Technical application guide

Ripple current and light flickering (modulation)

General
Light modulation describes how much the instantaneous value of the luminous flux is varying. Human perception strongly depends on the frequency. In practice, many drivers naturally deliver a high content of 100-Hz modulation. The root cause for this is the European 50-Hz mains supply, which results in 100 Hz after rectification.

Compared to traditional fluorescent light sources, LEDs instantaneously translate their operating current into light, without much smoothing effects. Already minor imperfections of the driver can lead to a low quality of light. The OPTOTRONIC LED driver family ensures that the 100-Hz light modulation is so small that it normally cannot be perceived by the human eye and is therefore not critical within a standard application.

The most common definition for the light modulation is:

In addition to economic and ecological aspects, the use of ECGs and LED drivers with low levels of light modulation has extremely positive effects on the working environment. In general, strong flickering at low frequencies should be avoided. It is known that a small subgroup of patients with epilepsy are photosensitive and react to flickering light. The risk for such reactions drastically reduces when the frequency of the light modulation is increased to over 70 Hz. Even when there is no obvious visible flicker, ergonomics studies with conventional

technology (CCG operation) have shown that latent flickering of the light is a load factor that affects a person’s work performance, particularly at VDU workstations. The consequences are rapid fatigue, lack of concentration and a greater number of errors in word processing and other such tasks. Practical VDU-based tasks have shown that the benefits of flicker-free light are considerable, not only for the people themselves but also for the quality of their work. Premium-quality LED drivers produce less than 5 % modulation, and are therefore often called “flicker-free” or “zero ripple”.

Definition and measurement (ripple current)
In most cases, the light modulation is proportional to the ripple current:

Measurement
In a first approximation, the light modulation is proportional to the current modulation, often called ripple current. This proportional behavior allows the light modulation to be judged by performing a current measurement. A low-pass filtered current measurement achieves the most accurate results. If no filter is available, a simplified measurement method can be used.

Simplified measurement

  • Measurement of module current
  • No filter necessary
  • Readings from upper envelope (100 Hz)
  • Typical accuracy : +/-15 % of result
  • Means: 20 % varies between 17...23 %
  • A light measurement with a photodiode, phototransistor or integrated light-to-voltage converter will deliver almost identical results
  • If PWM is used: modulation = 100 %
  • Most common behavior over time: increase of the ripple current due to capacitor aging

Picture below: Hybrid dimming

Picture above: Simplified measurement

Advantages and disadvantages of PWM

Advantages of PWM dimming:

  • Dimming without color differences among all LEDs
  • Same brightness of all LEDs

Disadvantages of PWM dimming:

  • Remaining risk of stroboscopic effects when rotating or fast-moving parts are illuminated; this risk can be reduced by increasing the PWM frequency to over 400–500 Hz
  • Interference effects with cameras are reduced with higher frequency, but for high demands on image quality or high-speed imaging, PWM is not recommended

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