LED lighting knowledge: PWM dimming technology
In the first part of this series, we learned the basics of LED light sources and their drive requirements. In the second part, we discussed why it can be your first choice when a constant current Buck converter can be used as an LED conversion mode driver. In the third part, we studied the large LED display and the application space in other conversion topologies. Now, in the summary of this series, the author turns his perspective on how best to implement dimming.
Whether you use Buck, Boost, Buck-Boost or a linear regulator to drive LEDs, their common idea is to use a driver circuit to control the output of the light. Some applications simply implement the "on" and "off" functions, but more application requirements are to adjust the brightness of the light from 0 to 100%, and often have high precision. Designers have two main choices: linearly adjusting the LED current (analog dimming), or using a switching circuit to operate at a frequency high enough relative to the human eye to change the average of the light output (digital dimming). Using pulse width modulation (PWM) to set the period and duty cycle (Figure 1) is probably the easiest way to implement digital dimming, and the Buck regulator topology often provides the best performance.
Figure 1: LED driver and its waveform using PWM dimming
Recommended PWM dimming
Analog dimming can usually be done very simply. We can proportionally change the output of the LED driver with a control voltage. Analog dimming does not introduce potential electromagnetic compatibility/electromagnetic interference (EMC/EMI) frequencies. However, PWM dimming is used in most designs due to one of the fundamental properties of LEDs: the characteristics of the emitted light are shifted with the average drive current. For a monochrome LED, its dominant wavelength will change. For white LEDs, the associated color temperature (CCT) will change. For the human eye, it is difficult to detect changes in the wavelength of a few nanometers in red, green or blue LEDs, especially when the light intensity is also changing. However, the color temperature change of white light is very easy to detect.
Most LEDs contain an area that emits blue spectral photons that provide a wide range of visible light through a phosphor face. At low currents, phosphorescence dominates and the light approaches yellow. At high currents, the LED blue light dominates and the light appears blue, achieving a high CCT. When more than one white LED is used, the difference in CCT of adjacent LEDs is clearly undesirable. The same problem arises when mixing multiple single-color LEDs into a light source application. When we use more than one light source, any difference in the LED will be noticed.