LED street lights problems and solutions

Author: jiangwei68 Source: Photo News blog column

In response to the global financial turmoil, the Central Development and Reform Commission has allocated 4 trillion yuan together with local financial support. It is estimated that more than 10 trillion yuan will be invested in national transportation, supporting enterprises, and improving infrastructure. Recently, the Ministry of Science and Technology launched the “Ten Cities and Ten Thousands” LED Street Lights Semiconductor Lighting Application Project in the country. Under this strong east wind, the semiconductor lighting industry, which has always received government support, will be an opportunity for great development. Many companies are optimistic about the development of this sunrise industry, especially the obvious energy-saving advantages of semiconductor outdoor lighting. In the past two years, many companies that have been rushing to the top, under the characteristics of semiconductor road lighting production, have simulated, counterfeit, and made a variety of "snake heads" in the shape of today's conventional lighting street lamps. At one time, "a hundred schools of thought contend, flowers bloom." The products have been introduced to the market. After two years of testing on the road, most of the products have different degrees of problems.

Specifically in:

1. Due to the lack of understanding of the working conditions of the LED light source, the light attenuation is severe or even dead.

2. Insufficient perception of road lighting requirements, scientific point light source optical light distribution difficulties and neglect of the importance of color temperature in road lighting, easy to cause glare, zebra effect and in the environment of serious air pollution, rain and foggy weather, The phenomenon that the light is not bright enough on the ground.

3. Insufficient understanding of the LED operating conditions and the technicality of the power supply, resulting in endless product failures on the power supply.

4. The application concept of the practical road of LED street lamps is unclear. Blind and high-power gas discharge lamps are “comparable” to produce super-power LED street lamps without cost, which makes it difficult to promote unrealistic and expensive street lamp products.

5. The requirements for road lighting are vague, and the actual use and maintenance are not considered, resulting in the direct use of the owner's boycott.

I will discuss the solution with you on the above issues:

The discussion of the working environment of the LED light source needs to have the basic knowledge of understanding the LED; the core of the LED is the PN junction. Therefore, it has the IN characteristic of a general PN junction, that is, forward conduction, reverse cutoff, and breakdown characteristics. In addition, it has luminescent properties under certain conditions. At the forward voltage, electrons are injected into the P region from the N region, and holes are injected into the N region from the P region. A part of the minority carriers (small children) entering the other area is combined with the majority carriers (multi-sub) to emit light. The current high-power LED luminous efficiency is about 30%, and 70% will be heat energy, which needs to be heat-dissipated. The junction temperature TJ of high-power white LEDs shows the relationship between life and lifetime when the brightness is attenuated by 70%: when TJ=50°C, the life is 90,000 hours, when TJ=80°C, the life is reduced to 34,000 hours, when TJ=115°C Its life expectancy is only 13,300 hours. TJ should propose the maximum allowable junction temperature value TJmax in the heat dissipation design. The actual junction temperature value TJ should be less than or equal to the required TJmax, ie TJ≤TJmax.

From the above test data, the ambient temperature at which the tested LEDs can work properly should be (<85 °C). Above this temperature range, the efficiency will be greatly reduced, even burning. It can be seen the importance of temperature directly affecting it. It is particularly worth mentioning that the heat balance speed of the heat dissipating material requires attention, and the heat of the light source is not effectively treated, resulting in serious light attenuation. Nowadays, many manufacturers' high-power LED heat sink heat-dissipating shell applications basically adopt different alloy aluminum materials, and their thermal conductivity is different. The heat dissipation rate of some materials is difficult to meet the LED working conditions. The non-negligible aluminum substrate and thermal silica gel, the thermal conduction of the silicone grease material, and the actual life quality of the material used will directly affect the working heat dissipation conditions of the LED. How to reduce the intermediate link, directly contact with the heat sink and close the heat to quickly achieve a balanced heat dissipation, is the direction that needs to be considered in the development of high-quality LED lighting products.

First from material analysis:

Table of heat transfer coefficient of metal:

Silver 429 copper 401 gold 317 aluminum 237 iron 80 tin 67 lead 34.8

Silver heat transfer coefficient is better, but the disadvantage is that the price is too high, the pure copper heat dissipation effect is second, but it is already very good. However, copper also has disadvantages: high cost, heavy weight, and resistance to corrosion. Therefore, most of the heat sinks are made of light and strong aluminum materials, among which aluminum alloys have the best heat transfer capability, and good air-cooled heat sinks are generally made of aluminum alloy. As for copper, there is also a pure copper radiator on the market. The thermal conductivity of copper is much faster than that of aluminum, but the heat of copper is not as fast as aluminum. Copper can quickly take away heat, but it can't be in a short time. Dissipating its own heat, and the oxidizability of copper is the biggest drawback of copper itself. When copper is in an oxidized state, it will greatly decrease in terms of heat conduction and heat dissipation.

