Keeping EMI from LED drivers under control

Author : Frederik Dostal | Field Applications Engineer | Analog Devices

01 May 2021

Analog Devices_Keeping EMI from LED drivers under control

Nearly all new lighting applications today now use LEDs. In a relatively short amount of time, they have become established as the clear preferred choice of illumination. However, in most applications, an LED cannot fulfill the required functionality by itself: it must be operated with a suitable power supply.

The full version of this article was originally featured in the May 2021 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.

Such driver circuitry should naturally be as efficient as possible to reduce energy consumption, which is why switch-mode power supplies are primarily used for this purpose. But as Frederik Dostal, Field Applications Engineer at data conversion, signal processing & power management semiconductor firm, Analog Devices explains here, lighting design engineers also need to consider how they can minimise electromagnetic interference (EMI) when using LED drivers…

For all power supplies, regardless of the type, electromagnetic compatibility (EMC) should be considered. This is especially true in the case of LED lights. Various standards for measurement, evaluation and documentation of the electromagnetic interference (EMI) generated by LED lights have become established over time.

Uncontrolled electromagnetic interference can have serious consequences. Just recently, I had a first-hand experience with one of them. An old E27 incandescent light bulb on my electric garage door opener burned out. After I replaced it with a modern LED light bulb, the light worked again. However, I could no longer open the garage door with the remote control. Thus, the radiated emissions from the LED light must have caused interference in the radio electronics of the garage door.

The emissions generated by a switch-mode power supply are partly conducted and partly radiated emissions. Electromagnetic emissions from an LED driver thus can be transferred via the power supply lines, as well as magnetically or capacitively coupled into adjacent circuit segments. These emissions are not usually destructive, but they can lead to improper functioning of adjacent circuit components.

Thus, it makes sense to minimise these generated emissions, but what requirements must be met in this regard? All electrical and electronic products in the European Union require CE marking. The CE mark proves that a product complies with EU rules on safety, health and environmental protection. As a result, transport of such compliant devices within the European Economic Area is permitted. In other parts of the world, there are other important requirements pertaining to radiated emissions. Examples include UL, CSA, and others (including now, post-Brexit, the UKCA mark).

There are numerous standards specifically related to the safety of and emissions from LED lights. A major one is CISPR 11. CISPR stands for International Special Committee on Radio Interference. There are many other rules and regulations, including ISO, IEC, FCC, CENELEC, SAE and more, that are based on the CISPR standards.

Conducted emissions can be reduced predictably with the appropriate measures, using additional supply line filters. These filters are designed to address common-mode or differential-mode noise. The frequency range that usually plays a role here is below 30 MHz. However, developing these filters is not all that simple. A filter is usually optimised for a particular frequency range. In other frequency ranges, parasitic effects and the resulting changes in behavior of the components used can cause problems. For example, a filter may reduce the emissions generated by a switch-mode power supply at 100 kHz very well. However, power supplies usually generate emissions in a wide frequency range, especially above 10 MHz. Here, the filter optimised for 100 kHz could even increase the emissions through parasitic effects and resonances.

The radiated emissions cannot be predictably reduced in this way. Here, the energy content of parasitic inductances and capacitances from the PCB traces, as well as passive circuit components, plays a decisive role. The frequency range usually lies above 30 MHz, up to an upper limit that is laid down in the respective standards. Reducing these radiated emissions is very difficult. It requires a great deal of experience and background knowledge.

Especially in the driving of LED lights, the level of radiated emissions can be extremely high. Usually a chain of LEDs is driven. This series circuit often requires a large amount of space on the board. Thus, the geometrical arrangement has the properties of an antenna, and generated emissions are radiated particularly effectively. Shielding electrical circuits is complex, expensive  and, in the case of LEDs, not even possible, in part because the desired light could not pass through a sheet metal shield. Thus, the solution lies in generating only a small amount of radiated emissions...

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