Main features of PWM dimming

PWM dimmer USES pulse width modulation signal to turn ON/OFF the LED driver repeatedly to adjust the average current of the LED. Compared with analog dimming method,PWM dimming has the following advantages:

(1) no matter how big the dimming ratio is, the LED always works under constant current condition.
(2) good color consistency and high brightness level. In the whole dimming range, because the LED current is either at the maximum value or turned off, the average LED current can be changed by adjusting the duty cycle of pulse, so the scheme can avoid color deviation in the process of current change
(3) can provide a larger dimming range and better linearity. Generally, the dimming frequency of PWM is 200Hz(low-frequency dimming)~ above 20kHz (high-frequency dimming). As long as the dimming frequency of PWM is higher than 100Hz, the flicker phenomenon of LED cannot be observed.
(4) the duty cycle adjustment range of low-frequency dimming is up to 1%~100%. PWM dimming ratio up to 5000:1.
(5) LED driver chips produced by most manufacturers support PWM dimming. This is divided into the following three cases: only with PWM dimming input pin, no analog dimming pin; Set pins for PWM dimming and analog dimming respectively; Share a dimming input pin with analog dimming.
(6) when PWM dimming is adopted, the conversion efficiency of LED driver is high.
The main disadvantages of pwM dimming are as follows: first, pwM dimming signal source is needed to make its cost higher than that of analog dimming; Secondly, if the PWM signal frequency is exactly in 200Hz~20kHz, the inductance and output capacitor in the LED driver will emit the noise that can be heard by human ear. High-end lighting systems should have a dimming frequency higher than 20kHz, but high frequency dimming reduces the dimming range of the LED driver.
Realization scheme of PWM dimming
There are three ways to use PWM signal dimming: (1) directly control with PWM signal; (2) through the collector open transistor (or leakage open MOSFET) for control (high level 1, low level 0); (3) the use of microcontroller (MU) generated PWM signal for control. Taking step-down constant-current LED driver MT7201 as an example, three PWM signal dimming modes are shown in figure 82-1(a)~(c) respectively. PWM dimmer end is generally used ADJ(or PWM), also some chips and analog modulation photosynthesis with a dimmer pin DIM. PWM dimming frequency should be above 100Hz to avoid human eyes to observe the phenomenon of LED flicker.
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