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There are many types of filters. In addition to some traditional inductors, capacitors and their combinations, there are also many new technology products with different uses. According to different applications, they can be divided into three categories:
① Filters used in AC and DC power supply parts: power supply filters, magnetic rings and magnetic beads, etc.;
② Filters used on signal lines: signal filters, magnetic rings and beads, feedthrough capacitors, filter connectors (i.e. filter arrays), etc.;
③ Filters used on printed circuit boards: decoupling capacitors, chip (surface mount) filters, magnetic beads, etc.
3) Inductors and inductor filters
The coil and its return part can form a traditional inductor, which is usually single-coil or multi-coil. Inductors can be classified according to the magnetic core they surround. The two most common types are air core and magnetic core. Magnetic core inductors (referred to as magnetic core inductors) can be further classified according to whether their magnetic core is open or closed. In addition, the widely used ferrite magnetic ring (or magnetic bead), although it plays the role of a transformer in physical terms, it is more like a variable resistor that changes with frequency, but people usually still consider it as an inductor.
In actual applications, the winding wire of the inductor must contain parasitic series resistance and distributed capacitance between the windings, so resonance will occur at certain frequencies in the application. The main parameters for measuring the performance of the inductor are: distributed capacitance, effective inductance, quality factor Q, self-resonant frequency and saturation current. These are all things that should be considered in the application.
① Ordinary coil inductor
An open-circuit core inductor with the same volume and number of turns has a much larger inductance and Q value than an air-core inductor, and a closed-circuit core is even better. An important characteristic of an inductor is the generation of stray magnetic fields and the resistance to stray currents.
Air core or open core inductors are most susceptible to interference because their magnetic flux extends from the inductor to a considerable distance. In terms of sensitivity to magnetic fields, core inductors are much more sensitive than air core inductors, and open cores are the most sensitive because the core (low reluctance path) concentrates the external magnetic field and causes more flux to flow through the coil.
Ordinary inductor filters are generally only used for low-frequency filtering. Under high-frequency conditions, their insertion loss begins to decrease. This is because as the frequency increases, when the frequency exceeds the self-resonant frequency of the inductor, the impedance of the parasitic capacitance begins to decrease, causing the impedance of the inductor to decrease. In this way, high-frequency noise cannot be well suppressed and causes noise leakage through the inductor.
② Ferrite ring inductor
The hollow ferrite bead can be placed on the wire, while the ferrite bead with lead is connected in series in the wire. The ferrite bead with lead has a simple structure, as shown in Figure 6, because a good return terminal can be provided through the magnetic core, so its parasitic capacitance is small. The situation is the same for the ferrite bead without lead. Therefore, the ferrite bead inductor has good high-frequency characteristics, and its operating frequency can reach 1GHz or higher. It can be used for high-frequency filtering and decoupling in low-impedance circuits.
4) Pulse voltage absorber
For the interference of transient pulse voltage (such as electrostatic discharge, surge, pulse group, etc.), filtering or absorption measures can be taken. However, the filter has limited ability to suppress transient voltage with large amplitude, and the effective way is to use pulse voltage absorber. Pulse voltage absorber includes lightning arrester, varistor and transient voltage absorption diode (TVS). Currently, sheet varistors and TVS arrays are available on the market. (Because strictly speaking, pulse voltage absorption technology does not belong to the category of filtering, it will not be introduced in detail here. If necessary, please refer to relevant materials and product manuals.)
5) Composite filter
In practical applications, if a single-element filter cannot achieve the desired filtering effect, a composite filter can be considered. A composite filter can be considered as a cascade of several single-element filters. Currently, there are also devices that combine several filter elements on the market. The following is a brief introduction to several commonly used composite filters.
① AC power filter
A typical AC power filter is shown in Figure 9. It can be seen that it can prevent noise from the external power supply from entering the device, and also suppress the device's own electromagnetic emission from entering the shared power grid. The AC power filter shown in the figure uses a common mode choke to suppress common mode noise, and also uses several capacitors. The capacitor directly connected between the two poles of the power line is used to suppress differential mode noise, commonly known as X capacitors, while a pair of capacitors connected between the live wire or neutral wire and the ground wire is used to suppress common mode noise, commonly known as Y capacitors.
