Radiation loss analysis of LED during use

- Feb 21, 2019-

Commonly used single-chip system RAM test method LED is called the fourth generation illumination source or green light source. It has the characteristics of energy saving, environmental protection, long life and small size. It can be widely used in various indications, displays, decoration, backlight, general illumination. And urban night scenes and other fields. In recent years, some economically developed countries in the world have launched a fierce technical competition around the development of LED.


The light output rate determines the degree of application of the LED light source


LED lamps and traditional lamps have completely different structures, and the structure has a crucial role in exerting its characteristics. Modern LED lamps are mainly composed of five parts: LED light source, optical system, driver, heat sink and standard lamp interface.


In Germany's one chip, by incorporating nano-phosphor in silica gel, the refractive index can be increased to 1.8 or higher, light scattering is reduced, LED light-emitting efficiency is improved, and light color quality is effectively improved. Generally, the phosphor has a refractive index of greater than or equal to 1.85 above 1um, and the refractive index of silica is generally about 1.5. The phosphor has a refractive index mismatch and the phosphor particle size is much larger than the light scattering limit (30 nm). There is light scattering on the surface of the particles, which reduces the light extraction rate.


At present, white LEDs are mainly implemented in three forms:


1. The combination of red, green and blue LEDs is a multi-chip white LED;


2, using blue LED chip and phosphor, from blue and yellow to obtain white light or blue LED chip with red and green phosphor, blue light emitted by the chip, red and green light emitted by phosphor Blending to obtain white light;


3. The white light is obtained by exciting the three primary color phosphors emitted by the ultraviolet LED chip.


At present, the second application is widely used. The blue LED chip and the phosphor are used to complement each other to obtain white light. Therefore, such a chip improves the lumen efficiency of the LED, which is determined by the initial luminous flux and the light maintenance rate of the blue chip.


The initial luminous flux of the blue LED chip is improved with the development of epitaxy and substrate technology. The lumen maintenance rate is maintained by the packaging technology. The key to maintaining the lumen maintenance rate is to improve the conductive and heat dissipation environment. This involves the key technologies of LED packaging: low thermal resistance packaging process and high light extraction package structure and process. .


At present, the current LED light efficiency level, because 80% of the input electrical energy is converted into heat, the heat dissipation of the chip is very critical. LED package thermal resistance mainly includes material internal thermal resistance and interface thermal resistance. The role of the heat-dissipating base is mainly to absorb the heat generated by the chip and conduct it to the thermal resistance to achieve heat exchange with the outside world; while reducing the interface and interface contact thermal resistance, and enhancing heat dissipation is also the key, so the thermal interface between the chip and the heat-dissipating base Material selection is important, and low temperature or eutectic solder paste or silver paste is currently used. The thermal conductive adhesive used in the LED chip used in Germany is a thermal conductive adhesive with nano-particles inside, which effectively improves the interface heat transfer, reduces the interface thermal resistance, and accelerates the heat dissipation of the LED chip.


In the process of using LEDs, there are three main aspects in the loss of photons generated by radiation recombination:


1. The internal structure defects of the chip and the absorption of the material, and the reflection loss of the photon at the exit interface due to the refractive index difference;


2. Total reflection loss due to the incident angle being greater than the critical angle of total reflection;


3. By removing a transparent adhesive layer with a relatively high refractive index on the surface of the chip, the loss of photons at the interface is effectively reduced, and the light extraction rate is improved.


Therefore, it is required to have high light transmittance, high refractive index, good thermal stability, good fluidity, and easy spraying, and it is required to have low hygroscopicity, low stress and aging resistance in order to improve the reliability of the LED package. Moreover, the white LED usually needs the blue light excited by the chip to excite the phosphor to synthesize the luminescence, and the phosphor needs to be added in the encapsulant for the color mixing, so the excitation efficiency and conversion efficiency of the phosphor are the key to high luminous efficiency.