Field of the Invention The present invention relates generally to electromagnetic devices, and, more particularly, to a coil bobbin for an electromagnetic device which provides increased mutual flux.

Background of the Invention Transformers are generally used to perform voltage conversions between two sets of windings. A schematic diagram of a particular transformer is illustrated in Figure 1 The illustrated transformer 100 generally includes two core members 102 which are disposed adjacent one another to form a magnetic core structure.

Each of the core members 102 include a center leg 104.

Together, the center legs 104 form a central core structure about which two windings 106 are formed. As current is passed through one of the windings 106, known as the

primary winding, flux 108 is carried by the magnetic core structure through the other winding 104, known as the secondary winding. The flux common to the two windings or mutual flux acts to induce a current in the secondary winding 104. In this manner, the transformer converts a voltage and current in the primary winding to a voltage and current in the secondary winding.

Although not illustrated in Figure 1, a coil former or bobbin is typically disposed about the center core of the magnetic core structure to facilitate the assembly of the windings and to thermally insulate the windings from the core structure. Coil bobbins are typically produced by molding of a thermoset or a thermoplastic resin. These moldings resins typically comprise a plastic material, along with a filler material, which is added to improve the thermal and physical characteristics of the resin. The filler material commonly includes glass or carbon fiber or minerals such as silica or talc. The molding resins typically have a very low permeability and are thus substantially magnetically inert.

By being disposed between the core structure and the windings, the magnetically inert conventional bobbin impedes the transmission of flux to the core structure.

This decreases the mutual flux between the two windings and degrades the power efficiency-to-size performance the transformer.

Coil bobbins typically include a central aperture for receiving the center of the core structure. The assembly tolerances associated with these apertures further

contribute to reducing the mutual flux between the two windings and further reduce the power efficiency-to-size performance the transformer.

Attempts have been made to increase the mutual flux and the power efficiency-to-size performance of a transformer by bringing the coil windings closer to the magnetic core structure. For example, coil bobbins fabricated from a polyamide film (rather than molded) have been used to decrease the thickness of the coil bobbin.

These fabricated coil bobbins are, however, typically difficult and expensive to manufacture. Self-supporting coils, which are wound directly on the magnetic core with only a single layer of thin film tape between the windings and the core structure, have also been used to bring the winding closer to the core. Self-supporting coils, however, are also typically cost prohibitive and, when used, are typically limited to low temperature applications due to the lack of thermal insulation between the coil winding and the magnetic core structure.

Summary of the Invention Generally, the present invention relates to increasing the mutual flux of an electromagnetic device.

In accordance with one embodiment of the invention, an electromagnetic device having coil bobbin is provided. The electromagnetic device includes a magnetic core, a coil bobbin disposed about the magnetic core, and at least one winding disposed about the coil bobbin. A magnetically active material is disposed between a portion of the

magnetic core and the winding in order to increase transmission of flux from the winding to the magnetic core structure.

In one embodiment of the invention, the magnetically active material is disposed in the assembly clearances between a portion of the magnetic core and the coil bobbin. In yet another embodiment of the invention, magnetically active material is provided within the coil bobbin itself.

The above summary of the present invention is ncr intended to describe each illustrated embodiment or every implementation of the present invention. The figures and detailed description of which more particularly exemplify these embodiments.

Brief Description of the Drawings The invention may be more completely understood in consideration with the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: Figure 1 is a basic schematic illustration of an exemplary transformer; Figure 2 illustrates an exemplary transformer in accordance with one embodiment of the invention; Figure 3 illustrates a coil bobbin according to one embodiment of the invention; Figure 4 is a side, cross-sectional view of an exemplary transformer according to another embodiment of the invention; and

Figure 5 is a side, cross-sectional view of yet another transformer in accordance with another embodiment of the invention.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description of the Preferred Embodiment The present invention is believed to be applicable to a number of devices having magnetic core structures for carrying flux. An appreciation of various aspects of the invention will be gained through the discussion provided below. Though the discussion below is generally directed to transformers, it will be readily appreciated by those skilled in the art that the invention is applicable to a number of other devices, including but not limited to, motors, solenoids, relays, LVDT's, and contractors.

Figure 2 illustrates an exemplary coil bobbin transformer 200. The transformer 200 includes a bobbin 210 disposed about two core members 220 which lie adjacent one another. The core members 220 are typically ferromagnetic members, which, together, make up the magnetic core

structure of the transformer. The core members 220 may both be E-shaped members having a center leg (not shown) and two outer legs 222. The coil bobbin 210 generally includes outer flanges 212 and inner flanges 214, which, together, define winding zones for a primary winding 232 and a secondary winding 234. The bobbin 210 further includes a core reception aperture (not shown) to accept the center legs of the core members 220.

In conventional bobbin transformers and other devices, the bobbin generally facilitates transformer (device) assembly, but also magnetically insulates the coil windings from the core structure. For example, even in relatively small power transformers, the thickness of the coil bobbin between the core structure and the windings is often on the order of 1 mm thick and the assembly clearances between the core structure and the coil bobbin are often another 0.5 mm. In larger transformers, both bobbin thickness and assembly clearances are further increased. As discussed above, in conventional transformers (as well as other devices), the thickness of the bobbin and the assembly clearances impede the transmission of flux to the magnetic core structure.

