Pulse transformer magnetic core

Abstract: A pulse transformer magnetic core comprising a magnetic core main body of a soft magnetic alloy ribbon having a thickness of 25 µm or less, the AL value of the magnetic core main body being 4.0 µH/N 2 or more when 0.1 V is input at 10 kHz. The magnetic core main body has an outer diameter of 10 mm or less and a thickness of 1.2 mm or less. The magnetic core main body may comprises by coiling the soft magnetic alloy ribbon or by stacking rings or E-shape and I-shape pieces which are made from the soft magnetic alloy ribbon by a punching process.

Description: The present invention relates to a pulse transformer magnetic core which exhibits excellent frequency characteristics of impedance and excellent pulse transmittance characteristics. Recent trends proceeding in electronic equipment fields include achievement of miniaturization, thinning and high performance. In particular, pulse transformers for interfaces of ISDN (Integrated Services Digital Network) and the like must satisfy electric characteristics defined by severe standards, e.g. ITU-T Recommendation I.430. In the electric characteristics defined in such a standard, a required impedance for the primary winding of the pulse transformer is at least 1,250 Ω at 10 kHz and 2,500 Ω at 100 kHz. These impedances correspond to inductances of 20 mH and 4 mH, respectively. The output pulse voltage waveform must be within a pulse mask range specified in the standard set forth above. Further, it is preferred that the primary winding have inductance characteristics that are as flat as possible. Miniaturization of pulse transformers is highly required in order to package PC cards and the like. For example, when packaging a pulse transformer to an internal board of a PCMCIA card (a type of interface standard cards for notebook personal computers), the height of the pulse transformer must be not more than 3 mm since the card itself has a thickness of approximately 5 mm. The packaging area in this case must be typically 14.0 mm by 14.0 mm or less. Under such circumstances, high permeability ferrite is mainly used as a core material of a pulse transformer for ISDN, at the present stage. The pulse transformer has an EI- or EE-shape magnetic core of which the butting faces are mirror-polished. The EI-shape magnetic core is made by integration of an E-shape core material with an I-shape core material by butting, and wiring is performed on the E-shape core material to form a transformer. The EE-shape magnetic core is made by integration of two E-shape cores butting each other. Although high permeability ferrite used in pulse transformer magnetic cores for ISDN has an official value of initial permeability of 10,000 to 12,000, the initial permeability of ferrite varies remarkably with the temperature and has a value which is approximately 40% lower than the official value at -20 °C. Thus, a significantly lower initial permeability than the official value must be taken into account in the design of a transformer using a ferrite core, when the operation of the pulse transformer must be assured at a temperature ranging from -40 °C to 100 °C. For achieving an ISDN required inductance for a pulse transformer, the effective cross-section of the magnetic core or turns must be increased. However, increased turns in a pulse transformer having a conventional structure results in increased leakage inductance and stray capacitance due to unavoidable approaching of sections having different voltages in the wound coil, for example, the start and end points of winding. Thus, the transmittance frequency region of the transformer is narrowed and the waveform transmission fidelity deteriorates. On the other hand, an increase in the effective cross-section of the magnetic core is incompatible with the miniaturization of the pulse transformer itself. Therefore, it is difficult to produce a pulse transformer with a ferrite core having a height of 3 mm or less and exhibiting excellent transmitting characteristics in accord with ISDN within the restriction of the packaging area set forth above. Some pulse transformers satisfy the demanded characteristics to a minimum by using a thin ferrite magnetic core and by increasing the turns to 100 or more. However, such pulse transformers do not satisfy the demanded characteristics when the turns are decreased to less than 100. EP-A-0 392 202 discloses a pulse transformer magnetic core comprising a toroidally coiled soft magnetic alloy ribbon. The magnetic core comprises a Fe-based fine crystal alloy the specific dimensions of the core are not indicated. The average crystal grain size of more than 50% of the material is less than 25 nm. EP-A-0 509 936 discloses a toroidally coiled soft magnetic alloy ribbon placed inside a magnetic core case. This document does not disclose the specific dimensions of the magnetic core. Patent Abstracts of Japan, Vol. 015 No. 139 (E-1053), April 9, 1991 & JP 03 019307 A discloses a magnetic core, wherein a gel-like resin is used for fixing a Fe radical soft magnetic alloy thin bend wound to obtain a magnetic core. The magnetic core is received in a case and then fixed by using said gel-like silicon rubber. It is an object of the present invention to provide a pulse transformer with a height of 3 mm or less and decreased turns which exhibits excellent frequency-impedance characteristics and excellent transmitting characteristics over a wide temperature range. It is another object of the present invention to provide a pulse transformer in which the cross-section area of the magnetic core is increased and stress is prevented by a coating resin. A pulse transformer magnetic core in accordance with the present invention comprises the features of any of claims 1, 2 and 3. Preferred embodiments are defined by the dependent claims. The magnetic core main body may comprise stacking rings made from the soft magnetic alloy ribbon and has an outer diameter of 10 mm or less and a thickness of 1.2 mm or less. The magnetic core main body may comprise any combination of an E-shape core and an I-shape core, a U-shape core and an I-shape core, and two U-shape cores, in which the E-shape core, the I-shape core and the U-shape core are formed by stacking E-shape thin pieces, I-shape thin pieces, and U-shape thin pieces, respectively, which are formed from the soft magnetic alloy ribbon with the magnetic core main body having a thickness of 1.2 mm or less. The magnetic core main body may comprise a toroidal ring formed by coiling the soft magnetic alloy ribbon having a width of 1.2 mm or less, and the outer diameter of the toroidal magnetic core main body being 10 mm or less. The rings are preferably packed within a covering member made of a resin at a packing rate of 50% or more. The soft magnetic alloy ribbon preferably has an absolute value of magnetostriction of 1×10 -6 or less. Variation in the AL value of the pulse transformer magnetic core over a temperature range of -40 °C to +100 °C from room temperature may be within ±20%. The magnetic core main body is preferably impregnated with a silicone rubber having a viscosity before curing of 1 Pa·s or less which is gelated by curing. The magnetic core main body is preferably impregnated with a silicone rubber having a viscosity before curing of 1.5 Pa·s or less and a JIS A hardness of 10 or less, the silicone rubber acting as an adhesive agent for fixing the magnetic core main body to a magnetic core case. The adhesive agent for fixing the magnetic core main body to the magnetic core case is preferably a silicone rubber having a viscosity before curing of 2 Pa·s or less and a JIS A hardness after curing of 25 or less. The adhesive agent is preferably applied to two to four sections on the bottom face of the magnetic core case. The soft magnetic alloy may be characterized in that 50% or more of the soft magnetic alloy essentially consists of body centered cubic fine crystal grains with an average crystal grain size of 30 nm or less, in which the soft magnetic alloy comprises Fe as a main component; at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Mo and W; and B. A pulse transformer magnetic core in accordance with a second aspect of the present invention comprises: a magnetic core main body comprising a toroidally coiled soft magnetic alloy ribbon, and a magnetic core case having an opening for holding the magnetic core main body. Both ends of the inner wall and outer wall of the magnetic core case preferably have curvature radii of 0.05 mm to 0.4 mm. The magnetic core main body is preferably packed in the magnetic core case at a packing rate of 50% or more. The magnetic core case may have an outer diameter of 10 mm or less, an inner diameter of 3.5 mm or more and a height of 1.3 mm or less, and an AL value of 6.0 µH/N 2 or more when 0.1 V is input at 10 kHz. