月份：2012年9月

Something you should know about LED dimming technology

Under such circumstances, you may probably choose to use step-down (wide voltage) constant-current sources, like using 10V-30V constant-current power suppliers to realize the LED dimming technology. However, this kind of step-down constant-current source still has defects. When the forward input voltage has been adjusted into a certain low level, the load current of LEDs will be forced down to a very low level. This would dramatically increase the entire potential (voltage) of load (LED lights), even exceeding the normal working range of this step-down constant-current source (causing flicker). Working in the low brightness conditions for a long time will decline the efficiency of step-down constant-current sources and increase the working temperature in the mean time. Since the efficiency of step-down constant-current sources has directly relations with the potential drop ratio. The larger the potential drop ratio, the lower the efficiency, and more power will be consumed, causing obvious damages to power suppliers and the service life of lighting sources.

Under such circumstances, you may probably choose to use step-down (wide voltage) constant-current sources, like using 10V-30V constant-current power suppliers to realize the LED dimming technology.

  Commonly used incandescent lamps and halogen lamps usually used SCR dimming technology. Since all of them are purely resistive devices, the input signal of these lamps are not necessarily sine waves like LED lighting devices required. The waveform of current will be indentified with the waveform of voltage forever. Thus regardless of the distortion of the input voltage waveform to a sine wave, changing the effective value of input voltage can realize the dimming effect.   However, the SCR will bring unexpected problems to LED dimming technology: LC filters in the input side will bring vibrations to SCR. This vibration has no effects on incandescent lamps, since the thermal inertia them hardly can be captured by human eyes. But this vibration will exert very obvious influences on LED power suppliers, causing audio noise and flickers. Besides, SCR dimming will also destroy the waveform of sine waves, reducing the power factor (normally less than 0.5). In order words, SCR dimming dramatically decline the lighting efficiency of LED lighting systems. Sometimes, SCR dimming technology will increase harmonic coefficients, generating non-sinusoidal waveforms, which will be a serious interference to single (EM), polluting the power grid. In serious conditions, the entire power grid will be failed.   Either step-down constant-current sources or SCR dimming sources are suitable for LED dimming technology. Some people may ask: what on earth type of dimming method is the most appropriate for this new lighting technology?

Some knowledge that you must know about LED dimming technology

How to comprehend LED dimming technology? How to know it well and master it? In order to answer these questions, we need first understand the voltage-current characteristic of LEDs.

 

Some knowledge that you must know about LED dimming technology

 

What is the voltage-current characteristic of LEDs? It is known by all that when voltage worked on PN junctions (typical construction of semiconductors), internal current will be varied with the vibration of input voltage. The characteristic curve can be cleared display on oscilloscope. A complete voltage-current curve includes the forward characteristic curve and the reverse characteristic curve. Typically, the reverse characteristic curve will be much steeper than the forward one. When input voltage surpassed a certain threshold value, the corresponding internal current will increase exponentially, and consequently, breakdown the entire LED PN junction. While the forward internal voltage is also determined by its forward internal current.

 

According to the voltage-current characteristic, we could conclude that LED dimming technology definitely cannot be realized through simply reducing input current or input voltage. In addition, LEDs’ sinusoidal waveform is different from the waveform of incandescent lamps, thus it is also impossible to vary the lighting effects (dimming effects) by changing their conduction angles.

 

For example, a LED lamp with an input voltage of 24VDC, with 8 pieces high-powered LED chips (1W/each) in series. Given that the forward current is about 350mA and the forward internal voltage for each LED chips supposed to be 3.3V, thus the total input voltage for the entire lighting sources (8 pieces LEDs connected in series) should be 26.4V. It is cleared that the load voltage (26.4V) is much higher than the input voltage (24V), thus here constant-current power suppliers should be applied. In order to realize the dimming effect, engineers have to reduce the internal current from 350mA to 100mA, causing the forward voltage in each LED chip reduced to only 2.8V. Then the total input voltage of the entire system is 22.4V (now the load voltage is smaller than the input voltage). So the constant-current is incompetent. That is the reason why many terminal users or buyers saw flickers of their LED lights. Theoretically, according to the working principles of LEDs, LED lighting applications will be free from flickers in ideal conditions.

