BACKGROUND
1. Field of the Invention
The invention relates to the field of magnetic core assemblies, and more
particularly to a process for assembling cores for transformers, inductors and other
magnetic devices with small but controllable gap spacing.
2. Description of Related Art
Transformers and inductors are often designed as one or more wire coils
wound around a core. The core defines a magnetic flux loop passing through the
centerhole of the coil and closing outside the coil. Because of the difficulty in winding
a wire around one side of a closed loop core structure, devices using closed loop
cores are typically manufactured using two core portions that are mated together
during assembly. Each core portion is itself open, so that a coil can be wound on one
or more legs of the core before the core portions are mated together. Typically the
coil or coils are pre-wound onto a non-magnetic bobbin, which is simply slipped onto
the desired leg of one core portion before the core portions are mated together.
Cores can have many different shapes. U-cores, for example, are assembled
from two U-shaped core portions, each with two legs protruding toward the opposite
core portion. One or more coils can be wound around one or both legs of the core
portion before the core portions are mated together, the end surfaces of the legs of
one core portion being placed into face-to-face contact with the end surfaces of the
legs of the other core portion. E-cores are assembled from two E-shaped core
portions, each with three legs protruding toward the opposite core portion.
Cylindrically shaped cores are assembled from two cylindrical core portions, each
having a center post protruding toward the other core portion. The coil(s) are
typically wound around the center post before mating. Numerous other shapes exist.
It will be appreciated that the two core portions that make up a complete core need
not be symmetrical, although they often are.
Cores intended for operation above about 100 kHz are often fabricated from
one of a number of ferrite materials. Certain ferrites advantageously have very high
permeability (µ), for example µ>10,000, allowing for very high inductance devices.
In order to use this high permeability in mated core portions, the mating surfaces are
ground to a high polish and the core portions are clamped together. The operation is
delicate because even a fingerprint can degrade the permeability. More recently a very
thin layer of interfacial epoxy adhesive has been used between the mating surfaces of
the core portions to hold them in place.
The effective permeability of magnetic devices with mated cores is very
difficult to control during the manufacturing process. And since the inductance of a
coil encircling the core is a function of the core permeability, the inductance of such
a coil is also very difficult to control. Typically the inductance can be controlled only
to within a tolerance of ±25% or so. In addition, the mating process itself degrades
controllability even further, perhaps by yet another ±5%. One way to control the
permeability more precisely is to introduce an air gap within one leg of the core,
typically the leg passing through the coil. Machinery currently exists which can grind
this leg down on one of the mating core portions to within a much tighter tolerance.
However, the introduction of an air gap also typically reduces the nominal
permeability of the core by 10 or more. Core permeability can alternatively be
controlled by the simple process of testing and rejecting those cores that are not
within the required tolerance specification. This process is expensive and wasteful,
however, especially since it cannot be performed until after the core has already been
assembled.
Accordingly, there i$ a great need in the industry for a process for
manufacturing magnetic devices using mated core portions which permits precise
control of the resulting effective core permeability.
SUMMARY OF THE INVENTION
According to the invention, roughly described, a magnetic device is assembled
by first applying an adhesive to the end surface of a leg of one core portion, slipping
a bobbin onto one of the legs (preferably a different leg) of the core portion, and
mating the two core portions together. Then while observing the inductance of the
coil, the two core portions are ground toward each other, gradually narrowing the
adhesive-induced gap between the mating surfaces, until the desired inductance is
achieved. The adhesive is then cured. Preferably the adhesive includes particulate
matter to help resist the narrowing of the gap during the grinding process. The result
is a "microgapped" core in which the effective permeability has been controlled to
within a very tight tolerance. The improvement in precision achieved due to the
assembly process described herein is not limited to a reduction of the tolerance
degradation that takes place during a conventional assembly process, but can also
correct for permeability imprecision that might have existed in the unmated core
portions themselves, prior to assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with respect to specific embodiments thereof,
and reference will be made to the drawings, in which:
Fig. 1 is a perspective diagram illustrating components of a transformer that
can be assembled in accordance with features of the invention. Fig. 2 is a flow chart illustrating the significant steps performed in the
assembly of the transformer of Fig. 1. Fig. 3 schematically illustrates a test setup that can be used during the
permeability observation step of Fig. 2. Fig. 4 is a perspective view of a magnetic device incorporating features of the
invention.
