Choosing the Right EMC Ferrite

There are a wide variety of EMC ferrites to choose from. This article outlines six types of ferrites and helps you select the best ferrite for your design.

Types of EMC Ferrites

Snap ferrites from Würth Elektronik

The snap ferrites of Würth Elektronik are suited for retrospective fitting on existing cable lines. The special lock, the so-called key technology, on the Würth Elektronik snap ferrites offers assurance against unauthorised opening – cue CE mark. At the same time, the plastic case protects against mechanical damage and its construction offers increased contact pressure on the ferrite halves.

The result is very high impedance, or in other words interference suppression. Typical applications are signal lines, monitor leads, printer cables, mouse cables, data transmission lines etc.

Flat and ribbon cable ferrites

RF is subject to its own laws. Hence a 2 cm ribbon lead can act as an effective antenna to pick up or transmit interference. For applications where a ”longer“ ribbon cable is used, interference suppression must still be achieved, ribbon cable ferrites or flat ferrites make an important contribution to compliance standard. To adapt the flat wire optimally to the body three different shape of cores are available. Fastening of the flat ferrites is done with plastic clips or with self-adhesive tape.

Ferrite sleeves and ferrite rings

Unlike snap ferrites or ferrites with nylon clamps, the ferrite sleeves or rings can be slid onto the cable prior to final assembly and fixed by using a heat shrinking tube or potting process. This solution is found on every computer monitor today. The closer the inner diameter is adapted to the cable diameter, the greater the suppression effect. Also with multiple winding turns, as with snap ferrites, the inductive part can be emphasised while achieving greater impedance.

6-hole ferrite beads

The 6-hole beads – commonly known as VHF chokes – or our SMD type WE-SUKW (5-hole ferrite bead) usually come with 2.5 windings and withstand high levels of current over long periods without going into saturation. They achieve effective RF interference suppression directly on the board. Typical applications are ground decoupling, supply voltage decoupling in association with blocking capacitors and data line filters.

Multiline ferrites

The multiline ferrite is a universally applicable suppression component: Firstly as a 4-turn current- compensated choke with a high current rating of typically 4 A. The impedance or the current can be increased with the appropriate circuit. Furthermore, a dual current-compensated wideband choke can also be achieved with the right connection (cf. Fig. 1).

SMD Ferrites of Würth Elektronik

Current is also passed through the ferrite in the SMD ferrites; for the signal, it acts as a low impedance connection (RDC → 0 Ω); however for the interference spectrum as a high loss resistance (impedances up to 5 kΩ).

With a multilayer construction current loads up to 6 A are achievable depending on the ferrite design. The following sizes are available 0402, 0603, 0805, 1206, 1210, 1806 and 1812 as standard.

In the pre-design stage, the engineer can design short-circuited solder pads at critical points, which can be separated and the appropriate SMD ferrites mounted after EMC measurement.

Choosing the right ferrite

The correct choice of ferrite can only be confirmed by measurements in the EMC lab. The calculation of the dynamic impedance (system impedance) of a high frequency electronic circuit is generally not possible. This is dependent on many factors, which are hard to describe mathematically, especially in the RF range.

Nevertheless, initial selection can be made with comparative measurements and experience:

Supply voltage lines and ground lines have a circuit impedance in the range 1–10 Ω, useful signal lines again have a circuit impedance between 50–100 Ω and video, clock and data lines have higher circuit impedance depending on the application (e.g. bus systems such as CAN, SCSI etc.)

Starting from this system impedance (ZA and ZB) and the required interference suppression, the ferrite impedance (ZF) can be calculated by using the equation above. According to the impedance of the ferrite and the application the correct part can be chosen now, e.g. a ferrite sleeve or an SMD ferrite.

Ferrite Design Kits

Want to learn more about Ferrites? Visit our catalog where you can purchase our ferrite design kit and ask questions from the ferrite experts.

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