Whether the LED is driven via a buck, boost, buck/boost or linear regulator, the most common thread connecting each driver circuit is the need to control the output of the light. Today, there are only a few applications that require simple functions to turn on and off, and most need to fine tune the brightness from 0 to 100%. Currently, for brightness control, the two main solutions are linearly adjusting the LED current (analog dimming) or switching the drive current from 0 to the target current value at high frequencies that are undetectable to the naked eye (digital adjustment) Light). Using pulse width modulation (PWM) to set the cycle and duty cycle may be the easiest way to implement digital dimming because the same technique can be used to control most switching converters.
PWM dimming can be used to match accurate color light
In general, analog dimming is easier to implement because the output current of the LED driver changes proportionally to the control voltage, and analog dimming does not cause additional electromagnetic compatibility (EMC) / electromagnetic interference (EMI) potential frequencies. problem. However, most of the reasons for PWM dimming are based on the basic characteristics of LEDs, ie the displacement of the emitted light is proportional to the average drive current (Figure 1). For monochromatic LEDs, the wavelength of the main light wave changes, and in the case of white LEDs, the relative color temperature (CCT) changes. For people's naked eyes, it is difficult to detect the change of nanometer wavelength in red, green or blue LEDs, especially when the intensity of light is also changing, but the color temperature change of white light is easier to detect. Most white LEDs contain a wafer that emits blue-spectrum photons that emit photons in various visible ranges after striking the phosphor coating. At lower currents, phosphorescence dominates and deflects light toward yellow; at higher currents, the LED emits more blue light, causing the light to deflect toward blue and also produce a higher CCT. For applications that use more than one white LED, the CCT difference between two adjacent LEDs can be significant and visually unpleasant, and this concept can further extend the source of multiple monochromatic LED rays. Once more than one light source is present, any CCT difference that occurs between them can be dazzling.
Figure 1 LED driver and waveform using PWM dimming
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