EP0291403A1 - Transformer with a strong coupling for chopped supply circuit and supply circuit comprising such a transformer - Google Patents

Abstract

High coupling transformer of the type comprising a primary circuit and a secondary circuit produced in several layers of printed circuits and magnetically coupled via a magnetic circuit, each printed circuit comprising at least one almost closed conductive track constituting an active turn of one of the primary or secondary circuits, characterized in that two neighboring turns of the same primary or secondary circuit are at potentials as close as possible, in that two neighboring turns belonging one to the primary circuit the other to the secondary circuit are at potentials as fixed as possible, and in that the turns brought to variable potentials are as far as possible from the turns brought to fixed potentials.

Description

The present invention relates to a transformer with strong coupling adapted to a switching power supply circuit. It also relates to a supply circuit implementing such a transformer.

The invention belongs to the field of manufacturing and optimizing transformers of multilayer technology.

The invention makes it possible to obtain a high reproducibility of the electrical and mechanical characteristics minimizing the manufacturing controls and the rejects.

In multilayer technology, a transformer has a primary circuit and a secondary circuit magnetically coupled to each other via a magnetic circuit; these two circuits consist of a stack of turns produced by printed layers on each of which is drawn an almost closed conductive track.

A variant according to the invention makes it possible to produce a transformer delivering larger currents by attaching to the multilayer printed circuit turns in cut metal, these turns in cut metal having a greater thickness than that of the printed layers.

The invention makes it possible to produce a multilayer transformer with very strong coupling. According to the invention, it is particularly well suited to a switching power supply circuit whose windings are traversed by currents with extremely rapid variations.

The transformer according to the invention is also intended to be mounted in switching power supplies whose dimension must be as small as possible. To this end, the transformer according to the invention is made as flat as possible.

For equipment developing a certain electrical power in a small volume, it is desired to carry out thermal optimization. We want a low thermal gradient between the core and the outside of the transformer. By dividing the stack of layers into N printed circuit boards, the heat exchange surface is increased in an N ratio.

Finally, in order to reduce the parasitic couplings, the transformer according to the invention is optimized from the electrical point of view with a view to minimizing the primary-secondary parasitic current.

In order to remedy these various problems not resolved in the prior art, the present invention relates to a transformer with strong coupling of the multilayer type. The invention is characterized in particular by the fact that two neighboring turns are at potentials as close as possible. When two successive turns belong one to the primary and the other to the secondary, they are at as fixed potentials as possible. The turns brought to variable potentials are as far as possible from the turns brought to fixed potentials.

• figure 1 : un schéma de connexion de spires secondaires dans un transformateur selon l'invention,
• figure 2 : un schéma d'empilement de spires dans un transformateur selon l'invention,
• figure 3 : un schéma de connexion des spires au primaire d'un transformateur selon l'invention,
• figure 4 : trois dessins possibles d'une spire spéciale disposée entre le primaire et le secondaire dans un transformateur selon l'invention,
• figure 5 : un plan d'empilement dans un mode de réalisation d'un demi bobinage à 14 couches,
• figure 6 : un dessin montrant l'optimisation de l'utilisation des isolants,
• figure 7 : un schéma électrique d'une utilisation possible,
• figure 8 : un transformateur selon l'invention,
• figure 9 : un plot de sortie,
• figure 10 : un plan d'empilement associant des circuits imprimés et des spires en métal découpé.

Other characteristics and advantages of the present invention will appear more clearly from the description and the appended figures which are:

• FIG. 1: a diagram of connection of secondary turns in a transformer according to the invention,
• FIG. 2: a diagram of the stacking of turns in a transformer according to the invention,
• FIG. 3: a diagram of connection of the turns to the primary of a transformer according to the invention,
• FIG. 4: three possible drawings of a special turn arranged between the primary and the secondary in a transformer according to the invention,
• FIG. 5: a stacking plane in an embodiment of a half-winding with 14 layers,
• FIG. 6: a drawing showing the optimization of the use of insulation,
• Figure 7: an electrical diagram of a possible use,
• FIG. 8: a transformer according to the invention,
• FIG. 9: an output pad,
• Figure 10: a stacking plan associating printed circuits and cut metal turns.