Thermal management solutions for high-brightness LED applications

With the increasingly serious in energy shortage and environmental pollution, LED lamps (especially for high-brightness LED applications) have become a hot topic among lighting applications providers and customers due to their energy-saving and environmental friendly characteristics. Normally, LED lamps are composed of LED modules, cooling sections, drivers, lens, and other optical/electrical components. It is the cooling section that the most important, since LED lamps with a poor heat dissipation effect will reduce their lifetime. According to the testing report of Cree XLamp XR-E, there is a closed relation between working temperatures and service life of LED lamps. Thus solving the problem of thermal management has become a key point to high-brightness LED applications.

 

Thermal management solutions for high-brightness LED applications

 

However, the heat generated by LED lighting sources is not the major factor for LED working conditions and service life. The accumulation of heat within chips (hot spots) is a more serious problem for LED lamps. For general standard LED devices, LED modules that have a power consumption of 1W will have a heat flux of 100W/cm2, 300W/ cm2 for 3W LED modules, while the average heat flux of CPU is only 60-130 W/cm2. A large amount of heat concentrated within the small-sized wafer, leading to the increase of temperature, the uneven distribution of thermal stress, and finally the reduction of wafer luminous efficiency and emission efficiency of the fluorescent powder. When the temperature exceeds a certain value (threshold), the failure rate of the device will increase exponentially. When multiple LED lights constitute the entire white light illumination system, the heat dissipation is even more serious.

 

Heat conduction and heat convection are two main path for heat dissipation of LED devices. Traditionally, the cooling structures of LED lamps include bases, heat sinks and radiators. Bases are directly connected with LED chips and typically used for conducting heat generated by chips, with thermal conductivity but not electrical conductivity. Heat sinks can spread the heat, used to avoid the heat accumulation on LED chips. They can also increase the efficiency of radiators. Heat radiator can effectively dissipate all heat outwards into the air. Among them, it is the base that needs to be taken careful consideration of. Bases made by materials with relatively large thermal conductivity will result in severe self heat effects, which is a devastating impact on the performance and reliability of LED devices.

High brightness LED emerged in the market

High brightness LED brought many compelling features to car manufacturers. In spite that most of these features were derived from the working principles and similar production process of LED indicators-products are much older than themselves and well-proved.

 

High brightness LED brought many compelling features to car manufacturers.

 

LED taillights, LED signal lamps, LED working lights, LED brake lights can overcome shortcomings that much common in incandescent lights. Usually, vehicles will meet with frequent moderate shocks and vibrations, which may probably cut down the service life of incandescent lamps. Besides, the instantaneous surge current caused by temperature coefficient of filament resistance will accelerate the destruction of bulbs. Thermal loop-an important feature of break lights will definitely shortened the service life of incandescent lamps.

 

The instantaneous surge current generated by incandescent bulbs made the circuit protection and fault detection be more complicated. Automobile manufacturers have to set the fuse rating and the fault detection threshold large enough, to adapt the inrush current magnitude and duration without causing fuse blows failures or detection failures.

 

In contrast, when cars in shocks and vibrations that in the normal range of amplitude and frequency, structures of high brightness LED would be much stronger than incandescent bulbs. LEDs have compact structures (small sized) and light weight, which can reduce the mechanical torque generated by shocks and vibrations. The compact construction of high brightness LED contributed to the flexibility in the design of vehicle lighting applications. Within such limited space, designers hope that each compartment can meet the requirements of the overall design of the car. For example, some small cars would not set the CHMSL (Center High Mounted Stop Lamp) module in the rear cover. Instead, manufacturers made use of the feature of compact construction of high brightness LED and embedded them into trunk lids.

 

Designers also found out several problems about automobile taillights and the control system. These problems were also emerged in other systems that control devices were far apart with devices that under control. LED is essentially a kind of current devices. The combination of electron-hole pairs can emit photons. When increasing the input current, the compound speed and output luminous flux will also be raised. Definitely, the efficiency of this process cannot reach up to 100%, thus the a certain proportion of electrical energy will convert into heat. Unless in very poor working conditions, LED Lights will not break down like incandescent lamps does. But working in a over-heated condition for a long time will also accelerate the light attenuation of LEDs and consequently the service life.

