Design and analysis of heat sink based on LED lighting fixtures

LED lighting has three main advantages: energy saving, environmental protection, and green lighting. This makes LED one of the new generation of light sources in the world today to replace traditional light sources. During the operation of LED, due to the PN junction, the LED chip will generate heat, so a good heat dissipation design must be made for this situation. The heat generated by LED lighting fixtures after emitting light is mainly conducted through the LED substrate and the heat dissipation device installed on the LED. A good heat dissipation design can greatly extend the service life of the LED, so the heat dissipation design plays a vital role in the performance of the LED light source.

There are two main ways to solve the heat dissipation problem: (1) improve the quality of the LED chip inside the lamp, increase the internal quantum efficiency of the chip's light emission under the same working current, and thus improve the chip's luminous efficiency; (2) improve the heat dissipation design outside the lamp, configure a reasonable heat dissipation device to speed up the heat dissipation process. This article mainly starts from the second point and analyzes the heat dissipation of LED lighting fixtures.

2 Structure and heat conduction path of high-power LED

Figure 1 shows the basic internal structure of an SMT LED package.

Here is a TOP LED, which is also a method of mounting on a printed circuit board to achieve heat dissipation. The LED consists of a chip mounted on a lead frame that is soldered or bonded. The leads are made of copper, a highly conductive material.

The main heat flow path from the PN junction is formed by heat conduction from the lead frame to the lead end. Another part of the transfer path is from the surface of the chip to the surface of the package. Heat is diffused from the lead end by heat conduction and heat extraction through surface convection and radiation of the circuit board. The efficiency of heat transfer from the PCB board to the air has a significant impact on the temperature difference between the chip and the air.

3 Analysis of the characteristics of LED heat dissipation substrates used in lighting

The LED chip has a small active area and a large operating current, which causes the operating temperature of the LED chip to rise in a short time. The increase in the junction temperature of the LED PN junction causes the LED output light power to decrease, accelerates chip aging, and shortens the device life. As the junction temperature rises, the wavelength of the LED will also "red shift" (the visual effect of orange-red and amber LED color drift is more significant). Therefore, considering the adverse effects of color drift in practical applications, thermal design must also limit the maximum junction temperature.

In the heat dissipation channel of high-power LEDs, the heat dissipation substrate is the key link connecting the internal and external heat dissipation paths. It has at least the following functions: (1) heat dissipation channel for LED chips; (2) electrical connection substrate for LED chips; (3) physical support for LED chips. The substrate material of high-power LEDs used in lighting must have high electrical insulation performance, high stability, high thermal conductivity, a thermal expansion coefficient similar to that of the chip, flatness, and high strength.

Traditional heat sink materials are aluminum alloy or copper. Aluminum has a thermal conductivity of up to 209 W/m·K, good processing characteristics and low cost, so it is widely used. Copper has a thermal conductivity of 390 W/m·K, which is 70% higher than aluminum, but its disadvantage is that it is about three times heavier than aluminum and difficult to process.

However, when the application is limited to the conductive properties as the focus, copper is usually used. In addition to these two materials, some materials that increase heat dissipation, such as carbon-based compound materials, metal powder sintered materials, compounded diamonds, and graphite, are currently attracting attention as thermal conductive materials.

AlSiC is the latest material, which is a mixture of various aluminum alloys to produce special physical properties, controlled thermal expansion, high conductivity and significant strength characteristics. Of course, due to cost, these new materials are generally used in the substrate at the bottom of the power module and in direct contact with the chip.

4 Design of heat sink used in LED lighting fixtures

There are many types of heat sinks. From the manufacturing method, air-cooled heat sinks can be divided into stamped heat sinks, extruded heat sinks, cast heat sinks, bonded heat sinks, folded heat sinks, improved cast heat sinks, forged heat sinks, cut heat sinks, machined heat sinks, etc.

The size and thickness of the heat sink directly affect the effective heat dissipation area and heat removal capacity. A good heat sink should be larger than the contact surface and have as much heat dissipation area as possible. The thermal pad is also an excellent thermal conductive material. Its combination with the heat sink can improve the heat dissipation effect.

The envelope volume refers to the volume occupied by the heat sink. If the heat generating power is large, the required heat sink volume is relatively large. The heat sink design can be preliminarily designed based on the envelope volume, and then the details of the heat sink can be designed. The relationship between the heat generating wattage and the envelope volume is shown in formula (1):

To increase the efficiency of the heat sink, the thickness of the bottom of the heat sink has a great impact. The bottom of the heat sink must be thick enough to allow the heat to be smoothly transferred to all the fins so that all the fins can be used with the best efficiency.

