Inductor devices – Combined – With connector
Reexamination Certificate
2000-09-13
2003-11-04
Enad, Elvin (Department: 2832)
Inductor devices
Combined
With connector
C336S196000, C336S206000, C029S602100
Reexamination Certificate
active
06642827
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to microminiature electronic elements and particularly to an improved design and method of manufacturing microminiature electronic components including toroidal transformers and inductive reactors (i.e., “choke coils”).
2. Description of Related Technology
For many years, electronic circuit boards have been fabricated by interconnecting a plurality of electronic components, both active and passive, on a planar printed circuit board. Typically, this printed circuit board has comprised an epoxy/fiberglass laminate substrate clad with a sheet of copper, which has been etched to delineate the conduct paths. Holes were drilled through terminal portions of the conductive paths for receiving electronic component leads, which were subsequently soldered thereto.
More recently, so-called surface mount technology has evolved to permit more efficient automatic mass production of circuit boards with higher component densities. With this approach, certain packaged components are automatically placed at pre-selected locations on top of a printed circuit board so that their leads are registered with, and lie on top of, corresponding solder paths. The printed circuit board is then processed by exposure to infrared, convection oven or vapor phase soldering techniques to re-flow the solder and, thereby, establish a permanent electrical connection between the leads and their corresponding conductive paths on the printed circuit board.
The increasing miniaturization of electrical and electronic elements and the high density mounting of such elements has created increasing problems with electrical isolation and mechanical interconnection. As circuit board real estate becomes increasingly more valuable, more and more components are put into increasingly smaller spaces, thereby generally increasing the heat generation per square millimeter of circuit board, as well as the likelihood of electrical and electromagnetic interference (EMI) between components in such close proximity. Such factors strongly militate in favor of components that utilize the absolute minimum footprint, and have acceptable heat and EMI signatures in addition to the desired electrical performance.
One very commonly used component is the transformer. As is well known in the art, transformers are electrical components that are used to transfer energy from one alternating current (AC) circuit to another by magnetic coupling. Generally, transformers are formed by winding one or more wires around a ferrous core. One wire acts as a primary winding and conductively couples energy to and from a first circuit. Another wire, also wound around the core so as to be magnetically coupled with the first wire, acts as a secondary winding and conductively couples energy to and from a second circuit. AC energy applied to the primary windings causes AC energy in the secondary windings and vice versa. A transformer may be used to transform between voltage magnitudes or current magnitudes, to create a phase shift, and to transform between impedance levels.
Another purpose for which microelectronic transformers are commonly used is to provide physical isolation between two circuits. For example, a transformer may be used to provide isolation between a telephone signal line and the Central Office (CO), and in the public switched telephone network and consumer equipment such as modems, computers and telephones, or between a local area network (LAN) and a personal computer. Often, the transformer must be able to withstand large voltage spikes which may occur due to lightning strikes, malfunctioning equipment, and other real-world conditions without causing a risk of electrical shock, electrical fire or other hazardous conditions.
In furtherance of these ends, the electrical performance of the transformer must be carefully considered. One means by which the electrical performance of transformers is gauged is the Dielectric Withstanding Voltage (DWV) or hi-pot test. A hi-pot test involves the application of AC or DC signals to the transformer to determine whether the breakdown of the core dielectric or other destructive failures occur at some chosen voltage level. A hi-pot test can also be used to demonstrate that insulation can withstand a given over-voltage condition (such as the aforementioned voltage spikes) and to detect weak spots in the insulation that could later result in in-service failures.
The International Electro-Technical Commission is an international standards body that develops the standards by which isolation transformers are categorized according to level of safety. Underwriter's Laboratories Standard 1950 (UL-1950) is the corresponding harmonized national adaptation for the United States. It specifies a minimum standard for dielectric breakdown between the primary and secondary windings of a transformer. Under UL-1950, insulation systems used in transformers are classified as Operational, Basic, Supplementary, or Reinforced. The most common classification for transformers used in telecommunications application is Supplementary.
In order to meet a standard such as UL-1950, it is critical that the primary and secondary windings are electrically isolated and/or physically separated from one another while remaining magnetically coupled to one another through the transformer core. The standard provides for (or allows) the use of: (1) required minimum spacing distances, (2) minimum thickness of solid insulating material, or (3) a minimum number of layers of a thin film of insulation for compliance. When the use of layers of a thin film of insulation is the means selected to provide electrical isolation between windings in the transformer, the standard states that a minimum of two layers must be used. Each of the layers must individually pass the DWV requirement. Three layers may also be used, in which case the DWV requirement must be met by testing combinations of two layers at a time. An option provided under the standard is to apply the thin films directly to a conductor as in the case of a wire having two or three extrusions of film material deposited directly over the copper conductor.
Magnet wire is commonly used to wind transformers and inductive devices (such as inductors or choke coils). Magnet wire is made of copper or other conductive material coated by a thin polymer insulating film or a combination of polymer films such as polyurethane, polyester, polyamide, and the like. The thickness and the composition of the film coating determine the dielectric strength capability of the wire. Magnet wire in the range of 31 to 42 AWG is most commonly used in microelectronic transformer applications, although other sizes may be used in certain applications.
Note that where Supplementary or Reinforced insulation is required by the cognizant safety agencies for specific applications, such as in the case of the aforementioned UL standard, the enamel insulation used on magnet wire is generally not sufficient. In these cases, the transformers need to be built such that additional insulation between the windings is provided. This is often achieved by adding insulating tape between the windings and additional tape in the margins of the winding form to provide spacing to ensure that the required minimum distance between the primary and secondary windings is maintained. While useful in certain types of transformers, such “margin” tape is not well adapted to very small transformers, and toroidal cores in particular.
Hence, under the prior art, the designer is left with the choice of using margin tape and layers of thin insulation or individually insulated wires in order to meet the dielectric requirements set forth in the applicable standards. One major disability with the use of individually insulated wires in transformer applications is space. Specifically, since each conductor is insulated with its own layers of insulation (typically on the order of a few mils thickness), it can be readily appreciated that the space required by many layers of such con
Jean George
McWilliams Michael D.
VanderKnyff Jacobus J. M.
Enad Elvin
Gazdzinski & Associates
Poker Jennifer A.
Pulse Engineering
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