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an assembly view of a transformer having a magnetic core comprising stacking rings stamped from a soft magnetic alloy ribbon in accordance with the present invention; Figure 2 is an assembly view of a transformer having a magnetic core formed by winding a soft magnetic alloy ribbon in accordance with the present invention; Figure 3 is an assembly view of a magnetic core for a pulse transformer in accordance with the present invention; Figure 4 is a cross-sectional view of line A-A of a magnetic core container illustrated in Figure 3; Figure 5 is a graph illustrating correlation between packing rate and impedance when soft magnetic alloy rings having a composition of Fe 86 Nb 3.25 Zr 3.25 B 6.5 Cu 1 are packed into a case; Figure 6 is a graph illustrating correlation between permeability and impedance when soft magnetic alloy rings having a composition of Fe 86 Nb 3.25 Zr 3.25 B 6.5 Cu 1 are packed into a case; Figure 7 is a graph illustrating change in the AL value at 10 kHz and 100 kHz with respect to thickness of a soft magnetic alloy ring; Figure 8 is a graph illustrating correlation between packing rate and impedance when each soft magnetic alloy ring having a composition of Fe 86 Nb 3.25 Zr 3.25 B 6.5 Cu 1 , Fe 84 Nb 3.5 Zr 3.5 B 8 Cu 1 and Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 , respectively, is packed into a case; Figure 9 is a graph illustrating correlation between permeability and packing rate of each sample set forth in Figure 8; Figure 10 is a graph illustrating correlation between AL value and packing rate of each sample set forth in Figure 8; Figure 11 a graph illustrating correlation between variation in permeability and temperature of each sample set forth in Figure 8 and a ferrite material; Figure 12 is a graph illustrating variation in AL value with temperature of a sample in accordance with the present invention and a sample for comparison; Figure 13 is a schematic view of an embodiment of a production apparatus of an alloy ribbon; Figure 14 includes DSC thermograms of a sample in accordance with the present invention and a sample for comparison; Figure 15 is a graph illustrating correlation between permeability and holding time; Figure 16 is a graph illustrating correlation between coercive force and holding time and between saturation magnetostriction and holding time; Figure 17 is a graph illustrating correlation between crystal grain size and holding time; Figure 18 is a graph illustrating correlation between permeability and holding temperature; and Figure 19 is a graph illustrating correlation between permeability and holding time. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be illustrated with reference to the embodiments and drawings. A magnetic core of a pulse transformer in accordance with the present invention has, for example, a toroidal shape. Such a toroidal magnetic core of the pulse transformer is formed by preparing a soft magnetic alloy ribbon having a composition set forth below by a quenching process, press-punching the ribbon to obtain rings, and stacking predetermined numbers of rings, or by coiling the soft magnetic alloy ribbon into a toroidal shape. The resulting magnetic core is coated with, for example, an epoxy resin or encapsulated into a resin case for insulation, and wiring is performed to obtain a pulse transformer magnetic core. An EI-shape magnetic core is made as follows: A plurality of E-shape thin pieces and I-shape thin pieces are prepared from the soft magnetic alloy ribbon set forth above by press-punching; an E-shape core and an I-shape core are produced by stacking E-shape thin pieces and I-shape thin pieces, respectively; and the E-shape core and the I-shape core are butt-jointed. Alternatively, after given sections of the E-shape core and the I-shape core are insulated by resin coating or by encapsulation into a resin case and wiring performed, the sides of the E-shape core and the I-shape core are butt-jointed. The combination of the magnetic cores is not limited to the E-shape core and the I-shape. For example, any combination such as two E-shape cores, a U-shape core and an I-shape core, and two U-shape cores are usable as the magnetic core. Figures 1 and 2 show embodiments of toroidal transformers. In Figure 1, the toroidal transformer comprises a circular upper case 1, a circular lower case 2 and a magnetic core main body 3 of soft magnetic alloy ribbon rings which are stacked in upper and lower cases 1 and 2. In Figure 2, the toroidal transformer comprises a circular upper case 1, a circular lower case 2, and a magnetic core main body 3 of a soft magnetic alloy ribbon 5 which is coiled within the upper and lower cases 1 and 2 and covered with a resin. The upper and lower cases are not always used, and thus the magnetic core may be made only by resin coating. Figure 3 shows another embodiment of the magnetic core of the pulse transformer and Figure 4 is a cross-section view of line A-A of a magnetic core case 7 in Figure 3. This pulse transformer has a toroidal shape and comprises a circular case 7 with a central cavity and a magnetic core main body 3 formed by toroidally coiling a soft magnetic alloy ribbon 5 which is placed in the circular case 7. The top of the magnetic core case 7 has an opening 7a which is not covered with a lid or the like. Such a magnetic core case 1 without a lid has a large volume relative to the size of the entire pulse transformer magnetic core. Thus, this pulse transformer exhibits improved inductance by increasing the cross-section area of the magnetic core main body 3 without changing the size of the entire pulse transformer, compared to that shown in Figure 1 comprising the upper case 1 and the lower case 2. Alternatively, the pulse transformer magnetic core can be miniaturized using a magnetic core main body 3 having the same cross-section area as that shown in Figure 1. The magnetic core case 7 is provided with an inner wall and an outer wall, the top and bottom ends 7b and 7c of the inner wall and the top and bottom ends 7d and 7e of the outer wall have curvatures of radii of 0.05 mm to 0.4 mm. When the radius of curvature is not more than 0.05 mm, the coating layer of the wound coil 9 may be damaged or the coil may be cut by the top and bottom ends 7b, 7c, 7d and 7e when coiling the wound coil 9 around the magnetic core case 7. On the other hand, a radius of curvature of over 0.4 mm causes increased thickness of the magnetic core case 7. As a result, the cross-section area of the magnetic core main body 3 and the AL value decrease. The magnetic core case 7 is preferably formed of a synthetic resin, for example, polyacetal resin or polyethylene terephthalate resin. An adhesive agent 4 is applied to two positions on the bottom 7f of the magnetic core case 7 to fix the magnetic core main body 3 to the magnetic core case 7. The adhesive agent 4 must be applied to at least two positions on the bottom 7f to securely fix the magnetic main body 3, while an excessive