PWM dimming modulates the regulated current through the LED at a much lower frequency than the switching frequency of the converter. However, the dimming frequency must be faster than the human eye can detect, 200 hertz or above, so there is no visible flickering in the LED light output.

The average LED current is proportional to the duty cycle of the PWM dimming command. Well-implemented PWM dimming generally results in a wider linear dimming range than analog dimming. With PWM dimming, the LED current is either zero or at the nominal LED current level, which minimizes color shift that can occur during analog dimming. However, PWM dimming can add complexity to the design compared to analog dimming.

There are three types of PWM dimming:

• Main FET dimming
• Series FET dimming
• Shunt FET dimming

Using main FET PWM dimming, dimming is achieved through the enabling and disabling the main switching MOSFET in the power converter. This dimming method is compatible with all converter typologies.

Figure 2-2 Three Types of PWM Dimming