Patent Publication Number: US-6714110-B2

Title: Sleeved case design for adjustably increasing creepage distance

Description:
This patent application claims priority from U.S. provisional application No. 60/234,267 entitled “Sleeved case design for adjustably increasing creepage distance” filed on Sep. 21, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to electrical component packaging and particularly to cases for toroidal inductors which are suitable for automatic insertion on circuit boards. 
     2. Background Description 
     Circuit board fabrication is often accomplished with the aid of automatic insertion equipment. This equipment automatically handles electrical components and inserts the component leads into holes in the circuit board. In order that electrical components, such as a wire-wound magnetic core, can be inserted on a circuit board with such equipment, the component is often placed on a supporting structure or “case”, that enables automatic handling. A typical existing case design for electrical components includes an open box with holes in the bottom for the wire leads. Once the leads are threaded through the holes, an adhesive or potting compound is used to secure the component to the case. Later the leads are sheared to length. 
     As technology advances, electrical components become smaller and are packaged more closely. However, it remains necessary to meet safety and operating requirements. As electrical components become smaller and are packaged more closely, the distance between conductors grows smaller and electrical effects which were not significant for larger distances become limiting factors in the design and packaging of electrical components. 
     Of particular concern are toroidal transformers for mounting on printed circuit boards, because of their relative bulk and higher voltages. An increasingly significant factor affecting the design of case mounted toroidal transformers is “creepage distance,” which is the shortest distance through air along the surface of an insulating material between two conductive parts. Minimum creepage distance requirements increase where air pollution generates high and persistent conductivity caused, for instance, by conductive dust or moisture. 
     To achieve minimum creepage distance requirements manufacturers have several options under current state of the art. First, they may raise the core to provide the required distance between windings and the terminations. Second, they may terminate the wires outside the case at a point some distance from where the wires exit the case. However, raising the core increases component height, which defeats an advantage of a smaller core. Furthermore, if the wires are terminated outside the case the burden of satisfying minimum creepage distance requirements passes to the circuit board designer who must pay special attention to the layout of the printed circuit board and the location of adjacent components. 
     What is needed is a way to provide minimum creepage distance without either raising the core or terminating wire leads some distance from where the wire exits the casing. Furthermore, it would be advantageous to provide an approach which is flexible enough to accommodate a wide range of creepage distance requirements. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a transformer casing design which increases the creepage distance. 
     It is a further object of the invention to provide a transformer casing design which allows the creepage distance to be varied to accommodate different limits in minimum creepage distance, and to do so without changing the size, footprint or pin placements of the case. 
     Another object of the invention is to have a casing design for minimum creepage distance which allows for smaller core sizes. 
     It is also an object of the invention to provide a casing design which does not require raising the case in order to achieve minimum creepage distance requirements. 
     A further object of the invention is to provide a casing design which does not require wire termination some distance from where the wire leaves the case in order to achieve minimum creepage distance requirements. 
     Yet another object of the invention is a casing design that is inexpensive to manufacture. 
     The present invention provides a sleeved case design wherein variable creepage distance is provided by sleeves protruding upward into the case and through which wire leads egress from the case at a mounting hole. This does not require raising the core or terminating the wire some distance from the mounting hole. This sleeved case can be built with different sleeve heights to meet different safety distance requirements. The sleeve allows production of a current sense device which is much smaller in physical size than is otherwise acceptable, much less expensive to manufacture, and still meet the creepage and clearance dimensions required by the Safety Agencies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
     FIGS. 1 a ,  1   b  and  1   c  are cutaway views of a core and case mounting in the prior art showing creepage distance between the core and the conductive lead in a flush mount ( 1   a ), an elevated mount ( 1   b ) and a mount where the leads are terminated some distance from the mounting hole where the wire leaves the case ( 1   c ). 
     FIG. 2 is a cutaway view of a core and a sleeved casing according to the present invention showing creepage distance between the core (where it most closely abuts the casing insulation) and the conductive lead (where the wire is stripped as it exits the mounting hole), there being a vertical sleeve extending up around the mounting hole so that the shortest path to the mounting hole along an insulating surface necessarily goes over the top of the sleeve and down the interior of the sleeve to the mounting hole. By varying the height of the sleeve it is possible to satisfy a range of minimum creepage distance requirements using the same casing construction and without changing the placement of the casing on the circuit board or the physical and electrical mounting procedures. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1 a , there is shown a cutaway view of a casing  11  within which there is a core  12  (windings not shown). Secondary windings (not shown) on the core are of insulated wire. This wire exits the casing  11  through a hole in the casing  19 , and during assembly is inserted through a corresponding hole in the printed circuit board  18 . At the point of exit  14   a  the insulation  16  on the wire is stripped, leaving bare conductor lead  17  for electrical connection to the printed circuit board  18 . 