DETAILED DESCRIPTION
Fig. 1 is a perspective diagram illustrating components of a transformer that
can be assembled in accordance with features of the invention. The components
include first and second core portions 110 and 112, a bobbin 114, and a spring clamp
116. The core 110/112 (collectively, 118) in the example of Fig. 1 has airindustry-standard
shape known as EP-13. In this example the two core portions are
symmetrical, so only core portion 112 will be described. Referring to Fig. 1, core
portion 112 includes a center post, or leg, 120, about which the bobbin 114 will be
placed during assembly. Partially surrounding the center post 120 is a wall 122,
partially cylindrical in shape, and spaced from the outer surface of the center post 120
sufficiently to permit insertion of the bobbin 114. Like the center post 120, the outer
wall 122 protrudes from an outer surface 124 of the core portion 112, toward the first
core portion 110. The end surface 126 of the center post 120, which will mate with
a corresponding end surface 128 of center post 130 on the first core portion 110, is
substantially co-planar with the end surface 132 of the wall 122 which will mate with
the corresponding end surface 134 of the wall 136 of the first core portion 110. The
core portions 110 and 112 in the example of Fig. 1 each have unitary construction,
and are made from a high-permeability ferrite material such as Nippon Ceramics NC-10
or TDK H5C2. These materials have a permeability of µ = 10,000, but they are
specified only with a tolerance of ±30%.
The EP-13 core in the example of Fig. 1 is an example of a partially cylindrical
core. As used herein, a "leg" of a core portion refers to any member of the core
portion that protrudes toward the mating portion. A "leg" need not be post-like, such
as the center posts 120 and 130. For example, as used herein, the walls 122 and 136
of the core portions 110 and 112 also constitute "legs" of such core portions. In
addition, as used herein, a "rim" of a core portion refers to any mating member of the
core portion other than a member that is encircled by the coil. A "rim" member does
not have to be circular in shape, or partially circular in shape such as the walls 122 and
136 of the core portions 110 and 112, nor must it even surround a central leg to any
extent. For example, in an E-core, in which the coil encircles the central leg, each of
the outer legs constitute "rim" members as that term is used herein.
The bobbin 114 includes a central tube 138 having a centerhole 140 into which
the center posts 120 and 130 can be inserted from opposite ends. Coils of wire (not
shown) are wound around the tube 138. To help manage the winding, end stops 142
and 144 are disposed on opposite ends of the tube 138. In the particular example of
Fig. 1, coils will be wound around the bobbin 114 so as to create a transformer having
a single primary and two secondaries. Electrical connections are made via surface
mount pins 148 on the bobbin 114. The shape of the bobbin 114 is an industry-standard
shape known as SEP-13, and it is made of a non-magnetic material such as
phenolic.
After the two core portions 110 and 112 are mated around the bobbin 114,
they are held in place by a spring clamp 116 in a well-known manner. Spring clamp
116 may be made, for example, from a nickel silver material.
Fig. 2 is a flow chart illustrating the significant steps performed in the
assembly of the transformer of Fig. 1. In step 210, a coil or coils are wound onto the
bobbin in a manner desired for the electrical purposes of the device. In the
embodiment of Fig. 1, three coils are wound: one primary and two secondaries. The
step 210 can be performed either at the same location as the following steps, or at
some other location and some earlier time.
In step 212, an adhesive 150 (Fig. 1) is applied to one of the mating surfaces
of one of the core portions 112. The adhesive 150 preferably, but not necessarily,
includes some hard particulate matter to improve controllability of the grinding
process described below with respect to step 218. An example adhesive is Zymet
517F, which is a thixotropic epoxy containing 3% by weight of 10 micron silica
particles. Zymet 517F is available in premixed syringes for application.
In step 214, the two core portions 110 and 112 are mated together around and
through the coils and bobbin 114 (Fig. 1), and clamped together with the spring clamp
116. At this point, the rim surface 132 of core portion 112 is "mated" with the
corresponding rim surface 134 of core portion 110, as that term is used herein,
although a very narrow gap spacing remains between the two surfaces due to the
adhesive 150. Similarly, since the end surface 126 of the center post 120 is co-planar
with the rim surface 132 of the wall 122 of core portion 112, and the end surface 128
of the center post 130 of the core portion 110 is co-planar with the rim surface 134
of the wall 136 of core portion 110, the two end surfaces 126 and 128 also remain
spaced by a narrow gap. These two surfaces are nevertheless considered "mated", as
the term is used herein, inside the bobbin tube 138.
In step 216, the effective permeability of the core 118 is observed. As used
herein, observation of a characteristic such as permeability includes indirect
observation of that characteristic, such as by observing an effect that is dependent
upon the characteristic. Permeability, for example, can be "observed", as the term is
used herein, by observing the inductance of a coil which is in sufficient proximity to
the core to be affected by the effective permeability of the core. Similarly, the term
"observing the inductance", as used herein, includes indirect observation of the
inductance, such as by observing an effect that is dependent upon the inductance. In
the embodiment of Fig. 2, the effective permeability of the core 118 is observed by
observing the inductance of the two secondary windings connected in series.
