LEDs are point sources that differ from current energy-saving lamps or incandescent lamps on the market. Point light source, as its name suggests, is a point of light, and this point of light also has another important feature, that is, only one side has light (all traditional back reflectors have no meaning for the LED itself). If we want to use this special light for lighting, we must process this light to meet our requirements and objectives, so we need to add optical design to change this relatively concentrated point source. It has a certain angle of astigmatism, which is known as the straw hat LED. The straw hat LED is a kind of package that is professionally used for lighting design. On the one hand, it can change the light from a concentrated point source to a certain angle. The light source, on the other hand, has less light decay and allows more light to be transmitted for us.
Although the straw hat LED has a qualitative leap in optical angle, it is still a point light source compared to the familiar surface light source. Because its brightness is too concentrated, if the human eye directly looks at the light source, it will still be ours. The eyes cause some damage. Therefore, in the application of LED lighting in home lighting, we have to further process its light, and then add a certain optical design to achieve the purpose of direct observation by the human eye, while softening the light, on the other hand, we must zoom in again. Irradiation angle, although the straw hat LED itself has been in the 120 degree range, but if it is directly applied to the home lighting, it gives us the feeling that the roof is black, we need to use the outermost transparent cover of the lamp to disperse the light again. To adapt to normal home lighting.
The difference between high power and low power LED
With the continuous development of LED technology, people have developed a single LED chip with more and more power to adapt to large-area lighting. At present, the world's most advanced edge can achieve a single LED power of more than 200W, although the power can be very Big, but its price/performance ratio is not good. Let's take an analysis of several common LEDs as an example.
At present, a large number of 0.06W power is used in the industry. The maximum brightness can be 7LM. We will calculate the 6LM according to the normal time. If we want to achieve 1W, we need 17 identical LEDs. It is 17*6=102LM, which means 100LM/W can be achieved. If we use a single chip for 1W power, the output brightness is up to 80LM, and the commonly used one is about 60LM, which is a brightness. The main difference, it can be seen that for home use, we still have to choose a low-power LED is more appropriate.
From the perspective of product cost, the cost of high-power LED is higher than the cost of low power. This comes from two aspects, one is the cost of LED itself, and the other is high-power LED with aluminum heat sink. The board, plus natural cooling, can meet the requirements.
From the point of view of the future maintenance cost of the product, if our luminaires fail during use, we can find any electrical repair shop to replace the damaged LED. A 0.06W LED costs up to 1 yuan, plus the maintenance cost does not exceed 5 yuan, if it is to replace a 1W LED, LED cost will be 8 yuan, plus maintenance costs will be about 15 yuan. Relatively speaking, a small power LED market can be purchased in an electronic market, and high power can not be bought anywhere.
LED power to high power is a market trend and the mainstream of future development, but at present it is not yet suitable for home lighting because it has not yet reached the desired result in terms of technology.
The importance of thermal design
The LED itself is a semiconductor device, and all semiconductor devices have certain temperature requirements for normal operation, including ambient temperature and operating temperature. Generally, the ambient temperature of a semiconductor device is normally lower than 80 degrees. When the PN junction temperature inside the LED reaches 140 degrees, it will fail. In normal operation, its own temperature will be radiated through the pin or the dedicated base, and then radiated to the surrounding air through the circuit board or aluminum substrate connected to the pin to ensure the normal operation of the LED. Generally speaking, if the single power is more than 0.2W, the aluminum substrate should be used for heat dissipation, and the aluminum shell and the aluminum heat sink should be added with high power. Of course, this is related to the number and density of LEDs in the entire lamp. The low-power LEDs that are too concentrated should also consider the heat dissipation design. This is like every piece of electronic products around you, such as: TVs, monitors, computer mainframes, etc. Incorrect heat dissipation design will directly lead to shortened LED life and faster light decay.
1. Most of the junction temperature can now be around 120 degrees. At present, CREE should be considered as high, 150 degrees.
2. The thermal resistance of the lamp bead varies according to the structure of the packaging material. Some multi-chips are up to several tens of times. The single chip is usually a single digit, at most a dozen. Of course, this is directly related to the junction temperature, which is related to the LED lifetime. Important parameters such as light efficiency.
3. At present, the design life of most LED lamps is 20,000~50000H, which is determined by many factors. Firstly, the life of ICs limits the life of the whole lamp.
4. The structural layout is actually a problem of contact thermal resistance and thermal conduction bottleneck in terms of heat dissipation. This involves more and is not listed. In general, the structure is integrated, and a large area is in good contact. The lamp beads on the PCB are distributed as evenly as possible to avoid concentration of heat sources.
5. Drive efficiency, of course, the higher the better, the maximum heat source on the layout as much as possible, that is, LED lamp beads. The method of filling glue can be used to achieve the effect of heat dissipation and fixed insulation.
Comprehensive consideration: good heat dissipation design temperature rise is best controlled below 35. The junction temperature is below 80. The theoretical life is more than 50,000.
Current thermal design challenges
1. Natural heat dissipation is limited by space (radiation heat dissipation itself is less affected and limited by space size).
2. The natural heat dissipation area has limited space for improvement (limited convection heat dissipation).
3. At present, metal parts are used as heat sinks, and heat conduction in a small space is sufficient. (The temperature difference of the radiator is small, and the thermal conductivity is less affected).
4. Most of the heat dissipation design can only focus on lamp bead selection, heat dissipation area, interface contact thermal resistance, and radiation dissipation. Almost dead end.
5. The current situation, the breakthrough point of heat dissipation design in the heat dissipation design of the lighting system, is the result of a combination of practical production, material accessories, structural modeling and heat dissipation design.
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