1. The importance of optical design
LED is a point light source, which is different from energy-saving lamps or incandescent lamps currently on the market. As the name suggests, a point light source is the light emitted by a point, and the light emitted by this point also has another important feature, that is, there is light in only one direction (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 achieve our requirements and purposes. Therefore, we need to add optical design to turn this relatively concentrated point light source into astigmatism with a certain angle. This is what everyone calls The familiar Straw Hat LED, Straw Hat LED is a package form professionally used in lighting design. On the one hand, it can change the light from a concentrated point source to a diffuse light source with a certain angle. On the other hand, it has less light attenuation effect and allows more light to be transmitted for our use.
Although Straw Hat LED has made a qualitative leap in optical perspective, compared with the surface light source we are familiar with, it is still a point light source. Because its brightness is too concentrated, if the human eye looks directly at the light source, it will still cause certain damage to our eyes.
So when LED lighting is used in home lighting, we need to further process the light and 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 need to enlarge its illumination angle again. Although straw hat LED itself has a 120-degree range, if it is directly applied to home lighting, it will give us the feeling that the roof is pitch black. We need to use the outermost transparent cover of the lamp to disperse the light again, thereby adapting to normal home lighting.
2. The difference between high-power and low-power LEDs
With the continuous development of LED technology, people have developed increasingly powerful single LED chips to adapt to large-area lighting. Currently, the world's most cutting-edge technology can achieve a single LED power of more than 200W. Although the power can be very high, its cost performance is not good. Let's do an analysis using several common LEDs as examples.
Currently, a large number of LEDs are used in the industry with a power of 0.06W, and the maximum brightness can be 7LM. We will temporarily calculate it based on the ordinary 6LM. If we want the power to reach 1W, we need to use 17 identical LEDs, with a total brightness of 17*6=102LM, which means it can be done. 100LM/W, if we use a single chip with a power of 1W, its output brightness can reach up to 80LM, and the commonly used ones are generally around 60LM. This is a major difference in brightness. It can be seen that for home use, we still have to choose low-power LEDs.
From the perspective of product cost, the cost of high-power LEDs is higher than that of low-power LEDs. This comes from two aspects. One is the cost of the LED itself, and the other is that high-power LEDs require aluminum heat sinks. For low-power LEDs, ordinary circuit boards and natural heat dissipation can meet the requirements.
From the future maintenance cost of the product, if our lamps malfunction during use, we can find any electrical repair shop to replace the damaged LED. The cost of a 0.06W LED is at most 1 yuan, and the maintenance fee is not more than 5 yuan. If it is to replace a 1W LED, the LED cost is 8 yuan, and the maintenance fee is about 15 yuan. Relatively speaking, low-power LEDs can be purchased in any electronic market, but high-power ones may not be available everywhere.
LED's trend toward high power is a market trend and the mainstream of future development. However, because it has not yet achieved the results we expect technically, it is not yet suitable for home lighting from a technical perspective.
3. The importance of thermal design
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 for normal operation of semiconductor devices is lower than 80 degrees. When the PN junction temperature inside the LED reaches 140 degrees, it will fail. During normal operation, its own temperature will be emitted through the pins or a special base, and then emitted to the surrounding air through the circuit board or aluminum substrate connected to the pins to ensure the normal operation of the LED.
Generally speaking, if the power of a single chip is greater than 0.2W, an aluminum substrate must be used for heat dissipation. If the power is higher, an aluminum shell and aluminum heat sink must be added. Of course, this is related to the number and density of LEDs in the entire lamp. Heat dissipation design must also be considered for low-power LEDs that are too concentrated. This is like every electronic product around you, such as televisions, monitors, computer hosts, etc. Incorrect heat dissipation design will directly lead to shortened LED life and accelerated light decay.
Nowadays, most junction temperature limits can reach about 120 degrees. Currently, CREE's is considered to be higher, at 150 degrees.
The thermal resistance of lamp beads varies according to the structure of the packaging material. Some multi-chip ones are as high as tens, while single-chip ones are generally single digits, at most a dozen. Of course, this is directly related to the junction temperature, which is an important parameter related to LED life, light efficiency, etc.
At present, the design life of most LED lamps is 20,000~50,000H, which is determined by many factors. First, the life of the IC limits the life of the entire lamp.
In terms of heat dissipation, the structural layout is actually a matter of contact thermal resistance and thermal conduction bottlenecks. This involves a lot and is difficult to list. In general, the structure is integrated and has good contact over a large area. Just distribute the lamp beads as evenly as possible on the PCB to avoid the concentration of heat sources.
Of course, the higher the driving efficiency, the better. The layout should be based on the largest heat source, which is the LED lamp beads. Glue filling and other methods can be used to achieve the effect of heat dissipation and fixed insulation.
Comprehensive considerations: For a good heat dissipation design, it is best to control the temperature rise below 35. The junction temperature is below 80. Theoretical life span is more than 50,000 years.
4. Current heat dissipation design problems
1. Natural heat dissipation is too limited by space (radiation heat dissipation itself has less impact and is limited by space size).
2. There is limited room for improving the natural heat dissipation area (convection heat dissipation is limited).
3. Currently, metal parts are used as radiators, and heat conduction is sufficient in a small space. (The temperature difference of the radiator is small, and the thermal conductivity coefficient has a small impact).
4. Most heat dissipation designs can only focus on lamp bead selection, heat dissipation area, interface contact thermal resistance, and radiation heat dissipation. Almost at the end of the road.
5. In the current situation, the breakthrough point of heat dissipation design is the heat dissipation design of the lighting system, which is the result of a combination of actual manufacturing, material accessories, structural modeling and heat dissipation design experience.
ANNA