Magnetic core structure and manufacturing method using a grinding post
A magnetic assembly includes a bobbin with two end flanges. A passageway extends longitudinally between the end flanges. An inner core extends through the passageway. The inner core has first and second end surfaces, each end surface proximate to one of the end flanges. A rectangular outer core is positioned around the bobbin with inner surfaces of the outer core close to the outer surfaces of the end flanges of the bobbin. A respective gap is formed between each end surface of the inner core and the adjacent inner surface of the outer core. A protrusion is formed on at least one inner surface of the outer core to control the cavity distance between the inner surfaces of the outer core proximate to the end surfaces of the inner core. The thickness of the protrusion is selectably reduced to increase the cavity distance to control the gaps.
1. A method for assembling a magnetic assembly, comprising:
providing a bobbin having a first end flange and a second end flange, the bobbin having a passageway between the first end flange and the second end flange, the passageway having a passageway length from the first end flange to the second end flange;
inserting an inner core into the passageway of the bobbin, the inner core having a length such that a first end surface of the inner core is proximate to the first end flange of the bobbin and a second end surface of the inner core is proximate to the second end flange of the bobbin, the inner core having an inner core length from the first end surface to the second end surface;
forming an outer core having an outer wall surrounding an inner cavity, the inner cavity having at least a first inner surface and a second inner surface, the second inner surface parallel to the first inner surface, at least the first inner surface having a first protrusion extending into the inner cavity toward the second inner surface, the first protrusion having a first protrusion face, the first protrusion face initially spaced apart from a portion of the second inner surface by an initial modifiable inner cavity distance;
removing a portion of the first protrusion at the first protrusion face to increase the initial modifiable inner cavity distance to a modified inner cavity distance selected to be greater than the inner core length by a total gap distance; and
positioning the outer core onto the bobbin with the first protrusion face proximate to the first end surface of the inner core and with the portion of the second inner surface proximate to the second end surface of the inner core.
2. The method as defined in claim 1 , wherein the first protrusion face is spaced apart from the first end surface of the inner core by a first gap distance and the portion of the second inner surface is spaced apart from the second end surface of the inner core by a second gap distance, the second gap distance and the first gap distance being substantially equal.
3. The method as defined in claim 1 , wherein the portion of the second inner surface of the cavity further includes a second protrusion extending into the cavity toward the first protrusion face, the second protrusion having a second protrusion face, the method further comprising removing a portion of the second protrusion at the second protrusion face to increase the modifiable inner cavity distance.
4. The method as defined in claim 3 , wherein the first protrusion face is spaced apart from the first end surface of the inner core by a first gap distance and the second protrusion face is spaced apart from the second end surface of the inner core by a second gap distance, the second gap distance and the first gap distance being substantially equal.
5. The method as defined in claim 1 , wherein the outer core is formed by a sintering process, and wherein the first protrusion extends from the first inner surface by a pre-sintering grinding post length, the pre-sintering grinding post length selected to be at least as long as the product of an expected sintering shrinkage factor F times a sum of the inner core length and the total gap distance, the expected sintering shrinkage factor F determined by a maximum expected sintering shrinkage percentage S during the sintering process, wherein F=S/(100−S).