Dimming by pulse-width-modulation (PWM) or operational current regulation

Modern solutions for all tasks in contemporary lighting technology require a high measure of calculability and replicability of results as well as reliability of the overall system and its components. One of those tasks is dimming of light sources, which must be executed in a defined manner to facilitate the precise setting of a required lighting level or a desired light colour (see also chapter 2.4.4.5 "DALI"). Fundamentally, there are two options for this:

  • continuous variation of lumen output with permanent light emission, and

  • variation of temporal interruptions in light emission with short intervals below the eye’s perceptual speed threshold.

Figure 2.44: Pulse-width modulation in an LED luminaire. The modulation frequency should be ≥ 400 Hz.

A variation in lumen output is usually achieved through LED operating current regulation. In many areas, the technical implementation of this principle is very manageable. However, it reaches its limits when it comes to small lumen outputs. This is due to the pronounced interconnection of the adjusting LED forward voltage and minor changes in current. It is expressed in the shape of the lower region of the LED’s current-voltage curve (see figure 2.41 in chapter 2.1.9.4 "Control gear for LED luminaires"). The consequence is a limitation in dimming range.

The periodic, temporal interruption of light emission is commonly referred to as pulse-width modulation (PWM). The timespan covering switching the LED on, switching it off and switching it on again is the cycle duration of the PWM. Its value should be below 2.5 ms, which corresponds to a frequency above 400 Hz. A visible flicker in the lighting is no longer perceived beyond ca. 100 Hz; however, physiological effects even below the perceptual threshold of vision are still possible up to a frequency of 400 Hz. Stroboscopic effects, e.g. on rotating parts can occur even in much higher frequencies if those coincide with the periodicity of the rotating movement.

In contrast to thermal radiators (incandescent lamps), which have to warm up after switching on and cool down after switching off, LEDs react very rapidly to switching actions. They develop their full luminous flux even during very short operating periods in the range of mere μs. For a cycle duration of ca. 2 ms, PWM can therefore be regarded as a sequence of switching cycles with operating times and operating interruptions. The temporally averaged luminous flux can thus be assumed to be (approximately) proportional to the temporal average of power consumption.

In reality, average power consumption affects the LEDs’ operating temperature. Due to increased luminous efficacy at low operating temperatures, the available average luminous flux in the lower dimming range of the PWM is even slightly higher than it would be in case of proportional interdependence.

In practice, LED are often dimmed using a combination of operating current regulation and PWM. Operating current regulation is applied in the upper power range. This approach provides the advantage that stroboscopic effects are prevented completely and securely in this range. Only in the lower dimming range, e.g. when a daylight-dependent regulation system has plenty of daylight available, PWM sets in, levelling off possible stroboscopic effects efficiently.

Overall, a broad dimming range with reliably replicable behaviour can be realised in this manner.

Control is managed via an interface such as DALI (Digital Addressable Lighting Interface, see also chapter 2.4 "Light management").

When measuring power consumption in dimmed operation, it should be considered that a reduced power factor (see chapter 2.1.8.4) might manifest.