[0001] The present invention relates to a so-called SMT transformer (Switch Mode Transformer),
to be used in electronic switch-mode power supply circuits for picture tubes, for
instance, or the like.
[0002] Preferably, SMT transformers are arranged substantially as disclosed in US-A-3 617
854, i.e. with at least a primary winding and at least a secondary winding consisting
of a coil of relevant metal bands which are wound with the interposition of an insulating
film. The coil including the windings is then fitted about a leg of a ferrite core.
[0003] In contrast with the transformers which are used to handle driving signals, the core
of a SMT transformer is a substantially closed core. In fact, even though such a core
is sometimes incorrectly referred to as an "open" core in order to distinguish it
over the complex cores including an inner core portion and an outer core cap surrounding
the transformer, it actually consists of two identical E-shaped or U-shaped portions
which are specularly fitted together to form a substantially closed magnetic circuit.
The coil including the windings is arranged about one of the legs of the core which
are defined by aligned pairs of legs of the E-shaped or U-shaped portions. Furthermore,
in order to limit the undesired phenomenon of saturation, the leg of the core which
is surrounded by the windings is provided with a central air gap, which however must
have a short length. As it is known, in fact, during operation magnetic losses in
the material of the core occur mainly in correspondence of such a gap, said losses
causing eddy currents to flow in the core with the result of the whole transformer
being undesirably heated up. In order to keep the consequences of such heating at
a minimum, it is necessary to size the various parts of the transformer in a critical
way, particularly taking care of the thermal insulation.
[0004] At any rate, the core consists of two specular portions which require particular
working steps to be at first manufactured as separate parts and then assembled together
with the windings, with the result of a relatively complicated and expensive manufacture
of the whole transformer.
[0005] It is the main object of this invention to provide a SMT transformer which, while
it has the same performances, has a particularly simple and compact construction and
may be readily assembled.
[0006] Another object of this invention is to provide a SMT transformer of the kind mentioned
above in which undesired heating phenomena caused by magnetic losses are kept at a
minimum.
[0007] These objects are attained, according to the present invention, in a SMT transformer
embodying the features recited in the appended claims.
[0008] The characteristics and advantages of the invention will be more clearly understood
from the following description, given by way of non-limiting example, with reference
to the attached drawings, in which:
Fig. 1 shows a cut-away perspective view of a preferred embodiment of the transformer
according to the invention; and
Fig. 2 shows a side view of the core of the transformer as in Fig. 1, in which the
magnetic flux lines during operation of the transformer are diagrammatically illustrated.
[0009] With reference in particular to Fig. 1, the SMT transformer according to the invention
comprises at least a primary winding 3 and at least a secondary winding 4 formed by
relevant metal bands 5 (made of aluminium, for instance) superimposed on insulating
film 6. Substantially as described in the above-mentioned US-A-3 617 854, metal bands
5 and insulating film 6 are arranged (preferably concentrically) about a portion of
a magnetic core 7, for example made of ferrite, so that the primary and secondary
windings form a coil 8 in which the core is inserted.
[0010] According to an aspect of the invention, and in contrast with the SMT transformers
already known, the core 7 only consists of one open, single-piece magnetic element
provided with a first leg 9 about which is arranged the coil 8, as well as with at
least a second leg 10 which is connected to the first leg through a crosspiece 11
of the core itself, so that the core has a substantially U-shaped configuration. In
the example herein described, however, the magnetic core 7 preferably includes two
of the said second legs, both illustrated at 10 and provided at opposite sides of
the first leg 9, so that the core 7 has a substantially E-shaped configuration. In
any case, with respect to the magnetic field produced during operation by the windings
3 and 4, the free ends of each leg 10 are separated from the free end of the leg 9
by relevant air gaps 12.
[0011] As it will be more apparent hereinafter, an end face of the coil 8 is preferably
disposed adjacent to the crosspiece 11 of the core 7, while the gap, or gaps 12 extend
substantially at the side of the coil which is opposite to said end face.
[0012] In a per se conventional manner, the transformer also comprises a plurality of electric
contact pins 13 and a shaped support frame 14 which is preferably located at the the
side of the coil 8 opposite to the crosspiece 11 of the magnetic core 7. The various
elements may be mounted in place in the most convenient way, as the case may be.
[0013] The constructive simplicity of the SMT transformer according to this invention is
apparent: the transformer comprises only one single-piece magnetic core 7 which may
have an elementary configuration and thus may be readily and quickly assembled together
with the windings 3 and 4, and further requires less material to be used and also
does not involve any critical step for assembling the various elements, as it would
be the case of specular magnetic elements to be coupled by glueing them together with
special materials.