amount of adhesive agent results in deterioration in the AL value. Thus, the adhesive agent 4 is preferably applied to two to four positions. An example of a preferable adhesive agent is a silicone rubber having a viscosity before curing of 2 Pa·s or less and a JIS A hardness after curing of 25 or less. When the viscosity before curing of the adhesive agent 4 is higher than this limit, the magnetic core main body 3 may rise from the bottom of the magnetic core case 7 and protrude from the magnetic core case 7. When the hardness after curing is higher than this limit, the AL value deteriorates due to shrinkage stress of the adhesive agent. Thus, the amount of the adhesive agent 4 is preferably decreased as much as possible within a range capable of fixing the magnetic core main body 3 to the magnetic core case 7. In this embodiment, the magnetic core main body 3 is formed as follows: A soft magnetic alloy ribbon 5 having a composition set forth below is prepared by a quenching process, coiled to a toroidal shape, and preferably impregnated with a silicone rubber followed by curing. The inductance of the pulse transformer can be improved by increasing the height of the magnetic core main body 3. However, when the magnetic core main body 3 has an excessive height, the wound coil 9 in contact with the upper side of the magnetic core main body 3 may be damaged due to friction of the wound coil 9. Thus, it is preferred that the height of the magnetic core main body 3 be 0 to 0.05 mm lower than the height inside the magnetic core case 7. The magnetic core main body 3 preferably has an outer diameter as large as possible and an inner diameter as small as possible within a range capable of being placed into the magnetic case 7. It is preferable that the silicone rubber impregnated into the magnetic core main body 3 have a viscosity before curing of 1 Pa·s or less and be gelled by curing. When the viscosity before curing is higher than this limit, the silicone rubber is barely impregnated between layers of the magnetic core main body 3. When the silicone rubber is excessively hardened by curing the AL value deteriorates due to strain on the silicone rubber. The silicone rubber impregnated into the magnetic core main body 3 may also be used as an adhesive agent to fix the magnetic core main body 3 to the magnetic core case 7. In this case, a silicone rubber having a viscosity before curing of 1.5 Pa·s or less and a JIS A hardness after curing of 10 or less may be preferably used. The magnetic core main body 3 may include no silicone rubber. However, an appropriate amount of silicone rubber impregnated in the magnetic core main body 3 can suppress the AL value deterioration due to stress occurring when fixing the magnetic core main body 3 to the magnetic core case 7 and due to heating. In this embodiment, the toroidal magnetic core main body 3 is formed by winding a soft magnetic alloy ribbon. The soft magnetic alloy ribbon may be punched into rings and a given number of rings may be stacked up to the magnetic core main body 3. The magnetic core main body 3 may be an EI shape. An EI-shape magnetic core is made as follows: A plurality of E-shape thin pieces and I-shape thin pieces are prepared from a soft magnetic alloy ribbon by press-punching; an E-shape core and an I-shape core are produced by stacking E-shape thin pieces and I-shape thin pieces, respectively; and the E-shape core and the I-shape core are butt-jointed. A magnetic core of a pulse transformer is formed by placing the magnetic core main body into a magnetic core case with an opening face. The combination of magnetic cores is not limited to the E-shape core and the I-shape core. For example, any combination such as two E-shape cores, a U-shape core and an I-shape core, and two U-shape cores are usable for the magnetic core. A soft magnetic alloy preferably used for the soft magnetic alloy ribbon set forth above comprises Fe as a main component; at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Mo and W; and B; and has a microstructure in which a large number of fine crystalline grains precipitate in an amorphous phase. The soft magnetic alloy comprises fine crystalline grains for a body-centered cubic lattice having a grain size of 30 nm or less in an amount of not less than 50% of the entire microstructure. Preferably, the soft magnetic alloy has any one of the following compositions: Fe b B x M y , Fe b B x M y X z , Fe b B x M y T d , or Fe b B x M y T d X z wherein, M is at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Mo and W; T is at least one element selected from the group consisting of Cu, Ag, Au, Pd and Pt; X is at least one element selected from the group consisting of Si, Al, Ge and Ga; and suffixes b, x, y, d and z indicating the stoichiometry satisfy 75≤b≤93 atomic percent, 0.5≤x≤18 atomic percent, 4≤y≤9 atomic percent, d of not more than 4.5 atomic percent and z of not more than 4 atomic percent. The b value indicating the Fe content in the soft magnetic alloy must be 93 atomic percent or less. When the b value exceeds 93 atomic percent, an amorphous phase is barely obtainable by a liquid-quenching process, and the microstructure of the alloy after annealing is inhomogeneous, resulting in decreased permeability. Further, it is preferred that the b value be 75 atomic percent or more to achieve a saturated magnetic flux density of 10 kG or more. Thus, the b value preferably ranges from 75 to 93 atomic percent. Boron (B) promotes amorphous phase formation in the soft magnetic alloy, prevents the crystal structure from coarsening, and reduces compound phase formation curing annealing which adversely affects magnetic properties. Although Zr, Hf, Nb and the like cannot be intrinsically dissolved into α-Fe, an amorphous alloy which is formed by a quenching process can dissolve such elements by oversaturation. A fraction of the dissolved elements is crystallized by annealing and precipitates as fine crystalline grains. The magnetostriction of the resulting alloy ribbon can be reduced and the soft magnetic properties are improved. The amorphous phase must still remain at grain boundaries in order to precipitate fine crystalline grains and to suppress coarsening of the fine crystalline grains. Because the amorphous phase at grain boundaries can dissolves M elements, such as Zr, Hf and Nb, which are excluded from α-Fe by temperature rise during annealing, this suppresses formation of Fe-M compounds which deteriorate the soft magnetic properties. Thus, it is essential to add B to Fe-Zr(Hf,Nb) alloys. When x indicating the B stoichiometry is less than 0.5 atomic percent, the amorphous phase at grain boundaries is not stabilized. On the other hand, when x is more than 18 atomic percent, B-M system and Fe-B system borides tends to form. Thus, the annealing condition for achieving a fine crystalline structure is limited and excellent soft magnetic properties are not obtainable. By appropriately adjusting the B content, an average size of the fine crystalline grains is adjustable to 30 nm or less. The amorphous phase can be more easily formed by adding any element of Zr, Hf and Nb which have high amorphous phase forming ability. A fraction of Zr, Hf and Nb may be replaced with any element of Ti, V, Ta, Mo and W among ocher Group 4A through Group 6A elements. Since these M elements having relatively low diffusion abilities may retard growth of fine crystalline nuclei, they are effective in fining the microstructure. When y indicating the M element stoichiometry is less than 4 atomic percent, the retardant effect of fine crystalline nucleus growth is lost, thus crystalline grains are coarsened and excellent soft magnetic properties cannot be achieved. In Fe-Hf-B alloys, average grain size is 13 nm at Hf = 5 atomic percent, but increases to 39 nm at Hf = 3 atomic percent. On the other hand, when y is more than 9 atomic percent, M-B and Fe-M compounds tend to