Transformer

This article deals with transmitters in the sense of communications engineering. For general transformers see transmission , technical transformers. See also: translation (technology)

Transformer (also single-phase transformer ) is an inductive component in communications engineering . Constructed similar to a transformer , it differs in that it usually has the same primary and secondary number of windings and its use exclusively for powerless signal transmission.

Transformers are optimized for power transmission with the highest possible degree of efficiency and usually only achieve the desired efficiency at a single frequency or in a narrow frequency band (e.g. at 50 Hz and / or 60 Hz). In contrast, transmitters are used for relatively broadband information transmission with the highest possible signal quality. Depending on the area of ​​application, transformers are specifically named, e.g. B. as an audio transmitter or balancing transmitter in the field of audio technology, as a matching transmitter in the field of audio and RF technology or as a pulse transmitter in the field of digital technology. Here are the names pulse transformer or pulse transformer consistently as these special transformers such as transformers are optimized for only a relatively narrow frequency range mostly similar.

Basics

Both types of transformers (for power and signal transmission) work on the same principles, but they perform different tasks and differ in their construction. In the case of transformers for power transmission, efficiency is primarily important, while in the case of transformers, the best possible preservation of the signal form is important. Important properties of a transformer for analog applications include: a. its linearity and the lowest possible distortion . In the English language, a prefix is ​​used to distinguish between transformer and transformer ; Transformers that are used in audio technology are called audio transformers . For digital signal transmission such as in Ethernet interfaces in English the term is pulse transformer used in the German pulse transformer .

Areas of application

SMD transformer of the type TG110, as used with Ethernet interfaces. Component from above (left) and from below (right)

Transmitter built into the Ethernet connection

Transformers are u. a. used:

• for adapting the wave impedance of a remote cable to the output impedance of the exchange equipment or the cable driver
• for matching the wave impedance of a coaxial cable to an antenna ; see resonance transformer
• for galvanic isolation of high-frequency connection points ( overvoltage protection , contact protection)
• for galvanic separation of audio signals, for separating ground loops (line transformer)
• for balancing electrical signals (microphone cables, antenna connections); see balun , balun , DI box
• for separating the alternating current component (signal component) in a signal which represents a mixed current with a direct current component , often also with adaptation
• for balancing and for voltage adjustment . ( Interface ) of input resistance or output impedance in sound engineering ; R _i  ≪  R _a
• for transforming the low microphone voltage and for galvanic separation of microphone and preamplifier in the microphone input transformer
• for the formation of phantom speech circles (multiple use of signal lines in telephone cables)
• to match the impedance of a tube amplifier stage to the impedance of the loudspeaker (output transformer) or the next stage (intermediate transformer)
• As an ignition transformer for the transmission of the ignition pulse for thyristors from the control circuit to the load circuit (with potential separation and possibly impedance matching)
• Ignition transformer for flash lamps (voltage transformation, no potential separation)
• Signal transmitter for controlling power semiconductors (square-wave signals for MOSFET , bipolar transistors , IGBT ) in switched-mode power supplies , inverters and frequency converters : potential separation
• Signal transmitters in local networks ( LAN , Ethernet ) etc. a. for electrical isolation and balancing

Designs

Transformer with pot core

The designs are basically the same as those of transformers for power transmission.

Some of the special features are:

• The windings are bifilar or trifilar (nested) in order to keep the leakage inductance small (increase of the upper limit frequency)
• Highly permeable core materials are used ( mu-metal , highly permeable ferrites ) to keep the lower limit frequency low.

For high frequency transformers, ferrite cores for high frequencies are required. Often double-hole cores are used from the VHF frequency range.

Other typical core shapes are ferrite toroidal cores and shell cores.

At high frequencies - from the higher short-wave frequencies - often no core made of ferromagnetic material is used for the coils . Such transformers consist of two air-core coils , which are either nested inside one another or axially attached to one another. In the latter design, there are also designs in which the second coil is rotatably arranged, for. B. to adapt the coupling of the two coils to the impedance of the antenna of a detector receiver or radio transmitter.

At even higher frequencies, the two “windings” can also consist of just one parallel wire pair (with and without a core).

