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Power conversion apparatus includes a circuit board with power conversion circuitry and a package having an upper portion and a lower portion that respectively enclose circuitry on a top surface and a bottom surface of the circuit board. The lower portion encloses a smaller region than that enclosed

Power conversion apparatus includes a circuit board with power conversion circuitry and a package having an upper portion and a lower portion that respectively enclose circuitry on a top surface and a bottom surface of the circuit board. The lower portion encloses a smaller region than that enclosed by the upper portion. The regions are arranged to define an overhang region on the bottom surface of the circuit board. Interface contacts are provided on the bottom surface in the overhang region for making electrical connections to the circuit board. A thermal extender includes a surface for mounting a heat dissipating power converter and a surface for mating with an external circuit board. Interface conductors mate with contacts on the power converter and with conductive regions on the external circuit board. A heat sink is thermally coupled to remove heat generated by the power converter.

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What is claimed is: 1. Apparatus comprising: a power converter including a circuit board having a top surface and a bottom surface, power conversion circuitry, and a package; the power conversion circuitry including a magnetic circuit having a magnetically permeable core, upper circuitry in a first

What is claimed is: 1. Apparatus comprising: a power converter including a circuit board having a top surface and a bottom surface, power conversion circuitry, and a package; the power conversion circuitry including a magnetic circuit having a magnetically permeable core, upper circuitry in a first region on the top surface of the circuit board, and lower circuitry in a second region on the bottom surface of the circuit board; the package including an upper portion defining an external top surface and a lower portion defining an external bottom surface; the upper portion enclosing the first region including the upper circuitry and connections between the upper circuitry and the circuit board; the lower portion enclosing the second region including the lower circuitry and connections between the lower circuitry and the circuit board; the second region being smaller than the first region; the first and second regions being arranged to define an external overhang region on the bottom surface of the circuit board, the overhang region extending along two or more sides of a periphery of the bottom surface of the circuit board; and interface contacts arranged within said overhang region on the bottom surface of the circuit board for making electrical connections to the power converter. 2. Apparatus comprising: a circuit board having a top surface and a bottom surface, circuitry, and a package; the circuitry including upper circuitry in a first region on the top surface of the circuit board, and lower circuitry in a second region on the bottom surface of the circuit board; the package including an upper portion defining an external top surface and a lower portion defining an external bottom surface; the upper portion enclosing the first region including the upper circuitry and connections between the upper circuitry and the circuit board; the lower portion enclosing the second region including the lower circuitry and connections between the lower circuitry and the circuit board; the second region being smaller than the first region; the first and second regions being arranged to define an external overhang region on the bottom surface of the circuit board, the overhang region extending along two or more sides of a periphery of the bottom surface of the circuit board; and interface contacts arranged within said overhang region on the bottom surface of the circuit board for making electrical connections to the circuitry. 3. The apparatus of claim 1 or 2 wherein said upper portion comprises an encapsulating material. 4. The apparatus of claim 1 or 2 wherein said lower portion comprises an encapsulating material. 5. The apparatus of claim 1 or 2 wherein the overhang extends along the entire periphery of the bottom surface of the circuit board. 6. The apparatus of claim 1 or 2, wherein: the lower portion comprises a first thickness defined by the maximum distance between the external bottom surface and the bottom surface of the circuit board; the upper portion comprises a second thickness defined by the maximum distance between the external top surface and the top surface of the circuit board; and the first thickness is at least 10% greater than the second thickness. 7. The apparatus of claim 1 wherein the interface contacts are adapted to make connection with the power converter at an elevation between the external bottom surface and the bottom surface of the circuit board. 8. The apparatus of claim 1 wherein the magnetic core extends through the circuit board. 9. The apparatus of claim 1 wherein said upper and lower portions enclose all of said power conversion circuitry. 10. The apparatus of claim 1 wherein said interface contacts further comprise conductive elements. 11. The apparatus of claim 10 wherein said conductive elements comprise a ball-grid array. 12. The apparatus of claim 10 wherein said conductive elements comprise a land grid array. 13. The apparatus of claim 10 wherein said conductive elements comprise pins. 14. The apparatus of claim 1 wherein said power conversion circuitry comprises heat dissipating power conversion components arranged on the top surface of the circuit board and further comprising an average thermal resistance between the top surface of the circuit board and said external top surface less than 3 C/Watt per square inch of the top surface of the circuit board. 15. The apparatus of claim 14 wherein said heat dissipating power conversion components comprise a MOSFET. 16. The apparatus of claim 1 further comprising: an external circuit board having a first surface and a second surface separated by a board thickness and having conductive regions on the second surface for making connections to the power converter; the power converter being mounted to the external circuit board. 17. The apparatus of claim 16 wherein the external circuit board comprises a board aperture, the lower portion extends into the board aperture, and the interface contacts are connected to the conductive regions. 18. The apparatus of claim 17 wherein the lower portion comprises a first thickness defined by the maximum distance between the external bottom surface and the bottom surface of the circuit board; the upper portion comprises a second thickness defined by the maximum distance between the external top surface and the top surface of the circuit board; and the first thickness is greater than said second thickness and a portion of the lower portion extends below the first surface. 