Four key technologies of LED tube lights

Tube lights are widely used in our daily life, which is can be say one of the most commonly used lamps in the world, in supermarkets, in schools, in offices, in subways. In recent years, LED tube lights started to be used in these places due to their advantages in energy-saving and environmental protection. How to improve the lifetime and brightness of these lamps to make them meet the requirements of general lighting is a significant subject. In order to maintain a longer service life and higher brightness, problems waiting to be solved are: power suppliers, LED lighting sources, cooling sections and safety.

Tube lights are widely used in our daily life, which is can be say one of the most commonly used lamps in the world, in supermarkets, in schools, in offices, in subways.

 

The first and foremost requirement of power supplier is high efficiency. LED tube lights with high efficiency will generate less heat and have higher stability. Normally used power suppliers can be divided into two groups: isolated drivers and non-isolated drivers. Generally, isolated power suppliers are superior to non-isolated power suppliers in many aspects. They are smaller, having higher efficiency and easier in packaging and installation. Overall, isolated drivers have wider prospects in the market.

 

When making LED lighting sources, designers usually placed chips on pins. These silver pins can conduct heat generated by PN junctions during their works. Different from the traditional DIP products and patch products in the heat dissipation aspect, chips packaged in this way will not collect heat within them, having longer service life and less light attenuation. Traditional patch-packaged LEDs connected with positive and negative electrodes through gold wires, which will be weaker than silver pins in heat conduction.

 

Introducing infrared heat radiation into LED tube lights is a crucial approach to improve their service lives. Normally, designers would like to separate LED lighting sources and power suppliers in case that the cooling sections of these two parts have interference and guarantee an effective heat dissipation. There are three ways of heat dissipation: convection, conduction and radiation. In a closed environment, the heat dissipated by convection and conduction is very limited, while radiation is the mostly commonly used cooling method among LED tubes.

 

PC flame-proof plastic tubes used in LED tube lights can easily be penetrated by infrared heat. When we design LED lamps, we need to take safety into account. Using all-plastic insulation, even in the use of non-isolated power suppliers, safety still can be guaranteed.

Difficulties of LED lighting sources used in residential lighting applications

Currently, LED lighting applications have been researched, developed and produced by countless LED companies. LED technology has occupied a very important position in the project lighting market and the outdoor lighting market. But for residential lighting market, it is only to say that LED technology is still an emerging market full of potentials and opportunities. How to bring LED lighting sources into ordinary families?

 

Difficulties of LED lighting sources used in residential lighting applications

 

The distribution of domestic LED lighting industry is relatively concentrated: the Pearl River Delta, the Yangtze River Delta, the southeast region and Beijing area. Each area has basically formed a relatively complete industrial chain. Dalian, Shenzhen, Xiamen, Nanchang have large-scale production bases for LED.

 

Most people in this country are still lacking of necessary knowledge about LED lighting technology. Furthermore, prices of LED products are much higher than that of other traditional lamps. As a result, it is difficult for LED lamps have a deep penetration in residential lighting market. On 20th of March this year, the Ministry of Finance, the National Development of Reform Commission and the Ministry of Science and Technology launched a public tender named “2012 financial subsidies and projects for promotion of LED semiconductor lighting products”. Nearly 300 bidders from 110 LED companies all around the country participated in the tender. Many LED lamps have been listed as tender products, like LED down lights, reflective self-ballasted LED lights are classified as indoor lighting applications, and LED street lights, LED tunnel lights used for outdoor purposes. The National Development and Reform Commission declared that in 2012 the Chinese government will spend 40 billion yuan for LED lights procurement and provide 30% financial subsidy for users.

 

At present, LED lighting sources are still in the stage of immature, frequently encountering bottlenecks both in technologies and in equipment producing. In the field of substrate materials and wafer equipments, domestic LED lighting industry had obvious bottlenecks. The independent innovation and patents of our LED industry are mainly concentrated on the packaging area. Those core technologies, like White-ray LEDs, heat dissipation of high-powered LED lamps, durable and efficient phosphor technologies, patents in these fields are mostly under the control of giant manufacturers from Europe, America and Japan. Most crucial parameters of domestic LED products are still lagged far behind the international level, like brightness, heat dissipation, lighting efficiency, evenness of light distribution, antistatic abilities, anti-leakage abilities, etc.