[0014] Anyway, it will be apparent to those skilled in the art that the switch mode transformer
according to the present invention is arranged in contrast with the technical prejudice
that the length of the gap provided in the magnetic core must be as short as possible.
It is known, in fact, that in the conventional SMT transformers the inductance values
of the windings depend not only on the characteristics of the material of which the
magnetic core is made, but also on the dimensions of the gap provided in the leg of
the core which bears the coil formed by the windings. In particular, such inductance
values are inversely proportional to the length of the gap.
[0015] Contrary to the above-mentioned technical prejudice, according to the present invention
the magnetic core 7 actually is an open core, the gaps 12 having a remarkable length
and the leakage flux being particularly high in correspondence of the free ends of
the legs 9 and 10.
[0016] It was experimentally found, however, that the SMT transformer according to this
invention can provide, under the same general dimensional conditions, performances
which are at least as good as those provided by a conventional SMT transformer.
[0017] Although the phenomenon not yet is known in detail to the applicant, it was in fact
experimentally found that the flux lines 15 of the magnetic field generated by the
windings 3 and 4 extend substantially as illustrated in Fig. 2, with a plurality of
closed paths, extending between each couple of legs 9 and 10, which are curvilinear,
mutually offset and enclosed into one another. Thus, a plurality of elementary magnetic
circuits are established in parallel between each couple of legs 9 and 10 of the open
core 7.
[0018] More particularly, the density of the flux lines 15 has a maximum value at the base
of the leg 9 (connected with the crosspiece 11) and decreases along the leg 9 to reach
a minimum value in correspondence of the gaps 12.
[0019] Hence, the magnetic flux leakages occurring in correspondence of the gaps 12 have
an effect which is limited thanks to the low density of the magnetic flux itself;
as a consequence, as it was experimentally found, in the SMT transformer according
to the invention the core heating phenomenon caused by induced currents is advantageously
reduced; in practice, it is perceptible only in correspondence of the base of the
leg 9 of the core 7.
[0020] In addition, since the windings 3 and 4 are formed using relevant metal bands - and
have, therefore, an even turn-to turn coupling - each turn of the windings is subject
to the whole density variation of the magnetic flux lines which are linked therewith,
said density decreasing, as already stated, from the base to the free end of the leg
9.
[0021] In other words, this means that, whereas the specific inductance A1 is constant in
a conventional transformer, in the SMT transformer according to the invention it changes
along the leg 9 of the magnetic core 7; in particular, the specific inductance A1
decreases from the base to the free end of the leg 9 bearing the coil 8 formed by
the windings. Therefore, bearing in mind that the inductance L of the windings of
the transformer is given by the known formula: L = N x N x A1, wherein N is the number
of turns of the windings, also the inductance L of the windings 3 and 4 of the SMT
transformer according to the invention decreases, along the axis of the coil 8, from
the crosspiece 11 of the core 7 to the free end of the leg 9.
[0022] In conclusion, in the SMT transformer according to the invention the inductance of
the windings is in practice independent of the dimensions of the gap, or gaps 12,
which are preferably spaced apart from the coil 8, anyway; on the contrary, the inductance
depends on the height H of the wound metal bands and on the axial position of the
coil 8 with respect to the leg 9 of the magnetic core. More particularly, the shorter
the height H of the relevant metal bands and the closer the coil 8 to the base of
the leg 9, the higher is the value of the inductance L of the windings 3 and 4. Indeed,
it is a particularly surprising effect, which however was found experimentally, as
already stated.
[0023] The consequent advantages are apparent; with the present invention, in particular,
it is posible to provide a SMT transformer the windings 3 and/or 4 of which have an
optimum inductance value while they are formed with metal bands having a reduced height
and wound with a relatively low number of turns.
[0024] As a result, the SMT transformer according to the invention can be made with a remarkably
compact construction, using a reduced amount of material for manufacturing the magnetic
core 7 and the windings 3 and 4.
[0025] Of course, the SMT transformer described above may undergo many modifications without
departing from the scope of the invention. For example, according to the desired dimensional
and operational characteristics, the axial position of the coil 8 with respect to
the support leg 9 of the magnetic core may be changed, or the leg 9 and/or 10 of the
magnetic core may be differently shaped, for instance with a different cross-section
and/or inclination. Moreover, the various primary and secondary windings may be mutually
offset axially, in a way known per se, as the case may be.