Step-by-Step Process for Selecting An EMI Filter

Today’s electrical and electronic devices employ high-frequency switching circuits, which create high-frequency electrical signals propagating through cables and air, called electromagnetic interference (EMI). EMI is a common problem for design engineers because electronic designs must meet industry-specific conducted and radiated emission (EMI) requirements. Electromagnetic interference (EMI) or electromagnetic noise can also disrupt the intended operation of a device, causing disturbance or malfunction. EMI can come from many other sources, from natural effects like static electricity, lightning storms, and solar flares to man-made sources like cell phone signals, radio signals, and even transportation systems. All these potential sources of EMI can cause similar problems for electronic equipment, which is why EMI filters have become a staple of the electrical and electronics industry.

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EMI filters, also called EMI suppression filters, mitigate electromagnetic noise by suppressing unwanted high-frequency signals/noise. When placed at the point of power entry to equipment, these filters prevent line noise from entering the equipment. However, an EMI line filter can also prevent noise generated by the equipment from exiting and entering the power lines. Selecting an appropriate EMI filter is critical to achieving optimum size, performance, and cost. Different types of EMI filters provide different suppression levels and have different characteristics. What works for one system might not work well in another. For this reason, it is essential to understand the considerations when selecting an EMI filter. This guide lists key parameters for selecting an EMI filter that best suits your specific application.

Electrical and Operational Parameters

The first step in selecting the right EMI filter for your application is reviewing the specification and system requirements. These include the electrical and operating requirements of your system. The crucial items in this category are below.

  • Rated voltage refers to the maximum line voltage the filter is designed to handle. Most single-phase EMI filters are rated 250VAC and will work at any AC voltage below that. The 3-phase filters are rated to 480VAC. Some standard single filters with 277/300VAC ratings for higher voltage applications are available. Similarly, 3-phase filters are available up to 600VAC and the DC filters up to 600/1200VDC. Filters can handle short-term voltage spikes/overvoltage, but consistently exceeding the rated voltage can severely damage the capacitors inside the filter. The rated voltage of the selected filter must be greater than or equal to the maximum input line voltage supplied to the device.
  • Rated current: This is the maximum steady-state current that the EMI filter is designed to carry without exceeding the safe temperature range. The rated current of the selected filter should be equal to or higher than the maximum steady-state input current the device will draw when powered. Filters can handle higher in-rush current, but the filter can fail if the rated current is exceeded for longer durations.
  • Ambient temperature: This is the highest temperature at which the filter is designed to carry its full rated current. Most filters are ambient rated for 40°C or 50°C. The ambient temperature can sometimes be higher at 65°C, 70°C, or 75°C. If the actual operating temperature exceeds the ambient temperature of the filter, the current must be de-rated.
  • Operating temperature: This is the temperature range where the filter can safely operate. For most commercial filters, the operating temperature range is -25°C to +85°C or -25°C to +100°C. For military applications, this can be -40°C to 100°C or -56°C to 100°C. The actual operating temperature in any application must fall within this range. Using the filter at a temperature outside this range can severely damage its components. Be sure to choose a filter that meets the operating conditions of your application.
  • Leakage current: This is the current that flows from ‘line’ and ‘neutral’ to ‘ground’ when the ‘line’ voltage is applied to the filter. This is caused by ‘line’ to ‘ground’ (Y) capacitors in the filter. The EMI filter with ‘Y’ capacitors will add leakage current on top of the device's contribution. In many applications, regulations limit the total leakage current of the equipment. Be sure to consider this to avoid compliance problems with safety standards, including EN 60950-1 for information technology equipment, IEC60601 for medical equipment, and EN 55014 for appliances. Some countries have unique power systems (such as corner grounded delta) that can result in very high leakage current even with low leakage filters. This needs to be considered. Non-compliance with leakage limits after the emissions and immunity testing is done will require a retest resulting in additional time and cost.
  • Power System: Utility power and power system configuration can vary from one country to another. Apart from the standard single-phase, 3-phase delta, and 3-phase wye configurations, there are other special configurations such as split phase, corner grounded delta, hi-leg delta, etc. EMI filters are typically designed for single-phase, 3-phase delta, 3-phase wye, and DC circuits, but these can be used in other configurations with proper selection or slight modifications. The input power type must be identified, and a matching filter must be selected. For special configurations, contact the filter specialists for recommendations on standard filter options that can work in those setups and still meet the requirements.
  • Number of Stages: Stages refer to the number of circuit repetitions (put in series) inside the filter. The single-stage filter has one circuit. If the circuit is repeated, it becomes a dual-stage filter, and so on. Increasing the number of stages improves filter performance while optimizing the filter package (size). It also helps in lowering the cut-off frequency.
  • Hi-pot voltage / Dielectric Strength: A high DC voltage is applied between the lines and ground to check the insulation strength. This can help determine any weak points in the insulation or manufacturing defects that can lead to the line voltage coming in contact with the filter chassis, causing unsafe conditions. The applied high voltage is a function of the rated voltage of the filter as specified by safety agencies. Certain critical applications require a higher hi-pot voltage than usual. The filter manufacturer must be contacted to ensure the filter meets the increased hi-pot requirement. If not, the filter can be modified using higher voltage-rated components to comply with these special conditions.

