A brief discussion on LED driver design ideas

LED has become an unshakable backlight source in portable products. Even in the backlight source of large-size LCDs, LED has begun to challenge the mainstream position of CCFL (cold cathode fluorescent lamp); in the field of lighting, LED, as the most critical component of semiconductor lighting, is sought after by the market because of its energy saving, environmental protection, long life, maintenance-free and many other halos. The drive circuit is an important part of LED (light-emitting diode) products. Whether in the field of lighting, backlight or display panel, the choice of drive circuit technology architecture should match the specific application.

The principle of LED light emission is to add a forward voltage across its two ends, so that the minority carriers and majority carriers in the semiconductor recombine and release excess energy, thereby causing the emission of photons. The main function of the LED driver circuit is to convert the AC voltage into a constant current power supply, and at the same time, to match the voltage and current of the LED according to the requirements of the LED device. In addition to meeting safety requirements, the LED driver circuit should have two other basic functions:

First, the constant current characteristic should be maintained as much as possible, especially when the power supply voltage changes by ±15%, the output current should still be able to change within the range of ±10%. When using LED as a display or other lighting equipment or backlight, it needs to be driven by constant current. The main reasons are:
1. Avoid the driving current exceeding the maximum rated value and affecting its reliability.
2. Obtain the expected brightness requirements and ensure the consistency of brightness and color of each LED.

Second, the driving circuit should maintain low power consumption, so that the LED system efficiency can be maintained at a high level.
PWM (Pulse Width Modulation) technology is a traditional dimming method, which uses simple digital pulses to repeatedly switch the LED driver. The system only needs to provide different widths and narrowness of digital pulses to simply change the output current, thereby adjusting the brightness of the LED. The advantages of this technology are that it can provide high-quality white light, simple application, and high efficiency, but a fatal disadvantage is that it is easy to generate electromagnetic interference, and sometimes even produce noise that can be heard by the human ear.

Boosting is an important task of LED driving circuits, and inductor boosting and charge pump boosting are two different topological modes. "Since LEDs are driven by current, and inductors are most efficient when converting current, the biggest advantage of the inductor boosting method is its high efficiency, which can exceed 90% if designed properly; however, its disadvantage is also obvious, that is, strong electromagnetic interference, and very high system requirements for mobile phones and other communication products. With the emergence of charge pumps, most mobile phones no longer use inductor boosting. Of course, the efficiency of the boosting method using a charge pump will be lower than that of the inductor boosting method.

Whether in lighting applications or backlight applications, improving the conversion efficiency of the drive circuit is a problem that product designers must face. Improving conversion efficiency is not only beneficial for extending the standby time of portable products, but also an important means to solve the heat dissipation problem of LEDs. In the field of lighting, due to the use of high-power LEDs, improving conversion efficiency is particularly important.

LED needs to have current and voltage stabilizing components when working, but such components should have the characteristics of high partial pressure but low power consumption. Otherwise, the efficiency of the overall system will be greatly reduced due to the high power consumption of the driving circuit of the LED with high efficiency, which is contrary to the purpose of energy saving and high efficiency. Therefore, it is necessary not to use resistors or series voltage stabilizing circuits as the main current limiting circuit of the LED driver as much as possible, but to use high-efficiency circuits such as capacitors, inductors or active switching circuits, so as to ensure the high efficiency of the LED system. The use of series integrated constant power output circuit can keep the light output of the LED constant within a wide power supply range, but the general IC circuit will reduce the efficiency. The use of active switching circuit can ensure constant power output when the power supply voltage changes greatly under high conversion efficiency.

At the current stage, the light efficiency of LED is far from being able to replace tri-color fluorescent lamps, but with its unique advantages, it can work efficiently under safe special voltage conditions (swimming pools, underwater lamps in paddling pools, mining lamps). In addition, it also has its unique advantages in directly using green electricity (solar energy, wind energy, etc.) and emergency lighting. Especially in dimming, LED can not only achieve 0-100% dimming, but also ensure that the light efficiency is maintained at a high level during the entire dimming process, and the life of the LED is not damaged, while gas discharge lamps are difficult to do this.