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 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 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. |
