Pure Iron Based Soft Magnetic Composite Core That Enables .

Figure 3 shows the correlation between theparticle diameter and iron loss of the iron-based softmagnetic powder by operation frequency. The eddycurrent loss (We) increases in proportion to the powderparticle diameter, because the increase in the particlediameter increases the area where the eddy current isgenerated. Meanwhile, the larger the powder particlediameter, the lower the hysteresis loss (Wh). This isbecause the increase in the particle diameter reducesthe percentage of the particle surface area (i.e.magnetic gap) in relative terms. Thus, the powderparticle diameter at which the iron loss value is reducedto the minimum is determined, depending on the operation frequency. In the actual manufacture of softmagnetic composite core materials, the insulationcoating on the particle surface is damaged due toplastic deformation of particles in the process ofcompacting or sliding friction when products areremoved from the die, resulting in an increased size ofthe area where an eddy current is generated (d) and anincrease in the eddy current loss. In manufacturing softmagnetic composite core materials, a key point ofdevelopment is how to minimize damage to the insulation coating on the particle surface in compacting andsubsequent processes.100Hysteresis loss (Wh)Eddy current loss (We)9030 kHz10 kHzIron loss W1/f (W/kg)8070605030 kHz40302010 kHz100050100150Powder particle diameter (μm)200250Fig. 3. Correlation between particle diameter and iron loss ofiron-based soft magnetic powder (example)When selecting the iron-based soft magneticpowder from which to best manufacture powder coresfor reactors, pure iron powder and alloy powder werethe candidates, as shown in Table 3. Pure iron powderhas three advantages: (i) high saturation magnetic fluxdensity that is suited to reducing the size of components, (ii) high powder compressibility and excellentcompactibility, and (iii) relatively low raw material cost.In contrast, alloy powder has the following three characteristics: (i) relatively low saturation magnetic fluxdensity (peculiar to the material), (ii) poor powdercompressibility that poses a difficulty in increasing thedensity, (iii) low eddy current loss at high frequenciesbecause the electrical resistance is higher than that ofpure iron.Table 3. Comparison of iron-based soft magnetic powders forsoft magnetic composite materialsSaturation magnetic flux densityElectrical resistancePowder compressibilityRaw materials costPure iron powder(Fe)Alloy lentPoorGoodFairWith the properties required of reactor cores inmind, we used pure-iron based powder that could beexpected to result in reduced sized components withhigh magnetic flux density. To reduce the loss, weworked to reduce the area where the eddy current isgenerated by reducing the size of the iron powderparticles, and developed a process to suppress theincrease in the eddy current loss by preventing thedamage to the insulation coating in the manufacturingprocess.3. Measures to Reduce Iron Loss in Soft MagneticComposite Cores for Reactors3 1 Manufacturing processThe reactor core discussed in this paper consists oftwo side cores (substantially semicylindrical components) and six middle cores (box-shaped components).Figure 4 shows the manufacturing processes forside cores and middle cores. The manufacturingprocesses are almost the same, consisting of thecompacting process (to compact the iron-based softmagnetic powder) and the heat treatment process (toremove the residual strain generated in the powder inthe compacting process). For the middle cores, weemployed the laser processing process to modify theproduct surface, as discussed below.To reduce the iron loss of soft magnetic compositecores for reactors, we decided to (i) reduce the intraparticle eddy current loss by reducing the size ofpowder particles (raw materials) and thereby reducingthe area where the eddy current is generated, as Manufacturing process of side cores Manufacturing process of middle cores Acceptance of raw materialsAcceptance of raw materialsCompactingCompactingHeat treatmentHeat treatmentLaser processingInspectionInspectionPackingPackingFig. 4. Manufacturing process of soft magnetic compositecores for reactors100 · Pure Iron Based Soft Magnetic Composite Core That Enables Downsizing Automotive Reactors

