LED application problems and solutions

In current practical applications, the problems of high-power LEDs are mainly manifested in the following aspects:

1. Lack of knowledge on ensuring LED working conditions and power supply technology has resulted in numerous power supply failures.

2. The concept of the practical application of LED street lamps is unclear, and the high -power LED street lamps are blindly produced at no cost to compete with high- power gas discharge lamps , resulting in unrealistic and expensive street lamp products that are difficult to promote.

3. Insufficient understanding of the requirements for road lighting, ignoring the difficulty of scientific point light source , optical, and light distribution, and the importance of color temperature in road lighting, which can easily cause glare, zebra effect, and the phenomenon that the ground is not bright enough when the lights are on in an environment with severe air pollution, rainy and foggy weather.

4. The requirements for road lighting are vague, and actual use and maintenance are not considered, resulting in resistance from direct users.

5. Due to the lack of understanding of the working conditions of LED light sources, the light attenuation is serious or even dead.

To discuss the working environment of LED light sources, we need to have basic knowledge of LEDs; the core of light-emitting diodes is PN junctions . Therefore, it has the IN characteristics of general PN junctions, that is, forward conduction, reverse cutoff and breakdown characteristics. In addition, under certain conditions, it also has luminescence characteristics. Under forward voltage, electrons are injected from the N region into the P region, and holes are injected from the P region into the N region. A part of the minority carriers (minority carriers) that enter the other region recombine with the majority carriers (majority carriers) to emit light. The current high-power LED luminous efficiency is about 30%, and 70% will be heat energy, which needs to be dissipated. The relationship between the junction temperature TJ of high-power white light LEDs and the lifespan when the brightness is attenuated by 70% can be seen: when TJ=50℃, the lifespan is 90,000 hours, when TJ=80℃, the lifespan drops to 34,000 hours, and when TJ=115℃, its lifespan is only 13,300 hours. In the heat dissipation design, the maximum allowable junction temperature value TJmax should be proposed. The actual junction temperature value TJ should be less than or equal to the required TJmax, that is, TJ≤TJmax.

The heat balance speed of heat dissipation materials is required to be paid attention to, resulting in the heat of the light source not being effectively processed, causing serious light attenuation. At present, many manufacturers use different alloy aluminum materials for the heat sink heat dissipation shell of high-power LEDs. Their thermal conductivity coefficients are different, and the heat dissipation rate of some materials is difficult to meet the working conditions of LEDs. The heat conduction link of aluminum substrates, thermal conductive silicone and silicone grease materials that cannot be ignored, and the actual life quality of the materials used will directly affect the working heat dissipation conditions of LEDs. How to reduce the intermediate links and directly contact the heat sink heat dissipation at a close distance to quickly achieve effective heat dissipation to achieve balance is the direction that needs to be considered in the development of high-quality LED lamps products.

Let’s start with material analysis:

Thermal conductivity of metals

Silver 429 Copper 401 Gold 317 Aluminum 237 Iron 80 Tin 67 Lead 34.8

Silver has a better thermal conductivity, but its disadvantage is that it is too expensive. Pure copper has the second best heat dissipation effect, but it is already very good. However, copper also has disadvantages: high cost, heavy weight, and corrosion resistance. Therefore, most heat sinks are now made of light and strong aluminum materials, among which aluminum alloy has the best thermal conductivity. Good air-cooled radiators are generally made of aluminum alloy. As for copper, pure copper radiators are also available on the market. Copper's thermal conductivity is much faster than aluminum, but copper's heat dissipation is not as fast as aluminum. Copper can quickly take away heat, but it cannot dissipate its own heat in a short time. In addition, copper's oxidizability is the biggest drawback of copper itself. Once copper is oxidized, its thermal conductivity and heat dissipation will be greatly reduced.

