Transient cooling LED
The previous discussion assumes that in a steady state, the LEDs are permanently energized and the heat sink continuously dissipates thermal energy into the surrounding air. This thermal model fails in both cases. One is when the LED is turned on, and more usually in pulsed operation. Surprisingly, a thermal path can be designed to keep the LED cool during continuous operation, but it will overheat when it is turned on. When this is done, the associated thermal excursion may cause the LED to suddenly fail, similar to the sudden breaking of a tungsten filament when it is turned on. Therefore, the thermal solution design of LEDs needs to consider transient operations and include both time and space variables.
Time dependent
The time component of transient cooling is due to the specific heat capacity of the material in the thermal path. This can be added as a capacitor to the thermal model of the electrical resistance (the time dependence of thermal conduction is due to the thermal capacity of the material in the system, and the electrical equivalent model is an RC low-pass filter.). Heat capacity refers to the nature of the material that absorbs (or releases) heat when it is heated (or cooled). The size of the heat capacity is expressed as specific heat capacity (abbreviated as specific heat).
The electrical model analogy means that thermal impedance is sometimes used to describe the time-dependent thermal properties of a material. Please note that the distinction should be made at this time, because the thermal resistance can also be used to describe the static thermal resistance of the entire system.
Space dependence
The spatial component of transient cooling stems from which direction the heat spreads. For example, an LED mounted on a large thin metal plate. Initially, the entire board is at ambient temperature. LED as a spot heat source. When switched on, the LED generates heat that transfers heat to the board through conduction. Heat quickly passes through the metal plate, increasing the temperature in the area under the LED. Therefore, the first time, a small part of the metal plate is to cool the LED. The conductivity of the metal plate means that some of the heat from the LED will spread laterally within the plate and eventually appear on the surface. Therefore, the volume of the metal plate involved in cooling the LED will increase with time, resulting in a significant change in thermal resistance and heat capacity.
Spatial dependence is particularly important when there is a high thermal resistance interface or layer in the path. By taking measures, heat is spread in the largest possible area before the barrier, so that the LED can achieve better cooling in steady state and pulsed operation.
Convection and radiation
Any material above ambient temperature loses its heat through convection and radiation. Although these are the main mechanisms for the cooling of tungsten lamps, they play a minor role in the thermal management of LEDs. However, convection and radiation should be included in any model to ensure that it is closest to reality.
In short, the LED must be cooled to achieve the best efficiency and ensure the stability and life of its light output. A simple heat conduction steady state model can be constructed using a model based on electrical components. However, in order to properly understand the thermal path, especially under transient conditions, it is best to use tools that can accommodate changes in time, space, and temperature.
The temporal and spatial dependence of heat conduction explains why there is a hierarchy in material selection. High specific heat capacity or thermal conductivity can vary depending on the location of the material in the thermal path and the intended mode of operation of the LED.
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