In this article, you can find the reasons for why high power LED heating, and how much heat it will generate.
Like the traditional light sources, the semiconductor light-emitting diode (LED) also produce heat during working, their heat depends on the overall luminous efficiency. Under the action of the applied electricity, electron and hole radiative recombination produce the electroluminescence, the light in the vicinity of the PN junction need to radiate out of the semiconductor media and package of LED chip, and then reach the outside. Integrated the current injection efficiency, luminescent quantum efficiency and chip emitting efficiency, ultimately only about 30-40% of the input energy is turned into light, the remaining 60-70% of the energy convert to thermal mainly by the way of the non-radiative and combinative generated lattice vibration.
The rise of chip temperature will enhance the non-radiative recombination, and further weaken the luminous efficiency. People subjectively believe that there is no heat in high-power led, but indeed there are heat, so may course problems when in use. In addition, many people who are the initial use of high power led, and don't know how to solve heat problem effectively, making the reliability of LED products a major problem. So, whether there is heat generated in led ? How much heat it can generate ?
When high power LED in the forward voltage, electrons obtain energy from the power supply, driven by the electric field to overcome the PN junction from the N zone transitions to the P zone, these electrons occur recombination with the electron hole in P zone. As the free electrons that was drift to the P region with the energy above the valence electron of P zone, when recombination, the electrons returned to low-energy states, excess energy is released in the form of photons. The emitted photon wavelength are related with energy difference (Eg). So light-emitting area is mainly in the vicinity of PN junction, light is due to the energy release when electron and hole recombination. In a semiconductor diodes, the electrons will encounter resistance from the time that it enter semiconductor area to the time it leave the area. Simply from the principle, the number of electrons sent from the negative source of semiconductor diodes is equal to the electrons that were sent back to the positive. When the ordinary diodes in recombination between electron and hole, due to the factors of energy difference (Eg), the released photon spectrum is not in the visible range.
When the electrons runing in the diodes, the power will be lost because of the resistance. The consumed power compliance with the electronics basic laws:
P = I2 R = I2 (RN + + RP) + IVTH
Formula: RN Represent the Resistance of N Zone.
VTH Represent turn-on voltage of PN junction.
RP Represent the resistance of P zone.
The heat generated by consumed power as follows:
Q = Pt
Formula: t Represent power time of the diode.
Essentially, LED is still a semiconductor diode. So when LED working in the forward current, its work process in line with the above description. The consumed electric power as follows:
P led = U led × I led
Formula: U led Represent forward voltage between the both ends of LED.
I led Represent the current flowing through the LED.
The consumed electric power convert into heat:
Q = P led × t
Formula: t Represent the power-time.
In fact, the power released by recombination between electron and hole is not provided directly by the external power supply, but because when the electron in N zone and without external electric field, its energy level is Eg higher than the valence electron in P region. When it reaches P zone, and occur the recombination, it will release such energy. Eg size is determined by the material itself, has nothing to do with the external electric field. The role of external power supply is only to push electron for directional movement, and to overcome the role of PN junction.
The heat production of LED is unrelated to light efficiency; It's not exist the relationship that a few percent of the electric power generated light, the remaining percent power generate heat. through the High power LED's theoretical formula and base concepts, such as heat generation, thermal resistance and junction temperature, we can study the actual package design, evaluation and product applications for high power LED. At present, the thermal management is key issue for LED products without high luminous efficiency, so reduce heat generation is the drastic move to improve the luminous efficiency of high power LED, this would require technology development of various sectors, such as chip manufacturing, LED packages and LED applications.
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