Monday, December 27, 2010

【 Weak current College 】 LED junction temperature causes and countermeasures 】

1. What is LED by TJ?
The basic structure of a LED is a semiconductor P-N junction. Experiments that when current flows through the LED components, the P-N junction temperature will rise, the strict sense, the P-N junction temperature is defined as LED's junction temperature. Usually because component chip has a small size, so we can put the LED chip temperature as TJ.
2, have LED junction temperature for what reasons?
When you work in, LED to the following five case prompted the junction temperature rise to varying degrees:
A, a symbol of electrode structure, window layer substrate or junction area of materials and conductive silver colloid, etc are there certain resistance values, the resistors are barrier plus, constitute LED component series resistance. When current flows through the P-N junction, also flow through these resistance, resulting in Joule heating, chip temperature or junction temperature increases.
B, because the P-N junction is not possible to extreme perfection, efficiency of note who will not be reached 100%, that is to say, apart from the LED work N P zone to zone into the charge (hole), N district to P note people charge (e), in General, the latter category of charge note people will not produce the photoelectric effect, and in the form of heat consumed. Even if the useful part of the charge injection, nor will all become part of the light, and the knot area of impurities or defects, and eventually become hot.
C, practice has shown that light efficiency limit is high temperature rise LED junction. At present, the growth of advanced materials and components manufacturing process has been able to make the LED most input energy into light radiation energy, however due to the LED chip materials compared with the surrounding medium, has a much larger number of refraction coefficient, chip inside of the vast majority of photon (> 90 per cent) could not be successfully overflow interface, whereas in chips and media interface generated total reflection, return-on-chip internal and through numerous internal reflection was chip materials or substrate absorption and lattice vibration of forms to become hot, knot temperature rise.
D, obviously, the LED heat dissipation capacity of components is to determine the level of the junction temperature and a critical condition. Heat capacity, the junction temperature drop, on the contrary, the thermal capacity when the junction temperature will rise. Due to the epoxy resin is a low calorie Guide material, P-N junction, it is difficult to heat generated through transparent epoxy upward emission into the environment, most of the heat through the substrate, silver paste, shell, epoxy adhesive layer, PCB and heat sink downward divergence. Obviously, the conductivity of the material will have a direct impact on the efficiency of the heat loss of components. A common type of LED, from P-N junction to ambient temperature of total thermal resistance in 300 to 600 ° c between the/w, for a good structure of power led components, the total thermal resistance of about 15 to 30 degrees centigrade/w. Huge difference indicates that standard thermal resistance of LED components only in very small input power you can normally work, and with a power-dissipation power is available elements to tile level even higher.
3, reduce LED junction temperature channels?
A, reduce the thermal resistance of the LED itself;
B, good secondary cooling bodies;
C, reduce LED and second thermal body thermal resistance between the installation interface;
D, control, rated input power;
E, lower ambient temperature

LED by the input power is the only source of thermal effect component, part of the energy into light, its remaining radiation section ultimately became hot, so as to uplift the temperature of the component. Obviously, reducing the effects of major LED temperature rise, one is to improve the conversion efficiency of electro-optic components (also knownas external quantum efficiency), so that as many input power into energy, another important way is to try to improve the capacity of heat loss of components so that the junction temperature heat generated, disseminated through various channels to the surrounding environment.

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