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Although common mode chokes are popular, another possibility is a monolithic EMI filter. If the layout is reasonable, these multilayer ceramic components can provide excellent common mode noise suppression.
Many factors increase the amount of “noise” interference that can damage or interfere with the functionality of electronic devices. Today’s car is a typical example. In a car, you can find Wi-Fi, Bluetooth, satellite radio, GPS systems, and this is just the beginning. In order to manage this kind of noise interference, the industry usually uses shielding and EMI filters to eliminate unwanted noise. But now some traditional solutions for eliminating EMI/RFI are no longer applicable.
This problem has caused many OEMs to avoid choices such as 2-capacitor differential, 3-capacitor (one X capacitor and two Y capacitors), feedthrough filters, common mode chokes, or combinations of these to obtain more suitable solutions. For example, in Monolithic EMI filter with better noise suppression in a smaller package.
When electronic equipment receives strong electromagnetic waves, unwanted currents may be induced in the circuit and cause unexpected operation-or interfere with the intended operation.
EMI/RFI can be in the form of conducted or radiated emissions. When EMI is conducted, it means that noise propagates along electrical conductors. When noise is propagated in the air in the form of a magnetic field or radio waves, radiated EMI occurs.
Even if the energy applied from the outside is small, if it is mixed with radio waves used for broadcasting and communication, it will cause reception failure, abnormal sound noise, or video interruption. If the energy is too strong, the electronic equipment may be damaged.
Sources include natural noise (such as electrostatic discharge, lighting, and other sources) and artificial noise (such as contact noise, use of high-frequency leakage equipment, harmful radiation, etc.). Generally, EMI/RFI noise is common mode noise, so the solution is to use EMI filters to eliminate unwanted high frequencies as a separate device or embedded in a circuit board.
EMI filter EMI filter is usually composed of passive components, such as capacitors and inductors, which are connected to form a circuit.
“Inductors allow DC or low-frequency current to pass, while blocking harmful unwanted high-frequency currents. Capacitors provide a low-impedance path to transfer high-frequency noise from the input of the filter back to the power or ground connection,” said Johanson Dielectrics Christophe Cambrelin said the company makes multilayer ceramic capacitors and EMI filters.
Traditional common-mode filtering methods include low-pass filters using capacitors that pass signals with frequencies 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, using a capacitor between each trace and the ground of the differential input. The capacitor filter in each branch transfers EMI/RFI to the ground above the specified cutoff frequency. Since this configuration involves sending signals of opposite phase through two wires, it improves the signal-to-noise ratio while sending unwanted noise to the ground.
“Unfortunately, the capacitance value of MLCCs with X7R dielectrics (usually used for this function) varies significantly with time, bias voltage, and temperature,” Cambrelin said.
“So even if these two capacitors are closely matched at room temperature and low voltage, at a given time, once the time, voltage, or temperature changes, they are likely to end up with very different values. This kind of between two lines A mismatch will cause unequal responses near the filter cutoff. Therefore, it converts common-mode noise into differential noise.”
Another solution is to bridge a large value “X” capacitor between the two “Y” capacitors. The “X” capacitor shunt can provide the required common-mode balancing effect, but will produce undesirable differential signal filtering side effects. Perhaps the most common solution and alternative to low-pass filters are common mode chokes.
The common mode choke is a 1:1 transformer in which both windings act as primary and secondary. In this method, the current passing through one winding induces the opposite current in the other winding. Unfortunately, common mode chokes are also heavy, expensive, and prone to failure caused by vibration.
Nevertheless, 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 caused by 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 the parasitic capacitance that comes with it, making high frequency filtering ineffective.
Mismatches in mechanical manufacturing tolerances between windings can cause mode conversion, in which part of the signal energy is converted into common mode noise, and vice versa. This situation will cause electromagnetic compatibility and immunity issues. The mismatch also reduces the effective inductance of each leg.
In any case, when the differential signal (pass) works in the same frequency range as the common mode noise that must be suppressed, the common mode choke does have a significant advantage over other options. Using common mode chokes, the signal passband can be extended to the common mode stopband.
Monolithic EMI filters Although common mode chokes are popular, another possibility is monolithic EMI filters. If the layout is reasonable, these multilayer ceramic components can provide excellent common mode noise suppression. They combine two balanced parallel capacitors in one package, which has mutual inductance cancellation and shielding effects. These filters use two independent electrical paths in a single device connected to four external connections.
To prevent confusion, it should be noted that the monolithic EMI filter is not a traditional feedthrough capacitor. Although they look the same (same package and appearance), their designs are quite different, and their connection methods are also different. Like other EMI filters, a monolithic EMI filter attenuates all energy above the specified cut-off frequency, and only selects the required signal energy to pass, while transferring unwanted noise to “ground”.
However, the key is very low inductance and matched impedance. For a monolithic EMI filter, the terminal is internally connected to the common reference (shielding) electrode in the device, and the board is separated by the reference electrode. In terms of static electricity, the three electrical nodes are formed by two capacitive halves, which share a common reference electrode, all reference electrodes are contained in 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 changes in temperature and voltage, so the components on the two lines have the same degree of aging. If these monolithic EMI filters have a disadvantage, they cannot be used if the common mode noise is the same frequency as the differential signal. “In this case, a common mode choke is a better solution,” Cambrelin said.
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