LEDs are compact, lightweight, low-drive, full-color, long-life, high-efficiency, vibration-resistant, and easy to control light, providing superior conditions for lighting systems designed for different locations and purposes. People are used to seeing things in the sun. For general lighting, people need mainly light sources close to the quality of sunlight, so white LEDs are an important indicator of semiconductor lighting technology. Since the single LED has low power and low brightness, it is not suitable for use alone. Therefore, it is necessary to assemble a plurality of LEDs together to design a practical LED lighting system. However, there is still a certain distance between the requirements of white LEDs and general lighting, and there are still many technical and cost issues that need to be solved urgently.
1. Overview of main technologies of semiconductor lighting system
1) Thermal management of the lighting system
LEDs are often referred to as cold light sources. This is because the principle of LED illumination is that electrons directly emit photons through recombination without the need for heat. However, due to the existence of Joule heat, LEDs also have heat accompanying illumination, and for high power and multiple LED applications, the heat accumulation is small and cannot be underestimated. LEDs are different from traditional illumination sources such as incandescent lamps and fluorescent lamps. Excessive temperatures can shorten or even terminate their service life. Moreover, LEDs are temperature-sensitive devices. When the temperature rises, their efficiency drops sharply. Therefore, system structure design and heat dissipation technology development are also issues that LED applications need to face. Since forced air cooling is generally undesirable in light sources, heat sinks and other methods of enhancing natural convection cooling play an increasingly important role in LED lamp and light source design as input power increases.
2) Improve color rendering
At present, white LEDs generally use a blue-emitting LED to superimpose a yellow-emitting yttrium aluminum garnet (YAG) phosphor excited by blue light, which is synthesized into white light. Since the luminescence spectrum contains only two spectra of blue and yellow, there is a problem that the color temperature is high and the color rendering index is low, which does not meet the general lighting requirements. The sensitivity of the human eye to chromatic aberration is much higher than the sensitivity to light intensity. For illumination, the color rendering of a light source is often more important than the luminous efficiency. Therefore, adding an appropriate amount of red-emitting phosphor and maintaining high luminous efficiency is an important issue in LED white light illumination.
3) Secondary optical design of the luminaire system
Conventional lamps have long used incandescent lamps and fluorescent lamps as reference materials to determine the optical and shape standards of lamps. Therefore, LED lamp systems should consider the abandonment of traditional lamps and LED light module assembly methods, fully consider their optical characteristics, and specialize for LED light sources. Design different luminaires. The design of the optical system mainly includes the following aspects: 1 According to the requirements of the illumination object and the luminous flux, determine the shape of the optical system, the number of LEDs and the size of the power; 2 design a plurality of LED lighting tubes into a point light source, a ring light source or The "secondary light source" of the surface light source calculates the illumination optical system according to the combined secondary light source; 3 it is important to control the light distribution of each LED tube on the "secondary light source" constituting the illumination optical system design.
Since the beam emitted by the LED is concentrated, it is easier to control, and the reflector is not required to be concentrated, which is advantageous for reducing the depth of the lamp. For example, with a flat mirror optical system, a large surface can be illuminated with only 1 or 2 LEDs, and the depth of the lamp is very thin. With the light guide technology, the LED is directly mounted next to the light pipe, which can greatly reduce the light source and other components. The volume occupied makes an ultra-thin luminaire.
4) Integration of power supply, circuit and luminaire
Design for the LED lamps, incandescent and fluorescent lamps need to pay attention to a few problems designers need to focus on is the power supply. Most incandescent lamps are powered directly from an AC line, so no power is required. Fluorescent lamps use ballasts to perform the function of the power supply. However, the LED needs a special power supply and drive circuit to match it. When designing the luminaire, the power supply and the luminaire system should be considered.
The biggest features of semiconductor lighting and solar power generation are environmental protection, energy saving, long life and safety. Solar energy and semiconductor lighting combine to achieve photoelectric to electro-optic conversion, which is the best combination. The natural energy-utilizing independent semiconductor lighting fixture with solar cell power generation as the power source saves energy, environmental protection and long life, and also saves related wires and supporting facilities, and has huge market space.
5) Improve system reliability
LED light source is called long-life lamp. As a solid-state light-emitting device, its theoretical life is more than 100,000 hours. Its service life is much longer than that of traditional light sources. Therefore, it can be greatly reduced in some occasions where it is difficult to replace and maintain. . However, in many practical applications, this advantage cannot be seen. Instead, the user sees that the light decay is severe and the life span is short. It takes less than 10,000 hours to break down. This is because the LEDs used by everyone are The Ф3~Ф5 mm LED, which is the most commonly used indicator function in the electronics industry, uses a simple electronic circuit to increase the current to increase its luminous intensity, but the LED only obtains a short high brightness, but loses the life expectancy of the LED. Therefore, LEDs should be carefully selected in the design of semiconductor lighting fixtures, and high-power LEDs should be used as lighting sources.
6) Increase luminous flux and lower prices
At present, a single 1W LED device has a luminous flux of about 25 lm, a good quality price of more than 5 US dollars, and a poor price of 3 US dollars. The luminous flux of 25 lm is too small for general illumination. The luminous flux of an ordinary 60W incandescent lamp is more than 700 lm, which means that instead of traditional lighting, multiple LED devices are needed, plus the driver circuit and lamp housing. Such a high cost is the biggest problem of white LEDs in general lighting. In particular, the efficiency is currently 25 to 45 lm/W, which needs to be further improved.
2. Technology development trend
1) Abandoning the current LED lamps is only the design method of traditional lamps and LED lighting modules. It is necessary to fully consider the optical characteristics of LEDs and develop LED-specific lamps.
2) The design of the power supply and control circuit, the power supply should change the application mode of the current commonly used capacitor buck and impedance divider, design a reasonable small current constant current source circuit, and adopt the clock cycle modulation method in the drive current to improve The stability of LED lamps, while further improving the efficiency of the power supply.
3) In the design of control circuit circuit, it is necessary to develop into three aspects: centralized control, standard modularization and system scalability.
4) In the current LED light effect and limited luminous flux, fully exploit the characteristics of LED color diversity, and develop control circuits for color-changing LED lighting.
5) Take advantage of LEDs and develop a lighting system that combines LED lighting with photovoltaic systems.
6) Develop an integrated flat light source system suitable for indoor lighting . The development trend is that LED lighting fixtures are integrated with the building.
7) Develop LED lighting simulation system to speed up product development.
8) Develop solar energy and high-brightness LED integration technology to solve the matching and control technology between solar cell system and LED lighting system.
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