EMI (Electromagnetic Interference) filters are used to filter the noise in electrical circuits. EMI Filter is an important mitigation equipment for suppressing undesired conducted electromagnetic interference (EMI). EMI filters are designed with passive components (resistor, capacitor, inductor) are called as passive EMI filters. EMI filters designed with semiconductor components (such as diodes, transistors, amplifiers) are called as active EMI filters.

Types of Passive EMI Filter Circuits

Passive EMI filters can be designed in any combination of passive components. The popular passive filters are:

  • C-Filter is the most common EMI filter. C-filter is a three terminal feed-thru capacitor, used to attenuate high frequency signals. C-filter circuits, do not suffer performance drops due to current saturation situations. The C-filter is the most reliable and lowest cost type.
  • L-filter consists of one inductive element and one capacitor. It is most commonly used in circuits where there is a high impedance load and low impedance source, or where the circuit has a high impedance source and low impedance load.
  • Pi-filter consists of two capacitors and one inductive element. Pi-filter presents a low impedance to both the source and load in a circuit. Because of the additional capacitor element, Pi-filter provides better high frequency attenuation than the C-filter and L-filters of the same capacitance. Pi-filters are not recommended for swithching applications due to the possibility of “ringing”.
  • T-filter consists of two inductive elements and one capacitor. This filter presents a high impedance to both the source and load of the circuit. It has similar filter performance to Pi-filter, without the possibility of ringing, and can be used in switching applications.

Types of Active EMI Filter Circuits

Active filters have many advantages over passive filters. All the varieties of passive filter configurations can also be found in active filters. Some of them are:

  • High-pass filters – attenuation of frequencies below their cut-off points.
  • Low-pass filters – attenuation of frequencies above their cut-off points.
  • Band-pass filters – attenuation of frequencies both above and below those they allow to pass.
  • Band-reject filters or Notch filters – attenuation of certain frequencies while allowing all others to pass.