As we all know, LED (Light Emitting Diode), light-emitting diode, is a solid-state semiconductor device, which can directly convert electrical energy into light energy. The heart of the LED is a semiconductor chip. One end of the chip is attached to a bracket, which is the negative electrode, and the other end is connected to the positive electrode of the power supply. The entire chip is encapsulated by epoxy resin.
The semiconductor wafer consists of two parts, one part is a P-type semiconductor, in which holes dominate, and the other end is an N-type semiconductor, where electrons are mainly. But when the two semiconductors are connected, a "P-N junction" is formed between them. When the current acts on the wafer through the wire, the electrons will be pushed to the P region, where the electrons and holes recombine, and then emit energy in the form of photons, which is the principle of LED light emission. The wavelength of light determines the color of light, which is determined by the material that forms the P-N junction.
In the process of converting electrical energy into light energy by the LED, when the current flows through the LED components, the temperature of the PN junction will rise, forming the so-called luminous heating, that is, the higher the current, the higher the temperature, and the higher the brightness of the light. Many people here may say, if this is the case, then why control the temperature of the LED?
To understand this problem, let's first take a look at how excessive temperature affects LEDs and LED lamps.
The effect of temperature on LEDs
①LED is mainly composed of bracket, silver glue, chip, gold wire (copper wire) and epoxy resin. Except for bracket and epoxy resin, the damage of the other three materials can directly cause the LED to stop working, and the excessively high The temperature of the LED is exactly the nemesis of the above three materials. In a high temperature environment, the light decay of the LED will increase, reducing the service life of the LED, and even the gold wire (copper wire) can be burned in an instant, causing the LED directly damaged.
② A single LED cannot directly meet the needs of use. Often accompanied by the LED, there is also an LED power supply (driver). The power supply itself is a heating body, and it has its own temperature resistance requirements. If the temperature is too high, it will cause the power supply Disability or even damage, resulting in LED lamps not working properly.
Therefore, it is urgent to solve the temperature problem of LED and LED lamps, that is, heat dissipation.
Principle of heat dissipation
Heat dissipation in the traditional sense is the so-called heat transfer, and the principle of heat transfer has the following three points:
As we all know, heat can be conducted through a medium, and it can be transferred from a position where the temperature is too high to a position where the temperature is too low. In this case, the conductivity of the material, the thermal resistance caused by the structure of the heat sink, and the shape and size have May interfere with heat transfer.
The high school physics teacher told us that heat goes upwards. In fact, this is the so-called radiation, which is very limited to the surrounding environment and the material of the radiator itself.
People who buy a building should have heard the concept of a one-story building. Sales generally say that such a building is very cool, and it is true. The so-called convection is to dissipate heat through the flow of gas or liquid, and its heat dissipation rate generally depends on the flow. If you want to achieve the speed on the lamp body, it is difficult (considering factors such as waterproof and safety regulations), but it is not without specific reference to the following products.
The above three points of heat dissipation, the most used in lamps now is heat conduction, which is also the topic we focus on today.
Friends who read the above text should notice that there are three points that affect heat conduction, material, structure and size.
When it comes to materials, we have to talk about the composition of LED lamps. First of all, one of the most common LED lamps is composed of heat sinks (lamp bodies), LEDs, substrates, power drivers, and optical components. Among them, the heating body includes LEDs and two drivers, and the heat sink includes a radiator, a substrate, or an aluminum optical element (the thermal conductivity is almost negligible).
The radiators of LED lamps are generally made of aluminum, which are divided into two categories, stretched aluminum mainly made of aluminum such as AL6060, and die-cast aluminum mainly made of ADC12. Among them, the thermal conductivity of ADC12 is 96.7w/m ℃, and aluminum materials such as AL6060 are 198~250w/m ℃. The latter is better than the former in terms of thermal conductivity. Some people may ask here, since aluminum materials such as AL6060 The heat conduction is so good, why does ADC12 appear?