In Figure 1 , there is shown a connection diagram of secondary turns. The secondary is produced in two identical halves, each comprising an odd number of turns. In order to reduce leaks caused by the separation between the two half-secondary, each turn of a half-secondary is connected to the terminals of a corresponding turn of the other half-secondary. Schematically, the turns (1), (2) and (3) of the semi-secondary (7) have terminals A, B, C, D, E, F. The second semi-secondary (8) has turns (4 ), (5) and (6) whose access terminals are respectively G, H, I, J, K, L. The terminals are connected in such a way that the turn (1) responds to the turns (4) and (5), and the turns (2) and (3) to turn (6). We therefore make the ADFGIL, CEK, BHJ connections. In the embodiments where the secondary has a greater number of turns, this device is repeated as many times as necessary.

In Figure 2 , there is shown the embodiment of a half-transformer according to the invention. According to the invention, a half-transformer is constituted by a stack of turns distributed between a semi-primary (14) and a semi-secondary divided into 2 parts (13) and (15) enveloping the semi-primary. The semi-secondary can be carried out as shown in Figure 1.

A part (13) of the semi-secondary is separated from the semi-primary (14) by a special whorl forming a screen (11). The second part (15) of the semi-secondary is separated from the semi-primary (14) by a second special turn (12) forming an electrostatic screen. On the right-hand side of FIG. 2, the direction of the variation of the potentials of turns inside the semi-primary and of the semi-secondary divided into 2 parts is shown. The tip of the arrow represents the increasing direction of a variable potential, the other end represents a fixed potential.

In order to reduce the potential variations between each semi-primary and semi-secondary, the turns are connected so that on both sides of a special turn forming a screen (11) or (12), the turns are at potential as fixed as possible, that towards the inside of the half-transformer, the turns of the semi-primary (14) are at the most variable potentials.

In Figure 3 , there is shown a primary formed by a stack of six turns. Outdoor coils (16) and (21) are intended to produce an electrostatic screen. These two turns are therefore connected to each other in parallel. The active turns (17), (18), (19), (20) are connected so that the potentials are as fixed as possible on the external faces of the stack. To this end, the output of the coil (17) is connected to the input of the coil (20), the output of which is connected to the input of the coil (18). Then, the output of the coil (18) is connected to the input of the coil (19) whose output (23) with variable potential constitutes a terminal of a semi-primary.

To formally represent the case of a primary of 2P turns, (-the turns being numbered successively in their stacking order from 1 to 2P-), not including the two screen turns, we consider that the connection is carried out in series of pairs of turns connected in series. The first pair is formed by the series 1 of the coil 1 and the coil 2P, and so on, the pair of row K being constituted by the series of the coil K and the coil 2P-K +1 , the last pair being formed by putting the P turn in series with the P + 1 turn.

Thus, the electrical connection of two turns to order K is noted (K, 2P-K + 1). It is represented in figure 3 for 2P = 4 and K = 1 with the pair 17.20 and K = 2, the pair 18.19. The symbolic writing of the serialization of the P pairs of turns is written:

Each pair of turns has an input on turn K and an output on turn 2P-K + 1. The serialization of two pairs are carried out as in the example in FIG. 3 from the output of the pair K to the input of the pair K + 1.

Such a distribution of potentials ensures that the capacitive currents - generated by the voltages between neighboring turns - between primary and secondary will have a minimum value.

In Figure 4 , there are shown three embodiments shown in Figures 4a, 4b and 4c of a special turn, the closest to a secondary turn shown in Figure 4a in a semi-primary. These turns forming an electrostatic screen have been shown schematically in (16) and (21) of Figure 3 or (11) and (12) of Figure 2. These three models allow with different efficiencies and complexities to minimize the primary stray current -secondary due to switching, when the transformer is mounted in a switching power supply. For maximum efficiency, the adjacent secondary turn of Figure 4d must have its two ends (24) and (25) diametrically opposite the two ends (26) and (27) of the special turn that it is made according to one or the other of the modes shown in Figures 4a, 4b, 4c.

The active secondary turn, adjacent to the screen turn and shown in Figure 4d, is composed of a wide conductive track almost closed which spares a central window. The central window makes it possible to stack the printed circuit of turn on a column of magnetic circuit. The turn is cut so as to release an input terminal 24 and an output terminal 25. The cut is preferably made so as to have two elbows so that the electrical resistance in the radial direction is increased on the cut . In general, the cut is obtained by at least two straight lines that are not aligned.

The end (26) of a special turn and the end (24) of the adjacent secondary turn must be brought to fixed potentials and decoupled by a capacitor of suitable value for the switching frequency when the transformer is mounted in a switching power supply.

