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White LED luminous efficiency improve the four key - (AddTime -2013-02-16)

In the past LED industry in order to gain the full white 200W led Flood light beam, ever developed large size LED chip through this way trying to reach goals, but in fact the white light LED on the power for more than 1 w above beam it will drop, luminous efficiency is relatively lower twenty to 30%, in other words the white light LED brightness if better than the traditional LED tarsus times, consumption power characteristic hope beyond the fluorescent lamp, it is necessary to overcome the following four topics, including, inhibition of temperature rise, to ensure that the service life, improve the luminous efficiency, and equal access to the luminescence properties. To solve the heat dissipation problem 1 package is basic method

Due to the increasing power but can cause encapsulated thermal impedance acute drop to below 10 k/W, so the foreign investors ever developed high temperature resistant white light LED by trying to improve the above problems, but in fact the calorific value of high power 200W led flood lights, but than small power LED high dozens of times above, and the temperature rise will make luminous efficiency plummets, even encapsulation technology allows high quantity of heat, but LED chip joint temperature could more than permissible value, and finally industry finally understood to solve encapsulated heat dissipation problems is the basic method.

The service life of the LED, such as convert siliceous packaging materials and ceramic packaging material, can make the LED the working life is raised a digit, especially the white 200W led Flood light spectrum contains less than 450 nm wavelength of short wavelength light, traditional epoxy resin encapsulation materials can be short wavelength light damage, high power white LED large quantity of light more accelerated packaging material degradation, according to industry test results show that less than ten thousand hours of continuous light, high power white LED brightness has reduced more than half, cannot satisfy the lighting light source long life of the basic requirements.

The LED luminous efficiency, improve chip structure and encapsulation structure, can achieve and low power white LED the same level, the main reason is the current density increase more than two times, not only not easy to take out from a large chip light, it will cause the luminous efficiency as low power white LED dilemma, if improve chip electrode structure, theory can solve the above problem take light.

2 try to reduce thermal impedance, improve heat dissipation problems

The luminescence properties uniformity, it is generally thought that as long as the improvement of white LED fluorescent body material density uniformity and phosphor production technology, should be able to overcome the above problems. As mentioned above to improve on the power at the same time, must try to reduce thermal impedance, improve heat dissipation problems, the concrete content respectively is: reduce the chip to the packaging of the thermal impedance, inhibition of packaging to printed circuit board thermal impedance, improve chip heat flow.

In order to reduce the thermal impedance, many foreign LED manufacturers will LED chip in copper and ceramic material made of cooling fins (heatsink) surface, and then use the welding way will be printed circuit boards with heat conductor, connected to the use of cooling fan forced air cooling of the cooling fins, according to the German OSRAMOptoSemiconductorsGmb experimental results confirmed that the structure of the LED chip to weld thermal impedance can reduce 9 k/W, is about 1/6 of the traditional LED around package after 2 W LED applied power, LED chip joint temperature than welding point high and k, even if printed circuit board temperature rise to 500 c, joint temperature at most only around 700 c; Compared to previous thermal impedance once reduce words, LED chip joint temperature will be printed circuit board temperature influence, so must try to reduce the temperature of the LED chip, in other words reduce LED chip to weld thermal impedance, can effectively reduce the LED chip cooling homework burden. On the contrary if white light LED have inhibition thermal impedance structure, if the heat from the package cannot be transmitted to printed circuit board, it LED temperature rise results luminous efficiency will drop rapidly, so matsushita electric development printed circuit board and packaging integration technology, the company will 1 mm square blue light LED to flipchip way packaging in the ceramic substrate, then will ceramic substrate copper paste in printed circuit board surface, according to the panasonic containing printed circuit board, module overall thermal impedance is about 15 k/W or so.

