Detailed explanation of the technical solution for achieving standardization of LED lighting modules

LED lighting is recognized as the next generation of lighting technology due to its power saving, environmental protection and long life, and will replace various existing lighting technologies. LED is a cold light source and is afraid of heat. As much as 80% of the electrical energy is converted into heat energy, heat dissipation measures must be taken. Although LED lighting technology has made great progress, with reports of 200lm per watt, LED heat dissipation is a very headache in LED lighting, but it has not been effectively solved.

The biggest problem that hinders the popularization of LED lighting applications is the high price of LED lamps. Although upstream LED chip manufacturers share most of the profits and have room for significant price cuts, in order to effectively allocate the entire social resources to the entire LED lighting industry chain, effectively reduce costs, and facilitate ordinary people to purchase and install, the module standardization of LED lighting is a must, just like the existing lighting (incandescent lamps, fluorescent lamps/energy-saving lamps). The obstacle to the standardization of LED lighting modules is the existence of heat dissipation problems.

Human beings have been studying heat transfer for hundreds of years, and heat transfer science and technology are very mature. However, in the electronics industry, the heat dissipation of electronic devices (especially chips) has only received more attention in the past ten years. Human beings' mature heat transfer knowledge has not been transplanted into the electronics industry, but has been created from scratch, creating many new terms: "active heat dissipation", "passive heat dissipation", "heat sink" sounds like something unknown, and the English word "sink" is also a very rare term in heat transfer science and technology.

LED heat dissipation only involves a very small part of heat transfer - heat transfer by conduction and heat transfer by convection (mainly natural convection heat transfer by air). It is a very simple problem. For heat transfer by conduction, a very accurate solution can be obtained using existing heat transfer calculation software, but for heat transfer by convection, a lot of experimental research is required. Calculation with computer software is only academically meaningful, but not meaningful in engineering technology because the error is too large.

Since there is no very clear research on the entire heat transfer process of LED heat dissipation in the industry, the convection (natural) heat transfer from the LED node to the air and the heat sink surface, the proportion of heat transfer temperature difference (i.e. thermal resistance) in each process, and the factors affecting each heat transfer process, and these research results must be known to structural engineers. Therefore, the current LED lighting structures are in various forms, and the heat dissipation technologies used are various. Some even proposed the use of reflux heat pipes, which is like killing a chicken with a butcher knife. There is also a Taiwanese company that invented "liquid immersion heat dissipation technology". This invention, which lacks basic convection heat transfer knowledge, actually won the gold medal at the International Invention Exhibition.

This article will propose a technical solution to achieve the standardization of LED lighting modules, including the strengthening and optimization design of the heat sink. The structure is very simple and the cost is low. It is a scientific way to effectively solve the heat dissipation problem of LEDs. Not only can the modularization and standardization of LED lighting be achieved, but the heat dissipation cost can be significantly reduced, and the aluminum used for heat dissipation can be less than 4 grams. In the future, there will be no need to consider the cost of heat dissipation. In short, LED heat dissipation is not difficult and will not be a problem.

Scientific division of modules

Figures 1 and 2 show LED lighting lamps launched by Toshiba and Sharp, respectively. They integrate LED chips, heat sinks and driver power supplies and use the same installation interface as existing incandescent lamps. There are many such structures on the market. Although such designs are easy for ordinary people to install and replace existing incandescent bulbs, they have a fatal flaw - unreliable heat dissipation.

The LED lamps shown in Figures 1 and 2 have different heat dissipation effects when placed horizontally, vertically or inverted. 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 be immediately manifested, and even damage will occur immediately. Therefore, this type of LED lamp will not be the development direction of LED lighting. In addition, the structural form of the heat sink shown in Figures 1 and 2 is not ideal, and the heat dissipation cost is not low.

Spherical incandescent bulbs and straight tube fluorescent lamps are adopted because of their easy production and manufacturing. 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 light source, so the design of LED lighting should start from the characteristics of LED light sources and establish a new model.

Figure 3 shows the division of the LED lighting module proposed in this article: the wick includes the LED core, the thermal core and the wick cover, the heat sink is classified as a component of the lamp, and the power supply is also a component of the lamp.

The scientific nature of this module division is:

1. Stable and reliable heat dissipation;

Second, it is easy to realize the module standardization of LED lighting, improve the entire industrial chain and reduce the cost.