② Signal line EMI suppression filter
Signal line EMI suppression filters are specially designed high-quality composite filter components. Figure 10 shows a product from Murata. The filter used on high-speed signal lines should have a "steep" insertion loss characteristic curve, which requires it to separate noise from the signal without distorting the signal waveform. This is because the noise frequency in the high-speed signal line is close to the signal frequency. If a three-terminal capacitor or other simple filter is used, the signal and noise may be compressed at the same time, causing distortion of the signal waveform. In this case, a special signal line EMI suppression filter should be used.
③ Block Type/Block Type Filter
This is also a high-performance filter device that combines ferrite beads, through-hole (bypass) capacitors and monolithic capacitors. It has a high rated current and operating reliability. Figure 11 shows a product from Murata.
④ SMT T-Type/SMT T-Type Filter
As shown in Figure 12, this type of T-type filter is a filter device that is mainly composed of a three-terminal capacitor and a ferrite bead. The special design and structure ensure that it has a higher rated current and rated voltage, and has a very wide temperature adaptability range and working reliability. Under special requirements, it is particularly suitable for use on DC power lines and signal lines.
⑤ EMI suppression filter with surge absorption function
This type of chip-type or lead-type three-terminal filter produced by Murata can be simply understood as a combination of varistor, capacitor and inductor.
⑥ Other special purpose filter components
There are also filter components specially designed for some specific purposes on the market. Please refer to their product manuals for details.
2) Capacitors and capacitive filters
According to the type of insulating dielectric material in the capacitor, capacitors can be divided into electrolytic capacitors, paper dielectric capacitors, polyester resin capacitors, ceramic (monolithic) capacitors, polystyrene polypropylene capacitors, etc. There are also new through-hole capacitors, three-port capacitors, etc. Different types of capacitors have different characteristics. It may meet a certain specification but not man
Meet other specifications. Sometimes, in order to provide filtering in a wider frequency band, two different types of capacitors are often used in parallel. The main technical parameters that characterize capacitors include: operating frequency, parasitic resistance, parasitic inductance, temperature sensitivity, failure mode, and the ratio of capacity to volume. Only by mastering these characteristics first can the type and parameters of filter capacitors be correctly selected.
① Aluminum electrolytic capacitors and tantalum electrolytic capacitors
Aluminum electrolytic capacitors have a relatively large capacitance, large series resistance, large inductive reactance, and are sensitive to temperature. They are suitable for occasions where temperature changes are not large and the operating frequency is not high (not higher than 25kHz), and can be used for low-frequency filtering. Aluminum electrolytic capacitors have polarity, and the correct polarity must be ensured during installation, otherwise there is a risk of explosion.
Compared with aluminum electrolytic capacitors, tantalum electrolytic capacitors have obvious advantages in series resistance, inductive reactance, temperature stability, etc. However, its operating voltage is lower.
② Paper dielectric capacitors and polyester film capacitors
Its capacitance is relatively small, the series resistance is small, and the inductive reactance is large. It is suitable for occasions with small capacitance and low operating frequency (such as below 1MHz), and can be used for low-frequency filtering and bypassing. When using tubular paper dielectric capacitors or polyester film capacitors, their shells can be connected to the reference ground so that their shells can play a shielding role and reduce the influence of electric field coupling.
③ Mica and ceramic capacitors
Its capacitance-to-volume ratio is very small, the series resistance is small, the inductance value is small, and the frequency/capacitance characteristics are stable. It is suitable for occasions with small capacitance and high operating frequency (frequency can reach 500MHz), and is used for high-frequency filtering, bypassing, and decoupling. However, this type of capacitor has a weak ability to withstand transient high-voltage pulses, so it cannot be randomly connected across the low-resistance power line unless it is specially designed.
