Flicker Effects in LED Applications

 

Flicker-free light is becoming more and more important in lighting design. It is therefore crucial to opt for high-quality LED switching power supplies and the advantages they bring.

The problem

Today, with the extremely fast reaction times of LED chips, flicker is becoming increasingly significant. Flickering is caused by current fluctuations in LED switching power supplies with pulse width modulated outputs, and is also caused by an AC current overlaying the DC output current of power supplies with linear outputs.

Example and solution

These days, modern sports facilities are use LED lighting solutions. In contrast to the ideal (100% pure) DC output current of an LED power supply (see image above), the output current is, in fact, overlaid with a proportion of AC current (ripple current). A change in amplitude occurs as a result, and at a frequency that instantly creates a perceptible fluctuation in brightness (flicker factor). Because of this:

• The dependency of the flicker factor of the lamp on the ripple current from the power supply has to be taken into consideration in lighting planning.
• It is important to choose an LED power supply with as little ripple current as possible.

The quality of an LED power supply is also crucial in fulfilling the flicker requirements of the advertising and film industries, UEFA and other organisations, because particularly with slow-motion recording at rates higher than 25fps, the flicker effect becomes visible.

The flicker factor to be expected can be derived in part from the information provided in the datasheets.

• With constant current power supplies (CC mode), such as Mean Well's HLG-320H-C series, the maximum expected current change – that is, the maximum percentage of current ripple – is noted on the datasheet. The actual measured values for this series, e.g. as shown in the test report for the HLG-320H-C1400, demonstrate even lower values – 1.14%. This power supply thus meets the requirements of UEFA "Elite Level A."

• Power supplies for constant voltage and constant current operation (CV+CC mode) show "ripple & noise" on their datasheets. An approximation of the flicker factor to be expected can thus be derived from the ripple & noise using a mathematical formula.

• For a purely constant voltage power supply (CV mode), a figure for ripple & noise is also noted on the datasheet. These devices, however, also require an LED driver to generate the constant current required by the LED. Determining the flicker factor means taking the complete system into consideration, including the LED driver.

What if the flicker factor requirements are even higher or if an unacceptable flicker factor has to be minimised?

• Tip: Excellent results can be achieved by the simultaneous illumination of one point by using three individual LED lamps. This is because, by operating separate LED power supplies on the three phases of the mains supply – due to the 120Hz phase shift between L1, L2 and L3 – the flicker factor compared to single-phase operation is greatly reduced.



Advantages of flicker-free illumination for filming:
• With professional film recording, expensive and time-consuming post-production processes can be avoided. For video recordings by consumers, too, it is also worth taking the flicker factor into account in lighting design.

Schukat electronic offers high-quality power supplies of all kinds. Schukat's sales team can advise on any aspect of flicker, including customer-specific questions, and can also answer any questions you may have regarding power supplies.