SEPARATION TYPE TRANSFORMER CORE

Abstract

 An isolation transformer core 1 having a coil 3 and a core member 2. The core member 2 comprises a mixed soft magnetic material which comprises an insulating material having an electrical insulating property and a soft magnetic material. The soft magnetic material content is in the range of 10 to 70 volume %.

Description

Technical Field

[0001] The present invention relates to an isolation transformer core, and more specifically to an isolation transformer applicable to an automobile component.

Background Art

[0002] An isolation transformer is a transformer in which cores having coils are arranged to face each other to transmit electric power or an electric signal between each other through electromagnetic coupling of the opposite coils in a contactless manner.

[0003] For example, a rotary transformer in which a primary core is fixed and a secondary core is rotatably arranged is an isolation transformer of this type, and a rotary transformer for a rotary head of a video tape recorder is generally known.

[0004] In the rotary transformer, in order to make a coupling coefficient of coils in cores large, cores having a high relative permeability are used and a gap between the cores is restricted to several µm. In the rotary transformer, when the coupling coefficient of the coils is very large, self-inductance and mutual inductance of the two opposite coils cancel each other, so that input-output impedance of the transformer is small. Therefore, in the rotary transformer, impedance matching between the coils and a load can be easily attained.

[0005] As cores of the rotary transformer, sintered ferrite cores are generally used. The sintered ferrite core is favorable as a core of a high frequency transformer in that it has a very high relative permeability and produces only a very small eddy-current loss.

[0006] In the rotary transformer in which primary and secondary cores are brought into a relative rotation, the size of a gap between the cores has a direct influence on manufacturing cost. In a rotary transformer having a large coupling coefficient of coils, in order to provide a gap between cores of several µm, high manufacturing precision and high assembling precision of components are required, which causes high manufacturing cost. In the case of an automobile, strict restriction is imposed on manufacturing cost and very strong vibration is produced during driving. Therefore, the rotary transformer for an automobile needs to have a gap of 0.5 mm or larger between the opposite cores.

[0007] The sintered ferrite core has favorable properties as mentioned above, but has a drawback peculiar to sintered oxide: fragility.

[0008] Therefore, when sintered ferrite cores are to be used as cores of a connector for an automobile, for example, cores of a connector for an air bag, various consideration is needed, for example, about how to prevent vibration, how to fix the cores and the like. Also in view of manufacturing cost, the sintered ferrite core is difficult to apply to an automobile component.

[0009] The present invention has been made in view of the above problems. The object thereof is to provide an isolation transformer core which is less fragile and easy to manufacture.

Disclosure of the Invention

[0010] In order to apply a rotary transformer to an automobile component, in particular to a connector for an air bag, the inventors have earnestly worked on a study.

[0011] First, an isolation transformer used as a connector for an air bag needs to be able to make large current flow to an air bag inflating unit under a low voltage of 12V (battery for an automobile) to transmit large power at a high speed. In this connection, impedance matching between a load and coils is very important.

[0012] In order to be able to transmit large power to an air bag inflating unit in a moment, the following conditions need to be satisfied:

(1) The allowable maximum delay time is about 1 msec. Therefore, the frequency of a transmission signal needs to be higher than several kHz.

(2) The diameter of a steering-wheel shaft of an automobile is about 30 mm. The inside diameter of a center through-hole of a core needs to be larger than the diameter of the steering-wheel shaft. Therefore, the diameter of a coil needs to be about 45 mm or larger. The inductance of a coil is proportional to a square of the diameter thereof. Therefore, in order to make the impedance of a coil small when a high-frequency signal is transmitted, it is the most effective to make the effective relative permeability in a magnetic circuit appropriately small. Normally, the inductance of two coils needs to be as small as several µH (the impedance of a load on the secondary side, that is, the inflating unit is about 2Ω). In order to meet this condition, it is important to make the effective relative permeability in the magnetic circuit appropriately small.

[0013] The inventors have researched on the effective relative permeability between coils of an isolation transformer (for example, using generally used sintered ferrite cores having a relative permeability of about 3000 to 10000).

[0014] First, in the case where the ratio of the length of the entire magnetic circuit between the coils to the size of the gap between the cores is approximately the same as the relative permeability of the core members (for example, the length of the magnetic circuit is 100 mm and the gap between the cores is several tens µm), the effective relative permeability in the magnetic circuit varies to a large extent, depending on the size of the gap. This means that the coupling state of the coils varies even when the gap between the cores varies only a little due to vibration of the automobile.

