Learn about the superior EMI filtering of feedthrough capacitors and how they’re used in C-type filters, LC-type filters, Pi-type filters and T-type filters.
This article is part of The engineer’s complete guide to capacitors. If you’re unsure of what type of capacitor is best for your circuit, read How to choose the right capacitor for any application.
What is a feedthrough capacitor?
Feedthrough capacitors use ceramic as a dielectric but are designed in such a way they are not just “simple” capacitors. They also exhibit coaxial cable properties. A feedthrough capacitor is a ceramic tube coated with a metal layer, forming two “plates” with one in the inside and the other on the outside. It is called a feedthrough capacitor because the porcelain tube is empty.
A feedthrough capacitor is essentially a three-terminal capacitor. However, when compared with an ordinary three-terminal capacitor, it has less ground inductance. It has virtually no lead inductance since it is mounted directly on a metal chassis panel. Further, the metal plate’s input and output terminals are isolated, which eliminates high-frequency coupling. The primary function of feedthrough capacitors is to eliminate electromagnetic interference (EMI).
Types of feedthrough filters
A feedthrough capacitor acts like a low-pass filter and is used to filter out EMI. It attenuates the EMI conducted on the power line(s) or on a signal input line. This reduces the possibility of external EMI disturbing proper equipment operation. Feedthrough capacitors can also be used to attenuate any EMI generated by that equipment on its output lines. This falls under the category of electromagnetic control (EMC).
Some feedthrough capacitors are used in assemblies that also include inductors. This permits the use of the various filter arrangements such C-type filters, LC-type filters, Pi-type filters and T-type filters (see below image).
It is important to note these are distributed capacitances and inductances as found in transmission lines. They are not discrete (lumped) reactances. Feedthrough capacitors can form a variety of RF filter configurations. These are sometimes called “feedthrough filters.” Because these tube-type filters have transmission line characteristics, no significant self-resonance will occur even as high as 10 GHz.
The C-type filter is a three-terminal device. It has input and output terminals, and its bushing makes a connection to ground. It is designed to attenuate high-frequency noise (EMI). Generally, these filters are used with high impedance sources and loads.
The LC- (and CL-), Pi-, and T-type filters serve as low-pass filters, but can also provide impedance matching. These feedthrough devices are classified by their number of reactive (energy-storage) elements. The C-type filter is first order, the LC-type filter is second order, and the Pi-type and T-type are third order. In the case of a low-pass filter, the roll off is -20n dB/decade, where n is the filter order. Consequently, a first order filter rolls off at -20 dB/decade, a second-order filter rolls of at -40 dB/decade, and a third-order filter rolls off at -60 dB/decade. As the roll off becomes steeper, the filter comes closer to being ideal.
LC-type filters are feedthrough filters which include an inductor to supplement the action of the capacitor. These filters are often used in circuits with low-impedance sources and high-impedance loads, and vice versa. The inductor is oriented to connect to the low-impedance source. (By convention, if the LC is reversed to CL, the inductor is then connected to the load.)
The Pi-type filter has three reactive elements. This filter interfaces well with low-impedance sources and low-impedance loads. Because it is of the third order, it provides better high frequency performance than the C-type and LC-type filters.
The T-type filter exhibits high impedance from either input end. Like the Pi-type filter, it is not used as widely.
Applications of feedthrough capacitors
Feedthrough capacitor filters are used in commercial, military and space applications. They are applied to medical equipment, rocket and missile launch systems, and radar and communication systems.
In typical (discrete component) interference filters, the effective filtering range runs from a few kilohertz to tens of megahertz. In contrast, the required filtering range for radio frequency interference (RFI) is from a few kilohertz to a gigahertz or more. Ordinary capacitors cannot filter out RFI successfully. Real (regular) capacitors possess a series equivalent inductance and a series equivalent resistance. The capacitance and the equivalent inductance will produce a series resonant frequency. The impedance of the capacitor is at its minimum at the series resonant frequency. As the noise frequency increases further, the capacitor will have a net inductive reactance that increases with further increases in frequency. Its bypassing to ground will become less effective. Parasitic capacitance between the leads serves as a coupling path, which further reduces the filtering effect.
In contrast, a feedthrough capacitor provides superior high-frequency filtering. The feedthrough capacitor has a very small parasitic inductance, a very low bypass impedance, and (because of its isolation mounting) it eliminates coupling between its input and output.
Alternatives to feedthrough capacitors
For simple, noncritical filtering, discrete filter circuitry can be used. However, for harsh EMI environments or critical EMC requirements, it is better to use a feedthrough filter. Note that feedthrough filters are also available in SMD packages. Not all feedthrough filters must have bushings and be wall mounted. It is important to understand the EMI/EMC requirements and discuss them with applications engineers.
