Basic principles and practical cases of LED driver design

Although high-power LEDs can't replace traditional incandescent lamps on a large scale, they are more and more widely used in indoor and outdoor decoration and special lighting. Therefore, master the design technology of high-power LED constant current driver, 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.

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: Conversion from constant voltage drive (left) to constant current drive (right) using the FB feedback terminal of the DC/DC regulator.

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: Place an op amp between the FB feedback and RFB to reduce power consumption.

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