LED chip quality
The LED chip is the LED light source core components, its life largely determines the LED lamp life. Effects of LED chip life from three factors, namely the lattice defects, chip packaging process and the quality of fluorescent powder.
First of all, consisting of LED chip material is crystal. Ideal crystal as shown in figure 3. If the lattice is not so good, some local missing row, as shown in Figure 4, these defects will affect the life of LED chip. In addition, if the LED chips by doping impurity is not what we need impurities, will also affect LED light source life.
Secondly, LED package is reasonable but also affect the life of important factors of chip. At present, the United States Cree ( Corey ) company, Lumilends company and Japanese Nichia CORPORATION Global several well-known company's packaging technology level is relatively high, LED packaging technology also have patent protection, the LED life is guaranteed. But most of the other enterprise technology products packaging imitation more, these products from the shape can also be, but the process structure and process quality is poorer, thus seriously affecting the life of LED. In addition, the LED chip performance of dispersion, before packaging should be based on the LED chip area, light efficiency and other indexes are classified. If the chip on the package was performed without preoperative grading, packaging the finished product rate of uneven light out of necessity. That is to say, the LED package and package before the classification is also important factors influencing LED life.
Finally, the quality of fluorescent powder also influences LED chip life. The current LED chip to produce white light method has many kinds, of which there are two method is by white light fluorescent powder. A method is LED blue chip coated with YAG ( Yttrium Aluminum Garnet, yttrium aluminum garnet fluorescent powder ). As has been noted, short wavelength photonic wave long photon energy, therefore, chip emits blue light ( short wavelength ) excited fluorescence powder can produce 500 nm~560 nm yellow-green light ( wavelength than the blue long ), blue and white Huang Lvguang synthesis. This method of white light fluorescent powder with low cost, but the uniformity is difficult to control, and the chip after a period of use, efficiency of light and color temperature will have corresponding change. Another method is to let LED chip emits ultraviolet light, ultraviolet and excitation of RGB ( red, green, blue three primary color phosphor to produce red, green ), blue three colors, three colors then mixed into white light. Because ultraviolet photon energy is larger, so that the packaging material of epoxy resin is easy to aging, this will also affect the life of LED chip.
If the LED chip LED lamps life-span " short board ", then, the above three factors which appear problem, which is the impact of the LED chip and the service life of the lamp "short board ",
2 LED cooling system
In refer LED lamps, the existence of infrared and heat quantity, also has a variety of views. In fact, LED light source to emit white light luminous flux theory value should be in above of 300 lm/W. High power LED light source at present only reached 80 lm/W ～100lm/W, i.e. power near 1/3 into visible light, most electric energy into heat energy. The heat energy to the heat conduction and heat radiation in the form of dissemination. Therefore, large power LED lamp heat dissipation system is crucial.
LED P-N junction temperature is very sensitive to light. High temperature, will enable the LED chip reduced life expectancy. As shown in Figure 6, it is Cree is a high-power white LED light
In accordance with international practice ( also the original source code ), the effective life of the light source is defined as: the luminous flux to the initial value of 70%, even if the end light source. LED light source P-N nodes at different temperatures the effective life of the figure in horizontal line and each curve intersection point. From the graph 6 in can see, the P-N junction temperature is 75 ℃, life is 51000 H ( green curve ); temperature is 85 ℃, life is 22000 H ( yellow curve ); temperature is 95 ℃, life is 18000 H ( pink line ); temperature is 105 ℃, life of 12000 H ( red curve ). In fact, high-power LED lighting P-N junction temperature is about 105℃, that is to say, the LED chip has a life span of 10000 hours. If the lamp heat dissipation conditions is not good, LED chip P-N junction temperature is too high, the LED chip can quickly damage. At this time, cooling system becomes LED lamp life "short board
Cooling system of LED chip with the same quality of life impact is very big, the chip is placed in a different light, life will differ severalfold even a few times. A lighting design is successful, except the light path system, cooling system plays a decisive role.
3 driving power supply
Driving power supply for LED lamp life plays a crucial role, this is an easily neglected problem, may also be the promotion of large power LED lamp bottleneck. Inspection lamp will often appear such circumstance: found LED chip and is not damaged, but the drive power supply fault. Usually, the LED chip life far outweigh the driving power of life. For example, Cree's XLamp series 7090XR-E is white LED chips, the typical operating parameters is 3.5 V, 700 mA ( 2.45 W ), P-N junction temperature of 80. C ( the temperature only very good heat dissipation system can achieve ), luminous flux to the initial value of 70%, life for the 50000 h chip. At present, the best driving power of life has been more than 30000 h, poor driving power for thousands of hours of life only. If the LED lamps to the high quality of the chip, while heat dissipation system do well, driving power of life may become the weakest link
4 on the effective life
Attenuation index using different criteria, are often put on the LED light source is less people introduce errors.
As we all know, the traditional life of the light source has two versions, a life, a life. The whole life span is defined as the light from light to the end ( bright ) so far all the cumulative time to ignition. Effective life refers to the light source, when the luminous flux to the initial value when the 70% light source accumulated ignition time. Because the light source of the luminous flux from an initial value of 70% to the end ( bright ) also points for a long time, so most of the effective life of the light source is far less than the full life.
LED light source is a kind of new light source, the new life of the light source adopts the effective life is not in dispute. At present on international normally will L70 as LED light source life assessment of a standard. The so-called L70, refers to the LED light source initial luminous flux as 1 ( i.e. 100% ), luminous flux to the initial value of 70% when counted as the effective life of the light source. While the domestic LED industry, some manufacturing enterprises to luminous flux to the initial value of 50% ( commonly known as half-life) as the LED lamp life standard. As you can see from Figure 6, when LED P-N junction operating temperature is 105 ℃, light to the initial value of 70% when life is 12000 h, light to the initial value of 50% when life is 21000 h, the two values differ a lot. Engaged in lighting applications and technical personnel, if not clear differences between two standards, may make wrong judgment.
5 on the color index and the color temperature
For film and stage LED lamp luminous flux, even without attenuation to the initial value of 70%, if the color rendering index Ra decreases too much or light source color temperature change is too large, should also be regarded as the end of life. Because, the color rendering index color temperature changes or excessive light color, each different, lamps and lanterns lost use value.
