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Filtering common-mode noise in automotive environments

Feature

By Jeff Elliott, technical writer

With growing design complexities in shrinking packages, several factors are combining to increase the amount of noise interference that impair the design’s functionality and even damage components.

Prime example are today’s automobiles. In a single vehicle there are Wi-Fi, Bluetooth, satellite radio, GPS systems, LED lights, air conditioning, power steering, anti-lock brakes, rear-view cameras and other systems that continue to grow in number and functionality. Many in-vehicle systems need DC motors, to power seats, windows, sunroofs, mirrors, windshield wipers and more.

To prevent interference between them, the industry typically employs shielding along with EMI filters in various configurations. Sadly, some of the traditional solutions for eliminating EMI/RFI are no longer sufficient, given increases in operating circuit frequencies, higher-frequency noise generation and the reduced distances between noise-impacting components.

If that wasn’t enough, many electronic devices are more sensitive to noise, even at low power, due to circuits operating at lower voltages. 

This has led OEMs to abandon certain options such as two-capacitor differentials, three-capacitor (one X-cap and 2 Y-caps) feed-through filters, common-mode chokes or any combination of these, for more appropriate solutions such as monolithic EMI filters that deliver superior noise suppression in substantially smaller packages.

EMI/RFI noise

Electromagnetic waves can cause unwanted currents in electronic devices, interfering with functions or causing unwanted operations altogether.

EMI/RFI emissions can be conducted or radiated. When conducted, the noise travels along the electrical conductors, whereas radiated EMI occurs when noise travels through the air as magnetic fields or radio waves.

Even if the energy applied from the outside is low, if mixed with broadcasting and communication radio waves, it can cause loss of reception, noise in the sound, or disrupted video. If stronger, this energy can damage electronic devices.

Sources of noise can be of natural origin, such as electrostatic discharge, lighting and other sources, or artificial, such as contact noise, leakage from high-frequency devices, unwanted emissions (e.g. harmonic emission from digital circuits, emission from switching power supplies), and others. Electronic devices also generate noise internally, causing interference to neighbouring circuits.

Usually, EMI/RFI noise is common-mode noise, so the solution will be to eliminate unwanted high frequencies with an EMI filter, either as a separate device or embedded in the circuit board. This also meets regulatory standards, which limit the amount of noise that can be emitted.

EMI filters

EMI filters normally consist of passive components, such as capacitors and inductors.

“The inductors allow DC or low-frequency currents to pass through, whilst blocking harmful, unwanted high-frequency currents. The 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,” explains Christophe Cambrelin of Johanson Dielectrics, a company that manufactures a variety of multi-layer ceramic capacitors and EMI filters.

Traditional common-mode filtering approaches include low-pass filters comprising 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 differential input. The capacitive filter in each leg diverts EMI/RFI to ground above a specified cutoff frequency. Because this configuration involves sending an opposite-phase signal through two wires, the signal-to-noise ratio is improved with unwanted noise sent to ground.

“Unfortunately, the capacitance value of a multi-layer ceramic capacitor with X7R dielectric (typically used for this function) varies significantly with time, bias voltage and temperature,” said Cambrelin. “So, even if the two capacitors are tightly matched at room temperature, with a low voltage, at a given time it’s very likely they end up with a very different value once time, 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 common-mode balancing – but, with the undesired side effect of differential signal filtering.

Common-mode chokes

Perhaps the most common solution and an alternative to low-pass filters is the common-mode choke. This is a 1:1 transformer where both windings act as both primary and secondary. 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 vibration-induced failure. Still, an ideal common-mode choke with perfect matching and coupling between the windings is completely transparent to differential signals, and offers 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 filtering, the greater the number of turns required and consequent parasitic capacitance that defeats high-frequency filtering.

Mismatch between windings from mechanical manufacturing tolerances can cause mode conversion, where a percentage of the signal energy converts to common-mode noise and vice versa, giving rise to electromagnetic compatibility and immunity issues. Mismatches also reduce the effective inductance in each leg.

Common-mode chokes do have a major advantage over other options when differential signals (to let through) 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 reject band.

Monolithic EMI filters

There are alternatives to common-mode chokes in the form of monolithic EMI filters. When properly laid out, those multilayer ceramic components provide superior rejection of common-mode noise. They combine two balanced shunt capacitors in a single package, with mutual inductance cancellation and shielding. These filters from Johanson Dielectrics use two separate electrical paths within a single device attached to four external connections.

It should be noted, however, 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, and only let through required signals whilst diverting others – including 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 a reference electrode.

Electrostatically, the three electrical nodes are formed by two capacitive halves that share common reference electrodes all contained in 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 piezo-electric effects are equal and opposite, canceling out.

“Compared to the common mode choke solution, this device provides significantly more RFI suppression in a substantially smaller package. It also rejects a much wider frequency band,” said Cambrelin.

The downside to monolithic EMI filters is that they can’t be used in setups where the common-mode noise is at the same frequency as the differential signal. In those cases, the common-mode choke is a better solution.

And on the final note, although monolithic EMI filters initially cost more than equivalent ordinary capacitors, their cost is a fraction of that of a common-mode choke.

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