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Multi-Chambered Planar Magnetics Design Techniques

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Abstract – New planar construction alternatives for blending the various power magnetic components of switch mode power processing circuits and systems are presented. These unique approaches are based on the use of core designs with multiple winding areas and core sections which provide common flux paths for transformer and inductor operations, arranged so as to minimize magnetic interactions and core material required. Construction examples for 125-watt & 50-watt DC-to-DC switch mode converters are detailed, the latter example consisting of a four-piece three-chamber cylindrical planar core structure that houses all power magnetic functions, including input filter inductance. Planar means to control leakage inductances between inductive windings are also presented for control and reduction of AC ripple current magnitudes.

ONE of the more interesting magnetic design techniques in practice today by power supply engineers is the “blending” or “mixing” of transformer and inductive functions of dynamic power conversion circuits on a single magnetic core structure. This technique has come to be known as Integrated Magnetics (IM) design. Although the origin of the technique can be traced to prior work performed in the early part of the last century, IM methods have ecome
increasingly popular since 1977, when “coupled-inductor” assembly concepts and ssociated converter designs [1][2][3] were more fully disclosed. When implemented correctly, IM
techniques can produce a significant reduction of the magnetic core content of related power processing systems, resulting in cost-effective designs of smaller weight and volume. Recently, IM packaging alternatives have been extended to include implementations in planar, or “flat”, forms, using modern printed-circuit approaches for windings together with low-profile ferrite core constructions.

It was subsequently demonstrated in 1984 [4][5] for circuit arrangements and in 1987 [6] on a system level that all power conversion circuit designs of the switch mode variety have one or more IM forms. Prior to these points in time, it was commonly believed that only those power-processing networks wherein transformer and inductor winding potentials are always dynamically proportional could have IM versions. Such special converter networks include
transformer-isolated versions of the .uk [2], SEPIC and ZETA topologies. However, by the use of core structures that possess more than one major material flux path [4], IM techniques can be applied to all switch mode power processing circuits and systems, and can be extended to include other magnetic elements often excluded. Examples of such elements are second stage input and output filter inductances used for reducing conducted AC current levels.

In this paper, two new approaches [8][9] for using IM techniques in conjunction with planar core and winding construction methods are described. These approaches require the use of core structures with more than one winding window area, and have the distinct advantages of minimal interactions between the various magnetic elements included
in the IM assemblies. The full benefits of reduced core system volume and weight by the IM process are also achieved by these unusual approaches. The resulting core structures can completely enclose all windings (except for access slots), providing excellent magnetic shielding capability. In one of these approaches, “off-the-shelf “planar core sections can be used to construct the IM system.

IM designs today typically use soft-ferrite E-I or E-E core structures. Fig. 1 is an collection of some of these “traditional” IM designs [1][4][6] for a “single-ended” converter circuit, which differ only in winding locations on the three “legs” of the core structure. For the core leg where inductor windings are situated, an air gap is added to obtain the desired inductance values. Here, the converter topology shown in Fig. 1(a) is a buck-derived “forward” configuration, found in many DC power conversion systems today, where output power needs typically range from 50 to 250 watts.

Effective core leg areas must be chosen in accord with the maximum flux levels that will occur as a result of converter operation so as to prevent saturation of any leg under
maximum loading conditions of the system. As can be seen in the alternative designs in Fig. 1, each leg will see different flux levels, depending on winding locations and phase
relationships, along with core leg reluctance values. For example, in the “split-winding” version shown in Fig. 1(c), the outer leg to the left of the center leg will see the sum of
the transformer produced by the inductor winding. In the right outer leg, the flux here will be the difference between the transformer AC flux and the remaining half of the AC and DC components of the inductive flux generated by the winding(s) mounted on the center leg of the core structure. In this particular IM variation, the window area needs on each side of the center leg are equal. However, in the variation shown in Fig. 1(d), an optimum core design would require unequal window areas on each side of the center leg. This disparity is also a
possibility for the IM designs in Figs. 1(b) and (e). In the variation of Fig. 1(e), the inductive leg is one of the outer legs of the core structure, and its area would need to be larger than the others, which is not a standard E-E or E-I core configuration. Finally, it is apparent that all of the alternative designs shown in Fig. 1 require winding bobbins that must be
fitted on one or both of the outside core legs. Presently, such bobbins are non-standard items, requiring custom manufacturing and added assembly cost.

