Inductor devices – Coil or coil turn supports or spacers – Printed circuit-type coil
Reexamination Certificate
1998-08-21
2001-09-11
Mai, Anh (Department: 2832)
Inductor devices
Coil or coil turn supports or spacers
Printed circuit-type coil
C336S223000, C336S232000, C336S083000
Reexamination Certificate
active
06288626
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to the field of electronic devices, and more particularly to the field of ferrite inductors, such as for circuit board mounting.
BACKGROUND OF THE INVENTION
Chokes are commonly used in electronic circuits to block signal frequencies above a desired range, while at the same time allowing DC or low frequency signals to pass. Thus, chokes have been employed to prevent electromagnetic interference (EMI) from disturbing various electronic devices. EMI is generated, for example, as a byproduct of switching regulators which have current and voltage waveforms with fast rise and fall times. Because switching regulators are typically contained in power supplies, EMI may be transmitted through an electronic device via the power supply conductors. Excessive EMI can lead to logic errors in a computer and can cause interference with other adjacent electronic components. Of course, there are many other applications where a choke may be needed to filter unwanted signals.
A choke is typically provided by a magnetic core through which, or around which, conductors or windings are positioned. Thus, a typical choke defines first and second mutually coupled magnetic paths. A choke may be schematically represented by a low pass filter. For any choke to function as intended, its inductance or inductive reactance, should not fall below a specific minimum, even though the current in a winding rises to a maximum value. Beyond the maximum current value, the reactance falls off significantly. The choke's ability to impede interference signals drops, thereby allowing the passage of unwanted signals. It is therefore typically desirable to prevent a choke from being driven into such a saturation condition.
Ferrite materials are commonly used as the core material for many chokes because, for example, ferrites have sensitive magnetic-frequency relationships. The ferrite material used to form the choke will determine which signal frequencies the choke will attenuate. Most ferrites having suitable inductance values for choke applications saturate at less than about 4,000 Gauss. Accordingly, when configured differentially, ferrites have a relatively low current carrying capacity before the choke is driven into saturation and its impedance level deteriorates at the desired filter frequency.
The techniques normally used to prevent this saturation are to provide a core air gap, use a larger cross-section core, or simply to limit the allowable current. An example of a choke with a core having an air gap is illustrated in U.S. Pat. No. 5,455,552, assigned to the assignee of the present invention. The choke represents a significant advance in technology and includes a ferrite body through which a pair of spaced apart vertical slotted openings are formed. Pairs of spaced apart electrical conductors extend from the bottom of the device, vertically to the top of the device through one slot, along the top of the device, and back down vertically through the other slot of the device. The conductors are configured for common mode operation to mitigate the saturation difficulties. In other words, by bringing the high side and ground return through the same core opposing fields are produced in the core which tend to cancel.
While chokes are commonly applied directly to printed circuit boards, in such applications it is typically undesirable to use chokes with large cores or gapped sections. Accordingly, the compact ferrite structure as disclosed in U.S. Pat. No. 5,455,552 is advantageous for circuit board mounting. Unfortunately, the physical positioning of the discrete conductors through the vertically extending slots requires additional efforts during manufacturing.
SUMMARY OF THE INVENTION
In view of the foregoing background, it is therefore an object of the present invention to provide a common mode choke adapted for circuit board mounting and having desirable impedance and saturation characteristics while being readily manufacturable.
This and other objects, features and advantages in accordance with the present invention are provided by a common mode choke comprising a plurality of joined together ferrite layers defining a generally rectangular body and having a cavity therein. The device includes a bottom ferrite layer, a top ferrite layer, and at least one intermediate ferrite layer between the bottom and top ferrite layers. More particularly, the at least one intermediate ferrite layer preferably has an opening therein defining the cavity in the generally rectangular body extending from adjacent a first end to adjacent a second end of the body. The common mode choke also preferably includes at least one pair of interior conductors extending on the at least one intermediate ferrite layer on opposite sides of the cavity. In addition, the choke may also include at least one first pair of end conductors on the first end of the generally rectangular body, and at least one second pair of end conductors on the second end of the body also connected to the pair of interior conductors. Accordingly, the common mode choke is a monolithic device that can be readily connected to a circuit board, for example. In addition, the choke provides high common mode impedance and low differential mode impedance.
The at least one pair of interior conductors may also have respective side edge portions exposed to the cavity in the generally rectangular body. In other words, the interior conductors communicate with the cavity or are immediately adjacent the cavity. In one embodiment, the cavity is filled with air and the at least one pair of interior conductors are positioned inwardly from respective sides of the generally rectangular body to improve the performance of the common mode choke. The interior conductors may include enlarged width end portions which connect to the end conductors and also provide enhanced performance.
In one advantageous embodiment, the opening in the at least one intermediate ferrite layer extends completely from the first end to the second end of the generally rectangular body. Accordingly, the cavity opens outwardly at the first and second ends of the generally rectangular body, and ambient air will fill the cavity in one embodiment. In a variation of this embodiment, a solid non-magnetic dielectric body, such as aluminum oxide or titanium dioxide, may be positioned in the cavity.
In another embodiment of the common mode choke, the at least one intermediate ferrite layer includes respective end portions extending between the first and second ends of the generally rectangular body and adjacent portions of the opening. In other words, in this embodiment, the cavity is closed at the first and second ends of the generally rectangular body.
The at least one pair of interior conductors may comprise a pair of generally linearly extending electrically conductive layers arranged in side-by-side relation. The at least one intermediate ferrite layer may be provided by a pair of intermediate ferrite layers. Thus, the at least one pair of interior conductors are positioned between the pair of intermediate ferrite layers. In addition, in one variation each interior conductor comprises first and second electrically conductive layers in stacked and joined together relation. Accordingly, this common mode choke may carry higher currents and provide lower DC resistance. The top and bottom ferrite layers are preferably generally continuous.
A method aspect of the invention is for making the common mode choke. The method preferably comprises the steps of: joining together a plurality of ferrite layers comprising a bottom ferrite layer, a top ferrite layer, and at least one intermediate ferrite layer between the bottom and top ferrite layers; forming an opening in the at least one intermediate ferrite layer defining a cavity in the body extending from adjacent a first end thereof to adjacent a second end thereof; and forming at least one pair of interior conductors extending on the at least one intermediate ferrite layer on opposite sides of the cavity. The method also preferably incl
Bradley Michael A.
Cameron Craig P.
Meadors Richard W.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Mai Anh
Steward, Inc.
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