What is Feedthrough Capacitor/Filter?
Update Time: 2023-03-09 17:24:50
Capacitive Equivalent Circuits
A feedthrough capacitor is a ceramic tube coated with a metal layer, forming two poles inside and outside. It is called a feedthrough capacitor because the porcelain tube is empty.
It is a kind of three-terminal capacitor, but compared with the ordinary three-terminal capacitor, it has less ground inductance and almost no lead inductance because it is mounted directly on the metal panel. In addition, the metal plate's input and output terminals are isolated, eliminating the high-frequency coupling.
These two features determine that the feedthrough capacitor has a filtering effect close to an ideal capacitor. Therefore, it is an ideal device for interference filtering.
A feedthrough capacitor (feedthrough filter, low-pass filter, EMI filter) is a kind of capacitors, known as an electromagnetic interference filter, is a new type of component that has been promoted and applied in recent years.
The feedthrough capacitor can effectively suppress grid noise, improve the anti-interference ability of electronic equipment and the system's reliability, and can be widely used in electronic measuring instruments, computer room equipment, switching power supply, measurement, control systems, and other fields.
Feedthrough Capacitors Classification
The C type is a low self-induction device to avoid noise interference from high frequency to ground, low cost, suitable for application in high impedance sources and high load.
The L type is a feedthrough capacitor with inductive and capacitive elements, typically used in circuits with low impedance sources and high impedance loads, and vice versa. Note: The inductor element should be oriented to the low-impedance source.
Pi-type filters are composed of two capacitive elements and an inductive element between the two capacitive elements and exhibit low impedance to both the impedance source and load. Pi-type filters provide better high-frequency filtering performance than C-type and L-type structures.
Characteristics and Applications
The crossover capacitors and various filters are mainly used for filtering signals, data lines, AC power lines, telecommunication equipment, microwave filters, industrial control machines, and composite circuit filter assemblies.
For example, it is effective in suppressing noise on the computer CPU high-speed data bus, and it is useful in suppressing interference on the transmitter.
Solder-type feedthrough capacitor
Solder-type feedthrough capacitors are ideal for small installation spaces.
Small size: effective use of space.
Rated voltage: up to 1000VDC.
High-temperature construction: prevents backflow during installation.
Certification: Filters available for MIL-F-15733 QPL and MIL-C-11015 (CK99) certification.
Circuit construction: C-type, Pi-type, L-type circuits.
Primarily for filtering signals, data lines, and AC power lines.
Telecommunications equipment, transmission equipment, microwave filters, industrial control machines, composite circuit filter assemblies.
Resin-sealed threaded feedthrough capacitors
Resin-sealed screw-type feedthrough capacitors can be easily mounted to the through-hole position with the supplied nuts and washers and are sealed with resin at both ends of the rugged housing to provide good environmental protection.
Voltage: Up to 2500VDC/240VAC.
Certification: MIL-F-15733 certified products are available.
Circuit construction: C-type, L-type, Pi-type.
Mainly used for signal, data line, and DC power line filtering.
Miniature extrusion mount or screw mount is ideal for applications where soldering is not desirable, suitable for microwave and other high-frequency applications.
High current and high voltage resin-sealed feedthrough capacitors
High current up to 100Amps.
Voltage: up to 2500VDC and 240VAC@400HZ.
High current switched mode power supplies, DC charging systems.
High voltage power supplies and rugged bolt-on construction for easy installation.
Glass-sealed high-performance feedthrough capacitors.
Glass-sealed excellent EMI filtering performance; the best choice for those who require high-reliability filtering in harsh environments.
Available for high-performance EMI filtering over a wide range of frequencies from 10KHz to >10GHz, the glass-sealed series are highly resistant to moisture, corrosion, and other harsh environmental influences that may be encountered in military applications.
Optimized design: various sizes and shapes and C, L, and Pi-type circuits to choose from, instantaneous suppression Pi, T, & TT circuit options.
Reliability: Manufactured in accordance with MIL-F-15733 and MIL-F-28861 standards, QPL compliant.
Based on MIL-F-28861, the "S" level for space applications.
Compliant with MIL-F-15733 and MIL-F-28861 standards.
Insertion loss range 0.01MHz-10GHz.
Capacitance and temperature characteristics: 1pF-1μF; NPO, X7R, Z5U, etc.
Temperature range: -55℃-+125℃; -40℃-+85℃.
Voltage value: 400VDC 240VAC@400Hz.
Power supplies, signal lines, rocket ignition devices, aircraft, military communications, medical equipment, and multi-segment filtering.
The higher the frequency of voltage or current, the more likely to generate radiation.
