Research on LED and semiconductor lighting
Abstract: In recent years, domestic and foreign countries have attached great importance to the research and development of light-emitting diodes (LEDs), briefly introduced the development status and characteristics of light-emitting diodes, and discussed some technologies that need to be solved for the development of LEDs for lighting The key issue is that such lighting systems will be widely used.
Status quo of the development of light-emitting diodes Light-emitting diodes, also known as light-emitting diodes (LightEmitTIng Diode, abbreviated as LED) is a semiconductor light-emitting device that can convert electrical energy into light energy, which belongs to solid-state light sources. LEDs came out in the early 1960s. At that time, LEDs were mainly red, with low luminous efficiency and low luminous flux, and could only be used as indicator lights and instrument displays. With the continuous advancement of die materials, structure and packaging technology, the variety of LED colors has increased and the light efficiency has been greatly improved. The current red LED light efficiency can reach 100lm / W, and the green LED can also reach 50lm / W. Luminous flux can reach dozens of lumens. Especially in recent years, the development of high-efficiency, high-brightness white LEDs has made the application of LEDs in the field of lighting possible. In the more than 100 years since Edison invented the incandescent lamp, electric light source lighting has gone through three important development stages. The representative light sources in these three stages are incandescent lamp, fluorescent lamp and high-intensity gas discharge lamp. Now people generally believe that LED will be expected to develop into the fourth generation light source, that is, semiconductor lighting.
Characteristics of semiconductor light-emitting diodes
LED is a semiconductor device that realizes electro-optical conversion through a PN junction. Its characteristics are:
1.1 Energy saving without causing environmental pollution
The energy consumption of LED is small, and with the advancement of technology, it will become a new type of energy-saving lighting source. At present, the luminous efficiency of white LEDs has reached 25lm / W, exceeding the level of ordinary incandescent lamps, and some people predict according to the current LED technology development speed. By 2005, the luminous efficiency of white LEDs can reach 50lm / W, and by 2010- In 2015, the luminous efficacy of white LEDs may reach 150-200lm / W, far exceeding the luminous efficacy of all current lighting sources. In addition, currently widely used fluorescent lamps, mercury lamps and other light sources contain mercury that is harmful to human health. The production process of these light sources and discarded lamp tubes will cause environmental pollution. LED does not have these problems, it is a "clean" light source.
1.2 Long service life Generally speaking, the service life of ordinary incandescent lamps is about 1000h, the service life of fluorescent lamps and metal halide lamps does not exceed 10,000 hours, and the service life of LEDs can be as long as tens of thousands of hours.
1.3 Solid structure
LED is a solid-state light source encapsulated with epoxy resin. There is no vulnerable parts such as glass bubbles and filaments in its structure. It is an all-solid structure, so it can withstand vibration and shock without causing damage.
In addition, LED has some valuable advantages as a light source for lighting, such as: the luminous body is close to the point light source, which is convenient for the design of the lamp; the response time of the light is short; it is easy to be made into a thin lamp, saving installation space, etc.
In summary, LED is a light source that meets the requirements of green lighting. The so-called "green lighting" concept refers to the use of scientific lighting design, the use of high efficiency, long life, safety and stable performance of lighting electrical products (electric light sources, lamp electrical accessories, lamps, wiring materials, and dimming control equipment And light control devices) to improve the conditions and quality of people ’s work, study, and life, thereby creating an efficient, comfortable, safe, economical, and beneficial environment that fully reflects the lighting of modern civilization.
2 The key to R & D of lighting LED technology
LED lighting can only be used for special occasions. The widely used illumination light source should be white LEDs, and the white LEDs developed at home and abroad are not high enough in light efficiency, nor large enough in power, and expensive. If it is required to completely replace the incandescent or fluorescent lamps currently in common use, there are still some technical problems that need to be studied and solved.
(1) First is the issue of luminous efficiency. The main method to improve the luminous efficiency of LEDs is to improve the structure and manufacturing process of semiconductor luminescent materials and LED wafers. Because this part of the work requires a solid theoretical research foundation and advanced semiconductor process equipment, it is not easy to carry out research in this area.
(2) The problem of high power. As lighting, the luminous flux output by a single LED must be large enough. To increase the luminous flux of the LED, first, sufficient electrical power must be injected. However, the temperature rise of the LED chip should not be too high, otherwise various performances, especially the service life, will be greatly affected. Obviously, in addition to designing larger input power LED devices and lamps, in addition to the larger area of ​​the chip, it must also have a good heat dissipation structure. Now some famous foreign companies have designed and developed some special LED device structures, and have achieved good results.
(3) Since the light emitted by the ordinary LED is relatively concentrated in a small solid angle range, the reflector in the general lamp is no longer a necessary optical element, and a lens is often used as a collimating optical element. For example, a convex lens or a Fresnel lens is used to generate parallel beams. Then, pincushion lenses, wedge prisms, etc. are used to re-diffuse and deflect the light beam to produce a light distribution that meets the requirements of various lighting standards. This requires a unique secondary optical system design for the LED lighting fixture.
(4) Since LED lighting needs to be composed of multiple LED tubes, its parameter dispersion is also a technical issue. In addition to pre-selection and classification to ensure consistency, it is also necessary to design a reasonable lamp structure (including the arrangement and placement of LEDs) and study a suitable driving circuit to prevent the occasional energy concentration from burning down some LEDs.
(5) In addition, when multiple LEDs form a lighting fixture, it is inevitable that the LEDs are connected in parallel or in series. As long as there is an LED short circuit or open circuit during use, it will cause the entire small piece or the entire LED to go out, affecting the lighting effect. For this reason, simple and inexpensive protection circuits must be studied to minimize such adverse effects.
(6) In view of the high requirements of LEDs for heat dissipation conditions, if the junction temperature of the die exceeds the standard limit value, it will cause the irreversible light intensity attenuation. Therefore, in addition to sufficient heat dissipation measures during use, a reasonable circuit design and circuit layout must also be provided to try to keep the LED in good working condition and give full play to its long life advantage.
3 Conclusion Although there are still a series of key technical problems that need to be solved in the development of light-emitting diode lighting systems, with the rapid improvement of light-emitting diode die light efficiency, the packaging of light-emitting diodes and the further improvement of the lamp manufacturing process, Some economic and technological obstacles will be gradually eliminated. I believe that in the near future, this type of lighting system can be widely used in many fields.
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