With the rapid development of electronic technology, the electromagnetic environment is deteriorating day by day, and a large number of electronic devices are difficult to work normally in this electromagnetic environment. On the other hand, the rapid increase of electronic devices has further led to the deterioration of the electromagnetic environment. Therefore, in modern electronic product design technology, how to select interference suppression filter devices, such as feedthrough capacitors, is one of the problems that every electronic product designer must face. As an efficient filtering element, feedthrough capacitors can effectively suppress high-frequency interference signals, and their superior electromagnetic compatibility performance has been widely recognized in many application scenarios.

1 Feedthrough capacitors ——— Feedthrough filters

Feedthrough filters are often used in some high-frequency processing modules such as mobile communication equipment and radar navigation. They cooperate with shielding structures to process input or output low-frequency signals. They are products that cannot be replaced by other forms of capacitors. Now the operating frequency of electronic circuits and the frequency of electromagnetic interference in the surrounding environment are getting higher and higher. The problem of insufficient high-frequency filtering exposed by installing filters on circuit boards is more prominent. In order to obtain better filtering effects in UHF or higher frequency bands, especially when protecting the shielding body from being penetrated, feedthrough filters must be used to solve them. The feed-through filter is installed on a metal panel, has a very low ground impedance, and uses the metal panel to isolate the input and output of the filter, so the filter has a very good high-frequency filtering effect.

The circuit structure of the feed-through filter is divided into C type (through capacitor), L type (one through capacitor plus one inductor), T type (two inductors plus one through capacitor), π type (two through capacitors plus one inductor), etc.; the more components of the filter, the shorter the transition band of the filter and the greater the insertion loss of the stop band. Among them, the C-type feed-through filter is generally called a Feedthrough capacitors.

The filtering effect of any capacitor with leads will be limited by the ground inductance. As shown in Figure 1, the capacitor is grounded by directly connecting the outer surface of the capacitor to the metal shield or panel by threading or welding. Since the ground current is dispersed within a range of 360° around the center conductor, there is actually no lead inductance, and the capacitor can maintain good performance in a very high frequency range.

Figure 1 Feedthrough capacitors

There are three ways to use feedthrough filters:

1) Installed on the panel of the shield (shielded box, shielded chassis, etc.) This is the most basic method of use. When a wire needs to pass through the shield, it is necessary to install a feedthrough filter on the panel of the shield so that the wire passes through the shield through the feedthrough filter. In this application, the feedthrough filter is usually in the form of a feedthrough capacitors to achieve more efficient filtering performance.

2) Installed on the ground layer of the circuit board, the ground layer of the circuit board can be used as an isolation layer and grounding layer.

3) Installed on the isolation board between circuits When conditions are not met and the feedthrough filter cannot be installed on the shield panel or ground surface, installing it on a metal partition also has a high-frequency filtering effect that ordinary capacitors (including three-terminal capacitors) cannot match.

Feedthrough filters have two types of welding installation and threaded installation. The advantage of welding installation is that it saves space and has reliable filtering performance. However, when the filter is welded to the panel, since the heat capacity of the panel is much larger than the heat capacity of the filter, the local temperature of the welding may reach a very high temperature, which is easy to cause damage to the feedthrough filter. When welding, pay attention to controlling the welding time and temperature. The threaded installation method is simple and easy. You can drill a hole on the panel and tighten the feedthrough filter with a nut; you can also drill a threaded hole directly on the panel and screw the feedthrough filter directly on the box or panel. There are two points to note when installing. First, the torque should not be too large. Although the feedthrough filter looks as strong as a screw, it is hollow inside and excessive torque will cause damage. Second, a serrated gasket should be used during installation to maintain good contact.

When selecting feedthrough capacitors and feedthrough filter circuit forms, the following conditions are generally considered:

1) Attenuation of interference: The more components of the filter, the greater the attenuation of the interference signal.

2) The difference in frequency between the useful signal and the interference signal: The smaller the frequency difference between the useful signal and the interference signal, the more components of the filter are needed.

3) The impedance of the circuit using the filter: A basic principle is that the capacitor in the filter faces the high-impedance circuit and the inductor faces the low-impedance circuit. The so-called high and low here can be referenced by 50Ω.

2 EMI Suppression Ferrites

Ferrites (as shown in Figure 2) are widely used in interference suppression of electronic equipment because of their high loss in high-frequency electromagnetic fields and strong absorption of interference energy. Due to the absorption of high-frequency waves by ferrites (not reflection), the Q value of the inductor suppression circuit in the circuit can be reduced, reducing signal resonance problems. EMI suppression ferrites have the opposite characteristics of low-loss inductor materials used in low frequencies or power supplies. Figure 3 shows how the resistance component determines the impedance characteristics at high frequencies.

