EP0378823B1 - Use of a magnet core in an interface transformer - Google Patents

Description

The invention relates to a magnetic core in which a low magnetostriction amorphous Co-base alloy is used as the magnetic core material.

Low magnetostriction amorphous Co-based alloys are also known for use in magnetic cores, for example from EP 21 101. By varying the alloying elements and the heat treatment after the alloy has been quenched into an amorphous band, largely different magnetic values can be set. Low-magnetostriction amorphous Co-based alloys are also described in DE-OS 30 21 536. By substituting the Co with small amounts of Fe and or Mn, particularly low magnetostriction values can be achieved. By tempering the amorphous strips obtained by quenching, permeabilities can be set, for example, in the range from 8,000 to 40,000. These permeabilities were measured in a magnetic field of 1 to 10 mOe at a frequency of 1 kHz. After heat treatments in an opposing magnetic field, the permeabilities were around 2,000.

It is an object of the present invention to provide a magnetic core which is particularly advantageous for interface transmitters in a digital transmission system. Such interface transmitters are used 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 takes place via a U _{kO 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 terminals can be, for example, telephone, screen telephone, screen text, facsimile, text fax, work station, etc. The terminals 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 interface 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. 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 capacitance 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 and the saturation induction Bs are limited. Typical values for this are »= 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 with 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 lying between the windings are required. This makes the winding complicated and costly.

The object of the invention is to obtain, by using a magnetic core with certain material properties, an S₀ interface transformer 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 transformer according to the ISDN requirements. The ISDN requirements should also be met, in particular, when the transformer is DC-magnetized.

The object is achieved by the characterizing features of patent claim 1. Co-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 transformers with small dimensions can be produced. The interface transformers also meet with a simple winding structure the requirements specified in the standards. In particular, the transmitters achieve the required values for the inductance even with a premagnetization, as is to be expected due to an asymmetrical current distribution in the ISDN network. With Co-based alloys>> 95,000, the permeability already decreases sharply with a low pre-magnetization, 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 is <25,000, the required inductance is also only achieved by the measures mentioned.

It has proven particularly advantageous to use a cobalt-based alloy which, in addition to cobalt, essentially contains iron and manganese with a total proportion of 3 to 8 atom% and metalloids with a proportion of 24 to 29 atom%. Amorphous cobalt-based alloys with a metalloid content in the range from 5 to about 35 atom% are known for example from EP-PS 21 101 and DE-OS 3 021 536. However, it has been found that cobalt-based alloys, after setting a flat magnetization curve by a heat treatment in the transverse field with a metalloid content of less than 24 atom% or more than 29 atom%, do not meet the requirements for the initial permeability. Boron, silicon, carbon and phosphorus can be used as metalloids.

$$\text{e = » (1 + 0,25 (a + c))}$$

$$\text{f = 0,25 (a - b) - 0,2 c - d + 3,2 x ist.}$$

A combination of boron and silicon has proven to be advantageous, wherein the boron can be partially replaced by carbon. By adding manganese, the magnetic cores according to the invention meet the ISDN standards of the S₀ interface even in the case of premagnetizations such as are expected in the transformers in the network termination. Furthermore, the amorphous cobalt base alloys can also contain nickel with a proportion of up to 15 atom% and one or more of the elements molybdenum, chromium or niobium with a proportion of up to 1 atom%. Magnetic cores with the highest permissible However, values for the premagnetization are achieved with the cobalt-iron-manganese-metalloid alloys if the manganese content is at least 0.5 atomic%. In extensive research and test series it was found that cobalt-based alloys in particular are suitable magnetic core materials for ISDN interface transmitters in which the metalloid content is dependent on the permeability, the iron content a, the manganese content b, the nickel content c and the molybdenum-chromium-niobium Content d and the boron or carbon content x is given by: $$\text{18 + 1.4 * lgA + B <z <18 + 1.7 * lge + f, where}$$

$$\text{e = »(1 + 0.25 (a + c))}$$

$$\text{f = 0.25 (a - b) - 0.2 c - d + 3.2 x.}$$

With the magnetic cores according to the invention, S₀ interface transmitters can be produced with an iron cross-section of less than 0.2 cm² for the network termination side or with an iron cross-section of less than 0.1 cm² for the end device side.

Fig. 1

die Schnittstellen und induktiven Bauelemente im ISDN-Netz,

Fig. 2

den Zusammenhang zwischen Metalloid-Gehalt und Anfangspermeabilität,

Fig. 3

den Zusammenhang zwischen der Feldstärke Ho, bei der die Permeabilität auf 70 % der Anfangspermeabilität abgesunken ist, und der Anfangspermeabilität,

Fig. 4

die Abhängigkeit der Induktivität von einer Gleichstrom-Vorbelastung im Bereich von 0 bis 10 mA,

Fig. 5

die Abhängigkeit der Induktivität von einer Gleichstromvorbelastung im Bereich von 0 bis 28 mA.

