subject: On The Led Driver Design [print this page] LED backlighting in portable products, the position has not wavered, even in large-size LCD backlight which, LED also began to challenge the CCFL (cold cathode fluorescent lamp) and mainstream; in lighting, LED lighting as most semiconductor key components, but also because withstand energy saving, environmental protection, long life, maintenance free, and many other ring and welcomed by the market. Drive circuit is the LED (light emitting diodes) products, an important component in terms of lighting, backlight or display area, driving circuit, the choice of architecture should be matched with the specific application.
LED light-emitting principle is that it ends with a positive voltage to the semiconductor in the minority carrier and majority carrier complex occurs, release excess energy, causing photons launches. LED drive circuit of the main function is to convert AC voltage constant current power supply, while LED devices in accordance with the requirements of complete with LED's voltage and current match. LED driver circuit to meet the safety requirements in addition to the other basic functions should be two aspects:
First, as far as possible to maintain constant current characteristic, especially in the power supply voltage changes 15% occurred when the output current should be able to keep within 10% change. Using LED as a display or backlight or other lighting source, need to be constant-current drive, the main reasons are:
1. To avoid the drive current beyond the maximum rating, affecting its reliability.
2. To achieve the desired brightness requirements, and ensure that all the LED brightness, color consistency.
Second, the drive circuit itself should be kept low power consumption, so as to enable efficient LED systems to maintain a high level.
PWM (pulse width modulation) technique is a traditional way of dimming, which uses a simple digital pulse, repeated switching LED driver, the system only need to provide wide and narrow pulses of different digital, you can simply change the output current to achieve, to adjust LED brightness. The technology is its ability to provide high-quality white light, and the application of simple, high efficiency, but a fatal drawback is prone to electromagnetic interference, and sometimes even cause the human ear can hear the noise.
Boost LED driver circuit is an important task, and the charge pump boost inductor boost topology are two different modes. "As the LED is driven by the current, while the inductor current during the maximum conversion efficiency, so the biggest advantages inductor step-up approach is efficient, if properly designed, can be more than 90%; but it is equally obvious disadvantage is the electromagnetic interference strong for mobile phones and other communication systems requested products is very high. With the appearance of the charge pump, many cell phones are no longer using inductive boost mode., of course, means using the charge pump boost its efficiency will be lower than the inductor boost.
Whether or backlighting applications in the field of lighting applications, to improve the conversion efficiency drive circuit are product designers must face. Improve the conversion efficiency will not only help to extend the standby time of portable devices, but also to solve an important means of LED heat dissipation. In the lighting field, the use of high-power LED, thus improving the conversion efficiency is particularly important.
LED at work, a steady flow, regulated components, but these components should have to bear the costs of the partial pressure of high, but the characteristics of smaller power, or will have a high efficiency LED driver circuit because work too much power, thus achieving a total efficiency of the system greatly reduced, contrary to the purpose of energy efficiency. So, as far as possible without using resistors or series regulator LED driver circuit as the main circuit current limit, but should use capacitors, inductors, or active and efficient circuit switching circuit, so as to ensure the high efficiency LED system. Series integration with constant power output circuit, you can make LED's light output over a wide constant power range, but in general the IC circuitry to degrade the efficiency decreased. Use of active switching circuit can guarantee a high conversion efficiency in the supply voltage to achieve constant power output when a substantial change.
LED at the present stage, the light effect is also far from the trichromatic fluorescent lamps can replace the degree, but its unique strengths, specifically in a safe voltage (swimming pool, program pool underwater lamps, miner's lamp) conditions to work efficiently. In addition, the direct use of green energy (solar, wind, etc.), as well as emergency lighting also has its unique advantages. Especially in the light-side, LED not only 0 ~ 100% dimmer and dimming can be guaranteed throughout the process to maintain a high photosynthetic efficiency, and without prejudice to the LED's life, while the gas discharge lamp is very difficult to do this.