Figure 2 EMI suppression ferrite

Figure 3 Impedance component of ferrite changing with frequency

Putting a ferrite ring on a wire or cable forms a simple, economical, and easy-to-install filter, as shown in Figure 4. The function of ferrites is to concentrate the magnetic field around the wire, thereby increasing the inductance of the wire by hundreds of times. The biggest advantage of ferrite chokes is that they do not require redesigning the circuit or structure, so they are widely used in equipment improvements. Manufacturers offer split ferrites with a variety of different inner diameter specifications, generally ranging from 5mm to 13mm.

Figure 4 Use of ferrite

When the signal line and return line pass through the ferrite at the same time, the ferrite has no effect on the signal (differential mode), but it will increase the impedance of the common mode current. The effect of the ferrite can be enhanced by winding the cable around the ferrite a few more times or using more ferrites. However, the improvement of increasing the number of turns is limited by parasitic capacitance.

The effect of ferrite increases with increasing frequency. The impedance of ferrite chokes is usually tens of ohms at 10MHz, and when the frequency exceeds 100MHz, the impedance increases to hundreds of ohms (the specific value depends on the shape and size, the larger the ferrite volume, the greater the impedance). Products from different manufacturers or different types of ferrites from the same manufacturer have different impedance changes with frequency. Figure 5 shows the characteristics of two ferrites of the same size (5mm outer diameter, 11mm length) but different materials.

Figure 5 Characteristics of ferrites made of two different materials

Since a ferrite choke is nothing more than a high-loss inductor, it is only effective in low-impedance circuits. It has little or no effect when used in high-impedance circuits. Most circuits, especially cables, have complex impedance variations with frequency and are usually in the range of 10 to 1000Ω. Therefore, the attenuation provided by a single ferrite is very limited, generally around 10dB and rarely exceeding 20 dB.

Ferrite chokes are particularly effective in reducing the fast rise rate of electrostatic discharge current pulses, which may be induced into internal cables. Transient parameters will be absorbed by the ferrite instead of being shunted or reflected to other parts of the system. To fully utilize the performance of ferrites, the following precautions are very important:

1) The effect of ferrite magnetic rings (beads) is related to the circuit impedance: the lower the impedance of the circuit, the better the filtering effect of the ferrite magnetic ring or bead. Therefore, in the product manual of general ferrite materials, the insertion loss of ferrite materials is not given, but the impedance of ferrite materials is given. The greater the impedance of ferrite materials, the better the filtering effect.

2) Influence of current: When a large current flows through the wire passing through the ferrite, the low-frequency insertion loss of the filter will become smaller, and the high-frequency insertion loss will not change much. To avoid this situation, when used on the power line, the power line and the power return line can be passed through the ferrite at the same time.

3) Selection of ferrite materials: According to the different frequencies of interference to be suppressed, ferrite materials with different magnetic permeabilities are selected. The higher the magnetic permeability of the ferrite material, the greater the impedance of the low frequency and the smaller the impedance of the high frequency.

4) Determination of the size of the ferrite magnetic ring: The greater the difference between the inner and outer diameters of the magnetic ring, the longer the axial direction, and the greater the impedance. But the inner diameter must wrap the wire tightly. Therefore, to obtain a large attenuation, try to use a larger magnetic ring on the premise that the inner diameter of the ferrite magnetic ring wraps the wire tightly.

5) Number of turns of common mode choke: Increasing the number of turns through the magnetic ring can increase the impedance of low frequency, but due to the increase of parasitic capacitance, the impedance of high frequency will decrease. Blindly increasing the number of turns to increase the attenuation is a common mistake. When the interference frequency band to be suppressed is wide, different numbers of turns can be wound on the two magnetic rings.

6) Number of ferrite magnetic rings on the cable: Increasing the number of ferrite magnetic rings on the cable can increase the impedance of low frequency, but the impedance of high frequency will decrease. This is because of the increase of parasitic capacitance.

7) Installation position of ferrite magnetic rings: Generally, try to get as close to the interference source as possible. For cables on shielded chassis, the magnetic ring should be as close to the cable inlet and outlet of the chassis as possible.

Better effect when used with capacitive filter connectors: Since the effect of ferrite magnetic rings depends on the impedance of the circuit, the lower the impedance of the circuit, the more obvious the effect of the magnetic ring. Therefore, when capacitive filter connectors are installed on both ends of the original cable, its impedance is very low and the effect of the magnetic ring is more obvious.

At the same time, the application of feedthrough capacitors can further optimize the filtering effect of the system, especially when dealing with high-frequency noise, it can work together with ferrite materials to provide stronger interference suppression.