Board mount EMI and RFI filters are electronic filters which mount on a printed circuit board (PCB). They attenuate radio frequency interference (RFI) or electromagnetic interference (EMI).

Electromagnetic Interference

Electromagnetic interference (EMI) can be classified into two types: conducted EMI, which derives from components and wiring within a circuit; and radiated EMI (or radio frequency interference), which travels through space as electromagnetic waves. EMI filters like those discussed here typically suppress conducted EMI, while radiated interference is corrected using shielding materials.

Conducted EMI is subdivided into common-mode noise (CMN) and differential-mode noise (DMN). EMI filters are designed to attenuate both noise types.

CMN, also known as symmetrical or line-to-ground noise, appears on both wires connected to the filter. In a three-wire power configuration, CMN would flow in the same direction on both power conductors and return via the ground wire. This type is caused by lightning, switching, and poor grounding. Filters suppress CMN using inductors and Y-capacitors.

DMN, also known as normal, symmetrical, or line-to-line noise, exists between two active conductors and is 180° out of phase with itself. DMN flows one way along a conductor and returns via another conductor in the opposite direction. Switching of large loads often causes DMN; X-capacitors within filters are frequently used to attenuate it.

Sources and Damaging Effects of Noise

EMI can be generated by almost any electronic or electrical device. Devices which switch or regulate current—such as switch-mode power supplies (SMPS), lighting dimmers, AC motors, and lamps—are particularly susceptible to unwanted EMI.

The image below shows the essential components of a typical switch-mode power supply; these devices are commonly used to power computers and other electronic devices. Unregulated interference in a SMPS circuit can damage the device in a number of ways.

A block diagram of a switch-mode power supply. Image credit: MasTec

Normal-mode interference is capable of doing gradual damage to power supply components, eventually causing the breakdown of the power supply itself as well as other related components. For example, noise of sufficient strength may break down capacitors, the p-n junctions within a rectifier, and transformer insulation. Failure of any of these components may lead to failure of the power supply.

Common-mode interference is capable of significantly more damage due to the fact that it continuously seeks a path from an active conductor to the ground wire. Two of these possible paths—high-frequency conductance through the transformer and on a stray capacitance path around densely packed components—exist within a SMPS. If CMN is conducted through the power supply, it will likely find its way into a computer's logic hardware, compromising processing and possibly destroying semiconductor components. Because these components can only withstand small amount of voltage and current, even minor CMN is enough to cause irreparable damage.

Design

A typical EMI filter schematic is shown below. As discussed above, filters typically use a combination of inductors and capacitors to dissipate CMN and capacitors alone to eliminate DMN.

Schematic view of a typical EMI filter. Image credit: Electronic Products

In this example, the pair of inductors (L_CM) eliminates most in-phase common-mode interference, while high- frequency noise is shunted to ground by a pair of capacitors (C_Y). The pair of C_X capacitors neutralize and suppress out-of-phase DMN between the active lines and are discharged after power is turned off.

Specifications

Form Factor

Board mount EMI and RFI filters are designed for mounting to a printed circuit board (PCB) or feedthrough. Three specific form factor types are listed below.

• Surface mount (SMT) devices are placed directly onto the surface of a board. Surface mount filters allow for smaller component and board sizes as well as operation at higher frequencies.
• Through hole (THT) devices use soldered leads or pins to connect to boards. THT is an older technology which results in larger components and boards.
• Feedthrough filters mount to a feedthrough using pins.

(left to right) An SMT, THT, and feedthrough filter.  Image credit: API Technologies Corp | Schaffner

Numerical Specs

When selecting a board mount filter, it is important to consider the device's rated current and voltage to ensure that they match the intended circuit or system.

Insertion loss is an important specification which defines the ratio between a signal without the filter installed and a signal with the filter installed. Insertion loss is effectively a filter's rated attenuation characteristic. The value, which is expressed in decibels (dB), can be found using the following formula.

where:

V₁ = signal level without filter

V₂ = signal level with filter

In most filters, V₂ is smaller than V₁, resulting in a positive value and establishing insertion loss as the measure of signal reduction during filter use.

Standards and Certifications

Board mount filters may be designed, tested, and used based on published standards and specifications. Example standards include:

• BS EN 60939 Passive filters for electromagnetic interference suppression (series)
• MIL-PRF-15733 General specification for radio frequency interference filters
• IEEE 1560 Standard measurement method of RF power line interference filters (100 Hz to 10 GHz)

Certifications

RoHS and WEEE certifications may be applied to qualifying EMI filters. 

The Restriction of Hazardous Substances directive (RoHS) is a European Union order which restricts the use of six hazardous materials in the production of electronic and electrical devices. RoHS products contain less than 0.1% of the following:

• Lead
• Mercury
• Cadmium
• Hexavalent chromium (Cr⁶⁺)
• Polybrominated biphenyls (PBB)
• Polybrominated diphenyl ether (PBDE)

This directive protects the health of electronics industry workers and those in close proximity to areas where high-tech waste is stored. Despite initial concerns about the reliability of lead-free solder, RoHS products have been favorably tested to withstand harsh environmental conditions and severe vibration and shock requirements.

While RoHS compliant products do not carry a standard mark or seal, the image at right presents a typical example.

The Waste Electrical and Electronic Equipment directive (WEEE) became European Law in 2003, at the same time as the RoHS directive. WEEE legislation sets collection, recycling, and recovery targets of electrical products, with minimum amounts for recycling recovery. Compliant manufacturers and distributors must provide users with the possibility of returning household electronic equipment to them for recycling free of charge.

References

IEE Transactions on Power Electronics - A Procedure for Designing EMI Filters for AC Line Applications (pdf)

Image credit:

Schaffner EMC Inc. | Advanced Digital Cable Inc. | MasTec | Electronic Products >| Wikimedia | European Union