form. Formation of these compounds deteriorates magnetic properties and results in embrittlement of the alloy ribbon after liquid quenching. Thus, the alloy ribbon is difficult to shape into a predetermined magnetic core shape. Accordingly, it is preferred that y ranges from 4 to 9 atomic percent. Among these elements, since Nb and Mo have small absolute values of free energy of oxide formation, they are thermally stable and barely oxidized during a production process. Addition of these elements results in ready production conditions with low production costs. It is preferable that at least one element selected from the group consisting of Si, Al, Ge and Ga be added in an amount of 4 atomic percent or less. These elements known as metalloid elements enhance amorphous phase forming ability, are dissolved into a bcc (body centered cubic) phase essentially consisting of Fe, and change the resistivity and magnetostriction of the alloy. When the content of these elements exceeds 4 atomic percent, magnetostriction increases, and saturation magnetic flux density or permeability decreases. The soft magnetic characteristics improve when 4.5 atomic percent or less of at least one element selected from the group consisting of Cu, Au, Pd and Pt is added. A trace amount of such an element, e.g. Cu, which is not dissolved in Fe, results in fluctuation of the amorphous alloy composition immediately after quenching, in which Cu forms clusters in an initial crystallization stage, and thus the formation rate of α-Fe nuclei increases due to occurrence of Fe-enriched domains in the alloy. Results of differential scanning calorimetry suggest the crystallization temperature of the alloy slightly decreases with addition of such elements, e.g. Cu and/or Ag. The amorphous phase may be de-homogenized due to such elements leading to decreased stability of the amorphous phase. In crystallization of the inhomogeneous amorphous phase, many partially-crystallizable domains result in inhomogeneous nuclei and thus a microstructure of fine crystalline grains. Thus, any elements which decrease the crystallization temperature other than the given elements will also exhibit the same effects. At least one of the platinum group elements, e.g. Cr, Ru, Rh and Ir, may be added in order to improve corrosion resistance of the alloy. However, the amount of these elements must be 5 atomic percent or less, because addition of over 5 atomic percent significantly decreases the saturated magnetic flux density. Other elements, such as Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Zn, Cd, In, Sn, Pb, As, Sb, Bi, Se, Te, Li, Be, Mg, Ca, Sr and Ba, may be added, if necessary, to adjust the magnetostriction of the resulting soft magnetic alloy. The soft magnetic alloy in accordance with the present invention may include incidental impurities, such as H, N, O and S, within a range not deteriorating its soft magnetic characteristics. A soft magnetic alloy in accordance with the present invention can be produced by atmospheric quenching of the alloy melt, while supplying an inert gas to a nozzle tip of a crucible according to demand. It is preferable that the soft magnetic alloy be produced in a vacuum chamber which can adjust its atmosphere. A soft magnetic alloy ribbon can be easily produced by spraying and quenching the alloy melt in the crucible on quenching equipment such as a revolving drum. The soft magnetic alloy ribbon after quenching essentially consists of an amorphous phase, and is annealed to precipitate numerous fine crystalline grains. The resulting alloy ribbon exhibits a high saturated magnetic flux density and excellent soft magnetic characteristics. Rings are made by punching the soft magnetic alloy ribbon with a pressing machine, and then stacking and placing into a container such as a resin case. Alternatively, the soft magnetic alloy ribbon is directly coiled, and placed into a container such as a resin case or fixed with a resin. A magnetic core having a high permeability can be produced in such a manner. The thickness of the usable soft magnetic alloy ribbon can be appropriately determined within a range of 10 to 40 µm. A soft magnetic alloy ribbon of thickness less than 10 µm is hardly produced by any current quenching processes. On the other hand, a thickness over 40 µm scarcely forms a microstructure of fine crystalline grains in an amorphous phase. A soft magnetic core obtained by such a process exhibits an AL value of 4.0 µH/N 2 or more at 10 kHz, and 2.0 µH/N 2 or more at 100 kHz, when 0.1 V is input, even when the soft magnetic core has a size of an outer diameter of 10 mm or less and height of 1.2 mm or less. Thus, the soft magnetic core satisfies the characteristics essential for a pulse transformer. In the magnetic core using the alloy in accordance with the present invention, variation in the AL value in a temperature range of -40 °C to +100 °C from room temperature can be controlled to be ±20%. Since the soft magnetic alloy ribbon has an absolute value of magnetostriction of 1x10 -6 or less, deterioration of magnetic characteristics due to magnetostriction scarcely occurs when it is covered with a resin or encapsulated into a resin case. Further, a pulse transformer with a packing area of no more than 14.0 mm by 14.0 mm and a height of no more than 3 mm can be fabricated by the configuration set forth above. Additionally, an alloy with a permeability of 40,000 or more at 10 kHz is easily obtainable within the composition range set forth above, and suitable for a high performance pulse transformer. The AL value set forth above means inductance per one turn of the coil and is represented by the equation: AL value = µ 0 µ'(S/1) (unit:H/N 2 ), wherein S indicates the cross-section area of the ring magnetic core, 1 indicates the magnetic path length, µ 0 indicates the permeability in the vacuum, and µ' indicates the specific permeability of the material. The magnetic core having a stable high inductance at 100 kHz or less can transmit undistorted rectangular pulse waves. In a toroidal magnetic core made by coiling a soft magnetic alloy ribbon, a magnetic core having a thickness of less than 1.0 mm will be difficult to make due to restriction of the ribbon width which can be made. In contrast, in a magnetic core comprising stacked rings, which are obtained by punching a soft magnetic alloy ribbon, a thickness of less than 1.0 mm can be readily achieved, resulting in a miniaturized magnetic core. It is preferable that the soft magnetic alloy ribbon be annealed as follows. The soft magnetic alloy ribbon is annealed at a temperature higher than a first crystallization temperature at which a first crystal phase precipitates, and lower than a second crystallization temperature at which a second crystal phase precipitates, for 0 to 20 minutes. No annealing time, i.e., 0 minute, is preferable in order to simplify the production process. The alloy ribbon after quenching essentially consists of an amorphous phase. Fine crystalline phases, which comprise bcc (body centered cubic) crystal grains essentially consisting of Fe and have an average particle size of 30 nm or less, precipitate by heating the alloy ribbon. In the present invention, the temperature, at which Fe fine crystalline phases having a bcc structure precipitate, is referred to as the first crystallization temperature. The first crystallization temperature varies with the alloy composition and generally ranges from 480 to 550 °C. At a temperature higher than the first crystallization temperature, a compound phase or second crystal phase, such as Fe 3 B or Fe 3 Zr when the alloy contains Zr, precipitates and deteriorates the soft magnetic characteristics. Such a temperature is referred to as a second crystallization temperature, in the present invention. The second crystallization temperature varies with the alloy composition and generally ranges from 740 to 810 °C. Thus, the annealing temperature of