Feedthrough Capacitor Definition

The capacitors used in EMI filters are generally ceramic capacitors. Due to the physical structure of their through-holes, this type of ceramic capacitor is also called a feedthrough capacitor or through-hole capacitor.

The self-inductance of feedthrough capacitors is much smaller than that of ordinary capacitors, so the self-resonant frequency is very high. At the same time, the through-core design also effectively prevents high-frequency signals from being directly coupled from the input end to the output end. This combination of low pass and high impedance provides excellent rejection in the 1GHz frequency range. Since it is directly installed on the metal panel, its grounding inductance is smaller and there is almost no influence of lead inductance. In addition, its input and output ends are isolated by the metal plate, eliminating high-frequency coupling. These two characteristics determine the through-core The capacitor has a filtering effect close to that of an ideal capacitor.

Structure of feedthrough capacitor

The internal electrode of  feedthrough capacitor uses a copper rod. The copper rod passes through the capacitor porcelain tube, and there are leads at both ends, which can flow very high current. The outer electrode is led out with an externally threaded metal tube or metal sleeve.

Working Principle of Feedthrough Capacitor

The feedthrough capacitor achieves filtering and EMI suppression through its unique structural design:

1. High-frequency signal bypass

The main function of the through-hole capacitor is to bypass the high-frequency signal to the ground, thereby preventing high-frequency noise from passing through. When the high-frequency signal passes through the capacitor, the high-frequency component is effectively bypassed to the ground because the capacitive reactance of the capacitor decreases with the increase of frequency.

2. Low impedance path

The through-hole capacitor provides a low impedance path for the high-frequency signal, so that the high-frequency noise signal can easily pass through the capacitor to the ground terminal without being transmitted to the load or subsequent circuit.

3. Electromagnetic interference suppression

The ground shielding layer of the through-hole capacitor effectively isolates the electromagnetic interference signal from the transmission signal, suppressing the influence of electromagnetic interference on the signal. This can significantly improve the electromagnetic compatibility (EMC) of the circuit.

4. Frequency selection characteristics

Due to the capacitive reactance characteristics of the capacitor, the through-hole capacitor exhibits different impedances to signals of different frequencies. The high-frequency signal is bypassed due to the low impedance of the capacitor, while the low-frequency signal continues to be transmitted through the center conductor. In this way, high-frequency noise is filtered out and effective signal transmission is achieved.

What is filtering?

Filtering is the process of removing or weakening unwanted frequency components in electrical signals through specific devices or circuits. The higher the frequency of voltage or current, the greater the probability of electromagnetic radiation, which may interfere with other electronic devices. In addition to optimizing circuit design and adding necessary magnetic rings, filters are also an important tool for reducing electromagnetic radiation interference. Filters can significantly attenuate high-frequency interference signals, thereby reducing radiation interference.

Filters can be divided into ordinary interference filters and radio frequency interference filters according to their effective filtering frequency range. The effective filtering frequency range of ordinary interference filters is from several kHz to tens of MHz, while the effective filtering frequency range of radio frequency interference filters can extend from several kHz to more than GHz.

Limitations of ordinary capacitors

Ordinary capacitors have the following limitations when filtering high-frequency noise:

Capacitor lead inductance: The lead inductance of the capacitor causes the capacitor to resonate at high frequencies, thereby showing a large impedance to high-frequency signals, weakening its bypass effect on high-frequency signals.

Parasitic capacitance: The parasitic capacitance between the wires causes high-frequency signals to couple, thereby reducing the filtering effect.

How does a feedthrough capacitor filter?

The through-hole capacitor performs well as a bypass capacitor in high-frequency filtering for the following reasons:

Low parasitic inductance: The through-hole capacitor has very small parasitic inductance, which makes its bypass impedance very small, thereby effectively filtering out high-frequency signals.

Isolated installation: The through-hole capacitor adopts an isolated installation method to eliminate the high-frequency coupling between the input and output ends.

Coaxial structure: Due to its coaxiality, the tubular through-hole capacitor will not produce obvious self-resonance even at a high frequency of 10GHz.

The medium of the through-hole capacitor is usually a ceramic medium, but the capacitance of the ceramic capacitor will change with the ambient temperature. This capacitance change will affect the filter cutoff frequency of the filter. Therefore, it is particularly important to choose an appropriate ceramic medium for the through-hole capacitor.

The shell of the through-hole capacitor serves as the other electrode of the capacitor and is connected to the “ground”. When the high-frequency electromagnetic interference signal passes through the center conductor, it will be short-circuited to the “ground”, thereby eliminating electromagnetic interference. This is the principle that the through-hole capacitor can effectively filter out noise.

Classification of feedthrough capacitors

Frequency-free capacitors can be divided into different types according to their internal structure and filtering characteristics, and each type is suitable for different circuits and application scenarios. The following are the main classifications and characteristics of feedthrough capacitors:

1. C type

• Features: C-type feedthrough capacitors are low self-inductance devices, mainly used to avoid high-frequency noise interference to the ground.
• Structure: Mainly composed of a capacitor element.
• Applicable occasions: Suitable for applications with high impedance sources and high impedance loads.
• Cost: Low cost.
• Filtering performance: Provides basic high-frequency filtering performance and has a certain suppression effect on high-frequency noise.

2.L-C type feedthrough capacitors

• Features: L-C type feedthrough capacitors combine inductor elements and capacitor elements to provide more efficient filtering performance and are suitable for applications requiring high impedance matching.
• Structure: Consists of an inductor element and a capacitor element to form an L-C type structure. There can be two configurations:
• Series configuration: Inductor and capacitor are connected in series.
• Parallel configuration: Inductor and capacitor are connected in parallel.
• Applicable occasions:

The series configuration is suitable for circuits with high impedance sources and low impedance loads, or circuits with low impedance sources and high impedance loads.The parallel configuration is suitable for circuits with high impedance sources and high impedance loads, or circuits with low impedance sources and low impedance loads.