From a comparative point of view, the best heat dissipation material is not aluminum. The comparison between copper and aluminum has formed a new type of process - copper-aluminum combination. The so-called copper-aluminum combination is to perfectly combine copper and aluminum together using a certain process, so that copper can quickly transfer heat to aluminum, and then the heat is dissipated by a large area of ​​aluminum. This not only increases the thermal conductivity of aluminum, which is not as good as copper, but also makes up for the fact that copper's heat dissipation is not as good as aluminum. The organic combination achieves the effect of rapid heat transfer and rapid heat dissipation.

Many articles have explained that heat dissipation depends on area rather than volume. Many companies understand this principle. The shell uses multi-layer fins for heat dissipation, but the fins of the heat sink shell are ignored for dust prevention and dust accumulation, which will affect the heat dissipation effect of the shell over time. It should be minimized under natural conditions to avoid dust accumulation, the natural scouring of wind and rain from different directions, and the adhesion of dust removal. Ensure that the heat dissipation effect of the heat sink shell is not affected by the harsh environment, the heat dissipation channel is unobstructed, and the real long life is achieved.

On the basis of increasing the heat dissipation surface to ensure the heat dissipation effect, the problems of natural scouring ease of wind and rain from different directions and the adhesion of dust removal are solved, ensuring that the heat dissipation effect of the heat sink shell is not affected by the harsh environment.

According to current metal processing technology, it is impossible to produce an ideal and absolutely flat surface by mechanical processing. Even a mirror surface has many tiny pits and dimples, but it is not easy to find them with the naked eye. In addition to the pits and dimples on the surface, there are also many tiny impurities, such as dust. When the surface of the heat sink contacts the surface of the chip, there are many grooves or gaps filled with air. The thermal conductivity of air is very poor, so other substances must be used to reduce the thermal resistance, otherwise the performance of the heat sink will be greatly reduced or even fail to function.

As a solution, thermal conductive media came into being. Its function is to fill the gaps between the two contact surfaces and increase the contact area between the heat source and the heat sink. Thermal grease is our most common thermal conductive medium.

Thermal grease is a material used to fill the gap between the aluminum substrate and the heat sink, which is also called thermal interface material. Its function is to conduct the heat emitted by the aluminum substrate to the heat sink, so that the temperature of the aluminum substrate can be maintained at a stable working level, prevent the aluminum substrate from being damaged due to poor heat dissipation, and extend its service life.

As a chemical substance, thermal grease has some relevant performance parameters that reflect its own characteristics. Understanding the meaning of these parameters can roughly determine the performance of a thermal grease product.

Thermal conductivity, the unit of thermal conductivity is W/m?K (or W/m?℃), which indicates the heat conduction power when the temperature difference of a column with a cross-sectional area of ​​1 square meter along the axial direction of 1 meter is 1 Kelvin (K=℃+273.15). The larger the value, the faster the heat transfer speed of the material and the better the thermal conductivity.

The thermal conductivity of mainstream thermal grease is greater than 1W/m?K, and the best ones can reach more than 6W/m?K, which is more than 200 times that of air. However, compared with metal materials such as copper and aluminum, the thermal conductivity of thermal grease is only about 1/100 of theirs. In other words, in the entire cooling system, the grease layer is actually the bottleneck of cooling. For a cooling system, it is not only the radiator, but also the thermal medium is a very important component:

The total thermal resistance of the cooling system = thermal resistance of the heat sink + thermal resistance of the thermal conductive medium

As the most commonly used heat-conducting medium, the importance of thermal grease is self-evident. To reduce its thermal resistance, it depends on the performance of the product itself on the one hand, and on the use of the product on the other. Therefore, we should try to use thermal grease with good thermal conductivity and low thermal resistance, and pay more attention to its use. Under the premise of ensuring that the grease completely fills the gaps on the heat source and the surface of the radiator, the grease layer should be applied as thinly as possible.

It is worth noting that ordinary thermal conductive silicone grease will "dry" or "harden" after being used in a high temperature environment for a period of time, which will greatly affect the heat dissipation effect. Therefore, the heat conduction link between the aluminum substrate and the heat sink needs to be paid attention to.

