Development and Future of High Power LED Lighting Dimming Technology

Development and Future of High Power LED Lighting Dimming Technology With the development of global industry, a large increase in carbon dioxide emissions during the industrialization process, the global climate is deteriorating, and flood and drought, hailstorms and snowstorms throughout the country have a record-breaking trend, which seriously threatens human survival. In order to jointly cope with the threat, in December 2009, heads of state gathered in Copenhagen to discuss major plans for energy-saving and emission reduction. Local governments actively formulated and implemented low-carbon plans. As green, environmental protection and energy-saving led lighting, more and more favored by countries, China's "ten cities and ten thousand", Los Angeles, "140,000 plan" is in full swing. The successful application of dimming technology in LED lighting will further increase the effect of energy conservation and emission reduction.

First, the need for dimming

As a kind of green energy, electric energy accounts for an increasing proportion of energy applications in various countries. According to Chen Lianghui, an academician of the Chinese Academy of Engineering, it is estimated that in 2010, the country's electricity consumption will reach 2.7 trillion kWh, of which lighting electricity consumption will exceed 300 billion kWh. Because LED lighting consumes only 1/3 of the power of traditional incandescent and HID lamps, replacing LED lamps with existing conventional lamps will hopefully save energy by 2/3. Its energy-saving effect is evident.

The application of dimming technology will further reduce energy consumption and save lighting power. At present, public lighting uses one-third of the entire lighting field, up to 100 billion degrees. In China, cities usually start from 5:30 or 6:00 in the evening. However, after 9 o'clock in the evening in small cities, and after 12 o'clock in the middle and large cities, there are few people on the roads, even in Beijing, Shanghai and Guangzhou. In the metropolitan area, pedestrians and vehicles are rarely seen on the road after 2:00 in the morning. From this period of time to 6 o'clock in the morning, the street light goes out, and it is obviously not necessary to maintain a high illuminance on the road with low traffic flow. If dimming technology is adopted, the output power of LED street lights will be reduced to half after 12 o'clock in the afternoon with the reduction of traffic flow, which will save electricity consumption by up to 50 billion kWh, equivalent to saving the annual power generation of the Three Gorges Power Station in 2009. About five-fifths of the total (the Three Gorges power station produced 79.85 billion kwh in 2009), and the energy-saving effect is very significant.

Using dimming technology to reduce output power will significantly improve the working environment of LED lamps, extend lamp life and improve reliability. The key factor affecting the reliability of LED luminaires is temperature. Whether it is LED driving power or LED chips, the higher the ambient temperature, the sharp decline in its life and reliability. Figure 1 shows the relationship between LED driver power supply life and temperature. When the ambient temperature is 25°C, the life of the LED power supply is up to 320,000 hours. When the temperature rises to 60°C, the life of the LED power supply will be shortened to only 35,000 hours.

The higher the temperature, the worse the LED chip's light fades. Figure 2 shows the light decay curve of the LED light source of Cree Corporation. As can be seen from the figure, the LED's light decay is closely related to its junction temperature. The so-called junction temperature is the temperature of the semiconductor PN junction. The higher the junction temperature is, the sooner the LED light source has light decay, that is, the shorter the lifetime is. As can be seen from the figure, if the junction temperature is 105 °C, the brightness of the LED light source reduced to 70% of the life of only more than 10,000 hours; the junction temperature of 95 °C LED light source life has 20,000 hours; and the junction temperature is reduced to 75 °C The life span can be as long as 50,000 hours, and the lifespan can be extended to 90,000 hours at 65°C. In summary, the key to prolonging the life of LED light sources is to reduce the junction temperature.

With dimming technology, as the output power is reduced, the loss of the driving power supply itself will be reduced, and the shell temperature will also decrease. As shown in Figure 3, the output power of the 200W LED driver power supply and the shell case temperature curve. When the ambient temperature is 25°C full-load output, the case temperature is 50°C, and when the dimming technology is used to control the output to only 50% power, the case temperature will drop to 40°C, and the lifetime will also be doubled, and the reliability can be increased by 25%- 40%.

