Information
-
Patent Grant
-
6507260
-
Patent Number
6,507,260
-
Date Filed
Thursday, April 27, 200024 years ago
-
Date Issued
Tuesday, January 14, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Donovan; Lincoln
- Nguyen; Tuyen T.
Agents
- Blakely, Sokoloff, Taylor & Zafman LLP
-
CPC
-
US Classifications
Field of Search
US
- 336 84 C
- 336 90
- 336 200
- 336 223
- 336 192
- 336 229
- 336 198
-
International Classifications
-
Abstract
A toroidal transformer for a communications application where a first winding is wound on the core. A separator comprising a pair of annular-shaped cups then encloses the core and first winding. A second winding is wound on the outer surface of the separator. The separator, fabricated from a low K material, provides substantial reduction in the capacitance between the first and second winding.
Description
BACKGROUND
1. Field of the Invention
The invention relates to the field of transformers, particularly transformers useful in connecting a twisted pair transmission line to a transceiver.
2. Prior Art
In many applications where transformers are used for isolation, there is a need to reduce common mode noise coupling between windings and at the same time minimize so-called “open frame” noise caused by magnetic flux from other devices inducing voltages in the windings of the transformer. This is particularly true where transformers are used to isolate a twisted pair communication line, such as a telephone line, from a transceiver.
One of the most effective ways to reduce noise pick-up from stray magnetic flux in transformers is to use a toroidal core with windings uniformly disposed around the full circumference of the toroid. Multiple windings are either wound on top of each other in layers or wound at the same time in a bifilar fashion. Uniformly spreading each winding around the full circumference of the toroid results in cancellation of stray magnetic field pick-up. This is true since windings on opposite sides of the toroid induce opposite polarity signals. Another advantage to tightly coupling primary and secondary windings is that leakage inductance is significantly reduced. Low leakage inductance provides wider transformer bandwidth.
Common mode noise coupling is generally the result of the parasitic capacitive coupling between the windings of the transformer. This capacitance can be most easily reduced by separating the windings, such as by having one winding disposed on one sector of the same core and a second winding disposed on another sector of a toroidal core. With this sector winding technique, the distance between windings is increased and consequently the capacitance between the windings is reduced. In some applications a transformer may need to provide a safety isolation barrier between windings of a transformer in addition to functional isolation requirements. Meeting stringent safety agency insulation and voltage breakdown requirements also leads to greater separation between transformer windings.
Thus, when windings are sector wound, common mode noise coupling is reduced and safety isolation requirements facilitated but noise pick-up from stray magnetic fields is increased and transformer bandwidth is reduced. Conversely, steps taken to reduce noise pick-up from stray magnetic fields tend to increase common mode noise coupling.
As will be seen, the present invention provides a transformer with a separator between the primary and secondary windings which reduces the capacitance and hence the common mode noise and at the same time minimizes noise pick-up from stray magnetic fields and transformer leakage inductance. This same construction also facilitates stringent safety isolation requirements without compromising other functional parameters.
SUMMARY OF THE INVENTION
A transformer is described having a toroidal core and a first winding disposed about the core. A winding separator comprising a pair of annular-shaped cups encircles the first winding. A second winding is disposed about the outer surface of the separator.
In one embodiment a tab extends adjacent an opening which provides access to the interior of the winding separator. The ends of the first winding pass through the opening and are separated from the second winding by the tab. In another embodiment, the interior of the separator includes spacers to provide a further air gap between the first and second windings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a plan view of the transformer with the separator cut away to reveal the first winding.
FIG. 2A
is a plan view illustrating one of the annular-shaped cups forming the separator.
FIG. 2B
is a perspective view of the cup of FIG.
2
A.
FIG. 3A
is a plan view illustrating the other annular-shaped cup forming the separator.
FIG. 3B
is perspective view of the cup of FIG.
3
A.
FIG. 4
is a cross-sectional elevation view of an alternate embodiment of the separator (without the core) generally taken through section lines
4
—
4
of FIG.
1
.
FIG. 5
is an enlarged view of the separator of
FIG. 4
taken through section line
5
—
5
of FIG.
4
.
DETAILED DESCRIPTION OF THE INVENTION
A transformer is described, particularly suited for use with a transceiver for providing isolation between a transceiver and a twisted pair line. In the following description specific details are set forth, such as materials and dimensions, in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known fabrication techniques, methods of winding, etc., are not described in detail in order not to unnecessarily obscure the present invention.
Referring first to
FIG. 1
the transformer includes a toroidal-shaped magnetic core
10
. Any one of numerous well-known magnetic materials may be used for the core
10
. A first winding
12
is wound about and disposed over the core
10
. In one embodiment, the winding
12
is wound about the entire core
10
, that is, the winding encompasses 360° of the core
10
to the extent possible.
A separator
14
comprising a pair of annular-shaped cups fit over the winding
12
. These cups define an interior space for receiving the core
10
and winding
12
. As will be seen, the cups include a tab
18
having an opening to allow the ends
20
of the winding
10
to exit the interior of the separator
14
. A second winding
16
is wound about the outer surface of the separator
14
. This winding again is disposed about the entire surface (all 360° less the area taken by the tab) of the separator
14
.
