Throttle (electrical engineering)
Chokes (engl. Choke ) are coils or inductors for limiting of currents in electrical conductors, for the intermediate storage of energy in the form of their magnetic field, for impedance matching, or for filtering . In contrast to transformers or resonant circuit inductors, they are usually connected in series with other components or the consumers.
They are used in the field of power supplies for electrical and electronic devices, in power electronics as well as in low and high frequency technology.
To increase the inductive resistance (reduction in size), chokes often contain a soft magnetic core.
Designs
Toroidal chokes
Toroidal chokes are wound on ferrite or powder toroidal cores . Toroidal cores can also consist of crystalline or amorphous metal strips. Toroidal cores form a closed magnetic circuit and therefore only have low magnetic stray fields. Low scattering contributes to better electromagnetic compatibility ( EMC ). An extreme form (one turn) of ferrite toroidal chokes are ferrite beads pushed over wires . Ferrite core chokes without an air gap are only used for current-compensated chokes or for saturation chokes .
Rod core chokes
Rod core chokes have an open magnetic circuit. They can therefore tolerate higher magnetization field strengths and - if they are wound in one layer - have a lower parasitic parallel capacitance than other designs, which makes them also suitable for very high frequencies (VHF chokes). Rod cores are made of ferrite for HF applications and of electrical sheet metal for mains voltage applications.
Air chokes
For very high frequencies, so-called air chokes are used, the core of which is free of ferromagnetic materials, so that no saturation , eddy current losses and hysteresis losses occur. However, air chokes require more turns than iron or ferrite core chokes for the same inductance , which increases the ohmic resistance of the coil winding.
If air chokes are wound in a single layer (see cylinder coil ), they have a particularly small parasitic capacitance. Such chokes are often used in high-frequency circuits. They are also well suited for high voltages, as the insulation strengths between the turns add up.
Iron or ferrite core
Most chokes have a ferromagnetic core because they then require significantly fewer turns for the same inductance than air chokes . However, with strong currents, the core can become saturated, which leads to a distortion of the current curve and a strong reduction in inductance. Another disadvantage is the occurrence of eddy currents in the coil core when alternating currents flow through the reactor. In order to suppress eddy currents, conductive core materials must be insulated from one another - the core then consists, for example, of several sheets of metal that are isolated from one another parallel to the magnetic field or of a ferromagnetic powder ( powder core ), as in transformers .
Since ferrite materials are ferromagnetic but not electrically conductive, ferrite core chokes show no eddy current losses and - depending on the material - can also be used for very high frequencies. At high currents, ferrite core chokes tend to show signs of saturation in the core than other materials, since ferrite has a lower saturation induction. You avoid saturation by providing the cores with an air gap or by creating an open magnetic circuit ( rod core choke , bobbin core).
Suppression chokes
Chokes should have little or no effect on direct current and low-frequency currents, whereas high-frequency alternating currents should effectively reduce them due to their high inductive resistance . The aim is to prevent high-frequency interference radiation . Because the full load current of the subsequent circuit flows through the winding, they often have a relatively large cable cross-section in order to keep ohmic losses low.
Chokes for radio interference suppression should have a high impedance in the widest possible frequency spectrum . To do this, they must have a high inductance and a low intrinsic parasitic capacitance. These requirements often cannot be achieved with a single design, but only by combining several chokes with different properties.
Current-compensated chokes
The current-compensated choke or common mode choke (short-CMC, from engl. Common mode choke ) has a plurality of identical windings that are oppositely flowed through by the working current, whose magnetic fields in the core of the choke so cancel. CMCs are often used to attenuate interference emissions. Such interference currents usually occur in the same direction in the forward and return lines (common mode). The current-compensated choke forms a very high inductance for this common-mode interference, since these interference currents are not compensated for in it. Current-compensated chokes are often found at the inputs and outputs of switched-mode power supplies and in line filters .
A particularly simple form of current-compensated chokes are toroidal cores pushed onto cables or so-called hinged ferrite; However, they only have an interference-suppressing effect at very high frequencies ( VHF range). For these small high-frequency coils for interference suppression in data bus systems or for mains voltage supply, there are many variants of perforated, cylindrical or flat, sometimes divisible (hinged ferrite ) ferrite cores that are threaded onto the stretched cable or the multi-core lines or enclose a few turns.
Normal- mode interference cannot be eliminated with common-mode chokes; a push-pull signal - like the useful signal - is allowed to pass through these chokes almost unhindered. In practice, however, the leakage inductance that is always present (which is usually also specified in the data sheets) is used to dampen differential mode interference through a clever arrangement of the filter components.
Current-compensated chokes are often made of one-piece, closed ferrite cores in ring shape, E-shape, frame shape or so-called D-shape by threading the winding wires through with toroidal cores and winding them on bobbins for other core shapes. Several chambers per partial winding reduce the self-capacitance and shift the self-resonance frequency and the effective range to higher frequencies.
