EMI/RFI Suppression Components Control Common-Mode Interference

The electronic revolution is in full swing as companies incorporate digital devices into every conceivable product from rugged engine controllers in automobiles to delicate medical equipment in hospitals. Everyday, manufacturers are developing new and faster electronic components that perform more functions more easily than their predecessors.

Many of these devices must perform in environments where there is a lot of electromagnetic interference. It is up to the engineer to determine the cause of the EMI and to select the suppression components that will filter out the offending frequencies. This process begins by considering the radiated and conducted forms of EMI.

Conducted EMI occurs in power lines in the common and differential modes. Common-mode interference is an unwanted signal comprised of currents flowing in the same direction along two or more conductors. Differential-mode interference is present between the line and neutral conductors, and is caused by currents flowing in opposite directions on the conductors. Common-mode currents flowing in a multiconductor cable or a tightly coupled grouping of PCB conductors induce a magnetic flux around the conductors. Differential-mode signals, however, do not produce interference because there is a zero net magnetic flux around the conductor pairs.¹

Common-mode EMI cannot be eliminated, but it can be controlled. Designers can implement filtering of conducted EMI or shielding of radiated emissions to reduce interference.

But what are the important criteria when considering suppression components? The critical performance parameters are decibel and frequency, said Ken Raina, President of NexTek. MIL-STD-220A, Method of Insertion Loss Measurement, is the benchmark for filter performance measurement.

The right filter is chosen by carefully considering the size, weight, cutoff frequency, passband, insertion loss, voltage rating and current capability. The size may be restricted by space or by the dimensions of the mounting surface. The weight is governed by the application.

The passband of the filter must be large enough to ensure proper operation of the rest of the system. The cutoff frequency should be chosen to adequately reject all undesired frequencies.

The filter performance is quantified in terms of attenuation or insertion loss. Typically, published insertion-loss data assumes that the power line and load have the same impedance and all data is generated from a 50-W circuit. Since this is rarely the case in real-world applications, specified insertion loss should be considered as a reference for comparison among different filters, or an evaluation of product conformity at incoming inspection.²

Simple filters use a frequency-selective impedance mismatch between a circuit’s source and load. Since maximum power is transferred from the source to the load when the magnitude of the source and load impedances are equal, an effective filter element must present an impedance that is orders of magnitude larger or smaller than that of the source or load.

The high-frequency performance of filters is limited by the non-ideal characteristics of filter components. In inductors, the interwinding capacitance present between turns establishes a series resonance. The impedance of the inductor decreases when frequency exceeds the series resonance.¹

The distributed-element model of any EMI suppressor is important to a designer, said Ron Demcko, Applications Engineering Manager at AVX. The significant element values are inductance, capacitance and resistance. Capacitance is meaningful because it sets the self-resonant frequency of the filter, while inductance affects the resonant trough and Q (the ratio of reactance to loss) of the filter. Resistance values influence the forward transmission loss of the filter.

When choosing an EMI suppression component, the first factor to consider is how much attenuation is required and at what frequency, said Ernest Niemisto, Chief Engineer at MMC Electronics. For example, capacitors and inductors suffer from self resonance, while chip ferrite beads become lossy at high frequencies. The combination of chip capacitors or feed-through capacitors with chip ferrite beads typically offers the best of both worlds (performance and package size).

The addition of series elements comprised of inductors and resistors is an easy and inexpensive technique to reduce interference, said Mr. Demcko. However, the interference is only reduced by -3 dB to -10 dB.

Filtering with capacitors and inductors is another method that attenuates EMI as much as -100 dB, continued Mr. Demcko. But size, weight and complexity limit the use of this method. The SMT feed-through capacitor is an excellent alternative because it eliminates these problems and offers an acceptable attenuation of -60 dB.

Capacitive filters are obviously dependent on capacitance, but inductance is also a factor, said Mr. Niemisto. The self-inductance of a capacitive filter may lower the resonant frequency of an EMI filter. It may reach a point where it not only eliminates the interference, but also cuts off the frequencies of interest.

Usually, it is better to use a lower value capacitor and add an inductor for additional rolloff, he said. Surface-mount feed-through capacitors used in conjunction with chip ferrite beads are very useful in EMI filtering, especially for small circuit-board applications.

For shielding of radiated EMI, the most important specification is conductance, said Mr. Niemisto. Other important product specifications are formability and sealing characteristics. However, EMI filters can help with both radiated and conducted interference by cleaning the signals before they enter the circuit.

Performance vs Specification

Filter performance is influenced by source and load impedances and load current effects, said Mr. Raina. A source impedance of < 50 W , or a load impedance of >50 W reduces the expected filtering effectiveness. The situation is mitigated by parasitic interconnection inductance or by using a discrete inductor between the source and capacitor, he said.

Filters that use series inductors can be susceptible to saturation, continued Mr. Raina. Saturation reduces the inductance of the circuit and dramatically reduces insertion loss. For best practice, use components evaluated at full-load insertion loss.

Attenuation depends on the existing circuitry, noted Mr. Niemisto. For example, long cable lengths or signal traces may add enough inductance to allow just a capacitor to be added for effective filtering.