the amorphous alloy ribbon is determined within a range from 500 °C to 800 °C according to the alloy composition, such that bcc fine crystalline phases essentially consisting of Fe precipitate and the compound phase set forth above does not precipitate. The annealing time of the amorphous alloy ribbon in accordance with the present invention may be within a time of 20 minute or less. An annealing time of 0 minute, i.e., cooling immediately after heating, can also achieve a high permeability, depending on the alloy composition. A composition not containing Cu and Si, and particularly Si, can acquire a high permeability with a short annealing time of 10 minutes or less. When Si is added, a longer annealing time is required to sufficiently dissolve Si into Fe. A further prolonged annealing time results in decreased productivity without improvement in magnetic characteristics. The heating rate of the amorphous alloy ribbon from room temperature to the annealing temperature ranges from 20 °C/min. to 200 °C/min. and preferably from 40 °C/min to 200 °C/min. Although a higher rate is preferable to shorten production time, a heating rate higher than 200 °C/min. is difficult to achieve with conventional heating apparatus. After annealing, the alloy ribbon is cooled in air or the like. As a result of annealing the amorphous alloy ribbon, an alloy comprising 50 percent or more of fine crystal phase bcc grains, which essentially consist of Fe and have an average grain size of 30 nm or less, is obtainable without precipitation of the compound phase, such as Fe 3 B, which deteriorates magnetic characteristics. The resulting microstructure essentially consisting of a crystal phase of fine crystalline grains and a boundary amorphous phase which is present in the grain boundary exhibits excellent soft magnetic characteristics. The reason that the annealed alloy exhibits excellent soft magnetic characteristics is as follows: Crystal magnetic anisotropy, which is one factor causing deterioration in soft magnetic characteristics of a conventional crystalline material, is evened due to magnetic interaction between fine bcc grains and apparent magnetic anisotropy is significantly decreased. If the average crystal grain size is larger than 30 nm, the soft magnetic characteristics deteriorate due to insufficient evenness of the crystal magnetic anisotropy. On the other hand, a fine crystalline phase of less than 50% causes lower magnetic interaction between grains and deterioration of soft magnetic characteristics. [Examples] Example 1 Rings, an outer diameter of 7.8 mm and an inner diameter of 4.8 mm, were made by punching from a soft magnetic alloy ribbon with a thickness of 15 to 25 µm having a composition of Fe 86 Nb 3.25 Zr 3.25 B 6.5 Cu 1 and annealed at a temperature of 510 to 540 °C. A predetermined number of annealed rings were placed into a circular PET (polyethylene terephthalate) resin case with an outer diameter 9 mm, an inner diameter of 4 mm and a height of 1.5 mm so that the height (ring thickness × numbers) of the core was 0.3 to 0.95 mm. The impedance and permeability were determined. The case used had an inner depth of 1.0 mm. ( Test Results 1 ) Figure 5 is a graph illustrating change in impedance (|Z|) to packing rate (%), calculated from the height of the magnetic core with a thickness of 15 µm having the composition set forth above and the inner depth of the case, using 20 turns of wire. Figure 6 is a graph illustrating change in permeability (µ') to packing rate (%). In a configuration of rings stacked in the case, vertical stress of rings and magnetostriction inherent in the material used generally decrease permeability. However, because the magnetostriction constant of the soft magnetic alloy ribbon having a composition of Fe 86 Nb 3.25 Zr 3.25 B 6.5 Cu 1 is significantly low, i.e., approximately -0.3×10 -6 after annealing at 540 °C for 30 minutes, permeability does not decrease due to stress even at a packing rate of approximately 90% and impedance increases with the packing rate as shown in Figures 5 and 6. Thus, it is preferable that the packing rate be as high as possible to achieve a high impedance. Table 1 shows thicknesses of the soft magnetic alloy ribbon for a packing rate of 92 to 93% and observed AL values (the AL value indicates inductance per one turn of coil), when 0.1 V is input. Thickness (µm) AL value (µH/N 2 ) at 10 kHz AL value (µH/N 2 ) at 100 kHz 15.2 5.73 3.33 15.7 6.09 3.27 16.3 5.73 3.04 16.6 5.58 3.00 19.5 5.93 2.77 Figure 7 is a graph illustrating variation in AL values at 10 kHz and 100 kHz with thickness of the soft magnetic alloy ribbon. It is known that, in a magnetic core using a soft magnetic alloy ribbon, eddy current loss generally increases with thickness of the soft magnetic alloy ribbon, and thus high frequency permeability and inductance decrease. The AL value at 100 kHz of the magnetic core using the soft magnetic alloy ribbon in accordance with the present invention also decreases with thickness of the soft magnetic alloy ribbon as shown in Figure 7. However, the AL value at 10 kHz does not substantially change until thickness of the soft magnetic alloy ribbon reaches 25 µm. It is preferable that a pulse transformer for the ISDN standard set forth above has an AL value of 2.0 µH/N 2 or more at 100 kHz. Such an AL value can be achieved when a toroidal magnetic core with an outer diameter of 7.8 mm, an inner diameter of 4.8 mm and a height of 0.92 to 0.93 mm is prepared from a soft magnetic alloy ribbon having a composition of Fe 86 Nb 3.25 Zr 3.25 B 6.5 Cu 1 and a thickness of 25 µm or less, as set forth above. Although the thickness of the soft magnetic alloy ribbon can be appropriately determined within a range from 10 to 25 µm, a preferable thickness ranges from 15 to 20 µm in view of simplified production conditions of the soft magnetic alloy ribbon, and the stacked thickness in the pulse transformer. Example 2 Using a soft magnetic alloy ribbon having a composition of Fe 84 Nb 3.5 Zr 3.5 B 8 Cu 1 and a thickness of 16 µm, rings with an outer diameter of 7.8 mm and an inner diameter of 4.8 mm were made by punching and annealing at 520 °C. The magnetostriction constant of the alloy ribbon was approximately +0.6×10 -6 . A given number of rings were placed into a resin case with an outer diameter of 9 mm, an inner diameter of 4 mm and a height of 1.5 mm to determined the impedance (|Z|) and permeability µ', such that the height of the magnetic core is 0.5 t 0.9 mm. [ Test Results 2 ] Figure 8 is a graph illustrating change in impedance to packing rate (%), calculated from the height of the magnetic core and the inner depth of the case, using 20 turns of a wire. Figure 9 is a graph illustrating change in permeability to packing rate (%). Figures 8 and 9 also show results of the soft magnetic alloy ribbon having a composition of Fe 86 Nb 3.25 Zr 3.25 B 6.5 Cu 1 used in Example 1. Permeability of the magnetic core in this example gradually decreases at a packing rate higher than 60% and significantly decreases at a packing rate higher than 75% due to magnetostriction inherent in the material and packing stress. Both impedances at 10 kHz and 100 kHz, which are proportional to the permeability and the cross-section area, have maximum values at a packing rate of approximately 70%. For comparison, Figures 8 and 9 show results of a fine crystalline soft magnetic alloy ribbon having a composition of Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 . This soft magnetic alloy ribbon having a thickness of 19.6 µm exhibited a magnetostriction constant of +1.3×10 -6 after annealing at 530 °C, and a permeability µ' of 80,000 at 1 kHz. The soft magnetic alloy ribbon was significantly brittle, permeability tends to decrease at lower frequency, and a permeability µ' at 1 kHz of a sample having a thickness of 15 µm was approximately 50,000. Thus, the sample having such a thickness was not used for