EP0392202A2 - Use of a fine crystalline iron-based alloy as a magnet core in an interface transformer - Google Patents

Abstract

In the new digital ISDN communication system, transformation between the mains terminal (2) and terminal units (3) takes place via the so-called S0 interface by means of interface transformers (6, 11). Since the power supply to the terminal units also takes place partly via said transformers, a current imbalance in the conductors (7, 8) or (9, 10) results in a premagnetisation of the transformers. The ISDN requirements imposed on the transformers consequently have to be met even with direct current premagnetisation. Compact transformers with a simple winding structure which meet the ISDN requirements have, according to the invention, a magnet core material consisting of a fine crystalline iron-based alloy containing more than 60 atomic % of iron, more than 50% of whose structure consists of fine crystalline grains with a grain size of less than 100 nm and which has a remanence ratio of less than 0.2 and a permeability in the range from 20,000 to 50,000. …<IMAGE>…

Description

The invention relates to a magnetic core for an interface transmitter according to the preamble of claim 1. Such an interface transmitter is used, for example, in the so-called S₀ interface of the ISDN network as a transmitter at the interface between the network termination and the individual terminals.

ISDN is a new, global, digital communication system. With ISDN, the connection between a digital local exchange and a so-called network termination is made via a U _k0 line _interface . The distance between the digital local exchange and a network termination can be max. 8 km. Up to 8 end devices can be connected to a single network termination. The end devices can be, for example, telephone, screen telephone, screen text, facsimile, text fax, work station etc. The end devices can in turn be up to 150 m away from the respective network termination. The interface between the network termination and the end devices is referred to as the S₀ user interface.

The requirements for such an S₀ interface are laid down in the international standard CCITT I.430 and in the FTZ 1 TR 230 standard of the Deutsche Bundespost. These standards set, for example, the impedance of the cut place depending on the frequency or a so-called. Pulse mask for the transmitted digital pulses. For example, the company publication PUBL 1101E by H. Hemphill, Using Pulse Transformers for ISDN Applications from Schaffner Elektronik AG, Luterbach, Switzerland, deals with the requirements for the magnetic and electrical properties of S₀ interface transformers resulting from these standards. In this publication, the requirements for the impedance and the pulse transmission according to the postal standards are also shown in FIGS. 2 and 3. Whether a digital pulse can be transmitted within the specified pulse mask essentially depends on the inductance and the capacitance values of the transmitter. The inductance L of the transmitter essentially determines the roof drop of the transmitted pulse. Roof waste is the undesired decrease in the voltage of the transmitted pulse during the pulse duration. In order to meet the ISDN requirements, the inductance of the transmitter must be greater than approximately 20 mH at 10 kHz. The capacitance values of the transmitter have an effect on the signal shape of the transmitted pulse, in particular when changing from the high to the low state. Here, the lowest possible values for the coupling capacity are required. The coupling capacitance is the capacity between two different windings of the transformer. The coupling capacity depends, among other things, on the number of turns applied and also on the arrangement of the windings. As magnetic cores for an S₀ - interface transmitter, for example, so-called. RM6 cores specified. Ferrite is mentioned as the core material. When using ferrite cores, the values for the permeability µ and the saturation induction Bs are limited. Typical values for this are µ = 10,000, Bs = 0.45 T (SIFERIT T38 from SIEMENS).

The inductance of the transformer is directly proportional to the permeability of the core material. In order to meet the ISDN requirements with regard to inductance with the values of the permeability and saturation induction of the ferrites, in particular also in the case of DC biasing of the transmitter, either a comparatively large magnetic core cross section or high numbers of turns are required. However, a larger magnetic core cross section means an enlargement of the magnetic core and thus an increase in the volume of the transformer. However, components that are as small as possible are desirable. A higher number of turns initially means an increase in the coupling capacity and thus a deterioration in the transmission behavior. To avoid this, complicated winding arrangements with insulating layers between the windings are required. This makes the winding complicated and costly.

