High-power LED street light cooling scheme in road lighting

The creation of a conservation-oriented society has become the consensus of the people. However, the overall efficiency of high-pressure sodium lamps that are still widely used in road lighting is not high, only about 70%, and the color rendering index is low, and the night lighting feels dim, which is not conducive to car driving. The resolution of targets and obstacles by personnel and pedestrians has a certain impact on road traffic safety.

At present, high-power white LEDs are unique in terms of luminous efficiency (>80lmPW), service life (>50000h), light output characteristics, color rendering performance (75-80), color temperature selection, controllability, and green pollution-free. Advantages, according to the requirements of urban road lighting design standards, convenient and flexible design of satisfactory street lights that meet the requirements of light output, and become a new high-quality light source with strong competitiveness. Among several key technologies that determine LED street light applications, heat dissipation design is a very important part and one of the technical bottlenecks that restrict its ability to be widely used. That is to say, the quality of the heat dissipation design will directly determine the performance and performance of the LED street lamp and whether the actual promotion and application can be successful.

2 high power LED cooling solution

High-power LED is the basic light source that constitutes LED street light. The current chip P-light conversion efficiency is very low, only 15%~20%, the physical size of the chip is 1~6125mm2, the area is small, the power density and heat generation are very large. 80%~85% of the consumed energy will be converted into heat energy and need to be dissipated. When the temperature of the chip exceeds a certain value, the wavelength of the light becomes longer and the color is red-shifted, which will lead to a decrease in the light-emitting efficiency of the chip and a decrease in the service life. Wait a lot of questions. Therefore, to ensure that high-power LEDs can be used normally and effectively, heat dissipation is the key problem that needs to be solved first.

2.1 The effect of temperature on the LED and the primary cooling solution of the package

The junction temperature of high-power LED chips works closely with the luminous flux and lifetime. In order to dissipate up to 80% to 85% of the heat, the LEDs are packaged with a scientific thermal process design and a fruitful packaging process. By applying a highly thermally conductive material (internal heat sink) to ensure that the high heat generated by the chip can be smoothly exported, the LED after the package molding has good heat conduction and heat dissipation performance.

2.1.1 Relationship between LED junction temperature and luminous flux and lifetime

Based on the working characteristics of high-power LEDs, the junction temperature has a direct interest in the size of the luminous flux and the length of the service life.

Figure 1 shows the relationship between junction temperature and luminous flux (Figure 1(a)) and lifetime (Figure 1(b)) for an international brand of LED chips.

It can be seen from Fig. 1 that as the junction temperature of the LED chip increases, the luminous flux of the output decreases regularly, and the service life also shows a rapid decline. Therefore, trying to maintain the temperature of the chip within the allowable range is the key technical problem to be solved first in LED applications.

2.1.2 Primary heat dissipation of the LED package

The primary heat dissipation design of the LED package is determined by the process of the LED production stage. Figure 2 shows the general flow diagram of the LED package heat dissipation design, which is mainly composed of the thermal design inside the chip and the thermal design of the package. In this way, satisfactory LED heat conduction and heat dissipation effects can be obtained through scientific and rational design.

Figure 3 shows a typical LED package structure. It can be seen from Fig. 3 that the package lens material is almost non-thermally conductive, and its function is to distribute and take out the light output of the chip. The heat of the chip is mainly discharged by the internal heat sink and then radiated through the external heat sink, so the LED package is once The heat dissipation design is based on the requirements and conditions of its use. The scientific design of the internal heat sink effectively exports and transmits the high heat generated by the chip to the heat sink.

2.2 LED secondary cooling solution

For the high-power LEDs that have been commercialized, the primary heat dissipation design built by the chip package has been fixed and cannot be changed during use. Therefore, when used as a light source in a street lamp, it is necessary to perform according to the actual working conditions and working conditions of the site. The design of the secondary heat dissipation scheme.

2.2.1 LED secondary heat dissipation design process

The LED secondary heat dissipation design process is shown in Figure 4. The main expressions are: calculate the thermal resistance and junction temperature, to see if it can meet the heat dissipation requirements of LEDs. If the heat dissipation requirements can be met, the results will be directly output. If the heat dissipation requirements of the LEDs are not met, the heat sink design should be performed, and then the design can be seen. To meet the heat dissipation requirements of the LED, it is necessary to carry out the next step of optimization design, if not, it is necessary to re-design the heat sink until it can meet the requirements.

The thermal resistance network diagram of the LED secondary heat dissipation design is shown in Figure 5. The inside of the dotted line in the figure is the heat dissipation of the primary package of the LED. The heat generated by the LED chip PD is transmitted outward through the internal thermal resistance Rj-c, and is diffused outward by the outer casing and the package lens, and the thermal resistance is RTP. The heat transfer process is expressed as follows:

The internal heat sink of the LED transfers heat to the metal circuit board through the bonding layer. The thermal resistance between the internal heat sink and the metal circuit board is Rc-b, and then the circuit board transmits the heat to the heat sink through the bonding layer. The thermal resistance is Rb. -s, the heat sink dissipates heat to the air through the thermal resistance Rs-a.

