Method of making a coil

Abstract
A method of assembling a coil includes forming a ferrite core having a top end, a bottom end, an inner opening extending from the top end to the bottom end, a cylindrical outer surface, and a step portion formed near the bottom end, the step portion extending past the outer surface. A first high dielectric material is applied on the outer surface of the ferrite core, then a conductive wire is wound onto the high dielectric material, whereafter a second high dielectric material is applied over the conductive wire.
Description




BACKGROUND OF THE INVENTION




An electrodeless fluorescent lamp (EFL) implements a coil design in its configuration. Such a coil design includes a cylindrical ferrite core, a bobbin and conductive insulative wire wound around a portion of the bobbin.

FIG. 1

illustrates a prior art high-temperature plastic threaded bobbin


10


which may be used in such a design. As depicted, bobbin


10


includes a high-temperature plastic base portion


12


and an integrated threaded high-temperature plastic chimney portion


14


. Chimney portion


14


is molded to include grooves


16


on the exterior cylindrical surface. A cylindrical ferrite core, not shown, is placed within the interior


18


of chimney


14


and conductive wire (not shown) is wound around chimney


14


by following the groove pattern


16


. A tape or shrink-tubing product would then be placed around the wound conductive wire to maintain the wire in position and maintain the integrity of the coil.




In the prior art coil, there are at least two ends of the conductive wire wound around the chimney


14


of bobbin


10


. The ends of these wires are passed through the base


12


for attachment to an electronic board or alternatively attached to plugs attached to the underside of base


12


. The plugs may be received by the electronic board for connection of the coil configuration. Threads


16


provide a built-in pitch wire spacing for the conductive wire.




Chimney


14


is a split element


20


whereby when conductive wire is wound around chimney


14


in the groove pattern


16


, chimney


14


is compressed around the ferrite core. Hook or holding elements


22


act to maintain the core securely within interior


18


. The underside of base


12


is formed such that the bottom portion of ferrite core is held within the chimney


14


. Bobbin


10


acts as an electrically insulating layer between the conductive wire and the ferrite core sufficient to prevent electrical breakdowns from occurring within the coil. The conductive wire itself may be insulated, and capable of continually withstanding temperatures approximately 250° C.




During operation of a coil, the highest temperature in the core body will occur in the middle height location of the core. Therefore, in

FIG. 1

the area having the highest temperature on bobbin


10


would be approximately at location


24


. For an RF coil assembly intended to work with EFL products in the 120-volt and 230-volt range, the temperature at this center point


24


could reach 250°. This being the case, it is necessary for bobbin


10


to be made of a material that has a maximum allowable service temperature capable of withstanding such a temperature level. Temperatures at the ends of the coil are around 200° C.




A drawback of a coil manufactured using bobbin


10


of

FIG. 1

, is the requirement of using the high-temperature material in order to withstand the temperatures generated during operation of the coil. This necessitates the use of expensive high temperature materials. Further, bobbin


10


uses a significant amount of such an expensive material due to the chimney feature. Additionally there is a significant amount of cost involved in manufacturing the bobbin


10


with threads


16


.




Therefore, the present invention looks to manufacture a simplified RF coil assembly with decreased costs as compared to existing coil assemblies, where the coil assembly meets expectations and operational requirements for use with an electrodeless fluorescent lamps.




BRIEF SUMMARY OF THE INVENTION




A cylindrical ferrite core includes a top-end, bottom-end and inner opening extending from the top end to the bottom end. An outer surface of the cylindrical core includes a step portion formed at the bottom end of the core, extending past the outer circumference of the non-step portion. A first high dielectric material is formed over at least a substantial portion of the outer surface of the cylindrical core to provide an insulative barrier. A length of conductive wire having a first end and a second end is wound around the first high dielectric material located over the outer surface of the cylindrical ferrite core. A second high dielectric material is then placed or located over the length of the conductive wire. This configuration seals the conductive wire between the two high dielectric materials and insulating the conductive wire from the ferrite core. A coil holder is provided having a base portion with a base opening formed substantially in a centered area in the base of the coil holder, the base opening is sufficiently sized to provide a passage way to the inner opening of the ferrite core. A snap-fit portion having a plurality of snap-fit fingers extending from the base portion engage the step portion of the cylindrical ferrite core, whereby the core is locked into engagement with the coil holder.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

depicts a prior art high-temperature plastic threaded bobbin;





FIG. 2

shows a cylindrical ferrite core having a step portion;





FIG. 3

illustrates a conductive wire used in the present invention;





FIG. 4

illustrates a first high-dielectric material formed over the ferrite core of

FIG. 2

;





FIG. 5

depicts the conductive wire wound around the insulative material of

FIG. 4

;





FIG. 6

shows the second insulative material formed over the conductive wiring;





FIG. 7

sets forth a coil holder of the present invention;





FIG. 8

shows a side view of a snap-fit finger of the coil holder.





