Switching mode power supplies are bound to emit noise when they encounter 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.
When dealing with electromagnetic compatibility (EMC) issues, they are typically modeled through three components: noise sources, paths, and receptors.
The noise source is the device or circuit node that generates the interference. In addition to the power supply itself, the noise source may include other devices such as microprocessors, video drivers, and RF generators.
Noise generated by a noise source can then be transmitted through two paths. The first is the radiation path, whereby electromagnetic energy is propagated into space and connected to other systems. The second is the conduction path, whereby the signal passes through the conductors of the system (e.g. PCB alignments and levels, component leads, input wiring, etc.). This path can return to the main power line and affect other devices that receive power from that line.
The receptor is the device that receives the noise from the noise source and is affected by the interference. Receptors can include almost all analog and digital circuits.
When testing EMC, regulators will test separately for conducted and radiated electromagnetic emissions. Each type of radiation has its own limits and frequency ranges as well as methods of suppression. Radiated electromagnetic emissions cover a much higher frequency range (typically 30 MHz to 1,000 MHz) and may be limited in how they can be controlled as noise propagates through space. In addition to using appropriate layout and circuit design techniques to attenuate noise at the noise source, shielding can be used to suppress radiated noise. Conducted electromagnetic emissions, on the other hand, cover a lower frequency range (typically 0.15 Mhz to 30 Mhz), and because they pass.
Engineers who choose off-the-shelf EMI filters may have some confusion on how to select the correct filter for their system. The first step is to ensure that the EMI filter meets the basic electrical requirements. Important items to review include.
After finding an EMI filter that meets the system's operating conditions, the actual filtering characteristics should be reviewed. The data sheet will typically have insertion loss graphs, one showing common mode loss and one showing differential mode loss. These charts show the user how much the signal frequency is attenuated between the input and output.
Insertion loss is the ratio of the signal between the filter input and output due to the large frequency range covered, usually measured in decibels and expressed as the following equation.
Insertion loss (dB) = 20Log 10 (unfiltered signal/filtered signal)
The equation can be rewritten to solve for the filtered signal using the division rule.
Filtered signal (dB) = Unfiltered signal (dB)-Insertion loss (dB)
—— common mode ------ differential mode
Sometimes a graph is not given, but rather the noise attenuation value is listed in a data table. This usually matches the frequency range to which the attenuation applies. For example, the datasheet might specify 30 dB of attenuation between 150 kHz and 1 GHz.
A final item to note when viewing filter data is that the noise source and load impedance can change the behavior of the filter. The insertion loss given in the data-sheet is derived using an impedance (typically 50 Ω) that may be very different from the impedance of the system to which it is applied. So, the filter shown in the datasheet may look good, but it is important to test the filter in the circuit to verify its performance under the actual noise source and load conditions of the end system.
When selecting an EMI filter, it is best to perform preliminary EMC tests for the power supply to be filtered to obtain a baseline value for conducted emissions. The test results will tell the designer the frequency of failure and the degree of failure of the equipment. This information can be compared with the insertion loss graph of the EMI filter to determine whether it can provide sufficient attenuation at the failure frequency to help pass the EMC test. For example, referring to the common-mode insertion loss graph of the EMI filter below, which shows an attenuation level of approximately -75 dB at 500 kHz, determine whether a common mode radiation test yielding a value of 64 dB at 500 kHz indicates a test failure. If this EMI filter is applied, it is expected to pass the EMC test with a margin of 11 dB at 500 kHz.
Because of the inconsistent attenuation across the spectrum, it is important to ensure that all fault or margin frequencies are properly attenuated. If the data sheet provides a single attenuation value rather than an insertion loss graph, it is important to ensure that this single value is greater than the maximum fault margin.
Switching power supplies are a major source of electromagnetic radiation (EMI), so their regulation is key to preventing interference with other electronic devices. Most, if not all, switching power supplies have filters on the input side, but because they are used in a wide range of applications, they are not always guaranteed to be adequate to pass the final EMC test when used for the entire system. Off-the-shelf EMI filters are a quick and easy way to assist in reducing electromagnetic emissions when internal filters are not sufficient and are more time efficient than designing a separate solution from scratch. cUI offers a wide range of ac-dc EMI power filters and dc-dc EMI power filters in board-mount, rack-mount, and DIN-rail configurations that can be optimized for the EMC needs of the system.
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