EP0165845A1 - Transformer with a strong primary-secondary coupling - Google Patents

Abstract

1. Electrical transformer comprising a first winding (2) coupled magnetically with a second winding formed by a conductive turn (1) of small thickness constituting a surface which forms an almost closed envelope which follows the surface of the said first winding (2), characterized in that the first winding (2) is a centrally hollow cylindrical coil and in that the surface of the second winding is a turn constitued : - by two concentric cylinders (1a, 1b) having radii such that the said coil (2) fits into the volume enclosed between the cylinders (1a, 1b), - and by an annular surface (3a) bearing on the edges of the cylinders (1a, 1b) at one extremity of these, - in that the annular surface (3a) is slotted along a radius and in that each of the cylinders is slotted along a generatrix passing through the slot (31) formed in the annular surface (3a).

Description

The present invention relates to transformers with high coupling. More particularly, it relates to transformers intended to be used in switching power supplies.

For reasons of gain in weight and volume of the magnetic components (transformer and inductance) and the filtering components (capacitor and filtering inductor), the operating frequency of the switching power supplies evolves towards higher and higher values. This development has been made possible by the development of high performance components and materials, in particular SCHOTTKY diodes for rectification, high voltage power MOS transistors for the cutter, very low series resistance capacitors for filtering and high frequency ferrites for transformers and inductances. Thus, in current switching power supplies, the switching frequency can now reach a few hundred kilohertz. It follows that it is necessary to design transformers having a very strong coupling to limit the voltage drop due to the leakage inductance between primary and secondary, to reduce the overvoltages due to switching, to improve the efficiency and to minimize electromagnetic radiation generated by the transformer.

There are a large number of transformers that are designed to achieve strong coupling. For example, French patent application No. 80 03978 describes an embodiment of plate transformers where a strong primary-secondary nesting allows excellent coupling. However, the high parasitic capacity and the characteristics of the necessarily thin insulator limit the application of this kind of transformers at moderate frequencies and powers. On the other hand, it can be shown that for rectified and filtered output voltages of less than 10 volts, and on condition of reaching switching frequencies of a few hundred kilohertz, it is possible to have only one turn in high school.

Transformers are known in the prior art, one of the windings of which consists of a single turn surrounding a magnetic core. However, these transformers do not have sufficient coupling to avoid electromagnetic radiation.

The invention therefore aims to remedy the aforementioned drawbacks of the prior art and relates to an electrical transformer comprising a first winding magnetically coupled to a second winding constituted by a turn characterized in that the turn is formed by a conductive element of small thickness constituting a surface which forms an almost closed envelope which follows the surface of said first winding.

• La figure 1 représente une vue éclatée en perspective d'un mode de réalisation du transformateur selon l'invention.
• La figure 2 montre une vue en perpective et en coupe selon un plan axial des spires en position sur un demi-noyau magnétique.
• La figure 3A représente une vue de dessus par rapport à la représentation de la figure 2 de la spire secondaire munie de connections électriques fixées sur cette spire.
• La figure 3B montre une coupe selon un plan axial d'une partie des éléments de la figure 3A.
• La figure 4 montre des variantes de réalisation d'une partie de la spire secondaire.
• La figure 5 montre le transformateur complet vu en coupe selon un plan axial.
• La figure 6 montre une section d'une variante de réalisation de l'enroulement primaire.
• La figure 7 montre un mode de réalisation particulier de la spire secondaire.
• La figure 8 montre un autre exemple de réalisation de la spire secondaire.
• Les figures 9A à 9D représentent différentes variantes de fabrication de la spire secondaire.
• La figure 10 représente en vue de dessus l'ensemble du transformateur muni d'ailettes de refroidissement.

Other characteristics will also appear in the following description, given by way of example only, and the attached drawings.

• FIG. 1 represents an exploded perspective view of an embodiment of the transformer according to the invention.
• Figure 2 shows a perspective view and in section along an axial plane of the turns in position on a magnetic half-core.
• FIG. 3A represents a top view with respect to the representation of FIG. 2 of the secondary turn provided with electrical connections fixed on this turn.
• Figure 3B shows a section along an axial plane of a part of the elements of Figure 3A.
• FIG. 4 shows alternative embodiments of part of the secondary turn.
• Figure 5 shows the complete transformer seen in section along an axial plane.
• Figure 6 shows a section of an alternative embodiment of the primary winding.
• Figure 7 shows a particular embodiment of the secondary turn.
• Figure 8 shows another embodiment of the secondary turn.
• FIGS. 9A to 9D represent different manufacturing variants of the secondary turn.
• Figure 10 shows a top view of the transformer assembly with cooling fins.

FIG. 1 represents an exemplary embodiment of a transformer adapted to a magnetic circuit constituted by two parts 6,7 in the shape of E mounted head to tail and with round central leg 6a, 7a. It is obvious that the same process could be used for any type of magnetic circuit, with a modification of the shapes of the secondary turn 1 and of the primary winding 2. The primary winding 2 is produced separately according to conventional techniques.