FIG. 9

illustrates a snap-fit engagement between the ferrite core and coil holder;





FIG. 10

depicts an EFL device designed using the coil of the present invention; and





FIGS. 11

,


12


and


13


show alternative connection concepts of the present invention.











DETAILED DESCRIPTION OF THE INVENTION




Turning to

FIG. 2

, illustrated is a first embodiment of a ferrite core or tube


30


designed in accordance with the teachings of the present invention. Core


30


includes a top end


32


, a bottom end


34


, an inner opening


36


extending from the top end


32


to the bottom end


34


. An outer surface


38


of a cylindrical formation with a step portion


40


extending past the outer surface


38


. In the present embodiment, step


40


extends in a cylindrical manner approximately 1 mm around the circumference past outer surface


38


of ferrite core


30


. It is understood that step


40


may be vary from the mentioned 1 mm. A notch


42


may be optionally provided in core


30


to assist in holding a coil winding in place. This concept will be discussed in greater detail below.




The core


30


of

FIG. 2

is manufactured by use of a form. Alternatively, the core could be machined by taking a larger dimension core and machining it down to a desired formation. If the core is machined, it is preferred to provide an annealing of the cores to maintain a quality factor (Q) desirable for operation of an EFL component. Another manner of forming the core is by an extrusion process.




Ferrite core


30


which may be used in a preferred embodiment of the present invention, has the following parameters. The core geometry and material must provide a given inductance value without causing the need for geometric changes in the EFL device in which it is used. Parameters for a core intended to be used with an EFL device previously described, has an outside diameter (OD) of 17±0.35 mm; an inside diameter (ID) of 8.6±0.25 mm; and a length of 30±0.7 mm.




In the present invention, a conductive wire


50


such as in

FIG. 3

, is to be wound around the ferrite core


30


(of FIG.


2


). Wire


50


, in one embodiment, is a bare copper magnetic wire. Winding wire


50


onto core


30


is conceptually different from prior art coils which incorporate a bobbin feature configuration to carry the wound wire. It is to be appreciated that winding the conductive wire directly onto the ferrite core


30


could result in conduction between windings of the wire through the ferrite core


30


. Particularly, there is a concern that even if an insulated wire is wound around the ferrite core, during the life of the coil assembly, the wire would break down causing conduction between the wire and core, causing a malfunction of the coil. This possibility emphasizes the need to provide some sort of insulating material between the ferrite core


30


and the conductive wire


50


.





FIG. 4

depicts a first high dielectric material


60


applied to ferrite core


30


. As can be seen, the step portion


40


and a small portion of the upper end


32


of core


30


are not encompassed within first dielectric


60


. It is to be appreciated that the windings of wire


50


will not be wound as far down core


30


to include step


40


or go to upper end


32


. Therefore the first dielectric material


60


does not need to cover these portions of the core


30


. However, in another embodiment, it is of course possible to include the dielectric material to cover core step


40


or upper end


32


.




In selecting the appropriate coating material for a first high dielectric material


60


, it is desirable to select a material which will maintain thermal stability at a continuous temperature substantially equal to or greater than 250° C., and will have a temperature expansion co-efficient which matches ferrite core


30


or otherwise be malleable. It is to be appreciated that some applications may be able to operate at lower temperatures, such as systems designed for table lamps instead of ceiling fixtures, and low wattage systems. Such material should also not adversely affect the ferrite material electromagnetic performance (i.e. dielectric strength, resistivity, magnetic flux density, permeability, and Q). Material


60


should also provide sufficient insulation between the coil formed by wire


50


, and core


30


, and between adjacent turns of wire


50


. The coating for the high dielectric material used in the present embodiment is also beneficially of a low cost, easy to apply and provides the appropriate material strength and adhesion to maintain the coil active for a life span of approximately 15,000 hours or more. Coatings which may be used include at least silicon/rubber/polymer coatings, ceramic coatings and vitreous/glass coatings. Specific types of coatings which meet the foregoing requirements include but are not limited to a material TSE 326, a silicon product from General Electric, PTFE and PFA which are Teflon products from Dupont, and Xydar G-930, a liquid-crystal polymer (LCP).




