Although common mode chokes are popular, an alternative might be a monolithic EMI filter.When properly laid out, these multilayer ceramic components provide excellent common-mode noise rejection.
Many factors increase the amount of “noise” interference that can damage or interfere with the functionality of electronic equipment.Today’s cars are a prime example.In a car, you’ll find Wi-Fi, Bluetooth, satellite radio, GPS systems, and that’s just the beginning.To manage this noise interference, the industry typically uses shielding and EMI filters to eliminate unwanted noise.But some traditional solutions to eliminate EMI/RFI are no longer enough.
This problem leads many OEMs to avoid using 2-capacitor differential, 3-capacitor (one X capacitor and 2 Y capacitors), feedthrough filters, common mode chokes, or a combination of these for a more suitable solution such as a monolithic EMI filter with better noise rejection in a smaller package.
When electronic equipment receives strong electromagnetic waves, unwanted currents can be induced in the circuit and cause unintended operation – or interfere with intended operation.
EMI/RFI can be in the form of conducted or radiated emissions.When EMI is conducted, it means that noise travels along electrical conductors.Radiated EMI occurs when noise travels through the air in the form of magnetic fields or radio waves.
Even if the energy applied from the outside is small, if it mixes with the radio waves used for broadcasting and communication, it can cause loss of reception, abnormal noise in sound, or interruption of video.If the energy is too strong, it can damage electronic equipment.
Sources include natural noise (eg, electrostatic discharge, lighting, and other sources) and man-made noise (eg, contact noise, leaking equipment using high frequencies, unwanted emissions, etc.).Typically, EMI/RFI noise is common mode noise, so the solution is to use an EMI filter to remove unwanted high frequencies, either as a separate device or embedded in a circuit board.
EMI Filters EMI filters typically consist of passive components, such as capacitors and inductors, that are connected to form a circuit.
“Inductors allow DC or low-frequency current to pass through while blocking unwanted, unwanted high-frequency currents. Capacitors provide a low-impedance path to divert high-frequency noise from the filter’s input to the power or ground connection,” Manufactures a Multilayer Ceramic said Christophe Cambrelin of capacitor company Johanson Dielectrics.EMI filter.
Traditional common-mode filtering methods include low-pass filters using capacitors that pass signals with frequencies below a selected cutoff frequency and attenuate signals with frequencies above the cutoff frequency.
A common starting point is to apply a pair of capacitors in a differential configuration, with one capacitor between each trace of the differential input and ground.Capacitive filters in each leg divert EMI/RFI to ground above the specified cutoff frequency.Since this configuration involves sending signals of opposite phases over the two wires, the signal-to-noise ratio is improved while unwanted noise is sent to ground.
“Unfortunately, the capacitance value of MLCCs with X7R dielectrics (typically used for this function) can vary significantly with time, bias voltage and temperature,” Cambrelin said.
“So even though two capacitors are closely matched at a given time at room temperature at a low voltage, they are likely to end up with very different values once the time, voltage or temperature changes. This mismatch between the two wires will result in unequal responses near the filter cutoff. Therefore, it converts common-mode noise to differential noise.”
Another solution is to bridge a large value “X” capacitor between the two “Y” capacitors.The “X” capacitive shunt provides ideal common-mode balance, but also has the undesirable side effect of differential signal filtering.Perhaps the most common solution and an alternative to a low pass filter is a common mode choke.
A common mode choke is a 1:1 transformer with both windings acting as primary and secondary.In this method, the current through one winding induces an opposite current in the other winding.Unfortunately, common mode chokes are also heavy, expensive, and susceptible to vibration-induced failure.
Nonetheless, a suitable common mode choke with perfect matching and coupling between the windings is transparent to differential signals and has 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 low frequency filtering, the more turns are required, thus resulting in parasitic capacitances that cannot pass high frequency filtering.
Mismatches between windings due to mechanical manufacturing tolerances cause mode switching, where a portion of the signal energy is converted to common mode noise and vice versa.This situation can cause electromagnetic compatibility and immunity issues.The mismatch also reduces the effective inductance of each leg.
In any case, common mode chokes do offer significant advantages over other options when the differential signal (pass through) operates in the same frequency range as the common mode noise that must be rejected.Using a common mode choke, the signal passband can be extended to the common mode rejection band.
Monolithic EMI Filters Although common mode chokes are popular, monolithic EMI filters can also be used.When properly laid out, these multilayer ceramic components provide excellent common-mode noise rejection.They combine two balanced shunt capacitors in one package for mutual inductance cancellation and shielding.These filters use two separate electrical paths within a single device connected to four external connections.
To avoid confusion, it should be noted that monolithic EMI filters are not traditional feedthrough capacitors.Although they look the same (same packaging and appearance), they are very different in design, and they are not connected in the same way.Like other EMI filters, monolithic EMI filters attenuate all energy above the specified cutoff frequency and select to pass only the desired signal energy, while diverting unwanted noise to “ground”.
However, the key is very low inductance and matching impedance.For monolithic EMI filters, the terminals are internally connected 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 a common reference electrode, all contained within a single ceramic body.
The balance between the two halves of the capacitor also means that the piezoelectric effects are equal and opposite, canceling each other out.This relationship also affects temperature and voltage variation, so components on both lines age equally.If there’s one downside to these monolithic EMI filters, it’s that they won’t work if the common-mode noise is at the same frequency as the differential signal.”In this case, a common-mode choke is a better solution,” Cambrelin said.
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Post time: Apr-19-2022
