Ferrite core

A ferrite core is a member of ferrite, which increases as the core of a coil ( inductor or transformer ) or the inductance of the magnetic field results.

In the simplest case, the " coil " or winding only one passed through a ring or perforated core wire or an electrical line.

Ferrite core as memory

The so-called core memory was invented in 1949 as a form of magnetic data storage. This invention replaced in part the Williams tube and was replaced by the end of the 1970s through the semiconductor memory. Core memory used magnetically hard, that is magnetizable ferrite toroids.

All other ferrite cores made ​​of magnetically soft ferrite, d. H of ferrite, which has no permanent magnet properties as possible.

Ferrite core noise filter as

Standing wave filter

Electronic devices which cause high-frequency interference, such as computer can radiate radio frequency energy via connected cable undesirably. The interference is carried out as a standing wave so as common-mode interference out of the device and radiated by the cable - the entire cable or shield acts as an antenna.

To dampen this interference, the cable in the entire cross-section are (possibly several times) through a ferrite core. For the common-mode noise, this arrangement works through its inductance as a choke with one or possibly several turns. However, the useful signal, which is transmitted as a push-pull signal, sees no inductance, as compensate for the magnetic fields of the on different cores inside the cable forward and return currents flowing. The mode of operation corresponds to that of the current-compensated chokes, which is often also made ​​up of toroidal cores.

Standing wave filters are used, for example, the following cable:

• USB cable (especially with USB 2.0)
• IEEE 1394 (FireWire) cable
• VGA cable
• Supply and output lines of switching power supplies
• Ribbon cable in equipment ( flat cable ferrites)

For retro- fitting of Entstörkernen order lines there are shared, lockable ring and ribbon cable ferrites, which can be attached to electric wires. This standing wave filter are also known as split ferrites.

Standing wave filters are usually made ​​of materials such as iron -nickel-zinc but they are also made ​​of iron - manganese - zinc. The former is suitable for interference between the 10 MHz to 2 GHz, temptation be applied as a filter for interference rather in the lower frequency range in the range of 500 kHz to 20 MHz.

For a trunk group between two devices, of which only one unit is grounded, a standing wave filter is ineffective in general. In such a configuration spread sheath waves as standing wave from. For standing waves Ferritkernmantelwellenfilter act only at the places where. Depending on frequency and wavelength, a current stomach, but not where to set a current node

Standing wave filters are available for different frequency ranges and wire cross-sections.

Suppression by ferrite beads

In order to attenuate high-frequency interference on lines that carry signals of low frequency or DC voltages, such as power lines or lines of EIA- 232 signals (serial interface ), the signal wires - in contrast to the standing wave filters - individually through ferrite beads. Other designations are damping Pearl (English ferrite bead ). There is therefor also nuclei with multiple holes through which are guided already wires.

Their identical on the purpose forth, but otherwise for construction counterpart in SMD style is called SMD ferrite.

Ferrite cores are guided through which conductors act like a choke with low inductance. At very high frequencies, the component acts as a large inductance and a large reactance losses. The losses prevent parasitic resonances and transmission capacity. For this reason, inductors can be used with high quality usually not good as a noise filter.

In addition, in comparison to the air high permeability of the ferrite material and thus lower number of turns at comparable inductance. Few or one turn means a lower parasitic capacitance and correspondingly lower coupling capacitance.

Ferrite beads attenuate high-frequency noise pulses on signal and power cables from the devices or reduce their leakage from the unit or the spread on the connected lines. Steep slopes and overshoots of the signal to flatten and reduce the interference caused by them. Also the wire legs of electronic components, if necessary, provided with it to reduce interference at the place of their origin. An example is the reduction of the effects of Sperrerholzeit of diodes.

This type of filtering is done on or in the appliance near the connectors. For this purpose, there are connectors with a built-in large pieces of ferrite, which is provided with holes for each pin.

In high-frequency coil, for example, in -band filters, ferrite cores are used to calibrate or adjust the inductance. They are for this purpose provided with threads to move them with a tool inside the coil can.

Cores for chokes, transformers and transformers

• Different types of ferrite cores for inductors

U-core

E-core

ER- core

EFD core

Toroidal

EP- core

RM core

At higher frequencies (from about several kHz ) are the nuclei of suppression and storage inductors, transformers and transformers expediently made ​​of ferrite. They are called ferrite core, although they usually are not only inside the windings, but this often surrounded as a closed magnetic circuit. Examples are:

• E-cores, even ELP Cores EFD cores ETD cores, etc., as EI- combination
• EQ cores, PM - cores, ER cores and EP cores ( intermediate form between E and pot core )
• Pot cores or shell nuclei

For these cores, there is usually a suitable bobbin or spool which allow the prefabrication, the insulated mounting of the winding on the core and the connection of the winding, for example, on a printed circuit board. However, the winding may be made especially for ELP cores also traces of a printed circuit board ( Planar ).

Wherein toroids ( an air gap closed magnetic circuit ), rod cores and drum cores ( each open magnetic circuit ), the winding is wound without a bobbin to the core, often helps to this purpose, an insulating layer.

UT and ET- cores are used for common mode chokes; there are not removable, no air gap or EE UU cores with large changing room and a thin center bar around which a split bobbin can be snapped, the wound is mounted by being rotated on the web. He has formed laterally to a gear.

UU or UI cores carry windings on one or both legs.

Depending on the application be divided cores, that is, except UT and ring cores, manufactured with or without air gap or mounted.

Ferrite cores for the assembly of printed circuit boards as an independent component in surface mount technology are referred to as SMD ferrites.