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How to Select an EMI Power Filter

August 6, 2019 by Ron Stull - 7 Minute Read

How to Select an EMI Power Filter

Switch-mode power supplies are inherently noisy with respect to electromagnetic emissions (EMI). The fast switching of high voltage and current nodes leads to relatively large di/dt and dv/dt values within the circuit causing noise to be emitted across a wide frequency range. Regulatory bodies in most countries set limits on the amount of electromagnetic noise that may be emitted. As a result, a lot of time and effort is given to mitigating noise sources and filtering out any noise that remains. However, while these power supplies will comply with regulations when tested alone, adding them to a system can lead to unintended electromagnetic emissions, which will require extra filtering to obtain regulatory approval. Off-the-shelf EMI filters, if properly selected, are an easy way to improve the emissions and comply with regulations.

EMI and Electromagnetic Compatibility Background

When dealing with electromagnetic compatibility (EMC), the problem is commonly modeled with three components: sources, paths, and receptors.

The sources are those devices or circuit nodes that produce the interference. In addition to the power supply itself, this may include other devices such as microprocessors, video drivers, RF generators, etc.

Noise generated by a source has two paths it can then travel. The first is a radiated path, which is electromagnetic energy propagating out into space and coupling into other systems. The second is a conducted path where the signal travels through the conductors of the system (e.g. PCB traces and planes, component leads, input wiring, etc.). This can get back into the mains power lines and affect other equipment being powered from that line.

Receptors are those devices which pick up noise emitted by the source and are affected by the interference. Receptors can include just about every analog and digital circuit.

When testing for EMC, the regulator will test conducted and radiated electromagnetic emissions separately. Each has its own limits and frequency range along with its own suppression method. Radiated emissions cover a higher frequency range (typically 30 MHz to 1,000 MHz) and as the noise travels through space it is limited in how it can be controlled. Besides using proper layout and circuit design techniques to attenuate the noise at the source, shielding can be used to contain the radiated noise. On the other hand, conducted emissions cover a lower frequency range (typically 0.15 MHz to 30 MHz), and, because they travel through conductors, can be controlled using electrical filtering components. The designer, when adding EMI filtering may choose to design it discretely or choose to go with an off-the-shelf EMI filter.

EMI Filters and System Requirements

For the engineers that choose an off-the-shelf EMI filter, there may be some confusion over how to choose the right filter for their system. The first step is making sure that the EMI filter meets the basic electrical requirements. Important items to review include:

  • Rated voltage, which is the maximum voltage that can be applied to the input. Exceeding this can damage components inside the filter.
  • Isolation voltage, which is the isolation rating measured between each input line and earth/chassis ground (there is no isolation between input and output).
  • Rated Current, which is the maximum current that can pass through the EMI filter within the specified operating temperature range.
  • Operating Temperature, which is the maximum temperature that the device may be operated.
  • Leakage Current, which is the current that flows through the earth/chassis ground. The EMI filter will contribute leakage current in addition to that of the power supply itself. Due to safety concerns leakage current has regulated limits and the contribution of leakage by the filter should be considered by the designer.

Example circuit of an EMI power filter
Internal filter circuit example

EMI Filtering Characteristics

After finding an EMI filter that meets the operating conditions of the system, the actual filtering characteristics should be reviewed. In the datasheet there will typically be insertion loss graphs, one for common mode and one for differential mode. These graphs show the user how much the signal will be attenuated between input and output with respect to frequency.

Insertion loss is the ratio of the signal at the input of the filter to the signal at the output, usually measured in decibels, due to the large frequency range covered, as shown in the following equation.

Insertion Loss (dB) = 20 Log 10 (Unfiltered signal / Filtered Signal)

This can be re-written, using the quotient rule, to solve for the filtered signal.

Filtered Signal (dB) = Unfiltered Signal (dB) - Insertion Loss (dB)

Examples of common mode and differential mode insertion loss graphs Examples of common mode and differential mode insertion loss graphs
Insertion loss graphs

In some cases, a graph is not given and instead a noise attenuation value is listed in the datasheet. This is usually paired with a frequency range over which the attenuation is applicable. For example, a datasheet may specify 30 dB of attenuation between 150 kHz and 1 GHz.

The final item to note when reviewing the filter data is that the source and load impedances will change the filter's behavior. The insertion loss given in the datasheet was obtained using an impedance (typically 50 Ω) that may be quite different from that of the system it is being applied to. So, while a filter may look good on paper, it is important to test the filter in circuit to verify its performance under the actual source and load conditions of the end system.

EMI Filter Selection

When choosing an EMI filter, it is ideal if the power supply to be filtered has gone through preliminary EMC testing in order to get a baseline of the conducted emissions. The test results will tell a designer at what frequencies the unit failed and by how much. This information can be compared to the insertion loss graphs of the EMI filter to determine if it offers enough attenuation at the failed frequencies to pass the EMC test. For example, if the common mode emissions test failed by 64 dB at 500 kHz, referencing the EMI filter's common-mode insertion loss graph below shows at 500 kHz an attenuation level of approximately -75 dB. If this EMI filter was applied, one could expect to pass the EMC test with 11 dB of margin at 500 kHz.

Example graph showing emissions before and after applying an EMI power filter and filter's insertion loss graph Example graph showing emissions before and after applying an EMI power filter and filter's insertion loss graph
Example of EMI graph before and after applying a filter (top) and EMI filter's insertion loss graph (bottom)

Because of the inconsistent attenuation across the frequency spectrum, it is important to make sure that all failed or marginal frequencies will be properly attenuated. If the datasheet provided a single attenuation value instead of an insertion loss graph, it is crucial to make sure that this single value was greater than the largest margin of failure.


Switching power supplies are a major source of electromagnetic emissions (EMI), which makes their regulation vital to prevent interference with other electronics. Most, if not all, switching power supplies will have a filter at the input, but due to the wide range of applications, this may not always be enough to pass final EMC testing once applied to a complete system. Off-the-shelf EMI filters are a quick and easy way to reduce electromagnetic emissions if the internal filter is not enough and can save time over having to design a discrete solution from the ground up. CUI offers several ac-dc EMI power filters and dc-dc EMI power filters in board mount, chassis mount, and DIN rail configurations readily optimized for a system's electromagnetic compatibility needs.

Categories: Product Selection

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Ron Stull

Ron Stull

Power Systems Engineer

Ron Stull has gathered a range of knowledge and experience in the areas of analog and digital power as well as ac-dc and dc-dc power conversion since joining CUI in 2009. He has played a key role on CUI’s Engineering team with responsibilities including application support, test and validation, and design. Outside of power engineering Ron can be found playing guitar, running, and touring the outdoors with his wife, where their goal is to visit all of the U.S. National Parks.

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