The object of the invention is to provide a magnetic core for an S₀ interface transmitter which has the smallest possible construction volume and which, with a simple winding structure and a low number of turns, allows the production of an S₀ interface transmitter according to the ISDN requirements. The ISDN requirements should in particular also be met when the transformer is DC-magnetized.

The object is achieved by the characterizing features of patent claim 1. Fine crystalline Fe-based alloys have very low magnetostriction values. This means that the permeability drop due to stresses in the material is very small.

With the magnetic cores according to the invention, compact interface transmitters with small dimensions can be produced. The interface transformers also meet the requirements of the standards with a simple winding structure requirements. In particular, the transmitters achieve the required values for the inductance even with a bias, as is to be expected due to an asymmetrical current distribution in the ISDN network. In the case of fine-crystalline Fe-based alloys with μ> 50,000, the permeability already decreases sharply with a low bias, so that the required inductance can only be achieved with a comparatively large magnetic core cross section or high number of turns. If the permeability µ <20,000, the required inductance is also achieved only by the measures mentioned.

Fine crystalline Fe-based alloys and processes for their production are known from EP-OS 271 657. These are in particular alloys which, in addition to iron, contain essentially 0.1 to 3 atom% of copper, 0.1 to 30 atom% of further metals such as Nb, W, Ta, Zr, Hf, Ti or Mo contain up to 30 atomic percent silicon and up to 25 atomic percent boron, the total content of silicon and boron being in the range between 5 and 30 atomic percent. Because of their good magnetic properties at high frequencies, these alloys are proposed for high-frequency transformers, chokes and magnetic heads. From EP-OS 299 498 magnetic cores made of a fine crystalline iron base alloy are also known, which largely retain their good magnetic properties even at elevated application temperatures. The fields of application mentioned are essentially the same as those already mentioned in EP-OS 271 657.

Surprisingly, it has now been found that fine-crystalline iron-based alloys with an initial permeability of more than 20,000 and less than 50,000 have only a very slight decrease in permeability when a DC field is present. These alloys are therefore extremely suitable for use as a magnetic core material in interface transformers, which have an inductance L of more than 20 mH, measured at 10 kHz coupling capacity should be as low as possible. The iron content of the suitable alloys is more than 60 atomic%. The alloys have a structure which consists of more than 50% of fine crystalline grains with a grain size of less than 100 nm, preferably less than 25 nm. The materials must have a flat hysteresis loop with a remanence ratio less than 0.2.

• Fig. 1 die Schnittstellen und induktiven Bauelemente im ISDN-Netz,
• Fig. 2 die Abhängigkeit der Permeabilität von einer Vor­magnetisierung bei 20 kHz und
• Fig. 3 die Abhängigkeit der Induktivität von einem Vor­magnetisierungsstrom bei 10 kHz.

The invention will now be explained in more detail with reference to the figures and examples. Show it:

• 1 shows the interfaces and inductive components in the ISDN network,
• Fig. 2 shows the dependence of the permeability on a bias at 20 kHz and
• Fig. 3 shows the dependence of the inductance on a bias current at 10 kHz.

1 shows the interfaces and inductive components in the ISDN network. These are the so-called. U _{K 0} - line interface between the digital exchange 1 and the network termination 2 (NT: Network Terminaton) and the S₀ subscriber interface between the network termination 2 and the terminals 3 (TE = Terminal Equipment). U _{K 0} - interface transmitter 4 are used to transmit the information between the digital switching center 1 and the network termination 2 . The processing of the digital signals in the network termination 2 is carried out by electronic components 5. The network termination also contains the NT interface transmitters 6 of the S₀ interface. The transmission of the digital signals between the network termination 2 and a terminal 3 takes place via the transmission lines 7, 8 and the receiving lines 9, 10. In the terminal 3 , the signals are converted via the TE interface transmitter 11 and further processing with electronic components 12. The terminal also contains current-compensated radio interference suppression chokes 13.

