Lighting control protocol: what is PWM dimming?

What is PWM dimming?

PWM is a lighting control method by reducing the average deliverable power of an electrical signal. It is accomplished by rapidly turning the light on and off at a fixed frequency, while varying the duty cycle, or the proportion of time the light is on versus off, to adjust the perceived brightness.

The basic principle of PWM dimming is to apply a series of electrical pulses to the LED driver circuit, where the width, or duration, of each pulse determines the amount of time the LED is turned on. The frequency of the pulses, usually in the range of several kilohertz to several megahertz, is chosen to be sufficiently high to render the flicker of the light source undetectable to the human eye.

When the duty cycle is increased, the light output is also increased, as the LED is turned on for a greater portion of each pulse. Conversely, decreasing the duty cycle reduces the light output. By adjusting the duty cycle in this manner, a smooth and gradual transition in brightness can be achieved.

PWM as A Dimming Signal

As we look to deepen our knowledge of pulse width modulation, it's important to recognize it as a distinct signal type. PWM signals are composed of square-wave-shaped pulses that alternate between peaks and valleys. During the on-time, the signal strength is high, and during the off-time, it's low. By understanding the fluctuations in the signal strength, we can utilize PWM in a variety of applications.

In the context of dimming, the duty cycle refers to the amount of time that a signal can stay high. A signal with a consistent "on" state would have a 100% duty cycle. The on-time of a PWM signal is adjustable, however. For example, when the duty cycle is set to 50%, the signal will alternate between being "on" and "off" for equal amounts of time.

PWM as LED Driver Output

Pulse width modulation (PWM) occurs when the PWM signal is converted to a DC voltage and utilized as an LED driver output. The PWM output circuit interrupts the DC LED currents at a high frequency between the on and off states, resulting in flicker that is imperceptible to the human eye and maintains the stability of the LED light output.

There is often confusion between PWM output and dimming signals, so it is important to note a few distinctions. The mechanism generates a PWM signal that is digital and consistent on the dimmable cable, while the driver extracts the output current by sensing the PWM duty cycle.

Advantages of PWM Dimming:

1. Offers precise output level to meet specific LED characteristics, such as efficiency, color and temperature.

2. Provides a wider range of dimming options.

3. Prevents color shifts by operating LEDs at their prescribed forward current and voltage operating point.

Disadvantages of PWM Dimming:

1. Requires expensive and complex drivers.

2. The rapid switching results in the emission of EMI radiation, which is caused by the swift ascent and descent of the switching cycles' edges.

3. At lower levels of brightness, a noticeable flicker may emerge, potentially leading to eye strain and headaches for certain viewers. To fully utilize the residual visual capabilities of the human eyes, it is recommended to have a working frequency of at least 100Hz, preferably 200Hz.

4. Eliminating the whistling sound that accompanies dimming bow is crucial. Although the human eye can't detect sounds above 200Hz, the human ear is sensitive to frequencies as high as 20kHz. Consequently, you may hear a buzzing sound at this range. There are two methods to resolve this issue. Firstly, increasing the switching frequency beyond 20kHz can keep it inaudible to humans. However, excessively high frequencies can cause issues due to parasitic parameters, leading to waveform distortion.

5. Using longer wires can create performance issues for drivers due to the inherent characteristics of the wire, such as inductance and capacitance, which can interfere with the fast edges of the PWM signal.

Overall, PWM dimming is a widely used technique for controlling the brightness of LEDs. While it has some limitations, its precise control and efficiency make it an attractive option for many lighting applications.