Abstract:
A magnetic device and a method of manufacture therefor. In one embodiment, the magnetic device includes: (1) a bobbin having a winding guide and molded-in margins proximate opposing inside flanges of the bobbin, each of the opposing inside flanges having at least one notch formed in an inside face of each of said opposing inside flanges; (2) an inner winding wound about the winding guide and between the molded-in margins; (3) an outer winding wound about the inner winding and the winding guide and between the flanges; and (4) an insulating plate, provided in the at least one notch and interposed between the inner and outer windings, a thickness of the insulating plate providing a predetermined creepage distance between at least one lead of the inner winding and the outer winding.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to magnetic devices, and, more specifically, to a magnetic device employing an isolation barrier and a method of manufacture therefor. 
     BACKGROUND OF THE INVENTION 
     Electronic manufacturers are constantly striving to make electronic components even smaller, especially in the field of magnetic devices. Miniature magnetic devices are used in a wide variety of electronic equipment such as telephones, televisions, computers, etc. With overall dimensions of the devices becoming smaller, the design of the magnetic devices presents unique challenges. The structure of the miniature devices must accommodate special features that are necessary to the manufacturability and electrical performance thereof. 
     A magnetic device is a device that uses magnetic material arranged in a defined structure for shaping and directing magnetic fields in a predetermined manner to achieve a desired electrical performance. The magnetic fields in turn act as the medium for storing, transferring and releasing electromagnetic energy. As a specific example of magnetic devices, transformers are composed of two or more windings wound about a bobbin with a magnetic core inserted through the bobbin. The bobbin may be made of virtually any suitable dielectric material. The insulated windings are wound about the bobbin in patterns to achieve specific electrical characteristics. The number of windings and the number of turns per winding is dictated by the function of the transformer in the intended circuit. 
     The bobbin may be manufactured separate from or integral with a base that provides the physical support for the bobbin. The transformer is electrically connected to the circuit by contacts extending from the base. A core of magnetic permeability, often ferrite, is inserted into the bobbin to shape the magnetic field. The core is often made in two pieces with an “E” shaped cross section. The central poles of the E-shaped core halves are inserted into opposite ends of the bobbin and the poles at the center of the bobbin. The complete transformer assembly is held together by various physical means such as an adhesive or spring clip. 
     Transformers work on the general principle that a change in current flowing through the primary winding, which is isolated from the secondary winding, creates a magnetic flux which causes a change in the current to flow in the secondary winding. The ratio of primary-to-secondary current is established by the number of windings in the secondary coil related to the number of windings in the primary coil. This, in turn, creates a voltage which is the product of the number of turns multiplied by the change in flux. This product is also proportional to a change in current multiplied by the inductance. 
     As the electronic devices employing magnetic devices continue to be made smaller, it is necessary to design a more compact and lower profile magnetic device (e.g., transformer). The limitation of “creepage,” however, can adversely affect the design and operation of the magnetic device. Creepage is generally defined as the transference of electrical current from one winding in a transformer to another winding in the same transformer by way of a conductive path forming a temporary bridge along a surface of a dielectric material separating the windings. The leakage current generally occurs as a result of ionization of air and insufficient creepage distance. Additionally, a minimum creepage distance is often required to comply with safety standards. 
     For example, a transformer may have a primary winding wound about a bobbin with a primary lead extending therefrom coupled to an input terminal of the transformer. In addition, the transformer may have a secondary winding wound concentrically about the same bobbin and around the primary winding. Although the primary winding and the secondary winding are separated by an insulator, the lead coming from the primary winding and terminating at the input terminal passes very close to the secondary winding. Because the primary lead passes very close to the secondary winding, there is the likelihood of creepage between the primary and secondary windings (e.g., at the point where the lead of the primary winding and the secondary winding are in the closest proximity to each other) thereby resulting in a potential short-circuit in the transformer. The distance between the point on the primary lead and the point on the secondary winding along the surface of which the creepage occurs is commonly known to those skilled in the art as the creepage distance. 
     As alluded to above, to help prevent creepage in magnetic devices, standards have been instituted defining minimum insulation permitted in a transformer. The standards are promulgated by administrative bodies such as the International Electrotechnical Commission (IEC) to, among other things, increase the safety of devices employing components such as magnetic devices (see, for instance, IEC Standard 60950, third edition, 1999). Included in these standards are the minimum creepage distances depending on the specifications of the transformer and the circuit into which the transformer is to be employed. The standards are becoming more universally accepted. Therefore, minimum creepage distance in magnetic devices is becoming a more important factor in the design of such devices. 