However, if the bottom is too thick, in addition to wasting materials, it will also cause heat accumulation, which will reduce the thermal conductivity. A good bottom thickness design must be thicker at the heat source and thinner towards the edge, so that the heat sink can absorb enough heat from the heat source and quickly transfer it to the surrounding thinner parts. The relationship between heat dissipation wattage and bottom thickness is shown in formula (2):

Acid-resistant aluminum or anodizing the heat sink surface can increase the heat radiation performance, thereby increasing the heat dissipation efficiency of the heat sink. Generally speaking, it does not matter whether the exterior color is white or black. The treatment of surface protrusions can increase the heat dissipation area, but in the case of natural convection, it may hinder air flow and reduce efficiency.

The above design method is for reference only. The actual heat sink design must also consider the coordination with the device and the environment, especially the design of high-performance heat sinks needs to be combined with experimental verification and computer analysis and simulation. At present, the design of heat sinks has gradually reached its limit. The air cooling method cannot meet the heat dissipation requirements of high-power LEDs. The heat sink design should be combined with the power number of LEDs to make the heat dissipation design more flexible and diversified. In any case, heat sinks are still the most common heat dissipation method for LED lamps. Making good use of heat sink design can improve the heating condition of LEDs.

5 Heat dissipation analysis of LED distribution on the heat sink surface

To analyze the heat distribution of LED chips on the heat sink, we analyze the effect of heat dissipation by placing LEDs in different relative positions on a heat sink. The experimental material is a heat sink of 200mm×60mm×15mm, and the thermal conductivity of two thermal pads is 6.0W/m·K, with an area of ​​20mm×30mm. It is equipped with two single aluminum-based PCB boards with LED chips welded on them (the size is 15cm×20cm, and each PCB board is welded with one LED chip). The test instrument is a FLUKE temperature sensor. The ambient temperature during the experiment is 26℃ and the relative humidity is 52%.

The experimental process is as follows: The position of the LED chip is placed every 20mm on the heat sink, and a thermal pad is added between each PCB board with LED chips and the heat sink. The power supply voltage is 6.3V, the current is 0.7A, and the power is 4.41W, and the heat dissipation test is carried out. Table 1 is the collected data. d represents the distance between the two LED chips (in mm), and t represents the temperature of the LED chip (in °C, and the test time for each t value is 4h, that is, the data after the temperature stabilizes).

It can be seen from Table 1 that as the distance between the two LED chips increases, the temperature of the LED chip gradually decreases. This change is quite obvious. With temperature as the ordinate and the distance between the two LED chips as the abscissa, a scatter plot of the two is obtained, as shown in Figure 2.

As can be seen from Figure 2, the two approximate to form a curve, and the mathematical statistics method can be used to fit this curve. The final mathematical model is:

It can be seen that a reasonable selection of the heat sink area and the correct placement of the LED chips can achieve better heat dissipation. If the power and the shape of the heat sink are changed, the following relationship will be obtained, namely:

In formula (4), the coefficients of a and b are different for different powers (it is also possible that a is different and b is the same). In practice, the values ​​of a and b can be calculated based on the power and the size of the heat sink.

The positions of LED chips on a given area of ​​heat sink should be as far apart as possible, that is, when designing the PCB board, the positions of LED chips should be dispersed as much as possible, and not concentrated together. If the size of the lamp allows, the area of ​​the PCB board should also be as large as possible, because the area of ​​the heat sink is generally similar to that of the PCB board, which helps to dissipate heat. But it cannot be too large, because from the above analysis, it can be seen that when the area is large to a certain extent, the heat dissipation effect is basically unchanged.

6 Conclusion

LED lamps need to use heat sinks to control the temperature of LED chips, especially the junction temperature T, so that it is lower than the safe junction temperature for normal operation of LED chips, thereby improving the reliability of LED chips. Conventional heat sinks tend to be standardized, serialized, and universal, while new products are developing in the direction of low thermal resistance, multi-function, small size, light weight, and suitable for automated production and installation. Through the analysis of the heating principle of LED chips and heat dissipation calculations, the design of heat dissipation methods and the selection of heat sinks can be guided, ensuring that the LED works within a safe temperature range and reducing quality problems. Reasonable selection and design of heat sinks can effectively reduce the junction temperature of LEDs and improve the reliability of LEDs. If the heat dissipation problem of LEDs can be solved, the advantages of LED lighting can be shown and they will soon replace traditional light sources.