Is A Ferrite Core Necessary

[Billi Mayhue]

Somecommon causes of EMI are switching-mode power supplies, arc welders, motor bushes, and electrical contacts. All of these can be problematic, not only in the equipment itself, but in other electronic equipment in the surrounding area.

If cabling is not properly installed and protected, high voltage surges are possible, generating electrical noise. These surges can damage hardware, often corrupting data. This has the potential to disrupt business operation on many levels.

Ferrite beads, cores and sleeves counteract these risks. Ferrite beads are made up of a wound coil and two terminals. Ferrite material typically comprises ceramic compounds with iron oxides integrated with nickel, zinc and manganese compounds.

First, they go by other names, such as clip-on ferrite, clip-on ferrite core, ferrite clip on, or even ferrite clips. These are typically hinged plastic casings that open up to permit the insertion of cable, then snaps together to secure the ferrite A5 core around the cable.

Ferrite sleeves are typically used in computer monitors, data cables, medical devices and other electronic equipment and circuits. Smaller sleeves are also embedded in printed circuit boards and electrical circuits.

Using properly installed and grounded shielded cables helps suppress EMIs. However, a ferrite core suppressor may also need to be installed on cabling as well. Ferrite cores come in different shapes. They attenuate any form of EMI emission and are often used either as a retrofit or for testing purposes when calculating ferrite core filter specifications and design requirements.

The core is a metallic component, which has a magnetic field attracted to the magnetic field of its electrode. A ferrite core suppresses electromagnetic emissions by blocking low-frequency noise and absorbing high-frequency noise to avoid electromagnetic interference.

When current flows to an inductor, in this instance, a ferrite core, the core generates magnetic flux. The current energy is then converted into magnetic energy. When the current changes, the magnetic flux changes to, by converting back into current by electromagnetic induction.

In some applications, EMI suppression can be achieved with a ferrite core transformer design. The transformer itself is constructed by using a magnetic core in which coil (inductor) windings are made on a ferrite core component.

A benefit of ferrite cores is their high resistance to high current. They also provide low eddy current losses over a range of frequencies. Factor in their high permeability, and you have the ideal solution for use in high-frequency transformers and adjustable inductors.

Ferrite cores have many applications and are most commonly found on cables. They are used to prevent a loss of electrical energy and their insulating properties can help reduce noise interference with the cable signals. Depending on the environment, ferrite cores may not be needed and other methods could be considered to prevent interference without a ferrite core.

These PCB multiline suppressor ferrite beads have a 236 Ω 25 MHz and 383 Ω 100 MHz ferrite bead impedance. They come with tinned copper jumper wires which complete the desired winding configuration on the PCB. The jumper wires are oxygen-free, high-conductivity copper with a 95/5 tin lead coating. Made of A5 material.

For outer cabling, a square plastic ferrite sleeve is often used. The core is contained in a hinged plastic casing that opens up to permit the insertion of the cable, then snaps together to secure the ferrite A5 core around the cable to suppress EMIs.

For installation on round power cables, you can also use a round nylon-casing ferrite sleeve. This example contains A5 material cores that attenuate any form of EMI emission. It has an impedance of 97 Ω at 25 MHz, and impedance of 207 Ω at 100 MHz.

Ferrite cores are typically used on a DC power cord to provide common-mode noise filtering. Due to the material properties and behavior of ferrites, they exhibit magnetic saturation and hysteresis, so physically larger ferrites are needed to accommodate larger powers and more intense noise filtering, particularly harmonic filtering in high power systems. More elaborate methods are used to ensure greater noise rejection than using a ferrite on its own, namely common-mode and differential-mode filter circuits on the board and PFC circuits.

In this instance, the ferrite bead is intended to produce low-pass filtering behavior, allowing only DC power to reach the load. The problem is that ferrite beads are not low pass filters, they are band stop filters. Because the transient current pulse drawn into a PDN has infinite bandwidth, the current draw excites a transient oscillation on the power bus that leads to high-frequency voltage ripple. This problem would have happened without the ferrite bead, and adding the ferrite actually makes this problem worse.