Because traditional incandescent lamps or HIDs could not be dimmed, in the 1970s Japan's energy crisis, the intermittent lighting method was used to try to save energy. However, due to the uneven illumination of the road surface, the visual problems of passers-by caused the public security and road traffic accidents to rise substantially, and they had to give up the interval lighting method. The rapid development of dimming technology will bring the LED lighting dimming system into a brand-new field, which will popularize and popularize green energy-saving LED lighting applications.

Second, LED lighting main dimming technology

When the LED was first used in public lighting such as roads, in the initial stage of promotion, some LED lamp factories used the idea of ​​interval lighting to achieve the purpose of dimming and energy saving. The LED chip of an LED lamp was divided into two parts and controlled separately. When the drive current is on and off, all LEDs are lit before 12 midnight, and are disconnected after 12 o'clock to save energy. Such a dimming method can achieve a certain energy-saving effect, and the uniformity of illumination is significantly improved compared with the conventional interval lighting method, but it will still affect the light distribution curve of the LED lamp. In order to further improve the efficacy and light distribution curve of LED lamps, some LED lamp factories also consider arranging the LED chips in a spaced manner. When dimming, only the intervals are bright or extinct, and the uniformity is improved to some extent. Light distribution.

The brightness of the LED chip is proportional to the LED driving current. Therefore, the LED lamp can be dimmed by adjusting the LED driving current. The second dimming method is generally called an analog dimming method or a linear dimming method. The advantage of this kind of dimming method is that when the driving current increases or decreases linearly, the influence of the driving current overshoot on the LED chip is relatively reduced, and the control circuit has strong anti-interference. The disadvantage is that the size of the drive current changes, which will have a certain influence on the color temperature of the LED chip. The typical application of this type of dimming method is LED tunnel lighting of Anhui Liuqian and Liuwu Expressway.

The third method of dimming is pulse-modulation (PWM). In this method, the drive current is square wave shaped through a certain control, the pulse width thereof is variable, the time for the continuous lighting of the LED lamp is changed by the adjustment of the pulse width, and the output power is also changed at the same time, thereby achieving the purpose of energy saving. The frequency is generally controlled at 200 Hz to 10 KHz. Due to the persistence of human eyes, the flickering of the light source during the dimming process is not felt. The advantage of this dimming method is that it can improve the heat dissipation of the LED. The disadvantage is that the overshoot of the driving current has a certain influence on the lifetime of the LED chip.

The above three types of dimming methods are classified according to the control of the LED drive current output terminal. The control circuit can also be divided into analog control and PWM control. The control circuit is usually implemented by a microcontroller (such as a single-chip microcomputer).

III. Composition and Future Development of Dimming System

Perfect intelligent control should have manual control, intelligent control, light control, time control and fault alarm control functions. When the sun goes down, it will automatically light up, and the illumination of the lamp will be halved automatically after 12 o'clock in the morning. After daylight, the light will automatically go out; in special circumstances, the person can control it manually; when the light fails, the control system Automatic alarm.

The intelligent dimming system is mainly composed of a server, a central controller, a single-chip controller, and a signal detection circuit, as shown in the figure.

Server: It is composed of upper computer and upper computer software. It mainly realizes monitoring operation and alarm function. The server is located in a control center such as a street lamp management, and the connection between the server and the central controller may be power carrier, network, or wireless.

The central controller is responsible for receiving the signals sent by the server and sending the signals to the unit controller after decoding. The control is realized and the result of the signal detection is transmitted to the server.

Signal detection circuit: Detects environmental brightness, driving power supply voltage, and current. The detected signal is sent to the unit controller or the central controller.

Unit controller: Each LED lamp is equipped with a unit controller. By receiving the instruction of the central controller, the LED is dimmed or turned on and off, and the LED lamp driving voltage and current are sent to the signal detection circuit. Central controller. When the LED lamp fails, the fault signal is transmitted to the central controller.

Currently, in the field of road lighting, the application of LEDs is rapidly gaining popularity. Long life and reliability are still the first factors currently considered. However, with the development of technology, when the problems of life and reliability have been solved to some extent, LED Lighting applications are bound to make great strides towards intelligent dimming systems.

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