chap14 - 14 Inductor Design This chapter treats the design...

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14 Inductor Design This chapter treats the design of magnetic elements such as filter inductors, using the method. With this method, the maximum flux density is specified in advance, and the element is designed to attain a given copper loss. The design of a basic filter inductor isdiscussed in Sections 14.1 and 14.1.5. In the filter induc- tor application, it is necessary to obtain the required inductance, avoid saturation, and obtain an accept- able low dc winding resistance and copper loss. The geometrical constant is a measure of the effective magnetic size of a core, when dc copper loss and winding resistance are the dominant constraints [1,2]. Design of a filter inductor involves selection of a core having a sufficiently large forthe application, then computing the required air gap, turns, and wire size. Asimple step-by-step filter inductor design procedure isgiven. Values of for common ferrite core shapes are tabulated in AppendixD. Extension of the method to multiple-winding elements is covered in Section 14.3. In applica- tions requiring multiple windings, it is necessary to optimize the wire sizes of the windings so that the overall copper loss isminimized. It is also necessary to write an equation that relates the peak flux den- sity to the applied waveforms or to the desired winding inductance. Again, a simple step-by-step trans- former design approach isgiven. The goal of the approach of this chapter is the design of a magnetic device having a given copper loss. Core loss is not specifically addressed in the approach, and is a given fixed value. In the next chapter, the flux density is treated as a design variable to be optimized. This allows the overall loss (i.e., core loss plus copper loss) to be minimized. 14.1 A filter inductor employed in a CCM buck converter is illustrated inFig. 14.1(a). In this application, the value of inductance L is usually chosen such that the inductor current ripple peak magnitude is a small fraction of the full-load inductor current dc component I , as illustrated in Fig. 14.1(b). As illustrated in Fig. 14.2, an air gap is employed that is sufficiently large to prevent saturation of the core by the peak FILTER INDUCTOR DESIGN CONSTRAINTS
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540 Inductor Design current Let us consider the design of the filter inductor illustrated in Figs. 14.1 and 14.2. It is assumed that the core and proximity losses are negligible, so that the inductor losses are dominated by the low-frequency copper losses. The inductor can therefore be modeled by the equivalent circuit of Fig. 14.3, in which R represents the dc resistance of the wind- ing. It is desired to obtain a given inductance L and given winding resis- tance R. The inductor should not saturate when a given worst-case peak current is applied. Note that specification of R is equivalent to speci- fication of the copper loss since The influence of inductor winding resistance on converter efficiency and output voltage is modeled in Chapter 3.
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14.1 Filter Inductor Design Constraints 541 It is assumed that the inductor geometry is topologically equivalent to Fig. 14.4(a). An equiva-
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chap14 - 14 Inductor Design This chapter treats the design...

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