The wick is designed and manufactured into a series of standard components, and different specifications of interfaces are divided according to the standard heat dissipation power, such as: 3W, 6W, 10W, 12W, 16W, etc., corresponding to different specifications of standard interfaces. The standard heat dissipation power is not necessarily equal to the power consumption of the LED core, which is related to the internal packaging of the LED core. Lamps are of various shapes and sizes, but their standard heat dissipation must reach the specified value. Lamps will be divided according to their standard heat dissipation, and their interfaces correspond to the wick interfaces. When designing, it can be done like this: a 10W (standard heat dissipation power) wick can be installed on a 12W lamp, but a 12W wick cannot be installed on a 10W lamp. This can be achieved through structural differences in the interface. Since each lamp has its corresponding fixed installation form, its heat dissipation performance is stable, so there is no need to worry about the user changing its heat dissipation performance during installation, that is, the heat dissipation is stable and reliable. Such module division makes it easier to formulate the thermal resistance and thermal conductivity testing standards of lamps and wicks as well as experimental operations. For lamps, it is only necessary to experimentally determine the heat dissipation performance curve of the relatively standard thermal conductive core and calculate the heat dissipation thermal resistance. For wicks, it is only necessary to experimentally determine the temperature difference between the LED node temperature and a standard heat sink and calculate the thermal conductivity thermal resistance of the wick.

Lamp manufacturers will focus on designing and manufacturing various lamps according to different needs; wick manufacturers will focus on chip packaging and wick manufacturing, aiming to reduce costs, improve production efficiency, and reduce the thermal resistance of chip packaging; chip manufacturers will focus on chip research and development and production, and invest more in how to reduce costs and improve luminous efficiency; power supply manufacturers will focus on power supply and the development of dedicated driver chips. By formulating unified standards, which include: mechanical interface standards and electrical interface standards between wicks and heat sinks (lamps), as well as power supply standards, various component manufacturers will be organically combined to form a complete industrial chain, social resources will be reasonably allocated to each chain, the price of LED lighting will be significantly reduced, and the popularization of LED lighting will be imminent.

It is easy to connect the wick to the lamp electrically, but it is not so easy to connect the wick to the heat sink thermally (heat conduction). Figure 3 shows an effective and simple technical solution to this problem: using a conical cylinder as the contact heat transfer surface. The conical cylinder and the conical hole are easy to process, the accuracy is easy to guarantee, and the processing cost is low. The significant advantage of using a conical cylinder as the contact heat transfer surface is to ensure that the contact pressure between the two contact surfaces of the heat-conducting core and the heat sink is large enough: as long as a small axial force is applied, the contact pressure can be magnified several times, so the heat transfer resistance between the wick and the heat sink is effectively controlled, that is, the heat conduction problem between the two is solved. The following calculation example further illustrates this point:

For example, the middle diameter of the thermal core is Ф=20mm, the height is h=15mm, the average gap between the thermal core and the conical hole surface of the heat sink is △=0.03mm, and ordinary thermal paste is used. λ=1.0W/m&mid dot;K, the wick power is Q=12W, and the average temperature difference between the thermal core and the heat sink root can be calculated:

△t=Q?△/λ?D?л?h=0.38℃, less than 0.4℃.

As shown in FIG3, the mechanical connection between the wick and the heat sink (lamp) adopts a screw buckle, and the electrical connection adopts a concentric plug type. Ordinary operators can easily install the wick correctly without any tools. The structure shown in FIG3 is very simple, easy to manufacture, and low in cost. The functions of the wick cover in FIG3 are: 1. Protect the LED core; 2. Facilitate installation by the operator; 3. Secondary optics, design and manufacture different light output lampshades, such as focusing type or diffusing type, to meet different places and applications.

About power supply standards:

This article believes that constant current drive should be used, and the LED chips in the wick are connected in series (partially in parallel), as shown in Figure 4. Each LED chip (or parallel group) is equipped with a bypass protection element. The function of this element is that once the LED chip is damaged and becomes disconnected, the voltage is too high (for example, twice the maximum voltage of the LED), and the element breaks down, forming a permanent short circuit, so that the entire wick will not be scrapped due to the damage of one or two LED chips. For example, a 12W LED lamp has 12 LED chips. If two are damaged and the brightness decreases, the current adjustment terminal is turned on to increase the current and compensate for the reduced brightness. Therefore, 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, such as setting a standard unified constant current of 350mA. The rated voltage of a 15W wick is 43V. The rated current of the chip is related to the chip area in the LED chip, and it is easy to adjust and design a chip that meets the unified rated current standard. In addition, it is also possible to achieve a unified rated current (such as 350mA) by connecting partial chips in parallel, such as two or three LED chips in parallel;

Second, the driving circuit is simple, with fewer components, low cost, and high power efficiency. Due to the low working current (350mA), the switching loss of the switch power tube BG is also small, so the power efficiency is high; using a unified standard constant current (350mA), the switch power tube BG can be integrated into the driver IC (as shown by the dotted line in Figure 4), and the rated power range is large, from 1W to 70W (AC 220V mains). The greater the output power (the more LED cores are connected in series), the higher the output working voltage of the power supply, so the smaller the switching voltage that the switch power tube BG bears, the smaller the switching loss, and the higher the efficiency of the power supply.

