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An encapsulated electronic device includes a magnetically permeable core structure which is exposed within and coplanar with a flat top surface of the device. A bottom surface of the core may be exposed within the bottom surface of the device. The bottom core surface may be recessed beneath, coplana

An encapsulated electronic device includes a magnetically permeable core structure which is exposed within and coplanar with a flat top surface of the device. A bottom surface of the core may be exposed within the bottom surface of the device. The bottom core surface may be recessed beneath, coplanar with, or protruding from the bottom surface of the device. Alternatively the bottom surface may be encapsulated within the device. A method for manufacturing the exposed core package includes positioning a first component relative to a second component before encapsulating the device. An improved planar magnetic core structure includes internal bevels having a radius greater than or equal to 15% and preferably 25%, 35%, or as much as 50% of the core thickness to reduce concentration of the magnetic field around the internal corners.

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1. An apparatus comprising: a printed circuit board having a winding to generate a first magnetic field; anda core structure having a first core piece having a back section and a first leg approximately normal to the back section for directing the first magnetic field along a flux path in the first

1. An apparatus comprising: a printed circuit board having a winding to generate a first magnetic field; anda core structure having a first core piece having a back section and a first leg approximately normal to the back section for directing the first magnetic field along a flux path in the first leg and the back section;the back section having an exterior back surface and an interior back surface separated by a back thickness, the back thickness being generally normal to the flux path;the first leg having an exterior leg surface and an interior leg surface separated by a leg thickness, the leg thickness being generally normal to the flux path;the flux path making a first turn between the first leg and the back section, the first turn being bounded on the inside by the interior leg surface and the interior back surface and forming a first angle that is approximately a right angle;wherein the first core piece includes a first feature for reducing flux crowding in the first core piece at the first turn, the first feature comprising a fillet formed between the interior back surface and the interior leg surface at the first turn;the fillet having a fillet surface comprising a rounded concavity or one or more segments approximating a rounded concavity;the fillet having an effective radius that is greater than or equal to 15% of the back thickness;wherein the effective radius is defined by the radius of a circle that is simultaneously tangent to the interior leg surface, the interior back surface, and an intersection of the fillet surface and a bisector of the first angle. 2. The apparatus of claim 1 wherein the bevel is rounded. 3. The apparatus of claim 1 wherein the bevel comprises a one or more segments approximating a rounded surface. 4. The apparatus of claim 1 wherein the first core piece further comprises a second leg having an exterior leg surface and an interior leg surface separated by a leg thickness, the leg thickness being generally normal to the flux path; andthe interior back surface and the interior leg surface of the second leg are joined by a bevel having a radius greater than or equal to 15% of the back thickness. 5. The apparatus of claim 4 wherein one or more of the bevels have a radius greater than or equal to 25% of the back thickness. 6. The apparatus of claim 4 wherein one or more of the bevels have a radius greater than or equal to 35% of the back thickness. 7. The apparatus of claim 4 wherein the core structure further comprises a second core piece adapted to mate with the first and second legs of the first core piece for directing the magnetic field along a flux path that includes in series the first leg, the back section, the second leg, and the second core piece. 8. The apparatus of claim 4 wherein the core structure further comprises: a second core piece having a back section, a first leg, and a second leg, for directing the magnetic field along a flux path in the core;the back section of the second core piece having an exterior back surface and an interior back surface separated by a back thickness generally normal to the flux path;the first and second legs of the second core piece each having an exterior leg surface and an interior leg surface separated by a leg thickness, the leg thickness being generally normal to the flux path; andwherein the interior back surface and the interior leg surfaces of the first and second legs of the second core piece include bevels having a radius greater than or equal to 15% of the back thickness,the first and second core pieces being adapted to mate together with the first and second legs meeting at first and second interfaces. 