Effective Solutions for LED Lamp Heat Dissipation Technology

LED Cooling Technology Solutions

This paper proposes a technical solution for standardization of LED lighting modules. The heat sink is classified as a component in the lamp. The wick composed of LED core and thermally conductive core will be designed and manufactured into a series of standards. The conical cylindrical thermally conductive core is used to effectively solve the problem. The problem of heat conduction between the wick (heat-conducting core) and the heat sink (lamp), the wick and the lamp can be easily disassembled and assembled, the structure is very simple, and the cost is low. It is a scientific way to realize module standardization, and it is considered that the constant current drive power supply More reasonable.

• Natural convection heat dissipation, no mechanical movement, high reliability, low cost, naturally preferred by LED lights. This article will explain the principle of natural convection heat dissipation, the maximum heat dissipation and the concept of optimized design, discuss the best application structure of LED lamp heat sink-sun fancy heat sink, and propose the use of convection cover to enhance heat dissipation by using the chimney effect. After a large number of experiments, analysis and research, the results of optimization and enhancement have been obtained, which can achieve less than 4 grams of aluminum per watt for heat dissipation. The cost of heat dissipation is significantly reduced. The cost of heat dissipation will no longer be considered in the future. In short, LED heat dissipation It is not difficult and will no longer be a problem.
• Scientific division of modules
  The LED lamp of a well-known company integrates the LED chip, heat sink, and drive power into one, and uses an installation interface like an incandescent lamp. There are many such structures on the market, although this design is convenient for ordinary people to install and replace the existing incandescent lamp. Light bulbs, but they are expensive, and have a fatal flaw-unreliable heat dissipation. Place the LED lamp horizontally, upright or upside down.
  The heat dissipation effect of the three postures is different. If a lampshade is added, the heat dissipation effect is closely related to the shape and size of the lampshade. If the lampshade is closed or the air circulation inside and outside is poor, The heat dissipation effect will deteriorate, the light decay will appear immediately, and even damage will appear immediately. Therefore, this type of LED lamp will not be the development direction of LED lighting. In addition, the structure of the heat sink itself is not ideal, and the heat dissipation cost is not low.
  Spherical incandescent bulbs and straight tube fluorescent lamps are used because of their ease of production. The accumulation of history makes people think of spherical bulbs and straight tubes when they mention lighting. People use lights for the purpose of light. LED is a new type of light source, so the design of LED lighting should start from the characteristics of the LED light source and establish a new model.
  The connection between the wick and the circuit of the lamp is easy, but the thermal connection (heat conduction) between the wick and the heat sink is not so easy. An effective and simple technical solution to solve this problem: use a conical column to contact the heat transfer surface. Conical column and conical hole, easy to process, easy to ensure accuracy, low processing cost. The significant advantage of using a conical column as the contact heat transfer surface is to ensure that the contact pressure between the two contact surfaces of the heat conduction core and the heat sink is large enough: as long as the axial force is small, the contact pressure can be magnified several times, so the wick The heat transfer resistance between the heat sink and the heat sink is effectively controlled, that is, the heat transfer problem between the two is solved. The following calculation example further illustrates this point:

For example, the middle diameter of the heat-conducting core is Ф=20mm, the height is h=15mm, and the average gap between the tapered hole surface of the heat sink is △=0.03mm, ordinary heat-conducting paste is used λ=1.0W/m·K, and the wick power is Q=12W. Calculate the average temperature difference between the thermal core and the heat sink root:

△t=Q·△/λ·D·л·h=0.38℃, less than 0.4℃

The problem of thermal connection (heat conduction) is solved, and the realization of universal standardization of wicks is close at hand. The wicks of different specifications will be divided according to the standard heat dissipation power, such as: 3W, 6W, 10W, 15W, 20W, corresponding to different specifications of standards The interface, unlike the current bulb (incandescent lamp), has nothing to do with power, there are only two interfaces. Therefore, there are many kinds of standard interface specifications for LED wicks, but the number is still limited. The lamps and lanterns involve decorations, they can be in various forms, but their standard heat dissipation must reach the specified value.

The lamps will be divided according to their standard heat dissipation, and their interfaces will be compared to their standards. The heat dissipation corresponds to the wick interface. The design can be like this. The 10W (standard heat dissipation power) wick can be installed on a 12W lamp, but the 12W wick cannot be installed on a 10W lamp. This can be achieved through the structural difference in the interface. Since each lamp has its corresponding fixed installation form, its heat dissipation performance is also stable, so there is no need to worry about users changing its heat dissipation performance when installing, that is, heat dissipation is stable and reliable.

In short, this article proposes the scientific aspects of the module division of LED lighting: ①The heat dissipation is stable and reliable, ②It is easy to realize the standardization of LED lighting modules, as well as corresponding testing standards and operating procedures, so as to improve the entire industry chain and reduce cost.

LED lighting heating problem

Because of its power saving, environmental protection and long life, LED lighting is recognized as the next lighting technology, which will replace the existing lighting technology. LED is a cold light source, which is afraid of heat. As much as 70% of the electric energy is converted into heat, so efficient heat dissipation measures must be taken to ensure the life of the LED lamp. Although LED light-emitting technology has developed by leaps and bounds, with reports of up to 200lm per watt, LED heat dissipation is a very headache in LED lighting, but it has not been effectively solved. It has become a roadblock on the road to popularization and development of LED lighting.