     The creepage distance  15  is the shortest distance between the core  12  along the surface of the insulating material that composes the casing  11 , from the point  13  where the core is closest to the insulating material to the point  14   a  at which the insulation  16  has been stripped leaving bare conductor lead  17 . In the configuration shown in FIG. 1 a , the creepage distance for cores in modern usage is increasingly insufficient to meet safety requirements. 
     To meet the safety creepage distance requirement for UL, VDE and IEC, manufacturers have several options under current state of the art. First, they may raise the core to provide the required distance between windings and the terminations. Second, they may provide additional distance between windings and the terminations by terminating the wires some distance from the point where the wire leaves the case. However, raising the core increases component height. Furthermore, if the wires are terminated some distance from the point where the wire leaves the case (in order to ensure enough creepage distance) the circuit board designer must pay special attention to the layout of the printed circuit board. 
     As shown in FIG. 1 b , the creepage distance may be increased by elevating the casing  11  above the printed circuit board  18 , allowing additional distance along insulator  16  (from  14   a  to  14   b ) to the point  14   b  at which the insulation  16  is stripped leaving bare conductor lead  17 . 
     Alternatively in the prior art, the creepage distance may also be increased, as shown in FIG. 1 c , by extending the wire beyond the exit hole  19  before terminating at a point  14   c  at which point the insulation  16  is stripped, allowing additional distance along insulator  16  (from  14   a  to  14   c ) before coming to bare conductor lead  17 . 
     Now turning to FIG. 2, there is shown a cutaway view of the sleeve  31 , through which the wire is inserted to go through the hole  30  exiting the casing  21 . The creepage distance  25  from the core  22  now goes along the surface of the insulating material that composes the casing  21 , from the point  23  (where the core is closest to the insulating material of the casing) to the point  24  where the insulation is stripped from the conductor  26  leaving bare conductor lead  27 . This distance is increased by twice the height  29  of the sleeve  31 , because the path along the surface must detour up and then down along the sleeve. 
     The sleeved case design provides creepage distance by means of vertical sleeves built into the case. A sleeve surrounds the hole so that the shortest distance from the case floor along a case surface to the hole is up the outside of the sleeve and down the inside of the sleve to the hole. The “creepage distance” is increased by the amount of traverse up and down the sleeve. This technique for increasing “creepage distance” does not require raising the core, and avoids the additional circuit board design concerns required where the added creepage clearance is provided by terminating the wire some distance from where the wire leaves the case. 
     In the best most of implementation of the invention the sleeve is a simple hollow cylinder vertically aligned over the mounting hole  30  constructed seamlessly as part of the casing  21 , so that the shortest path from inside the casing along the surface of the insulating material of the casing in order to exit the mounting hole  30  must necessarily go into the opening at the top of the sleeve  31  and down through the sleeve to the mounting hole  30 . Note that FIG. 2 is a cutaway that only shows the sides  31  of the sleeve, but it is understood that the sleeve itself is an enclosed cylindrical segment open at both ends. It will also be evident to those skilled in the art that a sleeve having a rectangular rather a cylindrical construction would also achieve the purposes of the invention. Nor is it necessary that the sleeve be vertically aligned, or that the sides of the sleeve be straight. It is only necessary that the sleeve be topologically equivalent to the cylindrical sleeve shown in FIG.  2 . Various topologically equivalent structures will readily appear to those skilled in the art, which could be used to increase the creepage distance. However, the simple vertically aligned cylindrical sleeve is the best most of implementation from a practical manufacturing point of view. 
     In this way the invention provides a structure and means for increasing the creepage distance without incurring the disadvantages of the prior art approaches illustrated in FIGS. 1 b  and  1   c . The invention has a further advantage in that, by adjusting the sleeve height  29  a range of creepage distance requirements can be accommodated, without altering the placement of the housing  21  upon the circuit board  28 , thereby extending manufacturing economies. This sleeved case can be built with different sleeve lengths to meet different safety distance requirements, but the design also allows for production efficiencies where a particular sleeve height meets minimum clearance requirements for a plurality of circuits. The sleeve itself may be a molded part of the case assembly, and may be in a variety of shapes. In the preferred embodiment, the sleeve is in the form of a hollow cylinder, but as indicated above any topologically equivalent structure (such as a hollow rectangular bar) which completely surrounds the hole will work. 
     It should further be noted that the measurement of creepage distance begins at that point  23  where the core  22  most closely touches the casing  21 . If the core  22  were not seated firmly in the casing with adequate clearance from other points of possible contact with the casing, the manufacturing process would not be able to achieve a desirable consistency in creepage distance, possibly leading to a reduced manufacturing yield. Consequently, in the best mode of implementation of the invention, the core is firmly seated by suitable means so that a selected point, chosen to meet creepage distance requirements, is the point where the core most closely touches the casing. Suitable means include adhesive at the point of contact  23 . Also, suitably firm seating may be achieved by use of a potting material to fill the casing, after the core has been placed as desired within the casing. 
     While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.