④ Polystyrene capacitor
Its series resistance is small, its inductance is small, and its capacitance is very stable with respect to time, temperature, and voltage. It is suitable for occasions requiring high frequency stability and can be used for high-frequency filtering, bypassing, and decoupling.
⑤ Feed-through capacitor (sometimes called feed-through/bypass capacitor)
The structure of the through-hole capacitor is that the ground electrode surrounds the dielectric and the signal line passes through the dielectric. This structure ensures that its inductance value is very small, the high-frequency performance is excellent, and the working current and working voltage can also be very high. It is suitable for high frequencies and for installation on shielded shells. At present, it is widely used in military equipment and mobile communication phones. When using a through-hole capacitor, it should be noted that its shell must be well grounded. Only in this way can the expected filtering effect be achieved.
⑥ Three-terminal capacitor
In high-frequency circuits, the lead wires of general capacitors have inductance components, which affects their high-frequency characteristics. The three-port capacitor can achieve a very small residual inductance component in series with the capacitor in terms of structure, so its insertion loss characteristics are better than those of the two-port capacitor, thereby improving the high-frequency characteristics of the capacitor. Three-port capacitors are available in leaded and sheet types.
⑦ Chip solid capacitor array
The chip solid-state capacitor array can be regarded as an integration of several three-terminal capacitors, and therefore has the same filtering characteristics as the three-terminal chip solid-state capacitor. It is also grounded through the "ground electrodes" at both ends. Murata currently supplies 4-wire, 6-wire and 8-wire chip solid-state capacitor arrays. The crosstalk between the signal lines in the chip solid-state capacitor array is very low, reaching more than -40dB. Obviously, the use of array filters can significantly simplify the design of printed circuit boards, reduce the area occupied by printed circuit boards, and also facilitate the installation of filters.
3) Capacitors and capacitive filters
According to the type of insulating dielectric material in the capacitor, capacitors can be divided into electrolytic capacitors, paper dielectric capacitors, polyester resin capacitors, ceramic (monolithic) capacitors, polystyrene polypropylene capacitors, etc. There are also new types of through-hole capacitors, three-port capacitors, etc. Different types of capacitors have different characteristics. They may meet a certain specification but not meet the requirements.
Meet other specifications. Sometimes, in order to provide filtering in a wider frequency band, two different types of capacitors are often used in parallel. The main technical parameters that characterize capacitors include: operating frequency, parasitic resistance, parasitic inductance, temperature sensitivity, failure mode, and the ratio of capacity to volume. Only by mastering these characteristics first can the type and parameters of filter capacitors be correctly selected.
① Aluminum electrolytic capacitors and tantalum electrolytic capacitors
Aluminum electrolytic capacitors have a relatively large capacitance, large series resistance, large inductive reactance, and are sensitive to temperature. They are suitable for occasions where temperature changes are not large and the operating frequency is not high (not higher than 25kHz), and can be used for low-frequency filtering. Aluminum electrolytic capacitors have polarity, and the correct polarity must be ensured during installation, otherwise there is a risk of explosion.
Compared with aluminum electrolytic capacitors, tantalum electrolytic capacitors have obvious advantages in series resistance, inductive reactance, temperature stability, etc. However, its operating voltage is lower.
② Paper dielectric capacitors and polyester film capacitors
Its capacitance is relatively small, the series resistance is small, and the inductive reactance is large. It is suitable for occasions with small capacitance and low operating frequency (such as below 1MHz), and can be used for low-frequency filtering and bypassing. When using tubular paper dielectric capacitors or polyester film capacitors, their shells can be connected to the reference ground so that their shells can play a shielding role and reduce the influence of electric field coupling.
③ Mica and ceramic capacitors
Its capacitance-to-volume ratio is very small, the series resistance is small, the inductance value is small, and the frequency/capacitance characteristics are stable. It is suitable for occasions with small capacitance and high operating frequency (frequency can reach 500MHz), and is used for high-frequency filtering, bypassing, and decoupling. However, this type of capacitor has a weak ability to withstand transient high-voltage pulses, so it cannot be randomly connected across the low-resistance power line unless it is specially designed.