Japan’s LED bulb market trapped in the doldrums

In Japan, the government, LED manufacturers, buyers, most of people are now promoting LED lighting technology in their life and work. After the earthquake in 2011, the needs for energy-saving became significant and necessary, which led to a vigorous development of LED lighting applications in Japan. In 2011, the spread of LED bulbs was far beyond the expectation of people. However, the supposed prosperous of LED bulb market has not come finally. LED manufacturers had to face up with rather naughty situations. LED technology has indeed spawned a new growth in the lighting area, bringing hope to the lighting market. But on the other hand, the long service life of these bulbs reduced the demand, plus the increasingly intense competitions, forcing large local LED companies to convert the business structure.

 

In Japan, the government, LED manufacturers, buyers, most of people are now promoting LED lighting technology in their life and work.

 

In early August, in one large home appliance store in Osaka, on the corner of floor one, reporters saw counters are piled up with LED bulbs. Labels of “low-price” were stick on boxes, like “4,650 yen for three”, “7,750 yen for five”…

 

According to the store’s sales, from May of this year, sales of LED bulbs started to trap in the doldrums. Due to the constant prices dropping, buyer chose to wait, it turned out that sellers’ expectations came to nothing.

 

It is really a hard time for LED bulb market in Japan. The re-construction of nuclear power plants in Japan still remained some distances off, some local governments required each ordinary household to save more than 10% daily electricity. Under such serious circumstance, LED bulbs are still unmarketable. Mr. Koji Sato, the registrar of the electrical appliances giant Toshiba, said, “the sales of LED bulbs is only 80% of that in 2011. It was expected previously that sales of LED blubs will climb up to the peak during 2013-2014. But now, it seems that 2011 is the peak year for the LED bulb market in Japan.”

 

The global general lighting market has a scale of 7 trillion to 10 trillion yen, and the scale of Japan is about 800 billion yen. In 2011, although that LEDs’ occupation has expanded to 17% in the general lighting market, the previously “hot” LED bulbs began to cool down.

Promote white-ray LED technology in indoor lighting applications

Although high-powered white-ray LED technology has made great breakthroughs in luminous efficiency, CRI and power consumption of single chip, and local LED manufacturers continued to improve their technologies and promote new products, white-ray LED technology still have a long way need to go before providing comfortable lighting environments practically. In order to improve the quality and also quantity of white-ray LED products in the indoor lighting area, the following points should be noticed by both producers and users.

 

Although high-powered white-ray LED technology has made great breakthroughs in luminous efficiency, CRI and power consumption of single chip,

 

In the view of the visual function, it is difficult for current LED products to meet basic needs for general lighting, both in luminous flux and lighting effect. Thus, reasonable lighting designs will be a key point to the indoor lighting. For example, using LED in local lighting and accent lighting, like spotlights; using LED as assisted lighting applications or in dimming designs, since they can be easily controlled. Some LED manufacturers even created intelligently controlled LED lighting system.

 

There is no doubt that the upstream business and midstream business are significant and crucial to the development of LED technology. But the downstream applications also played a lead role to the midstream business. Relations between scientists, engineers and designers should be strengthened, giving the LED industry a more reasonable and humanizing development.

 

LED secondary emitting is a key technology to LED lighting applications. Normally, lighting distribution can be optimized though LED secondary emitting designs. In this way, LED can emit much more reasonable light and meanwhile avoid glare.

 

Presently, testing departments in China have embarked on developing more effective test methods for LED lighting applications, including the development of relevant testing equipments. Standardization Technical Committee of National Lighting Appliance has already put the standardization work of LED lighting applications into agenda, including the standardization of LED technologies and also testing methods. The government hoped to promote the development of LED lights technologies and their applications in the field of lighting through the formulation of relevant LED standards. For LED indoor lighting standards, exerts believed that it is optical characteristics that should be improved at first, like luminous flux, lighting distribution, brightness, spectrum distribution, chromaticity coordinates, CRI, light attenuation, etc. Besides, the characteristics of electrical accessories, mechanical properties, temperature characteristics and appliance safety features should not be ignored, either.

