Talking about LED Drive Design Idea

The status of LEDs in portable products has been unshakable. Even in the backlights of large-size LCDs, LEDs have begun to challenge the mainstream status of CCFLs (cold cathode fluorescent lamps). In the field of lighting, LEDs as semiconductors The most critical components of lighting are also sought after by the market for their auras of energy saving, environmental protection, long life and maintenance-free. The driver circuit is an important part of the LED (light-emitting diode) product. Whether in the field of illumination, backlight or display panel, the choice of the driver circuit technology architecture should be matched with the specific application.

The principle of LED illumination is to apply a forward voltage across it to recombine minority carriers and majority carriers in the semiconductor, releasing excess energy, causing photon emission. The main function of the LED driver circuit is to convert the AC voltage into a constant current source, and at the same time complete the matching with the voltage and current of the LED according to the requirements of the LED device. In addition to meeting the safety requirements of the LED driver circuit, there are two other basic functions:

First, keep the constant current characteristics as much as possible. Especially when the power supply voltage changes by ±15%, the output current should be kept within ±10%. When using LEDs as displays or other lighting devices or backlights, they need to be driven with constant current. The main reasons are:
1. Avoid driving current exceeding the maximum rated value and affect its reliability.
2. Obtain the expected brightness requirements and ensure the consistency of brightness and chromaticity of each LED.

Second, the driver circuit should maintain its own low power consumption, so that the system efficiency of the LED can be kept at a high level.

PWM (Pulse Width Modulation) technology is a traditional dimming method. It uses a simple digital pulse to repeatedly switch the LED driver . The system only needs to provide wide and narrow digital pulse, which can easily change the output current. Thereby adjusting the brightness of the LED. The technology has the advantage of providing high-quality white light, as well as simple application and high efficiency, but a fatal disadvantage is that it is prone to electromagnetic interference, and sometimes even noises that can be heard by the human ear.

Boost is an important task for LED driver circuits, while inductor boost and charge pump boost are two different topologies. “Because the LED is driven by current, and the inductor is the most efficient in current conversion, the biggest advantage of the inductor boost method is high efficiency. If it is properly designed, it can exceed 90%; but its shortcoming is also obvious, that is, electromagnetic interference. Very strong, the system requirements for mobile phones and other communication products are very high. With the advent of charge pumps, most mobile phones no longer use the inductor boost mode. Of course, the boost mode using the charge pump will be less efficient than the inductor. Boost.

Whether in lighting applications or backlight applications, improving the conversion efficiency of the driver circuit is a problem that product designers must face. Improving the conversion efficiency not only helps the portable product to extend the standby time, but also is an important means to solve the LED heat dissipation problem. In the field of lighting, the use of high-power LEDs makes it especially important to increase conversion efficiency.

LEDs need to have steady current and voltage-stabilized components during operation, but such components should have their own high voltage divider, but the power consumption should be small. Otherwise, the LED with higher efficiency will work because of the driving circuit. The consumption is too large and the efficiency of the overall system is greatly reduced, which is contrary to the purpose of energy saving and high efficiency. Therefore, as far as possible, resistors or series regulator circuits should be used as the current limiting main circuit of the LED driver. Instead, high-efficiency circuits such as capacitors, inductors or active switching circuits should be used to ensure high efficiency of the LED system. By using a series-connected constant-power output circuit, the LED's light output can be kept constant over a wide range of power supplies, but the general IC circuit will reduce efficiency. The use of an active switching circuit ensures constant power output with a large change in supply voltage at higher conversion efficiencies.

At the current stage, LEDs are far from being able to replace the three-primary fluorescent lamps, but with their unique strengths, they can work efficiently under the conditions of safe special voltage (underwater, swimming pool, miner's lamp). . In addition, it has its unique advantages in direct use of green power (solar, wind, etc.) and emergency lighting. Especially in terms of dimming, LEDs can not only achieve 0~100% dimming, but also ensure high light efficiency throughout the dimming process without compromising the life of the LEDs, which is difficult to achieve with gas discharge lamps.

(Editor: Wen Jing)

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