Miniature high frequency power transformer

Abstract

 A power transformer comprises first and second circuit boards having formed thereon the primary and secondary windings of the transformer. To maintain close magnetic coupling between the windings, the boards are insulated from each other by a layer of an electrically insulating material deposited on one of the windings, and opposite halves of a bifurcated ferrite core extending through openings formed in the boards enclose the windings with virtually no air gap between the core and the windings.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

[0001] The present invention relates generally to power transformers and more specifically to miniature, high frequency, printed circuit power transformers.

2. Background Art

[0002] Conventional high frequency power transformers are usually relatively large devices requiring wound primary and secondary windings and customized core assemblies. Such devices are often unusable in modern printed circuit applications where size is a major factor in the design and manufacture of electronic equipment. The high costs relating to winding and core formation in conventional transformers further compound the problem.

[0003] Attempts to reduce transformer size have been made. For example, in U.S. Patent No. 4,507,640 a transformer formed with conductive ribbon windings placed between magnetic core halves is compact but requires close machine tolerances in the manufacture of the core and ribbons.

[0004] In U.S. Patent No 4,012,703 a transmission line matching transformer has primary and secondary winding turns separated by plastic sheeting and bonded to a printed circuit board. The transformer, which is designed to change the characteristic impedance of a transmission line, is relatively compact but is expensive as a result of customization that is characteristic of transmission line impedance matching transformers.

[0005] Therefore, it is an object of the present invention to provide a miniature high frequency power transformer that is compact and inexpensive to manufacture. It is a further object of the invention to provide a miniature high frequency power transformer that exhibits good magnetic coupling and high voltage isolation. Another object of the present invention is to eliminate the requirement of winding either of or both the primary and/or secondary windings of a miniature high frequency power transformer.

SUMMARY OF THE INVENTION

[0006] The above and other objects of the present invention are satisfied in accordance with a first embodiment by first and second circuit boards having formed thereon the primary and secondary windings of a transformer, positioned such that sides of the boards carrying the windings face away from each other. First and second halves of a bifurcated ferrite pot core extending through slotted openings formed in the circuit boards enclose the windings. To maintain close magnetic coupling between the windings, the windings are in contact with the core halves to avoid any high reluctance air gaps therein. The windings are insulated from the core and from each other by a layer of insulating material deposited on one of the windings. The first and second core halves are secured together either by bonding or by a non-metallic bolt extending through a center through-hole in the first and second core halves.

[0007] In accordance with a second embodiment of the invention, either the primary or secondary winding of the transformer is formed on the mother board of an electronic circuit, and the remaining winding is on a second circuit board parallel to and contacting the mother board. First and second core halves extending through corresponding slotted openings formed in the mother board and the second circuit board enclose the windings. The windings, on outer surfaces of the boards, are insulated from the core halves and each other [and] through the core halves by a layer of insulating material deposited on at least one of the windings, and the core halves are secured together to enclose the windings.

[0008] In the third embodiment of the invention, first and second ring-shaped circuit boards carrying respectively primary and secondary windings are enclosed within first and second core halves. The windings, on outer surfaces of the boards, are insulated from the core halves and from each other through the core halves by a layer of insulating material deposited on at least one of the windings.

[0009] In a fourth embodiment of the present invention, either the primary or secondary winding of the transformer is formed on the mother board of an electronic circuit, and the remaining winding is formed on the surface of a second ring-shaped circuit board parallel to and contacting the mother board. First and second core halves enclose the windings and a layer of insulating material deposited on one of the windings insulates the windings from the core and from each other.

[0010] In each embodiment, the primary and secondary windings may, as a variation, be printed on sides of the circuit boards facing each other, rather than on opposite faces as previously described. A layer of insulating material is deposited on at least one of the windings between the boards.

[0011] Magnetic coupling between the transformer windings in the invention is maximized since there is virtually no air gap between the windings and the core in any of the embodiments. This is especially important for efficient high frequency power transformation, and enables a more compact transformer to be realized. Furthermore, forming the windings directly on the circuit boards in the various embodiments of the present invention eliminates hand or machine winding, and reduces manufacturing costs.