For “push-pull” versions of converter circuits where twoquadrant B-H loop operations occur from transformer actions, the centermost leg is used for the inductive part of the converter, with dual primary and secondary windings placed on the outer core legs. In these situations, the window area requirement is balanced, like the core-and-winding arrangement shown in Fig. 1(c).

A variety of methods for core reset [1] due to the “singleended” transformer action of the converter in Fig. 1(a) is available. Some of these methods utilize resonant reset techniques involving the parasitic capacitances of T1, Q1, D1 and D2 and the self-inductances of the transformer windings.

The use of printed wiring methods for the windings of an IM can lower the height profile of the overall package of the magnetic component, and since the windings are mounted on
a laminate base, the cost of special bobbins mentioned earlier is eliminated. One example of a Planar Integrated Magnetic (PIM) design for a “split-winding” version of the forward
converter topology is shown below in Fig. 2.

In the PIM design of Fig. 2, the primary and secondary windings are interleaved using selected layers of the PCB on the outside portions, while the inductor winding is split into
parallel sections of the inside layers. Feed-thru vias in the PCB are used to interconnect the various parts of all three windings. Terminations of the winding ends can be accomplished in a number of ways, such as the use of solid pins running vertically from the PCB end patterns down to the PCB of the converter assembly. The use of “gull-wing” leads is another alternative, and this termination method is particularly useful when placing the PIM on a “motherboard” assembly with surface-mounted components.

“Conventional” IM constructions, whether they use PCBstyle or wire windings, do have some undesirable limitations. Because windings are required on the outer legs of the core system, it is not possible to completely surround all windings with core material to restrict magnetic leakage levels. This situation is easily seen from the PIM construction example shown in Fig. 2. Also, because conventional constructions using E-I or E-E cores restrict window locations for windings to two locales and material flux paths to a maximum of three, other power magnetic components in a conversion system (like input and added second-stage output filter inductances) cannot be easily accommodated in an IM arrangement without significant topology changes. This, in turn, often leads to undesirable compromises in power processing performance.

Rather than placing the various windings of a planar IM (PIM) in a conventional “open, side-by-side” manner, they can also be arranged in a “closed, top-to-bottom” core configuration [7][8], as shown in the cross-sectional diagram in Fig. 3(b). In this example, the upper chamber of the core serves as the location for the output inductive windings of the converter, while the lower chamber is reserved for the windings of the transformer. The core piece separating the two chambers provides a material path for both inductive and
transformer flux elements. With the windings phased as shown in Fig. 3(b), the effective flux level in this common path will be the difference between these flux values. Therefore, the core material needed for this common path can be reduced accordingly. Although the center post areas are shown in Fig. 3(b) to be identical, this condition is not an absolute design requirement. In fact, optimum size and unit height studies for the PIM form shown in Fig. 3(b) for a particular converter application may indicate otherwise.

A practical construction of the PIM concept of Fig. 3(b) is shown above in Fig. 4. Here, a “pot” format is used, consisting of three separate core pieces, stacked to form the two window areas, or winding “chambers”, needed for the transformer and inductor parts of the IM. Within the lower chamber are then placed the PCB assembly for the primary and secondary windings, with the upper chamber reserved for the PCB assembly for the inductor winding. Note that it is possible to use double-sided PCBs in place of a single
multi-layer PCB in each locale, and interconnect them by external wiring methods using termination “tabs” on the PCBs. These tab areas are accessed by means of “slots” cut
into the core sections, as can be seen in the example assembly in Fig. 4. Fig. 4. A “closed, top-to-bottom” PIM assembly, using three separate pieces of magnetic material. Only the center post (CP) of the core top is gapped to provide the desired filter inductance value.
In this construction example, a small hole through the center posts of the cores has been provided to permit a single non-conductive screw-and-nut combination to be used to
hold the assembly together.

Adding one or more inductive windings to a “stacked” PIM construction to accommodate additional filter inductances of a converter system is simply a matter of adding a third chamber area [7][8] to the core arrangement. This chamber can be located below the chamber in Fig. 3(b) where the transformer windings are housed. Fig. 5 is an illustration of this construction, wherein the lower chamber contains the winding(s) for the inductance of an input filter network for the forward converter topology shown in Fig. 1(a). Now the core piece below the transformer chamber serves as a differential flux path for the transformer AC flux and the DC+AC flux generated by the lower winding associated with the input filter inductance.

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