According to the first embodiment of Figure 4a, such a turn is constituted by two parts oriented in opposite directions from each other. The input (26) of the outer turn (28) is arranged at a fixed potential whose value is as close as possible to that of the next turn. Inside the loop formed by this turn, a second turn (29) is made in the opposite direction, one end of which is connected to the inlet (26) of the outer turn (28), the other end (30) is left free. The two turns are arranged as close to each other as possible. The outer turn (28) having ends (26) and (27) is in fact the first turn of the primary winding. It is therefore an active turn of the transformer.

The electric field appearing along the perimeter between this special turn and the adjacent secondary turn tends to reduce the primary-secondary stray current due to cutting.

This first embodiment is well suited to the production of small transformers, it gives an average efficiency.

According to a second embodiment shown in Figure 4b, the inner coil (31) has its ends (32) and (33) diametrically opposite the ends (26) and (27) of the active coil (34). The end (32) of the inner coil (31) is connected to the end (26) of the active turn by a link (35). The end (33) is left free. As for the first embodiment, the two turns must be as close as possible. The link (35) should be as close as possible.
This mode has a higher efficiency than the first embodiment and is suitable for medium power transformers.

According to a third embodiment shown in Figure 4c, the inner coil (36) is split into two equal parts (36a) and (36b). The ends (37), (38) are opposite one another and diametrically opposite the ends (39), (40) themselves opposite. The end (39) of an inner half-turn (36a) is connected to the end (26) of the active turn (43) by a link (41), the other end (37) of this same half -spire is connected to the end (38) of the second half-turn (26b) by a link (42).
The end (40) of the second half-turn (36b) is left free. The active turn and the two inner half-turns must be as close as possible, the connections (41) and (42) must be as close as possible. The link (41) is not a direct link which would erase the effect of the split cleavage of the internal coil (36). It consists of a narrow track which makes a complete turn around the common central region of the two internal (36) and external (43) turns.
This mode has the highest efficiency, it is suitable for high power transformers.

To make a transformer according to the invention, a stack of 2 printed circuits is provided, each consisting of 14 etched layers on which the connection pads are carried, a central window and a path almost closed intended to make a turn on each engraved layer.

In Figure 5 , there is shown a succession of 14 layers of a printed circuit intended to produce a transformer according to the invention. The 14 plates are of identical dimensions and each have at the bottom 6 metallized bores assembled two by two to make the connections ADFGIL, CEK, BHJ of FIG. 1 intended for the turns of the two semi-secondary. In the upper part of each printed circuit are arranged 8 contacts each consisting of a metallized hole numbered from 1 to 8 on the plates which use them. Thus, the connections of the semi-secondary are made at the bottom of the printed circuit and the connections of the primary are made in the high connections of this printed circuit.

The connections of printed plates to printed plates are therefore made via metallized holes. The plates are numbered successively from S1 to S14 by their stacking order in a transformer produced according to the invention. The first plate S1 and the last plate S14 are intended to provide mechanical and electrical protection of the stack. The semi-secondary divided into two parts, consisting of plates S2, S3, S4 on the one hand, of plates S11, S12, S13 on the other hand, surrounds the semi-primary.
The semi-secondary is made up of the series S2 winding in series with a parallel association of the S3, S4, S11, S12 and S13 turns.

The semi-primary consists of the stack of six plates S5 to S10. The end plates S5 and S10 are opposite the two parts of the semi-secondary. They form an electrostatic shield consisting of a half-width turn shown hatched on the plates S5 and S10.

This turn is wound in the opposite direction to the active turn in half the width of the plate in question. The half-primary plates are connected to the terminal block formed by the metallized holes greater than the number of eight on each plate. These metallized holes are numbered from left to right from 1 to 8 and only the terminals used for each plate are numbered in the drawing. Thus, it can be seen that the semi-primary consists of the serialization of the turns S5, S6, S9, S7 and S8 on the one hand, and the paralleling of the turn S10 with the turn S5 on the other hand. Finally, the access terminals of the semi-primary are constituted by the terminal 7 brought to the fixed potential and the terminal 1 brought to the variable potential.

Terminals 2 and 8 shown on plate S5 are not connected. When two printed circuits are associated, it allows simplified connections to be made.

The serial connections of the turns of the semi-primary (points 1-3-4-5-6-7) are all accessible, one can easily modify the transformation ratio.

In Figure 6 , there are shown two of the fourteen layers of the printed circuit marked (100) and (101). The etched copper surfaces (102) and (103) are opposite and insulated from each other by a prepreg insulator (104). The design of the copper has been optimized so that two edges, for example (105) and (106), never coincide. This arrangement makes it possible to reduce the thickness of the prepeg while avoiding the risks of shearing thereof during pressing of the printed circuit. The thickness of the transformer is thus minimized and the coupling between primary and secondary is improved.