[0012] Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] 

FIG. 1a is an exploded view of a transformer in accordance with one embodiment of the present invention;

FIG. 1b is a side view of an assembled transformer of FIG. 1a, showing virtually no air gap between the primary and secondary windings and between the windings and ferrite core halves;

FIG. 2 is a plan view of a first circuit board carrying a primary winding of the transformer of FIG. 1a;

FIG. 3 is a plan view of a second circuit board carrying the secondary winding of the transformer thereon;

FIG. 4a is a front view of one first ferrite core half shown in FIG. 1a; FIG. 4b is a rear view thereof;

FIG. 5 is an exploded view of a transformer in accordance with a second embodiment of the present invention;

FIG. 6a shows a first circuit board carrying a primary winding in accordance with the embodiment of FIG. 5;

FIG. 6b shows a second printed circuit board carrying a secondary winding in accordance with FIG. 5;

FIG. 7 is a side view of a portion of the transformer of FIG. 5, showing an insulating layer on the windings of one of the boards;

FIG. 8 is a perspective view of an assembled miniature high frequency power transformer in accordance with the invention;

FIG. 9 is a perspective view of a transformer in accordance with the invention, assembled to a mother board;

FIG. 10 is a variation on the embodiment of FIG. 9; and,

FIG. 11 is a variation on the preferred embodiments of the invention showing the primary and secondary windings facing each other with a deposited layer of insulating material therebetween.

BEST MODE FOR CARRYING OUT THE INVENTION

[0014] Referring to FIG. 1a, a miniature high frequency power transformer 14 in accordance with a first preferred embodiment of the invention comprises a first circuit board 20 (see FIG. 2), having a printed circuit primary winding 26 formed on one side thereof, positioned in a face-to-face relationship with a second circuit board 30 (FIG. 3) having a printed circuit secondary winding 36 formed thereon. The first circuit board 20 and second circuit board 30 have identical arcuate slotted openings 24 and 34 respectively, as well as a circular center aperture 22, 32 formed therein.

[0015] Primary winding 26 faces a first core half 40 and secondary winding 36 faces a second core half 42; windings 26, 36 are enclosed at opposite sides by the core halves 40, 42 as shown in Figures 1a, 1b.

[0016] Core halves 40 and 42, commonly known as "pot cores", are manufactured of ferrite material and are conventional. As shown in FIGs 4a and 4b, each of the identical core halves is substantially disc-shaped with a pair of arcuate peripheral lips 44 extending axially from one surface of the core and an annular spindle 48 projecting from surface 43 therebetween. A circular center through-hole 46 is formed within the spindle of each core half. The spindle 48 has a diameter equal to the diameter of the disc-shaped apertures 22 and 32 formed in the circuit boards. (See FIG. 2, 3).

[0017] In the assembly of the first preferred embodiment of FIG. 1a, the first and second circuit boards 20, 30 are positioned such that primary winding 26 formed on first circuit board 20 and secondary winding 36 formed on second circuit board 30 face away from other. Slotted openings 24 and 34 in the boards 20 and 30 are in registration with each other, and lips 44 of the first and second core halves 40, 42 extend through the slotted openings 24 and 34 to contact each other with annular spindles 48 on the core halves mating through disc-shaped apertures 22 and 32 in the boards. The lips 44 of each core half are located in registration with each other with primary winding 20 and secondary winding 30 facing and enclosing core halves 40 and 42 respectively. (See FIG. 8).

[0018] It should be understood that the depth of lips 44 and spindle 46 in each core half is substantially equal to the thickness of each circuit board 20, 30 so that the primary and secondary windings abut the core surfaces 43 of core halves 40, 42. This close fitting relationship between the windings and the core halves maximizes coupling of the windings to the core and minimizes leakage flux (See FIG 1b).

[0019] Although it is possible to achieve high frequency power transformation with the primary and secondary windings facing each other, separated by a discrete a layer of insulating material as in U.S. Patent 4,012,703, supra, it is believed that the highest level of efficient power transformation, i.e. a magnetic flux path having the lowest possible reluctance, is attained by placing the windings as close as possible to the ferrite cores, since ferrite is much more permeable than air. See Standard Handbook for Electrical Engineers, 9th Ed., McGraw Hill, 1957, p.321. Thus, in accordance with the invention, electrical insulation between primary winding 26 and secondary winding 36 is attained by the deposition of an insulating layer 13 on either or both of the two windings (the insulating layer 13 is deposited on secondary winding 36 in FIG. 1a). Insulating layer 13 is formed by applying Dry Film Solder Mask manufactured by the Dynachem Company, located in Tustin, CA, onto one of the circuit boards, as shown in FIG. 7, or preferably, by using vacuum lamination; however, any other permanent insulating depositing material that exhibits high voltage thin film characteristics may be used as well.

