A passive EMI filter is a type of EMI filter that uses passive components, such as inductors and capacitors, to suppress high-frequency noise currents (called conducted EMI) on power supply lines, ensuring the protection and reliability of electrical and electronic devices.

Conducted EMI is generated during the normal operation of electrical and electronic devices (e.g., SMPS, motors, and other electronic equipment) and can propagate through power lines, disrupting the operation of other devices connected to the same lines. Additionally, this EMI can cause the lines to act as unintended antennas, emitting electromagnetic radiation that may interfere with nearby devices.

The passive EMI/EMC filter design can utilize various circuit topologies, such as L filter, C filter, LC filter, CL filter, Pi filter, and T – filter, each offering unique noise suppression characteristics at different frequencies. The passive EMI filter functions as a low-pass filter that suppresses high-frequency noise currents and allows desirable currents (DC or AC low-frequency signal) to pass through. The filter diverts the noise currents to the ground, thereby filtering out conducted EMI and preventing radiated EMI caused by these currents. This ensures the protection and reliable operation of devices. 

Figure: Passive EMI filter for single phase system shown

Another commonly used passive EMI filter is the ferrite bead or ferrite choke/ferrite clamp (basically an inductor), commonly seen in laptop power cords. The ferrite bead absorbs the conducted EMI noise traveling on power lines/cables and dissipates the noise energy as heat, thereby preventing noise currents from reaching the load and also reducing radiation caused by these currents. 

The EMI filters are installed at the power entry point of a device/system, preventing the conducted EMI from entering or escaping from the filtered device. This ensures the device's electromagnetic compatibility (EMC). An EMC-compliant device can operate properly without disturbing other connected and nearby equipment in its intended real-world environment. 

Passive EMI filters are available to suppress both common mode and differential mode noise in single-phase and three-phase systems. They are widely used in applications like power supplies, UPS, motor drives, consumer electronics, industrial equipment, medical devices, robotics, and automated test equipment.