Product shapeCompacting methodCompacting reUpper punchShoulder1816Eddy current loss (We)14Hysteresis loss (Wh)1210864DieSlidingsurfaceLower punchうCompacting directionNewlydevelopedproductProduct in operationEddy currentforming surface whose electric insulation is maintained.When changing the compacting direction, we faced theissue of how to form the shoulders of side cores.Changing only the compacting direction and using thesurface facing the middle cores as the punched surfacewould require forming of the shoulders using a concavepunch; the stress in the compacting process wouldcause punching damage. Thus, we changed the productshape to the extent where interference with the reactorcase does not occur, as shown in Fig. 5. We decided toform the side core shoulders using a shoulder die whosestructure is simple, instead of using a concave punch.As shown in Fig. 6, we succeeded in suppressing thereduction in the electric insulation in the compactingprocess. Compared with the conventional shape, theeddy current loss has been significantly reduced, totake full advantage of the magnetic properties of softmagnetic composite core materials.Iron loss (W/kg)discussed above. It should be noted that (ii) the intraparticle eddy current loss is not generated if the powdersurface is completely covered by the insulation coating(i.e. theoretically, an electric current does not flow).However, the insulation coating on the powder surfaceis damaged due to the sliding friction with dies whenproducts are removed from the die or due to the plasticdeformation of the powder in the compacting process.Consequently, an electrically conductive layer is formed,resulting in generation of a large eddy current. In particular, pure iron powder is softer than alloy powder, andthe insulation coating is likely to be damaged due todeformation in the compacting process, resulting in ahigh eddy current loss.We decided to take measures appropriate for sidecores and middle cores, respectively, to prevent an eddycurrent from being generated on the product surface.3-2 Measures to reduce iron loss in side coresThe side cores are substantially semicylindricalcomponents that are arranged along the reactor caseshape. In general, when compacting such a shape, thesubstantially semicylindrical surface is used as thepunched surface, as shown as the conventional shape inFig. 5, to make the die structure as simple as possible.PunchedsurfaceSlidingsurfaceShape change(using a shoulder die)Upper punch2MiddlecoreMagnetic fluxThe eddy current is suppressedMiddlecore1. Product with2. Product withconventional shapenew shapeFig. 6. Iron loss reduction effect by changingthe compacting methodShoulder dieLower punch0MiddlecoreMagnetic fluxFig. 5. Reduction in eddy current loss by reviewingthe compacting methodWhen the above compacting methods wereselected, the substantially semicylindrical surface wouldnot be subject to sliding friction with the die becausethis surface would be formed by a punch. The electricinsulation of the product surface would be maintained.Instead, the side surface would be subject to slidingfriction when products were removed from the die,resulting in damage to the insulation coating on theproduct surface.As a result, if a reactor were operated, the surfacefacing the middle cores would become electricallyconductive. A large surface eddy current would begenerated, resulting in a significant increase in loss.To suppress the surface eddy current, we reviewedthe possibility of changing the forming direction so thatthe surface facing the middle cores becomes the punch3-3 Measures to reduce the iron loss for middlecoresThe middle core is a box-shaped componenthaving rounded corners along the coil shape insertedinto a coil. In general, the rounded corners arecompacted using a die shown in Fig. 7, to make the diestructure as simple as possible.When this compacting method were selected, thefour surfaces facing the coil became electricallyconductive surfaces due to the sliding friction thatwould be generated when removing products from thedie. When a reactor were operated, an extremely largeeddy current flowed along the circumference of thecore, resulting in a significantly increased loss.This problem could be solved by changing theforming method, as in the case of side cores. However,when the surface facing the coil was used as thepunched surface, the punch-end surface would becomeconcave; the stress in the compacting process wouldcause the punching damage. Thus, we reviewed aSEI TECHNICAL REVIEW · NUMBER 80 · APRIL 2015 · 101

Product shapeCompacting methodCompacting directionPunchedsurfaceProduct in operationPunchedsurfaceUpper punchEddycurrentうDieSlidingsurfaceLower punchMagnetic fluxSlidingsurfaceFig. 7. Middle core compacting method and issues in reducingthe iron lossmethod to remove and modify the electrically conductive layer generated in the forming process.Figure 8 shows the outline of the laser processingprocess. A slit-shaped laser beam is irradiated on thesliding surface of middle cores to melt and oxidize partof the electrically conductive layer, thereby blocking apotentially large eddy current along the sliding surfaceof the middle cores.Figure 9 shows the condition of the sliding surfaceon a middle core before and after laser processing. Onthe surface before laser processing, the intra-particleboundary is difficult to distinguish because the particleson the product surface are subject to plastic deforma-tion due to the sliding friction generated when theproduct is removed from the die; an electrically conductive layer is formed by metal particles that come intocontact with one another. On the surface after laserprocessing, the intra-particle boundary is clearly distinguishable. This proves that the electrically conductivelayer has been modified.Figure 10 shows the cross-sectional structure onthe sliding surface of a middle core before and afterlaser processing. The cross-sectional structure beforelaser processing shows an electrically conductive layer(near the sliding surface) formed by plastic deformationof particles due to sliding friction with the die. Afterlaser processing, the electrically conductive layer ismelted and spheroidized. This shows that the electrically conductive layer has been oxidized by l

As a result, we developed the pure iron-based soft magnetic composite core for automotive reactor that had been produced by electromagnetic steel sheet, and achieved 10% of downsizing and weight reduction with the same performance.-----Keywords: reactor, boost converter, soft magnetic compos