Multi-Chambered Planar Magnetics Design Techniques

Because the reluctance of the upper and lower air gaps is much larger than those posed by the inner and outer material paths of the core structure, flux produced by the MMF of the

transformer section of the PIM is restricted primarily to the inside sections of the core system. Therefore, PIM inductive and transformer-related operations will be largely

independent, with insignificant magnetic interaction. This situation also implies that there are no significant restrictions on turn ratios between transformer and inductor windings,

whereas in some IM converter designs [2], such restrictions are required to insure proportionality of winding driving potentials in order to permit their magnetic components to

utilize a common core structure.

In the cross-sectional view of the PIM construction in Fig. 5, the unshaded areas on the outside “walls” of the core structure represent open parts of the core for accessing the

ends of the PCB windings located in each chamber. Fig. 6 shows the overall construction approaches for viable PIM assemblies of the “stacked” variety [7][8], with typical access locations for PCB winding terminations illustrated.

Another PIM construction alternative is a “side-by-side” arrangement of transformer and inductive windings [9] as depicted in Fig. 7. In contrast to the design of Fig. 3, the inductive portion of the PIM lies in the center of the structure that surrounds the center portion. The core “wall” that separates the two “chambers” of the core system PCB Assembly then serves as a common flux path for inductive and transformer operations. Fig. 8 is a sketch of a practical implementation ofthe PIM concept [9] of Fig. 7, and a cross-sectional view of the design is shown in Fig. 9.

As indicated in Fig. 9, interactions between the magnetic operations of the system can be minimized further by the presence of a small air chamber placed in this common flux

path. It is also feasible to place a “shield” band of conductive material in this chamber. This shield technique is similar to a “belly-band” copper screen often added around the outside of a conventional inductor or transformer to reduce radiated magnetic emissions.

The PIM method of Fig. 7 has the advantage of keeping the overall height of the core structure low. However, it is obvious that the surface area of the PIM will be increased

over the “stacked” arrangement of Fig. 3, requiring more area for mounting. Also, to access the inductive winding, holes must be placed in the bottom part of the inner core chamber. It is also conceptually possible to add a third outer chamber and associated core walls to the outside part of this core system for another set of inductive windings. However, since the winding lengths will be much longer than those of the innermost chambers, the copper losses will be higher than those in the three-chamber “stacked” design approach illustrated in Fig. 5.

Many power converter systems require multiple inputs or outputs that, in turn, require additional inductances for filtering purposes. These inductances can be placed in a PIM

in “coupled-inductor arrangements ” [1][3] in the appropriate chambers. For example, in the PIM design shown in Fig. 5, leakage inductance values between windings mounted in the

upper or the lower chambers can be well controlled by the addition of thin disks of core material [11], often termed “magnetic shunts”, or “reluctance disks” when windings are

implemented in PCB formats. Such disks can be used to provide magnetic control and reduction of AC current levels in selected inductor windings [1][2][4]. Examples of reluctance disk forms are shown in Fig. 10. Suitable disk materials include inexpensive varieties of cold-rolled steel and low-permeability soft ferrite. Disks of non-magnetic materials can also be used in those instances where desired leakage inductance values needed are small.

V. PIM MODELING METHODS

To facilitate a better understanding of the magnetic operations of the PIM designs shown earlier in Figs. 5 and 7, equivalent circuit models can be developed, using the reluctance-to-inductance modeling methods described in Chapter 12 of [1]. These models can then be used to study the dynamics and magnetic interactions between the transformer and inductive sections of a PIM. For example, using the flux paths and directions defined

earlier in Fig. 6 for the three-chamber PIM structure of Fig. 5, a first-order reluctance model of the magnetic system is formed, along with MMF sources. This model is illustrated

in Fig. 11. Note that the symmetry of the basic model permits it to be simplified as indicated in this sketch.

With a base reluctance/MMF model established, it can be converted into one involving inductances and excitation sources. This new model is shown in simplified form in Fig.

12. In this model, all inductance values are referred to winding NP of the PIM system in Fig. 5.Values for the inductances indicated in Fig. 12 can be estimated, using the first-order reluctance and inductance relationships defined in Table I. Examination of the resultant circuit model in Fig. 12 shows that that the two inductances (Lit and Lib) formed by

the two core pieces separating the two inductive chambers from the transformer chamber will be much larger in value than those associated with the upper and lower core pieces

where air gaps are present (Lct and Lcb). For this reason, very little of the flux developed by the transformer actions within the center chamber will appear in the core areas ssociated

with the inductor chambers. This confirms that transformer and inductive operations in this PIM will indeed be largely independent, with very little interaction between them.

