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

Pi-type EMI filter is a passive EMI filter configuration consisting of two capacitors and an inductor, connected in the form of the Greek letter "π" (see figure). It removes high-frequency noise currents (called conducted EMI) on power lines and signal lines, thereby ensuring the protection and reliable operation of electrical and electronic devices.

 During normal operation of electrical and electronic devices like computers, SMPS, motor drive circuits, and other devices, high-frequency noise currents are emitted. These noise currents travel through the power lines/signal lines and can interfere with the operation of other devices connected to the same lines, which is known as conducted electromagnetic interference (conducted EMI). Additionally, the high-frequency noise currents traveling through power lines or cables can cause the lines to act as unintended antennas, resulting in radiated emissions from lines that can disrupt nearby devices in the surrounding space (called radiated EMI). Passive EMI filter configurations such as Pi-type EMI filter is designed to attenuate the conducted EMI. 

A Pi-type EMI filter circuit (as shown in the figure) functions as a low-pass filter that blocks high-frequency noise currents while allowing desirable 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 suppression of conducted EMI and prevents radiated EMI caused by noise currents, thereby ensuring the device's protection and reliable operation. Hence, the filter helps to enhance the electromagnetic compatibility (EMC) of the device. An EMC-compliant device can work properly without disturbing the operation of other connected and nearby devices in its real-world environments.  

The EMI filter is installed at the power entry point of a device to prevent the entry or leak out of conducted EMI from the device, helping to improve the EMC of a device.  The filter can be used separately or integrated with the device. 

The Pi-type EMI filter is classified as a third-order filter since it has three components. It offers steeper frequency attenuation characteristics than an LC filter, with a slope of 60 dB per decade. This makes it highly effective at suppressing high-frequency noise. Pi-type EMI filters are ideal for use in applications that have high source and load impedance (> 100 ohms). Typical applications include power supplies, uninterruptible power supplies (UPS), motor drives, DC/DC converters, consumer electronics, industrial equipment, medical devices, and more.