The first high dielectric material


60


is used to not only provide an insulative layer between the core and conductive wire, but also to provide space insulation.




It should be emphasized here that the required thickness will play a part in determining the method of coating ferrite core


30


. For example, spray coating techniques are able to apply up to 1 mil/per application. To build up a large thickness with spray coating, the process will need to be applied repeatedly. Dip-coating can build a thickness of up to approximately 50 mils per application. In this technique, the core is placed on a rod or other holder, is dipped into a coating material. Once removed from the material, core


30


now covered in the high dielectric coating, is spun to evenly distribute the coating on the core. Another technique includes brushing on the coating material. Therefore, when choosing the method of application, it may be useful, though not necessary, to have electromagnetic calculations made to establish the required insulation thickness for the first high dielectric layer


60


. The manner of obtaining such calculations are known in the art by one of ordinary skill.




With attention to ceramic coatings, ceramics can withstand very high temperatures and they provide a room temperature, short-time curability and high manufacturability if needed for winding. By controlling the chemistry and density (porosity) the dielectric properties can be optimized (low permitivity and losses) to match that of polymers. To promote adhesion, the reactivity between the ceramic coating and the ferrite core is optimized. Selected ceramics should not degrade the electromagnetic characteristics of the core. The material should be stable for the life of the lamp (i.e. greater than 15,000 hours) at the operating temperatures. The coefficient of thermal expansion of the coating in the core should be matched so that there is no cracking and spallation of the coating during the curing and the subsequent use cycles. The high dielectric strength and resistivity are required of the material to provide insulation between the coil wire and the core. Some ceramic adhesives and coating systems include but are not limited to Brewer AlPO


4


from General Electric, P-78 and No. 31 from Sauereisen and Ceramadip 538N from Aremco.




Turning to

FIG. 5

, core


30


is shown with a first covering of a high dielectric material


60


around which is wound wire


50


in the form of a coil


70


.




One embodiment of the present invention, the first high dielectric material


60


, is cured only to a point where it is still of a substantially tacky consistency. Conductive wire


50


which may be a bare copper wire is wound onto the partially cured high dielectric layer


60


using a known winding process. The tackiness of the partially cured layer


60


assists in maintaining the wire position on the ferrite core


30


as the coil is wound. Such a winding procedure will provide the required winding pitch, and also help hold the wire in place. However, if it is found the winding of conductive wire


50


in this process is too time-consuming, an alternative process is to fully cure the first high dielectric material


60


prior to the winding process.




Winding of conductive wire


50


on first high dielectric material


60


in a coil formation


70


, as shown in

FIG. 5

, results in a first end portion


72


and a second end portion


74


. These end portions will, eventually, be connected to an electronic circuit such as in an EFL assembly. To secure the winding, one of the first end and the second may be inserted into notch


42


, of core


30


. The winding of conductive wire


50


as coil


70


may be accomplished by one of many known winding techniques.




It is noted that in one embodiment, conductive wire


50


used to form coil


70


, may be a rectangular wire. Such an embodiment is considered to provide the benefit of maintaining desired wire spacing. Further, a benefit of rectangular wire over square wire is that square wire generally has thinner insulation at its corners and thus a lower voltage breakdown capability.




Once the coil


70


has been formed over material


60


and around core


30


, a second high dielectric material


80


is applied over wire coil


70


as depicted in FIG.


6


. The coil ends


72


and


74


are not encompassed within this second high dielectric material


80


. The second layer of high dielectric material assists in holding the wire coil


70


(

FIG. 5

) in place, and seals it from the environment to retard oxidation of the wire in the high-temperature environment.




The entire coil assembly


90


of

FIG. 6

, includes core


30


, first high dielectric material


60


, coil wire winding


70


, and the second high dielectric material


80


. This assembly is cured so dielectric coatings


60


and


80


form into a solid material. This solid maintains coil


70


in its precisely wound shape, forming the hermetic seal to prevent the oxidation of the wire, and electrically insulate it from the surface of ferrite core


30


to prevent electrical breakdown of the coil.