[0015] Second, in the case where the ratio of the length of the entire magnetic circuit between the coils to the size of the gap between the cores is much smaller than the relative permeability of the core members (for example, the length of the magnetic circuit is 100 mm and the gap between the cores is several mm), the effective relative permeability in the magnetic circuit almost exclusively depends on the size of the gap between the cores. Therefore, however high the relative permeability of the core members may be, the effective relative permeability in the magnetic circuit is almost determined by the size of the gap between the cores.

[0016] Thus, it has been found out that the effective relative permeability in the magnetic circuit formed between the coils is determined by the relative permeability of the core members and the size of the gap between the cores, and that the size of the gap between the cores is a factor having a particularly large influence on the effective relative permeability in the magnetic circuit.

[0017] From the above, the inventors have obtained a knowledge that an isolation transformer using cores of magnetic material of a low relative permeability (for example, mixed magnetic material) and having a larger gap between the cores shows an effective relative permeability in the magnetic circuit between the coils slightly lower than that of an isolation transformer using conventional sintered ferrite cores, but that it is suited to, transmit large power in a moment and has advantages of improved vibration resistance and lowered manufacturing cost (suited for mass production).

[0018] Based on the above knowledge, the present invention has been made to obtain an isolation transformer core suitable for a connector for an air bag which is installed in an automobile and needs to be able to transmit large power in a moment.

[0019] The isolation transformer core of the present invention comprises a coil and a core member, and is characterized in that the core member comprises a mixed soft magnetic material which comprises an insulating material having an electrical insulating property and a soft magnetic material.

[0020] In the isolation transformer core of the present invention, it is favorable that the soft magnetic material content is in the range of 10 to 70 volume %.

[0021] In the isolation transformer core of the present invention, it is favorable that the soft magnetic material is soft magnetic ferrite or Sendust.

[0022] In the isolation transformer core of the present invention, it is favorable that the insulating material is any one of thermoplastic resin, thermoplastic rubber, silicone rubber, thermosetting resin and adhesive.

Brief Description of the Drawings

[0023] 

FIG. 1 is a cross-sectional view of an isolation transformer core of the present invention, FIG. 2 is a graph showing the relation between the soft magnetic ferrite content of mixed soft magnetic material and the melt flow rate of the mixed soft magnetic material, FIG. 3 is a graph showing the relation between the soft magnetic ferrite content of mixed soft magnetic material and the relative permeability of a core member formed thereof, FIG. 4 shows volume resistivity characteristic curves indicating the relation between the soft magnetic ferrite content (volume %) of mixed soft magnetic material and the volume resistivity (Ω · cm) of the mixed soft magnetic material, and FIG. 5 shows relative permeability characteristic curves indicating the relation between the soft magnetic material (soft magnetic ferrite, Sendust, permalloy) content (volume %) and the relative permeability.

Best Mode of Carrying out the Invention

[0024] As shown in FIG. 1, an isolation transformer core 1 of the present invention comprises a core member 2 and a coil 3. The core member 2 is made of a mixed soft magnetic material which is a mixture of an insulating material having an electrical insulating property and a soft magnetic material, and formed into a desired core shape.

[0025] Here, if the soft magnetic material content of the mixed soft magnetic material is lower than 10 volume %, the relative permeability of the core member formed thereof is lower than 2, so that it is difficult to attain the required transmission efficiency of an isolation transformer. On the other hand, if the soft magnetic material content is higher than 70 volume %, the relative permeability of the core member formed thereof is high (it may be higher than 20, depending on the kind and grain diameter of soft magnetic material). This is favorable to raise the transmission efficiency of an isolation transformer, but the core itself is fragile. Further, if synthetic resin (described later) is used as the insulating material, flowability lowers, which makes injection molding difficult. Therefore, the soft magnetic material content of the mixed soft magnetic material is chosen in the range of 10 to 70 volume %.

[0026] In view of vibration resistance and formability, synthetic resin is favorable to be used as the insulating material. As the synthetic resin, for example, a thermoplastic resin such as nylon 6, nylon 66, nylon 11, nylon 12, polypropylene, polyphenylene sulfide or polyolefine, a thermoplastic rubber such as urethane, polyester or olefine, a thermosetting resin such as silicone rubber, epoxy resin, phenolic resin or diallyl phthalete, or two-liquid mixing adhesive can be used. When a synthetic resin as mentioned above is used as the insulating material, injection molding or the like can be applied to the mixed soft magnetic material. Therefore, a core member of a desired shape can be formed easily. Further, since the synthetic resin has flexibility, shock resistance of the formed core member is improved, and therefore the vibration resistance of the isolation transformer core itself is improved.