Relevant personnel are studying the direct installation of circuits by performing special ceramic treatment on heat sink materials. After such optimization, the heat conduction link of heat dissipation will be fundamentally solved.

Judging from the existing domestic manufacturers involved in the production of LED street lights, most of them have never produced street lights before, and the specific technical requirements for street light production are vague. They refer to and imitate each other, and most of them are produced in the conventional traditional "snake head" shape. The weight and wind resistance of the rectangular shape of today's LED street lights affect the renovation and installation of street lights. Compared with conventional gas discharge lamps, the optical light distribution and maintenance requirements are more difficult, especially the direct users, the owners, have strong reactions, which has brought difficulties to the application and promotion.

I got relevant information from the National Household Appliance Light Source Testing Center. Among the products sent by many manufacturers for inspection, most of them have problems in the IP protection project, and they are not aware of the problem. They think that our products have been soaked in water and there is no problem. Why do they have problems when they go to the testing center? I believe that some products have exposed moisture in the light source cavity after being used for a period of time. I feel heartbroken when I see many manufacturers queuing up to send products for inspection at the testing center. Many detours can be avoided. In terms of the sealing structure of the lamp body and the material selection of the sealing ring, Philips lamps have done special research in this regard.

In the design of street lamp shape and structure, do not follow the appearance of traditional street lamps. The function of street lamps is to meet the requirements of road lighting. They can give full play to the strengths of any combination of LED light sources, and can be made into "Transformers" to truly achieve the urban landscape effect of "viewing the scenery during the day and viewing the lights at night". Road lighting requires a certain illumination and road surface uniformity and average illumination. The defect of conventional lighting reflectors is that it is difficult to meet the standard requirements in terms of average road surface illumination. In terms of the effective use of the light output efficiency of lamps, the efficiency standard of traditional street lamps should be ≥70% to be qualified, but in fact, some of the scattered light cannot be accurately controlled on the irradiated surface and cannot be used, causing light pollution, and the effective utilization rate is about 50%, while the optical light distribution of LED street lamps can accurately control the direction of light, and the effective utilization rate is more than 60%.

Judging from the characteristics of today's high-power LED street lamp point light sources and light effects, in order to meet the requirements of road lighting, it is necessary to utilize the effective light intensity to the effective lighting range. The distance between lamp poles for road lighting is basically 30-40m. To ensure the average illumination requirements of the road, the light distribution of point light source LED lamps must ensure that there is no zebra effect on the road surface, and a corresponding secondary optical system supporting design is required.

The high-quality high-power LED street lamp uses a modular secondary optical system to effectively control the average illumination of the street lamp lighting. When it is 10 meters high, it is a 34m x 12m approximately rectangular light spot . The shape and uniformity of the light spot can meet the requirements of road lighting.

From the above two points, it can be seen that as long as the secondary optical design of LED street lamps is reasonable, the effective luminous flux projected to the target illumination surface is basically close to that of traditional street lamps.

At present, the lighting technology of LED street lamps has made great progress. Some manufacturers have developed a good secondary lighting system instead of relying solely on the arrangement of light sources. However, overall, the lighting distribution is not completely reasonable. Some lighting on the inner side of the road is brighter and the uniformity can meet the requirements, but the brightness of the ambient light on the sidewalk is obviously insufficient, and the SR value obviously does not meet the requirements. It is still that the manufacturer's designers do not understand the requirements of road lighting standards enough.

Color temperature problem of LED street lights:

At present, high-power LED street lamps basically use white light with a color temperature of about 5000K. As a road lighting source, the visual sense is too cold, and the observation ability will decrease when looking far away. In this regard, the street lamp users have the most say. Yellow light or warm white light of about 3000K is more suitable for road lighting, so daylight-colored LED street lamps are not suitable for use as street lamps.