The present invention provides portions of the area separating the coil windings from the core structure with a magnetically active material. This may be done by forming the coil bobbin from a magnetically active material or by filling assembly clearances or other voids between the core structure and the coil windings with a magnetically active material, as will be discussed below.

Figure 3 illustrates an exemplary transformer coil bobbin 300 formed in part by a magnetically active material. As noted above, the invention is applicable to devices other than transformers. The transformer is used for exemplary purposes only. The bobbin 300 is generally molded from a thermoset or thermoplastic resin which includes plastic, in addition to a magnetically active material. Alternatively, the bobbin may be fabricated.

The magnetically active material may include an iron- containing species, such as magnetite (Fe304) or an iron powder. In one exemplary embodiment, a 40% by volume level of magnetically active material is used in place of the conventional glass or other mineral fillers typically used to construct the bobbin 300. Greater or smaller volumetric ratios may be used in other embodiments.

By using a magnetically active material in a bobbin, the bobbin itself facilitates the transmission of flux to the magnetic core structure. This increases the mutual flux between the coil windings of a transformer and also enhances the power efficiency-to-size performance of a transformer. In effect, the bobbin can be considered as an extension of the magnetic core structure. In addition, using a magnetically active material, such as magnetite, the bobbin 300 may be provided with similar structural and thermal and electrical insulation properties as bobbins formed with typical fillers.

The magnetically active bobbin 300 may be entirely magnetically active or may be formed to include magnetically active regions and magnetically inert regions,

that is, regions having low permeability. This may be done by, for example, using well-known injection molding techniques. The magnetically inert regions may be formed from a resin which includes conventional filler materials, such as carbon glass or other minerals. In this manner flux transmission may be directed to the magnetic core structure and flux leakage may be reduced. In the illustrated embodiment, the inner flange 304 of the bobbin 300 is formed as a magnetically inert region while portions 306 and 301 of the bobbin between the flanges are formed as a magnetically active regions (as shown with shading).

This prevents flux from escaping outwardly from the inner flange member and focuses flux transmission toward the core structure. Bobbins with other magnetic profiles can also be formed.

Figure 4 illustrates a side, cross-sectional view of an exemplary transformer 400 having a magnetically active material 430 disposed between a coil bobbin 410 and a central portion 420 of a magnetic core structure.

Portions of the coil bobbin 410 (shown with shading) may also be formed from a magnetically active material as discussed above. The magnetically active material 430 typically includes an iron-containing species such as magnetite or an iron powder. However, it is intended that the present invention cover other types of magnetically active material.

In particular, the magnetically active material 430 is disposed in the assembly clearances between the central portion of the magnetic core structure and the

surface of central reception aperture 412 of the coil bobbin 410. This improves the transmission of flux from the windings 414 to the core structure and increases the power efficiency-to-size performance characteristics of the transformer.

In one embodiment, the magnetically active material 430 is an adhesive material which bonds the center core portion 420 to the coil bobbin 410. This facilitates the assembly of the transformer 400 and also immobilizes the magnetic core structure with respect to the coil bobbin 410. In doing so, the magnetically active adhesive may substantially reduce resonance in the transformer 400.

In one assembly method, the magnetically active material 430 is injected into the coil bobbin central aperture 412 prior to assembly of the core structure. The center core portion 420 is then disposed within the central aperture. The windings 414 are excited and the core members are reciprocated. The magnetic field generated by the winding excitation along with the reciprocation aligns and moves the magnetically active material 430 into the assembly clearances between the central portion of the core structure and the coil bobbin 410. The excitation may be performed by first passing an AC current through the windings followed by a DC current. This further ensures that the magnetically active material is aligned parallel to the flux field and enhances flux transmission. By exciting the windings 414 while the adhesive bonds the coil bobbin 410 to the magnetic core structure 420, the density

of the magnetic filler material is increased in areas where flux transmission is the greatest.

In an alternate embodiment, the center core portion 420 may be inserted within the reception aperture 412 prior to insertion of the magnetically active material.

This may be performed by, for example, inserting an injection nozzle into a molded recess adjacent the core reception aperture 412 of the bobbin 410 and injecting a mixture of magnetite and an adhesive under pressure to fill the assembly clearances.

Figure 5 illustrates another exemplary coil bobbin transformer 500 having a gapped core structure. In this embodiment, a magnetically active material 530 is supplied between the coil bobbin 510 and the core structure 520 and between the core structure 520 and a magnetically inert layer 540 which forms a gap between two core portions 522 and 524. The magnetically active material 530 may be an adhesive and may be supplied prior to or after inserting the core portions into the bobbin 510, as described above.

The magnetically active material 530 disposed between the core structure 520 and the bobbin 510 increases the mutual flux of the transformer as discussed above. The magnetically active material disposed between the core structure 520 and the separating layer 540 improves gap design, as more fully described in U.S. Patent Application Serial No. ~~~~~~~~~, , entitle entitled "Improved Gapped Core Structure" filed January 14, 1997, by inventor(s) hereof.

Though the above embodiments generally disclose the use of a magnetically active material in assembly

clearances of a molded coil bobbin transformer, the invention is not so limited. Coil bobbins for other types of devices are also covered. Moreover, magnetically active material may be supplied in assembly clearances or other voids in fabricated bobbins, self-supporting coil windings, etc.

As noted above, the present invention is applicable to a number of different types of electromagnetic devices which may benefit from a magnetically active material disposed between a coil winding and a core structure. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.