When choosing high-power LED lamps, attention should be paid  heat dissipation (Science)

　　1. Compared with previous lamps, why do LEDs need  dissipate heat?

　　1. Tungsten filament lamps, halogen lamps, fluorescent lamps, metal halide lamps: they are illuminated by high temperature heating. The higher the temperature of the filament, the brighter the light emitted.

　　2. Why are LEDs afraid of heat?

　　Overheating will cause the aging of the LED. When the current flows through the LED device, the temperature of the P-N junction will rise. Strictly speaking, the temperature of the P-N junction area is defined as the junction temperature of the LED. Usually because the device chip has a small size, we can also regard the temperature of the LED chip as the junction temperature.

　　The limitation of light extraction efficiency is the main reason for the increase of the junction temperature of the LED. It is known that the heat dissipation capacity of the LED device is also a key condition that determines the junction temperature.

　　As shown in the figure below: The higher the junction temperature, the smaller the luminous flux; the worse the working state. The shorter the life span.

　　2. Ways of heat dissipation: There are three basic ways of heat transfer: conduction, convection and radiation.

　　The main transmission path of the radiar is conduction and convection heat dissipation, and the radiation heat dissipation under natural convection.

　　1. Conduction: the way heat is transferred from the higher temperature part of the object  the lower temperature part along the object

　　Facrs affecting heat conduction: the thermal conductivity of the heat dissipation material, the thermal resistance caused by the heat dissipation structure, and the shape and size of the heat conduction material.

　　As shown below:

　　2. Radiation: a phenomenon in which high-temperature objects emit heat directly  the outside

　　Facrs affecting thermal radiation: the thermal resistance of the five mediums in the surrounding environment, and the characteristics of the thermal radiation itself.

　　3. Convection: a way of transferring heat by the flow of gas or liquid

　　Influencing facrs: gas flow and its velocity, liquid specific heat capacity and flow velocity and volume.

　　Three. How  judge whether the heat dissipation of the lamp is good

　　1. Use professional equipment  measure junction temperature,

　　2. How  measure junction temperature without professional equipment? If you uch the hot lamp with your hands, the heat dissipation is not good.

　　3. It may not be good if the lamp body is not hot when uched by hand. When LED lamps work normally, a good radiar must have a lower temperature, but a lower temperature radiar may not be good. As shown below:

　　4. Use the "half-hour illumination method"  judge whether the heat dissipation is good

　　Generally, when the junction temperature of the LED increases, the luminous flux will decrease. Then, we can inversely deduce the change of the junction temperature by measuring the illuminance change of the lamp at the same position. As shown in the figure below: "Half-Hour Illumination Method" indirectly tests the junction temperature change

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