     In the prior art, two commonly employed methods are employed to assure a minimum creepage distance. The first method is to manufacture two separate bobbins, each one with a winding wound thereabout and leads affixed to the respective terminals. One bobbin is then placed inside of the other so that the leads of one winding and the other winding are isolated from each other. Although effective for ensuring a minimum creepage distance, the use of two bobbins has several disadvantages. First, the two-bobbin method requires the manufacture of two bobbins rather than only one, thus resulting in increased parts and manufacturing costs. Second, the use of two bobbins is better suited for larger transformers because of the difficulty associated with manufacturing miniature bobbins that fit together, one inside the other. Third, the use of one bobbin inside of another commonly leads to a large leakage inductance as a result of the space between the windings. Thus, the two bobbin approach is not the design of choice. 
     The second and more commonly employed method to assure a minimum creepage distance is the use of sleeves placed on the wire leads of the inner winding. However, like the two bobbin approach discussed above, the use of wire sleeves also has major disadvantages. Although the use of wire sleeves is employable in the manufacture of small transformers, placing the wire sleeves on each of the leads of the inner winding is a labor-intensive process that must be completed by hand. As a result, the costs of manufacturing small transformers having wire sleeves on the leads of the inner windings is high. In addition, although placing sleeves on the large leads of large transformers may appear at first glance to be a trivial task, placing sleeves on extremely small leads of miniature transformers becomes a tedious and time-consuming chore. As a result, labor costs, and thus the overall costs of manufacturing, are again elevated. 
     Accordingly, what is needed in the art is a magnetic device, and related method, that maintains a predetermined creepage distance therein, but overcomes the deficiencies of the prior art. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides a magnetic device and a method of manufacture therefor. In one embodiment, the magnetic device includes: (1) a bobbin having a winding guide (or core tube) and molded-in margins proximate opposing inside flanges of the bobbin, each of the opposing inside flanges having at least one notch formed in an inside face of each of said opposing inside flanges; (2) an inner winding wound about the winding guide and between the molded-in margins; (3) an outer winding wound about the inner winding and the winding guide and between the flanges; and (4) an insulating plate, provided in the at least one notch and interposed between the inner and outer windings, a thickness of the insulating plate providing a predetermined creepage distance between at least one lead of the inner winding and the outer winding. 
     The present invention, in one aspect, introduces the broad concept of a magnetic device employing a bobbin having flanges adapted to receive an isolation barrier. The isolation barrier is interposed between inner and outer windings of the magnetic device to allow a creepage distance between the inner and outer windings to be above a predetermined amount. The magnetic device may thus meet insulation requirements promulgated by various standards bodies, such as the International Electrotechnical Commission (IEC). 
     In one embodiment of the present invention, the magnetic device further includes a magnetic core proximate the inner and outer windings. The magnetic core may thus impart a desired magnetic property to the inner and outer windings. In a related embodiment, the magnetic device further includes a spring clip that retains the magnetic core on the bobbin. The magnetic core may, in an advantageous embodiment, include first and second magnetic core portions. The spring clip may thus secure the first and second magnetic core portions to the bobbin. Those skilled in the pertinent art realize, of course, that the spring clip is not necessary to practice the present invention. 
     In one embodiment of the present invention, the isolation barrier includes an insulating plate. The insulating plate may be a molded plastic plate, a polyamide plate, or a nomex plate. Of course, the use of other materials for the insulating plate is well within the broad scope of the present invention. In a related embodiment, the isolating barrier further includes a layer of insulating tape placed over the insulating plate. The insulating tape may thus secure the insulating plate in place within the magnetic device. Of course, placing the insulating tape over the insulating plate is not necessary to practice the present invention. 
     In one embodiment of the present invention, the isolation barrier provides the creepage distance between a lead of the inner winding and a body of the outer winding. The magnetic device may thus avoid the use of sleeving to compensate for the lack of creepage distance between the lead of the inner winding and the body of the outer winding. 
     In one embodiment of the present invention, the predetermined amount is determined by a safety standard. The safety standards may be promulgated by any of various standards bodies, such as Underwriters Laboratories, Inc. (UL) and the IEC. Those skilled in the pertinent art are familiar with these and other relevant standards bodies. 
     In one embodiment of the present invention, the molded-in margins are slotted to allow at least one lead of the inner winding to terminate on a winding terminal of the magnetic device. In a related embodiment, the flanges are slotted to allow at least one lead of the outer winding to terminate on a winding terminal of the magnetic device. The leads of the inner and outer windings may thus be directly connected to the respective winding terminals while maintaining the predetermined creepage distance. 
