Analysis of suggestions for improving the heat dissipation level of LED lamps

　　The power of LED lamps, which LEDs need to consider heat dissipation, power LEDs need heat dissipation. Power LEDs refer to light-emitting diodes with an operating current of more than 100mA. It is defined by the ASSIST Alliance of the United States. According to the typical values ​​of the forward voltage of the two existing LEDs of 2.1V and 3.3V, that is, LEDs with input power of more than 210mw and 330mw are all power LEDs, and they all need to consider the heat dissipation of the device. Some people may have different opinions, but practice has proved that to improve the reliability (lifespan) of power LEDs, it is necessary to consider the heat dissipation of power LEDs.

　　Heat dissipation related parameters The main parameters related to LED heat dissipation are thermal resistance, junction temperature and temperature rise.

　　Thermal resistance refers to the quotient obtained by dividing the difference between the effective temperature of the device and the temperature of the external reference point by the steady-state power dissipation in the device. It is the most important parameter to indicate the degree of heat dissipation of the device. At present, the thermal resistance of power LEDs with good heat dissipation is ≤10℃/W, the best thermal resistance reported in China is ≤5℃/W, and the thermal resistance abroad can reach ≤3℃/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 part of the LED device. It reflects the temperature value that the LED device can withstand under working conditions. For this reason, the US SSL plan has set a goal to improve heat resistance. The heat resistance of chips and phosphors is still very high. At present, the chip junction temperature has reached 150℃ and the phosphor junction temperature has reached 130℃, which basically has no effect on the life of the device. This shows that the higher the heat resistance of the chip phosphor, the lower the requirement for heat dissipation.

　　There are several different temperature rises. What we are discussing here is: tube shell - ambient temperature rise. It refers to the difference between the temperature of the LED device tube shell (the hottest point that can be measured by the LED lamp) and the ambient temperature (on the light-emitting plane of the lamp, 0.5 meters away from the lamp). It is a temperature value that can be directly measured and can directly reflect the degree of heat dissipation of the LED device. Practice has proved that when the ambient temperature is 30℃, if the LED tube shell is measured to be 60℃, its temperature rise should be 30℃, at which time the life value of the LED device can be basically guaranteed. If the temperature rise is too high, the maintenance rate of the LED light source will be greatly reduced.

　　New heat dissipation issues for 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 to be dissipated. Second, the new packaging structure, as the power of LED light sources increases, multiple power LED chips need to be packaged together, such as COB structure, modular lamps, etc., which will generate more heat and require more effective heat dissipation structures and measures, which in turn raises 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 electrical energy needs to be converted into heat. Secondly, LED high current density and modular lamps will generate more concentrated waste heat, which needs to be well dissipated.

　　To improve the heat dissipation level, we provide the following suggestions:

　　1) From the perspective of LED chips, new structures and new processes should be adopted to improve the heat resistance of the LED chip junction temperature and the heat resistance of other materials, so as to reduce the requirements for heat dissipation conditions.

　　2) Reduce the thermal resistance of LED devices, adopt new packaging structures and processes, and select new materials with good thermal conductivity and heat resistance, including bonding materials between metals and mixed glue for phosphors, so that the thermal resistance is ≤10℃/W or lower.

　　3) Reduce the temperature rise, try to use heat dissipation materials with good thermal conductivity, and require good ventilation holes in the design to dissipate the waste heat as quickly as possible. The temperature rise should be less than 30°C. In addition, improving the heat dissipation level of modular lamps should be put on the agenda.

　　4) There are many ways to dissipate heat, such as using heat pipes, which is of course good, but cost factors must be considered, and cost-effectiveness should be considered during design.

　　In addition, the design of LED lamps should not only improve lamp efficiency, light distribution requirements, and beautiful appearance, but also improve heat dissipation and use materials with good thermal conductivity. It is reported that the heat dissipation body coated with certain nanomaterials can increase its thermal conductivity by 30%. In addition, it must have good mechanical properties and sealing, and the heat dissipation body must be dustproof, requiring the temperature rise of LED lamps to be less than 30℃.