Fig. 3 schematically illustrates the test setup. The transformer 310 includes
a primary winding 312 having terminals 314 and 316. The first secondary winding
318 has terminals 320 and 322, and the second secondary winding 324 has terminals
326 and 328. For purposes of the test setup, terminals 322 and 326 are connected
together and an inductance meter 330 is connected across terminals 320 and 328.
Since the inductance of any one of the windings 312, 318 and 324 is mathematically
related to the effective permeability of the core 118, observation of the inductance of
any one of the coils, or any two of the coils connected in parallel or series, or all three
of the coils connected in various non-canceling ways, will effectively constitute
observation of the effective permeability of the core 118. In addition, although the coil
whose inductance is observed in step 216 encircles the core 118, it will be appreciated
that in another embodiment, the coil might instead be merely in sufficient proximity
to the core so as to be affected measurably by the core permeability. It will also be
appreciated that observation of an inductance is a particularly advantageous method
of observing the effective core permeability, because usually it is the inductance, not
the effective permeability of the core, which is specified in electronic circuits.
Returning to Fig. 2, in step 216, it is determined whether the effective
permeability of the core 118 is within the desired tolerance. For the test setup of
Fig. 3, this determination is made by determining whether the inductance read from
inductance meter 330 is within a desired tolerance of a nominal inductance value. If
it is not, then in step 218, the gap spacing is adjusted and the effective permeability
of the core 118 is again observed (step 216). The process continues until the effective
permeability of the core 118 is within the desired tolerance. In one embodiment, the
observation step 216 and the adjustment step 218 are performed simultaneously and
continuously until a desired permeability is reached, whereas in another embodiment,
the two steps are performed in alternating manner.
The adjustment of gap spacing in step 218 can be performed in any of a
number of ways. In one embodiment, the two core portions 110 and 112 are held
tightly by apparatus which mechanically moves the cores toward or away from each
other in sufficiently fine increments. In another embodiment, the two core portions
110 and 112 are simply ground toward each other. For example, with the spring clip
116 urging the two core portions toward each other, and the adhesive 150 resisting
a narrower gap spacing, the two core portions 110 and 112 are either rotated relative
to each other or translated relative to each other, or both, in a plane parallel to their
mating surfaces. This motion effectively "grinds" the two core portions toward each
other. The rotation and/or translation can be uni-directional, bi-directional or
vibratory, and can be performed manually or by machine. In this connection, it will
be appreciated that the particulates in the adhesive 150 help resist the collapse of the
adhesive structure, thereby slowing the narrowing of the gap spacing and improving
controllability during the grinding process. When particulates are used, they should
preferably be of a non-magnetic material such as silica, so they do not saturate
magnetically during circuit operation.
After the gap spacing has been adjusted to provide the core 118 with the
desired effective permeability, in step 220, the adhesive is cured while the gap spacing
is maintained. Curing is performed according to the instructions of the adhesive
manufacturer, and may involve, for example, placing the assembly in a 170°C oven
for 10 to 15 minutes, or applying RF heating. Preferably, virtually no curing takes
place during the steps 216 and 218 of observing and adjusting, but in another
embodiment, partial curing during these steps can be accommodated. The curing
process typically increases the permeability of the core 118 by approximately 10%,
but the curing step does not significantly impact the precision of the device
manufacturing process because the permeability increase during cure is predictable.
Instead, the desired effective permeability targeted in steps 216 and 218 is merely
reduced by the known percentage increase that will take place during the curing step
220.
In step 222, the process is complete. It will be appreciated that not only has
the process overcome the tolerance degradation that takes place during a conventional
assembly process, but has also corrected for imprecision in the permeability of the raw
core portions 110 and 112 as originally manufactured. The process permits magnetic
devices to be tuned over a wide range, the upper limit being essentially the same
permeability as that which the core 118 would exhibit when tightly clamped together
without adhesive.
As mentioned above, in the embodiment of Fig. 1, the adhesive 150 is applied
only to the rim surface 132 of one of the core portions 112. As used herein,
application of adhesive to one mating surface can be performed either directly or
indirectly, for example by depositing adhesive onto the corresponding mating surface
of the opposite core portion and then mating the two together. Advantageously (but
not necessarily), no adhesive is applied to any mating surface that comes into close
proximity with the coil or bobbin 114, so as to prevent any accidental bonding
between the core 118 and the coil or bobbin 114. Otherwise the temperature
coefficient of expansion of the combined structure might be indeterminate, thereby
introducing an uncertainty into the circuit which may not be desired. Instead, one end
142 or 144 of the bobbin 114 can be adhesive bonded if desired to the inside end
section of the corresponding core portion 110 or 112. Of course if the device is
simply an inductor having a single central core member (i.e. the core has only one
"leg"), then the leg supporting the adhesive is the same as the leg encircled by the coil.