In these situations, the ferrite bead is usually added to solve an EMI problem that was created through bad layout practices. Some of these layout practices, like routing over uniform ground planes or designing with a low-impedance PDN, are now standard in almost every digital design. Although ferrites are sometimes misused in high speed/high frequency PCBs, there are some instances where the use of a ferrite bead or other ferrite materials are appropriate, particularly for providing EMI shielding.

Although there is a range of EMI shielding solutions on the market, the board layout should always be constructed with EMI in mind. From a CSWaP perspective, you should aim to forego these additional shielding materials as they will increase costs and weight in the system. An experienced design firm can help you weigh the different options and find the best solution to ensure EMC compliance.

A: Pick some that will look nice on the board, maybe in a colour that goeswith the solder mask. Your local craft supply store should have a goodselection of decorative beads, like these glass ones I got in a clearancesale.

Ferrite beads are often misunderstood, and they're included in a lot of SDIYdesigns by a sort of cargo cult process: people know they want some kind offiltering on power rails, they see ferrite beads used in others' designs, itis a general belief that putting ferrite beads on power rails is theaccepted practice, and so they write that into new designs too, whichcontributes further to the community's perception that that is Just How It'sDone. In this article I'm going to go into some detail on what ferritebeads actually are, and why they're useless when we see them on the powerrails in most analog synth designs.

Of course ferrite beads are not always useless. They exist as amanufactured product for a reason. I'll discuss below some of the caseswhere they do serve a useful purpose after all. But in cases where a beadis necessary or appropriate, it will matter what kind of bead to use(because there are different kinds and that makes a difference),and the careful designer who specifies a bead for a good reason, will alwaysgive you some details on what kind of bead is needed. Without those detailsyou might as well just use the decorative glass beads from the craft store;and if the details are missing from the design, it's a clue that maybe thedesigner didn't really understand what they were trying to accomplish whenthey put that symbol on the schematic.

The word "ferrite" has several different meanings in different fields. Inmetallurgy, it refers to a specific crystalline form of iron metal. But inelectronics, it means a ceramic material made of iron oxide combined withcertain other metal oxides, commonly used for inductive components. Thecoils in my Coiler VCF module are wound around ferrite cores.

If you want to build an electronic component with a high inductance, youface a tradeoff. To maximize the energy stored in the magentic field, youneed to pass that magnetic field as much as possible through a material witha high ability to accept magnetic field density (a property called permeability). Usually, that means winding a coil around or through a core made of this kind of material. The simplesthigh-permeability materials are iron and its alloys, which are mostlyclassified as different kinds of "steel."

But iron, like metals in general, conducts electrical current. If you winda coil on a solid iron or steel core, then a changing current in the coilwill induce an eddy current in the core material itself. The core becomes something like a transformer secondary winding, pluggedinto a short circuit, and that causes power loss and other problems. Theeddy currents create magnetic fields of their own, opposing the originalfield from the main winding, and the opposing fields tend to push theoriginal magnetic field out of the core, harming the intended operation ofthe component.

So a low-frequency power transformer will often be designed with a steelcore that is a stack of flat plates insulated from each other, instead of asingle solid piece. The laminated core splits the eddy currents into manysmall loops instead of a single large loop covering the whole cross section,and many small loops create much less power loss than a single large loop. At somewhat higher frequencies, transformer designers sometimes use powderediron combined with an insulator that separates the particles. The eddycurrent loops are limited to the size of the particles instead of the sizeof the core.

But another approach to avoiding eddy current issues is to use a materialwith fairly high magnetic permeability (even if not quite as high as ametallic alloy), but high resistance to electricity. That is the purpose offerrite: it is a substance that accepts a lot of magnetism, butbasically an insulator to electrical current. Ferrite is magnetic ironoxide with some alloying elements. The mineral magnetite can be thought ofas a primitive ferrite, though artificial substances made for the purposeare much more effective.

Ferrite achieves high permeability with low conductivity by containingiron nuclei in a crystal structure that allows them to be magnetic but doesnot have the conduction band of electron states that is present in metallicforms. Without the conduction band there's no easy way for a current topass through the crystal. A somewhat oversimplified intuition is that theoxygen atoms in the crystal are acting as insulators between the iron atoms.

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