Finally, as an example of the use of this circuit model in analyzing its use in conjunction with a converter network, TABLE I. Approximate relationships between the reluctances of Fig. 11

and the inductances shown in Fig. 12. airgap top airgap bottom mat center T B M cp top cp bottom mat cp center mat top mat middle sides mat mat top mat mat middle sides mat bottom Fig. 13. The forward converter circuit of Fig. 5 with the PIM equivalent model from Fig. 12. Fig. 13 is a circuit diagram of the forward converter system of Fig. 5, redrawn to include the PIM model of Fig. 12.

VI. PIM PROTOTYPE TESTS

To verify the “stacked” PIM approach illustrated in Fig. 5, a 200 kHz, 20-40VDC in, 5V-10A out, PIM forward converter system was designed, built and tested for performance, as a part of a recent NASA SBIR Phase II proposal effort. In this case, a three-chambered PIM was

constructed by using four separate cylindrical pieces of soft ferrite of the MN8CX variety made by Ceramic Magnetics. Overall height of the PIM structure was 16.8 mm (0.661 in)

and its diameter was 35.2 mm (1.386 in). Center post area in all chambers was set by design to 53.5 mm2 (0.076 in2 ), with the air gap lengths in the upper and lower chamber center posts cut to 254 mm (0.01 in). The primary and input inductor windings used 3 paralleled 8-turn double-sided PCBs, while the secondary and output inductor windings

used 3 paralleled double-sided 5-turn PCBs. Four-ounce copper patterns were used for all PIM PCB windings.

Measured inductance of the primary winding, the input filter inductor and the output filter inductor was 250 mH, 19.2 mH and 7.5 mH, respectively, very close to design projections. The measured efficiency of the power stage under maximum loading conditions was 87%, with total PIM core and winding power losses measured at nominally 1.2 watts. The measured temperature rise above ambient of the PIM was 30°C (no heatsink or forced-air cooling). As predicted by design, no discernible magnetic interactions were observed with regard to the transformer and inductive functions within the PIM during the testing of the converter. Core volume and weight savings over a conventional converter design having two individual planar inductors and one planar transformer were calculated to be 29.5%.

Similar verification testing of the “side-by-side” PIM system shown earlier in Figs. 8 and 9 have been reported by research engineers in Japan [9][10] with equal success. In one experiment, an off-line 100VAC-to-24VDC, 125W, 350 kHz forward converter system was built and tested. The PIM was constructed using a high-frequency low-loss ferrite of the 2500B2 variety made by TOKIN (Sendai, Japan). The outside diameter of the PIM was 53 mm (2.09 in), and its height was 8 mm (0.315 in). Total center post gap length was set at nominally 300 mm (0.012 in) to yield an output filter inductance of 16 mH. A 5-turn primary winding and a 3-turn secondary winding were used in the outer chamber, with an inner 6-turn inductor winding. Two-ounce copper patterns, nominally 70 mm (0.0028 in) thick, were used for all multilayer windings to minimize high-frequency copper losses.

The efficiency of the PIM of this converter system was measured to be on the order of 98% at an output power level of 125 watts. Noise reduction tests were also conducted, showing 10 to 20 dB reductions in 100-300 MHz radiated noise over that of a comparable converter design with separate “open-frame” inductor and transformer elements.

VII. CONCLUSIONS

The stacked “top-to-bottom” and “side-by-side” multichambered PIM constructions presented herein are new, volumetrically efficient IM techniques for blending the many

inductors and transformer functions of any dynamic power processing system without compromising electrical performance needs. The constructions can also include simple planar “disks” of magnetic material to control leakage inductances in selected areas of the structures. The form factors for the “top-to-bottom” construction variations can be either “open” or “closed”. In the former case, these PIMs can use standard circular core halves (e.g. pot, RM, PQ, DS shapes), or multiple E-I combinations of off-the-shelf lowprofile

rectangular cores. Research is continuing on these new PIM designs, including investigations associated with direct depositing of windings on selected parts of the core

structures to further reduce cost and assembly time.

ACKNOWLEDGMENTS

The author would like to acknowledge the electronic assembly funding support provided by the U.S. National Aeronautics & Space Administration (NASA) in the early phase of development [12] of the “stacked” PIM concepts described in this paper. A U.S. Patent [7] was issued to the author’s company in 1998 relative to this PIM method and other related enhancements.