Figure: Power line EMI filter concept

A power line EMI filter is an electronic filter designed to attenuate conducted electromagnetic interference (EMI) or radio frequency interference (RFI) present in the power line.  

Conducted EMI (or RFI) refers to unwanted electromagnetic signals emitted from an electrical or electronic device during operation, such as noise currents at radio frequencies. These signals travel via physical conductors like power lines or signal lines/PCB traces and can interfere with or damage other connected devices or equipment on the same line

The conducted EMI or high-frequency noise is caused by various man-made sources such as switching power supplies, electric motors, radio frequency (RF) transmitters, and other electronic devices, as well as due to natural events, such as lightning, electrical storms, and solar radiation. 

The purpose of the power line filter is to allow power frequency (DC or 50/60/400 Hz) current to pass freely while attenuating high-frequency noise current passing through the power lines or wires. 

By suppressing the high-frequency noise current, the filter protects sensitive electronic devices/systems connected to the line from the harmful impacts of such noise current, ensuring the reliable operation of these devices. The filter is connected between the input power line and the equipment or load. The filter prevents high-frequency noise (i.e., conducted EMI or RFI) from entering into or exiting from a device, keeping the device running smoothly without disrupting other connected equipment. 

Figure: Power line filter prevents EMI from entering into equipment and also prevents EMI emission leaking out from the equipment 

When carrying high-frequency noise currents, power lines can act as unintentional antennas, radiating electromagnetic energy, and potentially causing interference, such as to AM radio signals. By suppressing the noise current, the power line EMI filter prevents the power lines from acting as unintentional antennas, thereby reducing the risk of electromagnetic interference. 

The power line EMI filter is also known as EMC filter (Electromagnetic Compatibility filter)/ EMI filter (Electromagnetic interference filter) / RFI filter (radio frequency interference filter). The power line filter ensures that the connected electrical or electronic equipment operates without interference and is in compliance with electromagnetic compatibility (EMC) standards. The filter usually comes with a shield to protect circuits from radiated EMI.  Often, this filter is integrated into electrical and electronic devices or power supplies, or it can be used as a separate device. These filters are available to attenuate common mode and differential mode interference.

Construction and working principle of power line EMI filter:

A power line EMI filter is typically designed as a passive filter that consists of passive components such as capacitors, inductors, and resistors configured in a specific arrangement. The passive filter design can use various filtering circuit topologies, including L filter, C filter, LC filter, CL filter, T – filter, and Pi filter.  A simple EMI filter design uses LC filter topology. Figure shows a simple passive filter for single-phase application. 

Figure: A simple power line filter topology for single-phase application 

The EMI power line filter works as a low-pass filter that attenuates high-frequency noise in the power line while allowing the desired power frequency (DC or 50/60/400Hz) current to pass through freely. The filter works by diverting the high-frequency noise current away from the device to be protected to the ground. Some filters route the noise currents back to the EMI noise source or absorb them. After the filtering process, the filter provides clean output to the connected device. Cascading multiple filters can provide increased attenuation and sharper attenuation curves.

Types of power line EMI filters:

Depending on the network type or application, the power line filters are implemented with a different number of lines. 

• 2-line filters are ideal for DC or single-phase applications. 
• 3-line filters (no neutral conductor) ideal for three-phase AC applications
• 4-line filters (with neutral conductor) ideal for three-phase AC applications.

Where to place the power line EMI filter?

For EMI compliance, the power line EMI filter is placed at the power entry point (POE) of the equipment or device.  The filter is mounted in such a way that its metal case makes direct contact with the equipment enclosure. This arrangement prevents the conducted noises from exiting or entering the equipment effectively, thereby helping to comply with EMC standards.

Figure: Proper and improper mounting of power line filtersApplications:

The power line EMI filters or EMC/EMI filters are used in various applications, including the following:

• SMPS 
• Power supplies
• Computers
• Telecommunication equipment
• Home appliances
• Industrial machinery
• Medical devices
• UPS systems
• VFD 
• Test and measurement devices
• EMC testing chambers
• Energy management systems, military/aerospace systems, etc.

Power line EMI filter’s applications in EMC testing:

EMI Power line filters are one of the essential components in EMC (Electromagnetic Compatibility) testing setups, helping to ensure accurate and reliable testing by minimizing interference from external sources. Here are some common applications of power line filters in EMC testing are discussed below.

Anechoic Chambers: Anechoic chambers are used for EMC testing to provide a perfectly controlled electromagnetic environment free from external electromagnetic interference. The Power line EMI filter is placed at the power entry point (POE) of the chamber to attenuate unwanted conducted EMI noises or unwanted signals in the power lines. The filter helps to ensure accurate and reliable EMC testing.

Figure: Power line EMI filter is placed at the power entry point (POE) of the chamber

Semi-anechoic Chamber: Semi-anechoic chambers are used in EMC testing to provide an interference-free controlled electromagnetic environment with reduced reflections (i.e., simulating real-world environment). The Power line EMI filter is placed at the POE of the chamber to attenuate unwanted conducted noises or unwanted signals in the power lines. It ensures a noise-free clean power supply to the chamber for performing EMC testing.

Shielded cabinets: The shielded cabinets are used to provide an isolated interference-free environment for performing EMC testing of electronics. The Power line EMI filter is installed at the POE of the cabinet to attenuate unwanted conducted electromagnetic noises or unwanted signals in the power lines. It ensures a noise-free clean power supply to the shielded cabinets for performing EMC testing.

Laboratory Testing Equipment: Power line filters can be used in various testing equipment and instrumentation used in EMC laboratories, such as signal generators, spectrum analyzers, and oscilloscopes. By filtering out noise from the power supply, these filters help maintain the accuracy and reliability of measurements during EMC testing procedures.

Overall, power line filters play a crucial role in EMC testing environments by attenuating external conducted electromagnetic interference, ensuring accurate test results, and maintaining the integrity of testing setups and equipment.

How to select a power line EMI filter?

Number of Lines: The EMI power line filters are available in various options, such as 2-line filters (DC or single phase), 3-line filters (3-phase delta), or 4-line filters (3-phase wye). Select the appropriate filter based on the application requirement.

Rated Voltage (V): The filter should have a rated voltage more than or equal to the maximum line voltage applied to the equipment or load. 

Current Rating (A): The filter should have a current rating of more than or equal to the maximum current of the device or load.

Leakage current: When the filter is connected to an AC power source, the leakage current is the small current (in the mA range) from the 'line' and 'neutral' to the 'ground' due to the small current flow via the filter's line to ground (Y) capacitors. Consideration of leakage current is essential to avoid compliance issues with safety standards.

Insulation resistance (MΩ): Selecting a power line EMI filter with high insulation resistance is important to minimize leakage current and ensure the safety and reliability of electrical systems. 

Insertion loss: Selecting a filter with higher insertion loss provides greater attenuation of EMI signals.  The insertion loss is typically measured in dB.   

Package type: The power line EMI filters come in various package types, including Screw Mount, DIN Rail Mount, Chassis Mount, PCB Mount, and Panel Mount. The physical size and form factor of the filter also need to be considered, particularly in space-constrained applications.

EMC Directory has listed Power Line EMI Filters from multiple manufacturers. Click here to view power line EMI Filters.