Although high-power LEDs can't replace traditional lighting fixtures on a large scale, they have more and more applications in indoor and outdoor decoration and special lighting. Therefore, master the design technology of high-power LED constant current drivers, and develop high-power LEDs. The new application is crucial. LEDs can be divided into high-power LEDs, high-brightness LEDs, and ordinary LEDs according to power and luminance. In general, the power of high-power LEDs is at least 1W, and currently more common are 1W, 3W, 5W, 8W and 10W. There are 1W and 3W LEDs in high volume applications, while 5W, 8W and 10W LEDs are relatively rare. It is expected that high-power LED lights will be widely used in the 2010 Shanghai World Expo, so the electronics and lighting industries are very concerned about the development and application of new LED lighting technologies. Constant current driving and improving the optical efficiency of LED are two key issues in LED application design. This article first introduces the application of high-power LED and its selection guide for constant current driving scheme, and then takes National Semiconductor (NS) products as an example. Focus on how to skillfully apply the sampling resistor of the LED constant current driving circuit to improve the efficiency of high-power LED, and give attention to the design and heat dissipation design of high-power LED driver. Driver chip selection LED drivers account for only a small fraction of the cost of LED lighting systems, but they are related to the reliability of the overall system performance. At present, National Semiconductor's LED driver solutions are mainly positioned in the market of high-end LED lighting and lighting. Lighting is divided into indoor and outdoor. Since the power environment used for indoor LED lights has AC/DC and DC/DC converters, the choice of driver chips should also be considered from these two aspects. Figure 1: Variable current and variable voltage basic circuits 1. AC/DC converter The AC/DC is divided into 220V AC input and 12V AC input. 12V AC is the power source for halogen lamps that are widely used in hotels. Existing LEDs can be designed to retain the existing AC 12V. For the design of the replacement halogen lamp, the main advantage of National Semiconductor LM2734 is small size, high reliability, and output current up to 1A, which is suitable for the small diameter of the halogen lamp socket. After replacing the halogen lamp, the LED lamp is generally made 1W or 3W. Compared with halogen lamps, LED lamps have two advantages: (1) the light source is more concentrated, the brightness obtained by 1W illumination is equivalent to the brightness of a dozen watt halogen lamps, so it is more energy efficient; (2) the life of LED lamps is lower than that of halogen lamps. long. The main weakness of LED lights is that the angle of the light is too narrow and the cost is relatively high. However, in the long run, LED lamps have a very large cost advantage due to their long life. The 220V AC/DC converter (such as the LM5021) is primarily targeted at the stage and streetlight markets. Figure 2: RFB calculation 2. DC/DC converter At present, LED flashlights account for the vast majority of DC/DC converters. The LED power used in the flashlight is basically 1W, and the power supply method includes a lithium battery, a nickel-zinc battery, and an alkaline battery. The application of 3W flashlight has always had some difficulties, because the 3W LED lamp itself needs heat dissipation, and the heat sink has a large volume, which weakens the advantage of the small size of the LED lamp to a certain extent. In addition, since the current of the 3W LED lamp is as high as 700 mA, the battery life after one charge is shortened. Nevertheless, for the above applications National Semiconductor offers solutions such as LM3475, LM2623A and LM3485. Miner's lamp is also one of the main application fields of LED lamp. It belongs to the special lighting industry and requires professional certification standards. China has always attached great importance to the application of LED in the field of miner's lamp. At present, the LED design industry has a problem of insufficient understanding of the needs of special industries. Some unrealistic and novel design schemes are often used in the design. For example, the LED lamp and the battery are embedded in the helmet together, but the various needs of the special use environment of the miner's lamp are not taken into account, which may be an important reason for the LED to be used in the miner's lamp market. For miner's LED applications, National Semiconductor offers a wide range of DC/DC regulator products, including the LM3485, LM3478 and LM5010. The user has used a 1W LED lamp and placed 6 ordinary high-brightness LED lights around to form a miner's lamp with special flickering function. All in all, LED lamps have broad prospects in the lighting and specialty lighting industries, and National Semiconductor offers complete new LED driver solutions for this purpose. Figure 3: LM2734 based constant current drive circuit
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