LED lighting circuit design

There are mainly five Dimming modes for LED lamps: Analog Dimming, Pulse width Modulation Dimming, Digital Dimming, TRIAC Dimming (bidirectional thyristor Dimming, formerly known as tri-terminal bidirectional thyristor Dimming), Wireless Dimming. The digital dimming microcontroller sends digital signals to the LED driver through serial interfaces such as single-line interface, IC, SMBus, SPI, etc., to adjust the brightness of the LED lamp, which can achieve Gradual dimming. Progressive light modulation is a continuous way of light modulation, so that the current ILD exponential curve form gradually increased, can be a good compensation for the sensitivity of the human eye. In addition, digital potentiometer and microcontroller can be used to realize digital dimming. Wireless light modulation in the numerical control constant current driver based on the addition of infrared remote control transmitter, infrared remote control receiver to carry out dimming.
The realization scheme and application examples of analog dimming, PWM dimming, TRIAC dimming and digital dimming are introduced. Finally, the design method and example of passive and active LED drive power supply PFC circuit are introduced, which can provide help for the realization of optimization design of LED drive power supply.

The main features of analog dimming circuit and its realization scheme
Analog dimming is also known as Linear dimming. Analog dimming can realize linear dimming of LED by continuously changing the output current of LED driver with dc voltage signal. Its characteristic is that the dimming signal is analog, and the output current is continuously changing, so that the LED brightness can be continuously adjusted. The maximum analog dimming ratio can only reach 50:1, generally below 10:1. The advantage of analog dimming is that the circuit is simple, easy to realize, easy to operate, no flicker phenomenon, can avoid TRIAC dimming generated flicker, low cost, by adjusting the resistance value of direct sliding push-pull type linear potentiometer, to change the current through the LED, and then adjust the brightness of the LED. For example, the relation curve between relative luminous flux and forward current lF of XLampXP-(white LED), produced by Cree, USA, is shown in figure 8-1-1, and the relationship between them is approximately linear. As can be seen from the figure, if the relative luminous flux at the time when the forward current is 350mA is regarded as 100%, then the relative luminous flux at the time of 200mA and 100mA is about 60% and 25% respectively. Therefore, brightness adjustment can be realized by adjusting the driving current.
Simulation dimming mainly has the following disadvantages:
(1) when the current changes, the color deviation of LED will be caused. Because the color spectrum of LED is related to the current, the luminous quality of white LED will be affected. For example, at present, white LED is generated by stimulating fluorescent powder with blue LED. When the forward current increases, the brightness of blue LED increases and the thickness of fluorescent powder does not become thinner in the same proportion, resulting in the increase of the main wavelength of its spectrum. The main wavelength mentioned here refers to the wavelength perceived by human eyes. An example of the relation curve between the main wavelength and the forward current is shown in figure 81-2. When the forward current changes, it will also cause a change in the color temperature. An example of the relation curve between color temperature and forward current of white LED is shown in figure 813. As can be seen from the figure, when the forward current is 160mA, the color temperature is 5660K. When the forward current is increased to 320mA, the color temperature is shifted to 5731K. For white leds, the direction of the cold light is preferred. For rgb-led backlight, it will cause color deviation (referred to as color deviation). Because human eyes are very sensitive to color deviation, rgb-led backlight cannot adopt analog dimming.
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