Although the specific EP-13 core shape is illustrated in the example of Fig. 1,
it will be appreciated that the invention can be used with a wide variety of symmetric
and asymmetric core shapes. For E-cores, for example, if the bobbin is placed on the
center leg, then the adhesive can be applied to the end surfaces of the two outer legs.
For E-cores, cylindrical cores and the EP-13 core illustrated in Fig. 1, the surface(s)
supporting the adhesive exists on opposite sides of the leg that is encircled by the coil.
The substantially equal resistance exerted by the adhesive on both sides of the central
leg therefore maintains the two mating surfaces of the central leg in substantially
parallel planes. It is desirable, but not essential, that these two planes remain
substantially parallel to avoid local saturation of part of the core. In some
embodiments this may be difficult but not impossible to achieve during the grinding
process. For example, a U-core, which has two legs on each core portion, may
receive the adhesive on the end surface of one leg and receive the bobbin on the other
leg. In this case, the mating surfaces inside the bobbin tube can be maintained in
substantially parallel planes by applying the grinding force to the two core portions
substantially co-axially with the legs supporting the adhesive. Other accommodations
will be apparent for other kinds of core shapes.
It will also be appreciated that the invention can be used even where not all
of the mating surfaces are co-planar with each other. In the EP-13 core illustrated in
Fig. 1, for example, the invention can still be used even if the center post 120 or 130
of one or both of the core portions 110 and 112 has been shortened to create an
intentional air gap between them. As previously mentioned, equipment currently
exists to achieve very small permeability tolerance in this situation, but the
permeability tolerance can be improved even further through the use of the invention.
Fig. 4 is a perspective view of a magnetic device illustrating another aspect of
the invention. It comprises first and second core portions 410 and 412, held together
in an assembly 418 by a clamp 414. The coil is internal to the assembly of Fig. 4,
wound on a bobbin supported on a central leg similarly to the arrangement shown
unassembled in Fig. 1. A microgap spacing 416 between the mated surfaces of the
core assembly 418 is maintained by particulate matter disposed in the gap 416. Unlike
Fig. 1, however, there is no adhesive in the gap 416. Instead, the gap spacing is fixed
by the particulate matter resisting against sustained force provided by the clamp 414,
urging the two core portions 410 and 412 toward each other.
The device of Fig. 4 can be made by the same process as that set forth in the
flow chart of Fig. 2, except that the particulate matter is applied to the mating surface
in step 212 without adhesive, and the step 220 of curing the adhesive is replaced by
a step of clamping the two core portions together to apply sustained force urging the
two core portions toward each other.
As used herein, a given signal, event or value is "responsive" to a predecessor
signal, event or value if the predecessor signal, event or value influenced the given
signal, event or value. If there is an intervening processing element, step or time
period, the given signal, event or value can still be "responsive" to the predecessor
signal, event or value. If the intervening processing element or step combines more
than one signal, event or value, the signal output of the processing element or step is
considered "responsive" to each of the signal, event or value inputs. If the given
signal, event or value is the same as the predecessor signal, event or value, this is
merely a degenerate case in which the given signal, event or value is still considered
to be "responsive" to the predecessor signal, event or value. "Dependency" of a given
signal, event or value upon another signal, event or value is defined similarly.
Also as used herein, movement of two components "relative" to each other,
and movement of one component "relative" to another, does not imply any
restrictions about which component is moving relative to any absolute. In other
words, a statement that component A moves relative to component B is intended to
include all of the following possibilities and to not select among them: that component
A is stationary and component B is moves; that component B is stationary and
component A moves; and that component A moves and component B move
differently from component A.
The foregoing description of preferred embodiments of the present invention
has been provided for the purposes of illustration and description. It is not intended
to be exhaustive or to limit the invention to the precise forms disclosed. Obviously,
many modifications and variations will be apparent to practitioners skilled in this art.
For example, whereas the embodiments described above involve ferrite cores, it will
be appreciated that cores made of other materials, even amorphous materials, can also
benefit from the invention. Nor must the core material have high permeability; low
permeability cores can be used as well. In addition, whereas the above-described
embodiments use only two core portions to assemble an entire core, in another
embodiment, three or more core portions can be used. In such an embodiment the
adhesive can be applied in between any one or more of any pair of the mating surfaces
of any of the core portions, depending on requirements. Furthermore, and without
limitation, any and all variations described or suggested in the Background section of
this patent application are specifically incorporated by reference into the description
herein of embodiments of the invention. The embodiments described herein were
chosen and described in order to best explain the principles of the invention and its
practical application, thereby enabling others skilled in the art to understand the
invention for various embodiments and with various modifications as are suited to the
particular use contemplated. It is intended that the scope of the invention be defined
by the following claims and their equivalents.