[0001] The present invention relates to a transformer of the so- called SMT (switch mode
transformer) type, particularly for employ as a basic component for the proportional
control of static commutating elements.
[0002] Granted that by "proportional control" one normally understands the condition under
which the electronic control components associated with the primary winding of the
transformer operate at a substantially constant load current/ /control current ratio,
it is known that for satisfactory functioning of the entire circuit an SMT transformer
has to ensure a reliable separation of the circuits while presenting a minimum parasitic
inductance between its primary and secondary windings.
[0003] The circuit separation, required for safety reasons, is obtained by maintaining a
substantially loose coupling between the windings which are usually formed of insulated
copper wire. The loose coupling is at variance, however, with the requirement to minimize
the parasitic inductance in the transformer so as to permit a rapid signal transmission
between the windings free of spurious components and to thus ensure the correct control
of the circuit associated to the transformer.
[0004] For minimizing the occurrence of parasitic, inductance in spite of the above noted
loose coupling (required for safety reasons, as explained above) it is common practice
to provide SMT transformers of the type under discussion with a "closed" ferrite core
having an internal center core portion and an outer shield substantially enclosing
the transformer. In this manner the magnetic flux always circulates within the ferrite
core substantially without having to cross air gaps even externally of the transformer.
[0005] In practice, the closed core is usually formed of two components connected to one
another in a suitable manner and necessarily formed with windows for the passage of
the transformer terminals. These windows are basically undesirable and therefore kept
as small as possible.
[0006] It is obvious that SMT transformers having a closed core present considerable difficulties
as regards the formation of the windings, and in particular as regards the assembly
of the.various components. These difficulties are such as to render the full automatization
of the manufacture and assembly of the transformers impossible with all the resultant
consequences with respect to productivity and economy.
[0007] It is therefore an object of the present invention to provide an SMT transformer
of the type defined in the introduction, which present optimum safety and functional
characteristics and permits its manufacture to be substantially simplified and readily
automatized.
[0008] According to the invention, this object is attained in an SMT transformer for proportional
control of static commutating elements, comprising at least two primary windings and
at least one secondary winding, by providing that the primary windings and the secondary
winding(s) are formed respectively of metal strips wound, with interposition of an
insulating film, on at least one portion of a per se known ferrite core of the open
type.
[0009] The characteristics and advantages of the invention will become more clearly evident
from the following description of an exemplary embodiment with reference to the accompanying
drawing, the only figure of which shows a partially sectioned perspective view of
a preferred embodimetn of a transformer according to the invention.
[0010] The SMT transformer shown substantially comprises at least two primary windings 1
and at least one secondary winding 2 formed, in contrast to the windings of a conventional
transformer, of a metal strip (3) preferably of aluminum, superimposed on an insulating
film. This technique for the formation of windings is per se known for instance from
the field of coil-wound capacitors and from the manufacture of certain types of transformers,
but has never as yet been employed in the manufacture of SMT transformers of the type
under discussion, mainly because of the practical impossibility of forming a metal
strip winding in combination with the use of a core of the "closed" type. As ex- plaiend
above, the closed core has formerly been thought to be indispensable in an SMT transformer
for proportional control in order to minimize parasitic inductance and to thus ensure
a satisfactory overall functioning.
[0011] According to a fundamental aspect of the invention, the metal strips 3 constituting
the primary windings 1 and secondary winding 2 are wound in a concentric arrangement
with windings 4 of an insulating film interposed therebetween, on at least one portion
(not visible in the drawing) of a ferrite core 5 of the open type.
[0012] In the embodiment shown in the drawing, ferrite core 5 comprises two substantially
E-shaped portions 6 and 7 connected to one another in a mirror-symmetric arrangement,
with the windings 1, 2 supported on the central legs.
[0013] The ferrite core may obviously have other shapes, for instance a C-shaped configuration.
In each case, the employ of an open core greatly simplifies the assembly of the various
components of the transformer permitting such assembly to be readily and completely
automatized.
[0014] As generally known, the employ of an open ferrite core in an SMT transformer of the
type under discussion tends to increase the occurrence of parasitic inductance to
a degree unaccceptable for the stated applications. In the transformer according to
the invention, this phenomenon is unexpectedly compensated by the employ of metal
strips for forming the windings.
[0015] As in fact found in practical experiments, the metal strips permit the formation
of a more compact winding as compared to a conventional winding of copper wire, with
a reduced dispersion of the magnetic flux, so as to compensate in optimum manner the
hypothtical functional disadvantages deriving from the employ of an open core.
[0016] The transformer according to the invention may of course be modified in any suitable
manner within the scope of the basic characteristics of the present invention.
An SMT transformer for proportional control of static commutating elements, comprising
at least two primary windings and at least one secondary winding, characterized in
that said primary windings (1) and said secondary winding(s) (2) are formed respectively
of metal strips (3) wound, with the interposition of an insulating film, on at least
one portion of a per se known ferrite core (5) of the open type.
