1 Introduction
LED is recognized as the fourth generation of green light source. It is a solid cold light source with many advantages such as high efficiency, long life, safety and environmental protection, small size, high reliability and fast response. At present, the same lighting effect is achieved. The power consumption of LEDs is about 1/10 of incandescent lamps and 1/2 of fluorescent lamps. Many countries and regions have successively introduced various policies to support the development of the LED industry, in the hope that the industry will become an important part of the country's important industries, gestating huge business opportunities. The LED driver circuit is very important for the LED, and the LED dimming control can save energy. The driving and dimming of the high-brightness white LED is a hot spot in recent years. In this paper, the research on this aspect, designed a novel LED lamp driving circuit with dimming function for the deficiency of existing circuits.
2 LED characteristics
The theoretical luminous efficacy of LEDs is 300 lm/W. At present, the laboratory level is 260 lm/W, and the market level is above 120 lm/W. The high-brightness LED has a typical turn-on voltage of about 3.0 to 4.3V, and its core is a PN junction, which has the same volt-ampere characteristics as a normal diode. Its current-voltage relationship is as shown in equation (1):
Where, VF is the forward voltage of the diode, I0 is the reverse saturation current, which is a fixed value, q is the electron charge 1.6 & TImes; 1019, k is the Boltzmann constant, the size is 1.38 & TImes; 10-23, T is the thermodynamics The temperature, the constant β is approximately 2 . When the voltage applied to the LED is less than its turn-on voltage, almost no current flows through the LED. However, when the LED is turned on, its forward current changes exponentially with the forward voltage, and a small voltage fluctuation causes a large current change. When the conduction region voltage rises from 80% of the rated value to 100%, the current rises from 0% of its rated value to 100%.
Figure 1 shows the relationship between the relative luminous flux of a certain LED and its forward current IF. The relationship between the luminous flux of other types of high-power LEDs and the forward current is different, but the difference is very small. It can be seen that the white LED can effectively emit light when the forward current of the white LED is greater than a certain value, and the luminous flux of the LED is proportional to its forward current. Therefore, the light can be controlled by controlling the forward current of the LED. brightness. If the LED is driven by a constant voltage source, a small voltage change will cause a large current change. Therefore, the constant voltage drive is only suitable for low-power applications where high requirements are required. In high-demand applications and high-power applications, LEDs must be used. Constant current drive.
The research shows that the brightness of LED light decreases with the working time. After the brightness decreases, the light effect decreases with the increase of current. The brightness of the LED is saturated with the driving current. After the current of the LED reaches 70%~80% of its rated current, a large proportion of the current is converted into thermal energy, so the driving current of the LED should be 70%~80% of the rated current of the working current.
3 LED drive circuit classification
3.1 charge pump circuit
The charge pump circuit is also a DC/DC converter circuit. The charge pump circuit uses the cumulative effect of the capacitor on the charge to store the energy, and the capacitor acts on the energy coupling element to control the power electronic device to perform high-frequency switching, allowing the capacitor to store energy for a part of a cycle, and releasing the energy for the remaining time. . This kind of circuit obtains different output voltages through different connection modes of charging and discharging of capacitors, and the whole circuit does not need any inductance.
The charge pump circuit is relatively small in size, uses fewer components, and has lower cost. However, it uses relatively many switching components. When the input voltage is constant, the range of output voltage variation is relatively small, and the output voltage is mostly input. The voltage is 1/3~3 times, and the circuit power is small, and the efficiency will change with the relationship between the output voltage and the input voltage. Multiple LEDs must be driven in parallel. To prevent uneven distribution of branch currents, ballast resistors must be used, which can greatly reduce system efficiency. Therefore, charge pump drive circuits are limited in lighting drive applications for high power LEDs, which are often used in low power applications.
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