The magnetic cores according to the invention are used in the NT interface transformer 6 and the TE interface transformer 11 of the S₀ interface. The terminal devices are partially supplied with power by the digital switching center via the S₀ subscriber interface. This is the case, for example, if the terminal is a telephone. The remote supply of the terminals is not shown in FIG. 1. It takes place via the center tap 14 of the NT interface transmitter 6. In the ideal case which is not practical, the feed current is divided equally between the transmission lines 7, 8 and the reception lines 9, 10. In practice, however, the different current paths will have different resistances. Possible causes for this are, for example, different winding resistances of the transmitters and different resistances of the plug contacts of the lines or the connecting cord of a terminal. Such an asymmetry of the current in the transmission lines 7, 8 or in the reception lines 9, 10 leads to a premagnetization in the NT interface transmitters 6 or the TE interface transmitters 11 of the S₀ interface. Intensive investigations and calculations have shown that a bias current of approximately 3 mA must be expected in the TE interface transformer 11. The expected maximum bias current in the NT interface transformer 6, on the other hand, is much higher, since up to eight terminals can be connected in parallel to a network connection. A bias current of up to approximately 12 mA is expected for this.

In order to ensure the transmission of a digital pulse within the specified pulse mask as required in the standard, the transformer must also have the specified values magnetizing currents have an inductance of more than 20 mH. Furthermore, the coupling capacity should be low. The upper limit for this is about 100 pF.

Embodiments

The magnetic core materials mentioned in the following examples were produced in the form of thin strips by the method known from EP-OS 271 657. Toroidal cores were then wound out of the tapes. The toroidal cores were then subjected to a heat treatment in the transverse field, ie in a magnetic field parallel to the rotational symmetry axis of the toroidal cores. This resulted in flat hysteresis loops with a remanence ratio B _r / B _s of less than 0.2, B _r indicating the remanent induction and B _s the saturation induction. For comparison, toroidal cores were also heat-treated in a longitudinal field or without a magnetic field. This results in magnetic core materials with values for the initial permeability and the remanence ratio outside the claimed range. Finished transformers were manufactured with toroidal cores measuring 0̸ 14 X 0̸ 7 x 6 mm and the dependence of inductance L on a bias current at 10 kHz was measured.

Example a):

A magnetic core containing 73.5 atomic% iron, 1 atomic% copper, 3 atomic% niobium, 13.5 atomic% silicon and 9 atomic% boron was subjected to heat treatments of 1 h, 540 in a transverse field ° C and 3 h, 280 ° C, subjected. This magnetic core had an initial permeability of 23,000. 2 shows the dependence of the normalized permeability (permeability with premagnetization divided by permeability without premagnetization) as a function of the premagnetization. There is a slight dependence of the permeability and thus also the inductance on the premagnetization (curve A). In Fig. 3, curve A is the Dependence of the inductance on a bias current for a transformer with a total number of turns 2 N = 48 plotted. This magnetic core is ideally suited for use in an interface transformer that is subjected to a direct current bias. Even with a direct current preload of 12 mA, the inductance is still 33 mH. The required inductance of the transformer of at least 20 mH can be achieved with this core even with a pre-magnetization of 12 mA with a total number of turns of 2 N = 36. This low number of turns results in a low value for the coupling capacitance even with a simple winding construction of only about 35 pF .

Example b):

Magnetic materials with the same composition as in example a) were subjected to a heat treatment in the transverse field of 1 h, 540 ° C. with subsequent cooling of 10 K / min in this field. The toroidal cores made from it had an initial permeability of 31,000. The dependence of the permeability on the premagnetization is in turn plotted in FIG. 2 (curve B). These magnetic cores also showed only a very low dependency of the permeability on the premagnetization. Finished transformers with a total number of turns of 2 N = 40 had inductance values clearly above the required minimum value (FIG. 3, curve B).

Example c)

Magnetic core materials with the same composition as in examples a) and b) were subjected to a heat treatment in the transverse field of 1 h, 540 ° C. with subsequent cooling in air. This heat treatment achieved a somewhat higher initial permeability value of around 35,000. As can be seen from FIG. 2, curve C, the permeability falls with increasing premagnetization in this Fall off a little more. However, even with this core, the requirements placed on the interface transformer could be met well, as can be seen from FIG. 3, curve C for transformers with a total number of turns 2 N = 38.

