LEDS are complex devices. LED lighting not only has common problems related to semiconductor design and operation, but also is mainly used for luminescence.
Therefore, optical coating, beam management devices such as reflector and lens, wavelength conversion fluorescer and so on have further system complexity. Nevertheless, heat management is critical for reliable solid state lighting (SSL) products. In addition, you need to understand how to cool leds in both static and transient Settings.
For leds, there are two thermal management parameters to follow. One is the required working temperature, and the other is the maximum working temperature. Generally, the required operating temperature needs to be as low as possible. This can ensure high electro-optical efficiency, good spectral quality and long device life. Operating at high temperatures will not only reduce the light produced by LED, but also reduce the quality and quantity, which will eventually trigger many failure mechanisms.
LED manufacturer for these defects are very proficient in, can design the products as high as 130 ° C junction temperature. Due to the thermal resistance of LED packaging, printed circuit board (PCB) temperature of about 10 ° C. If higher than the rated junction temperature, every 10 ° C, the LED life reduced to half.
By converting electrons into phonons, leds are relatively inefficient. High-brightness white leds can achieve 40 percent efficiency, while UVC leds may only have 5 percent efficiency. In both cases, excess heat must be removed by conduction to prevent overheating. This is the responsibility of the LED light source or lighting designer.
Static cooling LED
A common way to keep leds cool is to attach them to radiators. Heat from the LED is transmitted to the radiator and then released into the air. If quantity of heat is removed by water or other fluid, radiator is called cold plate sometimes, because the heat sink system that is associated often wants to design working fluid to be in the fixed temperature below indoor environment.
The effective transport of heat from LED to radiator depends on the high thermal conductivity of the material. For example, as can be seen from the diagram in figure 1, copper is superior to aluminum and brass, and superior to stainless steel.
Figure 1. The material has different degrees of thermal conductivity.
Although copper is the best thermal conductor of these metals, the thermal conductivity is independent of the thickness of the material. The ability to transfer heat through material conduction is mainly related to thermal resistance. The thicker the thickness, the greater the thermal resistance.
Dielectric and air flow
For example, a medium-high power LED flood light array is usually built on a heat-conducting PCB. At the top, copper is electrically connected to the LED, while at the bottom, an aluminum plate conducts heat. There is a dielectric layer between copper and aluminum to prevent electrical short circuit of copper plate to aluminum. Manufacturers have taken different approaches to the selection of dielectric materials, ranging from organic materials to inorganic compounds, covering the entire spectrum. As shown in figure 2, the dielectric material with the smallest thermal resistance is almost an order of magnitude, and the thinest dielectric material can be applied while still providing the required insulation isolation.
Figure 2. The thickness of the dielectric material affects the heat resistance.
However, figure 2 does not tell the whole story. Assuming the device is air-cooled, there will be many interfaces in the thermal path between the LED and the radiator. Some are bridged by solder, some by adhesive, and others will be pressed together (for example, using screws). These joints present additional barriers to heat transfer, which can be large, unpredictable, and change over time.
The series/parallel addition of all thermal resistance and interface resistance in the system is called thermal resistance, and the conduction path is designed to keep the LED cool. Calculations are analogous to resistance networks. In figure 3, the voltage is essentially the temperature, the current is the heat flux, and the resulting resistance is the heat resistance.
In figure 3. In development, you can rely on the equivalent resistance of the heat conduction path. In order to obtain a complete thermal impedance system model, thermal interface resistance must be added at each transition point between materials.
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