19. The apparatus of claim 17 wherein the circuit board comprises a multilayer circuit board including alternating layers of electrically conductive and insulating materials and the interface contacts are connected to the multilayer circuit board. 20. The apparatus of claim 19 wherein the interface contacts comprise BGA contacts. 21. The apparatus of claim 20 wherein the top and bottom surfaces of the multilayer board are generally rectangular, the rectangular surfaces having a short side and a long side, the BGA contacts being arranged along the long side. 22. The apparatus of claim 21 wherein the dimensions of the rectangular surfaces are approximately 21 mm��32 mm and the BGA contacts are on a 1 mm pattern. 23. The apparatus of claim 19 further comprising a thermal resistance between the top surface of the multilayer board and the conductive regions on the second surface of the external circuit board that is less than 40 C/Watt per cm of package perimeter. 24. The apparatus of claim 16 further comprising an interconnect extender having interface conductors for connecting the interface contacts to the conductive regions and having a first surface area for mating with the overhang region and a second surface area for mating with the external circuit board; and wherein the power converter is mounted to the first surface area and the second surface area is mounted to the external circuit board. 25. The apparatus of claim 24 wherein the interconnect extender further comprises an extender aperture, and the lower portion extends into the extender aperture. 26. The apparatus of claim 25 wherein the interconnect extender further comprises an outer periphery generally matching the outer periphery of the overhang. 27. The apparatus of claim 25 wherein the extender aperture generally matches the outer periphery of the lower portion. 28. The apparatus of claim 27 further comprising a card-edge connector mounted to the external circuit board; and wherein the second surface area comprises contact fingers for mating with the card-edge connector, and the extender is mounted to the external circuit board using the connector. 29. The apparatus of claim 25 wherein the lower portion comprises a first thickness defined by the maximum distance between the external bottom surface and the bottom surface of the circuit board; and the interconnect extender comprises a thickness between the first surface area and the second surface area; and the thickness is greater than the first thickness. 30. The apparatus of claim 25 wherein the external circuit board further comprises a board aperture, the lower portion comprises a first thickness defined by the maximum distance between the external bottom surface and the bottom surface of the circuit board; and the interconnect extender comprises a thickness between the first surface area and the second surface area and the thickness is less than the first thickness, and the lower portion extends into the board aperture. 31. The apparatus of claim 25 further comprising a connector having conductive sockets connected to the conductive regions. 32. The apparatus of claim 31 wherein a first end of the interface conductors connects to the interface contacts and a second end of the interface conductors comprises pins which connect with the sockets. 33. The apparatus of claim 25 wherein the first surface area is generally perpendicular to the second surface area and the aperture generally matches the outer periphery of the lower portion. 34. The apparatus of claim 24 wherein a first end of the interface conductors is adapted to connect to the interface contacts and a second end of the interface conductors is adapted to connect to the conductive regions. 35. The apparatus of claim 34 wherein the conductive elements comprise a ball-grid array. 36. The apparatus of claim 34 wherein the conductive elements comprise a land grid array. 37. The apparatus of claim 34 wherein the conductive elements comprise pins. 38. The apparatus of claim 34 wherein the conductive elements comprise J-leads. 39. The apparatus of claim 16 wherein the interface contacts further comprise conductive elements. 40. The apparatus of claim 39 wherein the conductive elements comprise a ball-grid array. 41. The apparatus of claim 39 wherein the conductive elements comprise a land grid array. 42. The apparatus of claim 39 wherein the conductive elements comprise pins. 43. The apparatus of claim 1 or 2 further comprising: an external circuit board having a first surface and a second surface separated by a board thickness and having conductive regions on the second surface for making connections to the circuitry; and an interconnect extender having interface conductors for connecting the interface contacts to the conductive regions and having a first surface area for mating with the overhang region and a second surface area for mating with the external circuit board; the interconnect extender having a heat sink; wherein the circuit board is mounted to the first surface area and the second surface area is mounted to the external circuit board. 44. The apparatus of claim 43 wherein the heat sink is attached to the first surface area. 45. The apparatus of claim 43 wherein the interconnect extender comprises a thermally conductive layer and the heat sink comprises an extension of the thermally conductive layer. 46. The apparatus of claim 1 or 2 further comprising: an external circuit board having a first surface and a second surface separated by a board thickness and having conductive regions on the second surface for making connections to the circuit board of the apparatus; and a heat sink located on the external circuit board and thermally coupled to the external circuit board; wherein the circuit board is mounted to the external circuit board and the circuitry is thermally coupled to the heat sink through the circuit board and the external circuit board. 47. The apparatus of claim 46 wherein the heat sink is mounted to the second surface of the external circuit board. 48. The apparatus of claim 46 wherein the package comprises a perimeter and further comprising a thermal resistance between the heat sink and the top surface of the circuit board of less than 80 C/Watt per cm of the package perimeter. 