[0020] As can be readily appreciated, forming printed circuit primary and secondary windings on first circuit board 20 and second circuit board 30 eliminates winding and reduces costs. The first and second circuit boards 20 and 30 in the preferred embodiment comprise epoxy resin impregnated fiberglass; however other suitable materials can be used.

[0021] Once assembled, transformer 14 can be bonded together by any suitable bonding agent (not shown) applied to the core lips 44. Alternately, as shown in FIG. 1a, although bonding is preferable a non-metallic bolt 12a may secure the tranformer assembly together, passing through first core half 40 and second core half 42 via through-­holes 46 and retained by a non-metallic nut 12b.

[0022] It should be understood that one winding of the transformer can be formed directly on a circuit board carrying components of a complex electronic circuit, rather than on a dedicated circuit board. Thus in FIG.9, the transformer 90 has either its primary winding 26 or its secondary winding 36 on a mother board 92 of a piece of electronic equipment with the remaining transformer winding formed on a second circuit board 94.

[0023] In another embodiment of the invention, shown in FIG. 5, the primary and secondary windings 64, 66 of a transformer 50 are formed on first and second ring-­shaped circuit boards 60, 62, each having a disc-shaped central aperture 68 formed therein. (FIG. 6a,6b). The diameter of aperture 68 is identical to that of annular spindle 48 formed on the ferrite core halves, shown in FIG. 4a, 4b and FIG. 6a,6b. Both ring-shaped circuit boards conform to the surface 43 in each of the ferrite core halves, and a layer of insulating material is deposited on one or both of the windings.

[0024] In the assembly of transformer 50, first ring-­shaped circuit board 60 and second ring-shaped circuit board 62 are brought together with the circuit boards oriented such that primary winding 64 and secondary winding 66 face away from each other. The boards 60, 62 are located within the core halves 40, 42 as the core halves are assembled around the boards, with primary and secondary windings 64, 66 facing their respective core halves. A layer of insulating material 13 is deposited on one or both windings 64, 66 (only one deposited layer is shown). The entire assembly is next bonded together utilizing any appropriate bonding material (not shown), with spindles 48 and lips 44 of the respective core halves 40, 42 interfitting with each other (See FIG. 5). As in the embodiment of FIG. 1a, there is no air gap between the windings and the core halves of the transformer, thereby optimizing the power transfer capacity of the device.

[0025] Referring now to FIG. 10, in a further preferred embodiment of the invention, either the primary winding 104 or secondary winding 106 of a transformer 100 is formed on a mother board 92 and the remaining transformer winding is formed on a second, ring-shaped circuit board 101. The ring-shaped circuit board 101 is formed so as to fit entirely within first and second core halves 40, 42, and in contact with surfaces 43 of the core halves, as in the other embodiments of the invention.

[0026] In the assembly of transformer 100, ring-shaped circuit board 101 is located in contact with mother board 92, with primary winding 104 and secondary winding 106 of the two boards facing their respective core halves 40,42. Core halves 40 and 42 are brought together with one core half enclosing ring-shaped circuit board 101, and the other core half inserted into the mother boadrd 92 in the same manner as in the embodiment of FIG. 1a. The entire assembly is secured into place by bonding together or otherwise joining core halves 40, 42 by other means.

[0027] As a variation in each embodiment of the invention the primary and secondary windings are formed on sides of the circuit boards facing each other, rather than on opposite faces as previously described. A layer of insulating material is deposited on at least one of the windings between the boards. Thus, in FIG. 11, primary winding 26 on circuit board 20 faces secondary winding 36 on circuit board 30, insulating layer 13 is deposited on secondary winding 36.

[0028] There has accordingly been described a miniature high frequency power transformer comprising first and second circuit boards having formed thereon the primary and secondary windings of the transformer, positioned such that sides of the boards carrying the windings face away from each other. The boards are insulated from each other by a layer of an electrically insulating material deposited on at least one of the windings. Opposite halves of a bifurcated core extending through openings formed in the boards enclose the windings with virtually no air gap between the core and the windings. Both boards can be dedicated to the transformer, or one board can be a mother board of an electronic circuit. Further, one or both boards can be shaped to be enclosed by one or both core halves.

