Hi , I would like to know the time taken by LED Lighting system to reach their maximum operating temperature( say 90 - 120 deg), Once they are switched on.
It is dependent on: 1) the electrical resistance of the LED die; 2) the voltage drop across the die; 3) the drive current through the die; 4) the thermal resistance between the die and the heat sink; 5) the ambient air temperature; 6) the thermal mass (capacity) of the heat sink; and whether the surrounding air is still or moving (such as driven by a fan).
Thinking of this intuitively, the electrical current flow through the LED die is generating thermal power, which is being absorbed by the heatsink and the ambient air. Similar in principle to an resistor-capacitor network, the heatsink temperature will rise until the flow of heat (i.e., the flow of thermal energy) into the heatsink equals the flow of heat out of the heatsink into the surrounding air.
The analogy is useful in that you can reason how the thermal "circuit" will respond to changes in the thermal resistances of the die-heatsink and heatsink-air, the thermal capacity of the heatsink., and the temperature differences (equivalent to voltages) between the die and heatsink and between the heatsink and air.
Is there any equation governing the heat flow and thermal convection in LED lighting system?
Ya sir, I have gone through the above response. But i want to know the time taken for the LED Lighting to reach the operating temperature and not the Junction temperature sir.
There is no definitive answer to this question, short of performing finite element analysis on a detailed thermal model of whatever the LED module or luminaire represents. Once the system reaches equilibrium, there will be a continuum of temperatures from the LED die junction to the surface of the heatsink and its supports. Determining the time needed to achieve equilibrium would require knowledge of the geometry and thermal properties of each and every component.
Sounds like you are actually talking about a complete luminaire? It really does vary depending on the module types and wattages, heat management, luminaire package etc. making it to hard to give a definitive answer but in general, they seem to start settling down after 5-10 minutes (depending on the bare module or luminaire) but I have noticed output variance as recorded by a spectrometer up to approx. 15 minutes after initiating.
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