EP1054416A1

EP1054416A1 - Process for manufacturing transformers, in particular transformers for battery chargers and transformers obtained with said process

Info

Publication number: EP1054416A1
Application number: EP99830306A
Authority: EP
Inventors: Pasquale Forte
Original assignee: Eldor Corporation SpA
Current assignee: Eldor Corporation SpA
Priority date: 1999-05-18
Filing date: 1999-05-18
Publication date: 2000-11-22

Abstract

A process for manufacturing transformers, in particular miniaturized transformers for battery chargers, provides the steps of associating with a support frame (3), a primary winding (2) arranged to receive electricity supply and a secondary winding (6) insulated from the primary winding and arranged to supply electricity. Once a core of ferromagnetic material (7) passing through the cavities defined inside the primary and secondary windings has been associated with the support structure, an outer protection structure (11) is made for the frame, windings and core which is associated with the structure of the transformer itself while it is being made.

Description

The present invention relates to a process for manufacturing transformers, in particular transformers for battery chargers, and a transformer obtained by said process.

Preferably, transformers in accordance with the invention are of a miniaturized type with insulation from the mains and power lower than 15 watt.

Their main use is as battery chargers for electric devices such as cellular or cordless telephones.

It is known that transformers for battery chargers are manufactured starting from a base structure of plastic material defined by a pin-carrying bobbin.

Practically the pin-carrying bobbin is made up of a first and a second side shoulders centrally joined by a tubular portion.

During the transformer manufacturing step a primary winding, of enamelled copper for example, is wound around the tubular portion and its electric terminals are connected with the respective pins carried by one of the two shoulders.

Subsequently the primary winding is insulated by superposition of one or more strips of a tape having appropriate insulating features.

Then a secondary winding is added on top of the primary winding still on the tubular portion and the respective electric terminals are connected with the pins carried by the other shoulder.

At this point closure of the magnetic circuit is carried out by positioning an appropriate core of ferromagnetic material in the structure.

The core is generally made of two identical halves having an E-shaped conformation, which are positioned on the pin-carrying bobbin in such a manner that the central leg of both E's is fitted inside the tubular portion on which the primary and secondary windings are located.

In detail, once in place, the core appears to be formed of three columns only the central one of which is surrounded by the primary and secondary windings.

In order to keep the two core halves in place, the end surfaces of both of them intended to come into contact with each other are coated with appropriate adhesive materials.

In addition, in order to ensure a better structural holding, soft glue is injected into the hollow space of the tubular portion of the bobbin.

Transformers of this typology however have caused arising of some problems, which are all substantially connected with the increasing requirement of miniaturizing the component itself.

In fact, for the purpose of observing the safety rules in force, distances between the primary winding wires and the secondary winding wires are to be maintained higher than predetermined limit values, unless appropriate insulating materials are interposed therebetween.

In order to solve this drawback, according to a first known technique the transformer manufactured as briefly described above is inserted into a plastics cover having an upwardly turned cavity and such sizes that pins alone project from the overall dimensions for electrical connection.

The structure consisting of a cover and the inserted transformer is then put into appropriate furnaces in which the vacuum is created and the hollow spaces generated between cover and transformer are subsequently filled by casting with an epoxy resin until the cover is almost completely filled up.

The presence of the vacuum has the function of ensuring correct filling of the hollow spaces and the absence of air bubbles or others.

The epoxy resin is then allowed to polymerize by an appropriate heat and time cycle and the finished product thus made enables transformers of small sizes to be obtained in which distances between the primary winding and secondary winding wires can be relatively small because a further insulation supplied by the resin is present.

However, a transformer manufactured following this process has many drawbacks.

Firstly, it is to note that for obtaining a correct polymerization of the epoxy resin a controlled cooling of the resin itself over a time period of many hours (12-16 hours) is required.

In addition, control of the heat cycles is necessary in order to avoid arising of internal stresses or possible cracks in the material that could cause tests to be carried out on the finished device, mechanical strength tests for example, not to be overcome.

On the other hand, since furnaces in which the vacuum is to be created are to be employed, there is an important increase in costs connected with the machineries to be used, which makes the transformer insulating operation rather expensive.

In order to overcome at least partly these drawbacks, a second transformer typology has also become widespread.

These second-type transformers are of very reduced sizes and, in order to comply with the safety rules concerning distances between the primary and secondary windings, they use an electric wire provided with a three-layered insulation for the secondary winding.

