basic introduction
1. LED illumination mechanism: The terminal voltage of the PN junction constitutes a certain barrier. When the forward bias voltage is applied, the barrier decreases, and the majority carriers in the P and N regions diffuse toward each other. Since the electron mobility is much larger than the hole mobility, a large amount of electrons diffuse into the P region, which constitutes injection of minority carriers in the P region. These electrons recombine with the holes on the valence band, and the energy obtained during recombination is released as light energy. This is the principle of PN junction illumination.
2, LED luminous efficiency: generally referred to as the external quantum efficiency of the component, which is the product of the internal quantum efficiency of the component and the extraction efficiency of the component. The internal quantum efficiency of a component is actually the electro-optic conversion efficiency of the component itself, which is mainly related to the characteristics of the component itself (such as the energy band, defects, and impurities of the component material), the composition and structure of the component. The extraction efficiency of the component refers to the number of photons that can be measured outside the component after the photons generated inside the component are absorbed, refracted, and reflected by the component itself. Therefore, the factors concerning the extraction efficiency include the absorption of the component material itself, the geometry of the component, the refractive index difference of the component and the packaging material, and the scattering characteristics of the component structure. The product of the internal quantum efficiency of the component and the extraction efficiency of the component is the luminous effect of the entire component, which is the external quantum efficiency of the component. Early component development focused on improving its internal quantum efficiency. The main method is to improve the quality of the barrier crystal and change the structure of the barrier crystal, so that the electrical energy is not easily converted into thermal energy, thereby indirectly improving the luminous efficiency of the LED, thereby obtaining a theory of about 70%. Internal quantum efficiency, but such internal quantum efficiency is almost close to the theoretical limit. Under such circumstances, it is impossible to increase the total amount of light of the module by increasing the internal quantum efficiency of the module. Therefore, it is an important research subject to improve the extraction efficiency of the component. The current methods are mainly: changes in grain shape - TIP structure, surface roughening technology.
3, LED electrical characteristics: current control type device, load characteristics similar to the PN junction UI curve, the minimum change in forward conduction voltage will cause a large change in forward current (exponential level), reverse leakage current is small, there are Reverse breakdown voltage. In actual use, it should be chosen. The LED forward voltage becomes smaller as the temperature increases, with a negative temperature coefficient. The LED consumes power and a part of it is converted into light energy, which is what we need. The rest is converted to heat, which causes the junction temperature to rise. The amount of heat (power) emitted can be expressed as.
4, LED optical characteristics: LED provides a half-width large monochromatic light, because the energy gap of the semiconductor decreases with the rise of temperature, so its peak wavelength emitted with the increase of temperature, that is, the spectrum red shift The temperature coefficient is +2~3A/. The LED light-emitting luminance L is approximately proportional to the forward current: K is a proportional coefficient. As the current increases, the luminance of the light also increases approximately. In addition, the luminance of the light is also related to the ambient temperature. When the ambient temperature is high, the composite efficiency is lowered and the luminous intensity is decreased.
5, LED thermal characteristics: under small current, LED temperature rise is not obvious. If the ambient temperature is high, the main wavelength of the LED will be red-shifted, the brightness will decrease, and the uniformity and consistency of the light will be deteriorated. In particular, the temperature rise of the dot matrix and the large display screen has a more significant effect on the reliability and stability of the LED. So thermal design is critical.
6, LED life: LED long-term work will cause aging caused by light decay, especially for high-power LED, the light fade problem is more serious. When measuring the life of an LED, it is not enough to use the damage of the lamp as the end of the life of the LED. It is more meaningful to specify the lifetime of the LED by the percentage of light attenuation of the LED, such as 35%.
7, high-power LED package: mainly consider heat dissipation and light. In terms of heat dissipation, copper-based thermal lining is used to connect to the aluminum-based heat sink, and the solder joint is used as a connection between the die and the thermal lining. This heat dissipation method has better effect and high cost performance. In terms of light output, the chip flipping technology is adopted, and the reflective surface is added on the bottom surface and the side surface to reflect the wasted light energy, so that more effective light can be obtained.
8. White LEDs: There are three main types of natural spectrum white LEDs:
The first is a relatively mature and commercialized blue chip + yellow phosphor to obtain white light. This white light has the lowest cost, but the shift of the blue light crystal wavelength, the change of the intensity and the change of the thickness of the phosphor coating will be Affects the uniformity of white light, and the spectrum is narrower in band shape, color is incomplete, color temperature is high, color rendering is low, and the light is not soft and uncoordinated to the eyes. The human eye is most adapted to the sunlight. The continuous spectrum of incandescent lamps is the best. The color temperature is 2500K and the color rendering index is 100. Therefore, this white light needs to be improved, such as adding a multi-lighting process to improve the spectrum to make it continuous and wide enough.
The second type is ultraviolet or violet chip + red, blue and green phosphors to obtain white light. The principle of illumination is similar to that of fluorescent lamps. This method has better color rendering, and UV-LED does not participate in white color matching, so UV- The fluctuation of the wavelength and intensity of the LED is not particularly sensitive to the white light that is dispensed, and the white light of acceptable color temperature and color rendering can be modulated by the selection and ratio of the phosphors of the respective colors. However, there is also a problem that the effective conversion efficiency of the phosphor used is low, and in particular, the efficiency of the red phosphor needs to be greatly improved. Such phosphors have poor luminous stability, large light decay, selection of phosphor wavelengths for phosphors, difficulty in UV-LED fabrication, and development of anti-UV packaging materials.
The third is to use the three primary color principle to mix RGB three ultra-high brightness LEDs into white light. The advantage of this method is that white light can be directly dispensed without conversion of the phosphor, which is better than avoiding the loss of phosphor conversion. In addition to the luminous efficiency, the luminous intensity of the red, green and blue LEDs can be separately controlled to achieve a full-color color-changing effect (variable color temperature), and a better color rendering property can be obtained by selecting the wavelength and intensity of the LED. However, the problem with this method is that the conversion efficiency of green light is low, the light mixing is difficult, and the design of the driving circuit is complicated. In addition, since these three light colors are all heat sources, the heat dissipation problem is three times that of other package forms, which increases the difficulty in use. Polarized LEDs and three-wavelength full-color white LEDs will be the future direction.
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