[0027] In view of heat resistance and the like, ceramic is favorable to be used as the insulating material. Zirconia ceramic or silicon nitride ceramic which have high strength and high toughness can be used. As the zirconia ceramic, partial stabilized zirconia ceramic is in particular favorable. When the ceramic is used as the insulating material, powdered ceramic and powdered soft magnetic material are mixed to produce a mixed soft magnetic material. Then the mixed soft magnetic material is formed into a desired shape and subjected to press sintering or HIP (hot isostatic pressing) to produce a desired isolation transformer core. The isolation transformer core produced this way has better heat resistance and wear resistance due to the ceramic.

[0028] Among the above mentioned insulating materials, nylon is favorable in that it is inexpensive, fuses well with the soft magnetic material, and exhibits good flowability in injection molding.

[0029] As the soft magnetic material, for example, soft magnetic ferrite, Sendust, permalloy, high-permeability amorphous material or the like can be used.

[0030] As the soft magnetic ferrite, for example, spinel ferrite represented by a general expression MO·Fe₂O₃ (where M is at least one element chosen from Zn, Mn, Ni, Cu and Fe), or compound ferrite made of several kinds of the above spinel ferrites can be used. Mn-Zn ferrite, Ni-Zn ferrite and Ni-Zn-Cu ferrite are in particular favorable. The favorable compounding ratio of Mn-Zn ferrite is MnFe₂O₄:ZnFe₂O₄=1:1(mole % ratio), and the favorable compounding ratio of Ni-Zn ferrite is NiO:ZnO:Fe₂O₃=15:35:50 (mole % ratio). The soft magnetic ferrite is used in a powdered state, and powdered soft magnetic ferrite whose maximum grain diameter is 100 µm or smaller is favorable. Powdered soft magnetic ferrite having an average grain diameter of 3.8 µm is more favorable.

[0031] As the Sendust, Fe-Si-Al alloy containing about 6 to 11 weight % of Si and about 4 to 6 weight % of Al can be used. 9.62 weight % Si-5.38 weight % Al-bal.Fe alloy is in particular favorable. The Sendust is used in a powdered state. Powdered Sendust having an average grain diameter of 10 µm or smaller is favorable.

[0032] As the permalloy, Fe-Ni alloy containing 35 to 80 weight % of Ni can be used. 78 weight % Ni permalloy, 48 weight % Ni permalloy, and supermalloy (79 weight % Ni-5 weight % Mo-0.3 weight % Mn-bal.Fe) are favorable. The permalloy is used in a powdered state. Powdered permalloy whose maximum grain diameter is 100 µm or smaller is favorable.

[0033] As the high-permeability amorphous material, Fe amorphous material or Co amorphous material can be used. The high-permeability amorphous material is also used in a powdered state having an average grain diameter of 1 to 500 µm.

[0034] In the present invention, an insulating material and a soft magnetic material are mixed and fused to produce a mixed soft magnetic material 2. If synthetic resin is used as the insulating material, the mixed soft magnetic material 2 exhibits good flowability when it is heated to fuse. Therefore, it can be easily formed by injection molding into a desired shape, for example, into a disc-shaped core member 2 having a though-hole 2a at the center and a coil groove 2b for receiving a coil 3 in the disc face, as shown in FIG. 1.

[0035] A coil 3 having a predetermined number of turns is placed in the coil groove 2b of the formed core member 2 to form an isolation transformer core 1. Alternatively, an isolation transformer core may be molded from the mixed soft magnetic material together with the coil 3 having a predetermined number of turns.

[0036] The isolation transformer cores each having a coil placed therein are arranged to face each other to form an isolation transformer. The isolation transformer is used, for example, as a connector for an air bag.

[0037] Next, how the isolation transformer cores of the present invention are used for a connector for an air bag will be explained.

[0038] First, in a steering section of an automobile, a primary transformer core is set on a fixed portion (a column side) and a secondary transformer core is set on a rotary portion (steering portion). Here, in view of vibration produced on an automobile and the like, the primary and secondary transformer cores are arranged to face each other with a gap of 1mm±0.5mm therebetween. Here, a primary-side coil is connected with a control unit for controlling an air bag inflating unit, and a secondary-side coil is connected with the air bag inflating unit.

