1 Introduction
At present, many power-type driver current reaches 70mA, 100 mA or even 1A, which will cause the internal heat of the chip to concentrate, result in a series of problems such as luminous wavelength drift, decreased lighting efficiency, fluorescent powder acceleration, and shortened service life. The industry has made a lot of effort to improve the problem of high-power LEDs: by optimizing the chip epitaxial structure, use surface roughening technology, improve the quantum efficiency of the chip, reduce the lattice
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oscillation of radiation-free composite production, fundamentally reduces heat dissipation Component load; by optimizing packaging structure, material, selective metal core printed circuit board (MCPCB), using ceramic, composite metal substrate, etc., and accelerate heat from the epitaxial layer to the heat radiating substrate. Most manufacturers also recommend using heat sinks in high performance requirements, which can promote high-power LED heat dissipation in terms of strong convection heat. Despite this, a single LED product is currently only in the level of 1 to 10 W, and the heat dissipation capacity is still in urgent need. Quite many studies focus on looking for high thermal conductivity heat sinking materials, however, when the LED power reaches LOW, this focus encounters considerable resistance. Even if the wind-cooled strong convection mode is applied, the cost advantage is sacrificed, and it is not possible to obtain satisfactory changes.
Discussing factors in existing structures, LED packages and thermosomal thermal conductivity such as the thermal conductivity of LEDs on their maximum power, find key factors affecting LED heat dissipation. The research method is a finite element heat analysis. The method has experimentally verified the difference between the LED finite element model and its real devices, proves that it is accurate in the range of error licenses.
2, establish model
2.1 Finite element heat analysis theory
Transient temperature field variables t (x, y, z, t) in three-dimensional right angle coordinate system satisfy:
Where: T / X, T / Y, T / Z are temperature gradients in the X, Y, Z direction; λxx, λYY, λzz are thermal conductivity; q0 is thermally generated by unit volume; ρc is density and a specific heat capacity Product: DT / DT is the rate of change with time.
Where: Vx, Vy, Vz is the medium conduction rate.
For steady-state thermal analysis, T / T = 0, formula (1) can be simplified:
According to the formula (3), boundary conditions and initial conditions, it is used to obtain heat analysis results.
2.2 Establishment of Geometric Model
Figure 1 is established and simplified in accordance with common 1W high power LEDs, and the seagull wing encapsulating aluminum heat sinking high-power LED pattern, the base is connected to the MCPCB aluminum substrate. Main data: The chip size is 1 mm × 1 mm × O.25MM, and the lens is a half-ball having a diameter of 13mm. The silicon substrate is a hexagonal aluminum substrate having a three-dimensional edge length of 17 mm, a height of 0.25 mm, and the MCPCB is a hex star aluminum substrate having a diameter of 20 mm and a high 1.75 mm.
2.3 Establishment of finite element model
The model uses ANSYSL0.0 calculation, for convenience analysis, assuming model:
The LED display input power is 1W, and the optical efficiency is 10%; the components of the package (including the MCPCB, ceramic package, and heat sinking) pass through the convection heat of the air; the heat parallel coefficient of the device and the outside is 20. The working ambient temperature is 25 ° C; the device satisfies the conditions for the use of ANSYS software for steady-state finite element heat analysis; the maximum junction temperature is selected to 125 ° C. Parameters of various materials are shown in Table 1.
3. Analyze the effects of various factors for heat dissipation capacity
3.1 Influence of thermal radiation coefficient on LED heat dissipation
Figure 2 is a temperature cloud map when the surface is 0.8. According to Stephen-Bolzman’s law, the relationship between the irradiation J * and the temperature T is: j * = εσt4. The ε is the radiation coefficient of the black body; σ = 5.67 × 10-8w / (m2 · k4), called Stephen – Bolzman Constant. Therefore, it is understood that the higher the temperature, the greater the irradiance. When the input power is 1W, the thermal energy scattered via surface radiation is 7.63 × 10-4W, only 1.63 ‰ of the total thermal power; when the power reaches 2W, the thermal energy scattered by radiation is only 6.33 ‰. Therefore, the key to improving the heat radiation coefficient is not large, and the cooling of heat dissipation is to improve the additional two heat dissipation: heat transfer and hot alignment. Despite this, there are still some manufacturers to coat the outer surface of the LED device to black to maximize radiation heat dissipation.
3.2 Influence of thermal conductivity on the heat dissipation of LED
Only thermal conductivity and convection are considered, and different packaging fillers such as silicone resins are changed. The results are obtained, as shown in Figure 3. Even when a thermal conductivity is as high as 7WM-1 k-1, the chip temperature drops more, and the chip temperature is not much. The aluminum substrate temperature only reduces 2.271 ° C, and the maximum power is only 0.69W. In fact, the thermal conductivity value exceeds 7WM-1k-1, and commercially used transparent silicone packaging materials have no literature. The distribution cloud is shown in Figure 4.
When the lens thermal conductivity is 0.2 wm-1k-1, the thermal conductivity of different heat sink materials is affected by the maximum power of the LED. It is seen from Table 2 that the thermal sink material has a small impact on the maximum heat dissipation capacity of the LED.
In summary, the thermal conductivity change is weak to the maximum power of the LED.
3.3 Increase the effect of heat dissipation area on LED heat dissipation
Table 3 is the effect of three different heat dissipation methods on the temperature distribution and maximum power of the LED. It can be seen that increasing the heat dissipation area is a good heat dissipation method, which can easily improve the heat dissipation capacity of the LED device, which is one of the heat dissipation methods commonly used by the LED product. However, the disadvantage is also clear: affecting cost, increasing product weight, affecting package density. Evergraduately improved the area of ??the LED heat sink is clearly unreal, so the maximum power of the LED product is usually used to increase the maximum power to 10W, and it cannot be improved for factors such as cost.
3.4 Influence of convection mode on LED heat dissipation
There are two common convective temperatures: natural convection and mandatory convection. The heat dissipation of the fixed structure is related to the surface heat transfer coefficient. When the air cooling method, the effect of different heat transfer coefficients on the maximum power is shown in Figure 5. Strong abant streaming will greatly improve the heat dissipation capacity of LED products in a certain speed, helping to improve heat dissipation.
In summary, both the heat dissipation area or the increase in convection speed can not limit the heat dissipation capacity, the reason is that when the heat dissipation structure, the method is fixed, even if the LED thermal conductivity is rising, the chip cannot be truly lowered. Temperature; facts prove to increase heat dissipation area, which can promote heat dissipation. However, due to cost limit, and it is impossible to increase the heat dissipation area without limitation, it is necessary to improve the heat dissipation capacity of the LED product, and the key to increase the heat dissipation of the upper surface heat to the upper surface heat.
4, conclusion
Three-dimensional finite element heat analysis is performed on high-power LED display, and the temperature cloud map of the device is in different factors. By comparing various factors on heat dissipation performance, it is concluded that after necessary After optimization, the pursuit of material thermal conductivity is only modified to improve the end of the LED heat dissipation capacity, and the key to greatly improve the heat dissipation capacity of LEDs is to increase heat dissipation area and change the heat dissipation.