The quality of LED chips LED chips are the core components of LED light sources, and their lifespan largely determines the lifespan of LED lamps. There are three factors that affect the life of an LED chip, namely, the lattice defects of the chip, the packaging process and the quality of the phosphor. First of all, the material that makes up the LED chip is crystal.
An ideal crystal is shown in Figure 3. If the lattice arrangement is not so good, and some places lack rows, as shown in Figure 4, this defect will affect the life of the LED chip. In addition, if the impurities doped in the LED chip are not what we need, it will also affect the life of the LED light source.
Secondly, whether the packaging of the LED is reasonable is also an important factor affecting the life of the chip. At present, several world-renowned companies such as Cree, Lumilends, and Nichia of Japan have relatively high levels of packaging technology. LED packaging technology is also protected by patents, and the life of its LEDs is guaranteed. However, most of the other companies have many imitations of product process packaging. These products are OK in appearance, but the process structure and process quality are poor, which seriously affects the life of LEDs.
In addition, the performance of LED chips has a certain degree of dispersion. Before packaging, they should be classified and classified according to the area of LED chips, light efficiency and many other indicators. If the chip is not binned before packaging, the light output rate of the finished product after packaging will inevitably be uneven. That is to say, the packaging of LEDs and the binning before packaging are also important factors that affect the life of LEDs.
Finally, the quality of the phosphor also affects the life of the LED chip. At present, there are many ways for LED chips to produce white light, two of which are through phosphor powder. One method is to coat YAG (Yttrium Aluminum Garnet, yttrium aluminum garnet) phosphor on the LED blue light chip.
As mentioned earlier, photons with short wavelengths are more energetic than photons with long wavelengths. Therefore, the blue light (short wavelength) emitted by the chip excites the phosphor to produce yellow-green light of 500 nm~560 nm (longer wavelength than blue light), blue light and yellow-green Photosynthesis of white light. The cost of obtaining white light by this method is relatively low, but it is difficult to control the uniformity of the phosphor, and after the chip is used for a period of time, the light effect and color temperature will change accordingly. Another method is to let the LED chip emit ultraviolet rays, which then excite RGB (red, green, and blue primary colors) phosphors to produce red, green, and blue primary color lights, and then mix the three primary color lights into white light.
Because the photon energy of ultraviolet rays is relatively large, the epoxy resin of the packaging material is easy to age, which will also affect the life of the LED chip. If the LED chip becomes the "short board" of the life of the LED lamp, then which of the above three factors has a problem, which one is the "short board" that affects the life of the LED chip and the lamp. 2 LED heat dissipation system When talking about LED lamps, There are also various theories as to whether infrared is present and how much heat is generated. In fact, the theoretical value of the luminous flux of the LED light source to emit white light should be above 300 lm/W.
The high-power LED light source currently only reaches 80 lm/W~100 lm/W, that is to say, nearly 1/3 of the electric energy becomes visible light, and most of the electric energy becomes heat energy. The heat energy should be dissipated in the form of heat conduction and heat radiation. Therefore, the heat dissipation system of high-power LED lamps is very important.
The P-N junction of LED light source is very sensitive to temperature. High temperature will greatly reduce the life of the LED chip. As shown in Figure 6, it is the light decay of high-power white LEDs given by Cree. According to international practice (also stipulated in the original light source specification), the effective life of the light source is defined as: when the luminous flux decays to 70% of the initial value, the light source Even at the end of life.
The effective life of the P-N junction of the LED light source at different temperatures is shown in the intersection of the green horizontal line and each curve in the figure. It can be seen from Figure 6 that when the temperature of the P-N junction is 75°C, the lifetime is 51,000 h (green curve); when the temperature is 85°C, the lifetime is 22,000 h (yellow curve); when the temperature is 95°C, the lifetime is 18000 h (pink curve); when the temperature is 105 °C, the lifetime is 12000 h (red curve). In fact, the P-N junction temperature of high-power LED lamps is currently about 105°C, that is to say, the life of the LED chip is only more than 10,000 hours.
If the heat dissipation condition of the lamp is not good, the temperature of the P-N junction of the LED chip will rise too high, and the LED chip will be damaged quickly. At this time, the heat dissipation system becomes the "short board" of the life of LED lamps. The heat dissipation system has a great influence on the life of LED chips. If chips of the same quality are placed in different lamps, the life span will vary several times or even dozens of times. A lamp Whether the design is successful, in addition to the optical system, the heat dissipation system plays a decisive role.
3 Driving power The driving power plays a key role in the life of LED lamps. This is a problem that is easily overlooked, and it may also be the bottleneck in the current promotion of high-power LED lamps. When checking the lamps, it often happens that the LED chip is not damaged, but the driving power supply is faulty. Generally, the lifetime of the LED chip is much longer than that of the driving power supply.
For example, Cree's XLamp series product 7090XR-E is a white light LED chip, its typical operating parameters are 3.5 V, 700 mA (2.45 W), and the P-N junction temperature is 80. C (this temperature can only be achieved by a very good heat dissipation system), when the luminous flux decays to 70% of the initial value, the chip life is 50,000 h. At present, the life of the best driving power has exceeded 30,000 h, and the life of the poor driving power is only a few thousand hours.
If LED lamps use high-quality chips and the heat dissipation system is well done, the life of the driving power supply may become a weak link. 4 Different standards are used for the indicators of effective life and light decay, and little is often known about LED light sources. people introduce misunderstandings. As we all know, there are two versions of the life of traditional light sources, one is called the full life, and the other is called the effective life. Full life is defined as the total cumulative ignition time of the light source from ignition to end of life (not lit).
The effective life refers to the cumulative ignition time of the light source when its luminous flux decays to 70% of the initial value after the light source is ignited. Because the luminous flux of the light source can last for a long time from 70% of the initial value to the end of life (not bright), the effective life of most light sources is far less than the full life. The LED light source is a new light source, and the effective life of the new light source is not controversial.
At present, L70 is usually listed as a standard for evaluating the life of LED light sources in the world. The so-called L70 refers to the effective life of the light source when the initial luminous flux of the LED light source is regarded as 1.0 (ie 100%), and the luminous flux decays to 70% of the initial value. In the domestic LED industry, some manufacturers regard the luminous flux attenuation to 50% of the initial value (commonly known as the half-life) as the life standard of LED lights.
It can be seen from Figure 6 that when the working temperature of the P-N junction of the LED is 105 °C, the lifespan is 12 000 h when the light decays to 70% of the initial value, and the lifespan is 21 000 h when the light decays to 50% of the initial value. The two values are quite different. Technicians engaged in lighting applications may make wrong judgments if they do not know the difference between the two standards. 5 About the color rendering index and color temperature For LED lamps in the film and television stage, even if the luminous flux has not decayed to 70% of the initial value, if the color rendering index Ra decreases too much or the color temperature of the light source changes too much, it should be regarded as the end of its life.
Because the color rendering index or color temperature changes too much, and the color temperature of each lamp is different, the lamp loses its use value.