testing. The impedance of the alloy for comparison started to decrease at a lower packing rate. This is probably due to a great influence of the magnetostriction on the permeability when the packing rate was increased in the case. In contrast, in the magnetic core made of the soft magnetic alloy ribbon in accordance with the present invention, the impedance started to decrease at a definitely higher packing rate. Figure 10 is a graph illustrating correlation between AL value and packing rate of the same samples illustrated in Figures 8 and 9. Figure 10 demonstrates that when a magnetic core has a configuration comprising soft magnetic alloy rings packed in a resin case, the packing rate is preferably 50% or more, and more preferably 55 to 80% in order to clear both lower limits at 10 kHz and 100 kHz. Example 3 [Test Results 3] Figure 11 is a graph illustrating change in permeability with temperature of the magnetic cores having a packing rate of 80% used in Examples 1 and 2, as well as a ferrite magnetic core. Figure 11 demonstrates that transformers using soft magnetic alloys in accordance with the present invention encapsulated into resin cases (□ : Example 1, ○: example 2) exhibit an extremely small change in permeability over a wide temperature range from -20 to +100 °C, approximately ±5% over a range from -20 to +70 °C, and +5 to -10% over a range from -20 to 100 °C. Therefore, change in permeability of the transformer in accordance with the present invention is evidently small compared with that of the comparative example. Example 4 A toroidal magnetic core main body was made by coiling a soft magnetic alloy ribbon with a width of 0.9 mm having a composition of Fe 84 Nb 3.5 Zr3 .5 B 8 Cu 1 , and annealed at a temperature of 650 to 690 °C, so that the magnetic core main body have a size with an outer diamecer of 8.8 mm, an inner diameter f 4.2 mm and a height of 0.9 mm. The annealed magnetic core main body was impregnated with a silicone rubber (TSE3051 made by Toshiba Silicone Co., Ltd.) having a viscosity of 0.7 Pa·s and heated to cure the silicone rubber at a temperature of 110 to 140 °C. A magnetic core case with an opening set forth in Figure 3 was made of a polyacetal resin, and a silicone rubber (TSE3991 made by Toshiba Silicone Co., Ltd.) having a viscosity before curing of 1.5 Pa-s and a JIS A hardness after curing of 19 was applied to the bottom in two locations with an area of 1 mm 3 each. The magnetic core case has a size with an outer diameter of 9.5 mm, an inner diameter of 3.5 mm, a height of 1.15 mm and a thickness of 0.15 mm. Both ends of the inner wall and both ends of outer wall of the magnetic core case have a curvature radius of 0.1 mm, respectively. The magnetic core main body was placed into the magnetic core case, and the silicone rubber on the bottom of the magnetic core case was cured at room temperature to fix the magnetic core main body. A pulse transformer was prepared in such a manner. Example 5 A pulse transformer was prepared as in Example 4 except that the magnetic core was not impregnated with the silicone rubber. [Test Results 4] Transformers were made by coiling wound coils around magnetic core main bodies of Examples 4 and 5 which were not placed into the magnetic core cases. Transformers were also made by coiling wound coils around magnetic core main bodies placed into the magnetic core cases of Examples 4 and 5. AL values at 10 kHz of these transformers were measured when 0.1 V was input, and the change rates of the AL values after fixing to the case to before placing into the case were determined. The results are shown in Table 2, wherein the units for the AL values are µH/N 2 . Resin immersion AL value before placing AL value after fixing Change rate Example 4 Done 6.98 6.87 -1.58 Example 5 Not done 6.70 6.48 -3.28 [Test Results 5 ] Pulse transformers were made by coiling wound coils around the magnetic core prepared in Examples 4 and 5. Their AL values at 10 kHz were measured when 0.1 V was input, while varying the atmospheric temperature within a range from -50 to 100 °C. The change rates of AL values at each temperature to those at 20 °C were determined. The results are shown in Figure 12, in which a solid line indicates the pulse transformer magnetic core in Example 4 and a broken line indicates that in Example 5. Experimental results 4 demonstrate that, in Example 4, the stress occurring when the magnetic core main body is fixed to the magnetic core case is relaxed by immersing the magnetic core main body in a silicone rubber which is gelated after curing, resulting in further improvement in AL deterioration, although a high AL value and a low change rate for the AL value can be achieved in Example 5. As set forth in Experimental results 5, the AL deterioration at high temperature is further suppressed by immersing the magnetic core main body in a silicone rubber which is gelated by curing compared with Example 5. Example 6 A pulse transformer magnetic core was made as in Example 4, except that sizes of the magnetic core main body and the magnetic core case were changed as set forth in Table 3 below. Example 7 A pulse transformer magnetic core using a magnetic core case comprising an upper case and a lower case as set forth in Figure 2 was made. Sizes of the magnetic core case and the magnetic core main body were set forth in Table 3. A toroidal magnetic core was made by coiling a soft magnetic alloy ribbon with a width of 0.7 mm having a composition of Fe 84 Nb 3.5 Zr 3.5 B 8 Cu 1 , and annealing at a temperature of 650 to 690 °C. The annealed magnetic core main body was placed into a magnetic core case of a polyacetal resin. [ Test Results 6 ] Transformers were fabricated by coiling wound coils around the pulse transformer magnetic cores prepared in Examples 6 and 7 and AL values at 10 kHz were measured when 0.1 V was input. The change rate in AL values of Example 7 to Example 6 was determined. The results are shown in Table 3. In Example 6, the AL value at 10 kHz of the magnetic core main body before placing into the magnetic core case was 8.6 µH/N 2 when 0.1 V was input as an average of 10 magnetic core main bodies. Example 6 Example 7 Case size (mm) Outer diameter 9.5 9.5 Inner diameter 3.5 3.5 Height 1.2 1.2 Thickness 0.15 0.15 Magnetic core main body size (mm) Outer diameter 8.8 8.4 Inner diameter 4.2 4.2 Height 0.9 0.8 AL value (µH/N 2 ) 7.79 6.49 Change rate 20.0 0.0 Results set forth in Table 3 demonstrate that a high AL value is achieved in Example 7. The magnetic core main body size (outer and inner diameters and height) in Example 6 is larger than that in Example 7 because no upper case is used and the thickness of the magnetic core case is decreased. As a result, the cross-section area of the magnetic main body is increased and the AL value further improves 20% or more compared to Example 7. Example 8 A magnetic core main body as in Example 4 was placed into a magnetic core case (outer diameter: 9.5 mm, inner diameter: 3.5 mm, height: 1.15 mm, and thickness: 0.15 mm) and impregnated with a silicone rubber (TSE3250 made by Toshiba Silicone Co., Ltd.) having a viscosity before curing of 1.3 Pa·s and a JIS A hardness after curing of 9, followed by curing. A pulse transformer was fabricated by coiling a wound coil around the magnetic core and the AL value was determined. Since the transformer in Example 8 does not exhibit characteristic deterioration due to stress of the silicone rubber, the AL value (an average of 10 transformers) is further improved to 8.6 µH/N 2 to 10 µH/N 2 compared with Example 4. Preferred examples for production of soft magnetic alloys for the pulse transformer magnetic core will now be explained. Example 9 An amorphous alloy ribbon having a composition of Fe 84 Nb 3.5 Zr 3.5 B 8 Cu 1 was produced as an alloy example in accordance with the present invention using a production apparatus set forth in Figure 13. In the production apparatus in Figure 