In many applications, EMI suppression can be difficult to design, said John Nemec, Director of Applications at California Micro Devices. The designer has to model the entire system, including signal sources and radiators, to determine what frequencies cause problems.

The radiating elements such as the cable and the signal sources also must be modeled, continued Mr. Nemec. Software, such as Hewlett-Packard’s Momentum and Ansoft’s Maxwell, predicts the radiated fields from the physical environment. This information, combined with spectral analysis of the sources, can identify the frequency components that must be filtered.

Then selection of an EMI filter is based on s-parameter frequency response characteristics, said Mr. Nemec. The designer uses a program such as SPICE to model the signal sources, the filter and the radiating elements and to determine if the desired suppression was achieved.

In simple cases, matching the characteristics of the filter to the characteristics of the circuit may be enough. In extreme cases, however, the combination of the signal path and the EMI filter may cause an unexpected result. The best way to prevent EMI is to fix it during the design phase rather than to add a bandage in production, said Mr. Niemisto.

References

1. “EMI Suppression Ferrites, Product and Applications Guide,” Steward, 1993.

2. “EMI/RFI Power Line Filters,” Product Guide Brochure #1F, Qualtek Electronics Corp., 1994.

Suppression Components

Multiple RJ45 Jack Connectors

Available With Filtering

The Multiport Modular Jack Series of filtered RJ45 modular jack connectors protects equipment from conducted and radiated EMI. They are interchangeable with existing standard products. Filtering options include ferrite or capacitive film. The series provides from one to eight connector positions. Typical insertion loss is 35 dB at 200 MHz. Applications include LANs, WANs, network cards, broadband transmission equipment, fax/modems, PCs and copy machines. AMP, (800) 522-6752.

Feed-Through Capacitors Used

For Broadband I/O Filtering

The 1206 Feedthru Capacitors are an addition to the W3F Series and are used for EMI suppression, broadband I/O line filtering and high-impedance data lines. They are standard EIA sizes and fit 8-mm tape-and-reel equipment. Capacitance values range from 22 pF to 22,000 pF with voltage ratings of 50 V and 100 V and a current rating of 200 mA or 300 mA. AVX, (919) 878-6200.

Surface-Mounted Filter

Suppresses Noise at I/O Ports

The IPEC T Series EMI/RFI Filter suppresses noise at I/O ports of PCs, peripherals, workstations, LANs, WANs and ATM networks. It features eight EMI/RFI protection lines per package. The filter has a low parasitic inductance and suppresses noise at frequencies >1,000 MHz. The power rating is 100 mW and insulation resistance is >100 MW . Applications include low-pass filtering and an LCD panel filter. It is available as a 20-pin SOIC or QSOP. California Micro Devices, (408) 946-9111.

Two Capacitors Placed

On Single Package

The Dual Chip Capacitor has two capacitors on a single 0805 package. It can be used with a ferrite bead to create a Pi filter. Crosstalk between the capacitors ranges from 70 dB @ 10 MHz to 40 dB @ 1 GHz. The chip size is 1.25 mm ´ 2.0 mm ´ 0.8 mm. MMC Electronics America, (800) 323-6237.

Feed-Through Filter Reduces

Noise from 6 kHz to >1 GHz

The HPR family of feed-through filters accommodates high-current applications. It has current ratings of 32 A, 63 A, 125 A and 250 A. Voltage ratings are 50 VDC, 100 VDC, 200 VDC and 500 VDC. The filter reduces noise at frequencies from 6 kHz to >1 GHz. It features a bolt-on style installation and a wire-bus hookup. Applications include power-supply output, power-system input, generator or bus lines, telephone or medical systems, computers, military equipment and test equipment. NexTek, (508) 486-0582.

Power-Line Filters Meet

FCC and VDE Requirements

A series of EMI/RFI power-line filters meets international safety-agency requirements. The filters also help customers meet FCC and VDE emission standards. The series consists of many standard configurations for power-line connectors and multifunction modules as well as block types and medical versions. Qualtek Electronics, (216) 951-3300.

Filter Integrated

With Power-Entry Module

The COMBIFIT integrates the functions of power-entry and line filters for use on PCBs. The filter snaps onto the rear of the power-entry module. The design eliminates potential crosstalk between filter components. AC interference is reduced from 10 kHz to 300 MHz. A second module attaches for dual-stage filtering. It is approved by UL, CSA, VDE, SEMKO and SEV. Schurter, (800) 848-2600.

Voltage Suppressor Diverts

And Attenuates Disturbances

The STABILINE^® Power Quality Interface diverts and attenuates all modes of power disturbances before they reach the computer circuitry. The rack-mount unit has a multistage suppression and filtration design and uses MOVs, gas tubes, inductors and filter capacitors. It provides bidirectional protection from source or load power disturbances. It is available in two versions: Model PQI-2115R is designed for 120-VAC, 60-Hz single-phase operation and Model PQI-2310R is for 220/240-VAC, 50-Hz single-phase operation. Loads up to 10 A can be connected to six conditioned output receptacles. Superior Electric, (203) 585-4500.

June 1996