ferritekeloja

Ferrite coils, known in Finnish as ferritekeloja, are inductors or chokes consisting of windings of conductor around ferrite magnetic cores. They are used to filter high-frequency noise, suppress electromagnetic interference (EMI), and in some cases store energy in power conversion circuits. The ferrite core is a ceramic compound of iron oxide with other metals such as manganese, zinc, or nickel, chosen for its magnetic permeability and high electrical resistivity.

Coils can be built on toroidal, E-core, or bead-shaped ferrite cores and are available as through-hole or surface-mount devices. Ferrite beads, sometimes called ferrite sleeves, act as broadband EMI suppressors on data and power lines. MnZn ferrites offer high permeability suitable for lower frequencies, while NiZn ferrites provide better performance at higher frequencies due to greater resistivity and lower magnetic loss.

Key design considerations include the target frequency range, the desired impedance versus frequency, saturation current, DC resistance, and temperature stability. Increasing the number of turns raises inductance but also increases DC resistance and potential saturation. In practice, ferrite coils are used in switching-mode power supplies, input/output filters, RF receivers, and automotive electronics to reduce conducted and radiated noise.

Typical specifications cover impedance at a specified frequency, self-resonant frequency, current rating, size, and packaging. While effective for EMI suppression, ferrite coils introduce parasitics and must be chosen to avoid undesired resonance in the signal path.