0 Introduction Due to the impact of the current greenhouse effect and energy crisis, people are paying more and more attention to energy-saving technologies. LED lighting has the advantages of energy saving and long life. As a new type of green lighting technology, LED lighting technology is becoming more and more widely used. The white light illumination of LEDs is usually composed of multi-chip LED modules of blue, green and red. Since the aging characteristics of each LED chip in this LED module are different, the color temperature changes with time and temperature. There is a limit to the use of white LEDs. In this paper, the driving mode of white LED module is analyzed, and a design scheme of a new white LED module driving circuit is proposed.
1 White LED module drive For the traditional light source driving method, most of them exist in the form of constant voltage, which makes the driving of traditional light source very simple. In the linear range, the luminous intensity of an LED is proportional to its drive current and forward voltage drop and varies with temperature, which requires a constant current to drive the LED. In addition, the white LED module has an important indicator, which is the color temperature. The color temperature is expressed by the absolute temperature K. When the color of a light source and the black body are the same, the absolute temperature of the black body is called the color temperature of the light source. The white LED color temperature depends on the composition ratio of the three color components in the white light emitted.
In order to adjust the color temperature of the white LED module light source, it is necessary to separately control the driving currents of the blue, green and red LED chips in the white LED module, and three independent constant current driving circuits are needed. At present, the constant current driving circuit is generally implemented by a PWM controller of a switch mode, and the adjustment of the driving current is mainly divided into an analog adjustment mode and a PWM adjustment mode.
The analog adjustment mode is to use the PWM controller to adjust the drive current to stabilize the LED drive current and obtain a stable output light intensity. Since the feedback control signal of the PWM controller is usually a voltage value, the analog adjustment uses a measuring resistor to sample the output driving current and convert it into a feedback voltage signal, and stabilize the feedback voltage through the PWM controller. The purpose of stabilizing the output drive current is shown in Figure 1.
There is a big problem with the method of adjusting the output current to adjust the output light intensity of the LED: since the output wavelength of the LED changes as the driving current of the LED changes, so that under different output light intensities, the white LED mode The color temperature of the group will change. In order to overcome the above shortcomings, a PWM adjustment method for the LED drive current has been developed.
2 PWM control method of LED drive current PWM regulation mode refers to the method of driving LEDs with pulse drive waveform of specific duty cycle in one cycle. The average driving current of the LED depends on the duty cycle of the pulse waveform and the rated driving current of the LED. In the case of fixing the rated driving current of the LED, by changing the duty ratio, the average driving current of the LED can be changed, thereby changing the output light of the LED. strength. In order to avoid flickering, the frequency of the pulse waveform is usually greater than 200 Hz. Linear Technology's LED driver chip LT3474 is an LED constant current drive control chip with adjustable output current. The maximum drive current can reach 1 A. It offers two drive modes: analog adjustment mode and PWM adjustment mode. In the PWM adjustment mode, a dimming ratio of 400:1 can be achieved by changing the duty ratio of the applied PWM signal. A typical application circuit for its PWM regulation is shown in Figure 2, where the external PWM control signal is applied by the PWM pin.
Figure 3 shows the measurement results of the PWM pulse drive signal and the output drive current in the PWM regulation mode. It can be clearly seen from Figure 3 that the LED drive current is controlled by an applied PWM signal. The output reference voltage of the REF pin is 1.25 V. If the VADJ is shorted to the REF pin, the rated LED drive current of the output is the maximum output current (1 A). The regulation of the rated drive current ILED of the LED is achieved by changing the voltage of the VADJ pin, which can be done by dividing the 1.25 V reference voltage output by REF. The rated current of the output LED is obtained by:
The average current of the LED is obtained by:
Where: DDIM is the duty cycle of the applied PWM signal.
3 color temperature adjustable white light drive circuit design In order to stabilize the light intensity and color temperature of LED output light, and realize the adjustment of light intensity and color temperature, a white LED module drive circuit is designed. The block diagram is shown in Figure 4, in which The light intensity detectors with blue, green and red filters respectively monitor the light intensity of the three LEDs constituting the white LED module, and change the duty ratio of the corresponding PWM control signal by the control of the AVR microcontroller. Stabilizing the light intensity of the three LEDs of blue, green and red stabilizes the light intensity and color temperature of the white LED module. The driving circuit of each LED uses LT3474 as the driving control chip.
The adjustment of the output white light can be divided into light intensity and color temperature adjustment. If only the light intensity is changed, the duty ratio of the PWM signal in the three LED driving circuits is changed proportionally; if the color temperature needs to be changed, the blue, green, and red LEDs are controlled by using different duty ratios, thereby realizing white light. The color temperature is adjusted by adjusting the ratio of the three color components. Of course, it is also possible to adjust the output light intensity and color temperature at the same time. Through the circuit, the real-time control of the light intensity and color temperature of the white LED module is realized, and the feedback control of the light intensity detector is used to overcome the light intensity and color temperature change caused by the aging of the LED, and can be Personal preference to adjust the light intensity and color temperature of LED lamps, to achieve a good lighting effect.
4 Conclusion This paper discusses the two adjustment methods of LED driver, introduces the PWM adjustment circuit, and proposes a drive control circuit for white LED module. By using the three-color intensity detector, the light intensity and color temperature are realized. Real-time adjustment control. The circuit can overcome the light intensity and color temperature change caused by the aging of the LED, and can adjust the light intensity and color temperature of the LED lamp according to personal preference, and achieve a good lighting effect.
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