①ADC12 die-casting aluminum, which can be mass-produced quickly and can meet the general requirements of design aesthetics (almost any shape you can think of can be presented), but because of low changeability and high design mold cost, it is generally used when the size is not very large. , on products that do not need to be modified, and also because of its relatively cheap price, it has long occupied the shell market.
②Stretched aluminum, strong thermal conductivity, low mold cost is generally one-tenth of that of die-casting molds, flexible in length changes, and fast processing and mass production. However, the reasons why it cannot fully occupy the market of the shell are as follows: insufficient hardness, It is easy to deform, and the appearance can only be stretched, with few changes, which cannot meet the market appearance requirements.
Combining the above two points, it can be concluded that the heat dissipation stretched aluminum is better than the die-casting aluminum, and the appearance practicality of the latter is better than the former, which depends on the cost requirements and appearance requirements of the design.
In addition to the shell, the substrate is also one of the important heat sinks. The common heat dissipation substrates that can be used on LED lamps on the market include copper substrates and aluminum substrates. Among them, the thermal conductivity copper substrate is better than the aluminum substrate, but the price is relatively high. , Most designers or enterprises will choose aluminum substrates as heat sinks.
①Appearance can use the fin design shown in the figure below.
②In the early stage of design, you should think about the placement of electronic components to avoid common heat sources as shown in the following diagrams.
③With the permission of the structure, it is best not to fix the position of the power supply to the bottom of the light source, because according to the principle of heat flow, the heat goes up. In the case that it is really impossible to control, it is best to reserve the conduction distance to More than 3CM (as for why, please Baidu heat conduction related knowledge). The design plane is as shown below.
I won't go into details about this. For example, one square meter of material can export 1W of heat, and one square meter of material can export 100W of heat. The concept is very clear, and there is no need to go into details. It can only be recommended here, within a reasonable range. Choose a reasonable size inside.
Heat dissipation misunderstanding
After reading the above information, many people should understand that thermal conductivity is very important for the heat dissipation of lamps, so many people will choose better heat dissipation materials, so they are superstitious about those so-called high prices that have not been verified by many parties. Material, in fact, it has been experimentally found that using ordinary aluminum for heat dissipation, after many tests, the temperature of the heat sink is only 3-5° higher than some so-called high-tech heat dissipation materials. A material with excellent thermal conductivity can only reduce the temperature by 3-5° when the thermal resistance is zero, but the cost is another matter.
In addition to the material, there are also many misunderstandings on the heat pipe. First of all, what is a heat pipe? A heat pipe (called a heat pipe) is a special material with fast temperature uniformity. The characteristics of heat pipe, and its characteristics of rapid temperature uniformity make it have excellent thermal superconducting performance; Natural geothermal references, etc., play the role of rapid heat conduction, and are the most common and efficient thermal (non-heat-dissipating) components in today's electronic product heat-dissipating devices.
This introduction sounds great, but the heat from the heat sink eventually needs to be taken away by air convection. Without the support of the cooling fins, the heat pipe will soon reach thermal equilibrium and the temperature will rise together with the heat sink. That doesn't make much sense. If you add fins to the heat pipe, you will eventually use the fins to dissipate heat, and the contact point between the fins and the heat pipe is not as good as other methods, and it will also lead to an increase in cost and heat dissipation. The effect is not improved. Therefore, when using heat pipes, try to consider the rationality of their structure and not use them blindly.
There are three points of heat dissipation, conduction, convection, and radiation. I have explained conduction and convection. Next, let’s talk about radiation. In fact, it does not make much sense for the heat dissipation of lamps. However, many people will listen to what manufacturers say about nanometers. Radiant materials or something, in fact, this is really negligible for lamps. Why? First of all, for lamps, the temperature range is about 50-80°, and in this case, even if the radiation material reaches the radiation capacity of black body radiation, it can play a maximum of a few percent of the heat dissipation, and the coating It itself hinders heat conduction, thus affecting the heat dissipation of lamps.
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