Used in transformers bopet each isolated embodiment from 1.3 to 3.6 kV, and only up to 130 ° C resistant. With 5 kV insulation strength, three layers are glued on top of each other. In the case of transformers in switched-mode power supplies, a thermal fuse can be dispensed with, since the line resistance does not adapt and heat up in the event of a short circuit, but rather triggers the upstream fuse. Such transformers only have 19 to 40 turns of the corresponding wire size.

theory

The most important property of transformers is the current or voltage transformation ratio:

${\displaystyle {\frac {U_{1}}{U_{2}}}={\frac {n_{1}}{n_{2}}}={\ddot {u}}\,}$

${\displaystyle {\frac {-I_{2}}{I_{1}}}={\ddot {u}}\,}$

${\displaystyle {\frac {I_{1}}{-I_{2}}}={\frac {n_{2}}{n_{1}}}\,}$

With

${\displaystyle n_{1}}$= Number of coil turns of the primary winding

${\displaystyle n_{2}}$ = Number of turns of the secondary winding

${\displaystyle {\ddot {u}}}$= Transmission ratio or ratio of the number of turns

${\displaystyle U_{1}}$and are the primary and secondary voltage and and the primary and secondary amperage . ${\displaystyle U_{2}}$ ${\displaystyle I_{1}}$ ${\displaystyle I_{2}}$

The ratio between primary and secondary impedance can be calculated from the square of the transformation ratio of the transformer:

${\displaystyle R_{1}=R_{2}\left({\frac {n_{1}}{n_{2}}}\right)^{2}}$

or

${\displaystyle R_{2}=R_{1}\left({\frac {n_{2}}{n_{1}}}\right)^{2}}$

The transformation ratio required for an impedance transformation can therefore be calculated as follows:

${\displaystyle {\ddot {u}}={\frac {n_{1}}{n_{2}}}={\sqrt {\frac {R_{1}}{R_{2}}}}}$

${\displaystyle {\ddot {u}}^{2}={\frac {R_{1}}{R_{2}}}}$

${\displaystyle R_{1}={\ddot {u}}^{2}\cdot R_{2}}$

or

${\displaystyle R_{2}={\frac {R_{1}}{{\ddot {u}}^{2}}}}$

Another important characteristic of many signal transmitter is the integral of the voltage over time until the core in saturation falls. It is called the voltage-time area or voltage-time product, also because the voltage curves of many transformers are rectangular. It is determined by the length of a square wave signal that can still be transmitted at a given voltage.

The voltage-time product (unit volt-second ) is calculated from the inductance  L and the saturation current  I _sat :

${\displaystyle \int u(t)\mathrm {d} t=L\cdot I_{\mathrm {sat} }}$

The voltage curve can have positive and negative partial areas. For a rectangular voltage curve, starting from the core induction zero, the integral is simplified to

${\displaystyle U\cdot t=L\cdot I_{\mathrm {sat} }}$

During a square-wave pulse , the current rises gradually at first, and then continues to rise very steeply when core saturation occurs. This causes the voltage to collapse and the shape of a square wave signal is falsified. For this reason, core materials with a high permeability number are also used for such transformers (e.g. to control power MOSFETs ) . ${\displaystyle U\cdot t}$

A positive or negative initial magnetization (integration constant) must often be taken into account, which is given by a current that flows at the beginning of the integration. The maximum voltage-time area also determines the lower transmission frequency limit of an AC voltage signal. The mean value of the magnetic flux and the mean current are set to zero: the end value of the integral of the previous half-wave is first integrated. The minimum frequency may therefore be lower than the time integral of the half-wave of this alternating voltage, which is equal to the voltage-time product. ${\displaystyle I_{\mathrm {0} }}$

Further parameters of transformers concern the parasitic inductance and capacitance. The former is the leakage inductance (not to be confused with the leakage field), which is created by the field components of the magnetic flux passing the windings. It is important for the accurate transmission of higher frequency components and should be as low as possible, i.e. H. you want a high coupling factor. It is improved by nested windings and a small spacing between the windings. Bifilar windings are also common. However, the increasing capacitance of the windings to one another due to the local proximity of the two windings is likewise undesirable. Since the distance is given by the thickness of the insulation and therefore cannot be arbitrarily small, high insulation strength, high coupling factor and low parasitic capacitance can only be achieved with difficulty together.

See also

• Matching transformer
• Isolating transformer
• Balun (balancing transformer)

Web links

Commons : Transmitter  - collection of images, videos and audio files

• The transmitter, the unknown being jogis-roehrenbude.de

Individual evidence

1. ↑ In: Electrical engineering. Communication electronics tables. Formula collection. Westermann Schulbuchverlag, 3rd edition 2002. Page 57
2. ↑ Ing: GdE: Models of the transformer on Wikibooks