Practically the two first layers supply the required appropriate insulation and the third layer acts as a safety layer to avoid discharges or sparks between the two windings being generated during operation.

Use of this wire typology makes insulation by the epoxy resin no longer necessary, thereby eliminating the polymerization and heat control steps, so that the production rates are greatly increased.

However, this second transformer typology as well has proved to suffer from many and serious drawbacks.

Firstly it is to note that the ferrite core is no longer surrounded by, and incorporated into any insulating structure, so that some powdered material detaching from the core may be scattered inside the device and cause shortcircuits and consequently malfunctions or breaking of the device itself.

In addition, the structural holding between the two halves forming the core is ensured by the only presence of adhesive materials.

Since the glued surfaces are minimum, in case of an insufficient amount of glue or if the glue is not conveniently positioned, the core may have problems connected with vibrations or separation of the parts, as well as problems connected with leakage of the magnetic flux, these parameters greatly affecting the final transformer efficiency.

It is finally to note that the wire provided with a three-layered insulation is very expensive, its cost being much higher than the usual cost of a copper wire.

Therefore the present invention aims at substantially solving all the above mentioned drawbacks.

It is a fundamental object of the invention to provide a process for manufacturing transformers, and in particular transformers of small sizes in which observance of the safety rules is made possible by suitably insulating the primary winding from the secondary winding without, on the other hand, involving high production times and high costs for process control.

It is a further object of the invention to avoid gluing of the adjacent core portions of ferromagnetic material while obtaining a correct positioning and a structural steadiness of the core itself.

The invention also aims at avoiding, in case of core breaking or damage, shortcircuits resulting from scattering of the ferromagnetic material within the electric device.

It is therefore a further object of the invention to manufacture a transformer which is structurally strong and capable of withstanding even severe mechanical efforts.

The foregoing and further objects that will become more apparent in the course of the present description are substantially achieved by a process for manufacturing transformers, in particular transformers for battery chargers, in accordance with the features set forth in the appended claims.

Further features and advantages will be best understood from the detailed description of a preferred but non exclusive embodiment of a transformer manufactured in accordance with the process of the present invention. This description will be taken hereinafter with reference to the accompanying drawings, given by way of nonlimiting example, in which:

Fig. 1 is a perspective view of a transformer manufactured by a process in accordance with the present invention;
Fig. 2 shows the whole transformer seen in Fig. 1 with the outer protection structure in chain line;
Fig. 3 is a perspective view of a lower mould half arranged for carrying out the process in accordance with the present invention;
Fig. 4 is a cross-sectional view of a forming mould incorporating a transformer before injection of the plastic material;
Fig. 5 is a cross-sectional view similar to the one shown in Fig. 4, but obtained along an axis rotated through 90° relative to the preceding one; and
Fig. 6 is a perspective view of an upper mould half arranged for carrying out the process in accordance with the present invention.

With reference to the drawings, a transformer obtained by the process being the object of the invention has been generally denoted by 1.

As one can see from Fig. 2, the inner structure of transformer 1 consists of a support frame 3, preferably of plastic material, manufactured by moulding and having two side portions 3a, 3b joined together by a tubular central portion 3c.

Associated with each of the two side portions 3a, 3b is a first and a second series of

conductor pins

4, 5 fastened to the support frame at a lower portion thereof.

Frame 3 practically defines a pin-carrying bobbin on which a primary winding 2 is wound, which primary winding is set in electrical connection with the first series of pins 4 and directly wound on the tubular central portion 3c.

The primary winding 2 is generally made using an enamelled copper wire and is intended for receiving electricity supply from the domestic network.

One or more layers of insulating material, a wound-up tape of an appropriate material for example (not shown in the accompanying figures) are present on the upper surface of the primary winding.

A secondary winding 6 is present on top of the primary winding 2 and the insulating layer and it is electrically insulated from the primary winding 2 and connected to the second series of pins 5, said secondary winding being arranged for supplying said pins with electricity.

The magnetic circuit is completed with the presence of a core of ferromagnetic material 7 having at least one leg 7a, in particular the central leg, arranged to be fitted at least partly into a cavity 8 defined by the primary and secondary windings of the tubular portion of frame 3.

The ferromagnetic material core 7 is divided into two

halves

9, 10, of which at least one first body 9 has the mentioned projecting leg 7a.

In the embodiment herein shown the ferromagnetic material core is made up of two

identical halves

9, 10 substantially having an E-shaped conformation.