[0029] As a variation in each embodiment, the primary and secondary windings may be printed on sides of the circuit boards facing each other rather than on opposite faces. A layer of insulating material is deposited on at least one of the windings between the boards.

[0030] In this disclosure, there is shown and described only the preferred embodiments of the invention, but, as aforementioned, it is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.

Claims

1. A high frequency power transformer, comprising:
first and second printed circuit boards, primary and secondary printed circuit windings of said transformer formed respectively on said first and second boards;
said first and second boards being positioned so that surfaces thereof carrying said primary and secondary windings face away from each other;
first and second core halves;
said first and second printed circuit boards having slotted openings formed therein partially surrounding said primary and secondary windings, said openings in said boards being in registration with each other;
said first and second core halves having portions extending respectively through said slotted openings, said core halves substantially enclosing said primary and secondary windings, retaining together said first and second printed circuit boards with said primary and secondary windings in contact with respective core halves to provide close magnetic coupling between said windings; and
a layer of an electrically insulating material deposited on one of said printed circuit primary and secondary windings to electrically insulate said one winding from said core halves and from the other winding.

2. The transformer of claim 1, wherein the other one of said electrical printed circuit windings is uninsulated.

3. The transformer of claim 1, wherein said core halves together form a pot core.

4. The transformer of claim 4, wherein axially extending side portions of said core halves and said slotted openings formed in said printed circuit boards are arcuate.

5. The transformer of claim 1, wherein one of said first and second printed circuit boards is larger in surface area that the other and carries at least one electrical component in circuit with said transformer.

6. The transformer of claim 1, wherein said core halves are formed of ferrite.

7. The transformer of claim 1, wherein said core halves each have a circular center through-hole therein, and including means extending through said through-holes of said core-halves for securing said core halves together.

8. The transformer of claim 7, wherein said securing means comprises a non-metallic bolt.

9. The transformer of claim 1, wherein said core halves are bonded together.

10. A high frequency power transformer, comprising:
first and second printed circuit boards, primary and secondary printed circuit windings of said transformer formed respectively on said first and second boards;
said first and second boards being positioned so that sides thereof carrying said primary and secondary windings face away from each other;
first and second core halves substantially enclosing said primary and secondary windings;
an electrically insulating material deposited on one of said printed circuit primary and secondary windings for electrically insulating one of said printed circuit windings from said core halves and from the other winding;
said first and second printed circuit boards having central openings formed therein in registration with each other;
said first and second core halves having portions extending respectively through said central openings into contact with each other, said core halves retaining together said first and second printed circuit boards, said primary and secondary windings in contact with respective core halves to provide close magnetic coupling between said windings and said core halves.

11. The transformer of claim 10, wherein the other one of said printed circuit windings is uninsulated.

12. The transformer of claim 10, wherein said core halves together form a pot core.

13. The transformer of claim 10, wherein said core halves are formed of ferrite.

14. The transformer of claim 10, wherein said core halves each have a circular center through-hole therein, and including means extending through said through-holes of said core-halves for securing said core halves together.

15. The transformer of claim 10, wherein said securing means comprises a non-metallic bolt.

16. The transformer of claim 10, wherein said core halves are bonded together.

17. The transformer of claim 10, wherein at least one of said circuit boards is ring-shaped.

18. The transformer of claim 10, wherein a first of said first and second printed circuit boards is ring-­shaped and the other printed circuit board is larger in surface area than the first and carries at least one electrical component in circuit with said transformer.

19. A high frequency power transformer, comprising
first and second circuit boards, planar primary and secondary circuit windings of said transformer formed respectively on said first and second boards;
said first and second boards being positioned so that sides thereof carrying said primary and secondary windings face each other, said boards having openings formed therein in registration with each other;
a deposited layer of electrically insulating material between said facing sides of said boards for electrically insulating said primary and secondary windings from each other;
first and second core halves;
said first and second core halves having portions extending respectively through said openings, retaining together said first and second circuit boards so that said primary and secondary windings are in close magnetic coupling with each other, said core halves substantially enclosing said primary and secondary windings.

Drawing