These parameters serve as a "first pass" in selecting the correct EMI filter. These will also help narrow down to fewer EMI filter options that are suitable for the specific application. The next steps are to specify the application/system requirements further and determine the best EMI filter for the application.


Application and System Requirements

The further down selection of the filter can be made based on the application and system needs. These specific parameters will dictate which EMI filter might be a better choice. Some factors to consider include the following.

  • Equipment type: The type of equipment and its components can significantly impact the EMI filter selection. AC/DC converters, variable speed drives, appliances, industrial equipment, RF modules, medical equipment, and cabling. All these types of equipment can potentially require EMI filtering. A solution that works in one application might not work in the other due to unique performance, space, and interconnection requirements. Other characteristics of equipment electronics, such as switching frequencies, clock frequencies, harmonic spectrum bands, and signal rise/fall times, can significantly impact the emission profiles.
  • Industry requirements: Every industry has established emissions and safety standards that the equipment must comply with. For example, military standards (MIL-STD-) govern the requirements for military applications, while FCC and UL/CSA standards are the required guidelines for North America. Be sure to select the proper EMI filter by considering the industry requirements.
  • Filter type: Certain filter designs may be more desirable for an application than others. These include PCB filters, chassis mount filters, IEC inlets, and bulkhead mount filters. Be sure to research the available filter types and consider the one that will work best in the application.
  • Size restrictions: If there are any spatial (size, space, or form factor) limitations for the application, these must be considered in the early stages. Standard filters are available in various shapes, sizes, form factors, performance, interconnection, and mounting types. Usually, one of these standard designs or a pre-existing modified solution can meet the application's needs. In other cases, a modified or distributed solution can be designed. Call the filter specialists to discuss these.
  • Terminations: Filters are typically offered with several different interconnection options, including fast-on/quick disconnects, wires, threaded studs, connectors, etc. Every termination/connection type may or may not suit every piece of equipment. Be sure to select an interconnection that is compatible with the system and its application needs.
  • Grounding: Grounding is essential for the safety and high-frequency performance of the filter. An alternate filter design must be considered if a chassis ground is unavailable for the filter.

The above selection process will be useful in down-selecting filters that meet the electrical, operational, and application requirements. However, the most important role of a filter is to help the equipment meet conducted (and sometimes radiated) emissions requirements as directed by the applicable EMI regulation. The insertion loss performance of a filter is critical for equipment compliance.



Level of Attenuation and Insertion Loss

Reviewing the filtering characteristics of an EMI filter and analyzing its efficacy for the application is one of the essential factors when selecting an EMI filter. This requires EMI (conducted and radiated emissions) testing of the equipment without and with a filter.

1. Collect Device Data

The first step in this process is to run the initial conducted emissions scan on the equipment, preferably without a filter. Ideally, this should be done early, as it can significantly affect the selection of EMI filters. The initial testing will establish the raw conducted emissions data. This data will help a designer determine where (frequency) and to what extent (dB amplitude) the unit requires suppression. There are two primary ways to run this test, as described below.

  • Full Scan: If possible, scan the entire conducted frequency spectrum. Doing so provides a fully conducted emission test plot.
  • Limited/Partial Scan: If a complete scan is impossible due to time or test equipment constraints, select a few worst-case frequencies or smaller ranges (based on judgment or prior experience) and measure the noise at each.

2. Analyze the Datasheet

An important step in the EMI filter selection is correctly reading filter datasheets, which typically feature insertion loss graphs or tables/charts. Insertion loss is the proportion of loss in signal power between the filter input and output. This expresses the loss the filter provides to a signal at a given frequency. Typically, this is measured in decibels (dB). Insertion loss graphs and charts show users how much attenuation/loss the signals get in a range of frequencies when passing through the filter and measured with 50ohm input and 50ohm output impedance.

The conducted noise can be common or differential based on where it is present and how it propagates. The common mode (CM) currently refers to noise or signals that flow in the same relative direction in a pair of lines or all the lines. The differential mode (DM) current refers to noise or signals that are present in one of the lines and flow in opposite directions in a pair of lines. There are usually two graphs or charts for insertion loss: one for common mode and the other for differential mode. This implies that a filter typically attenuates common and differential modes noise. So, knowing the noise type can help choose a more appropriate EMI filter.

3. Pick a Filter

Once the conducted emission test data is collected, compare it to the insertion loss graphs provided by the filter manufacturer. Look at the failure margin at each failed frequency and determine if the filter provides enough attenuation to push the noise amplitude below the limit line. For instance, if the conducted emission test failed by 25dB at 500kHz, the attenuation/insertion loss provided by the EMI filter at 500kHz must be 30dB or more. This will provide about 5dB of safety margin.

