June, 2019

Page 57

Filtering out Common Mode Noise in Congested Environments with Monolithic EMI Filters

Continued from previous page

ter, either back into the power supply or to the ground connection,”

explains Christophe

Cambrelin of Johanson Dielectrics, a company that manufactures a variety of multilayer ceramic capacitors and EMI filters. Traditional common mode filtering approach-

es include low pass filters that consist of capacitors that pass signals with a frequency lower than a selected cutoff frequency and attenuate signals with frequencies higher than the cutoff frequency. A common starting point is to apply a pair of

capacitors in a differential configuration, with one capacitor between each trace and ground of the dif- ferential input. The capacitive filter in each leg diverts EMI/RFI to ground above a specified cutoff frequency. Because this configuration involves sending a signal that is opposite in phase through two wires, the signal-to-noise ratio is improved, while unwanted noise is sent to ground. “Unfortunately, the capacitance value of an

MLCC with X7R dielectric (typically used for this function), varies significantly with time, bias volt- age and temperature,” explains Cambrelin. “So even if the two capacitors are tightly matched at room temperature with a low voltage, at a given time, it is very likely that they end up with a very different value if time or voltage or temperature have changed. This mismatch between the two lines will cause the response near the filter cutoff to be unequal and therefore it will convert common mode noise to differential noise.” Another solution is to bridge a large value “x”

capacitor across the two “y” capacitors. The “x” capacitor shunt delivers the desired effect of com- mon mode balancing, however, with the undesired side effect of differential signal filtering. Perhaps the most common solution and an

alternative to low pass filters is the common mode choke. A common mode choke is a 1:1 transformer, where both windings act as both primary and sec- ondary. In this approach, current through one winding induces an opposing current in the other

winding. Unfortunately, common mode chokes are also large, heavy, expensive, and subject to vibra- tion-induced failure. Still, an ideal common mode choke with perfect matching and coupling between the windings is completely transparent to differen- tial signals, and presents very high impedance to common mode noise. One disadvantage of common mode chokes is

the limited frequency range, due to parasitic capacitance. For a given core material, the higher the inductance used to obtain lower frequency fil- tering, the greater the number of turns required and consequent parasitic capacitance that defeats high-frequency filtering. Mismatch between windings from mechanical

manufacturing tolerance can cause mode conver- sion, where a percentage of the signal energy con- verts to common mode noise and vice versa. This gives rise to electromagnetic compatibility and immunity issues. Mismatches also reduce the effective inductance in each leg. Common mode chokes do have a major advan-

tage over other options when differential signals (to pass) operate in the same frequency range as the common mode noise that must be suppressed. With a common mode choke, the signal pass band can extend into the common mode reject band.

Monolithic EMI Filters Despite the popularity of common mode

chokes, a better alternative may be monolithic EMI filters. When properly laid out, these multi- layer ceramic components provide superior rejec- tion of common mode noise. They combine two bal- anced shunt capacitors in a single package, with mutual inductance cancellation and shielding effect. These filters from Johanson Dielectrics uti- lize two separate electrical pathways within a sin- gle device attached to four external connections. To prevent confusion, it should be noted that

a monolithic EMI filter is not a traditional feed- through capacitor. Although they look identical (same package and external look), their design is

very different and they are not connected in the same way. Like other EMI filters, monolithic EMI filters attenuate all energy above a specified cutoff frequency, selecting to pass required signal energy while diverting unwanted noise to ground. The key, however, is the very low inductance and matched impedance. With monolithic EMI filters, the terminations connect internally to a common reference (shield) electrode within the device, and the plates are separated by the reference electrode. Electrostatically, the three electrical nodes

are formed by two capacitive halves that share common reference electrodes all contained in a sin- gle ceramic body. “Being very well balanced, a monolithic EMI

filter introduces almost no conversion of common mode noise to differential signals, or vice versa. Furthermore, having a very low inductance makes it particularly effective at high frequencies,” says Cambrelin. The balance between capacitor halves also means piezoelectric effects are equal and opposite, canceling out. This also affects tempera- ture and voltage variation, so components age equally on both lines. “Compared to the common mode choke solu-

tion, this device provides significantly more RFI suppression in a substantially smaller package. It also rejects a much wider frequency band,” says Cambrelin. If there is a downside to these monolithic

EMI filters, it is that they cannot be used if the common mode noise is at the same frequency as the differential signal. “When this is the case, the common mode choke is a better solution,” says Cambrelin. “Although monolithic EMI filters initially

cost more than equivalent ordinary capacitors, our customers tell us that their cost is a fraction of the cost of a common mode choke alone.” Contact: Johanson Dielectrics, 4001 Calle

Tecate, Camarilla, CA 93012 % 805-389-1166 E-mail: Web: r

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