Conducted EMI are the high-frequency noise currents that can be generated during the electrical and electronic device's operation that travel via power lines or signal lines and can interfere with the operation of other connected devices.

An EMI feedthrough filter is an electronic component or circuit designed to suppress high-frequency noise currents (conducted EMI/RFI) on power and signal/data lines. By filtering out the noise currents, the filter protects electrical and electronic devices from electromagnetic interference (EMI)/radio-frequency interference (RFI), ensuring reliable device operation. 

Figure: EMI feedthrough filters

The circuit design of an EMI feedthrough filter can comprise a single feedthrough capacitor or capacitors and inductors combinations. These filters come in various circuit topologies, such as C, LC, Pi, and T circuits. The filter functions as a low-pass filter, diverting high-frequency noise currents to the ground while allowing desirable clean signals to reach the equipment or load. By diverting or attenuating noise currents, the filter prevents conducted EMI signals from entering and exiting the connected device via power or signal lines, ensuring smooth device operation without disturbing other nearby or connected devices. 

Note: Wires or lines can unintentionally function as antennas when high-frequency noise currents pass through them, emitting electromagnetic radiation into the surrounding environment. This radiated signal can interfere with other nearby devices, known as radiated EMI. Hence, the conducted EMI can cause for radiated EMI. Additionally, wires acting as unintentional antennas can pick up electromagnetic radiation from the surrounding environment and convert it into noise currents that interfere with the connected device. By suppressing high-frequency noise currents on power and signal/data lines, the EMI feedthrough filter prevents both conducted EMI and radiated EMI (caused by conducted EMI). Therefore, the EMI feedthrough filters improve the EMC performance of electrical and electronic equipment. 

These filters are typically installed at points where cables or conductors enter or exit a shielded enclosure or equipment metal chassis panel to attenuate both incoming and outgoing EMI noises. The filters come in various styles or configurations such as solder-in, bolt-in, press-in, hermetically sealed style, etc. In the case of bolt-in filters, the filter is installed on the shielded enclosure or equipment chassis panel using a nut and lock washer. For solder-in filter styles, the filter is directly soldered to the shielded case or panel. Also, SMT feedthrough filters are available for use with PCB designs. Individual line feedthrough filters and multiline feedthrough filters are the various package options of these filters.

What are Feedthrough Capacitors?

A feedthrough capacitor is a type of capacitor with three terminals. In this capacitor geometry, the ground electrode surrounds the dielectric, and the signal terminal (feedthrough terminal) passes through the dielectric. Typically, ceramic dielectric material is used in these capacitors. Compared to conventional capacitors, feedthrough capacitors have a different geometry, lower grounding inductance, and virtually no lead inductance on the signal terminal. These features enable feedthrough capacitors to suppress high-frequency noise currents or electromagnetic interference (EMI) more effectively than conventional capacitors.

Figure: Feedthrough capacitors

Types of Feedthrough Capacitors

• Tubular Ceramic Feedthrough Capacitors
• Discoidal (donut shape)  Feedthrough Capacitors
• Plastic Film Feedthrough Capacitors
• SMD Feedthrough Capacitors

These capacitors can be used alone or in conjunction with inductors to design RF filters or feedthrough filters in various circuit topologies such as C, LC, Pi, and T. When only one feedthrough capacitor (C) is used, it is known as a C-circuit feedthrough filter.

 Feedthrough filter circuit topologies:

The design of feedthrough filters can use the following circuit topologies. 

Capacitive filters or C filters:

Capacitive-type filters use a single feedthrough capacitor as a filtering element to attenuate high-frequency noises. This type of filter has an insertion loss equal to 20 dB / decade. It is a low-cost filter solution, ideal for use in applications with a high-impedance source and load. The C filter can be used when sharp variation in the insertion loss curve is not required.

Figure: C-filter

L-C filters:

This filter circuit comprises one inductive element and one capacitive element. The theoretical insertion loss of an L-C filter type equals 40 dB / decade. The L-C feedthrough filter is ideal for circuits or applications with varying source and load impedances. 

Figure: Pi-filter 

Pi-Filter:

Pi-type feedthrough filter features two capacitors and one inductive element in between them. This filter has a theoretical insertion loss equal to 60 dB / decade. This filter topology is ideal for circuits with high source and load impedances. This filter provides better high-frequency noise attenuation than C and L-C filter circuits.

Image Caption

Figure: Pi-filter

T-filters:

T-filter consists of two series inductors separated by one feedthrough capacitor. This filter has a theoretical insertion loss equal to 60 dB / decade.  It is ideal for circuits with low source and load impedances.

Figure: T-filter

Different styles or configurations of feedthrough filters:

Solder-In Filters:

The solder-in filter style is designed for directly soldering into a package, bracket, or bulkhead (shielding enclosure/wall). It is ideal for applications where space does not allow for the use of mounting tools or hardware. This filter style comes in C, L, and Pi circuit designs. They are often utilized in AC power lines and signal/data lines.

Miniature Solder-in Filters: This filter style is used in applications where space is at a premium.

Bolt-In Filters: This filter is designed for mounting in the through holes on the shielding wall using the standard nut and lock washer supplied. Miniature bolt-in style is also available.

Resin Sealed Filters: 

In a resin-sealed filter, both ends are sealed with resin to ensure effective environmental protection. This filter can be easily mounted with a tapped hole or through hole position using nuts and washers supplied. This filter style can consist of C, Pi, and L circuit designs. 

High Current Resin Sealed Filters:

The high current resin sealed filters feature a rugged bolt-in style for easy installation. They are ideal for use in high-current applications such as high-current switch mode power supplies and DC charging systems.

Figure: EMI feedthrough filter styles

Hermetically Sealed Filters: These filters feature hermetic glass seals to provide reliable operation in harsh environmental conditions such as high moisture, solvents, or other conditions commonly encountered in military applications. The hermetically sealed filter is designed for bulkhead mounting in a slotted hole using nuts and lock washers supplied. These filters are supplied with C, L, Pi, T, and double T circuit designs. 

Press-fit style: Inserted into a hole in a metal chassis or bulkhead and held in place by friction or a retention mechanism. This filter style allows for fast and secure mechanical bonding without the need for soldering.

Spec Spin Filters: These filters are used in applications where multiple lines need to be filtered in close proximity to one another.

Package options:

Ceramic feedthrough Filters come in various package options including individual line feedthrough filters and multiline feedthrough filters.

• Individual line filters are designed for individual signals or power lines. Examples of individual line filter packages include solder-in filters, bolt-in filters, press-in filters, and SMT filter configurations.  
• Multiline filters are designed to perform filter operation in multiple I/O or power lines. Examples of multiline filter packages include filtered terminal blocks, filtered Mil-circular connectors, D-sub connectors, and filter plates.

Key features:

• Available in compact sizes with various styles such as solder-in, bolt-in, press-in, hermetically sealed style, etc. 
• Offers high contact reliability
• Easy to install
• Come in AC feedthrough and DC feedthrough filter configurations.
• The feedthrough filters are available with a wide range of voltage, current rating, attenuation, and capacitance options.

Applications of feedthrough filters:

• AC/DC supply lines
• Telephone exchanges, base stations
• Secured rooms
• Power supplies (e.g., UPS)
• Broadband interference suppression up to GHz range.
• Machines and electrical systems
• Computers
• Instrumentation
• Radar
• PCMCIA Cards
• Medical instrumentation
• Test Equipment
• Transceivers/Cell Phones
• Used with electronics in aircraft, satellite telecommunications, military communications, etc.