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LED lamp heat dissipation design (2)

- Mar 19, 2019-

Thermal analysis tool

Thermal analysis software can realistically simulate the thermal state of the system and can be thermally simulated during the product design phase to determine the highest temperature in the model。 If the allowable temperature is exceeded, the heat dissipation measures should be improved to meet the requirements for use。 This reduces the cost of design, redesign and reproduction, and increases the success rate of the product。

At present, there are many kinds of thermal analysis software, such as ANSYS, FLUENT, EFD, ICEPAK and FloTherm.


1) From the operational point of view, EFD is embedded in PRO/E, SW, CATIA software. The software model is easy to set up and simple to set up. It is useful for rough calculation in engineering, but the software occupies more system resources. It should not be divided too much, and the calculation accuracy is not high.

2) In terms of applicability, ICEPAK is embedded in ANSYS WB12。1, which can handle general complex surfaces。 The model import function has been greatly improved, and the software operation is simple。 Like EFD, there is no need to manually calculate the flow state。 Handling complex meshes is fast。

3) From the tradition of electronic cooling, FloTherm is wider than ICSPAK and has long occupied the electronic thermal analysis market, but it is difficult to process curved surfaces。 Modeling is a key issue for designers of LED luminaires。

4) Professionally speaking, ANSYS and FLUENT are the preferred software for academic and essays, and they emphasize accurate calculations from theory. The ANSYS software is based on the finite element method, and the calculation result has high precision. It is suitable for those with higher theoretical basis and can easily realize manual programming and optimization design.

Based on the above comparison, the ANSYS software was used for thermal analysis。


Thermal analysis of a high power LED lamp

In this paper, a high-power LED lamp is used as a model for research. The thermal simulation analysis of the luminaire is carried out using ANSYS software. The analysis steps are: establishing a simplified model, setting boundary conditions, dividing the grid and calculating.


Physical model of the luminaire

A high-power LED tunnel light developed by a certain LED company in Zhejiang was selected as the research model。 The lamp consists of LED lamp beads, lamp covers, leather pads, mirrors, circuit boards, heat sinks and power supplies。

Among them, the lamp bead is shown in Figure 2, which belongs to the integrated package structure。 Each lamp bead is packaged with 9 GaN blue-light chips, three strings are connected in series, and the surface of the chip is coated with phosphor for light compensation。 The chip is encapsulated with epoxy resin and fixed on the copper heat sink with silver glue。 Install the lens and connect the electrode with a gold wire。 The heat sink and the circuit board are connected by the thermal conductive silica gel, and the circuit board and the heat sink are fixed by screws on the four sides of the heat sink。 To reduce the contact thermal resistance, the middle layer of the circuit board and the heat sink is also coated with the thermal conductive silica gel。


Luminaire thermal network model

There are three main ways to analyze the heat dissipation of the LED lamp by the structure of the lamp:

1) chip - phosphor layer - epoxy resin - lens - environment;

2) chip - gold wire - bracket - circuit board - thermal silica gel - heat sink - environment;

3) Chip - silver glue - copper heat sink - thermal silica gel - circuit board - thermal silica gel - heat sink - environment.

Since the thermal conductivity of the epoxy resin for encapsulation is only 0.2 W/(m·k), the heat treatment is performed here. In addition, the area of the gold wire is very small, and the heat transfer effect is minimal. Therefore, the main heat dissipation path is the third, that is, the heat emitted by the chip is conducted by the heat sink, the thermal conductive silica gel, the circuit board, the thermal conductive silica gel to the heat sink, and then the heat sink. Enter the air in a convective manner.

This study first simplifies the model based on the main heat dissipation methods of the above analysis: simplifying the LED lamp lens into a rectangular parallelepiped to reduce the amount of calculation; simplifying the silver paste between the chip and the heat sink to a thin plate of 0.1 mm; lamp beads and heat dissipation The thermal silica between the sheets is simplified to a 0.3 mm thin plate.


Thermal analysis of lamps

Then determine the boundary conditions according to the application situation of the luminaire and the actual working conditions as follows:

1) Each chip has a power of 1.5 W and a luminous efficiency of 20%, so the heating power per chip is 1.2 W, that is, the total heat of each source is defined as 1.2 W.

2) The luminaire is a tunnel light, and the maximum temperature does not exceed 100 °C, so solar radiation is not considered.

3) In the actual use process, the luminaire is directly installed in the outside air, which belongs to the natural convection。 Therefore, the six faces of the enclosure are defined as opening, and the ambient temperature is assumed to be 25 °C。

The maximum temperature of the luminaire is concentrated at the chip, and the maximum temperature is as high as 76.23 °C. Considering a certain error, it is very likely to exceed the maximum allowable junction temperature of 80 °C. It can be seen that the current heat dissipation of the LED luminaire is relatively poor, and it is necessary to carry out the current cooling system. improve. The main reason for analyzing the high temperature of the lamp is that the thickness of the circuit board is large and the thermal conductivity is poor, and the heat dissipation layer is too much, which causes a heat conduction bottleneck, and the heat is not well transmitted.


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