The LED is a solid-state light source. When a forward voltage is applied across the LED, the minority carrier and the majority carrier in the semiconductor are recombined, and the excess energy emitted will cause photon emission. Different materials can be used to make light emitting diodes in different colors. As a new light source, in recent years, major companies and research institutes have been in the ascendant in LED research, which has greatly improved their light efficiency. Lumiled has developed and produced a white LED with a luminous efficiency of 90 Lm/W, which has reached incandescent light. s level.
LED drive power status
Standard lights are undergoing a revolution. In order to protect energy and cope with global warming, some states and some other countries in the United States have banned the use of incandescent light bulbs with low energy efficiency. Various new technologies are being used to replace incandescent bulbs, of which the compact vacuum fluorescent lamp (CFL) is the main alternative. Although this type of CFL lamp consumes only 20% of the power of an incandescent lamp, it does contain the toxic substance mercury. In contrast, LED lights can provide more efficient and more environmentally friendly solutions.
The initial commercial application of LEDs appeared in the 1970s, but due to its extremely low light output, the scope of application was limited to areas such as indicator lights and calculator displays. Today, high-power LEDs capable of producing white light continue to increase in efficiency, and prices are declining year by year, so it has become one of the choices for mainstream lighting applications. It is expected that with the development of LED technology, the luminous efficiency will reach 150 lumens/watt by 2012, and the cost of 1000 lumens will be less than 5 US dollars (source: OIDA). By then, LED is expected to become the main source of indoor lighting.
The Beijing Olympic Games in 2008 was a perfect interpretation of the concepts of green Olympics, high-tech Olympics, and humanistic Olympics through the application of numerous specific product forms and new technologies. LED display products, widely used in the 2008 Beijing Olympic Games, are even more glittering, adding splendor to the 2008 Beijing Olympic Games.
In recent Olympic Games, LED display screens are indispensable facilities for Olympic venues. The number of stadiums newly built and reconstructed in the 2008 Beijing Olympic Games is numerous and unprecedented. The Olympic venues all use a large number of LED display screens as equipment for live broadcast and score display of the games. The display screen products are full-color, high-resolution, digital, etc. Mainly. It can be said that the LED display products adopted in the 2008 Beijing Olympic Games have reached a new milestone from quantity to technical level and application. Many domestic LED display product manufacturers play an important role.
Through the efforts of global LED technology leaders in improving materials, processes and packaging technology, the luminous efficiency and performance of high-brightness LEDs have been significantly improved. In addition to the traditional backlight and display panel markets, high-brightness LEDs have begun to move toward indoor and outdoor general lighting, Automotive interior and exterior lighting, searchlights, traffic lights and other new applications. These all indicate that there will be a broad application prospect of LED driving power.
the problem we are facing
While the LED shines, the LED drive power supply is the guarantee for the development of the LED industry chain. The quality of the LED power supply directly restricts the reliability of the LED product. Therefore, in the LED industry chain gradually perfect today, the LED drive power supply Mature is also crucial. As far as LED power companies are concerned, the growth rate of the high-power power supply market around 100W in 2009 will be greatly improved. LED lights sourced from about 100W have more outstanding cost performance compared with traditional lighting sources, and soon formed an industrial-scale LED street lighting and landscape lighting market. Since the LED is a characteristic-sensitive semiconductor device and has a negative temperature characteristic, a stable operation state and protection are required in the application process, thereby creating a concept of driving.
The LED device's requirements for the driving power supply are almost harsh. The LED, like an ordinary incandescent light bulb, can directly connect to 220V AC commercial power. The LED is a low voltage driver of 2 to 3 volts. It is necessary to design a complicated conversion circuit. Different types of LED lamps must be equipped with different power adapters. In the international market, foreign customers have very high requirements on efficiency conversion, effective power, constant current accuracy, power supply life, and electromagnetic compatibility of LED driving power. Designing a good power supply must take these factors into consideration, because the power supply is in the entire luminaire. Its role is just as important as the human heart.
