Figure: EMI filter and T-type EMI filter circuit shown

Conducted EMI refers to high-frequency noise currents generated during the normal operation of electrical and electronic devices like motor drive circuits, SMPS, computers, drilling machines, and other electronic devices.  These noise currents can travel through the power lines and disturb the operation of (or sometimes damage) other connected devices on the same lines. Additionally, high-frequency noise currents traveling through power conductors or cables can cause the lines to act as unintended antennas, leading to radiated emissions that disrupt nearby devices in the surrounding area (called radiated EMI). Passive EMI filter configurations such as T-type EMI filter is designed to suppress the conducted EMI disturbances.  

The T-type EMI filter circuit (as shown in the figure) operates as a low-pass filter that blocks high-frequency noise currents while permitting desired low-frequency signals, such as DC or AC power at 50/60/400 Hz, to pass through. The inductor blocks the noise, while the capacitors divert the noise to the ground. This dual mechanism of blocking and grounding high-frequency noise ensures the filtration of conducted EMI and prevents radiated EMI caused by noise currents. It ensures the electromagnetic compatibility (EMC) of the devices. 

An EMC-compliant device can operate properly without disturbing any other connected and nearby devices in its real-world environment. The EMI filter is installed at the device’s power entry point to prevent conducted EMI from entering or exiting the device, helping to improve the EMC performance of the device. The filter can be used as a standalone or integrated with equipment.

The T-type EMI filter is classified as a third-order filter because it comprises three components. It provides steeper frequency attenuation characteristics, with a slope of 60 dB per decade, making it highly effective in suppressing high-frequency noise. T-type EMI filters are typically used in applications with low source and load impedance (< 100 ohms). Common applications include noise removal in consumer electronics, office equipment, industrial equipment, medical devices, and more.