Structural design of integrated high-power LED street lamp radiator

Light Emitting Diode (LED), as a new generation of green and environmentally friendly solid lighting source, has become the focus of people's attention. It has a series of advantages such as low power consumption, pure light color, all solid state, light weight, small size, and environmental protection. When LED emits light, part of the energy will be converted into heat, which will increase the temperature of the LED chip. The temperature has a great influence on the working performance of the LED chip. High temperature will lead to a decrease in the number of photons emitted by the chip, a decrease in color temperature quality, accelerated chip aging, shortened device life and other serious consequences. Therefore, in order to ensure the normal operation of the LED, the heat emitted by it must be dissipated in time. At present, more and more high-power LED chips are used. According to data, high-power LEDs can only convert about 10% to 15% of the input power into light energy, and the remaining 85% to 90% into heat energy, so the heat dissipation problem is more serious.

At present, high-power LED light sources are divided into two types. One is the array-distributed high-power LED light source, which arranges several LEDs in an array, as shown in Figure 1. The other is the integrated high-power LED light source, which integrates and packages several LEDs together, as shown in Figure 2. These two types of LED lamps have different light distribution curves, occupied space and heat dissipation due to the different LED chip layout methods. Relatively speaking, lamps made of integrated high-power LED light sources are lighter in weight, use less packaging materials, and can meet the requirements of street lighting in terms of light distribution compared with array-distributed high-power LED light sources. It is the future development trend of street lamps. However, because heat dissipation is more difficult than array-type, the life span is shortened, which has become a key problem hindering the development of integrated high-power LED light sources.

Figure 1 Array distributed high-power LED light source

Figure 2 Integrated high-power LED light source

This paper mainly uses ANSYS finite element software to optimize the structure of the integrated high-power heat source LED street lamp radiator. The operating temperature of high-power LED lamps is required to be below 75°C, so the purpose of this optimization is to reduce the quality of the radiator while striving to reduce the junction temperature of the LED chip to a minimum of less than 75°C.

1 Heat transfer theory and thermal analysis

1.1 Basic theory of heat transfer

There are three main methods of heat transfer: heat conduction, heat convection and heat radiation. In the heat dissipation system of LED street lights, all three heat transfer methods are present, but heat conduction and heat convection are the main methods. The strength of thermal conductivity depends on the product material. Many articles have been studied in this regard, and studies have shown that the key to solving the LED heat dissipation problem is not to find materials with high thermal conductivity but to change the LED heat dissipation structure or heat dissipation method. Therefore, this article mainly considers the difference in heat dissipation effect caused by different radiator structures.

The basic calculation formula for convective heat transfer is Newton's cooling formula. Let the temperature difference be △t, and agree that it is always a positive value. Then the Newton cooling formula is:

Where h is the surface heat transfer coefficient, unit is W / (m 2 K).

A heat exchange area, unit: m2 .

It can be seen from the convective heat transfer rate equation (1) that the convective heat transfer can be increased by increasing the temperature difference, increasing the surface heat transfer coefficient and increasing the heat transfer area. For LED street lights with natural convection heat transfer, the method of increasing the temperature difference and surface heat transfer coefficient is not convenient to use, so this article mainly increases the heat transfer surface area.

Fins are an effective way to increase the heat exchange surface. They allow the heat flux to be conducted along the rib height direction while dissipating heat to the surrounding environment by convection or convection plus radiation. The larger the heat dissipation area, the better the heat dissipation effect, but it is not a simple proportional relationship.

1.2 Radiator model establishment

The straight fin heat sink used in the preliminary design of this paper is shown in Figure 3. Its structural parameters include fin thickness, height, length, and substrate length, width and thickness. These six parameters are analyzed using ANSYS software to perform the structural design of the heat sink.

Figure 3 Preliminary heat sink model.