In contrast, the best heat sink material is not aluminum. A new type of process, copper-aluminum combination, has been formed in the comparison of copper and aluminum. The so-called copper-aluminum combination is a perfect combination of copper and aluminum with a certain process, so that copper can quickly transfer heat to aluminum, and then the heat is dissipated by a large area of ​​aluminum, which not only increases the heat conduction of aluminum. Copper also compensates for the heat dissipation of copper, which is not as good as aluminum, and the organic combination achieves the effect of rapid heat transfer and rapid heat dissipation.

In many articles, the heat dissipation is based on the area rather than the size of the volume. Many companies have learned the truth. The shell uses multiple fins to dissipate heat, but the fins of the heat sink heat sink are neglected. And accumulated dust, the accumulation of time will affect the heat dissipation effect of the shell. It is necessary to avoid the minimization of dust under natural conditions, the natural scouring of wind and rain in different directions, and the adhesion of dust. Ensure that the heat dissipation effect of the heat sink housing is not affected by the harsh environment, and the heat dissipation channel is unblocked to achieve a true long life.

Let's discuss a design approach: heat dissipation is obvious by area. Is it possible to make the heat-dissipating surface into a columnar or multi-faceted cone?

On the basis of increasing the heat dissipation surface to ensure the heat dissipation effect, the problem of the natural scouring ease of wind and rain in different directions and the stickiness of removing dust are solved, and the heat dissipation effect of the heat sink shell is not affected by the harsh environment. .

In the current metal processing technology, machining is impossible to make an idealized flat surface. Even the mirror surface has many small pits, but the naked eye is not easy to find, except for the pits on the surface. There will also be many small impurities, such as dust or something. When the surface of the heat sink is in contact with the surface of the chip, there are many gullies or voids in the air. The thermal conductivity of air is very poor, so other materials must be used to reduce the thermal resistance, otherwise the performance of the heat sink will be greatly reduced or even impossible to function.

As a solution, a heat-conducting medium has emerged. Its function is to fill the gap 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 medium.

Thermal grease is a type of material used to fill the gap between the aluminum substrate and the heat sink, and this material is also referred to as a thermal interface material. Its function is to transfer the heat radiated from the aluminum substrate to the heat sink, so that the temperature of the aluminum substrate is maintained at a level that can work stably, preventing the aluminum substrate from being damaged due to poor heat dissipation and prolonging the service life.

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

Thermal Conductivity

The unit of thermal conductivity is W/m•K (or W/m•°C), which indicates the heat conduction when the temperature difference of the column with a cross-sectional area of ​​1 m2 is 1 Kelvin (K=°C+273.15). power. The larger the value, the faster the heat transfer rate of the material and the better the thermal conductivity.

At present, the thermal conductivity of mainstream thermal grease is greater than 1W/m•K, and excellent can reach more than 6W/m•K, which is more than 200 times that of air. However, compared with the metal materials such as copper and aluminum, the thermal conductivity of thermal grease is only about 1/100 of them. In other words, in the entire heat dissipation system, the silicone grease layer is actually the bottleneck of heat dissipation. For a cooling system, not only the heat sink, but also the heat transfer medium is an important part:

Total thermal resistance of the heat dissipation system = heat sink thermal resistance + thermal resistance of the thermal medium

The importance of thermal grease as our most commonly used thermal medium is self-evident. To reduce its thermal resistance depends on the performance of the product itself and on the other hand. Therefore, we should try to use thermal grease with low thermal conductivity and low thermal resistance, and pay more attention to the use. Under the premise of ensuring that the silicone grease completely fills the heat source and the surface of the heat sink, the application method is as thin as possible.

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

The relevant person is studying the special ceramization treatment on the heat sink material to directly install the circuit. After such optimization, the heat conduction part of the heat dissipation will be fundamentally solved.

From the perspective of the domestic manufacturers who are involved in the production of LED street lamps, most of them have not produced street lamps. The specific requirements for the production of street lamps are relatively vague and cross-referenced. They are made in the shape of conventional traditional "snake heads". The rectangular weight and wind resistance of LED street lamps affect the retrofit installation of street lamps. Compared with conventional gas discharge lamp lamps, optical light distribution and maintenance requirements are more difficult, especially for direct user owners. It is difficult.

The information I got from the National Electric Light Source Testing Center is in the IP protection project of many manufacturers. Most of the problems exist, and I don’t realize the problem. I think our products are in the water. There is no problem after soaking. How can I go to the testing center to have problems? It is believed that some products have exposed the water in the light source cavity for a period of time. I have seen many manufacturers lining up for inspection products in the testing center. I feel heartache and many detours can be avoided. In the lamp body sealing structure, Philips' lamps have been specially researched in this regard in the material selection of the sealing ring.