Referring first to
FIGS. 2A and 2B
, the cup
14
a
comprising one-half of the separator
14
is illustrated. The annular-shaped cup
14
a
includes a generally cylindrical outer wall
32
and an inner cylindrical wall
30
. The space between these walls and the bottom surface of the cup define an interior space which houses the core and the first winding. A tab
21
expands from the bottom surface of the cup
14
a
and mates with a tab on the other cup.
The other half of the separator
14
comprising cup
14
b
is shown in
FIGS. 3A and 3B
. It again is an annular-shaped cup with an outer cylindrical wall
34
which slides over wall
32
of cup
14
a.
A tab
18
extends from the outer wall
34
and includes a notch
22
extending from the very furthest end of the tab
18
to the interior space of the cup
14
b.
When the cup
14
b
is mated to the cup
14
a
, the surface
46
of tab
18
contacts the surface
44
of the tab
21
. These two tabs then define an opening notch
22
through which the ends of the first winding pass. This is important since it provides separation between the second winding wound about the exterior of the separator
14
and the ends of the first winding disposed within the interior of the separator
14
. This spacing, as will be discussed is at least 0.4 mm and is sufficient to meet certain safety regulations mentioned later.
Preferably, the separator
14
is fabricated from a material having a low dielectric constant (low K) such as polypropylene, polyethylene, Teflon or other low K materials. This spacing between the winding with the low K material reduces the common mode noise transfer including both the sinusoidal and transient noise as well as the common mode emitted noise. Also safety isolation is easier.
Referring to
FIG. 4
when the cup halves
14
a
and
14
b
are assembled, they define an interior space
30
for receiving the core and first winding.
By way of example, in one embodiment when the first and second windings are directly on one another without the use of the separator
14
, approximately 45 pF of capacitance was found to exist between the windings. Where a polypropylene separator provides spacing of approximately 0.5 mm between the windings, this capacitance was reduced to approximately 6 pF. Additional decrease of capacitance may be achieved by using the ribs described later in conjunction with
FIGS. 4 and 5
. Also using the separator with the winding wound as discussed (approximately over 360° of the toroid) provided an improvement of 28 dB in the magnetic field induced noise when compared to the sector windings. Often with sector windings some of the common mode noise is converted to differential noise because of the imbalance in the winding technique. This problem is also greatly reduced with the invented transformer.
The tab
18
of
FIG. 1
which is formed by the tabs
20
and
18
of
FIGS. 2 and 3
, respectively, provide sufficient separation between the ends of the first winding and the second winding to meet the standards of the Underwriter Laboratories (UL 1950) and the European equivalent (EN 950).
In the alternate embodiment of
FIGS. 4 and 5
, the bottom surface of the cup
14
a
includes a circular rib
28
upon which the core and winding rest in order to provide additional air space between the first winding and the second winding. The interior of the cup
14
b
includes a circular rib
24
similar to the rib
28
of cup
14
a
to provide air space between the core and first winding and the second winding.
Additionally, a rib
40
is disposed about the inside surface of wall
34
of the cup
14
b
and engages a groove
41
disposed about the outside surface of wall
32
of the cup
14
a.
The rib
40
is taller than the depth of the groove
41
leaving an air gap
50
. This provides a partial air dielectric along the sides of the core when used in conjunction with the ribs
24
and
28
. In one embodiment the air gap
50
is approximately 2 mm.
Thus, a transformer with a separator is disclosed which can be easily fabricated and assembled.
Claims
- 1. A transformer comprising:a toroidal core; a first winding disposed about the toroidal core; a winding separator fitted over the first winding, the winding separator comprising a pair of annular shaped cups defining an interior space encircling the first winding separator; a second winding disposed about an outer surface of the winding seperator; and a pair of tabs, each tab extended from an outer wall of each annular shaped cup, only one tab from the pair of tabs comprising a notch extended from a furthest end of the tab to an interior space of the annular shaped cup.
- 2. The transformer defined by claim 1 including ribs on interior surfaces of the winding separator to provide additional separation between the first winding and the interior space of the winding separator.
- 3. The transformer defined by claim 1 wherein the first winding overlays substantially the toroidal core and the second winding overlays substantially the entire outer surface of the winding separator.
- 4. The transformer defined by claim 1 wherein the winding separator is fabricated from a low K material.
- 5. The transformer defined by claim 1 wherein the pair of tabs define an opening to permit ends of the first winding to exit the interior space of the winding separator.
US Referenced Citations (3)
Number |
Name |
Date |
Kind |
3964009 |
Bernstein et al. |
Jun 1976 |
A |
4578664 |
Kinzler et al. |
Mar 1986 |
A |
5214403 |
Bogaerts et al. |
May 1993 |
A |
Foreign Referenced Citations (1)
Number |
Date |
Country |
04-297005 |
Oct 1992 |
JP |