This paper includes the discussion about the calculation methods of the lighting’s heat transfer. Also the usage of the calculation method of heat resistances in equivalent electrical circuits, by which the difference between the temperature of the attention point inside the lighting and environment temperature can be directly obtained. Such a method is advanced on estimating the feasibility of a heat transfer’s structure. The calculation process has been demonstrated by an example of cooling of LED lighting in this paper.
Key word: lighting, heat transfer, heat resistance, LED
High power LED is much better than ordinary small power LED in the cooling performance, it has separated electricity and heat channels, its LED chips are connected in a metal embedded chip, radiating performance has improved a lot.
However, high power LED is used for special lamps and lanterns, or lamps and lanterns which are used in severe environmental, shell protection class of these lamps always above IP 65, if the case is nonmetal materials (such as plastic), although LED was connected with aluminum plate (MCPCB), if the heat of aluminum plate cannot be effectively transmission to shell surface, the quantity of gathered heat will make aluminum plate temperature rising sharply, resulting in high temperature, it increased the possibility of disabled LED, resulting LED light failure aggravate, shorten service life.
In theory heat dissipation calculation of lamps, there are a lot of difficulties in calculating the heat conduction theory of lamps, the main difficulty is the calculation when conduction and convection both play a role in heat transfer, however, convection is in the closed cavity, boundary conditions are very complex. Transmission will through multi-layer heat conduction materials, multi-layer interface, and sectional area is usually ranging, leading to the distribution of the heat flow line is very hard to obtain through analysis before the calculation.
Due to lamps and lanterns warm up gradually after turned on, and finally achieve heat stable state, that is, all points hold highest temperature when heat stability state , so in general the calculations of heat dissipation of lamps only consider the steady state, transient temperature distribution is not important. Regarding steady heat source for containing in isotropic single medium heat conduction obey Poisson equations :
k is thermal conductivity of the medium. q’’’ is heating power of the heating source.
Because the structure of lamps are a variety of medium, so in the practical calculation, a must for every kind of medium in solving one type, the calculation of temperature field distribution in the lamps and lanterns is very difficult, and it is not necessary. In fact, the thing we are concerned is whether the body temperature of some part ranges in the temperature tolerance, it is ok after the thermal stable temperature of these parts calculated.
In this paper, we discussed the thermal resistance calculation of circuits, the disadvantages of thermal resistance calculation is that we should not know exactly what the environment temperature, also need not solving the temperature field in the lamps, calculated directly within the rise of temperature of the lamps concerns, difficulty is the distribution of heat flow line must be through the analysis and not calculated, however, the process is very complicated.
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