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

Fig. 1

the interfaces and inductive components in the ISDN network,

Fig. 2

the relationship between metalloid content and initial permeability,

Fig. 3

the relationship between the field strength Ho, at which the permeability has dropped to 70% of the initial permeability, and the initial permeability,

Fig. 4

the dependence of the inductance on a DC bias in the range of 0 to 10 mA,

Fig. 5

the dependence of the inductance on a DC bias in the range of 0 to 28 mA.

1 shows the interfaces and inductive components in the ISDN network. These are the so-called. U _K ₀ - line interface between the digital switching center 1 and the network termination 2 (NT: Network Terminaton) as well as the S₀ subscriber interface between the network termination 2 and the terminals 3 (TE = Terminal Equipment). U _K ₀ interface transmitters 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 transformers 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 processed with electronic components 12. The The terminal also contains current-compensated radio interference suppression chokes 13.

The magnetic cores according to the invention are used in the NT interface transmitter 6 and the TE interface transmitter 11 of the S₀ interface. The terminal devices are partly supplied with power from 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 transformer 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. The reasons for this include, for example, different winding resistances of the transformers and different resistances of the plug contacts of the lines or the connecting cord of a terminal into consideration. 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 transformers 6 or the TE interface transformers 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 by the standard, the transformer must have an inductance of more than 20 mH even with the specified bias currents. Furthermore, the coupling capacity should be low. The upper limit for this is about 100 pF.

Embodiments

In the following examples, the amorphous magnetic core materials were produced in the form of thin strips by the melt spin process. This method is well known and does not form the subject of this invention. Toroidal tape cores were then wound from the amorphous 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. For this purpose, the cores were heated to a temperature of about 420 ° C. and then cooled at a cooling rate of 0.1 to 3 K / min. The magnetic cores according to the invention have magnetostriction values of less than 0.3 * 10⁻⁶. This means that the permeability drop due to stresses in the material is very small.

Example 1: Co-Fe-M alloys

A magnetic core with the alloy composition Co _68.2 Fe₄Si _16.8 B₁₁ was produced by the method described above. Different cooling speeds of 0.2, 0.4 and 1.0 K / min were selected. Table 1 lists the values for the saturation induction Bs, the initial permeability », measured at a frequency of 20 kHz, and the magnetic field strength Ho, at which the permeability has dropped to 70% of the value of the initial permeability. The magnetic field strength Ho, together with the initial permeability, provides information about the suitability as a transmitter material in the case of a pre-magnetization. Small Ho values mean suitability only with a small bias.

It can be seen that the initial permeability increases with increasing cooling speed, while the field strength Ho decreases accordingly. In order to meet the requirements of the ISDN standard with regard to the transmission of a digital pulse via an S₀ interface, it is assumed on the basis of the expected bias that Ho is not less than 70 mA / cm in the case of the NT interface transmitter 6 and in the case of the TE Interface transformer 11 must not be less than about 20 mA / cm. Because of its Ho values, the alloy mentioned is therefore suitable for both interface transformers.

Example 2: Co-Fe-Me-M alloys

In Table I, under Leg.No. 2.1 to 11.1 Alloys listed, which now contain manganese in addition to Co, Fe and metalloids. The magnetic cores with the numbers 2.1 to 2.3 have values for the initial permeability of over 95,000 and are therefore outside the claimed one Protection area. The corresponding Ho values are well below 20 mA / cm, so that these magnetic cores are not suitable for the interface transformers mentioned. The magnetic cores 3.1 to 10.3, on the other hand, have all values of the initial permeability within the range from 25,000 to 95,000. With these magnetic cores too, the heat treatment has the same influence on the permeability and on Ho as with the magnetic cores according to Example 1. However, the value for Ho always remains above 20 mA / cm, regardless of the cooling rate during the heat treatment. A magnetic core is listed under No. 11.1, which has an alloy composition outside the claimed range. The metalloid content in this example is 20 atom%. The value for Ho is extremely high with Ho = 5060, but the initial permeability with »= 1000 is too low, so that with this core, the requirements for the interface transformer mentioned can only be met with an enlarged magnetic core cross section or an increased number of turns.

