Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
1999-02-17
2001-04-17
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S185000, C307S105000
Reexamination Certificate
active
06218913
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to reduction of conducted electromagnetic interference (EMI) on alternating current (AC) power lines utilizing frequency domain filters of lumped parameters having significant physical structure comprising inductor devices with core(s) surrounding linear conductors.
The use of inductor-capacitor (L-C) filtering to remove conducted EMI noise in the frequency band previously mentioned is widely accepted.
Schematics of three-phase filters, such as that shown in
FIG. 1
, are relatively standard in the industry and it is well understood that the inductors have increasing reactance and the capacitors have decreasing reactance as frequency increases, thereby impeding and shunting the high frequency noise to minimum values.
Some basic formulas should be noted which will further the design discussions which follow:
I
max
=10*B
max
*A
e
/N*A
l
(1)
Where:
I
max
=Peak saturation current (A)
B
max
=Maximum flux density (G)
A
e
=effective core area (sq. cm)
A
l
=inductance index (mH/1000 Turns)
N=Turns
L=n*A
l
(N/1000)
2
(2)
Where:
L=inductance (mH)
n=number of magnetic cores
C=1/(6.28f
0
)
2
L (3)
Where:
f
0
=center frequency (Hz)
C=capacitance (F)
L=inductance (H)
The physical layout and component types in 3-phase filters usually adhere to standard filter design and construction methods which have certain disadvantages. An example of such a prior art filter is shown at
FIGS. 2
a
and
2
b
, where a pair of cores
1
are shown positioned in a housing
2
.
FIG. 2
a
is a top plan view, while
FIG. 2
b
is a cross-sectional view of the filter shown in
FIG. 2
a
, taken generally along line
2
b
—
2
b
, to have the effect of a side elevational view of the filter with the side of the housing
2
removed. In the filter shown in these figures, wires
3
are wound or wrapped around cores
1
multiple times to result in an inductor
7
having an inductance determined by the properties of the cores and the number of wraps. Capacitors
4
,
5
and
6
are connected between the inductors
7
and neutral, and between the inductors and ground, to constitute a completed filter circuit as shown schematically in FIG.
1
. Mylar
9
provides necessary insulation, and encapsulation material
8
provides both component mounting and insulation. Resistors
10
act as bleed resistors to bleed any residual voltage off the capacitors
4
, while not affecting filter performance.
In higher current applications (180-600 A), inductors are usually constructed of several parallel turns of magnet wire wound around a medium (1-4k) permeability core. The wires themselves can be heavy and difficult to bend to form windings around brittle ferrite cores without breaking the wires, the cores, or both. The soldered, clamped or crimped connections to these inductors can also contribute to filter overheating.
Inductor and capacitor component values must be selected such that proper low pass cutoff frequencies are realized. To keep leakage current low, line-to-ground capacitors are usually kept small, thus driving up the size of the inductance, and consequently the size of the inductors.
The placement of the capacitors is driven by the inductor lead location, which often requires long leads. Leads that are too long can compromise high frequency filtering and poor location can cause high voltage isolation problems.
The present invention is directed toward relieving the aforementioned problems.
SUMMARY OF THE INVENTION
To avoid the limitations with conventional devices, the present invention provides a combination of insulated wires, large ferrite toroid cores and a capacitor ring, which provides significant (20-120 dB) attenuation of EMI noise in a compact, integrated, shielded package.
To eliminate the need for winding magnet wire, the inductors used herein are comprised of a group of toroidal ferrite cores, with power conductors simply passing through the center, once. Permeabilities of the ferrites are as high as presently possible in this size (greater than 4k) (referred to as “high &mgr; permeability”), generating proper saturation fluxes even with only one turn. The single pass-through also eliminates solder or crimp connections in the power conductor, which allows the filter to run cooler or use smaller power conductor wire sizes.
The utilization of toroidal ferrite cores as common mode inductors, by passing through the center once, takes advantage of the physical properties of toroids which allow a single pass-through to create one full turn. Additionally, all conductors which carry current are enclosed in the toroid center. By means of basic closed circuit analysis, it can be seen that the net current, and consequently the net flux, is therefore equal to zero. This allows all normal circuit currents to create canceling flux. Thus, unbalanced noise current contributes to the toroid core saturation, in turn allowing higher inductance from higher permeability materials.
The utilization of toroidal ferrite cores as differential inductors takes advantage of the increased leakage inductance resulting from the reduced coupling which accompanies the single turn pass-through configuration. Therefore, differential mode filtering is achieved without the primary use of separate differential inductors. However, the differential inductance can be enhanced by use of separate properly sized and chosen iron powder cores on each of the conductors singly.
To achieve minimum component size, and thus minimum filter size, the inductor and capacitor values are selected and matched so that, as well as meeting the appropriate low pass cut-off frequencies, the two part types remain volumetrically similar. The best volumetric match will likely lead to higher than usual capacitor values. Higher capacitor values result in higher leakage currents, making the filter provided by this invention possibly less suitable as an appliance filter. As a facility or power device filter, however, this configuration is well suited. To improve the high frequency filtering and high voltage isolation, a pass-through square axisymmetric capacitor ring is created, mimicking the physical arrangement of the pass-through inductors in a coaxial fashion. As the line-to-line and line-to-ground capacitor values are the same, a compact ring of capacitors of a uniform size and shape is formed around the conductors. This capacitor ring allows component leads to remain short to minimize series inductance to the capacitors, addressing high frequency concerns and still providing sufficient length and separation to maintain required conductor to conductor spacings, addressing high voltage concerns.
To reduce EMI radiated emissions, a shield is fitted tightly around the coaxial components. This arrangement not only reduces outgoing radiated noise, but also minimizes the tendency of electromagnetic fields to bypass internal components, which would compromise high frequency performance.
These and other objects of the invention are provided by a novel integration of coaxial inductors and axisymmetric capacitor rings surrounding linear conductors.
REFERENCES:
patent: 4725739 (1988-02-01), McCartney et al.
patent: 4761623 (1988-08-01), Schneider
patent: 5083101 (1992-01-01), Frederick
patent: 5153539 (1992-10-01), Hara et al.
patent: 5243308 (1993-09-01), Shusterman et al.
patent: 5444609 (1995-08-01), Swamy et al.
patent: 5461351 (1995-10-01), Shusterman
patent: 5635890 (1997-06-01), Yamaguchi et al.
patent: 5650759 (1997-07-01), Hittman et al.
patent: 5838216 (1998-11-01), White et al.
Baxter William K.
Bettendorf Justin P.
Curtis Industries, a division of Powers Holatings, Inc.
Godfrey B Kahn, S.C.
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