• Filtering performance: L-C type feedthrough capacitors can provide very superior filtering performance and are suitable for use in complex electromagnetic interference environments. The inductance and capacitance values ​​can be adjusted according to the specific application to optimize the filtering effect.

3. L type

• Features: L-type feedthrough capacitors combine inductance elements and capacitance elements, and are suitable for circuits with low impedance sources and high impedance loads, or vice versa.
• Structure: It consists of a capacitance element and an inductance element to form an L-shaped structure.
• Applicable occasions: Typically used in circuits with low impedance sources and high impedance loads, or circuits with high impedance sources and low impedance loads.
• Note: The inductance element should face the low impedance source.
• Filtering performance: It can provide better filtering performance and is suitable for more complex electromagnetic interference environments.

4. Pi type

• Features: Pi type through-hole capacitors are composed of two capacitor elements and an inductor element located between the two capacitor elements, providing superior high-frequency filtering performance.
• Structure: It is composed of two capacitor elements and an inductor element to form a Pi type structure.
• Applicable occasions: It exhibits low impedance to both the impedance source and the load, and is suitable for occasions requiring high-performance filtering.
• Filtering performance: It provides better high-frequency filtering performance than the C type and L type structures, and can more effectively suppress high-frequency noise and electromagnetic interference.

Different types of through-hole capacitors are suitable for different circuit requirements and application scenarios according to their structure and the combination of inductor and capacitor elements. The C type is suitable for basic high-frequency filtering, the L type is suitable for circuits with different impedance sources and loads, and the Pi type provides the best high-frequency filtering performance, which is suitable for high-demand filtering occasions. Choosing the right type of through-hole capacitor can significantly improve the filtering effect and electromagnetic compatibility of the circuit.

Features and advantages of feedthrough capacitors

1. Three-terminal design: The through-hole capacitor has three terminals, namely the input terminal, the output terminal and the grounding point. The signal or power line enters from the input end, passes through the center conductor of the capacitor, and is transmitted from the output end, while the high-frequency interference signal is bypassed to the ground point through the capacitor layer.

2. Directly installed on the metal panel: This installation method greatly reduces the influence of ground inductance and lead inductance, which can be almost ignored, thereby improving the filtering effect.

3. Good high-frequency filtering effect: The structural design of the through-hole capacitor makes it perform well at high frequencies, and can effectively filter out high-frequency noise and electromagnetic interference, especially suitable for electronic equipment with higher frequencies.

4. EMI suppression: The through-hole capacitor can effectively suppress electromagnetic interference, improve the electromagnetic compatibility (EMC) of the equipment, and ensure the normal operation of the equipment in a complex electromagnetic environment.

Application

Welding Feedthrough Capacitors:

Fit in Compact Spaces: Best

Uses: Mainly used for filtering signals, data lines, AC power lines in telecommunications, transmission equipment, microwave filters, industrial computers, composite circuit filter components.

Small Size: Maximizes use of available space.

Rated Voltage: Up to 1000VDC.

Circuit Structure: Available in C-, Pi-, and L-Type configurations.

High Temperature Design: Prevents backflow during installation.

Resin Sealed Threaded Feedthrough Capacitors:

Easy to Install: Directly mount in through-hole locations using nuts and washers.

Environmental Protection: Resin-sealed rugged housing ensures strong environmental protection.

Applications: Filters signals, data lines, and DC power lines for microwave and high-frequency applications where soldering is not possible.

Voltage Handling: Up to 2500VDC/240VAC.

Circuit Options: Supports C-, L-, and Pi-Type configurations.

High Current, High Voltage Resin Sealed Feedthrough Capacitors:

High Current Applications: Filters high currents in switching power supplies and DC charging systems.

High Voltage Applications: Filters high voltages in power supplies with rugged bolt-on construction for easy installation.

Features: Capable of handling currents up to 100Amps and voltages up to 2500VDC and 240VAC @ 400Hz.

Glass Sealed High Performance Feedthrough Capacitors:

Excellent EMI Filtering: Glass sealing provides excellent EMI filtering performance.

Applications: Ideal for demanding environments requiring reliable filtering such as power supplies, signal lines, rocket ignition systems, aircraft, military communications, medical equipment, and multi-stage filtering.

Design Options: Available in a variety of sizes, shapes, and circuit types (C, L, Pi) with optional transient suppression Pi, T, and TT circuits.

Performance Features: Available with insertion loss ranging from 0.01MHz to 10GHz, capacitance ranging from 1pF to 1μF, and different temperature characteristics (NPO, X7R, Z5U).

Temperature Range: Reliable operation from -55℃ to +125℃ or -40℃ to +85℃.

Voltage and Current: Handles voltages up to 400VDC and 240VAC @ 400Hz, and currents up to 30Amps.

Each type of capacitor is tailored to specific requirements to ensure efficient performance in a variety of electronic and industrial applications, taking into account space constraints, voltage ratings, environmental conditions, and frequency response needs.

Summary

Feedthrough capacitors have become an important component for electromagnetic interference (EMI) suppression due to their unique structure and excellent high-frequency filtering performance. Through their low self-inductance design and direct installation on metal panels, feedthrough capacitors achieve efficient high-frequency signal bypass and low-impedance paths, significantly improving the electromagnetic compatibility (EMC) of the circuit. They are widely used in scenarios that require high-frequency filtering and EMI suppression, such as communication equipment, computers, power systems, avionics, and medical equipment. Choosing the right type of feedthrough capacitor can greatly improve the filtering effect of the circuit and the stability of the equipment in complex electromagnetic environments.