The reliability (life) of LED lighting fixtures depends largely on the level of heat dissipation, so increasing the level of heat dissipation is one of the key technologies. It is mainly to solve the problem that the chip generates excess heat through the heat sink and the heat sink. This is a very complicated technical problem.
The power of LED lamps, which LEDs need to consider heat dissipation, power LEDs need to dissipate heat. The power LED refers to a light-emitting diode with an operating current of 100 mA or more. According to the definition of the current ASSIST alliance in the United States, the typical forward voltages of the two types of LEDs are 2.1V and 3.3V, that is, the LEDs with input powers above 210mw and 330mw are all power LEDs. Some people may have different opinions, but practice has proved that to improve the reliability (life) of power LEDs, it is necessary to consider the heat dissipation problem of power LEDs.
The main parameters related to heat dissipation related parameters and LED heat dissipation are thermal resistance, junction temperature and temperature rise.
Thermal resistance is the quotient of the difference between the effective temperature of the device and the external specified reference point temperature divided by the steady state power dissipation in the device. It is the most important parameter that indicates the degree of heat dissipation from the device. At present, the heat dissipation of the LED with good heat dissipation is ≤10°C/W, the best thermal resistance reported in China is ≤5°C/W, and the thermal resistance of foreign countries is ≤3°C/W. If this level is achieved, the life of the power LED can be ensured. .
Junction temperature refers to the temperature of the semiconductor junction of the main heat generating portion of the LED device. It is the temperature value that the LED device can withstand under working conditions. To this end, the US SSL plan sets targets for improving heat resistance. The heat resistance of the chip and the phosphor is still very high. At present, the junction temperature of the chip is 150 ° C, and the phosphor at 130 ° C basically has no effect on the lifetime of the device. It shows that the higher the heat resistance of the chip phosphor, the lower the requirement for heat dissipation.
There are several different temperature rises for temperature rise. What we are discussing here is: shell-environmental temperature rise. It refers to the temperature difference between the temperature of the LED device package (the most hot spot that can be measured by the LED luminaire) and the environment (0.5 meters from the luminaire on the illuminating plane of the luminaire). It is a temperature value that can be directly measured, and can directly reflect the heat dissipation degree of the LED device. It has been proved that when the ambient temperature is 30 °C, if the LED tube is 60 °C, the temperature rise should be 30 °C. At this time, the lifetime value of the LED device can be basically ensured. If the temperature rise is too high, the maintenance rate of the LED light source will be greatly reduced.
New problems with heat dissipation of LED lamps:
With the development of LED lighting products, there are two new technologies: First, in order to increase the luminous flux of a single tube, a larger current density is injected, as mentioned below, so that the chip generates more heat and needs heat dissipation. Second, the new structure is packaged. As the power of the LED light source increases, multiple power LED chips are required to be packaged together, such as a COB structure or a modular lamp, which generates more heat and requires a more efficient heat dissipation structure. Measures, which in turn raise new issues for heat dissipation, otherwise it will greatly affect the performance and life of LED lamps.
At present, the total heat dissipation efficiency of LED lamps is only 50%, and a lot of electric energy has to be turned into heat. Secondly, LEDs with high current density and modular luminaires will generate more concentrated heat and require good heat dissipation.
To improve the heat dissipation level, we offer the following suggestions:
1) From the perspective of LED chips, it is necessary to adopt new structures and new processes, improve the heat resistance of the junction temperature of LED chips, and the heat resistance of other materials, so that the requirements for heat dissipation conditions are lowered.
2) Reduce the thermal resistance of the LED device, adopt a new structure and a new process, and select a new material with better thermal conductivity and heat resistance, including a bonding material between the metal and a mixed rubber of the phosphor, so that the thermal resistance ≤ 10 ° C / W or lower.
3), reduce the temperature rise, try to use heat-dissipating materials with good thermal conductivity, and design a better ventilation hole, so that the residual heat can be dissipated as soon as possible, the temperature should be less than 30 °C. In addition, increasing the level of heat dissipation of modular luminaires should be mentioned on the agenda.
4) There are many ways to dissipate heat. For example, the use of heat pipes is of course good, but cost factors should be considered. Cost-effectiveness should be considered in design.
In addition, in addition to improving the efficiency of the luminaire, the light distribution requirements, and the beautiful appearance, the design of the LED luminaire should improve the heat dissipation level and use a material with good heat conductivity. It has been reported that the heat sink is coated with some nano materials, and its thermal conductivity is increased by 30. %. In addition, to have better mechanical properties and sealing, the heat sink should be dustproof, and the temperature rise of the LED lamp should be 30 °C.
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