feedthrough capacitors for Interference Suppression

Feedthrough capacitor is a three terminal capacitor used to reduce or bypass high frequencies. It is directly installed on a metal panel or body, thereby resulting in low grounding inductance, and almost no effect on lead inductance. Hence it provides good filtering, and helps eliminate or minimize electromagnetic interference (EMI) in circuits. It has two poles, one on inside of tube, and one outside. It is called feedthrough because the center is hollow. Feedthrough capacitors also exhibit coaxial cable properties.

Feedthrough capacitor is a tube of ceramic material, coated with metal layers on its outside and inside, forming two electrodes of a capacitor. Inside of tube is empty, through which a conductor passes. This means very low insertion loss compared to most other types of capacitors. A typical feedthrough capacitor has a conductor, working as an electrode, passing right through the center of a grounded tubular housing. Ceramic dielectrics used are class 1 low permittivity low loss materials, and have good ability to stand high RF currents. This leads to almost zero inductance, resulting in extremely good high frequency characteristics, and better filtering performance. Being directly mounted on metal chassis or panel, there is virtually no lead inductance.

Feedthrough capacitor is an excellent device for interference (RFI or EMI) filtering. It can suppress grid noise and is widely used in electronic industry, computer equipment, power supplies, switching circuits, control systems, and several other fields.

Current carrying conductor of feedthrough capacitor is connected to an electrode that passes through the center of component, while the outer electrode is connected to case. This ensures excellent performance at both low and high frequencies. Near zero inductance ensures ideal capacitor behaviour and excellent high frequency filtering performance. Hence feedthrough capacitor is ideal for high current and high frequency filtering applications. Since ceramic dielectric properties vary with temperature to lesser or higher degrees, selection of ceramic material for desired properties is critical.

Structure of feedthrough capacitor

Feedthrough capacitors differs from regular capacitor in several ways. They have three terminals, and one terminal is direct mounting on panel or metal sheet, which reduces lead impedance and has very low ground inductance. Low inductance means high self-resonance frequency.

Physical construction of feedthrough capacitors can be of four different types.

1- Solder –type feedthrough capacitors

• Good for small mounting spaces
• Small size, high voltage up to 1000 V, and high temperature suitability
• C, Pi and LC type construction available
• Mainly for filtering signals, transmission equipment and industrial applications

2- Resin sealed threaded feedthrough capacitors

• Mounting easy with nuts supplied
• High voltages up to 2500 VDC / 240 VAC, and circuits C, LC, Pi-type construction
• Suitable where soldering not desirable / possible
• For use in DC power line filtering, data line and signals.

3- High current High voltage resin-sealed feedthrough capacitors

• Can handle currents up to 100 Amp. And voltages up to 2500 VDC /240 VAC 400 Hz.
• Applications include high current switch mode power supplies, DC charging systems

4- Glass-sealed feedthrough capacitors

• Excellent EMI performance, can stand harsh environments
• High moisture and corrosion resistance
• Suitable for high-performance filters from 10 kHz to 10 GHz (ideal for military applications)

Mounting-wise constructions

Mounting-wise feedthrough capacitors can have following structures:

1- Panel mount feedthrough capacitor: Mounted directly on panel, providing low inductance and no lead inductance

2- Surface mount Feedthrough capacitors : These are suitable for direct PCB mounting, where space is limited, and high frequency performance is desired.

3- Screw mount feedthrough capacitors : Mounting is with nuts and washers, and suitable where welding or soldering is not desirable / possible.

4- Through hole feedthrough capacitors : Mounted on plated through holes of PCB, and where strong connections are needed, and are available in both axial lead and radial lead type.

Ceramic feedthrough capacitors may be tubular or discoidal.

Types of feedthrough capacitors

Feedthrough capacitors are classified according to their construction and filtering function, as C type, LC type, Pi Type or T type.

C type

This type has low self-inductance which avoids high frequency noise interference to ground. It is suitable for high current load, high impedance source and is most cost-effective.

LC type

LC typehas an inductance and a capacitor element, and is used for low impedance source with high impedance load, or vice versa. Inductor is placed on low impedance side.In similar fashion, LC circuit can be made by using ferrite core on one side.

Pi type

Pi type capacitor has two capacitor elements, with an inductive element between them. It is useful for low impedance sources with high impedance loads.

T Type

T typefeedthrough capacitor is made by having two inductors and a capacitor. T type feedthrough capacitor has high impedance from both ends.

The four types of filters are useful for different frequency response requirements. Both pi type and T type designs are not used very widely.

The discussion has been mostly centered around ceramic dielectric capacitors. However, metallized plastic film construction is also used for feedthrough capacitors. Further, surface mount (SMD) capacitors are also commonly available. Metallized plastic film feedthrough capacitors are used for high reliability devices. These give additional benefit of self-healing in high transient conditions. Being cost-effective, these are often preferred choice. Metallized feedthrough capacitors are also made in solderless or screw type terminals. These capacitors are susceptible to high temperatures, where ceramics have a preference.