Example 3: Co-Fe-T-M alloys

Table II lists the magnetic values of magnetic cores made of manganese-free alloys. The alloys also contain 1.5 atomic% molybdenum. A comparison shows that these magnetic cores have smaller Ho values than the manganese-containing magnetic cores according to Example 2. The manganese-free magnetic cores with a molybdenum additive can therefore be used advantageously for TE interface transformers 11. As magnetic core no. 14.1 shows, suitable heat treatment can also achieve high Ho values that allow the use of these magnetic cores in the NT interface transformer 6.

$$\text{e = » (1 + 0,25 ( a + c))}$$

$$\text{f = 0,25 (a - b) - 0,2 c - d + 3,2 x.}$$

The relationship between the metalloid content and the initial permeability is shown graphically in FIG. The pairs of values represented by rectangles relate to an alloy of the composition Co _rest Fe _3.2 Mn 1 (Si _0.6 B _0.4 ) _z The pairs of values represented by stars refer to an alloy of the composition

Co _rest Fe _3.8 Mo _1.5 (Si _0.6 B _0.4 ) _z.


From such an application, particularly advantageous range limits for the metalloid content can be specified depending on the content of other metal alloy components and on the initial permeability. For a magnetic core according to the invention with the alloy composition Co _Rest Fe _a Mn _b Ni _c T _d (Si _1-x (B, C) _x ) _z
where T is at least one of the elements molybdenum, chromium or niobium, the individual metal alloy components of a magnetic core according to the invention are given by: 3 <a + b <8, c <15, d <1, 0.3 <x <0.7 . The permissible metalloid content of the magnetic cores is: $$\text{18 + 1.4 * lge + f <z <18 + 1.7 * lgA + B with}$$

$$\text{e = »(1 + 0.25 (a + c))}$$

$$\text{f = 0.25 (a - b) - 0.2 c - d + 3.2 x.}$$

3 shows the relationship between Ho and the initial permeability for manganese-containing and manganese-free magnetic cores according to Examples 2 and 3, respectively. The illustration shows that the Ho values are increased by the addition of manganese. In particular, the manganese-containing magnetic cores can achieve Ho values that are significantly higher than those of the manganese-free magnetic cores. The manganese-containing alloys are therefore the preferred magnetic core materials for the claimed interface transformers. The manganese-containing alloys are therefore preferred as magnetic core materials, especially for interface transformers with a high DC bias.

Example 4:

Amorphous cobalt-based alloys containing various combinations of iron, manganese, nickel and molybdenum additives were examined in a further extensive series of tests. The results are summarized in Tab. 3. The initial permeability was again measured at a frequency of 20 kHz. As the results show, permeability values in the range from 25,000 to 95,000 can also be achieved with these magnetic cores. However, this often requires a relatively high cooling rate during heat treatment, which is technically more difficult to implement. The elements chrome or niobium have the same effect as molybdenum. Suitable magnetic cores can therefore also be produced with amorphous cobalt-based alloys which, in addition to cobalt, iron, manganese and metalloids, also contain nickel, molybdenum, chromium or niobium. However, preference is given to alloys which are free from the latter alloy elements.

Example 5:

Finished transformers were produced with the aid of magnetic cores with alloy compositions according to Examples 2 and 3. The transformers had two windings with the same number of turns. The finished components had the dimensions 9.8 x 6.8 x 5 mm. The inductance of the transformers was measured without DC bias L (0) and with bias with a DC current of I = 2.5 mA and the coupling capacitance C. The results are summarized in Table 4. With a magnetic core of No. 3.2 from Example 2, an inductance of more than 20 mH was achieved even with a total number of turns of 2 N = 34, even with a direct current preload of 2.5 mA. The inductance value could be increased by increasing the number of turns; at the same time, however, the coupling capacity increased slightly.

Sufficient inductance values and low coupling capacitance values could also be achieved with magnetic cores No. 5.3, 6.3 and 7.3. The magnetic cores mentioned have permeability values between 67,000 and 86,000. The Ho values are in the range between 26 and 45 mA / cm.

Furthermore, transformers with manganese-free magnetic cores No. 12.1 from example 3 were produced. These magnetic cores have a Ho value of only 15 mA / cm. For this reason, the required inductance value was not achieved with a DC preload of 2.5 mA with a total number of turns of 2 N = 34, 38 or 42. However, the specifications were met for transformers with a total number of turns of 2 N = 46, 50 and 54. However, due to the higher number of turns, these transformers are less favorable for economic reasons.

In Fig. 4 the dependence of the inductance on the DC bias for transformers with magnetic cores No. 3.2 and 12.1 is shown graphically. 2 x 19 turns are applied to the magnetic cores. The transformer with the manganese-containing magnetic core 3.2 shows a significantly higher direct current load than the transformer with the manganese-free core No. 12.1. The transformer with the manganese-containing core no. 3.2 fulfills the ISDN requirements in the specified embodiment up to a direct current bias of approximately 5 mA. It can therefore be used in particular as a TE interface transformer 11.

