Transformer with controlled interwinding coupling and controlled leakage inducances and circuit using such transformer

• US 5,719,544 A
• Filed: 07/12/1996
• Issued: 02/17/1998
• Est. Priority Date: 09/13/1991
• Status: Expired due to Term

• Current Assignee: VLT Incorporated (Vicor Corporation)
• Original Assignee: VLT Incorporated (Vicor Corporation)
• Inventors: Vinciarelli, Patrizio, Prager, Jay M.
• Primary Examiner(s): Gellner, Michael L.
• Assistant Examiner(s): MAI, ANH T
• Application Number:
  US08/679,190
• Time in Patent Office: 585 Days
• Field of Search: 336/84 R, 336/84 C, 336/84 M, 336/184
• US Class Current: 336/84.00C
• CPC Class Codes: H01F 27/346 Preventing or reducing leakage fields using magnetic shields H01F27/36; using auxiliary windings H01F27/38
• CPC Class Codes: H01F 27/36 Electric or magnetic shields or screens movable for varying inductance H01F21/10
• CPC Class Codes: H01F 27/363 made of electrically conductive material
• CPC Class Codes: H01F 29/14 with variable magnetic bias amplitude modulation by means of variable impedance element H03C1/08 ; magnetic amplifiers H03F; circuits for automatic telephonic communication H04M3/00

Abstract

A transformer in which a magnetic medium provides flux paths within the medium, two or more windings enclose the flux paths at separated locations along the paths, and an electrically conductive medium, arranged in the vicinity of the magnetic medium and the windings, defines a boundary within which flux emanation from the magnetic medium and the windings is confined and suppressed. In a transformer constructed in accordance with the present invention, both controlled values of leakage inductance and the benefits of separated windings can be achieved. The conductive medium can be configured to reduce the leakage inductance of a controlled-leakage inductance transformer (e.g. for use in a zero-current switching power converter), having separately located windings, by at least 25%, and can be configured to reduce the leakage inductance of a low-leakage inductance transformer (e.g. for use in a PWM power converter), having separately located windings, by at least 75%.

Claims

1. A transformer having a controlled value of leakage inductance comprising

a magnetic core comprising a magnetic material arranged to form at least one loop and to carry magnetic flux longitudinally in the loop, the magnetic core having a surface through which a portion of the magnetic flux leaks as leakage flux,

at least two windings that surround sections of the magnetic core to carry currents associated with the magnetic flux, and

a conductive medium that surrounds, at substantially all locations along the loop except at sections surrounded by the windings, at least a portion of the surface of the magnetic core.

2. The transformer of claim 1 wherein the conductive medium also surrounds at least a portion of the surface of the magnetic core at locations within the sections surrounded by the windings.

3. The transformer of claim 1 or 2 wherein the conductive medium surrounds substantially all of the surface of the magnetic core at each location along the loop.

4. The transformer of claim 3 wherein the conductive medium includes a gap that runs longitudinally along the loop to prevent the conductive medium from forming a shorted turn around the magnetic core.

5. The transformer of claim 4 wherein the gap runs along an inner circumference of the loop.

6. The transformer of claim 4 further comprising

an insulating strip laid over at least a portion of the gap in the conductive medium, and

a conductive strip laid over the insulating strip.

7. The transformer of claim 1 or 2 wherein the conductive medium comprises at least one conductive cup fitted over a portion of the loop.

8. The transformer of claim 7 wherein the loop comprises at least two substantially parallel leg sections connected at each end by one of at least two base sections, and wherein the conductive cup is shaped to fit snugly over one the base sections.

9. The transformer of claim 7 wherein the conductive medium further comprises a conductive material surrounding at least a portion of the magnetic core at locations within the sections surrounded by the windings.

10. The transformer of claim 1 or 2 wherein the conductive medium comprises a conductive tape.

11. The transformer of claim 10 wherein the conductive tape further comprising a layer insulating material.

12. The transformer of claim 10 wherein the conductive tape surrounds all of the surface of the magnetic core without forming a shorted turn.