49. A method comprising: packaging a power converter including, providing a circuit board having a top surface and a bottom surface; arranging power conversion circuitry on the top and bottom surfaces of the circuit board with upper circuitry in a first region on the top surface of the circuit board and lower circuitry in a second region on the bottom surface of the circuit board; enclosing the first region, the upper circuitry, and connections between the upper circuitry and the circuit board in an upper portion enclosure; enclosing the second region, the lower circuitry, and connections between the lower circuitry and the circuit board in a lower portion enclosure, the second region being smaller than the first region; arranging the first and second regions to define an overhang region on the bottom surface of the circuit board, the overhang region extending along two or more sides of a periphery of the bottom surface of the circuit board; and providing interface contacts on the bottom surface of the circuit board in the overhang region for making electrical connections to the power converter. 50. The method of claim 49 wherein enclosing the first region comprises encapsulating. 51. The method of claim 49 wherein enclosing the second region comprises encapsulating. 52. The method of claim 49 wherein enclosing the first region comprises enclosing all of the power conversion circuitry on the top surface of the circuit board. 53. The method of claim 49 wherein the enclosing of the first and second regions comprises enclosing all of the power conversion circuitry. 54. The method of claim 53 wherein the enclosing of the first and second regions further comprises encapsulating. 55. The method of claim 54 wherein the providing interface contacts comprises attaching interface contacts to the circuit board before encapsulating the second region. 56. The method of claim 55 wherein the attaching further comprises attaching a ball grid array to the circuit board. 57. The method of claim 56 wherein the encapsulating further comprises using a mold having a step over cavity for accommodating the interface contacts. 58. The method of claim 49 further comprising: providing an external circuit board having a top surface and a bottom surface separated by a board thickness; providing conductive regions on the top surface of the external circuit board for making connections to the power converter; mounting the power converter to the external circuit board. 59. The method of claim 58 further comprising providing an aperture in the external circuit board for accommodating the lower portion enclosure and wherein the mounting comprises placing the lower portion enclosure in the aperture and connecting the interface contacts to the conductive regions. 60. The method of claim 59 wherein: the lower portion enclosure comprises a first thickness defined by the maximum distance between an external bottom surface and the bottom surface of the circuit board; the upper portion comprises a second thickness defined by the maximum distance between an external top surface and the top surface of the circuit board; and the method further comprises: making the first thickness greater than the second thickness and mounting the power converter with a portion of the lower portion enclosure extending below the bottom surface of the external circuit board. 61. The method of claim 59 further comprising providing a thermal resistance between the top surface of the circuit board and the conductive regions on the top surface of the external circuit board less than 40 C/Watt per cm of package perimeter. 62. The method of claim 58 further comprising providing an interconnect extender for mating with the overhang region and the top surface of the external circuit board, the interconnect extender having interface conductors for connecting the interface contacts to the conductive regions. 63. The method of claim 62 further comprising providing the interconnect extender with an extender aperture for accommodating the lower portion enclosure. 64. The method of claim 63 further comprising providing the interconnect extender with an outer periphery generally matching the outer periphery of the overhang. 65. The method of claim 63 further comprising matching the extender aperture with the outer periphery of the lower portion enclosure. 66. The method of claim 62 wherein: the lower portion enclosure comprises a first thickness defined by the maximum distance between an external bottom surface and the bottom surface of the circuit board; and further comprising providing the interconnect extender with a thickness greater than the first thickness. 67. The method of claim 62 wherein: the lower portion enclosure comprises a first thickness defined by the maximum distance between an external bottom surface and the bottom surface of the circuit board; and further comprising: providing a board aperture in the external circuit board, providing the interconnect extender with a thickness less than the first thickness, and placing the lower portion enclosure into the extender aperture and in the board aperture. 68. The method of claim 62 further comprising mounting the power converter to the interconnect extender and mounting the interconnect extender to the external circuit board wherein the overhang surface is generally perpendicular to the top or bottom surface of the external circuit board. 69. The method of claim 68 further comprising mechanically stabilizing the interconnect extender to the board. 70. The method of claim 62 further comprising mounting the power converter to the interconnect extender; and providing a card edge connector on the external circuit board for receiving the interconnect extender; wherein the overhang surface is generally perpendicular to the top or bottom surface of the external circuit board. 71. The method of claim 58 further comprising connecting the interface contacts to the conductive regions using a ball grid array. 72. Apparatus comprising: a power converter including a circuit board having a top surface and a bottom surface, and power conversion circuitry; the power conversion circuitry including a magnetic circuit having a magnetically permeable core, upper circuitry in a first region on the top surface of the circuit board, and lower circuitry in a second region on the bottom surface of the circuit board; the second region being smaller than the first region; the first and second regions being arranged to define an overhang region on the bottom surface of the circuit board, the overhang region extending along two or more sides of a periphery of the bottom surface; interface contacts arranged within said overhang region on the bottom surface for making electrical connections to the power converter; and an external circuit board having an aperture larger than the second region of the power converter and conductive regions for making connections to the power converter; the power converter being connected to the external circuit board and the second region of the power converter extending into said aperture. 73. The apparatus of claim 72 wherein the interface contacts comprise a BGA. 74. The apparatus of claim 72 wherein the interface contacts comprise a LGA. 75. The apparatus of claim 72 wherein the interface contacts comprise an interconnect extender.