Cooling solutions for LED lighting designs

LED lighting has attracted widespread attention as a new generation of lighting. LED packaging alone cannot produce good lighting fixtures. This article mainly explains how to use LED characteristics in design from the aspects of electronic circuits, thermal analysis, optics, etc.

In recent years, with the high density and high integration of electronic products, the importance of thermal solutions has become increasingly high. LED lighting is no exception and also requires thermal solutions. Although incandescent lamps and fluorescent lamps have large energy losses, most of the energy is directly radiated through infrared rays, and the light source generates less heat; while LEDs, except for the energy consumed as visible light, all other energy is converted into heat. In addition, due to the small packaging area of ​​LEDs, less heat is dissipated through convection and radiation, thus accumulating a large amount of heat.

What is the thermal solution?

Next, let’s consider how to develop a thermal solution. Simply put, a thermal solution is to solve various problems caused by heat. The main ones are:

1. Warping and cracking due to thermal expansion

2. Operational problems of electronic circuits

3. Deterioration of material quality

In addition, you may also worry about whether the device will be damaged if it gets hot. In order to avoid these problems, the temperature of the electronic equipment should be controlled as much as possible, that is, effective heat dissipation is important. The key is to consider the machine's use environment and installation method to develop the best thermal solution. The following are the 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 made up of multiple parts, each of which is made of different materials and has different thermal expansion and contraction rates. Therefore, when various materials are combined together, they may bend or expand, and cracks may occur at the joints of the product due to excessive stress.

2. Operational problems of electronic circuits

Generally speaking, semiconductor components, which are heat sources, have a characteristic that when the temperature of semiconductor components in electronic equipment rises, the electrical impedance will decrease. This can easily lead to a vicious cycle of "temperature rise-impedance decrease-current increase-heat increase-temperature rise", which can easily cause burnout.

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 subjected to high-temperature oxidation, their lifespan will be shortened. At the same time, repeated heating, multiple expansion and multiple contraction of the materials will reduce the strength of the materials, thereby destroying the materials.

Thermal Solutions for LEDs

Taking LED lamps as an example, the following will discuss the thermal solution of LEDs in detail.

There are many ways to avoid heating of electronic equipment. For example, add a heat sink and place a fan that can provide cooling air around the heat source. The former is to increase the heat dissipation channel by increasing the heat dissipation area, and the latter is to prevent heat from accumulating around the heat source. However, as shown in the general diagram of LED lamps in Figure 1, the LED package cannot be directly connected to the heat sink, and there is no place to install a fan. In addition, the internal power circuit board will also generate heat, so the heat dissipation problem of LED lamps 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 Overview of LED lights

So how can we effectively utilize LED mounting materials and heat sinks? First, we must understand the heat transfer path that generates heat.

The heat generated by the LED element moves to the circuit board through the package wires, and then dissipates through the heat sink. The same is true for the heat generated by the power supply circuit board, which is dissipated to the outside through the heat sink through the air around the circuit board and the filling material.

It is important to eliminate factors that hinder heat transfer in the heat transfer path in thermal solutions. For example, you can consider using materials with good thermal conductivity in the heat transfer path, expanding the cross-sectional area of ​​the path (for example, thick copper wire is easier to conduct heat than thin copper wire), and applying thermal conductive lubricant to ensure that there are no gaps in the connection parts of the product.

In addition, even if the heat conduction characteristics are improved through these methods, if the heat sink does not dissipate heat to the outside, a lot of heat will still accumulate inside. Therefore, the heat dissipation characteristics of the heat sink surface must also be improved. The typical method is to install several more heat sinks on the surface to expand the heat dissipation area of ​​the heat sink.

Use CAE tools to verify thermal solutions through simulation

Application of CAE

So how do we verify whether the thermal solution is effective? One way is to measure the temperature through experiments, but once the conditions change, we have to re-measure, which is inefficient. Therefore, we need to use CAE software for simulation. Figure 2 Using ANSYS analytical software, the heat and air flow around the LED lamp is simulated when the LED lamp is placed horizontally. (?。?(??) is the temperature distribution diagram of the entire lamp. The red part represents high temperature and the blue part represents low temperature. (?#?(?ぃ┦堑朴胩) is the natural convection diagram of the ED package periphery (inside the lid). The red arrow part indicates fast convection speed and the blue part indicates slow convection speed. Compared with the actual situation, this example is just a very simple model, but it can verify the temperature distribution of the product and the natural convection of the air to a certain extent. From the temperature distribution of the entire 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 a certain extent. Most of the heat generated on this surface is transferred to the heat sink, and there are no obstacles in the transmission path. The heat sink can play a role in heat dissipation, but if the heat dissipation characteristics are not good, the temperature of the entire lamp will rise, so you must pay attention to the shape of the heat sink (the size, shape, number of installed heat sinks, etc.).

In simulation, various information such as the shape of the object to be analyzed, product characteristics, and conditions are required. However, the information you want to confirm can be used to distinguish between simple analytical models and detailed analytical models, so that the quality of the thermal solution you want to verify can be effectively grasped. For example, this example is a simple modeling of the entire light bulb, and the detailed temperature distribution inside the LED package cannot be grasped. However, if this part is modeled in detail, the actual temperature of the component can be confirmed.

Repeated experiments can be performed simply by modifying some information through simulation. For example, it is easy to understand the influence of the shape and number of fins in the radiator on the temperature. As simulation software, CAD information can be directly used for analysis. In a unified environment, a wide range of analyses can be performed on structure, heat conduction, thermal fluid, etc., and various combined analyses can be performed. In design, not only heat issues must be considered, but other factors must also be considered. The difficulty of combined analysis is a key point in proficient simulation, which we will discuss later.

This time we only discussed the heat problem, but there are cases where even if the heat problem is solved, the light and electricity problems cannot be solved. The product is important for long life, no performance damage, and safe use, so our task is to achieve the overall optimal design. Next time we will discuss the circuit and optical design issues.