In 2009, although the financial crisis had a great impact on the world economy, the demand for high-power LED power supplies in the international markets such as Europe and the United States was still very large, and the corresponding exports of high-end LED products were less affected by the financial crisis. In 2008, the output value of LED application products in China has exceeded 45 billion RMB. LED demonstration application street lamps, LED full-color display display devices, solar LED, landscape lighting, consumer electronics backlight, signals, indications and other applications are still the main application areas.
However, under the background of a prosperous market, the quality of LED products is uneven, and the requirements for driving power are confusing. Under the development trend of LED products in the market, there are several challenges for LED driver power companies. The first is the overall life of the driver circuit. In particular, the life of a critical device such as a capacitor at high temperatures directly affects the life of the power supply. Secondly, LED drivers should challenge higher conversion efficiency, especially when driving high-power LEDs, because all power that is not used as light output is dissipated as heat, and the power conversion efficiency is too low, which affects the energy-saving effect of LEDs. The play. Third, dimming the LED with high efficiency at a large dimming ratio while ensuring constant color characteristics at high and low luminance. At the same time, it is necessary to reduce costs. At present, in applications with low power (1 to 5 W), the proportion of the cost of constant-current driving power is already close to one-third, which is close to the cost of the light source, which has affected the market promotion to some extent.
Looking to the future
From the current market understanding point of view, LED fluorescent tube structure applications will be better developed. From the perspective of home lighting, the appropriate lighting intensity still needs more than 10W energy-saving lighting products. Most fluorescent lamps have 7 to 15W power, and energy-saving fluorescent lamps have 20 to 40W. If we use LED to design more than 10W of power, heat dissipation will cause us headaches and an important factor affecting the cost. Obviously, products with a bulb structure are not suitable for LED applications. It is unlikely that products such as spotlights will be mass-produced for civilian use. The fluorescent tube structure meets the LED heat dissipation requirements and also conforms to the existing lamp interface mode.
Manufacturers from the cost considerations, most of the design of power in the 20W LED lamp, from the increasing brightness of the LED is 20W in line with the future development of LED lighting source requirements, to be LM value of 150LM / W, is the LED lighting shouting energy-saving brilliant era! The future way is to first constant pressure, then linear and constant current integration. The voltage is guaranteed to meet the load requirements within a certain range. According to the LED, there are different Vf values between 3 and 3.6V. Calculate the lowest value by multiplying the actual number of LEDs by 3V. Calculate the maximum possible voltage by multiplying the voltage by 3.6V. The value ultimately determines the voltage range that the power supply section needs to adjust. The constant current of the back-end constant current source can be used in parallel with multiple constant current sources, and the current can also be increased by using multiple constant current sources. The front-end voltage source part detects the voltage difference of the constant current source and adjusts the appropriate load demand voltage so as to achieve an efficient and flexible driving line demand.
The constant current source needs a low-dropout linear constant current device, and the linear constant current source has a good current error and also has a good grayscale performance. In the case of a small current, there may be a pressure difference of 1 to 3 V. In the case of a large current, a low pressure difference of 200 to 300 mV is required to have a high efficiency, and the linear constant current source needs to be supplied with power. In short, with the continuous improvement of technology, our daily lives have become colorful due to LEDs.
With the further in-depth study of LED, its light efficiency will be further improved, and its cost will further decline. In the near future, LED will replace incandescent or even fluorescent and develop into a major lighting source in the 21st century will become a kind of trend.
Unlike fluorescent electronic ballasts, the main function of the LED drive circuit is to convert the AC voltage into a constant current power supply and simultaneously complete the matching of the voltage and current of the LED.
From the LED's volt-ampere curve and digital model, the LED's small positive voltage change after forward conduction will cause a large change in the LED current, and environmental temperature, LED aging time and other factors will also affect the LED's electrical performance. The light output of the LED is directly related to the LED current, so the LED drive circuit is preferably able to control the size of the LED current when the input voltage and the ambient temperature change. Otherwise, the light output of the LED will change with input voltage and temperature and other factors, and, if the LED current is out of control, long-term operation of the LED at high current will affect the reliability and life of the LED, and may fail.
