1.High frequency suppression effect of feedthrough capacitor in feedthrough filters

A feedthrough capacitor is a three-terminal capacitor that is mainly used to eliminate high-frequency noise. Unlike ordinary three-terminal capacitors, feedthrough capacitors are directly mounted on metal panels, which can effectively reduce ground inductance and almost ignore the influence of lead inductance. Therefore, feedthrough capacitors have excellent filtering performance, especially in feedthrough filters, which can significantly suppress electromagnetic interference. For electronic equipment working at high frequencies, the suppression effect of feedthrough capacitors is particularly significant, and it is an important component that effectively improves the electromagnetic compatibility of the system.

2.What is filtering?

We all know that the higher the frequency of voltage or current, the easier it is to generate radiation. In addition to changing the circuit board and adding necessary magnetic rings, there are actually filters. In many cases, corresponding filters are sometimes added to reduce interference caused by radiation, which can greatly attenuate high-frequency interference signals. The effective filtering frequency range for ordinary interference filters is from several kHz to tens of MHz, while the effective filtering frequency range for radio frequency interference filters is from several kHz to more than GHz.

Since ordinary capacitors are not ideal capacitors, they cannot effectively filter out high-frequency noise. This is because: ① The inductance of the capacitor leads causes capacitor resonance, presenting a large impedance to high-frequency signals, weakening the bypass effect on high-frequency signals; ② The parasitic capacitance between the wires causes high-frequency signals to couple, reducing the filtering effect.

3.How does the through-hole capacitor filter?

Using a through-hole capacitor as a bypass capacitor can make the high feedthrough filters effect very good. The through-hole capacitor has a very small parasitic inductance and a very small bypass impedance. In addition, due to the isolation installation method, the high-frequency coupling between the input and output ends is eliminated. Through-hole capacitors can form various RF filters suitable for high-frequency occasions, which we also call “feed-through filters”. Due to the coaxiality of the tubular through-hole capacitor, even at a frequency of 10GHz, there will be no obvious self-resonance phenomenon.

The medium of the through-hole capacitor is a ceramic medium, and the capacity of the ceramic capacitor will change with the ambient temperature. This capacity change will affect the filter cutoff rate of the filter. Therefore, it is particularly important to choose an appropriate ceramic medium for the through-hole capacitor. Since the shell of the through-hole capacitor is the other electrode of the capacitor and is connected to the “ground”, the high-frequency electromagnetic interference signal is short-circuited to the “ground” when it passes through the center conductor, eliminating the electromagnetic interference. This is the principle of through-hole capacitors to filter noise.

Insertion loss

Insertion loss is the main performance indicator of the filter and determines the performance of the filter. As shown in the figure below, the through-hole capacitor is closer to the ideal capacitor.

4.Several forms of feedthrough filters

Feedthrough filters, sometimes also called RF filters. The appearance structure of this filter is similar to that of the through-hole capacitor, but a ferrite core is added. In order to meet the requirements of different occasions, this filter generally has five forms: C-type filter, LC-type filter, π-type filter, T-type filter, and double T-type filter.

4.1. C-type filter

The C-type filter is composed of a three-terminal capacitor or a through-hole capacitor, which is suitable for suppressing high-frequency signals. It is a low self-inductance device that avoids high-frequency noise interference to the ground. It is low-cost and suitable for applications with high impedance sources and high loads.

4.2. LC filter

LC filter is a Feedthrough filters composed of inductor and capacitor. This filter is typically used in circuits with low impedance source and high impedance load, or vice versa. Note: The inductor should face the low impedance source.

4.3. π filter

π filter is composed of two capacitors and an inductor between the two capacitors. It shows low impedance to both the source and the load.π filter provides better high feedthrough filters performance than C and LC structures.

4.4. T filter

T filter is composed of two inductors and one capacitor. Its circuit structure shows that the input from either end is high impedance. It is similar to the π filter structure, but it is not as widely used as the π filter. It can be used in the field of switch conversion.

4.5. Double T filter

Double T filter is composed of two T filters. It is a high-performance filter for circuits with strict requirements.

It is worth noting that the simple through-core structure is composed of one (C type) or two capacitors (π type) composed of inner and outer electrodes and ceramics. The capacitance of this capacitor can be from 10pF and the working voltage can reach 2000VDC. The following figure shows a C-type feedthrough filtering, which is a plug-in. However, for electronic products with narrow installation space, the use of welded feedthrough filter is particularly important.

5.Application occasions of feedthrough filters

Through-hole capacitors and various filters are mainly used for filtering signals, data lines and AC power lines, telecommunications equipment, microwave filters, industrial computers, composite circuit filter components, etc. For example, adding through-hole capacitors to the high-speed data bus of a computer CPU can effectively suppress noise, and adding through-hole capacitors to the transmitter has a great effect on suppressing interference.

6.Installation precautions

Due to the special structure of through-hole capacitors and feedthrough filters, in order to ensure the normal operation of the filter, the housing of the filter must be sufficiently connected to the ground of the panel, thereby providing an effective path for interference. Therefore:

1) The through-hole filter is a disc-shaped multilayer or tubular ceramic capacitor. Like other ceramic items, it will be damaged by sudden temperature changes, mechanical vibrations and excessive voltages. When installing a through-type filter on a board, welding the filter’s guide pins and shaping the filter, care must be taken to minimize all types of stress;

2) For threaded through-type filters, the recommended installation rotation force relative to the housing should be used when installing them on a partition or panel. Otherwise, the deformation of the housing may cause damage to the capacitor inside.