced LED compact fluorescent lamps, large-scale into the home lighting, and asserted that the best white LED is the current road lighting.

Beyond doubt that after 20 years of development, LED in the display area has achieved complete success and has become a free and on the ratio of the display can not be replaced. In the decorative lighting and fully demonstrate its features to obtain half of the 2008 Olympic Games opening ceremony on the outstanding performance of LED is more shocking the world. But must address the fact that the function of white LED lighting is still great limitations. China is the worlds largest LED production base and consumer market, LED broad areas of application, but because of the current technical level and the actual performance of the constraints, such a device is not used for functional lighting with its directors. The so-called high-power LED, can be found on the market today, but also the maximum power devices 3W, even research and development in the 10W device can be mass produced, but for general lighting, road lighting, especially the use of hundreds of watts of light source, the single grain loss of power is too small. Using a white LED lighting design the dosage must be great, whether it is difficult to design or cost would be worth the candle.

Ⅱ LED light effect

General evaluation of LED is a high light efficiency and long life, many manufacturers claim their products luminous efficiency of up to 90lm / W or higher, but these data only certain laboratory tests when the LED is still in the initial stages of cold highest when measured data. The so-called 50,000 hours or 100,000 hours of life is just an early estimate of low-power single-color LED results.

LED light-emitting mechanism from the theoretical light efficiency can be estimated. As is well known is the use of LED carrier recombination and light, the carrier recombination time all of its potential energy into light energy, this process alone, in terms of their internal quantum efficiency of 100%. But in the medium carriers and the lattice energy of the collision and the loss of the composite when the carrier kinetic energy carried by, as well as to overcome the external circuit resistance and energy consumption are not translated into light. Take into account additional energy consumption, the actual carrier recombination within the quantum efficiency does not exceed 90%.

Carrier recombination photons produced when a 50% chance for the outgoing radiation, which some 50% of the photons in the collision with the lattice and the lattice absorption into heat. Part of the photon transmission at the interface of different media will be reflected back into the original medium was absorbed, in addition to the output window of the outer electrode as a metal or transparent conductive film retina will also allow some reflection by absorbing photons, these processes will reduce the LEDs Quantum extraction rate, set this part of the photons quantum extraction rate of 80%.

Carrier recombination photons produced when a 50% chance within the radiation, this part of the photons reaching the bottom doubles as a mirror reflection conductive film will be part of the output light into the outward, but inward in this part of the photon or outward transfer process will be partially absorbed by the collision with the lattice, and in the interface through different media, also will be part of the reflected or refracted back to the original media was eventually absorbed. The reflection mirror base efficiency and makes the process within the limitations of photon radiation into the quantum of light extraction rate of output will not exceed 40%.

Based on the above analysis we can estimate the total LED electro-optical conversion efficiency is about 54%, which is very ideal circumstances the estimated results. Any omissions in the manufacturing process, materials are any defects on the energy conversion efficiency will result in the decline. With visible light conversion efficiency compared to less than 5% of the incandescent lamp, and even with the highest conversion efficiency of the current high pressure sodium, ceramic metal halide lamp than (electro-optical conversion efficiency of about 30%) is also very high, this is a great lure LED The reason people foreground.

As is well known, all the energy into the 1W 555nm wavelength of yellow light flux generated by up to 683lm / W (683lm / W optical power that is equivalent), if all is converted to white light is about 360lm / W. As described in the previous estimate, made in a very ideal case the maximum LED 555nm yellow light may reach about 300lm / W maximum light efficiency, the above estimate is far from being achieved at present. Reported in the current maximum light efficiency LED actually only reached half the ideal value, but actually less than the 1 / 4, which is one theory as there is great potential to be LED, on the high hopes reason. Improve the efficiency of LED light

key is to improve the extraction rate of the quantum, which is to minimize the internal photon absorption, which is the development of ultra-thin LED to the cause. The highest luminous efficiency of the ultra-thin (thickness of tens of nanometers) GaN LED, the conductive film using a high reflective mirror film, and the output window Zhicheng rough surface structure, to reduce the photon inward reflection, but even so the present report However, the maximum luminous efficiency 150lm / W. [3]

Above analysis is carried out monochromatic LEDs will have to re-monochromatic light into white light through a quantum conversion, now the center of most used wavelength of emission 470nm blue LED to excite the central wavelength of radiation spectrum 560nm broadband yellow phosphor to blue and yellow made from a mixture of white LED, but this will reduce the LED light effect taking a step forward. The photon efficiency can be estimated as follows:

If you leave the LED radiation output of about 20% of the blue, with the remaining 80% of the blue excitation yellow phosphor, which in the best case, about 80% of the Blu-ray photons will be more than 20% the phosphor absorption, the remaining 80% in the transfer to the center wavelength of 560nm as the broadband yellow radiation, a process that produces an average of 16.1% of the quantum energy loss. Data can be combined with the previous estimate of total energy efficiency white LED does not exceed 40%, converted to white light when the total efficiency white LED at most 150lm / W. If no other breakthrough, this is the current white light LED may reach the highest efficiency, the luminous efficiency and the highest luminous efficiency of the current source, such as the high pressure sodium HID lamp or ceramic metal halide lamp is higher than some. But now efficiency has not actually achieved, and the half light.