The biggest problem hindering the popularization of LED lighting applications is the high price of LED lights. Although upstream LED chip manufacturers share most of their profits and have room for substantial price reductions, they must effectively allocate the entire social resources to the entire LED lighting industry chain to effectively reduce The cost is convenient for ordinary people to purchase and install. The standardization of LED lighting modules is the only way to go, just like existing lighting (incandescent lamps, fluorescent lamps/energy-saving lamps). The obstacle to standardization of LED lighting modules is the existence of heat dissipation problems.

Heat dissipation is part of heat transfer. Human research on heat transfer has a history of hundreds of years. The 1960s to 1970s were the peak period of people’s research on heat transfer. The main driving force was the demand for human development of aerospace. At that time, many outstanding talents were gathered in the field of heat transfer technology, and many heat transfer researchers became famous people. After that, people’s enthusiasm for heat transfer research gradually decreased.

At present, there are very few professionals in heat transfer and technology. Heat transfer and technology are already very mature, like a ripe fruit that falls on the ground and is covered by leaves, and is not seen by people nowadays, so that when the electronic industry, mainly the CPU in the computer, the heat is suddenly increased, People did not pull the leaves on the ground, pick up the ripe fruits, and transplant the mature knowledge of human heat transfer into the electronics industry.

Regarding the heat dissipation of LED lamps, the current industry lacks clear research results on every heat transfer detail in the entire heat transfer process. The analysis points out: the convective (natural) heat transfer from the LED junction to the air and the surface of the heat sink, the proportion of the heat transfer temperature difference (ie thermal resistance) in each process, which process has the largest temperature difference, that is, the main contradiction, and the impact For each heat transfer process factor, how to reduce the technical direction of its thermal resistance, especially the heat transfer process with the largest thermal resistance, the technical direction of reducing its thermal resistance is more important. Even with these research results, it must be well-known by structural engineers, because heat transfer is ultimately achieved through structure.

In terms of heat transfer and technology, LED heat dissipation is not complicated, it only involves a very small part of heat transfer-heat conduction and convection heat transfer (mainly air natural convection heat transfer), of which heat conduction and heat transfer can be used Ready-made heat transfer computer software can get very accurate answers, such as analyzing the temperature distribution in the LED package chip (heat transfer process); analyzing the internal temperature distribution from the LED chip to the heat sink. However, special attention should be paid to convective heat transfer, where air flow is involved, and a large number of experimental studies must be carried out. Computer software calculations have only academic significance and no practical engineering significance. Because the error is too large, there are still many Of companies are keen to promote such software.

How is the LED heat dissipation problem complicated?

The reasons that cause the simple problem of LED heat dissipation to be complicated are:

1. Knowledge gap,
2. People with mature heat transfer knowledge have rarely participated in the research of LED heat dissipation.
3. Lack of professional LED heat dissipation research institutions, clear and correct guiding ideology for the industry, there are many seminars, but the academic atmosphere is less, and the commercial flavor is strong.

At present, many of the professional heat dissipation technicians working in the industry have transferred from the aspect of computer heat dissipation, and naturally brought the commonly used technology and commercial activities in that area. For example, heat pipe technology is widely used in high-power LED lighting (such as street lights). ), creating new business opportunities for heat pipe manufacturers that originally served computer chip radiators.

There is even a proposal to use a reflux heat pipe, which is undoubtedly an overkill. A company in the European Union invented liquid immersion heat dissipation technology. This invention lacks basic knowledge of convective heat transfer. The creators of these inventions inspired by car water tanks do not know why the car engine uses water (liquid) cooling technology, and water In the heat dissipation process, this technology is transferred to the field of LED heat dissipation, which lacks the most basic scientific spirit and judgment ability.

About Power supply Standards

This article believes that constant current drive should be used. The LED chips in the wick are in series (partially in parallel), and each LED chip (or parallel group) is equipped with a bypass protection component. The function of this component, once the equipped LED chip is damaged, In the off-circuit state, due to the high voltage (for example, twice the maximum voltage of the LED), the component breaks down and forms a permanent short circuit, so that one or two LED chips will not be damaged and the entire wick will be scrapped. For example, a 12W LED lamp has a total of 12 LED chips. If two of them are damaged and the brightness decreases, the current adjustment terminal can be connected to increase the current to compensate for the decreased brightness, so the reliability of the lamp is high.

The advantages of using constant current drive power supply include:

1. It is easier to achieve a unified standard power supply. For example, the standard uniform constant current current is set to 350mA. 15W wick, and the rated voltage is 43V. The rated current of the chip is related to the chip area in the LED chip, so it is easy to adjust the design. Chips that meet the unified rated current standard can be produced. In addition, local chips can also be connected in parallel, such as two or three LED chips in parallel, to achieve a unified rated current (such as 350mA);
2. The driving circuit is simple, the components are few, the cost is low, and the power supply efficiency is high. Due to the low operating current (350mA), the switching loss of the switching power tube BG is also small, and the power supply efficiency is high; using a unified standard constant current (350mA), the switching power tube BG can be integrated into the driver IC (as shown in Figure 4) Shown by the dotted line), and the rated power range is large, from 1W to 70W (mains power is AC220V). The greater the output power (the more LED cores are connected in series), the higher the output working voltage of the power supply, the smaller the switching voltage that the switching power tube BG bears, the smaller the switching loss, and the higher the efficiency of the power supply.
3. The principle and optimization of natural convection heat dissipation. In the heat dissipation process, heat is finally transferred to the air, and the heat is taken away by air flow (convection). The radiation heat transfer of the heat sink occupies a very low component, so it is not considered.