13, a chamber comprises a prismatic main section 13 providing a cooling roll 35 and a crucible 12, and a holding section 14 coupled with the prismatic main section 13. The main section 13 and the holding section 14 are hermetically coupled to each other through flange sections 13a and 13b with bolts. The main section 13 of the chamber 10 is provided with an exhaust tube 15 which is connected to a vacuum exhaust system. The cooling roll 35 is supported by a rotating shaft 11 which crosses both side walls of the chamber 10, and is driven by a motor (not shown in the drawing). A nozzle 37 is provided at the bottom end of the crucible 12 and a heating coil 38 is provided at the lower section of the crucible 12. Molten metal 34 is reserved in the crucible 12. The upper section of the crucible 12 is connected to a gas supply source 18 for supplying, for example, gaseous Ar through a feeding pipe 16 with a pressure-control valve 19 and a solenoid valve 20, and a pressure gauge 21 is provided between the pressure-control valve 19 and the solenoid valve 20. The feeding pipe 16 is provided with a bypass pipe 23 in parallel with a pressure-control gauge 24, a flow control valve 25 and a flow meter 26. The molten metal 34 in the crucible 12 is sprayed on the cooling roll 35 through the nozzle 37 by means of pressure due to gaseous Ar supplied to the crucible from the gas supply source 18. The top wall of the chamber 10 is provided with a feeding pipe 32 with a pressure-control valve 33 connected to a gas supply source 31 for supplying, for example, gaseous Ar to the chamber 10, An alloy ribbon is produced using the production apparatus as follows: The chamber 10 is exhausted to a vacuum while a non-oxidative gas such as gaseous Ar is fed from the gas supply source 31 to the chamber 10. The molten metal 34 is sprayed on the top of the cooling roll 35 rotating at a high speed through the nozzle 37 by means of the pressure of gaseous Ar which is fed into the crucible 12 from the gas supply source 18. The molten metal 34 flows along the surface of the cooling roll 35 to form a thin ribbon 36. A long thin ribbon 36 is continuously produced by continuously spraying the molten metal 34 on the cooling roll 35 from the crucible 12. The thin ribbon 36 is drawn out from the cooling roll 35, and held in the holding section 14 of the chamber 10. Since the chamber 10 is filled with gaseous Ar, the thin ribbon 36 which is still hot due to the heat inertia, can be prevented from oxidizing. When the thin ribbon 36 is cooled to near room temperature after the thin ribbon production, the thin ribbon 36 is removed by detaching the holding section 14 from the main section 13 of the chamber 10. The crystallization temperature of the resulting amorphous alloy ribbon with a width of 15 mm and a thickness of 20 µm was determined by differential scanning calorimetry (DSC) at a heating race of 40 °C/min. A DSC thermogram which is shown with a solid line in Figure 14 was obtained. The result suggests that this amorphous alloy ribbon has a first crystallization temperature T x of approximately 508 °C at a heating rate of 40 °C/min. Comparative Example 1 An amorphous alloy ribbon having a composition of Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 was produced as an example alloy out of the range of the present invention in Example 9. The crystallization temperature of the resulting amorphous alloy ribbon was determined by DSC at a heating rate of 40 °C/min. A DSC thermogram which is shown with a broken line in Figure 14 was obtained. The result suggests that this amorphous alloy ribbon has a first crystallization temperature T x of approximately 548 °C. The amorphous alloy ribbons obtained in Example 9 and Comparative Example 1 were annealed for various holding times t to prepare soft magnetic alloys. The resulting soft magnetic alloys were used for evaluation of magnetic characteristics, i.e., permeability µ', at 1 kHz, coercive force Hc (Oe), saturation magnetostriction λs and average crystal grain size D (nm). The heating program is as follows: Each amorphous alloy ribbon was heated until a given holding temperature Ta at a heating rate of 40 °C/min., held at the holding temperature for a given time, and then cooled. The holding temperature Ta was set at a temperature slightly higher than the first crystallization temperature of the alloy, i.e., 510 °C in Fe 84 Nb 3.5 Zr 3.5 B 8 Cu 1 (Example 9), and 550 °C in Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 (Comparative Example 1). The results are shown in Figures 15 through 17, in which  represents Example 9 and ○ represents Comparative Example 1. Figure 15 demonstrates that, in Example 9, a high permeability is achieved for a relatively short holding time of 0 to 20 minutes, whereas the sample of Comparative Example 1 has a maximum permeability for a holding time of approximately 30 minutes and the permeability drastically decreases for shorter holding times. Figure 16 demonstrates that coercive forces in Example 9 and Comparative Example 1 do not substantially change with holding time and are almost level. The saturation magnetostriction λs increases with decreased holding time in Comparative Example 1, whereas the sample in Example 9 always has a lower saturation magnetostriction for short holding times of 0 to 20 minutes than that of Comparative Example 1. Figure 17 demonstrates that average diameters D in Example 9 and Comparative Example 1 do not substantially change and the sample in Example 9 has a smaller average diameter than that in Comparative Example 1. These results illustrate that the sample in Example 9 exhibits almost the same coercive force as that in Comparative Example 1 for a relatively shorter holding time of 0 to 20 minutes, and superior permeability and saturation magnetostriction to that of Comparative Example 1. Further, in Example 9, the smaller average size of crystal grains contributes to such improvement in soft magnetic characteristics. The amorphous alloy produced in Example 9 was annealed at various holding temperatures Ta for a holding time of 0 minutes and changes in permeability µ' of the resulting soft magnetic alloy were measured at 1 kHz. Annealing was performed by heating the amorphous alloy ribbon to a given holding temperature Ta at a heating rate of 40 °C/min. and then immediately cooling. The holding temperature Ta was varied within a range of 480 to 800 °C. The results are shown in Figure 18. Figure 18 demonstrates that the amorphous alloy ribbon of Example 9 exhibits high permeability by annealing at a temperature of 500 to 700 °C without a holding time. Example 10 An amorphous alloy ribbon in accordance with the present invention having a composition of Fe 84 Nb 7 B 9 was produced as in Example 9. Example 11 An amorphous alloy ribbon in accordance with the present invention having a composition of Fe 90 Zr 7 B 3 was produced as in Example 9. The amorphous alloy ribbons produced in Examples 10 and 11 were annealed for various holding times t and the permeability µ' of each soft magnetic alloy after annealing was evaluated at 1 kHz. The annealing program included heating to a given holding temperature Ta at a heating rate of 180 °C/min., holding for a given time, and cooling. The holding temperature Ta of each sample was set at a temperature higher than the first crystallization temperature and lower than the second crystallization temperature of the sample, i.e., 650 °C for Fe 84 Nb 7 B 9 (Example 10) and 600 °C for Fe 90 Zr 7 B 3 (Example 11). Results are shown in Figure 19, in which  represents Example 10 and ○ represents Comparative Example 11. Figure 19 demonstrates that the sample in Example 10 exhibits a high permeability for a holding time of 1 minute to 120 minutes, and preferably 2 minutes to 30 minutes, and the sample in Example 11 exhibits a high permeability for a holding time of 0 minute to 120 minutes, and preferably 2 minutes to 30 minutes.