(Tc - the temperature of the internal heat sink; Ts - the highest point temperature of the heat sink; Ta - ambient temperature).

2.2.2 Factors affecting secondary heat dissipation

By analyzing the scheme and mechanism of LED secondary heat dissipation, it can be seen that the main factors affecting LED heat dissipation are:

(1) The function of the heat-dissipating substrate is to connect with the internal heat sink of the LED to derive and dissipate the heat. Common ones are:

Metal PCB circuit board - a technical means used to solve the problem that the circuit connection between the unit LEDs and the heat dissipation channel are independent of each other. The problems are large expansion coefficient, large specific gravity, heavy weight, and the like.

Commonly, there are metal low-temperature sintered ceramic substrates formed by combining ceramics and metals.

Metal-based composite board - The metal-based composite board is an improved version of the metal PCB circuit board. The high thermal conductivity of the metal material is combined with the low expansion property of the reinforcing material, and the expansion coefficient is adjustable, the specific gravity is small, and the thermal conductivity is high.

(2) Temperature equalization plate

The heat of the high hot spot between the LED units is derived and diffused to obtain a uniform temperature distribution on the heat dissipating surface, thereby improving the heat dissipation effect and facilitating the overall heat dissipation of the heat sink.

(3) Bonding layer There are three kinds of bonding materials commonly used for LED chips and heat sinks:

Thermal adhesive - hardening temperature below 150 ° C, low thermal conductivity, poor thermal conductivity.

Conductive silver paste - hardening temperature below 200 ° C, good thermal conductivity and good bonding strength.

Tin paste - Tin paste should be preferred over the above two adhesives because of its optimum thermal conductivity and excellent electrical conductivity.

(4) There are many design schemes and forms of heat sink cooling devices, which are mainly divided into two categories:

Passive heat dissipation - features no need to consume additional energy (electric energy) when dissipating heat, but the overall heat dissipation capacity is limited, suitable for medium and small power LED street light cooling.

Active heat dissipation - features the need to consume additional power when dissipating heat, but the heat dissipation effect is good, suitable for heat dissipation of larger power LED street lights.

(5) Improved heat dissipation design In order to minimize the overall thermal resistance of the LED, that is, to reduce the number of thermal resistance, some improvements have been proposed in the literature, which are summarized as follows:

Thin film integrated package - the metal PCB board is eliminated, and the insulating film and the electrode film are directly formed on the metal heat sink. The heat dissipation effect obtained thereby is far superior to the conventional metal PCB board, and the overall thermal resistance of the LED can be further reduced.

The chip is directly packaged on the heat sink—the conventional LED internal heat sink is eliminated, and the chip is directly packaged on a pre-designed metal heat sink surface with a special structure, and then packaged in its entirety. This also further reduces the thermal resistance.

2.2.3 LED passive cooling solution

The passive heat dissipation of LEDs is mainly suitable for medium and small power LEDs to dissipate heat. Since no additional power is required, the overall efficiency of the application is not affected.

(1) Natural heat dissipation

The principle of natural heat dissipation is to add a heat sink to the outside of the substrate, and heat the chip to conduct heat through heat conduction, and then exchange heat with the air to dissipate heat from the heat sink.

The basic formula for heat conduction:

In the formula, Q is heat, that is, heat of heat conduction; K is the heat transfer coefficient of the material, the higher the heat transfer coefficient, the lower the specific heat value; A is the heat transfer area (or the contact area of two objects) ; ΔT is the temperature difference between the two ends; ΔL is the distance between the two ends. Therefore, it can be found from the formula that the heat transfer is proportional to the heat transfer coefficient, the heat transfer area, and the temperature difference between the two ends, and inversely proportional to the distance. That is, the material of the heat sink has a high thermal conductivity, and its own temperature rise is low, and it is larger than heat, and is generally made of a material (copper, aluminum) having high thermal conductivity and large heat capacity (see Table 1).

The basic formula for heat convection:

Where Q is heat, that is, the heat taken away by heat convection; H is the value of the heat convection coefficient; A is the effective contact area of heat convection; ΔT is the temperature difference between the solid surface and the regional fluid. Therefore, in the heat convection transfer, the magnitude of heat transfer is proportional to the thermal convection coefficient, the effective contact area, and the temperature difference. In order to increase the heat dissipation effect, that is, to increase the contact area of the surface with the air, the outer surface of the heat sink may be formed into a fin shape. The shape of the fins is also varied, and the number, position, size, angle of inclination, and thickness of the fins need to be carefully studied. In addition to the common linear shape, there are wavy, spiral, cylindrical, and tapered shapes. The shape of the table, etc., is designed to facilitate air convection and rain wash for optimal heat dissipation. In the materials used, the thermal conductivity of copper is much faster than that of aluminum, but the heat dissipation of copper is not as fast as aluminum, thus forming a new type of copper-aluminum composite heat sink - combining the advantages of copper and aluminum. Copper can quickly transfer the high heat of the LED chip to the aluminum, and then the heat is dissipated by the large-area aluminum fins, thereby achieving a better heat dissipation effect.

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