9. The apparatus of claim 8 wherein one or more of the bevels comprise a radius greater than or equal to 25% of the back thickness. 10. The apparatus of claim 8 wherein one or more of the bevels comprise a radius greater than or equal to 35% of the back thickness. 11. The apparatus of claim 8 wherein the first and second core pieces each include a center leg between the first and second legs, each center leg having interior leg surfaces separated by a center leg thickness, each interior leg surface being connected to the interior back surface by a bevel having a radius greater than or equal to 15% of the back thickness, and wherein the center legs meet at a center interface. 12. The apparatus of claim 11 wherein at least one of the interfaces includes a gap in magnetic permeability. 13. The apparatus of claim 11 wherein the exterior back surface is essentially flat. 14. The apparatus of claim 11 wherein the first and second core pieces comprise a magnetically permeable material. 15. The apparatus of claim 13 wherein the exterior of the core structure along an outside perimeter of the flux path comprises a generally rectangular shape. 16. The apparatus of claim 15 wherein the leg thicknesses are approximately equal to the back thicknesses, and the center leg thicknesses are approximately double the back thickness. 17. The apparatus of claim 11 wherein the bevels comprise a radius greater than or equal to 25% of the back thickness. 18. The apparatus of claim 11 wherein the bevels comprise a radius greater than or equal to 35% of the back thickness. 19. The apparatus of claim 11 wherein the bevels comprise a radius greater than or equal to 50% of the back thickness. 20. The apparatus of claim 8 wherein the flux path comprises an interior perimeter, the interior perimeter of the flux path comprises one or more bends, each bend comprises a radius, and each radius is greater than or equal to 25% of the back thickness. 21. The apparatus of claim 11 wherein the flux path comprises one or more interior perimeters, each interior perimeter of the flux path comprises one or more bends, each bend comprises a radius, and each radius is greater than or equal to 25% of the back thickness. 22. The apparatus of claim 1 in which the first leg of the core structure extends through an opening in the printed circuit board. 23. The apparatus of claim 1 in which the first leg of the core structure extends along a direction substantially perpendicular to a surface of the printed circuit board. 24. An apparatus comprising: a planar magnetic core structure comprising one or more loops of magnetically permeable material for directing a magnetic field along a flux path having an inner perimeter and an outer perimeter defined by its respective loop, the loop having a loop thickness defined by the distance between its respective inner perimeter and outer perimeter;the core structure having a first generally flat exterior surface along and parallel to a first section of the outer perimeter of the flux path;wherein each inner perimeter comprises one or more bends, each bend is between a first generally flat section and a second generally flat section of the inner perimeter, the first and second sections form a first angle that is approximately a right angle, each bend comprises a rounded concavity or one or more segments approximating a rounded concavity, and each bend has an effective radius that is greater than or equal to 15% of the loop thickness;wherein the effective radius is defined by the radius of a circle that is simultaneously tangent to the first section, the second section, and an intersection of the bend and a bisector of the first angle. 25. The apparatus of claim 24 wherein the structure further comprises a second generally flat exterior surface along and parallel to a second section of the outer perimeter of the flux path, the second generally flat exterior surface being generally parallel to the first generally flat exterior surface. 26. The apparatus of claim 25 wherein each radius is greater than or equal to 25% of the loop thickness. 27. The apparatus of claim 25 wherein each radius is greater than or equal to 35% of the loop thickness. 28. The apparatus of claim 27 further comprising power conversion circuitry including an inductive element having a conductive winding comprising one or more turns encircling a portion of at least one of the one or more loops; wherein the turns have a conductor width and the conductor width is less than or equal to the loop thickness. 29. The apparatus of claim 24, further comprising a printed circuit board having a winding to generate the magnetic field. 30. The apparatus of claim 29 in which the second generally flat section of the inner perimeter extends through an opening in the printed circuit board. 31. The apparatus of claim 29 in which the second generally flat section of the inner perimeter extends along a direction substantially perpendicular to a surface of the printed circuit board.