The filter must be rested with the equipment to ensure compliance with conducted emissions. The radiated emissions test must also be run to uncover any areas where filter placement might help.

4. Filter Insertion Loss Data v/s Actual Performance

Finally, one key thing to consider when reviewing the EMI filter insertion loss data is that the filter behavior will not precisely match that provided in the datasheet. Manufacturers typically measure the filter’s insertion loss in a standard 50ohm input/50ohm output impedance setup. However, the system impedance can be different and much more complex in real-world applications. This can alter the actual performance of the filter. For this reason, it is essential to test the filter with the equipment to verify compliance. Run a test that, at a minimum, looks at the failure frequencies identified during the initial scan and see if adding the filter provides enough attenuation to pass the test.

5. What to do If You Lack an Insertion Loss Graph

In some cases, the manufacturer may not provide an insertion loss graph. Instead, the datasheet contains a frequency-insertion loss chart which provides only a few specific frequency data points. While this is not ideal, this will not majorly impact the EMI filter selection.

If only the insertion loss table /chart is available, the insertion loss values for the frequencies falling between the given data points can be estimated by linear extrapolation (as a linear function). Use the device data collected from the preliminary emissions testing (initial scan) to identify where the failing frequencies and failure margins fall. Now, try to extrapolate the filter's insertion loss at these frequencies using the adjacent data points available on the filter insertion loss table/chart. For example, if a datasheet specifies that a filter provides 30dB of attenuation at 150kHz and 50dB at 500kHz, and the initial scan shows a failure of 30~35dB (or less) at 300kHz, the filter should work. But if the test data showed a failure of 45dB or higher at 300 kHz, the filter may not be the appropriate solution, and a different filter must be selected.

Additional EMI Filtering Characteristics

A few other parameters may not be critical in every application but must be understood and considered case by case.

  • DCR/Power loss: The filter components have inherent resistance, collectively represented as the filter's DC resistance (DCR). When current flows through the filter, it can cause I2R losses resulting in power loss/heat dissipation.
  • Resonance / Harmonic distortion: If the filter's resonant frequency is close to the control loop bandwidth, it can cause harmonic distortion, resulting in equipment malfunctions. A filter can also create a resonant circuit with other components in a device that can severely undermine its performance or cause an audible hum.
  • Temperature rise: Temperature rise testing demonstrates that a filter, despite the increase in its temperature during operation, remains within a temperature range that is safe for the internal components and ensures a longer lifespan. Although safe for the components, the filter body temperature may not be safe for human touch. This must be considered while deciding the filter mounting location.
  • EMI Requirements: The EMI/emissions standards are equipment/system-level requirements. Filters are not required to meet these. The filter only helps equipment comply if it fails the test.
  • Agency Certifications: A filter must meet agency safety requirements to be considered for use in any application. The most common ones are UL 1283, UL 60939, IEC 60939, CSA 22.2, etc. At a minimum, a filter must be designed to meet these standards. Otherwise, the equipment will not pass the final safety testing. It is a good idea to select filters already tested to these standards and carry safety agency approval marks such as UL recognition, CSA/cUL approval, ETL mark, IEC/Semko/VDE mark, etc. This will simplify the equipment safety approval process.

Also, consider the filter placement carefully. Crosstalk is a common problem in system design, where noise crosses between wires or traces placed nearby or from the input to the output side, causing emissions failure. The designer should monitor this and other related issues during the design and testing processes.

Custom Solutions from Astrodyne TDI

Custom Solutions from Astrodyne TDI

The above selection guide serves as a baseline for selecting EMI or EMC filters, but it may not cover every possible scenario. There is always a possibility that an off-the-shelf solution may not meet all the application criteria. In these cases, custom solutions may become necessary. Whether you need assistance determining how to find the best EMI filter for your power supply or your application or if you are seeking a custom filter solution, Astrodyne TDI can help.

Astrodyne TDI provides durable and dependable EMI filters for various applications, including industrial, professional-grade appliances, medical, aerospace, semiconductor, and military. Whether you need specialized EMI filters for military or medical applications or cost-effective designs for an appliance, Astrodyne TDI has numerous quality options. Our team of EMI filter engineers can design an EMI filter to meet your specific requirements.

With more than 60 years of experience, Astrodyne TDI is a trusted manufacturer of high-quality EMI filters. Our customers count on us for quality products, unparalleled technical support, and custom product design. We work hard to create EMI filter designs that meet and exceed industry standards and achieve the highest level of quality. Explore our selection of EMI filters if you're looking for an off-the-shelf solution, or submit a quote request to get started on a custom EMI filter designed for your unique needs. For more information about custom and standard EMI filters from Astrodyne TDI and how to make your selection, contact us today.