Research on key issues of LED lamp heat dissipation modeling and simulation

　　0 Preface

　　LED is a semiconductor light-emitting device. Limited by the current manufacturing level of LED chips , only about 20% to 30% of the input power of LED high-power products is converted into light energy, and the remaining 70% is converted into heat energy. The increase in junction temperature will affect the life, light efficiency, light color (peak wavelength), color temperature, light distribution, reliability, luminous intensity, forward voltage, etc. of LEDs, which are all important factors affecting lighting quality.

　　In order to control the temperature rise of LED lamps and ensure the life and reliability of the lamps, domestic and foreign scholars have conducted a lot of research on the heat dissipation design of LED lamps for lighting, especially using finite element fluid mechanics CFD simulation software for heat dissipation simulation analysis, which can comprehensively analyze the heat conduction, heat convection and heat radiation of LED lamps, analyze and solve the temperature field and flow field inside and outside the LED lamps, etc., which is very suitable for the current heat dissipation simulation of LED lighting lamps.

　　This paper will analyze the key issues in the heat dissipation simulation modeling of LED lighting fixtures from several aspects, including boundary conditions (ambient temperature, gravity direction, etc.), thermal resistance calculation, heat load distribution and form, thermal conductivity and emissivity of heat dissipation materials, and verify the accuracy of the model simulation results through laboratory temperature measurement.

　　1 Boundary conditions

　　1.1 Ambient temperature

　　The temperature field distribution of 5WHL-A60LED bulbs at ambient temperatures of 20, 25, 30, 35, 40, 45 and 50℃ is simulated and analyzed. Figures 1 to 3 show the temperature trend of LED operating temperature (in the figure, max indicates the maximum operating temperature of LED, avg indicates the average operating temperature of LED, the same below), average temperature of radiator, and power supply temperature as the ambient temperature changes. It can be seen from the simulation result that the maximum temperature and average temperature of LED, average temperature of radiator and ambient temperature are linearly related, that is, the higher the ambient temperature, the higher the maximum temperature of LED and the average temperature of radiator. However, the relationship between them is not a pure linear superposition, and the proportionality coefficient is about 0.8.

1.2 Direction of gravity

　　Heat has a tendency to transfer in the opposite direction of gravity. Figure 4 shows the temperature simulation analysis results of 5WHL-A60 LED bulbs with three different installation methods. From the figure, it can be found that the temperature field of the lamp changes significantly due to the different directions of gravity. Therefore, in the simulation process, it is necessary to clarify the installation position and method of the LED lamp.

　　2 Thermal resistance

　　Thermal resistance (Rth) refers to the resistance encountered by heat in the heat path, which can be calculated by the thermal conductivity of the material (K):

　　Where L represents the length of the heat channel path, and A represents the effective cross-sectional area of ​​the heat channel.

　　Thermal resistance is divided into thermal conduction resistance and contact resistance. When heat is transferred in the same object by heat conduction, the thermal resistance encountered is called thermal conduction resistance. When heat flows through the interface of two contacting solids, the interface itself presents obvious thermal resistance to the heat flow, which is called contact resistance. The main reason for the contact resistance is that for any two objects with good surface contact, the actual contact area is only a part of the interface, and the rest is gaps. Heat is transferred by heat conduction and heat radiation of the gas in the gap, and their heat transfer capacity is far less than that of general solid materials.

　　For some thermal channel material layers, due to their small thickness, they may not be reflected in the modeling process, but are replaced by equivalent surface contact thermal resistance to facilitate CFD simulation analysis of heat dissipation modeling. For example:

　　(1) The LED light source is soldered to the aluminum substrate using the reflow soldering process, and a contact thermal resistance is set between the LED light source lamp bead and the aluminum substrate. The main material component of the reflow soldering layer is tin (96%), the thickness is generally 0.1~0.15mm, and the thermal conductivity is 60W/(K·m).

　　(2) As shown in Figure 5, the aluminum substrate consists of a conductive layer, a thermal insulation layer, and a metal base layer. The conductive layer has a small thickness and good thermal conductivity, so it can be ignored. The main thermal resistance is determined by the thermal insulation layer, which has a small thickness and poor thermal conductivity, while the metal base layer has a large thickness and good thermal conductivity. If the two are set as the same material body, the simulation results will have a large deviation.

　　The thermal resistance of the aluminum substrate insulation layer and the reflow solder layer is converted into an equivalent thermal resistance R equivalent, and the calculation formula is as follows:

　　Furthermore, R equivalent can be expressed by the equivalent thermal conductivity r equivalent, and r equivalent can be calculated as follows:

　　Where ri is the thermal conductivity of each layer of material, and hi is the thickness of each channel.

　　The lamp in this article uses Bergquist aluminum substrate (insulation layer thickness 0.076mm, thermal conductivity 1W/(K·m)), so the equivalent thermal conductivity K is equivalent to 2.88W/(K·m) and the thickness is 0.226mm.

