introduction
In recent years, semiconductor light sources are gradually entering the field of illumination with the role of new solid-state light sources. According to the principle of solid-state luminescence, LED luminous efficiency can be close to 100%, with low working voltage, low power consumption, short response time, high luminous efficiency, impact resistance, long service life, pure light color, stable and reliable performance and low cost. Etc. As the price of LEDs continues to decrease and the brightness of illumination continues to increase, semiconductor light sources have shown broad application prospects in the field of illumination. The volt-ampere characteristics of LEDs are the same as those of ordinary diodes, and small fluctuations in forward voltage result in sharp changes in forward current. The magnitude of the LED forward current changes with the ambient temperature. When the environment reaches a certain temperature, the LED allows the forward current to drop sharply. In this case, if the current is still passed, it is easy to cause the LED to age and shorten the service life. In the application process, an LED driving system with constant temperature and constant current control and reliable protection is needed. This article describes the design of a smart LED drive system
Method.
2 LED model
The LED is a low-voltage-driven PN junction optoelectronic semiconductor device, and its voltage-current relationship can be expressed by equation (1).
q———Electronic charge.
After the forward voltage of the LED exceeds the threshold voltage, the forward current of the LED increases rapidly with the increase of the forward voltage, and the small fluctuation of the voltage causes a large change of the current.
The volt-ampere characteristics of LEDs have nonlinear and unidirectional conductivity. When the forward voltage is less than a certain value, the current is extremely small and does not emit light. When the forward voltage exceeds a certain value, the forward current follows the forward voltage. The increase and increase rapidly, the volt-ampere characteristic model of the LED can be expressed by the formula (2).
The simplified LED model reflects the volt-ampere characteristics of the LED and is used to analyze the effect of the circuit drive current on the electrical characteristics of the LED to verify the correctness of the analysis and design.
3 system design
The performance of the LED drive system directly affects the operating life and operational stability of the light source system. The drive system is mainly composed of three main parts: switching power supply, constant current drive circuit and single-chip constant current control. The system block diagram is shown in Figure 2.
The system adopts the combination of hardware circuit design and software program design, takes the single-chip microcomputer control as the core, adjusts the output current through negative feedback, and thus completes the brightness adjustment. It is an intelligent drive control system suitable for various LED lamps, so that the LED The performance is improved and improved, and the output stability and reliability of the LED light source are solved.
3. 1 switching power supply
LED lighting can only be powered by DC. In the case of AC220V power supply, the AC / DC conversion problem must be solved first. The switching power supply has the advantages of high conversion efficiency, small size and light weight, wide input range, flexible use and energy saving. The switching power supply generally uses a semiconductor power device as a switch to convert one power supply form into another form, and automatically controls the stable output during conversion, and has various protection circuits.
The structure of the switching power supply is shown in Figure 3. The 220V AC is low-pass filtered and bridge-rectified to obtain an unregulated DC voltage. This DC voltage is used for active power factor correction to improve the power factor of the power supply. The DC voltage is then passed through the inverter to become a high-frequency square wave pulse voltage that meets the requirements, and is rectified and filtered to become a DC voltage output. After the output voltage is divided and sampled, it is compared with the reference voltage by the comparison circuit, and the error is amplified. The pulse width is adjusted by the pulse width modulation. The pulse width modulation (PWM) is the most commonly used method in the switching power converter.