what is passive filter?

The passive filter circuit uses passive components such as inductors, capacitors, and resistors to suppress the conducted EMI in an electronic circuit.

A simple passive EMI filter is designed with LC network topology. The inductor (L) allows DC and low-frequency signals to pass through while blocking high-frequency current (i.e., noise). The capacitor (C) provides a low impedance path for high-frequency current (i.e., noise) and directs the noise into the ground or earth. The multiple LC filters can be used in cascade form (i.e., multiple stages/higher order filter) to achieve a higher attenuation effect and a steeper rise in the attenuation curve.   EMI filters for power supply typically utilize passive components.

There are various circuit configurations are used for passive filters, including L filter, C filter, LC filter, CL filter, T – filter, and Pi filter. Each filter configuration has its unique performance and characteristics that determine the level of noise suppression (attenuation) required at various frequencies. The following table provides various passive filter circuit configurations (C, L, Pi, T, 2Pi, and 2T filters) and their advantages and applications.

Another commonly used passive EMI filter is the ferrite bead or ferrite choke/ferrite clamp. The ferrite bead or ferrite clamp is a type of choke commonly used on a cable and wire to suppress high-frequency radiated noise. It absorbs the conducted EMI on power lines/cables and dissipates the noise in the form of heat. The laptop’s power cord uses a ferrite bead/ferrite clamp to remove high-frequency noise on the input power supply line.