　　(3) The aluminum substrate is connected to the heat sink through thermal grease or silicone gasket. This channel layer is set to surface contact thermal resistance, with a thickness of 0.5 mm and a thermal conductivity of 1.5 W/(K·m). Different bonding layer material thickness and thermal conductivity will affect the LED operating temperature, as shown in Figures 6 and 7.

　　Analysis shows that the smaller the thickness of the bonding layer, the higher the thermal conductivity of the bonding material, the lower the operating temperature of the LED, and the better the heat dissipation of the lamp.

　　3 Thermal load

　　3.1 Heat load distribution

　　The heat load is mainly distributed in two places, the LED light source and the power supply. The heat generated by the LED light source is the main heat source of the LED lamp. The current photoelectric conversion efficiency of the LED for lighting is about 30%, that is, about 70% of the LED input power PLED is converted into heat. The LED heat QLED is:

　　The electronic components in the LED lamp driver are also a heat source. The total power consumption (Ppower) can be obtained by subtracting PLED from the total lamp input power (Plamp). Then, based on the power efficiency, the heat generated by the power supply, Qpower, can be calculated:

　　3.2 Heat load form

　　There are two forms of heat sources: body heat source and surface heat source. The heat load of a 25W LED downlight is 17.5W. Heat dissipation simulation is performed according to the two forms of heat sources. The simulation results are basically the same, as shown in Figure 8. Therefore, different forms of heat sources do not have a great impact on CFD heat dissipation simulation analysis.

　　4 Thermal conductivity and emissivity of heat dissipation materials

　　4.1 Thermal conductivity of heat dissipation materials

　　The thermal conductivity of a material reflects the strength of the material's heat conduction ability. Heat conduction is the most fundamental factor affecting heat dissipation. It determines whether the heat generated by the LED lamp can be effectively and quickly transferred to the heat dissipation surface of the lamp. The thermal conductivity of different materials varies due to their physical properties, production processes, etc. The simulation analysis of the 14WLEDPAR30 spotlight uses heat dissipation materials with different thermal conductivities to see how it affects the operating temperature of the LED lamp. The simulation results are shown in Figure 9, which shows that the higher the thermal conductivity of the material, the lower the final operating temperature of the LED lamp and the better the heat dissipation effect.

　　4.2 Emissivity of heat dissipation materials

　　The thermal radiation coefficient γ of different materials is different. Even for the same material with different surface treatment processes, its thermal radiation coefficient is not exactly the same [14]. Therefore, in CFD heat dissipation simulation, the material and its surface treatment must be clearly defined. The temperature field of the heat sink surface radiation coefficient of the 7WLEDPAR16 spotlight was simulated and analyzed to be 0.95, 0.9, 0.85, 0.8, 0.7, 0.6, and 0.5 respectively. Figures 10 and 11 show the variation trend of the LED operating temperature and the average heat sink temperature with the radiation coefficient of the heat dissipation material. Observing the simulation results, it can be found that when the material emissivity changes above 0.80, the LED operating temperature and the average heat sink temperature do not change significantly, indicating that for aluminum heat sinks, the material emissivity of 0.80 is sufficient; when the material emissivity is below 0.80, the maximum LED temperature and the average heat sink temperature change linearly with the material emissivity. The lower the emissivity, the higher the temperature. Therefore, when selecting product heat dissipation materials, the surface emissivity of 0.80 can be used as a reference.

　　5 Simulation Data and Laboratory Measurement Verification

　　CFD simulation software was used to simulate the heat dissipation of 7WLEDPAR16 spotlights and 14WLEDPAR30 spotlights. According to the laboratory ambient temperature, the room temperature and the initial temperature of the solid were set to 29°C. The simulation results are shown in Figures 12 and 13. The laboratory temperature was measured using an 8-channel thermocouple thermometer TP700. The measurement environment was an unmanned, constant-temperature, closed laboratory with an ambient temperature of 29°C. The laboratory temperature measurement results were compared with the CFD simulation results, as shown in Tables 1 and 2.

　　By comparing Table 1 and Table 2, it can be concluded that the maximum error between the simulated temperature and the laboratory measured temperature is only 4.17℃, and the minimum is 0.17℃, which shows that the LED lamp heat dissipation model established in this paper is more in line with the actual working conditions and the simulation accuracy is relatively high. At the same time, through simulation, it is also found that the operating temperature of the LED lamp driver power supply is too high. In the subsequent product development process, the power supply heat dissipation problem can be solved in a targeted manner to improve the life and reliability of LED lighting products.

　　6 Conclusion

　　Boundary condition setting, thermal resistance calculation, heat load analysis and heat sink are key steps in CFD simulation analysis of LED lamps. They need to be verified and corrected in combination with laboratory temperature measurement to obtain more accurate heat dissipation simulation analysis results. CFD heat dissipation simulation results have important reference value and guiding role in the development and design of LED lamps, which can shorten the R&D cycle, reduce development and design costs, and improve the reliability and competitiveness of LED lamp products.