④ Polystyrene capacitor
Its series resistance is small, its inductance is small, and its capacitance is very stable with respect to time, temperature, and voltage. It is suitable for occasions requiring high frequency stability and can be used for high-frequency filtering, bypassing, and decoupling.
⑤ Feed-through capacitor (sometimes called feed-through/bypass capacitor)
The structure of the through-hole capacitor is that the ground electrode surrounds the dielectric and the signal line passes through the dielectric. This structure ensures that its inductance value is very small, the high-frequency performance is excellent, and the working current and working voltage can also be very high. It is suitable for high frequencies and for installation on shielded shells. At present, it is widely used in military equipment and mobile communication phones. When using a through-hole capacitor, it should be noted that its shell must be well grounded. Only in this way can the expected filtering effect be achieved.
⑥ Three-terminal capacitor
In high-frequency circuits, the lead wires of general capacitors have inductance components, which affects their high-frequency characteristics. The three-port capacitor can achieve a very small residual inductance component in series with the capacitor in terms of structure, so its insertion loss characteristics are better than those of the two-port capacitor, thereby improving the high-frequency characteristics of the capacitor. Three-port capacitors are available in leaded and sheet types.
⑦ Chip solid capacitor array
The chip solid-state capacitor array can be regarded as an integration of several three-terminal capacitors, and therefore has the same filtering characteristics as the three-terminal chip solid-state capacitor. It is also grounded through the "ground electrodes" at both ends. Murata currently supplies 4-wire, 6-wire and 8-wire chip solid-state capacitor arrays. The crosstalk between the signal lines in the chip solid-state capacitor array is very low, reaching more than -40dB. Obviously, the use of array filters can significantly simplify the design of printed circuit boards, reduce the area occupied by printed circuit boards, and also facilitate the installation of filters.
④ Chip Ferrite Inductor Array
Like chip solid capacitor arrays, chip ferrite inductor arrays also have 4-wire, 6-wire and 8-wire types, and also have similar excellent characteristics.
⑤ AC and DC chokes
A common mode choke is an inductor consisting of two windings with the same winding direction and the same number of turns and a magnetic core. It is usually made of two wires in parallel, as shown in Figure 8. When the signal current or power current flows through the two windings, the directions are opposite, and the magnetic flux generated cancels each other out, and the choke presents a low impedance. When the common mode noise current (including the disturbance current caused by the ground loop, also called the longitudinal current) flows through the two windings, the directions are the same, and the magnetic flux generated is added in the same direction, and the choke presents a high impedance, thereby suppressing the common mode noise.
In some special cases, the same core closed loop symmetrical differential mode choke may also be used in the power supply filter circuit to suppress the higher frequency differential mode noise in the line. Chokes are used in AC and DC power supply filter circuits and signal circuits.
Ferrite rings are very useful for suppressing common-mode current. If the two paired wires (signal line and ground line) pass through the ferrite ring, the ferrite ring only suppresses the unwanted EMI current and has no effect on the useful differential mode current. Ferrite rings are also effective when placed on coaxial cables.
The most obvious disadvantage of ferrite magnetic ring is its low impedance. In addition, the following points should be noted in the application:
a) When the length of the ring or bead approaches λ/4 (λ is the wavelength corresponding to the noise to be filtered), it will become ineffective.
b) The end-to-end capacitance of the ferrite bead (typically 1~3pF) bypasses its series impedance at certain frequencies, making it ineffective in attenuating noise.
c) When the magnetic induction intensity exceeds a certain range, the ferrite ring becomes saturated and the efficiency decreases.
d) When ferrite rings are placed on multiple cables, they may increase inductive crosstalk between adjacent wires.
③ Chip ferrite bead inductor
Figure 7 is a schematic diagram of a chip ferrite bead inductor. The internal leads of a simple chip ferrite bead are straight, so its impedance value is small. However, the chip ferrite bead with a coiled internal lead has a relatively high impedance value.
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