The two

bodies

9, 10 are positioned on the support frame 3 in such a manner that they have corresponding end faces 9a, 10a facing each other (see Figs. 4 and 5), so as to define a final three-column structure of the core.

In particular, the central column is fitted into the cavity 8 defined by the tubular central portion 3c and around which the primary 2 and secondary 6 windings are wound.

The transformer is also provided with an outer protection structure 11 for the frame 3, windings 2, 6 and core 7 from which only the first and second series of

pins

4, 5 project (Fig. 1).

From a construction point of view, the process for manufacturing the transformers briefly described above is as follows.

Firstly the support frame of plastic material is made, by moulding for example. Associated with the thus obtained frame is first the primary winding 2 around the tubular central portion 3c, then the insulating layer and afterwards the secondary winding 6.

In general, during the support frame moulding step the first and second series of

conductor pins

4, 5 are provided to be rigidly associated with the frame itself.

Then welding of the terminals of the primary 2 and secondary 6 windings to the

corresponding pins

4, 5 is carried out so as to accomplish electric connections of the transformer. It is to note that the primary and secondary windings are electrically insulated from each other.

Then the ferromagnetic core 7 is associated with the frame provided with windings and in more detail the first and second

E-shaped bodies

9, 10 are positioned in such a manner that at least the central column of the core is fitted into the tubular central portion 3c.

At this point the outer protection structure 11 for the frame 3, windings 2, 6 and core 7 is made, and said protection structure is associated with the previously assembled transformer.

Advantageously, the steps of manufacturing and associating the outer protection structure are carried out simultaneously during one and the same step of the manufacturing process.

Practically the bobbin with the different elements associated therewith is positioned within a mould half of the type shown in Fig. 3 so that the first and second series pins 4, 5 are fitted at least partly in corresponding cavities 12 present in the inner surface of the mould half.

Then the mould is closed with an upper mould half (shown in the accompanying figures only in section; Figs. 4, 5) so that the transformer is housed in a cavity 13 conforming in shape to the outer protection structure 11 to be made.

Under this situation a plastic material intended for completely filling all hollow spaces generated between the mould and frame is introduced, preferably injected, into the mould. This material is injected into the mould through channels 14 highlighted in Fig. 3 and generally consists of a thermoplastic material in the liquid state, a polycarbonate or polyethylene terephthalate for example.

Prior to the plastic material injection step, a step of mechanically locking the first and

second bodies

9, 10 defining core 7 to the desired position is also provided;

bodies

9, 10 are such arranged that the contact surfaces 9a, 10a of same are exactly superposed on each other.

In detail, the mechanical-locking step is obtained by use of pushers 15 which operate by moving the first and second bodies along a mutual approaching/moving apart direction, as shown by arrows 16 in Fig. 5.

As viewed from Figs. 3 and 4, the mould has appropriate guides defined by through holes 17 to enable pushers 15 to reach the first and

second bodies

9, 10 defining the ferromagnetic material core 7 once the mould has been closed.

These pushers 15 are fitted into holes 17 before injection of the thermoplastic material, in order to ensure a correct positioning of the core the position of which will be subsequently maintained by the thermoplastic material itself, once hardened.

On the other hand, in order to keep the first and

second bodies

9, 10 defining the core conveniently aligned, a given number of support elements 18 operating in a plane parallel to the approaching/moving apart direction 16 of the first and second bodies is provided.

These support elements can be actuated along the direction of arrows 20 (Fig. 4) orthogonal to direction 16 and, once in place within the mould, have the function of supporting the core 7 halves in a plane on which they are moved close to each other.

In fact a mould half has through cavities 19 arranged to receive the support elements 18 used for ensuring matching of the contact surfaces 9a, 10a of the first and second bodies defining the ferromagnetic material core.

Once the thermoplastic material has hardened (this hardening process having a duration in the order of ten seconds), the mould is opened and the thus made transformer is removed therefrom.

It is finally to note (see in particular Figs. 2, 4 and 5) that the first and second series of conductor pins 4, 5 are only partly inserted in the cavities present in the mould halves. This aims at enabling electrical connection of the transformer with external circuits but, at the same time, at ensuring better structural engagement of the pins with the complete structure of the transformer itself.

In fact, a free portion 4a, 5a of these pins that is not directly engaged with the support frame 3 is coated with thermoplastic material and therefore the whole structure of the frame and pins is stiffened.

The invention achieves important advantages.