Optimization and strengthening of heat sink

The heat dissipation process ultimately transfers heat to the air, where it is carried away by air flow (convection). Air flow (convection) is very important in the heat dissipation process. The greater the air flow, the greater the heat that can be carried away (i.e., the amount of heat dissipated). The heat sink is set vertically up and down, with the fins upright, so that natural convection air flows from bottom to top and passes through the fins. The air flow resistance is small, which is conducive to increasing the heat dissipation.

In the process of natural convection heat transfer, the driving force for air flow is: the buoyancy generated by the increase in air temperature and the decrease in specific gravity. The buoyancy is proportional to the volume and the temperature difference of the air (the difference between the air temperature in the heat sink and the ambient air temperature). That is to say, the higher the air temperature in the heat sink, the greater the buoyancy and the greater the natural convection of the air. However, the convection heat transfer between the heat sink fins and the air is proportional to the difference between the fin temperature and the air temperature in the heat sink, and to the fin area (heat dissipation area). The air flow resistance generated by the heat sink is proportional to the distance (i.e., fin height) through which the air flows and the fin density. The fin height and fin density are expressed as the heat dissipation area. These analyses show that in the natural convection heat transfer in the fin structure, there are opposite and contradictory factors in increasing the heat dissipation surface by increasing the fin height or fin density (reducing the gap between the fins) to increase the heat dissipation amount, so the heat dissipation amount is limited, and it is even possible to reduce the heat dissipation amount and get the opposite result.

Through theoretical analysis and a large number of experimental studies, it is concluded that the space occupied by the heat sink is certain, there is a maximum natural convection heat dissipation, corresponding to the optimal fin structure (fin density), and the maximum heat dissipation is proportional to the flow area of ​​the heat sink (the cross-sectional area of ​​the air flowing through). In the sunflower-type heat sink, the fins extend around the central heat-conducting column, and the LED core is concentrated on the end surface of the central heat-conducting column. The distance from the LED core (heat source) to the root of the fin is short, so the thermal resistance is low. The area occupied by the heat-conducting column and the LED core is small, so the flow area is large. Therefore, the sunflower heat sink is the best structure for LED heat dissipation. Increasing the height of the space occupied by the heat sink can help increase the maximum heat dissipation, but it is not a proportional relationship. The higher the height, the less the benefit. Therefore, the heat sink structure should be optimized to find the optimal fin density. This article points out that the optimal structure can only be found through experimental research and analysis.

A convection hood is set above the heat sink, as shown in Figures 5 and 6. The chimney suction principle is used to increase the flow of air through the heat sink, thereby enhancing the heat dissipation. The lampshade can be used as a convection hood. The suction intensity of the convection hood is proportional to the effective volume inside the convection hood, and the external size is proportional to the height of the convection hood.

When the convection cover is set upright, the suction effect of the convection cover is most effective, the heat sink adopts a sunflower style, and the LED core can only face upward or downward, as shown in Figures 5 and 6. The LED lamp shown in Figures 7 and 8 solves the problem of flat light. The convection cover is made of transparent material. At this time, the convection cover is the lampshade, the LED core faces upward, and a reflector is arranged inside the convection cover. The upward light emitted from the LED core is reflected by the reflector into flat light.

The heat dissipation of a single sunflower heat sink is limited, and the power (i.e., illuminance) of a corresponding single LED wick is also limited. For high-power lighting lamps such as street lamps, several sunflower heat sinks are combined into one. Each sunflower heat sink has an LED wick, which can be combined into lighting lamps with a variety of illuminances (powers) according to needs.

FIG. 9 shows a street lamp, wherein the heat sink is 10 equal hexagonal sunflower-shaped heat sinks assembled in a honeycomb structure.

After a lot of experimental analysis and research, it is concluded that the height of the convection cover is 120mm (suitable for street lamps and downlights), the heat sink structure is optimized and the best structure is adopted. When the temperature difference between the heat conducting core and the ambient temperature is 30℃, the aluminum used for heat dissipation per watt is less than 4 grams, which is a significant improvement compared with the aluminum used for more than 100 grams per watt of current products. It can be said that the cost of LED heat dissipation should no longer be considered, and LED heat dissipation will no longer be a problem.