According to this embodiment, the secondary turn 1 is made from two cylindrical parts concentric 1a, 1b in copper for example, of small thickness and each provided with a slot 10a, 10b. These slots 10a, 10b are arranged according to a generatrix of these cylinders and placed opposite. At one end of these cylinders (not visible in Figure 1) is placed an annular piece 3a also of copper which allows to close on one side the volume between these two cylinders la, lb so as to form a bowl annular U-shaped section. The primary winding 2 has the shape of a cylindrical ring of the same height as the cylinders la, lb so as to be able to be housed in the space between the two cylinders la, lb. A second conductive annular piece 3 can be arranged so as to completely close the space including the primary winding 2. These annular pieces 3, 3a are also split according to the radial slots 31 opposite the slots 10a 10b made in the cylinders la, lb. Insulating washers 5,8 provided with a cylindrical part covering at least partially the legs 6a, 7a complete this assembly to provide electrical isolation between the magnetic core 6, 7 and the windings 1,2 of the transformer. Connection plates 4 are provided to ensure the electrical connection between the secondary 1 and the outside.

In order to allow the passage of feeder conductors 9 for the primary winding 2 and possibly for the connection of guard rings, openings 32 are made in one of the annular parts 3. The same is true for one of the insulating washers 5.8 which can also be split to facilitate mounting of the plates 4.

FIG. 2 represents a section of the assembly comprising the secondary coil 1, the primary winding 2 and a magnetic half-core 6. The annular part 3a constitutes the lower part of the bowl formed by the secondary turn 1. This element 3a makes it possible to produce the electrical and thermal junction between the inner part lb of the turn and its outer part 1a. The slots 10a, 10b formed in the inner and outer cylinders 1b of the coil 1 provide electrical insulation between the lips of these slots. The lips of the outer slot 10a are used as the output terminal of the transformer.

FIG. 3A represents a top view of the secondary turn 1 placed on the magnetic half-core 6. The plates 4 are fixed by welding to the lips of the slot lOa formed on the outer cylinder 1a. Advantageously, these plates can be produced in the form of a multilayer printed circuit, which allows the external circuits of use to be placed there directly.

Figure 3B shows another sectional view of this assembly and shows the coil 1 closed by the annular part 3 and the connection between the plates 4 and the outer cylinder 1a of the secondary coil.

Figure 4 shows various embodiments of the annular part 3. It is necessary that this annular part has an opening allowing in particular the passage of the wires 9 of the primary winding 2 contained inside the two cylinders. According to Figure 1, this annular part 3 can be a complete ring with the exception of a slot and several openings 32 in number equal to the number of wires that it is necessary to exit from the volume contained between the two cylinders. However, we can be satisfied with simplified embodiments where we simply practice a cutting in the ring 3. This cutting can take the form of a sector, a half ring or be obtained by cutting along a cord. The electrical connection between the inner cylinder and the outer cylinder of the secondary coil is provided by the remaining part of the ring 3.

Figure 5 shows a section of the transformer assembly once mounted. The two magnetic half-cores 6,7 are mounted end-to-end so as to completely close the magnetic circuit. The primary turn 2 is entirely contained between the two cylinders 1a, 1b of the secondary turn 1. There are also provided guard rings 11a, 11b serving to reduce the capacitive currents between the primary and the secondary. These cylindrical guard rings are interposed between the primary winding 2 and the cylinders 1a, 1b and are intended to be electrically connected to a point of the circuit with low voltage excursion by conductors also passing through the openings 32 of the annular part 3. The washers 5,8 isolate the coil 1 from the core 6,7.

FIG. 6 shows a section of a particular embodiment of the primary winding 2. This winding is produced using an insulating strip and a conductive metal strip and wound together on themselves so as to form a coil. In this case, the first or the last turn can serve as a guard ring.

FIG. 7 shows another embodiment of the secondary turn 1. This turn is formed as previously by a first external cylinder 1a. On the other hand, the inner cylinder has an external spiral groove 12 intended to receive the conductors of the primary winding. This embodiment makes it possible to obtain a tighter magnetic coupling.

FIG. 8 shows yet another embodiment of the secondary turn 1. The part is first executed internal cylindrical lb possibly provided with rings 3a, 3. Next, a strip of good conductive metal is used which is preformed to constitute both the outer cylinder 1a and the two connection plates 4. After having placed the primary winding on the internal cylindrical part 1b, the external cylinder 1a, provided with the plates 4, is assembled by welding it on the annular part or parts 3a, 3.

It is obvious that various processes can be used to manufacture this secondary turn. It can be produced for example by molding, stamping, stamping or machining. It is also possible to make the secondary turn by electrolytic growth on the insulating surface which surrounds the primary winding. In the latter case, masks will of course have been provided to provide the slots 10a and 10b.

In the case where the two cylinders 1a, 1b and one of the annular parts 3, 3a are manufactured in one piece, these elements of the secondary turn 1 can serve as a mold thus facilitating the setting and the manufacture of the insulation of the primary coil 2.