     The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the pertinent art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the pertinent art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the pertinent art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an exploded isometric view of an embodiment of a magnetic device constructed according to the principles of the present invention; 
     FIG. 2 illustrates a cross-sectional view of an embodiment of a magnetic device constructed according to the principles of the present invention; and 
     FIG. 3 illustrates a flow diagram of the method of manufacturing a magnetic device according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, illustrated is an exploded isometric view of an embodiment of a magnetic device  100  constructed according to the principles of the present invention. The magnetic device  100  includes a terminal bobbin  110  formed about a plurality of terminals (one of which is generally designated  112 ) having a winding terminal  114 . The magnetic device  100  further includes an isolation barrier  120 . In the illustrated embodiment, the isolation barrier  120  is an insulating plate. The magnetic device  100  further includes first and second core halves  130 ,  140 , formed as an E-shaped core. The magnetic device  100  still further includes a spring clip  150 . 
     The terminal bobbin  110  is manufactured of dielectric material. An inner winding (not shown) is wound about a winding guide  132  of the terminal bobbin  110  between molded-in margins (one of which is generally designated  134 ) thereof. The isolation barrier  120  is placed atop the inner winding, in between opposing inside flanges (one of which is designated  122 ) of the terminal bobbin  110 . In a preferred embodiment, the opposing inside flanges  122  of the terminal bobbin  110  include a notch  126  to support and align the isolation barrier  120 . Also in a preferred embodiment, the isolation barrier  120  is composed of molded plastic, polymide, or nomex. Of course, the present invention is not limited to a specific dielectric material. 
     Additionally, an insulating material (see FIG.  2 ), perhaps insulating tape, is placed atop the inner winding to decrease creepage between the inner winding and an outer winding. The outer winding is then wound about the winding guide  132  and between the flanges  122 . The molded-in margin  134  and flange  122  are slotted (e.g., slot  136 ) to allow at least one lead of the inner winding to terminate on a winding terminal (for instance, terminal  114 ) and at least one lead of the outer winding to terminate on a winding terminal of the magnetic device  100 . 
     The first and second core halves  130 ,  140  are placed into corresponding sides of an aperture in the terminal bobbin  110 . In a preferred embodiment, the first and second core halves  130 ,  140  are composed of a ferrite material, but any material suitable for use as a core is within the broad scope of the present invention. The spring clip  150  is placed over the first and second core halves  130 ,  140  to secure the core halves  130 ,  140  within the aperture and about the terminal bobbin  110 . Although the magnetic device  100  in FIG. 1 is illustrated having a spring clip  150  to secure the placement of the first and second core halves  130 ,  140 , any means used to hold the assembly together is within the broad scope of the present invention. The use of the spring clip  150  is preferred over adhesives or other means because of the simplicity of assembly of the magnetic device  100 . 
     Referring now to FIG. 2, illustrated is a cross-sectional view of an embodiment of a magnetic device  200  constructed according to the principles of the present invention. The magnetic device  200  includes a terminal bobbin  202  having molded-in terminals, one of which is designated  204 . The terminals  204  extend from the terminal bobbin  202  and are distal from one another. The terminal bobbin  202  is constructed from a dielectric material, such as molded plastic. The terminal bobbin  202  further includes a winding guide  206  for winding inner and outer windings  212 ,  214  thereupon. 
     In addition, the winding guide  206  constrains the placement of the windings within opposing inside flanges (one of which is designated  203 ) proximate the winding guide  206 . The terminal bobbin  202  further includes molded-in margins  208  created therein located proximate the opposing inside flanges  203 . The flanges  203  of the terminal bobbin  202  include lead slots  210  for the winding leads of the inner and outer windings  212 ,  214  to terminate on the terminals  204 . The lead slots  210  are created through both the opposing inside flanges  203  and the molded-in margins  208  of the winding guide  206 . The terminal bobbin  202  still further includes notches (one of which is designated  220 ) created in the dielectric material on the opposing inside flanges  203  of the terminal bobbin  202  for supporting an isolation barrier. 
     The magnetic device  200  further includes the inner winding  212  and the outer winding  214 . Either the inner winding  212  or the outer winding  214  may be used as the primary winding or the secondary winding of the magnetic device  200 , depending on the application which the magnetic device  200  is employed. The magnetic device  200  still further includes an isolation barrier (e.g., insulation tape  216  and insulating plate  218 ) placed between the inner winding  212  and the outer winding  214 . 