Turning now to

FIG. 7

, shown is a coil holder


100


according to concepts of the present invention. Coil holder


100


includes a base portion


110


having a base opening


120


formed substantially at a centered area of the coil holder


100


. The base opening


120


is sufficiently sized to provide a passageway to the inner opening of the ferrite core


30


once attached to holder


100


. Also included is a snap-fit portion comprising a plurality of snap-fit fingers


130


, which extend from the base portion


110


. In one embodiment the snap-fit portion consists of four evenly spaced snap-fit fingers


130


. However, more or less fingers may also be used. Snap-fit fingers


130


are designed to receive step


40


of core


30


. This concept is depicted in more detail in

FIG. 8

which provides a side view of one of snap-fit fingers


130


for engaging step


40


of ferrite core


30


. As can be seen from this figure, step


40


fits into snap-fit finger


130


, which has a bottom ledge portion


140


and an upper support or top tab


150


. To allow for more flexibility, snap-fingers


130


are designed such that the top tabs


150


are tapered in a vertical direction.




In one preferred embodiment of the present invention, the overall core height is 30 mm, where the step is 3 mm. The step outer diameter is 19.02 mm, and the core body outside diameter is 17.02 and the inner opening is 8.56 mm in diameter. Each of the dimensions have a ±2% tolerance. The snap-fit finger connection's preferred dimensions for the present embodiment include an inner groove diameter of 19.50 mm±0.1% (i.e. a diameter corresponding to the four snap fingers), an overall individual snap finger height of 9.3 mm±0.5% (


152


), a snap finger inner opening height dimension of 3.2 mm±0.05% (


154


), an upper depth of 0.8 mm±0.05% (


156


), and a lower depth of 1.0 mm±0.05% (


158


).




Coil holder


100


is secured to the coil assembly


90


as shown in FIG.


9


. Since coil holder


100


is far simpler in design than a prior art bobbin, and since it does not need to endure temperatures nearly as high as the prior bobbin designs, it may be manufactured at a much lower cost.




Through-holes such as


160


are provided as passageways for first end


72


and/or second end


74


to pass through the bottom side of base


110


. It is to be understood that in the wiring process, first end wire


72


may pass through the inner portion


36


of core


30


and therefore not be required to use a through-hole but rather will pass through the back side of base


110


via center portion


120


. The back side of base


110


, can have pins


162


, attached to which are connected the first and second ends


72


and


74


. Connection between pins


160


and ends


72


,


74


can be made by a clamp connection, soldering or other known connection technique. Pins


136


, are then capable of being inserted into female receptacles of a larger electronic component.




Turning to

FIG. 10

, depicted is an EFL configuration. A lighting element


170


is shown inserted into the inner opening


36


of core


30


of coil assembly


90


. Pins


160


connected to at least ends


72


,


74


are inserted into a power source


180


causing the lamp to function as an electrodeless fluorescent lamp.




It is to be appreciated that in addition to the snap-fit technology described, the present invention may also include the use of a coil holder using a press-fit assembly. The press-fit assembly such as shown in

FIGS. 11 and 12

include both an outside press fit and an inside press fit. Particularly, core holder


190


of

FIG. 11

includes prongs


200


spaced such that they are slightly outside the outer dimension of core


210


. As illustrated, core


210


is similar to core


30


, but is symmetrical throughout its length. As core


210


is pressed to holder


190


, pressure from impingement of core


210


with pins


200


, hold core


210


in place. Turning attention to

FIG. 12

, inside press-fit construction is shown with prongs


220


of core holder


230


, spaced so as to exert a holding force on the inside passageway


240


of core


200


. Again, core


280


is symmetrical throughout its outer surface.




Turning to another embodiment, shown in

FIG. 13

, is a snap-fit assembly using a grooved ferrite core


250


. In this arrangement, in place of the ledge or step portion


40


of core


30


, core


250


includes a groove


260


. In this embodiment, snap finger


270


is designed to snap into engagement with groove


260


of core


250


.




While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.