We have conducted this experimental study, we use a companys 1W high brightness blue LED, the spectral center wavelength of 470nm radiation, see Fig.3, except to the different thickness of the coating after packaging, respectively, the radiation central wavelength of 560nm of the yellow phosphor coating, the emission spectrum measured after treatment. More than twenty different from the thickness of the samples selected four typical cases, the results are shown in Fig.4 a, b, c, d and Schedule. Best case (Fig.4 c) of the luminous flux output of 38lm (initial cold about 50lm), color rendering index of 78.

A well-known multinational companies and technical personnel to present their initial 1W white LED luminous efficiency reached 70lm / W, the normal operation of pn junction temperature rise due to light effect reduced to 50lm / W.

2007, Japan Institute of Materials Science, Nano Ceramics Center for the result is the highest [3], they used the center wavelength of 450nm InGaN blue LED and the photon conversion efficiency, in the range of 450nm blue light has a strong absorption peak low temperature coefficient of the three phosphors tested, these three phosphors were: α-sialon: Yb2 +, α-sialon: Eu2 + and Sr2Si5 N8: Eu2 +. The light-emitting phosphors are characterized by low efficiency of thermal decay, remains at 150 ℃, 85% of the initial value. The emission spectra of the three phosphor display such as Fig.5.

InGaN blue LED with phosphor, and 1 white LED made of the emission spectrum shown as Fig.6, the luminous efficiency of 55lm / W, color temperature 4500K. I know this is the highest luminous efficiency white LED report (according to the testing of domestic 1W white LED luminous flux of 40lm mostly below). 2 and 3 with mixed trial phosphor emission spectrum of white light LED display, such as Fig.7, it increased to 82 color-rendering index, color temperature 4200K, but the light effect is indeed reduced to 20lm / W.

As mentioned earlier, LED light-emitting n-type semiconductor by the load current (electrons) in the role of foreign potential to overcome the potential barrier into the p-type semiconductor and the hole capture to generate composite luminous. Usually the larger the temperature coefficient of the semiconductor, the resistivity with increasing temperature decreased, the barrier potential will also be down at the end, while its long-wave radiation direction of the center wavelength will shift (redshift) leaving the phosphor luminous efficiency decreased.

Current white LED running up to 20% of the energy in the form of photon radiation, the remaining 80% are converted into heat energy to make the chip and the phosphor coating and heating. For low-power LED, 80% of the input power heating the chip temperature rise will not make too much light on the LED efficiency, lifetime and operating conditions have little effect. 0.5W or less power, but the white LED, more than 80% of its power concentrated in very small pn junction area, will make the chip and the phosphor coating substantially warming, which not only affects the light efficiency, and could lead to rapid lumen and to shorten the life span.

5 million or 10 hours of long-life LED to determine the early development in single-color LED for low power estimation for high-power LED, especially the white LED, the chip and phosphor long baking at high temperatures, will the life of the decline. This is poorly designed LED lighting project in the operation of 3 to 4 months after the dramatic decline of the light source so that the designer had to be replaced often the reason.

Ⅲ LED and road lighting

White LED because of the huge investment and excessive publicity, to make this area of the white LED to send some personnel to high expectations, eager to push the conventional function of white LED lighting, and many people in this area directly extended to the road lighting, the past few years the country completed LED Solar LED street lighting and road lighting no less than dozens of model projects (too much of the study, be used as the light source LED road lighting project) has made more than enough experience and lessons to be summarized. I believe that this test should be a temporary slowdown, and then restart when conditions are ripe.