     The terminal bobbin  202  is formed about the plurality of terminals  204 . The terminals  204  are molded into the terminal bobbin  202  (e.g., the terminal bobbin  202  is composed of a moldable dielectric material) as a one-piece assembly. Of course, other materials and assembly types for the construction of the terminal bobbin  202  are well within the broad scope of the present invention. 
     The following features of the terminal bobbin  202  provide several advantages, but are not required to conform with the broad scope of the present invention. The orientation of the terminals  204  facilitates the use of automatic equipment to dispose the inner and outer windings  212 ,  214  about the winding guide  206  of the terminal bobbin  202 . The orientation of the terminals  204  also facilitates coupling (e.g., through a soldering process) of all the leads of the inner and outer windings  212 ,  214  to the respective terminals  204 . The terminals  204  are typically long enough to accommodate more than one winding lead thereby allowing more sophisticated winding patterns to be employed with the magnetic device  200  for enhanced high frequency component performance. 
     The lead slots  210  on the terminal bobbin  202  also assist in the arrangement of the leads of the inner and outer windings  212 ,  214  on the terminals  204 . The lead slots  210  facilitate a better connection by trapping the leads of the inner and outer windings  212 ,  214  within the lead slots  210  and isolating particular leads from one another where such isolation is advantageous. In addition, the lead slots  210  permit the leads of the inner winding  212  to pass through, rather than over, the molded-in margins  208  in the winding guide  206 . By passing through rather than over the molded-in margins  208 , the leads of the inner winding  212  pass farther away from the outer winding  214  than in transformers found in the prior art. The magnetic device  200  also includes a magnetic core (see FIG. 1) having a first core half and a second core half. The terminal bobbin  202  includes a core aperture (not shown), formed through the terminal bobbin  202 , to guide and constrain the core halves on the magnetic device  200 . 
     The inner winding  212  is disposed about the winding guide  206  of the terminal bobbin  202  on an axis parallel to the terminals  204 . This arrangement allows an automatic winding machine to wind and terminate the inner winding  212  easily. The leads of the inner winding  212  are typically soldered to appropriate terminals  204  located closest the center of the terminal bobbin  202 . In addition, the leads of the inner winding  212  are positioned within respective lead slots  210  to distance them from other leads, the outer winding  214 , and the magnetic core, as well as to allow the leads to pass through both the molded-in margins  208  and the opposing inside flanges  203 . As mentioned above, the terminals  204  may be adapted to receive multiple winding leads. Those skilled in the art understand that allowing various patterns of winding leads is advantageous when the magnetic device  200  is to handle high frequency electrical signals or the magnetic device  200  is employed in other advantageous embodiments. 
     The notches  220  formed in inside faces of the opposing inside flanges  203  of the terminal bobbin  202  are included to position and support the isolation barrier therein. The isolation barrier is positioned within the notches  220  across the winding guide  206  of the terminal bobbin  202  and atop the inner winding  212 . In the illustrated embodiment, the insulation tape  216  is placed around the inner winding  212  to insulate the inner winding  212  from both the outer winding  214  and the core. In addition, the insulation tape  216  is placed around the center of the isolation plate  218  to secure its position within the notches  220  of the terminal bobbin  202 . Although the isolation barrier is illustrated as an insulating plate  218 , the broad scope of the present invention is not so limited. In fact, the isolation barrier may itself simply be constructed of insulation tape  216  guaranteeing a minimum creepage distance by extending past the molded-in margins  208  and past the opposing inside flanges  203  of the terminal bobbin  202 . 
     Although the insulating tape  216  is placed around the entire inner winding  212  to insulate it from the outer winding  214 , the leads of the inner winding  212  remain uninsulated from the outer winding  214  when passing through their respective lead slots  210  and terminating at their respective terminals  204 . The point at which the leads of the inner winding  212  pass closest to the outer winding  214  indicate the point where creepage between the inner and outer windings  212 ,  214  is most likely to occur. As discussed above, in an effort to decrease creepage between the inner and outer windings  212 ,  214  standards have been established to assure a minimum creepage distance depending on the specifications of the magnetic device  200  and the application into which the magnetic device  200  is employed. By positioning the isolation barrier (e.g., the insulation plate  218 ) within the notches  220  in the terminal bobbin  202  and between the inner and outer windings  212 ,  214 , a minimum creepage distance between the inner and outer windings  212 ,  214  is achieved. 