[0039] The core members of the present invention have a relatively low relative permeability (for example, the relative permeability of a core member made of a mixed soft magnetic material comprising soft magnetic ferrite (MnFe₂O₄-ZnFe₂O₄) and nylon 6 is about 3 to 12). Therefore, the inductance of the coils is small, and therefore impedance matching between the coils and a load, that is, the inflating unit can be easily attained. Thus, the isolation transformer using the isolation transformer cores comprising the core members described above is suited to transmit large power in a moment.

[Embodiments]

[0040] As soft magnetic materials, Mn-Zn soft magnetic ferrite (MnFe₂O₄-ZnFe₂O₄) powder and Ni-Zn soft magnetic ferrite (NiO-ZnO-Fe₂O₃) powder whose maximum grain diameter was 50 µm were prepared. As insulating materials having an insulating property, nylon pellets (nylon 6) and polypropylene pellets as used in ordinary injection molding and the like were prepared. Using these materials, several kinds of mixed powders having different soft magnetic ferrite powder contents were prepared. Each mixed powder was then fused, so that several kinds of mixed soft magnetic materials having different soft magnetic ferrite contents were prepared.

[0041] The melt flow rate of mixed soft magnetic materials containing nylon 6 as an insulating material was measured by a melt index test in accordance with JIS K 7210. Measurement was performed under the condition that measurement temperature was 270°C and a load was 10.0 kg·f. When the soft magnetic ferrite content was 5 volume % or lower, the soft magnetic ferrite content had little influence on the melt flow rate. When the soft magnetic ferrite content was 70 volume % or higher, mixing to produce a mixed soft magnetic material was difficult. Therefore, the melt flow rate of mixed soft magnetic materials having the soft magnetic ferrite content of 5 to 65 volume % was measured by the melt index test. The results are shown in FIG. 2.

[0042] Next, using the above mixed soft magnetic materials, core members were formed as follows:

[0043] Using an injecting molding machine, each mixed soft magnetic material was formed into a core member of a predetermined shape, that is, a disc shape having a through-hole 2a at the center and a circular coil groove 2b in the disc face. Injection molding of mixed soft magnetic materials containing nylon 6 as an insulating material was performed under the ordinary condition of injection molding using nylon 6, and injection molding of mixed soft magnetic materials containing polypropylene as an insulating material was performed under the ordinary condition of injection molding using polypropylene.

[0044] Next, the relative permeability of formed core members was measured in accordance with JIS C2561. The results are shown as the relation between the soft magnetic ferrite content (volume %) and the relative permeability of a core member in FIG. 3, where black circles represent core members using nylon 6 as an insulating material and white circles represent core members using polypropylene as an insulating material.

[0045] Further, the volume resistivity of mixed soft magnetic materials was measured in accordance with JIS H 0505. The results are shown as the relation between the soft magnetic ferrite content (volume %) and the volume resistivity (Ω · cm) of mixed soft magnetic materials in FIG. 4, where black circles represent mixed soft magnetic materials using Mn-Zn ferrite as a soft magnetic ferrite and white circles represent mixed soft magnetic materials using Ni-Zn ferrite as a soft magnetic ferrite.

[0046] Further, FIG. 5 shows the relation between the soft magnetic ferrite content (volume %) and the relative permeability, the Sendust content (volume %) and the relative permeability and the permalloy content (volume %) and the relative permeability. This was obtained by calculation based on the measurement results of the soft magnetic ferrite content (volume %) and the relative permeability shown in FIG. 3, using general data on Sendust and permalloy. Soft magnetic ferrite, Sendust and permalloy were used as soft magnetic materials.

[0047] From FIGS. 2 and 3, the following has been found out. The higher the soft magnetic ferrite content (volume %) is, the higher the relative permeability of a core member is. The kind of insulating material contained in a mixed soft magnetic material has no influence on the permeability. The higher the soft magnetic ferrite content (volume %) is, the lower the flowability of a mixed soft magnetic material is.

[0048] When the soft magnetic ferrite content is higher than 70 volume %, mixing is difficult, and injection molding is difficult due to low flowability. Further, due to an increase of ferrite component having high hardness, a mold for injection molding wears quickly, the mechanical strength of a formed isolation transformer core is much lower, and a core is more difficult to form. Thus, the mixed soft magnetic material having the soft magnetic ferrite content higher than 70 volume % is unsuitable for a transformer core.