FIGS. 9a to 9d relate to other alternative embodiments of the secondary turn 1. Each of these figures represents only a section of the turn 1. The whole of the turn is deduced from the representation of these figures by rotation around the 'axis of revolution XX of the secondary turn.

According to FIG. 9a, the secondary turn 1 is produced from two cups lc, ld with a U-shaped section, which are placed end to end so as to enclose the primary turn.

FIG. 9b represents an embodiment close to the previous one but according to which the bowl lg particularly covers the bowl lh on its outer surface.

According to FIG. 9c, each cylinder 1a, 1b and one of the rings 3, 3a are made in one piece 1c, 1f.

According to Figure 9d, the inner cylinder 1b and the two rings 3, 3a are made in one piece li. In this case, the primary winding 2 must be wound directly on the inner cylinder 1b.

All these embodiments show that by its design, the transformer according to the invention allows simple manufacture of the secondary turn 1 as well as easy assembly and manufacture of the primary winding 2.

Figure 10 shows a top view of the transformer assembly. According to another advantageous characteristic, there are provided cooling fins 13 directly fixed, for example by welding or by molding, on the external surface of the secondary turn l. This embodiment provides maximum cooling of the transformer, which increases its operating power.

It should be noted that all of the embodiments of the secondary turn described above make it possible to obtain optimum cooling of the transformer. Indeed, the part which it is essential to cool as a priority is the interior of the transformer, or according to the design of the primary turn produced in the form of conductive sheets, the internal cylinder 1b, the annular part 3a and the cylinder exterior have very good thermal conduction. So the calories that are released inside the magnetic core are easily evacuated to the outside. Furthermore, the fact that large plates 4 are used as external connection further promotes heat exchange and therefore the cooling of the transformer, even in the absence of fins.

It should be noted that the embodiments presented in FIGS. 9a to 9d also make it possible to make series connections of two half-turns. In this case it is obvious that the half-turns must be isolated by construction from one another. A series connection, using external connections, then makes it possible to obtain a secondary voltage double compared to the case where these turns are in parallel.

The invention can be used to power computer circuits. A secondary flow of 120 amperes can be obtained at a voltage of 5 volts by using a ferrite with a central section of 1.5 0 2 Cm ₂ and having a dimension of 4 to 5 cm. The transformer thus produced can work at a frequency of 400 kilohertz. It has excellent primary-secondary coupling and connections with the outside at very low impedance. The parasitic capacity between primary and secondary is low. Finally the secondary turn being almost closed, this constitutes a shielding compared to the high primary voltages which reduces the electromagnetic radiation.

Claims (10)

1. Electric transformer comprising a first winding (2) magnetically coupled to a second winding constituted by a turn (1) characterized in that the turn (1) is constituted by a conductive element of small thickness constituting a surface which forms an almost envelope closed which follows the surface of said first winding (2). 2. Transformer according to claim 1 characterized in that the first winding (2) is a coil in the form of a cylinder hollowed out in its center and in that said surface of said turn consists: - by two concentric cylinders (la, lb) having radii such that said coil adjusts in the volume between said cylinders (la, lb) - and by an annular surface (3a} resting on the edges of said cylinders (la, lb) at one end thereof, - In that said annular surface (3a) is split along a radius and in that each of said cylinders is split along a generatrix passing through the slot (31) formed in said annular surface (3a). 3. Transformer according to claim 2 characterized in that said surface comprises a second annular surface 3 removable split arranged in the same way as the first annular surface (3a) on the edges of the other end of said cylinders (la, lb). 4. Transformer according to claim 2 or 3 characterized in that at least one of said annular surfaces (3a, 3) is perforated to allow the passage of electrical conductors (9). 5. Transformer according to one of claims characterized in that it comprises a magnetic core (6, 7) having a first part (6a, 7a) filling the recess of said first winding (2) and a second part closing the circuit magnetic of said core (6, 7) and in that insulating elements (5, 8) are provided for electrically insulating the magnetic core (6, 7) from said coil (1). 6. Transformer according to one of claims 2 to 5 characterized in that the electrical connection of said coil (1) with the outside is carried out by means of two conductive plates (4) respectively in electrical contact with the two lips of the slot (10a) of the outer cylinder (la) of said turn (1). 7. A transformer according to claim 6 characterized in that said plates (4) are constituted by a multilayer printed circuit. 8. Transformer according to one of claims 2 to 7 characterized in that the inner cylinder (lh) of the turn (1) has a spiral groove (12) on which is housed the insulated conductor of the first winding (2) . 9. Transformer according to one of the preceding claims, characterized in that cooling fins (13) in thermal contact with the ambient air are fixed directly to the turn (1) of the second winding. 10. Transformer according to one of claims 2 to 9 characterized in that said cylinders (la, lb) constituting the turn (1) of the second winding are constituted by two half-cylinders (lc, ld, lg, lh) arranged end -by end and by construction without direct electrical contact, so as to allow a series connection of the two half-cylinders.

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