LED lighting design (2) indispensable "heat dissipation solution"

(From MONOist) (CCS Compilation)

CYBERNET Machinery Department Masuda

LED lighting design (1) LED lighting basic LED lighting design (2) indispensable "heating solution"
LED lighting design (3) LED electrical characteristics and simple drive circuit LED lighting design (4) pulse modulation PWM circuit detailed

LED lighting has received wide attention as a new generation of lighting. Simply relying on LED packaging does not make good lighting fixtures. This article mainly explains how to use the LED characteristics design from the aspects of electronic circuit, thermal analysis and optics.

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 directly radiated through infrared rays, and the light source generates less heat. In addition to the energy consumed by the visible light, the LEDs are converted into heat. In addition, since the LED package area is small, heat dissipation by convection and radiation is small, thereby accumulating a large amount of heat.

What is the thermal solution?

Next, consider how to develop a thermal solution. The thermal solution is simply to solve the various problems caused by heat. There are:

1. Bending and cracking due to thermal expansion 2. Operational obstacles in electronic circuits 3. Deterioration of material quality

In addition, you will also worry if the heat will damage the device? In order to avoid these problems, it is necessary to control the temperature of the electronic equipment as much as possible, that is to say, effective heat dissipation is very important, and the focus is to consider the machine environment and installation method to develop an optimal thermal solution. The problems caused by heat are listed below. The latter part uses LED lights as an example to explain LED-related solutions.

Heat caused problems

1. Bending and cracking due to thermal expansion

Electronic equipment consists of multiple parts, each of which has a different material, and the scale of thermal expansion and contraction is also different. Therefore, when various materials are combined, it is possible to bend the material. When expanding, the product will crack at the joint due to excessive stress.

2. Operational obstacles in electronic circuits

In general, a semiconductor element as a heat source has such a characteristic that when the temperature of a semiconductor element in an electronic device rises, the electrical impedance becomes small. In this way, it is easy to fall into a vicious cycle of "temperature rise - impedance drop - current increase - heat increase - temperature rise", and the blown phenomenon is likely to occur.

3. Deterioration of material quality

In general, materials used in electronic devices are easily oxidized, and the higher the temperature, the faster the oxidation. If these materials are repeatedly subjected to high temperature oxidation, their life will be shortened. At the same time, repeated heating, multiple expansion of the material, multiple shrinkage, will reduce the strength of the material, thereby destroying the material.

LED thermal solution

Let's take the LED light as an example to discuss the thermal solution of LED.

There are many ways to avoid the heat of electronic devices. For example, add a radiator and place a fan that can provide cold air around the heat source. The former is to increase the heat dissipation area by increasing the heat dissipation area, and the latter is to make the heat not gather around the heat source. However, as shown in the general diagram of the LED lamp of Fig. 1, the LED package cannot be directly connected to the heat sink, and the position of the fan is not installed. Moreover, the internal power supply board also generates heat, so the heat dissipation problem of the LED light can be said to be a very difficult problem. In this way, how to effectively use LED mounting materials and heat sinks becomes very important.

Figure 1 LED lamp overview

So how to effectively use LED mounting materials and radiators? First, we must grasp the heat transfer path that generates heat.
The heat generated by the LED elements moves through the packaged wires to the board and then dissipates heat through the heat sink. The same is true of the heat generated by the power board, which is radiated to the outside through the heat sink through the air and the filling material around the board.

It is important in the thermal solution to eliminate the factors that hinder heat transfer in the heat transfer path. For example, it is conceivable to use a material with good thermal conductivity in the heat transfer path and to enlarge the cross-sectional area of ​​the path (for example, a thick copper wire is thinner than a thin copper wire). It is easier to conduct heat.) Apply thermal grease to leave no gaps in the joints of the product.

In addition, even if the heat conduction characteristics are improved by these, if the heat sink does not dissipate heat to the outside, the inside will accumulate a lot of heat. Therefore, it is also necessary to increase the heat release characteristics of the surface of the heat sink. The typical method is to install several heat sinks on the surface to enlarge the heat release area of ​​the heat sink.

Verify thermal solutions with simulations using CAE tools

Use of CAE

So how do you verify that the thermal solution is working? One is to measure the temperature by experiment, but once the condition changes, it is re-measured, and the efficiency is relatively low. Therefore, it is necessary to use CAE software for simulation. Figure 2 uses ANSYS analysis software to simulate the flow of heat and air around the LEDs when the LEDs are placed laterally. (i) (ii) is the temperature profile of the entire lamp, with the red portion representing the high temperature and the blue portion representing the low temperature. (iii) (iv) is the natural convection diagram of the lamp and the periphery of the LED package (inside the cover), the red arrow indicates that the convection speed is fast, and the blue portion indicates that the convection speed is slow. Compared to the actual situation, this example is a very simple model, but to some extent it can verify the temperature distribution of the product and the natural convection of the air. From the temperature distribution of the whole lamp, although the temperature of the cover 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 heat sink and there are no obstacles in the transfer path. The heat sink can act as a heat sink, but if the heat dissipation characteristics are not good, the temperature of the whole lamp will rise. Therefore, the shape of the heat sink (the size, shape, number of heat sinks, etc.) must be paid attention to.

Figure 2 Results of thermal fluid analysis based on ANSYS

In the simulation, various information such as the shape, product characteristics, and conditions of the object need to be analyzed. However, the information to be confirmed can distinguish between the simple analytical model and the detailed analytical model, thereby effectively grasping the quality of the thermal solution to be verified. For example, this example is a simple modeling of the entire lamp, and can not grasp the detailed temperature distribution inside the LED package, but if the part is modeled in detail, the actual temperature of the component can be confirmed.

Repeated experiments can be easily performed by modifying some of the information by simulation. For example, it is easy to grasp the influence of the shape and number of the heat sink in the heat sink on the temperature. As a simulation software, CAD information can be directly used for analysis, and structural, thermal, and thermal fluids can be extensively analyzed in a unified environment, and various combinations can be analyzed. In the design, not only the heat problem must be considered, but other factors must also be considered. The difficulty of combined analysis is a key point for skilled simulation, which we will discuss later.
This time, we only discussed the issue of heat, but there are cases where the problem of light and electricity cannot be solved even if the problem of heat is solved. The product is characterized by long life, no performance damage, and safe use. Therefore, our goal is to achieve an overall optimized design. Next time we will discuss the issues of circuit and optical design.