[0049] On the other hand, when the soft magnetic ferrite content is lower than 10 volume %, the relative permeability of a core member is low. Therefore, with an isolation transformer using isolation transformer cores comprising core members of this type, it is difficult to transmit power with a high efficiency.

[0050] When the soft magnetic ferrite content is in the range of 60 to 70 volume %, the relative permeability of a formed core member is high, but the flowability of a mixed soft magnetic material is relatively low. The mixed soft magnetic material having the soft magnetic ferrite content of this range is suitable for a core which is used in an isolation transformer requiring a relatively high transmission efficiency and does not have a very complicated shape.

[0051] When the soft magnetic ferrite content is in the range of 10 to 60 volume %, the relative permeability of a formed core member is relatively low, but the flowability of a mixed soft magnetic material is high. The mixed soft magnetic material having the soft magnetic ferrite content of this range is suitable for a core which is used in an isolation transformer not requiring a high transmission efficiency and has such a complicated shape that it can be formed only of material having a high flowability.

[0052] From FIG. 4, the following has been found out.

[0053] The higher the soft magnetic ferrite content (volume %) is, the lower the volume resistivity (Ω · cm) of a mixed soft magnetic material is. A mixed soft magnetic material containing Ni-Zn ferrite has a high volume resistivity, though it is expensive. It is desirable to use a mixed soft magnetic material containing Ni-Zn ferrite when a mixed soft magnetic material containing Mn-Zn ferrite does not satisfy a required volume resistivity.

[0054] When a mixed soft magnetic material has a low volume resistivity, grains composing the mixed soft magnetic material are not insulated well, so that eddy-current is easily induced by an ac magnetic field. Thus, the intended transmission efficiency of a transformer cannot be attained.

[0055] From FIG. 5, it has been found out that like soft magnetic ferrite, Sendust and permalloy also have properties required for use in an isolation transformer.

[0056] For an isolation transformer core used in a connector for a air bag as an automobile component, mixed soft magnetic material having the Mn-Zn soft magnetic ferrite content of 50±3 volume % is particularly favorable. This mixed soft magnetic material has a good flowability and a relatively high melt flow rate, and injection molding thereof is easy. The relative permeability of a core member formed thereof is about 10. Thus, an isolation transformer core formed of this mixed soft magnetic material is suitable for a connector for an air bag which has two cores arranged to face each other with a gap of 1 mm therebeteween and needs to be able to surely transmit large power in a moment even if a gap varies in the range of ±0.5 mm.

Industrial Applicability

[0057] In an isolation transformer core of the present invention, a core member is made of a mixed soft magnetic material comprising an insulating material having an electrical insulating property and a soft magnetic material. Thus, the isolation transformer core has an improved vibration resistance and a lowered fragility. Further, the relative permeability of a coil is relatively low. Therefore, the isolation transformer cores are suited to be arranged to face each other with a gap of about 1 mm therebeteween and transmit large power in a moment.

[0058] Further, when the soft magnetic material content is in the range of 10 to 70 volume %, the isolation transformer core of the present invention has the relative permeability required for transmitting large power in a moment, and at the same time a mechanical strength higher than that of a core made of sintered ferrite alone.

[0059] The isolation transformer core of the present invention uses, as a soft magnetic material, soft magnetic ferrite or Sendust. The isolation transformer core using soft magnetic ferrite is suitable for a high-frequency transformer, because it has only a small eddy-current loss. The isolation transformer core using Sendust is advantageous in that it can be of a small size because it has a high saturation magnetic flux density (twice as high as that of ferrite).

[0060] The isolation transformer core of the present invention uses, as an insulating material, any of thermoplastic resin, thermoplastic rubber, silicone rubber, thermosetting resin and adhesive which all have flexibility and good formability. Therefore, the isolation transformer core has large shock resistance, and is easy to form even when it has a complicated shape. Thus, the vibration resistance of the isolation transformer core is much improved and manufacturing cost is lowered.

Claims

1. An isolation transformer core comprising a coil and a core member, characterized in that said core member comprises a mixed soft magnetic material which comprises an insulating material having an electrical insulating property and a soft magnetic material.

2. An isolation transformer core according to claim 1, characterized in that the soft magnetic material content is in the range of 10 to 70 volume %.

3. An isolation transformer core according to claim 1 or 2, characterized in that said soft magnetic material is soft magnetic ferrite or Sendust.

4. An isolation transformer core according to any one of claims 1 to 3, characterized in that said insulating material is any one of thermoplastic resin, thermoplastic rubber, silicone rubber, thermosetting resin and adhesive.

Drawing