Claims: A pulse transformer magnetic core comprising a magnetic core main body (3) of a soft magnetic alloy ribbon (5) having a thickness of 25 µm or less, the AL value of said magnetic core main body being 4.0 µH/N 2 or more when 0.1 V is input at 10 kHz, the AL value being defined by AL = µ 0 µ' (S/l) [H/N 2 ], wherein S indicates the cross-section area of the magnetic core, 1 indicates the magnetic path length, µ 0 is the permeability in vacuum and µ' is the permeability of the core material, wherein said magnetic core main body (3) comprises stacking rings made from said soft magnetic alloy ribbon and has an outer diameter of 10 mm or less and a thickness of 1.2 mm or less, and wherein 50% or more of said soft magnetic alloy essentially consists of body centered cubic fine crystal grains with an average crystal grain size of 30 nm or less, and said soft magnetic alloy comprises Fe as a main component; at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Mo and W; and B. A pulse transformer magnetic core comprising a magnetic core main body (3) of a soft magnetic alloy ribbon (5) having a thickness of 25 µm or less, the AL value of said magnetic core main body being 4.0 µH/N 2 or more when 0.1 V is input at 10 kHz, the AL value being defined by AL = µ 0 µ' (S/l) [H/N 2 ], wherein S indicates the cross-section area of the magnetic core, 1 indicates the magnetic path length, µ 0 is the permeability in vacuum and µ' is the permeability of the core material, wherein said magnetic core main body comprises any combination of an E-shape core and an I-shape core, a U-shape core and an I-shape core, and two U-shape cores, in which said E-shape core, said I-shape core and said U-shape core are formed by stacking E-shape thin pieces, I-shape thin pieces, and U-shape thin pieces, respectively, which are formed from said soft magnetic alloy ribbon with said magnetic core main body having a thickness of 1.2 mm or less, and wherein 50% or more of said soft magnetic alloy essentially consists of body centered cubic fine crystal grains with an average crystal grain size of 30 nm or less, and said soft magnetic alloy comprises Fe as a main component; at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Mo and W; and B. A pulse transformer magnetic core comprising a magnetic core main body (3) of a soft magnetic alloy ribbon (5) having a thickness of 25 µm or less, the AL value of said magnetic core main body being 4.0 µH/N 2 or more when 0.1 V is input at 10 kHz, the AL value being defined by AL = µ 0 µ' (S/l) [H/N 2 ], wherein S indicates the cross-section area of the magnetic core, 1 indicates the magnetic path length, µ 0 is the permeability in vacuum and µ' is the permeability of the core material, wherein said magnetic core main body comprises a toroidal ring formed by cooling said soft magnetic alloy ribbon having a width of 1.2 mm or less, and the outer diameter of said toroidal magnetic core main body being 10 mm or less, and wherein 50% or more of said soft magnetic alloy essentially consists of body centered cubic fine crystal grains with an average crystal grain size of 30 nm or less, and said soft magnetic alloy comprises Fe as a main component; at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Mo and W; and B. A pulse transformer magnetic core in accordance with claim 1, wherein said rings are packed within a covering member made of resin at a packing rate of 50% or more. A pulse transformer magnetic core in accordance with any of claims 1 to 4, wherein said soft magnetic alloy ribbon has an absolute value of magnetostriction of 1x10 -6 or less. A pulse transformer magnetic core in accordance with any of claims 1 to 5, wherein variation on the AL value of said pulse transformer magnetic core over a temperature range of -40°C to +100°C from room temperature is within ±20%. A pulse transformer magnetic core in accordance with any of claims 1 to 6, wherein said magnetic core main body is impregnated with a silicone rubber having a viscosity before curing of 1 Pa•s or less which is gelated by curing. A pulse transformer magnetic core in accordance with any of claims 1 to 6, wherein said magnetic core main body is impregnated with a silicone rubber having a viscosity before curing of 2 Pa•s or less and a JIS A hardness of 25 or less, said silicone rubber acting as an adhesive agent for fixing said magnetic core main body to a magnetic core case. A pulse transformer magnetic core in accordance with claim 7, wherein said adhesive agent for fixing said magnetic core main body to said magnetic core case is a silicone rubber having a viscosity before curing of 1.5 Pa•s or less and a JIS A hardness after curing of 10 or less. A pulse transformer magnetic core in accordance with claim 8 or 9, wherein said adhesive agent is applied to two to four sections on the bottom face of said magnetic core case. A pulse transformer magnetic core according to any of claims 1 to 10, comprising a magnetic core case (7) having an opening for hilding said magnetic core main body. A pulse transformer magnetic core in accordance with claim 11, wherein said case (7) comprises an inner wall and an outer wall wherein the top and bottom end of said walls have curvature radii of 0.05 mm to 0.4 mm. A pulse transformer magnetic core in accordance with claim 11 or 12, wherein said magnetic core main body is packed in said magnetic core case (7) at a packing rate of 50% or more. A pulse transformer magnetic core in accordance with any of claims 11 to 13, wherein said magnetic alloy ribbon has an absolute value of magnetostriction of 1x0 -6 or less. A pulse transformer magnetic core in accordance with any of claims 11 to 14, wherein said magnetic core main body is impregnated with a silicone rubber having a viscosity before curing of 1 Pa·s or less which is gelled by curing. A pulse transformer magnetic core in accordance with any of claims 11 to 14, wherein said magnetic core main body is impregnated with a silicone rubber having a viscosity before curing of 2 Pa·s or less and a JIS A hardness of 25 or less, said silicone rubber acting as an adhesive agent for fixing said magnetic core main body to a magnetic core case. A pulse transformer magnetic core in accordance with claim 16, wherein said adhesive agent for fixing said magnetic core main body to said magnetic core case is a silicone rubber having a viscosity before curing of 1.5 Pa•s or less and a JIS A hardness after curing of 10 or less. A pulse transformer magnetic core in accordance with claim 16 ro 17, wherein said adhesive agent is applied to two to four sections on the bottom face of said magnetic core case. A pulse transformer magnetic core in accordance with any of claims 11 to 18, wherein said magnetic core case has an outer diameter of 10 mm or less, an inner diameter of 3.5 mm or more and a height of 1.3 mm or less, and an AL value of 6.0 µH/N 2 or more when 0.1 V is input at 10 kHz.

Inventors: HANGAI KATSUKAI, HATANAI TAKASHI, INOUE AKIHISA, MAKINO AKIHIRO, MASUMOTO TSUYOSHI, NAITO YUTAKA, SAITO YUICHI, WADA SHINJIRO, HANGAI, KATSUKAI, HATANAI, TAKASHI, INOUE, AKIHISA, MAKINO, AKIHIRO, MASUMOTO, TSUYOSHI, NAITO, YUTAKA, SAITO, YUICHI, WADA, SHINJIRO
Applicants: ALPS ELECTRIC CO LTD, INOE AKIHISA, JAPAN SCIENCE & TECH CORP, MASUMOTO TSUYOSHI, ALPS ELECTRIC CO., LTD., INOUE, AKIHISA, JAPAN SCIENCE AND TECHNOLOGY CORPORATION, MASUMOTO, TSUYOSHI
Application year: 1997
Main classification: H01F19
IPC codes: H01F17, H01F3, H01F1, H01F27