White-ray LED lamps: meet the need of users psychologically and physiologically

During normal indoor activities, people not only need their visual function to be satisfied, but also need to feel comfortable visually, which is defined as a psychological needs for the lighting environment. Normally these good-quality lighting environments should not be bothered with direct glares and indirect glares, with right color temperatures, with comfortable arrangement of lamps. In the aspect of glare control, white-ray LED lamps have both advantages and challenges. Since LEDs have smaller emitting points and beam angles, it is relatively easy to control the direction of the beam. But on the other hand, as a kind of highly concentrated lighting sources, if the maximum light distribution of LED lamps cannot be disposed properly, the entire lamps will send out some glares which exert bad influence on human eyes. It is showed that the discomfort glare will not only case a decline in the efficiency of your works, but can also cause visual fatigue, headaches and other adverse reactions. If choosing RGB LED chips as lighting sources, manufacturers can produce white-ray LED lamps with arbitrary color temperatures. But this will affect the comfort and aesthetics of the whole field of view. Blue-ray LEDs with YAG powder will generate high color temperature rays. In order to maintain good illumination effects, the color temperature of lighting sources must be adapted to the illumination requirements. Higher color temperatures will definitely impact the indoor white-ray LED lighting applications. Because it requires a correspondingly higher illuminance to make people feel comfortable. Besides,

 

White-ray LED lamps: meet the need of users psychologically and physiologically

 

People’s physiological needs for indoor lighting are mainly in the health part. It is cleared that a long-term UV radiation can have skin adverse effects on human bodies. Besides, there are two other health-hazard optical influences that can be easily ignored: visual effects and effects on the circadian rhythm system. There are two effects that closely associated with white-ray LED lights for indoor lighting: the blue-ray hazardous effect on retina and the light affect the circadian rhythm system by suppressing the secretion of melatonin. Usually, UV caused damages on the front of our eyes (cornea). The wavelength of blue rays is longer than that of UV rays. Thus they can easily get through the eye ball and reach end to the retina. Furthermore, blue-rays have the highest unit energy among visible ranges, can easily burn the retina and lead to cataracts. On the other hand, the melatonin secretion is affected by light. The disorder of melatonin secretion will cause physiological imbalance, even cancers or other diseases in some serious conditions.

The visual requirements for white-ray LED lights

The color temperature and CRI of white-ray LED lights are close related to production process of LEDs. It was Nichia that created white-ray LEDs for the first time by using InGaN blue-ray LED chips (yttrium aluminum gamet) with phosphor powders. After that, people also created white-ray LEDs through light mixing of RGB chips.

 

The color temperature and CRI of white-ray LED lights are close related to production process of LEDs.

 

Blue-ray LEDs plus YAG phosphor is relatively simple than other methods, and cost less. It is the most commonly used method to make white-ray LED lights. But relatively, LEDs produced through this way have very low CRI. Adding some red phosphor powder and green phosphor powder can increase the CRI level, but the lighting efficiency will be declined. Theoretically, ultraviolet LED chips combined with RGB tri-chromatic phosphor powder can generate multiple color temperatures and white-rays with higher CRI. But the problem is the current ultraviolet LED technology is still in an immature stage. Indeed, multi-chips can obtain arbitrary color temperature and white-ray LEDs with high level of CRI theoretically. However, the input voltage and output light of multi-LED chips are different. Besides, the temperature characteristic and the light-keeping characteristic are different, either. Thus, it requires very sophisticated circuit designs. The current immature technologies will lead to the huge variations in color temperatures.

 

In order to satisfy a certain level of illuminance, LEDs should provide sufficient luminous flux. Take a small reading room for instance, assumed that the area of this reading room is 6m x 8m. Normally, lamp with a luminous flux around 20000lm – 30000lm is fit for reading needs. If taking no consideration of other factors, even using single LED chip with a luminous efficiency of 200lm/W, 100-150 pieces are necessary. However, currently, the common luminous efficiency of single white-ray LED chip is only about 30lm/W-40lm/W, far less than the assumed 200lm/W. It is cleared that no both in power consumption and in production costs, compared with energy-saving lamps, white-ray LED lights is in no match. In addition, the immature design of optical technologies reduced the stability of the lighting distribution of LED lamps. Obtaining a much more even lighting distribution, manufacturers have to increase production costs. In terms of spot lighting applications, LEDs with a small amount of energy consumption can meet the needs of visual, as well as evenness. In CRI aspect, the current white-ray LEDs can meet the needs of the general indoor activities.