We will generally change the circuit board and add the necessary magnetic ring for this situation. And in addition to this approach, there are filters to reduce the radiation brought about by interference; sometimes, add the appropriate filter so that the high-frequency interference signal can play a large attenuation effect.
In ordinary interference filters effective filtering frequency range of a few kHz to tens of MHz, while the effective filtering frequency range of RF interference filters from a few kHz to GHz or more.
Ordinary capacitors are not ideal and cannot effectively filter out high-frequency noise, which is due to.
Capacitive resonance is caused by the inductance of the capacitor leads, which presents a large impedance to the high-frequency signal and weakens the bypass effect on the high-frequency signal.
The parasitic capacitance between the leads makes the high-frequency signal coupled, which reduces the filtering effect.
How to filter through the core capacitor?
A feedthrough capacitor as a bypass capacitor can make the high-frequency filtering effect very good.
This is because the feedthrough capacitor has a very small parasitic inductance and a very low bypass impedance and eliminates high-frequency coupling between the input and output due to the isolation mounting method.
Feed-through capacitors can form a variety of RF filters for high-frequency applications, which we also call "feedthrough filters." Due to the coaxiality of the tube-type feedthrough capacitor, no significant self-resonance is generated even at 10GHz.
The dielectric of the feedthrough capacitor is ceramic, and the capacity of the ceramic capacitor changes with the ambient temperature. This capacity change affects the filter cutoff rate of the filter. Therefore, choosing the proper ceramic dielectric for the feedthrough capacitor is especially important.
Since the case of the feedthrough capacitor is the other electrode of the capacitor and is connected to "ground," the high-frequency electromagnetic interference signal is shorted to "ground" when it passes through the center conductor, and the electromagnetic interference is eliminated, which is the principle that the feedthrough capacitor can filter out noise.
5 types of feedthrough filters
In general, there are five feedthrough filters to meet different needs: C-type filter, LC-type filter, π-type filter, T-type filter, and Double T-type filter.
C-type filters are composed of three-terminal or feedthrough capacitors and are suitable for suppressing high-frequency signals. It is a low-intensity device to avoid noise interference from high frequency to ground. It is low-cost and suitable for applications with high-impedance sources and high loads.
Basic structure of LC type filter
LC type filters are feedthrough filters with inductive and capacitive elements. These filters are typically used in circuits with low-impedance sources and high-impedance loads, and vice versa. Note: The inductor element should be oriented toward the low-impedance source.
Basic structure of π-type filter
The π-type filter consists of two capacitive elements and an inductive element between the two capacitive elements, which exhibits low impedance to both the impedance source and the load. π-type filters provide better high-frequency filtering performance than C-type and LC-type structures.
Basic structure of T-type filter
T-type filters consist of two inductive elements and one capacitive element. The circuit structure exhibits high impedance from either end of the input. It is similar to the π-type filter structure but is not as widely used as the π-type filter, which can be used in switching conversion applications.
Double T filters
Basic structure of Double-T filter
Double-T filter comprises two T-type filters, a high-performance filter with more stringent circuit requirements.
Structure and Usage
In the structure of feedthrough capacitors, the ground electrode surrounds the dielectric, and the signal terminals pass through the dielectric.
The feedthrough capacitor is used by cutting a mounting hole in the shield case and soldering the ground electrode directly to the shield case.
Feedthrough Filter Selection Method
Rated voltage is the power EMI filter used in the specified power frequency when the operating voltage and the filter's highest allowable voltage value.
For example, used in a 50Hz single-phase power supply filter, rated voltage of 250V; used in a 50Hz three-phase power supply filter, rated voltage of 440V. If the input voltage of the filter is too high, the internal capacitor will be damaged.
The rated current (Ir) is the continuous operating current allowed under the rated voltage and specified ambient temperature conditions.
As the ambient temperature rises or the operating temperature is higher than room temperature due to copper loss of inductor wires, core loss, and ambient temperature, it is difficult to ensure the insertion loss performance.
We should select the current filter rating based on the actual possible operating current and the operating ambient temperature.
Unless otherwise specified, the rated current given in the EMI filter manual are room temperature +25℃ (nominal temperature) values; the same given typical insertion loss or curve also refers to the value of +25℃.
Large operating current (Imax), rated current and temperature between the existence of the following relationship:
Imax is the high operating current, Ir is the rated operating current at room temperature, Tmax is the high operating temperature +85°C, Ta is the actual operating temperature, and Tr is the room temperature +25°C. According to Equation 2.0, the relationship between Imax/Ir and Ta is illustrated as follows.
In addition, some filter companies specify +40℃ (nominal temperature) as the operating current value Ir.
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