Choke (electronics)

Chokes (English: choke) are low-impedance coils to reduce high-frequency currents on electrical wires. They are used in the field of power supplies electrical and electronic equipment in power electronics and high frequency technology. To increase resistance of the inductive reactors often contain a soft magnetic core.

• 2.1 Common Mode Chokes

Designs

Toroidal Chokes

Toroidal chokes with ferrite ( such as manganese zinc ) or ( carbonyl iron ) produced powder toroidal cores. Toroids may be made of crystalline or amorphous metal strips. Ring cores form a closed magnetic circuit and therefore have only low magnetic stray fields on. Low scattering contributes to better electromagnetic compatibility ( EMC) and results in lower magnetic losses by a higher degree of coupling. An extreme form ( 1 turn ) of ferrite toroidal chokes are ferrite beads on wires pushed.

Rod Core Chokes

Contrast, rod-core inductors have an open magnetic circuit. However, they tolerate higher magnetization field strengths and have - if they are wound in one layer - a small self-capacitance, which makes them suitable for very high frequencies ( VHF chokes ). Rod cores exist in RF applications of ferrite and for mains voltage applications from electrical sheet.

Air chokes

For very high frequencies using so called air reactor, whose core is free from ferromagnetic materials to avoid saturation, eddy current losses and hysteresis losses occur. However, air chokes for the same inductance need more turns than iron or Ferritkerndrosseln, which increases the ohmic resistance of the coil winding.

Iron core

Most chokes have a ferromagnetic core, because then they need much less turns for the same inductance as air chokes. However, the core may become saturated at high currents, which leads to the distortion of the current waveform and the great reduction of the inductance. A further disadvantage is the occurrence of eddy currents in the coil core when AC currents flow through the inductor. In order to suppress eddy currents, conductive core material must be isolated from each other - the core is then, for example as transformers of a plurality of longitudinal magnetic field to the underlying insulated from one another metal sheets or of a ferromagnetic powder ( powder core).

Since ferrite materials are ferromagnetic, but not electrically conductive, show Ferritkerndrosseln no eddy current losses and can - depending on the material - also be used for very high frequencies. However Ferritkerndrosseln show at high currents rather saturation phenomena in the core than other materials because ferrite has a lower saturation induction. To avoid the saturation, by providing the core with an air gap or an open magnetic circuit design ( rod core Bobbinkern ).

Suppression Chokes

Chokes to DC and low-frequency currents do not affect or only slightly reduce high-frequency alternating currents, however effective due to its high inductive resistance. The aim is to prevent RF emissions. Because the winding of the full load current of the following circuit flows, they often have a relatively large conductor cross-section in order to keep the ohmic losses low.

Inductors for radio interference suppression should have in the broadest possible spectrum of high impedance. You need this, have a high inductance and low parasitic self-capacitance. These requirements are often not achievable with a single design, but only by the combination of several reactors with different properties.

Common Mode Chokes

The current-compensated choke or common mode choke ( CMC short, of Engl. Choke common mode ) has a plurality of identical windings, which are in opposite directions crossed by the current work, so cancel out their magnetic fields in the core of the reactor. CMCs are often used to attenuate noise emissions. Such interference currents usually occur in the same direction in return line on ( common mode, German: Common mode ). For these common-mode noise current- compensated choke forms a very high inductance, since this interference does not compensate in it. Common Mode Chokes are often found on the inputs and outputs of switching power supplies and line filters.

A particularly simple form of current-compensated chokes on cable deferred toroids or so-called split ferrites; However, they act only at very high frequencies ( VHF range ) störunterdrückend. For these small RF coils for interference in data - bus systems or for mains voltage injections, there are many variants of perforated, cylindrical or flat, sometimes divisible ( ferrite bead ) ferrite cores, which are threaded onto the stretched cable or on the multi-core cables or a few turns enclose.

Mode noise can be personalized with Common Mode Chokes not fix a push-pull signal - as well as the useful signal - is allowed to pass almost unhindered by these reactors. In practice, however, always present leakage inductance (which is usually given in the data sheets in addition ) used by a clever arrangement of the filter components for damping mode interference.

Chokes are often made ​​from one-piece, closed ferrite cores in a ring shape, E- shape, form the framework of so-called D-shape, or by winding the wires are threaded in the ring cores and wound at the other core shapes on bobbins. Several chambers per part winding reduce the self-capacitance and shift the natural resonant frequency and the effectiveness of range toward higher frequencies.

Chokes

Gas discharge lamps always require a ballast. This often has a throttle which limits the current through its reactance and the other fluorescent lamps produced with the aid of an additional initiator, the required high ignition voltage. A sample calculation of the inductive reactance is shown here.

Chokes conventional control gear (CCG ) have a laminated iron core with an air gap. Electronic ballasts ( EVG) use a Ferritkerndrossel. In large rectifiers commutating reactors are used to increase the conduction angle and reduce network harmonics.

Chokes

Chokes are required in switching power supplies low power to store magnetic energy. In these reactors, the magnetic circuit of the ferrite core is frequently interrupted by an air gap. The energy stored in the choke then inserted almost completely in this air gap. The core serves only to guide the magnetic field. The air gap serves to reduce the magnetic flux density. This avoids the saturation of the core material and ensures a more linear inductance curve even at high magnetization. Therefore, the core of chokes is either a magnetic circuit interrupted frequently or sintered metal ( powder core ), or nanocrystalline or amorphous -wound metal strip ( toroidal core ).

To prevent core saturation of the solid iron or ferrite core chokes are often provided with an air gap. The air gap is a slit-shaped interruption of the magnetic core, and is often filled with mechanical stabilization of non-magnetic material such as paper, plastic, or resin. Iron chokes are used as ballast inductor ( inductive ballast resistor ) of discharge lamps or in line filters. The calculation of the inductance takes place here.

A powder of iron or iron alloys (such as Sendust, high flux, MPP) are usually used. Characteristics of this powder cores are compared to massive cores higher energy storage capacity and the magnetic field more linear inductance curve without sharp transition into saturation. This is also called a distributed air gap. Powder core chokes are as compact storage chokes in switch mode power supplies, switching regulators and PFC stages ( PFC: Power Factor Compensation, power factor correction filter ) as well as noise suppression chokes with push-pull disturbances (eg dimmers ) is used.

Inductive resistors

Air chokes or air coils are used in crossovers of speakers and for very high frequencies as in transmission amplifiers, which fail ferrites. In low-pass filters and crossovers reactors are used for the separation of alternating currents of different frequencies. You are this, combined with capacitors for example, in line filters and speaker points. When the throttle eliminates high-frequency components, steep current rise flanks are flattened and smoothed rectified alternating currents.

Saturable reactors

Saturable reactors and amplifier chokes use of the effect of magnetic saturation of the core material: saturable reactors limit the rate of current rise in thyristor circuits at the beginning of current flow and lose their later by entering the saturation inductance almost completely.

Amplifier chokes

Amplifier chokes allow the control inductance or reactance by means of a direct current bias. The bias shifts the saturation operation, thereby alternating voltages and currents can be controlled by means of direct current.

PFC chokes

PFC chokes work in series with the supply mains voltage in switching power supplies, to reduce the harmonic distortion of the supply system. You either work at mains frequency as a passive PFC on transformer lamination core, or as storage choke in a special active PFC circuit at about 10 ... 100 kHz.