In the design of the street lamp structure, do not follow the traditional street lamp. The function of the street lamp is to meet the requirements of road lighting. It can exert the strength of any combination of LED light sources, and can be made into the characteristics of “Transformers”, which can truly achieve the urban landscape effect of “day view and night view”. The requirements for road lighting are: certain illumination and road uniformity and average illumination. The defects of conventional lighting reflectors are difficult to meet the standard requirements on the average illumination of the road. In the effective utilization of the light-emitting efficiency of the lamp, the efficiency standard of the traditional street lamp should be ≥70% as qualified, but in fact, some of the spill light cannot be accurately controlled on the illuminated surface and cannot be utilized, resulting in light pollution, and the effective utilization rate is about 50%. The optical light distribution of the LED street light can accurately control the light direction, and the effective utilization rate is over 60%.

From the characteristics of today's high-power LED street light point light source and light effect, in order to meet the requirements of road lighting, it is necessary to use effective light intensity to the effective lighting range. The distance between the light poles of road lighting is basically 30-40m. To ensure the average illuminance requirement of the road, the point light source LED luminaire should be equipped with light to make the road surface have no zebra effect, and the corresponding secondary optical system design must be carried out.

Take a certain lamp as an example: see the picture; the high-quality high-power LED street lamp can effectively control the average illumination of street lighting with the modular secondary optical system. At 10 meters high, it is an approximate rectangular spot of 34 meters × 12 meters. The spot shape and uniformity can meet the road lighting requirements.

It can be seen from the above two points that as long as the secondary optical design of the LED street lamp is reasonable, the effective luminous flux to the target illumination surface is substantially close to that of the conventional street lamp.

At present, the light distribution technology of LED street lamps has been greatly developed. Some manufacturers have developed a good secondary light distribution system, instead of relying solely on the arrangement of light sources to distribute light, but comprehensively, the light distribution cannot be achieved. It is completely reasonable. Some of the light distribution on the inner side of the road is brighter, and the uniformity can meet the requirements. However, the brightness of the ambient light on the sidewalk side is obviously insufficient, the SR value is obviously not in compliance with the requirements, and the manufacturer's design personnel have insufficient understanding of the requirements of the road lighting standard.

Color temperature problem of LED street light:

At present, high-power LED street lights are basically white light with a color temperature of about 5000K. As a road illumination source, the visual sense is too cold and cold, and the observing ability will decrease when it is farsighted. In this respect, the street lamp use unit has the most say, and the yellow or warm white light of about 3000K is more suitable for road lighting, so the daylight color LED street light Not suitable for street lighting.

Today's LED street lamps lack uniform technical standards in terms of product specifications. At present, there is no unified technical standard for LED street lamp production. Each manufacturer produces its own products according to its own standards. It is difficult to be common to each other. In addition, LED street lamp manufacturers The lamps are self-contained, the light source electrical assembly and the lamps are bundled together, and their different working characteristics are difficult to maintain. The original products must be used for maintenance. Due to the unification of technical standards, high-pressure sodium lamps have strong versatility in lamps, light sources and electrical appliances. Products from different manufacturers can be used interchangeably. This is very important from the perspective of maintenance, and maintenance is also important for street lighting management units. It is the most important thing. Only in this way can the faulty street lamp be maintained in time to ensure the perfect lighting rate (the most important part of the street lighting management assessment by the superior department), and greatly reduce the maintenance cost. It can be said that under the current development of LED street lights, in the event of a fault, only the entire luminaire can be replaced. In addition to the high cost, the maintenance time should be greatly increased, which is very unfavorable for the user. For the maintenance of lamps, conventional lighting fixtures are much simpler, and the light source appliances can be directly replaced at the site, and the cost is also low. However, many manufacturers' LED street lights may be considered maintenance-free by the manufacturers, so the on-site maintenance cannot be carried out, and the real burden of maintenance is still given to the manufacturer. As everyone knows; the bottleneck that makes the product difficult to promote is still from the direct use of the unit.

Two major challenges for today's LED streetlight applications:

Difficulties in maintenance

2. Bottleneck of high-power LED power supply

According to these two difficulties, I personally recommend; from the safety, maintenance and easy replacement, to avoid the quality risks of high-power power. Modular modular design must be the direction of development. Made into standard modular, such as 30w, 40w, 50w together with the power supply integration. Not only solves the risk of high-power power supply, but also facilitates the replacement of faults. The single-group module can also be used for a variety of lamps, which can be converted into LED tunnel lights, LED ceiling lights, LED wall washers, LED flood lights , LED tunnel lights, etc.

The heat-dissipating material, the heat-conducting link and the scientific shape structure design are worthy of everyone's understanding in the production of high-power LED street lamps. From the doubts and even rejection of the LED street lamp market, to the current trial and acceptance, we have seen the dawn of the future, but we still need some way to go. With the continuous improvement of high-power LED light efficiency and the rapid development of silicon-based LEDs, it will fundamentally solve the bottleneck of high price of semiconductor lighting sources. Let us join hands to explore and discuss. I believe that with everyone’s efforts, use the fourth. Generation of light source to make better LED street lighting, replacing the high-pressure sodium street lamp this road will not be very far!

For the original text, please refer to: http://blog.ofweek.com/jiangwei68/DiaryDetail.do?diaryid=48d6e4ccf6a73374

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