Some basic knowledge about LED

1. LED is not a cold light source

Many people know that LED is a cold light source. Literally, it should be a light source without temperature. In fact, it is not the case. Cold light source does not mean no temperature. It is just a cold light source compared to most current light sources. When LED is working normally, it will also generate its own heat. The amount of heat is proportional to the LED's own power . In other words, the greater the power of a single LED, the more heat it will generate when working.

2. The importance of optical design

LED is a point light source, which is different from the energy-saving lamps or incandescent lamps currently on the market. As the name suggests, a point light source is a light emitted from a point, and this point light also has another important feature, that is, there is light in only one direction (all traditional back reflectors are meaningless for LEDs themselves). If we want to use this special light for lighting, we must process this light in a certain way to achieve our requirements and goals, so we have to add optical design to turn this relatively concentrated point light source into scattered light with a certain angle. This is the straw hat LED that everyone is familiar with. The straw hat LED is a packaging form professionally used for lighting design. On the one hand, it can turn light from a concentrated point light source into a scattered light source with a certain angle. On the other hand, it has less light decay effect and allows more light to be transmitted for our use.

Although the straw hat LED has made a qualitative leap in optical perspective, it is still a point light source compared to the surface light source we are familiar with. 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. Therefore, when LED lighting is used in home lighting, we need to further process its light and add certain optical designs to achieve the purpose of direct observation by the human eye, soften the light, and on the other hand, enlarge its irradiation angle again. Although the straw hat LED itself has a 120-degree range, if it is directly used for home lighting, it gives us the feeling that the roof is dark. We need to use the outermost transparent cover of the lamp to disperse the light again to adapt to normal home lighting.

3. The difference between high power and low power LED

With the continuous development of LED technology, people have developed single LED chips with higher and higher power to adapt to large-area lighting . At present, the world's most advanced single LED power can reach more than 200W. Although the power can be very high, its cost performance is not good. Let's take several common LEDs as examples for analysis.

At present, the industry uses a large number of single 0.06W power, and its maximum brightness can reach 7LM. We will temporarily calculate based on the ordinary 6LM. If we want to reach 1W power, we need to use 17 identical LEDs, and the total brightness is 17*6=102LM, which means it can reach 100LM/W. If we use a single chip with a power of 1W, the output brightness can reach up to 80LM. 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 LED is higher than that of low-power LED. This comes from two aspects. One is the cost of the LED itself, and the other is that high-power LED needs to be equipped with an aluminum heat sink, while low-power LED only needs to use an ordinary circuit board and natural heat dissipation to meet the requirements.

From the perspective of future maintenance costs, if our lamps break down during use, we can go to any appliance repair shop to replace the damaged LED. A 0.06W LED costs at most 1 yuan, and the repair fee is no more than 5 yuan. If we replace a 1W LED, the LED cost is 8 yuan, and the repair 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.

The trend of LED towards high power is a market trend and the mainstream of future development. However, since the technology has not yet achieved the results we expect, from a technical point of view, it is not yet suitable for home lighting.

4. The importance of heat dissipation design

LED itself is a semiconductor device. 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 should be 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 dissipated through the pins or special base, and then dissipated into 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 LED is greater than 0.2W, an aluminum substrate must be used for heat dissipation. For higher power, 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. Low-power LEDs that are too concentrated also need to consider heat dissipation design. This is like every electronic product around you, such as: TVs, monitors , computer hosts, etc. Incorrect heat dissipation design will directly shorten the life of LEDs and accelerate the speed of light decay.

1. Now most of the junction temperature limits can reach about 120 degrees. Currently, Cree 's is considered to be relatively high, at 150 degrees.

2. The thermal resistance of lamp beads varies according to the packaging material structure. Some multi-chip ones are as high as dozens, while single-chip ones are generally in the single digits, at most a dozen. Of course, this is directly related to the junction temperature, which is an important parameter related to the LED life, light efficiency , etc.

3. Currently, most LED lamps have a design life of 20,000~50,000 hours, which is determined by many factors. First of all, the life of the IC limits the life of the entire lamp.

4. From the perspective of heat dissipation, the structural layout is actually a problem of contact thermal resistance and thermal conductivity bottleneck, which involves a lot and is difficult to list. In general, the structure is integrated and the contact is good over a large area. The lamp beads on the PCB should be distributed as evenly as possible to avoid heat concentration.

5. Drive efficiency The higher the better, the layout should be as far away from the largest heat source, that is, the LED lamp beads. Gluing and other methods can be used to achieve the effect of heat dissipation, fixing and insulation.

Comprehensive consideration: For a good heat dissipation design, the temperature rise should be controlled below 35°C, the junction temperature below 80°C, and the theoretical lifespan should be above 50,000°C.

5. Current heat dissipation design problems

1. Natural heat dissipation is too limited by space (radiation heat dissipation itself has a smaller impact and is limited by the size of the space).

2. There is limited room for improvement in the natural heat dissipation area (convection heat dissipation is limited).

3. Currently, metal parts are used as heat sinks, and heat conduction is sufficient in a small space. (The temperature difference of the heat sink is small, and the thermal conductivity coefficient has little effect).

4. Most heat dissipation designs can only focus on lamp bead selection, heat dissipation area, interface contact thermal resistance, and radiation heat dissipation. This is almost a dead end.

5. At present, the breakthrough point of heat dissipation design lies in the heat dissipation design of the lighting system, which is the result of combining various experiences of actual production and manufacturing, material accessories, structural modeling and heat dissipation design.