Present, China and developed countries, the main road lighting high pressure sodium light source is still a small number of metal halide lamp, the former light efficiency is 100 ~ 120lm / W, but the color rendering index of only 20 or lower, the latter optical efficiency 80 ~ 100lm / W, color rendering index of 80 or so, taking into account the energy consumption of electrical accessories, the use of such sources in the actual system optical efficiency 75lm / W ~ 90lm / W between. As used in road lighting lamps light reflection rate is not high, and is often below 60%. When using such fixtures direct light around 40% of the ground, the remaining 60% of the light reflection is required by the lamps after the shooting at the ground. After removing all rough estimate losses total conventional road lighting does not exceed 76% of light utilization, which is estimated around the end of the light efficiency 60lm / W ~ 70lm / W. Furthermore, since the lighting design of the unreasonable and high pressure sodium light special slender column of light caused by uneven distribution, for the road alignment and the dark area between two lights to lighting standards, the design had to use more power, more High luminous flux of light illumination of dark areas to meet the requirements, thus resulting in more energy waste. Based on the above road

LED lighting designers that the use of directional radiation performance LED road lighting design, can significantly reduce energy waste, they think white LED is more energy than the high-pressure sodium lamp. They designed a number of use of high-power (1W, 3W) white LEDs street lighting program, and made a model project. However, most of these model projects have not been carefully tested and certified. Lighting design is one of the 120 1W white LED matrix distributed in 200 × 250mm rectangular planar circuit board, after the wiring board, and then the whole street lamps installed in the conventional. This design is very unscientific, the program selected LED single light of not more than 40lm, estimated to 40lm on it, taking into account the supporting electrical loss (10%) and 90% of the light utilization efficiency, the actual use of the system of light rate of less than 32lm / W, 120 LEDs total effective flux Zhi about 4300lm. If the LED light emission angle of 120 °, according to this design 4300lm light emission angle of 120 ° will be fired below. Imagine this design would make the ground very uneven distribution of light, middle of the street and the poles are completely dark. To these facilities on the LED substrate did not consider to take effective measures to cool, so much focused on such a small LED space, more than 80 watts of power will continue to accumulate the chip temperature is too high, stop the pressure drop decreased, the fat shift towards longer wavelength blue light, which will inevitably lead to a serious decline in their photosynthetic efficiency. Investigated the outskirts of Beijing Road, a photovoltaic lighting model (the total power source 105W) lamp illumination of only 3.5 lux, It is said that after two and a half, when the project has just completed lamp illumination 7 lux. The lamp outside diameter of 15 meters is not any lighting effects. In Jiangsu, Zhejiang, Shandong, Guangdong, similar to those project templates, for example, somewhere in Shunde, Guangdong and Shenzhen road somewhere in the LED lighting demonstration only dimly lit street, and every 3 to 5 months, must replace the lamp.

LED road lighting test must be under the guidance of experts, after a rigorous certification and design before implementation. Some specialize in Guangdong

LED road lighting engineering enterprises from access lessons, they have taken measures to 1W LED status of work in the 0.5 ~ 0.6W or 1W to 3W LED is designed to use, so that although the costs are high light efficiency is very low, poor lighting effects, but basically solved the problem of bad light and life.

A multinational view of Chinas white LED hot, specifically for the Chinese developed a white light for road illumination LED, 1W LED they will design the front of the packaging materials for a particular convex structure, alleged use of such LED without the second and third optical design can be uniform lighting, easy to use. The company claims that this LED luminous efficiency of the cold state of 70lm / W, compared with the normal use 50lm / W.

A company in Zhejiang for road lighting design integrated white LED, they will be more blue LED chip assembly on the same substrate, with a large convex lens specially designed for output windows, walls painted in the output window deposited phosphor, excited all the same LED chip, phosphor coating together. The combined LED light-emitting characteristics are modified and conventional spherical light source was similar to using a more convenient, but the structure of the substrate LED heat problem still exists.

Note that multiple LED lighting design of roads together, in addition to the flux and reasonable enough optical design in order to ensure a reasonable distribution of light, the more important is the heat. As mentioned earlier, the energy consumption of LED current of approximately 80% converted to heat, but not high temperature semiconductor devices, and if the heat generated can not be guided walk is bound to cause immediate serious consequences.

Carried out with the LED road lighting conditions is not yet mature, it only remains to be LED light efficiency improvements and increased power and more cooling to be effective LED programs and measures to solve, of course, the optical design is very reasonable important.

Ⅳ U.S. Department of Energy on the commercial assessment of the effectiveness of LED products (CALiPER)

LED light efficiency and lifetime in the indicators, there is a big difference of opinion at present, one important reason is the absence of uniform standards and testing methods, such as lighting industry has always been to stabilize the running of the flux output and luminous efficiency as its product parameters indicators, while the LED industry is generally the initial (cold) and the maximum luminous flux and luminous efficiency as the standard color index in the measurement methods and standards there are also many differences, life standards and measurement methods are different, the data far, the evaluation also quite different.

