LED has established its unshakable position in the backlighting of portable devices. Even in the backlighting for the large-sized LCD panel, it has started to challenge the mainstream CCFL. In lighting, LED is especially popular in the market due to its highlighted features like energy-efficiency, environmental friendliness, long lifetime, and low maintenance. The driver circuit is a crucial and integral part of the LED. Whether in lighting, backlighting or the display panel, the choice of the technical architecture of the driver circuit must correspond to specific applications.
The LED lighting mechanism goes as follows: when the forward voltage is applied to both ends, the minority carrier and majority carrier in the semiconductor recombine to release surplus energy, giving off photons. The main functions of the LED drive circuit are to transfer the AC voltage into a constant power supply and match the voltage and current according to the requirements of LED devices. Apart from safety need, the LED driver circuit must also include two other basic features:
First, a constant current should be kept as long as it is possible, thus the output current variation is able to be maintained between the range of ±10％ especially when the power supply change goes beyond ±15％range. Here are the reasons for having a constant current driver when using LED as the monitor, other lighting devices or backlighting:
1.To prevent the drive current from exceeding the maximum rate and further affecting its reliability.
2.To meet expected brilliance requirements and ensure the color and brilliance homogeneity of each LED.
Second, the driver circuit should keep low power consumption so that the LED system efficiency can remain at a high level.
PWM (Pulse Width Modification) is a traditional light adjusting technology, which uses simple digital pulses to switch on and off LED driver from time to time. The system only needs to supply wide and narrow digital pulses in order to easily change the output for adjusting LED brilliance. The advantage is that the technology is able to provide high quality white light with high efficiency through easy application. But there is a fatal disadvantage: it is susceptible to EMI (electromagnetic interference), sometimes even produces audible noises.
Voltage boost is an important task of the LED driver circuit, divided into two different topological modes, namely, voltage boost via the inductor and charge bump. As the LED is current-driven, and the inductor is most efficient at the moment of current transfer, so the greatest strength of voltage boost via inductor lies in the high efficiency, which can reach 90% if properly designed. However, its weakness is as much remarkable, i.e., strong EMI, which imposes high requirements on systems of telecommunication products such as mobile phones. With the appearance of charge pumps, most mobiles don't boost voltage via the inductor. Of course, the efficiency of voltage boost via the charge pump is lower than that in the other way.
No matter in lighting or backlighting applications, the product designer has to face the problem of raising the driver transfer efficiency. To improve the transfer efficiency is not only beneficial for portable products to extend standby time but is also an important means to solve the LED thermal dissipation problem. In lighting, the use of the high power LED also underlines the problem of enhancing the transfer efficiency.
LED needs current and voltage stabilizing components at work, which should be featured with high divided voltage and low power consumption, otherwise, the highly efficient LED will lower the overall system efficiency because of high working consumption, contradicting the principle of energy-saving and high efficiency. Therefore, the main current limiting circuit should use highly efficient circuits like the capacitance, inductor or switching circuit with power supply as it is possible to ensure the high efficiency of the LED system instead of the resistor or the series voltage stabilizing circuit. The series constant power output circuit can keep the LED light output constant in a wide range of power supply, but normal IC circuits will lose some efficiency. The adoption of switching circuit with power supply is able to guarantee constant power output with high transfer efficiency under dramatic voltage variation of the power supply.
At present, LED with its luminous efficacy is far from replacing the three band fluorescent lamps, yet LED lights can efficiently work under safety extra low voltage (SELV), for instances, underwater lights in swimming pools or paddling pools, mining lamps. Besides, LED has unique advantages in the direct use of green energy like solar energy, wind energy, or emergency lights. Particularly in light adjusting, LED cannot only realize zero to one hundred percent adjusting, but also maintain high efficacy during the whole adjusting process without hurting the durability, which is a difficult task for gas discharge lights.