LED lighting as a new generation of lighting has been widely concerned. Relying on LED packaging alone does not make a good lighting fixture. This paper mainly explains how to design LED characteristics from electronic circuit, thermal analysis, optics and other aspects.
In recent years, with the high density and high integration of electronic products, the importance of thermal solutions is increasing, and LED lighting is no exception, and thermal solutions are also needed. Although the energy loss of incandescent lamps and fluorescent lamps is large, most of the energy is emitted directly through infrared light, and the heat of the light source is less; LED, in addition to the energy consumed as visible light, other energy is converted into heat. In addition, due to the small LED package area, the heat dissipation through convection and radiation is small, thus accumulating a lot of heat.
What is the thermal solution?
Let's consider how to develop a thermal solution. The thermal solution is simply to solve various problems caused by heat. The main ones are:
1. Bending and cracking due to thermal expansion
2. Operation obstacles of electronic circuits
3. Deterioration of material quality
In addition, will you worry if the heat will damage the equipment? In order to avoid these problems, to try to control the temperature of the electronic equipment, that is, effective heat dissipation is very important, the focus is to consider the use of the machine environment and installation methods to develop thermal solutions. The following is a list of problems caused by heat. The second half takes LED lights as an example to explain LED related solutions.
Problems caused by heat
1. Bending and cracking due to thermal expansion
Electronic devices are composed of multiple parts, each of which is made of different materials, and the scale of thermal expansion and contraction is not the same. Therefore, when various materials are combined together, it is possible to make the material bend and expand, and the product will crack at the joint because of too much stress.
2. Operation obstacles of electronic circuits
In general, the semiconductor component as a heat source has such a characteristic that when the temperature of the semiconductor component in the electronic device rises, the electrical impedance becomes smaller. In this way, it is easy to fall into a vicious cycle of "temperature rise - impedance decline - current increase - heat increase - temperature rise", and then it is easy to burn.
3. Deterioration of material quality
Generally speaking, the materials used in electronic devices are easy to oxidize, and the higher the temperature, the faster the oxidation, if these materials are repeatedly oxidized at high temperatures, it will shorten their life. At the same time, repeated heating, multiple expansion of the material, multiple cold shrinkage, will reduce the strength of the material, thus destroying the material.
Thermal solutions for leds
The following takes LED lights as an example to specifically discuss the thermal solution of LED.
There are several ways to avoid heat from electronic devices. For example, add radiators and place fans around the heat source to provide cool air. The former is to increase the heat dissipation channel by increasing the heat dissipation area, and the latter is to make the heat not around the heat source *. However, as shown in the general diagram of the LED light in Figure 1, the radiator cannot be directly connected when the LED is packaged, and there is no place for the fan to be installed. And the internal power circuit board will also generate heat, so the heat dissipation problem of LED lights can be said to be a very tricky problem. In this way, how to effectively use LED installation materials and heat sinks becomes important.
So how to effectively use LED installation materials and radiators? First of all, we must grasp the heat transfer path that generates heat.
The heat generated by the LED component moves through the encapsulated wire to the circuit board, and then the heat is released through the radiator. The same is true of the heat generated by the power circuit board, through the air around the circuit board and the filling material, through the heat sink to the outside.
In the thermal solution, it is important to eliminate the factors that hinder heat transfer in the heat transfer path, such as considering the use of materials with good thermal conductivity in the heat transfer path, expanding the section area of the path (for example, thick copper wire is easier to conduct heat than thin copper wire), and applying thermal lubricant to leave no gap in the connection part of the product.
In addition, even if the thermal conductivity characteristics are improved through these, if the radiator does not dissipate heat to the outside, the inside will still be very hot. Therefore, it is also necessary to improve the heat release characteristics of the radiator surface. The typical method is to install several heat sinks on the surface to expand the heat release area of the radiator.
The thermal solution is verified by simulation using CAE tools
Application of CAE
So how do you verify that thermal solutions work? One is to measure the temperature by experiment, but once the conditions change, it has to be re-measured, which is relatively low efficiency. Therefore, it is necessary to use CAE software for simulation. FIG. 2 Simulation of heat and air flow around LED lights when they are placed horizontally with ANSYS analysis software. (? . ? (??) Is the temperature distribution map of the whole lamp, the red part represents the temperature is high, the blue part represents the temperature is low. (? #? (? ぃ┦ Natural convection diagram surrounding the carbylamine package (inside the lid), red arrows indicate fast convection, blue indicates slow convection. This example is only a very simple model compared to the actual situation, but it can verify the temperature distribution of the product and the natural convection of air to a certain extent. From the temperature distribution of the whole lamp, although the temperature of the lid is low and the temperature of other parts is high, it is still in an equal temperature distribution to some extent. Most of the heat generated by this surface is transferred to the radiator, and there are no obstacles in the transmission path. The radiator can play a role in heat dissipation, but if the heat dissipation characteristics are not good, the temperature of the whole lamp will rise, so you must pay attention to the shape of the radiator (the size, shape, number, etc.).
In the simulation, various information such as the shape of the object, product characteristics, and conditions need to be analyzed, but the simple analytical model and the detailed analytical model can be distinguished through the information to be confirmed, so as to effectively grasp the quality of the thermal solution to be verified. For example, this example is a simple modeling of the entire lamp, and it is not possible to grasp the detailed temperature distribution inside the LED package, but if this part is modeled in detail, it is possible to confirm the actual temperature of the component.
Repeated experiments, through simulation to modify part of the information can be simple operation, such as easy to grasp the shape and number of the heat sink in the heat sink influence on the temperature. As a simulation software, you can directly use CAD information for analysis, you can carry out a wide range of analysis of structures, heat conduction, thermal fluids, etc. in a unified environment, and you can do a variety of combination analysis. In the design, not only the heat problem should be considered, but other factors must also be considered. The difficulty of combination analysis is a key point for skillful simulation, which we will discuss later.
This time, only the problem of heat was discussed, but there are cases where the problem of light and electricity cannot be solved even if the problem of heat is solved. Products focus on long life, no performance damage, safe use, so our topic is to achieve the overall design. Next time we will discuss the problems of circuit and optical design.