Series chokes
Gas discharge lamps always require a ballast . This often contains a choke that limits the current through its reactance and, on the other hand, generates the necessary high ignition voltage with fluorescent lamps with the help of an additional starter . An example of the calculation of the inductive resistance is shown here .
Ballast chokes of conventional ballasts (CCG) have a sheet iron core with an air gap. Electronic ballasts (EVG) use a ferrite core choke. Commutation reactors are used in large rectifiers to increase the angle of current flow and reduce mains harmonics .
Storage chokes
In switching power supplies certain topologies such as forward converter as well as in switching regulators ( buck and boost converter (. E.g. active PFC), buck-boost converter , SEPIC converter , Cuk converter ) are magnetic for storing energy storage inductors needed. In these chokes, the magnetic circuit of the ferrite core is often interrupted by an air gap . The air gap is a gap-shaped interruption in the magnetic core and is often filled with non-magnetic material such as paper, plastic or resin for mechanical stabilization. The energy stored in the choke is then almost completely in this gap. The core only serves to guide the magnetic field. The gap serves to reduce the magnetic flux density . This avoids saturation of the core material and ensures a more linear inductance curve even with high magnetization. Other designs for storage chokes with a reduced tendency to saturation are rod and thread spool cores.
The core of storage chokes consists either of an interrupted magnetic circuit as described above, made of sintered metal ( powder core ) or nanocrystalline or amorphous wound metal strip (toroidal tape core ).
Iron or iron alloys (e.g. Sendust, High Flux, MPP) are usually used as powder. Features of these powder cores are the higher energy storage capacity compared to solid cores as well as the inductance curve which is more linear to the magnetic field without a sharp transition to saturation. One also speaks of a distributed air gap. Powder core chokes are as compact inductors in switch mode power supplies , switching regulators and PFC stages (PFC: Compensation, power factor power factor correction ) as well as suppression chokes in normal mode noise (. Eg in dimmers ) used.
Frequency-dependent resistance
Air-core coils are used for impedance matching or direct current injection in higher-frequency transmission amplifiers where ferrites fail.
In high and low pass filters and crossovers , chokes are used to separate alternating currents of different frequencies. For this purpose they are combined with capacitors . Examples of this are line filters , loudspeaker switches and antenna switches .
Since the choke limits high-frequency current components, steep current rising edges are flattened and rectified alternating currents are smoothed.
Saturation chokes
Saturation chokes and transductor chokes use the effect of magnetic saturation of the core material: Saturation chokes limit the rate of current rise in thyristor circuits at the beginning of the current flow and later lose their inductance almost completely when saturation occurs. In line deflection circuits, premagnetized saturation chokes are used in series with the horizontal deflection coil in order to linearize the deflection current.
Transductor chokes
Transductor chokes allow the inductance or reactance to be controlled by means of direct current bias. The premagnetization shifts the saturation point, so alternating voltages and currents can be controlled by direct current. In the past, line-frequency transducer chokes were used for B. for dimming the room lighting in the cinema.
PFC chokes
PFC chokes, for English Power Factor Correction , work in series with the supplying mains voltage in switched-mode power supplies in order to reduce the harmonic load on the supplying network. They work either at mains frequency as a passive PFC on a sheet metal transformer core or as a storage choke in active power factor correction circuits at approx. 10 ... 100 kHz.
literature
- Joachim Franz: EMC, fail-safe construction of electronic circuits . Teubner, Stuttgart Leipzig Wiesbaden 2002, ISBN 3-519-00397-X .
- Electronics manual . Franzis-Verlag, Munich 1979, ISBN 3-7723-6251-6 .
- Lexicon electronics and microelectronics . VDI-Verlag, 1990, ISBN 3-18-400896-7 .
- FF Mazda: Understanding and using electronic components . Telekosmos-Verlag, 1984, ISBN 978-3-440-05324-9 .
- Zinke, Since then: resistors, capacitors, coils and their materials . Springer-Verlag, 1982, ISBN 3-540-11334-7 .
- Dieter Nührmann: work book electronics . Franzis-Verlag, 1981, ISBN 3-7723-6543-4 .
- The Brockhaus, Science + Technology . 2003, ISBN 3-7653-1060-3 .
Web links
- Information on radio interference suppression in general and on interference suppression and storage chokes as well as EMC interference suppression filters (PDF file; 1 MB)
- Information on EMC suppression chokes
Individual evidence
- ↑ Archive link ( Memento of the original from February 19, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. page 8
- ↑ http://www.epcos.com/inf/30/db/ind_2008/b82734r_w.pdf
- ↑ Magnetically stored energy also occurs in flyback converters and in ignition coils , but both are not chokes
