A multitude of factors are conspiring to increase the amount of “noise” interference that can disturb the functionality of (even damage) electronic devices, starting with the sheer number used in our vicinity at any given time.
Today’s automobiles provide a prime example. In a single vehicle, you can find Wi-Fi, Bluetooth, satellite radio, GPS systems, LED lights, air conditioning, power steering, anti-lock brakes, rearview cameras, and other instrumentation. Numerous items also operate using DC motors, including power seats, adjustable mirrors, windshield wipers, power windows, and sunroofs.
Similar situations also apply to items ranging from Wi-Fi or Bluetooth-enabled appliances like washing machines to espresso machines to medical instruments and medical implants.
To mitigate against this interference, the industry has typically employed shielding along with EMI filters in various configurations to eliminate unwanted noise. However, some of the traditional solutions for eliminating EMI/RFI are no longer sufficient given increases in operating circuit frequency, noises of higher frequencies that expand the affected frequency range, and the miniaturization of electronic devices that shrinks the distance between source and victim.
If all of this wasn’t enough, many electronic devices are more easily affected by noise, even with less energy, due to today’s circuits operating at lower voltages.
This is leading many OEMs to avoid traditional options such as two-capacitor differential, three-capacitor (one X-cap and two Y-caps), feed-through filters, common-mode chokes, or combinations of these and instead use more effective solutions such as monolithic EMI filters that deliver superior noise suppression in a substantially smaller package.
When electronic devices receive strong electromagnetic waves, unwanted electric currents can be induced in the circuit and cause unintended operations or interfere with intended operations.
EMI/RFI can arrive in the form of conducted or radiated emissions. When EMI is conducted, it means that the noise travels along the electrical conductors. Radiated EMI occurs when noise travels through the air as magnetic fields or radio waves.
Even if the energy applied from the outside is small, if it is mixed with the radio waves used for broadcasting and communication, it can cause loss of reception, abnormal noise in sound, or disrupted video at places where the radio waves for broadcasting and communication are weak. If the energy is sufficiently powerful, electronic devices can be damaged.
Sources of noise includes natural causes (e.g., electrostatic discharge, lighting) and artificial originators like contact noise, leakage from devices that employ high frequencies, unwanted emissions (e.g., harmonic emissions from digital circuits, emissions from switching power supplies), and others.
Noise can even be generated from a circuit inside an electronic device itself, thereby causing interference with other circuits in the same device.
Usually, EMI/RFI noise is of the common-mode variety (i.e., present with one sign on all considered conductors), so the solution to all but eliminate unwanted high frequencies is to use an EMI filter, either as a separate device or embedded in the circuit boards. This also helps OEMs meet regulatory standards that exist in most countries that limit the amount of noise that can be emitted.
EMI filters normally consist of passive components, such as capacitors and inductors, connected together to form circuits.
“The inductors allow DC or low-frequency currents to pass through while blocking harmful unwanted high frequency currents,” explains Christophe Cambrelin of Johanson Dielectrics, a company that manufactures a variety of multi-layer ceramic capacitors and EMI filters. “The capacitors provide a low-impedance path to divert the high-frequency noise away from the input of the filter, either back into the power supply or to the ground connection.”
Traditional common-mode filtering approaches include low-pass filters comprised 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 differential 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 voltage, 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 likely that they will 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, 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, which is a 1:1 transformer in which 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 while presenting very high impedance to common-mode noise.
One disadvantage of common-mode chokes is 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 the high-frequency filtering.
Mismatch between windings from mechanical manufacturing tolerance can cause mode conversion, wherein a percentage of the signal energy converts 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 advantage 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 rejection of common-mode noise. They combine two balanced shunt capacitors in a single package with mutual inductance cancellation and shielding effect. Filters of this type, such as those from Johanson Dielectrics, utilize two separate electrical pathways within a single 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 may appear identical (e.g., same package and external look), their internal implementation 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 pass required signal energy while diverting unwanted noise to “ground.”
The real key to these components’ success, however, is the combination of 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 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,” says Cambrelin. “Furthermore, having a very low inductance makes it particularly effective at high frequencies.”
The balance between capacitor halves also means that piezo-electric effects are equal and opposite, thereby canceling out. This also affects temperature and voltage variation, so components “age” equally on both lines.
“Compared to the common-mode choke solution, this device provides significantly more RFI suppression in a substantially smaller package,” says Cambrelin. “It also rejects a much wider frequency band.”
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.”
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