3 each industry show thermal design capability

Due to the cooling fins and printed circuit board between the close to nature about direct heat conduction effect, therefore the printed circuit board design become very complicated, in view of the United States and Japan Lumileds CITIZEN and lighting, LED packaging manufacturers, one after another, to the development of the high power LED with a simple cooling technology, CITIZEN started in 2004 samples of shipment white LED packaging, do not need special joint technology can also will be thick about 2? 3 mm cooling fins heat direct emissions to the outside, according to the CITIZEN said although LED chip joint point to cooling fins 30 k/W heat resistance than OSRAM 9 k/W, and in general environment under room temperature can make the thermal impedance increase 1 W or so, but even if is the traditional printed circuit board without cooling fan forced air cooling condition, the white light LED module can be used for lighting. Lumileds began in 2005 sample shipment of high power LED chip, joint permissible temperature higher up to + 1850 c, than any other company the product high 600 c, the use of traditional RF4 printed circuit board packaging, the ambient temperature 400 c range can enter the equivalent of 1.5 W power current (about 400 ma). So Lumileds and CITIZEN to take to improve joint permissible temperature, Germany OSRAM company is LED chip in cooling fins surface, reach 9 k/W low thermal impedance record, the record in the past than OSRAM development the product of thermal impedance reduced by 40%, and it is the LED module encapsulation, the traditional method of the same flipchip mode, but LED module and thermal fins joint, then select the most close to the LED chip luminous layer as the joint surface, thereby make luminous layer heat can in the most short distance conduction emissions. In 2003, Toshiba Lighting once in 400 mm square aluminum alloy surface, laid luminous efficiency is sixty lm/W low thermal impedance white light LED, no cooling fan and other special cooling module premise, try for beam for 300 lm LED module, because Toshiba Lighting has rich try experience, so the company that the simulation analysis technology progress, after 2006 more than sixty lm/W white LED, can easily use of lamps and lanterns, frame body improve thermal conductivity, or use of cooling fan forced air cooling mode design Lighting equipment cooling, do not need special cooling technology module structure can also use white light LED.

4 change packaging material suppression material degradation and light penetration rate reduce speed

The LED longevity change, at present, the LED manufacturers, the countermeasure is to change the packaging materials, at the same time will be scattered fluorescent material in packaging material inside, especially siliceous packaging material than traditional blue and black light LED chip above epoxy resin encapsulation materials, can be more effective suppression material degradation and light penetration rate reduce speed. Due to the epoxy resin absorption wavelength for 400 ~ 450 nm light percentage reaches as high as 45%, the siliceous packaging material is lower than 1%, luminance half time epoxy resin is less than ten thousand hours, siliceous packaging materials can be extended to forty thousand hours, almost and lighting equipment design life the same, this means that the lighting equipment use period without the need to change the white light LED. But siliceous resin belong to high elastic soft materials, processing the necessary use will not scratch the surface of silicon resin production technology, in addition process on siliceous resin easily attached cuttings, therefore the future must development can improve the surface characteristics of the technology.

Although the siliceous packaging material can ensure that LED forty thousand hours of service life, however lighting equipment industry there are different views, the main argument is the traditional incandescent lamp and fluorescent lamp life, has been defined as a "brightness drop to below 30%," brightness by half time for forty thousand hours of LED, if the conversion into brightness down to 30% of the words, only about twenty thousand hours. There are two prolong the service life of components countermeasures, it is respectively, inhibition of white LED the rise of temperature of the whole, and stop using resin encapsulation.

Generally think that if more than two prolong life thoroughly carry out countermeasures, can achieve the requirement of 30% brightness forty thousand hours. Inhibition of white LED temperature rise can use cooling LED packaging printing circuit board method, the main reason is the packaging resin high temperature condition, plus glare will rapid degradation, in accordance with the law, "ray new temperature decreases c life will extend 2 times. Stop using resin packaging can eliminate degradation factors, because of the light from the LED in packaging resin internal reflection, if use can change the chip side light course of resin material reflection plate, due to the reflection plate can absorb the light, so the light out quantity will decrease rapidly, this also is LED manufacturers in the ceramic system and metal packaging materials is the main reason.