The LED is a semiconductor device, and all semiconductor devices have certain temperature requirements for normal operation, including ambient temperature and operating temperature. Generally, the ambient temperature for the normal operation of semiconductor devices is lower than 80 degrees. When the temperature of the PN junction inside the LED reaches 140 degrees, it will fail. So how important is the heat dissipation design of the lighting fixtures?
During normal operation, its own temperature will be emitted through the pins or dedicated base, and then emitted to the surrounding air through the circuit board or aluminum substrate connected to the pins to ensure the normal operation of the LED.
Generally speaking, if the power of a single chip is greater than 0.2W, an aluminum substrate must be used for heat dissipation. For high-power LED chips, aluminum shells and aluminum heat sinks must be added. Of course, this is related to the density of LEDs in the entire lamp, and the heat dissipation design should also be considered for low-power LEDs with a concentrated layout design. This is like every electronic product around you, such as televisions, monitors, computer hosts, etc. Incorrect heat dissipation design will directly lead to shortened LED life and speed up light decay.
Heat dissipation design of the lighting system, which is the result of a combination of actual manufacturing, material accessories, structural modeling, and various experiences in heat dissipation design.
- Most of the upper limit of junction temperature can reach about 120 ℃, the current CREE LED chips should be regarded as higher150 degrees.
- The thermal resistance of the lamp bead varies according to the structure of the packaging material. Some multi-chips are as high as tens of degrees Celsius, and single-chips are generally lower. Of course, this is directly related to the junction temperature, which is an important parameter related to the life of the LED and the lighting efficiency.
- Currently, the design life of most LED lamps is 20000~50,000H, which is determined by many factors. First, the life of IC limits the life of the entire lamp.
- From the perspective of heat dissipation, the structural layout is actually a problem of contact thermal resistance and thermal conductivity bottleneck. In general, the structure should be integrated design and large areas within good contact. The lamp beads on the PCB should be distributed as evenly as possible to avoid the concentration of heat sources.
- Of course, the higher the driving efficiency, the better. Glue filling and other methods can be used to achieve the best effect of heat dissipation.
Generally speaking, a good heat dissipation design temperature rise is best controlled below 35℃. The junction temperature is below 80℃. The theoretical life span is more than 50,000hrs.
Current thermal design challenges
- Natural heat dissipation is limited by space (radiation heat dissipation itself has little effect and is limited by space size).
- The space for improvement of natural heat dissipation area is limited (limited heat dissipation by convection).
- Metal parts are used as radiators, and heat conduction in a small space is sufficient. (The temperature difference of the radiator is small, and the thermal conductivity has a small effect).
- Most of the heat dissipation design can only focus on lamp bead selection, heat dissipation area, interface contact thermal resistance, and radiation heat dissipation.
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