Example d)

Magnetic core materials containing 73.5 atomic% iron, 1 atomic% copper, 3 atomic% niobium, 16.5 atomic% silicon and 6 atomic% boron were subjected to the same heat treatment as in Example a). An initial permeability of 28,000 was measured on these materials. As can be seen from FIG. 2, curve D, these magnetic cores also showed only a slight dependence of the permeability on a premagnetization. The requirements for inductance were again met very well with a transformer with a total number of turns of 2 N = 42 (FIG. 3, curve D).

Example e):

Magnetic core materials of the same composition as in Example d) were subjected to a heat treatment as in Example b). The dependence of the permeability on the bias is shown in FIG. 2, curve E, the dependence of the inductance on a bias current for a transformer with 2 N = 38 in FIG. 3, curve E.

Example f)

Cores with the same composition as in Examples d) and e) were subjected to a heat treatment as in Example c). A permeability of 38,000 was determined. The decrease in permeability as a function of a bias was again somewhat greater than in Examples d) and e) and is shown in FIG. 2, curve F. As can be seen from FIG. 3, curve F, an inductance of even more than 30 mH with a bias current of 12 mA was also achieved here with a total number of turns 2 N = 36.

As can be seen from the examples mentioned above, all magnetic cores according to the invention from the examples are therefore very well suited for use in interface transformers.

For comparison, magnetic core materials of the same composition as in Examples a) to c) were subjected to a heat treatment without a magnetic field for 1 h at 540 ° C with subsequent air cooling (Example g)) and a heat treatment in a longitudinal field of 1 h, 540 ° C with a then subjected to a cooling rate of 1 K / min (example h). The core heat-treated without a magnetic field had an initial permeability of 58,000 and the core treated in the longitudinal field had an initial permeability of 6,000. As can be seen from FIG. 2 (curves G and H), these comparison cores showed a very strong decrease in the permeability in the case of a DC bias. Finished transformers with the material treated without a magnetic field (example g), which had a total number of turns of 2 N = 28, achieved an inductance of approximately 35 mH comparable to the transformers according to the invention without a bias current, but only an inductance of 7 with a bias current of 12 mA mH, as can be seen from FIG. 3, curve G. Transformers that contained toroidal cores with the material from Example h) tempered in the longitudinal field also showed a sharp drop in inductance with increasing bias current, as can be seen in FIG. 3, curve H for a transformer with a total number of turns of 2 N = 42 .

In contrast, the magnetic cores according to the invention can be used to produce very compact transmitters which meet the ISDN requirements. In particular, they can also be used for the NT interface transformers 6, in which a bias current of up to approximately 12 mA is expected.

Claims (4)

1. Magnetic core for an interface transformer, which has an inductance L of more than 20 mH with the lowest possible coupling capacity for use in a digital transmission system, characterized in that the magnet core material is a low-magnetostriction Fe-based alloy with an iron content of more than 60 atomic % is used, the structure of which consists of more than 50% of finely crystalline grains with a grain size of less than 100 nm and which has a remanence ratio B _r / B _s of less than 0.2 and a relative initial permeability in the range from 20,000 to 50,000 having. 2. Magnetic core according to claim 1, characterized by a grain size of less than 25 nm. 3. Magnetic core according to claim 1, characterized in that an alloy is used which, in addition to iron, contains essentially 0.1 to 3 atom% of copper, 0.1 to 30 atom% of other metals, such as niobium, tungsten, tantalum, zircon , Hafnium, titanium or molybdenum, contains up to 30 atomic% silicon and up to 25 atomic% boron, the total content of silicon and boron being in the range between 5 and 30 atomic%. 4. Interface transformer which contains a magnetic core according to claim 1 and which has an inductance L of more than 20 mH with a coupling capacitance as low as possible up to a direct current bias of 12 mA.

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