In fact a transformer of small sizes manufactured with a process in accordance with the description of the present invention enables an important material saving essentially resulting from the possibility of reducing the overall sizes of the device to a great extent.

The manufacturing process is greatly streamlined and speeded up, as long times connected with possible polymerizations of the materials and others are no longer required.

The obtained transformer is a very strong product, capable of overcoming severe mechanical-strength tests and in addition, if the core of ferromagnetic material should break, no scattering of the powder within the device would occur and therefore the risk of possible shortcircuits is avoided. In addition, due to the presence of the appropriate insulating material occupying all hollow spaces defined by the transformer in the mould, all safety rules can be observed while enabling the transformer miniaturization to become increasingly more marked.

It is finally to point out that the particular positioning procedure of the ferromagnetic material core allows any type of gluing operation to be avoided, because the transformer structural holding is ensured by the thermoplastic material.

Thus, all problems resulting from the minimum contact surfaces and therefore possible problems of vibrations or stray magnetic flux are avoided.

Claims

A process for manufacturing transformers, in particular transformers for battery chargers, comprising the following steps:

arranging a support frame (3);

associating a primary winding (2) arranged for receiving electricity supply with the support frame (3);

associating a secondary winding (6), electrically insulated from the primary winding (2) and arranged for supplying electricity, with the support frame (3);

associating a core of ferromagnetic material (7) with the support frame (3), said core (3) passing through cavities (8) defined internally of the primary and secondary windings,

characterized in that it further comprises the steps of:

making an outer protection structure (11) for the frame (3), windings (2, 6) and core (7); and

associating the protection structure (11) with the frame, windings and core, the steps of making and associating the protection structure (11) with the frame, windings and core being carried out simultaneously.
A process for manufacturing transformers as claimed in claim 1, characterized in that it further comprises the steps of:

engaging a first and a second series of conductor pins (4, 5) with the support frame (3);

carrying out electrical connection of the first series pins (4) with the primary winding (2); and

carrying out electrical connection of the second series pins (5) with the secondary winding (6).
A process for manufacturing transformers as claimed in anyone of the preceding claims, characterized in that the step of associating the ferromagnetic material core (7) comprises the sub-steps of:

arranging a first body (9) defining part of the ferromagnetic material core (7) and having at least one projecting leg (7a);

arranging a second body (10) capable of defining the core (7) together with the first body (9);

positioning the first body (9) on the frame (3) by fitting the projecting leg (7a) at least partly into the cavity defined by the primary (2) and secondary (6) windings; and

positioning the second body (10) on the frame (3) to define the whole structure of the core (7).
A process for manufacturing transformers as claimed in anyone of the preceding claims, characterized in that the steps of associating a primary winding (2) and a secondary winding (6) are successive in time, said secondary winding (6) being wound on top of the primary winding (2) after an intermediate step of insulating the primary winding from the secondary one.
A process for manufacturing transformers as claimed in anyone of the preceding claims, characterized in that the step of making and associating the outer protection structure (11) comprises the sub-steps of:

positioning the frame (3) with the windings (2, 6) and core (7) associated therewith, inside a forming mould conforming in shape to the outer protection structure to be made;

introducing, and preferably injecting, a plastic material into the mould, which material is intended for filling the hollow spaces generated between the mould and frame; and

removing the transformer (1) from the mould.
A process for manufacturing transformers as claimed in claims 2 and 5, characterized in that the sub-step of positioning the frame (3) with the windings (2, 6) and core (7) associated therewith, involves an at least partial insertion of the first and second series of pins (4, 5) into corresponding cavities present in an inner surface of the mould.
A process for manufacturing transformers as claimed in claims 3 and 5, characterized in that the sub-steps of positioning the frame with the windings and core associated therewith, involves the step of mechanically locking the first and second bodies (9, 10) defining the core to a desired position, said locking being carried out before the step of introducing the plastic material and being at least partly maintained during said step.
A process for manufacturing transformers as claimed in claim 7, characterized in that the mechanical-locking step is obtained by means of pusher means (15) operating along a mutual approaching/moving apart direction (16) of the first and second bodies.
A process for manufacturing transformers as claimed in claim 8, characterized in that the positioning sub-step comprises the further step of keeping the first and second bodies defining the core in alignment with each other by use of support elements (18) movable along a direction (20) transverse to that of the pusher means (15) and operating in a plane parallel to the approaching/moving apart direction (16) of the first and second bodies.
A transformer, in particular for battery chargers, manufactured with a process in accordance with anyone of the preceding claims.