The outer surface of the heat sink in contact with the air is set to natural convection, the convection coefficient is 7.5W/(m2 · K), and the ambient temperature is set to 40℃, so that the working temperature of the LED street lamp can be guaranteed to be below 75℃ under normal circumstances. Due to the sealing effect of the lampshade, the other surfaces of the model are defined as heat insulation. The volume of the light source is 60mm×60mm×8mm. The power of the LED street lamp is 50W, of which 15% is converted into light energy and 85% is converted into heat energy, so the heat generation rate load of (1.47×106 ) Wm-3 is applied to the chip entity. The heat sink material is ZL104 aluminum alloy, with a thermal conductivity of 147W/m and a density of 2650kg/m3 . Under normal pressure and surface roughness, the contact thermal resistance between aluminum and aluminum is taken as 4.55×10-4m2 · K / W.

1.3 Optimization design

The orthogonal experimental design method has the advantages of a small number of experiments required to complete the test requirements, uniform distribution of data points, and the ability to analyze the test results using corresponding range analysis methods.

In order to reduce the computational scale of the simulation and analyze the influence of the structural dimension changes of the heat sink on its temperature field, this paper designs an orthogonal test to conduct multiple thermal analyses on the parameterized model. The six heat sink structural parameters that affect the final temperature field distribution are taken as factors, and each factor takes 5 levels (see Table 1). The heat sink quality and the maximum temperature of the chip are taken as test indicators, and the orthogonal table L25 (5 6 ) is selected.

Taking into account the size of the LED wick and the design structure of the entire lamp body, as well as the requirements for the quality and volume of the radiator, the number of fins A is (5-17), the fin height B is (20-60) mm, the fin thickness C is (1-3.8) mm, the substrate thickness D is (1-3) mm, and the substrate length E and width F are both (150-250) mm. The specific five horizontal values ​​are shown in Table 1.

Table 1 Parameters of orthogonal test

1.4 Analysis of test results

The experimental results and analysis are shown in Table 2.

Table 2 Test results data.

From Table 2, we can see that the number of fins has the greatest impact on the chip junction temperature, followed by the fin height, followed by the substrate length, substrate thickness, fin thickness and substrate width. That is, A > B > E > D > C > F.

The fin thickness has the greatest impact on the quality of the heat sink, followed by the fin height, and then the number of fins, base plate length, base plate width, and base plate thickness. That is, C > B > A > E > F > D.

Based on the analysis results, a graph showing the impact of different levels of various factors on the temperature target is drawn, as shown in Figure 4.

According to the quality formula, when other parameters remain unchanged, the parameter value is directly proportional to the quality result. The larger the value, the greater the quality, so no curve graph is drawn.

Figure 4 The impact of different levels of six factors on the maximum temperature of the chip

From the results of the range analysis, we can know that different factors have different effects on the two targets, and the same factor has different effects on the two targets. Therefore, the selection of the values ​​of different factors should be based on the principle of keeping the highest temperature of the chip as the lowest as the main goal and the minimum heat sink quality as the secondary goal. For example, the fin thickness ranks sixth in terms of the impact on the highest temperature of the chip, but has the greatest impact on the quality. Therefore, a smaller fin thickness can be selected to reduce the quality while minimizing the temperature.

In the 25 experiments, it can be seen that the best effect is achieved at the 25th time, that is, A5B 5C 4D3E 2F 1. At this time, the temperature is 59.61 ℃ and the mass of the radiator is 1.61 kg. The result is shown in Figure 5. The result after optimization is A5B 5C1D 5E5F 1. It has been verified that in this case, the temperature can be reduced to 58.09 ℃ and the mass of the radiator can be reduced to 0.98 kg. The result is shown in Figure 6.

It can be seen that the purpose of dual-objective optimization design has been achieved through orthogonal analysis.

Figure 5 Steady-state temperature field under the A 5B 5C4D 3E 2F 1 heat dissipation structure.

Figure 6 Steady-state temperature field under the A 5B 5C1D 5E 5F 1 heat dissipation structure.

2 Conclusion and Outlook

This paper studies the integrated high-power light source LED street lamp radiator by combining orthogonal test method and simulation experiment. With a small number of simulation experiments, the test data that can basically reflect the overall situation is obtained, and the degree of influence of different parameters on LED heat dissipation and quality is studied, and then a set of optimized parameter combinations is obtained. This optimization method is also applicable to other fin forms, and has great significance for the promotion and application of high-power centralized heat source LED lamps.