Example 6:

Transformers were manufactured with the magnetic cores No. 1.2, 7.1, 9.2, 10.2, 11.1 and 27.1. The transformers in turn had two windings of the same number of turns. The dimensions of the finished component were 14 x 7 x 6 mm. The inductance L (0) without premagnetization and the Inductance with a DC bias of 12 mA and the coupling capacitance C. The measurements were carried out at a frequency of 20 kHz. The results are summarized in Tab. 5.

The transformers with the magnetic cores No. 1.2, 7.1, 9.2 and 10.2 already showed very good values for the inductance and the coupling capacitance with a small total number of turns of 2 N = 38, for example. You thus meet the ISDN requirements. Because of their high DC current load capacity, these transformers can be used in particular as NT interface transformers 6. The permeability of the cores used as an example is between 34,000 and 39,000. The Ho values are in the range between 90 and 108 mA / cm. It can be seen from the comparison with the values from Example 5 that magnetic cores with a lower initial permeability and a higher Ho value must be selected for higher direct current preloads. This can also be seen from the transformers with magnetic core no. 11.1 and 27.1 also listed in Tab. 5. With these magnetic cores not according to the invention, the requirement for the inductance value was only met when the total number of turns was 120 or 200. However, these high numbers of turns in turn result in very high coupling capacities. To reduce the coupling capacity, a complex and thus expensive winding structure would be necessary to reduce the coupling capacity in these transmitters, for example by the application of insulating layers lying between the windings.

5 shows the dependency of the inductance on the DC bias for two transmitters with the magnetic cores No. 9.2 and 14.1. The transformer contained 2 x 19 turns (2 N = 38). The transformer with the manganese-free core No. 14.1 fulfills the ISDN requirements for inductance up to a DC current load of around 10 mA. The transmitter with the manganese-containing magnetic core No. 9.2, on the other hand, fulfills the ISDN requirements with regard to Inductance up to a DC bias of about 14 mA. With the specified size and number of turns, it can thus be used as an NT interface transformer 6. FIG. 5 again illustrates the superiority of the magnetic cores containing manganese with a high direct current preload.

The magnetic cores according to the invention can thus be used to produce very compact transmitters which meet the ISDN requirements. The suitable magnetic cores for the different direct current preloads can easily be selected on the basis of the examples given.

Claims (5)

1. Use of a magnetic core in which the magnetic core material consists of a low-magnetostriction, amorphous Co base alloy, characterised in that the magnetic core is used in an interface transformer which is also suitable for direct current magnetic biasing and which for use in a digital transmission system exhibits an inductance L of more than 20 mH at 20 kHz with the lowest possible switching capacitance, in that, in addition to Co, the amorphous Co base alloy essentially contains Fe and Mn with a total content of 3 to 8 gram-atomic % and metalloids with a content of 24 to 29 gram-atomic % and optionally up to 15 gram-atomic % of Ni and up to 1 gram-atomic % of Mo, Cr and/or Nb and in that the magnetic core material has been subjected to a heat treatment in a transverse field so that a permeability of more than 25,000 and less than 95,000 results.
2. Use of a magnetic core according to claim 1, characterised in that the Co base alloy exhibits the composition
   Co_remainderFe_aMn_bNi_cT_d(Si_1-x(B and/or C)_x)_z, in which T is at least one of the elements Mo, Cr or Nb and in which the following relations apply to the alloy contents (in gram-atomic %): $$\text{3 < a + b < 8}$$

   

   $$\text{c < 15}$$

   

   $$\text{d < 1}$$

   

   $$\text{0.3 < x <0.7}$$

   

   $$\text{18 + 1.4 · lg A + B < z < 18 + 1.7 · lg e + f in which}$$

   

   $$\text{e = » (1 + 0.25 · (a + c))}$$

   

   $$\text{f = 0.25 · (a - b) - 0.2 · c - d + 3.2 · x}$$

   

   with the remainder Co and impurities, » denoting the initial permeability and lg e the decimal logarithm of e.
3. Use of a magnetic core according to claim 2, characterised by an Mn content of more than 0.5 gram-atomic % (b > 0.5).
4. Use of a magnetic core according to claim 2, characterised in that the magnetic core material contains no Ni (c = 0).
5. Use of a magnetic core according to claim 2 or 4, characterised in that the magnetic core material contains no Mo, Cr or Nb (d = 0).

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