     The notches  220  in the terminal bobbin  202  extend past the molded-in margins  208  on the opposing inside flanges  203 . As a result, the isolation barrier extends the insulation between the inner and outer windings  212 ,  214  to an acceptable aspect. In addition, because the lead slots  210  pass through the molded-in margins  208  as well as the opposing inside flanges  203 , the leads of the inner winding  212  pass through the sides of the terminal bobbin  202  farther away from the outer winding  214  than magnetic devices found in the prior art. Thus, as the leads of the inner winding  212  pass from the winding guide  206 , under the outer winding  214 , and through their respective lead slots  210 , a minimum creepage distance can be assured between the inner and outer windings  212 ,  214  by a thickness of the isolation barrier (e.g., the insulation plate  218 ). 
     Moreover, the isolation barrier can be constructed to various specifications, thus conforming to the various standards regarding creepage distance that may be imposed. In addition, because the notches  220  designed to position the isolation barrier can be created during the same manufacturing process used to initially create the terminal bobbin  202  (e.g., plastic injection molding), a magnetic device  200  constructed according to the principles of the present invention can be manufactured in less time and with less cost than a magnetic device found in the prior art using wire lead sleeves to insulate the leads of the inner winding  212  from the outer winding  214 . 
     Turning now to FIG. 3, illustrated is a flow diagram of the method of manufacturing a magnetic device according to the principles of the present invention. The method begins at a start step  305 . Then, a terminal bobbin is first created at a create bobbin step  310 . The terminal bobbin is formed about a plurality of terminals (e.g., injection molding a plastic material about the terminals) during the create bobbin step  310 . In addition, the terminal bobbin includes a winding guide for receiving inner and outer windings. An aperture is also developed through the terminal bobbin to accommodate a center leg of a magnetic core. Also, opposing inside flanges are formed on a surface of the terminal bobbin. The opposing inside flanges are notched in at least one place and are adapted to receive an isolation barrier. 
     The inner winding is wound about the terminal bobbin at an inner winding wound step  315 . The inner winding is disposed on the terminal bobbin (e.g., machine-winding of the wire about an axis parallel to the terminals) between molded-in margins proximate the winding guide during the inner winding wound step  315 . The leads of the inner winding then terminate at the terminals molded into the terminal bobbin at an inner winding terminates step  320 . 
     An isolation barrier (e.g., an insulating plate) is then installed atop the inner winding at an install isolation barrier step  325 . The isolation barrier is adapted to rest in the notches created in the opposing inside flanges during the create bobbin step  310  discussed above. While positioned in the notches created in the opposing inside flanges, the isolation barrier is adapted to extend past the winding guide and molded-in margins of the terminal bobbin on both ends. Once the isolation barrier is in place, an insulator (e.g., insulation tape) is installed around the inner winding during an install insulator step  330  to further isolate the inner winding from an outer winding. The insulator is also installed around the isolation barrier to prevent its movement or displacement from the notches made in the opposing inside flanges. 
     The outer winding is wound about the inner winding at an outer winding wound step  335 . Because the insulator and isolation barrier were both installed atop the inner winding in prior steps of the manufacturing process, the outer winding is wound around the insulator and isolation barrier during the outer winding wound step  335 . Like the inner winding, the outer winding is disposed on the terminal bobbin (e.g., machine-winding of the wire about an axis parallel to the terminals) under the constraint of the winding guide between the opposing inside flanges. The leads of the outer winding then terminate at other terminals molded into the terminal bobbin at an outer winding terminates step  340 . 
     The leads of both the inner and outer windings are then coupled (e.g., through a soldering process) to their respective terminals at a couple inner and outer winding leads to terminals step  345 . The magnetic core is then placed on the terminal bobbin under constraint of the core aperture at an install core pieces on bobbin step  350 . Of course, for some applications (e.g., “air-core” magnetic devices) it is unnecessary to include a magnetic core on the bobbin, and such magnetic devices are within the broad scope of the present invention. A spring clip is then placed over the two halves of the E-core of the magnetic device in an install spring clip step  355 . 
     Next, a label, if necessary, is applied to the magnetic device in an apply label to magnetic device step  360 . The label is applied to the magnetic device to identify the specifications of the device, the model number, or to disclose other similar information regarding the magnetic device or its operation. Of course, applying a label to the magnetic device is not necessary to the broad scope of the present invention. Once manufactured through the above-mentioned steps, the magnetic device may be tested as required for proper operation at a test magnetic device step  365 . The method concludes at an end step  370 . 
     For a better understanding of magnetic devices (including bobbin structures) and construction techniques therefor see Soft Ferrites, by E.C. Snelling, Butterworth (1988). For a general reference regarding electronics including communication systems employing magnetic devices see  Reference Data for Engineers: Radio, Electronics, Computers and Communications , 7th edition, Howard W. Sams &amp; Company (1988). The aforementioned references are herein incorporated by reference. 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.