13. The transformer of claim 1 or wherein the conductive medium comprises a metal plated onto the magnetic core.

14. The transformer of claim 1 or 2 wherein the magnetic core comprises two substantially U-shaped segments connected end-to-end to form the loop.

15. The transformer of claim 1 or 2 wherein the magnetic core comprises a single loop.

16. The transformer of claim 1 or 2 wherein the magnetic core comprises multiple loops.

17. The transformer of claim 1 or 2 wherein the conductive medium includes an aperture that allows flux to leak from the magnetic core.

18. The transformer of claim 17 wherein the magnetic core comprises a leg protruding through the aperture.

19. The transformer of claim 18 wherein at least a portion of the leg is covered by a conductive medium.

20. The transformer of claim 17 wherein the conductive medium also includes a second aperture that, in conjunction with the first aperture, creates a path for leakage flux outside the magnetic core.

21. The transformer of claim 20 wherein the magnetic core comprises a leg protruding through each aperture.

22. The transformer of claim 21 wherein at least a portion of each leg is covered by a conductive medium.

23. A transformer having controlled value of leakage inductance comprising

a magnetic core comprising a magnetic material arranged to form at least one loop and to carry magnetic flux longitudinally in the loop, the magnetic core having a surface through which a portion of the magnetic flux leaks as a leakage flux

at least two windings that surround sections of the magnetic core to carry currents associated with the magnetic flux, and

a conductive cup that covers at least a portion of the surface of the magnetic core at one end of the loop.

24. The transformer of claim 23 wherein the magnetic core comprises two substantially U-shaped segments connected end-to-end to form the loop.

25. The transformer of claim 24 wherein the conductive cup covers one of the U-shaped segments at an area opposite the end connected to the other U-shaped segment.

26. The transformer of claim 23 wherein the loop comprises at least two substantially parallel leg sections connected at each end by one of two base sections, and wherein the conductive cup is shaped to fit snugly over one of the base sections.

27. The transformer of claim 26 wherein the conductive cup covers at least a portion of at least one of the leg sections.

28. The transformer of claim 27 wherein the conductive cup covers at least a portion of both leg sections.

29. The transformer of claim 28 further comprising a second conductive cup shaped to fit snugly over the other base section.

30. The transformer of claim 29 wherein both conductive cups each cover at least a portion of at least one of the leg sections.

31. The transformer of claim 29 wherein both conductive cups each cover at least a portion of both of the leg sections.

32. The transformer of claim 29 wherein the conductive cups cover substantially all of the leg sections.

33. A transformer having a controlled value of leakage inductance comprising

a magnetic core comprising a magnetic material arranged to form multiple loops and to carry magnetic flux longitudinally in each of the loops, the magnetic core having a surface through which a portion of the magnetic flux leaks as leakage flux,

at least two windings that surround sections of the magnetic core to carry currents associated with the magnetic flux, and

a conductive medium formed over at least a portion of the surface of the magnetic core forming the loops, the conductive medium being structured and arranged to preclude formation of shorted turns with respect to flux being carried longitudinally in said loops.

34. The transformer of claim 37, wherein the conductive medium is formed over portions of the core forming each of the loops.

35. The transformer of claim 33, wherein the conductive medium comprises a slit.

36. The transformer of claim 33, wherein the magnetic core comprises three coupled legs, each of the legs being surrounded by one of the at least two windings.

37. A method of controlling leakage inductance in a transformer having a magnetic core formed by a magnetic medium configured to form at least one loop, sections of which are surrounded by at least two windings, comprising the steps of inducing a magnetic flux to flow longitudinally in the loop,

allowing a portion of the magnetic flux to leak from a surface of the magnetic core,

providing a conductive medium surrounding, at substantially all locations along the loop except at sections surrounded by the windings, at least a portion of the surface of the magnetic core, and

restricting the leakage of flux from the surface of the magnetic core with the conductive medium.

38. The method of claim 37 further comprising restricting the leakage of flux from at least a portion of the magnetic core at locations within the sections surrounded by the windings.

39. The method of claim 37 or 38 further comprising restricting the leakage of flux from substantially all of the surface of the magnetic core at each location along the loop.