This issue is not only our country, in foreign countries is equally serious, the U.S. Department of Energy had discovered them from December 2006 to September 2008 conducted a total of seven market product survey, only in 2008, respectively, on a continuous January, May and September to three checks, and in September announced its "commercial LED products, the effectiveness of assessment and reporting" CALiPER (Commercially Available LED Product Evaluation & Reporting), their published data and concluded that the negative , in the 24 products tested, only two categories (under shoot down light) data consistent with product specifications, except for a few LED luminous efficiency of 62lm / W, the average, only 32lm / W, low-less than 10lm / W, far away far from being able to reach Energy Star standards. For information on the data exaggerate the confusion and made a very harsh and negative conclusions, including the optical efficiency, color temperature, color quality, light distribution and lighting system power factor. Especially the light effect, they found that publicity tend to exaggerate more than 2 to 3 times, and the average user does not understand this. A U.S. Department of Energy concluded that the need to establish uniform standards to regulate product quality and indicators, in our more prominent and important these issues.

Ⅴ Conclusion

LED is a good promising light source, light-emitting mechanism of its specific features and given it a range of features. Because of its light is small, medium density, light is very concentrated, very high brightness. Even if the power is very small, small LED luminous flux but also to give the human eye in a clear, bright, accurate optical signals to indicate, show, have no advantage compared with the theory, so its application in the display area has been extremely broad, everywhere. Low-power single-color LED decorative lighting and art lighting also has demonstrated the unique features and charm, the 2008 Beijing Olympic Games give full play to the LEDs such a function.

White LED LED is an outstanding member of the family, has many features and characteristics. Development of one of the important objectives of white LED is used to replace conventional power sources such as incandescent, fluorescent, high intensity discharge lamps, etc., but it seems its rapid development as expected. White LED on the market today is about the actual luminous efficiency 40lm / W, a maximum of 50lm / W, for the function of lighting, these parameters are to be increased. To promote the white LED is more important is cheap, efficient cooling systems.

As is well known, in addition to LED, all the light sources are working at high temperatures, several thousand degrees Celsius to ensure its efficient light conditions, but the LED light-emitting devices such as semiconductor temperature rise of the luminescence center is extremely harmful, if the problem can not be resolved, increased efficiency LED light, the power of the increase is limited.

Low power white light LED (1W and the following products) in a wide range of applications has shown a tremendous advantage, for example, in a small space, local lighting, garden lights, close-low-power flashlights, lamps and emergency lighting, etc. has a very broad scope for development. White LED current does not need, nor the conditions and competition in the high-power conventional light sources for road lighting market.

With different interior lighting, street lighting around the open space there is no reflective objects, the greater the flux requirements of road lighting, street lighting which is currently mostly used 250W, 400W high pressure sodium lamps because of high luminous flux, this lamp flux in the 28000lm to 48000lm range, Ruoyi 1W, 3W white LED to replace, the LEDs requirements too costly to the difficulty of design is also great, and the effect is difficult to perfect. LED lighting in the road in recent years has made a lot of lessons learned, to the current white LED light efficiency is not high, life is not long, I think that using LED for street lighting test should be suspended, pending successful development of more efficient, more powerful White LED and cheap and efficient cooling system before making progress.

Road lighting for the huge energy consumption, but also have great potential to be tapped, the transformation imperative, and Fudan University, recently co-Philips, which had a very useful research [5], they concluded that Ceramic metal halide high pressure sodium lamps are very good replacement.

In literature

[1] Hans Nikol etl; "Worldwide National Solid State Lighting Programs and Standardization Efforts. An Overview from Industrys Perspective"; "Proceedings of the 11th International Symposium on the Science and Technology of Light Sources May 20th-24th, 2007, shanghai, china "; P.475 - 478

[2] Wu Ling; "National SSL Program in China"; "Proceedings of the 11th International Symposium on the Science and Technology of Light Sources May 20th-24th, 2007, shanghai, china"; P.479 - 485 < / P>

[3] Volker H? Rle, B. Hahn, J. Baur; "With ThinGaN to 1000lm white"; "Proceedings of the 11th International Symposium on the Science and Technology of Light Sources May 20th-24th, 2007, shanghai , china "; P.487 - 488

[4] R.-J. Xie, N. Hirosaki; "White Light-Emitting Diodes Using (Oxy) nitride Phosphors"; "Proceedings of the 11th International Symposium on the Science and Technology of Light Sources May 20th-24th , 2007, shanghai, china "; P.491 - 492

[5] Lighting Engineering. 2008 VOL. 19 NO.3 back

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