Claims
  • 1. A method of assembling a coil comprising:forming a ferrite core having a top end, a bottom end, an inner opening extending from the top end to the bottom end, a cylindrical outer surface, and a step portion formed near the bottom end, the step portion extending past the outer surface; applying a first high dielectric material onto the outer surface of the ferrite core, the first high dielectric material being in a partially cured state such that the first high dielectric material has a tacky compliant quality; winding a conductive wire onto the partially cured first high dielectric material, including embedding at least a portion of the conductive wire into the partially cured first high dielectric material, holding the wound conductive wire in a secure position; applying a second high dielectric material over the conductive wire; and completing the curing of the first high dielectric material, including forming a hermetic seal around the conductive wire.
  • 2. The method according to claim 1, wherein:the curing causes the first high dielectric material and the second high dielectric material to form into a single solid mass, wherein the conductive wire is held in a fixed position.
  • 3. The method according to claim 1 further including:forming a coil holder having, i) a base portion with a base opening formed substantially at a centered area of the coil holder, the base opening being sufficiently sized to provide a passage way to the inner opening of core, and (i) a plurality of snap fit fingers extending from the base portion.
  • 4. The method according to claim 3 further including:inserting the step portion of the cylindrical ferrite core into the snap fit fingers of the coil holder, wherein the core is locked into engagement with the coil holder.
  • 5. The method according to claim 1 wherein the first high dielectric material is a material different from the second high dielectric material.
  • 6. The method according to claim 1 wherein the steps of applying the first and second high dielectric materials include at least one of coating, spraying, dripping and brushing.
  • 7. A method of assembling a coil comprising:forming a ferrite core having a top end, a bottom end, an inner opening extending from the top end to the bottom end, a cylindrical outer surface, and a step portion formed near the bottom end, the step portion extending past the outer surface; applying a first high dielectric material, in a partially cured state having a tacky compliant quality, onto the outer surface of the ferrite core; winding a conductive wire onto the first high dielectric material, including embedding at least a portion of the conductive wire into the partially cured dielectric material, thereby holding the wound conductive wire in a secure position; and applying a second high dielectric material over the conductive wire.
  • 8. The method according to claim 7 further including:curing the first high dielectric material and the second high dielectric material into a single solid mass, wherein the conductive wire is held in a fixed position.
  • 9. The method according to claim 8 whereinthe step of curing includes forming a hermetic seal around the conductive wire.
  • 10. The method according to claim 7 further including:forming a coil holder having, i) a base portion with a base opening formed substantially at a centered area of the coil holder, the base opening being sufficiently sized to provide a passage way to the inner opening of the core, and (i) a plurality of snap fit fingers extending from the base portion.
  • 11. The method according to claim 10 further including:inserting the step portion of the cylindrical ferrite core into the snap fit fingers of the coil holder, whereby the core is locked into engagement with the coil holder.
  • 12. The method according to claim 7 wherein the first high dielectric material is a material different from the second high dielectric material.
  • 13. The method according to claim 7 wherein the steps of applying the first and second high dielectric materials include at least one of coating, spraying, dripping and brushing.
  • 14. A method of assembling a coil which includes a core having a top end, a bottom end, an inner opening extending from the top end to the bottom end, and a cylindrical outer surface, the method comprising:applying a first high dielectric material onto the outer surface of the core; winding a conductive wire onto the first high dielectric material; applying a second high dielectric material over the conductive wire; curing the first high dielectric material and the second high dielectric material into a single solid mass, wherein the conductive wire is held in a fixed position; inserting the core into a coil holder, the coil holder including a base portion with a base opening, the base opening being sufficiently sized to provide a passage way to the inner opening of the core.
  • 15. The method according to claim 14 wherein the first high dielectric material is a material different from the second high dielectric material.
  • 16. The method according to claim 15 wherein,the step of applying the first high dielectric material further includes applying the first high dielectric material in a partially cured state such that the first high dielectric material has a tacky compliant quality; the step of winding the conductive wire onto the partially cured first high dielectric material including embedding at least a portion of the conductive wire into the partially cured high dielectric material, thereby holding the wound conductive wire in a secure position; and the step of curing includes forming a hermetic seal around the conductive wire.
  • 17. The method according to claim 14 wherein the steps of applying the first and second high dielectric materials include at least one of coating, spraying, dripping and brushing.
  • 18. The method according to claim 14 wherein the step of inserting includes inserting a step portion into snap fit fingers of the coil holder.
US Referenced Citations (3)
Number Name Date Kind
3638155 Combs Jan 1972 A
4978712 Bair et al. Dec 1990 A
5600294 Buenconsejo et al. Feb 1997 A
Foreign Referenced Citations (1)
Number Date Country
2-223204 Mar 1990 JP