Figure: Feedthrough capacitor, construction, and circuit diagram

Conducted EMI refers to the high-frequency noise currents generated during the normal operation of electrical and electronic devices, such as SMPS, motors, and other equipment. It can travel through power or signal lines, interfering with the operation of other connected devices on the same lines. The noise currents can also cause the lines to act as unintended antennas, leading to radiated EMI emissions that may interfere with other nearby devices in the same environment.

A feedthrough capacitor filter is an EMI filter designed to suppress high-frequency noise currents (conducted EMI) on power and signal lines. It uses a feedthrough capacitor, which is a three-terminal capacitor with ceramic as the dielectric material (figure). In this capacitor design, the signal terminal (feedthrough terminal) is surrounded by the ceramic dielectric, which, in turn, is enclosed by the ground electrode. Compared to regular capacitors, feedthrough capacitors feature a unique geometry, lower grounding inductance, and negligible lead inductance on the signal terminal. These characteristics make them highly effective at suppressing conducted EMI/radio frequency interference (RFI) noise on power and signal lines. 

The feedthrough capacitor filter works as a low-pass filter that filters out high-frequency noises while allowing desirable signals (DC or AC signals) to pass through it. It works by directing the noise currents to the ground.  By suppressing the noise currents, the filter prevents both conducted EMI and radiated EMI (caused by conducted noise currents), thereby, protecting devices from these EMI. The EMI filter is installed at the power or signal line entry point of a device, preventing noise currents from entering or leaving the device. This ensures the electromagnetic compatibility (EMC) of the device, which means the device can work satisfactorily without disturbing any other nearby devices in its real-world environment. The feedthrough capacitor filter plays an essential role in improving the EMC of the device.

The feedthrougth capacitor can be used individually or combined with inductors to form various feedthrough filter circuit configurations, including C, LC, Pi, and T topologies (figure). Types of feedthrough capacitors include tubular ceramic feedthrough capacitors, discoidal (donut shape) feedthrough capacitors, plastic film feedthrough capacitors, and SMD feedthrough capacitors. They are ideal for use in harsh EMI environments or critical EMC requirements. The feedthrough capacitor filter applications include commercial and medical equipment, rocket and missile launch systems, radar and communication systems, and military and space applications.

Figure: Various configurations of feedthrough filter circuit