Talking about the Design and Analysis of High Power LED Constant Current Drive Circuit

Although high-power LEDs cannot yet replace traditional lighting fixtures on a large scale, they are more and more widely used in indoor and outdoor decoration and special lighting. New applications are crucial. LEDs can be divided into high-power LEDs, high-brightness LEDs and ordinary LEDs according to power and luminous brightness. Generally speaking, the power of high-power LED is at least above 1W, and the more common ones are currently 1W, 3W, 5W, 8W and 10W. 1W and 3W LEDs have been applied in large quantities, while 5W, 8W and 10W LEDs are relatively few. It is expected that high-power LED lights will be widely used at 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 drive 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 the selection guide of constant current drive scheme, and then takes the products of National Semiconductor (NS) as an example. Focus on discussing how to use the sampling resistance of the LED constant current drive circuit to improve the efficiency of high-power LED, and give the high-power LED driver design and heat dissipation design considerations.

Driver chip selection

The LED driver only accounts for a small part of the cost of the LED lighting system, but it is related to the reliability of the performance of the entire system. Currently, National Semiconductor's LED driver solutions are mainly positioned in the mid-to-high-end LED lighting and lighting markets. Lighting is divided into indoor and outdoor. Since the power environment used in indoor LED lights has AC / DC and DC / DC converters, the choice of driver chip should also be considered from these two aspects.

Figure 1: Basic circuit of variable current and variable voltage

1. AC / DC converter

AC / DC is divided into 220V AC input and 12V AC input. 12V AC is the power source of halogen lamps widely used in hotels. Existing LEDs can be designed while retaining the existing AC 12V. For the design of alternative halogen lamps, the main advantages of National Semiconductor LM2734 are small size, high reliability, and an output current of up to 1A, which is suitable for the small diameter of halogen lamp.

After replacing the halogen lamp, the LED lamp is generally made into 1W or 3W. Compared with halogen lamps, LED lamps have two major advantages: (1) the light source is more concentrated, and the brightness obtained by 1W lighting is equivalent to the brightness of a dozen-watt halogen lamp, so it is more power-saving; (2) the life of LED lamps is more than halogen lamps long.

The main weakness of LED lights is that the angle of light is too narrow and the cost is relatively high. But in the long run, due to the long life of the LED lamp, it still has a very large cost advantage. 220V AC / DC converters (such as LM5021) mainly target the stage and street lights market.

Figure 2: RFB calculation

2. DC / DC converter

At present, LED flashlights occupy most of the demand for DC / DC converters. The LED power used in the flashlight is basically 1W, and the power supply includes lithium batteries, nickel-zinc batteries, alkaline batteries, etc. There are still some difficulties in the application of 3W flashlight, because the 3W LED lamp itself needs heat dissipation, and the volume of the heat dissipation device is large, thus weakening the advantage of the small size of the LED lamp to a certain extent. In addition, because the current of the 3W LED lamp is as high as 700mA, the battery life after a single charge is shortened. Nevertheless, for the above applications National Semiconductor offers LM3475, LM2623A and LM3485 and other solutions.

Miner's lamp is also one of the main application fields of LED lamps. 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 lamps. 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 lights and batteries are embedded in the helmet together, but the various requirements of the special use environment of the miner's lamp are not taken into account. This may be an important reason why the application of LED in the miner's lamp market has not opened up.

For miner's lamp LED applications, National Semiconductor provides a wealth of DC / DC regulator products, including LM3485, LM3478 and LM5010. Users have used a 1W LED lamp, and then placed 6 ordinary high-brightness LED lights around it to form a miner's lamp with a special blinking function.

All in all, LED lights have broad development prospects in the lighting and special lighting industries, and National Semiconductor provides complete new LED drive solutions for this purpose.

Figure 3: Constant current drive circuit based on LM2734