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