Feature: EMI filters
“Te inductors allow DC or low-
frequency currents to pass through, whilst blocking the harmful unwanted high-frequency currents. Te capacitors provide a low-impedance path to divert high-frequency noise away from the filter’s input, either back into the power supply or to ground,” said Christophe Cambrelin of Johanson Dielectrics, a company that manufactures multi-layer ceramic capacitors (MLCC) and EMI filters. Traditional common-mode filtering
approaches include low-pass filters comprised of capacitors that pass a frequency lower than a selected cutoff frequency and attenuate signals with frequencies higher than the cutoff. 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 differential input. The capacitive filter in each leg diverts EMI/RFI to ground above a specified cutoff frequency. Because this configuration involves sending opposite-phase signals through two wires, the signal-to-noise ratio is improved, with the unwanted noise sent to ground. “Unfortunately, the capacitance
value of an MLCC with X7R dielectric (typically used for this function) varies significantly with time, bias voltage and temperature. So, even if the two capacitors are tightly matched at room temperature, using low voltage at any given time it’s very likely they’ll end up with a very different value, once voltage or temperature has changed. This mismatch between the two lines will cause unequal response near the filter cutoff, converting common-mode noise to differential noise,” said Cambrelin. Another solution is to bridge a large-
value X capacitor across the two Y capacitors. Te X capacitor shunt delivers the desired effect of common-mode balancing, although differential signal filtering is an undesirable side effect. Perhaps the most common solution
and an alternative to low-pass filters, is the common-mode choke – a 1:1 transformer where both windings act as primary and secondary windings. In this
Today’s automobiles are a prime example of growing electronics into smaller spaces, with many on-board instruments
approach, current through one winding induces an opposing current in the other. Unfortunately, they are also large, heavy, expensive devices and subject to vibration-induced failure. Still, an ideal common-mode choke
with perfect matching and coupling between the windings is completely transparent to differential signals, and presents very high impedance to common-mode noise. One disadvantage of common-mode
chokes is their limited frequency range due to parasitic capacitance. For a given core material, the higher the inductance used to obtain lower frequency filtering, the greater the number of turns required, leading to consequent parasitic capacitance that defeats high-frequency filtering. Mismatch between windings from
loose manufacturing tolerance can cause mode conversion, where a portion of the signal converts to common- mode noise, and, vice versa, giving rise to electromagnetic compatibility and immunity issues and ineffective inductance in each leg. However, a major advantage
of common-mode chokes is that 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 passband can extend into the common-mode rejection band.
Monolithic EMI filters Despite the popularity of common- mode chokes, a better alternative may be monolithic EMI filters. When
properly laid out, these multilayer ceramic components provide superior rejection of common-mode noise. Tey combine two balanced shunt capacitors in a single package, with mutual inductance cancellation and shielding effect. Johanson Dielectrics’s monolithic filters use two separate electrical pathways within a single device, with four external connections. To avoid confusion, it should be
noted that a monolithic EMI filter is not a traditional feedthrough capacitor. Although the two filters look identical (same package and external look), their design and connections are very different. Like other EMI filters, monolithic
EMI filters attenuate all energy above a specified cutoff frequency and only pass certain signals whilst diverting unwanted noise. The key, however, is their very low inductance and matched impedance. With monolithic EMI filters, the terminations connect internally to a common reference (shield) electrode within the device, separating the plates. Electrostatically, the three electrical nodes are formed by two capacitive halves that share common reference electrodes, all within a single 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,” said Cambrelin. The balance between capacitor halves
also means that piezoelectric effects are equal and opposite, cancelling out. This also affects temperature and voltage variations, so components age equally on both lines. “Compared with common-mode
choke solutions, this device provides significantly more RFI suppression in a substantially smaller package. It also rejects a much wider frequency band,” said Cambrelin, adding that if there’s a downside to monolithic EMI filters, it is that they can’t be used if the common- mode noise is at the same frequency as the differential signal. In those cases common-mode chokes are a better solution.
www.electronicsworld.co.uk April 2022 29
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