In Figure 7 , there is shown an electrical diagram of a transformer according to the invention. The semi-primary (44) or (45) is associated with the semi-secondary (46) or (47) in the same printed circuit described in FIG. 5. The example shows that by combining two printed circuits, a transformer can be produced for a Push-Pull assembly. The common points (49) and (50) or (53) and (54) common are joined to the primary or secondary fixed potential. Points (48) and (51) or (52) and (55) are joined to the primary or secondary variable potential. The phase match is represented by four dots. The possibility of easy series or parallel connection of the turns offers a large number of possible combinations as well as modular power.

In FIG. 8 , a complete transformer is shown fulfilling the functions described in the diagram in FIG. 7 Two identical layers (56) and (57) each comprising a semi-primary and a semi-secondary are associated by two rows of studs ( 58) and (59). One layer is mounted with its upper side up, the other layer having directed it down. In this way, the two semi-secondary are opposite. A free space (60) between the two layers of printed circuits (56) and (57) allows better cooling by circulation of a cooling fluid. The size of this space can vary depending on the speed and the nature of the heat transfer fluid available to optimize cooling. Finally, the transformer is completed by a magnetic circuit (61), a core (62) of which dips into the central windows of two layers. In an exemplary embodiment, the magnetic circuit consists of a core (62) mounted in the middle of a closed part (63). The assembly is cut by the median plane (64) so as to allow mounting.

In Figure 9 , there is shown the drawing of an output pad. This part fulfills 3 functions:
. the height of the cylinder (65) makes it possible to fix the spacing between the two layers of printed circuits for the passage of the coolant;
. the cylinder (66) emerges from the upper printed circuit layer through a terminal hole and allows the improvement of the cooling by draining in the external ambient flow the calories dissipated in the heart of the printed circuit;
. the cylinder (67), which makes the connection with the homologous terminal of the lower layer, has a sufficient height to allow connection to the printed circuit constituting the power supply, when the latter is mounted on a printed circuit.

In FIG. 10 , two layers of printed circuits (66) and (69) have been shown as described above, each comprising a semi-primary and a semi-secondary with strong coupling.

To increase the current available at the secondary, coils of cut metal (70, 71, 72, 73) having a thickness greater than a layer of printed circuits are added. The strong coupling is preserved thanks to the secondary turns contained in the printed circuits (68) and (69).

Insulating parts (74) and (75) make it possible to isolate the cut coils closest to the magnetic circuit (76) and (77) relative to the latter.

The isolation between the printed circuits (68) and (69) and the cut turns (70-73) is ensured by the closing layer of the printed circuits.

The positioning of the cut turns (70-73) is ensured by the studs (78) as described in FIG. 9. The size of the interior (or windows) (79) and exterior (80) cuts of the layers (68-74) is calculated in such a way as to insulate the passage of the magnetic circuit.

The stacked layers (68) and (69) are all identical and can be mounted in two possible directions depending on the configuration imposed by the electrical diagram.

Claims (16)

1. A highly coupled transformer of the type comprising a primary circuit and a secondary circuit produced in several layers of printed circuits and magnetically coupled by means of a magnetic circuit, each printed circuit comprising at least one almost closed conductive track constituting a turn active of one of the primary or secondary circuits, characterized in that two neighboring turns of the same primary or secondary circuit are at potentials as close as possible, in that two neighboring turns belonging one to the primary circuit l ' others at the secondary circuit are at potentials as fixed as possible, and in that the turns brought to variable potentials are as far as possible from the turns brought to fixed potentials. 2. Transformer according to claim 1, characterized in that the secondary circuit is produced in at least two parts containing the same number of turns and surrounding the primary circuit. 3. Transformer according to claim 2, characterized in that, in order to reduce the leaks caused by the separation between the parts of the secondary circuit, each part of the secondary has at least one turn which responds to each of the turns of the other parts, the turn of response being connected in parallel with the turns of the other party. 4. Transformer according to claim 2, characterized in that the transformer is produced in two halves, each half comprising a semi-primary (14) surrounded by two parts of semi-secondary (13, 15), two turns adjacent to a primary part and a secondary part being separated by a special turn (11 or 12) forming an electrostatic screen and in that the potentials of turns of the primary or secondary parts are increasing on either side of the fixed potentials at the screen turns (11, 12). 5. Transformer according to claim 4, characterized in that the two screen turns (16, 21) are connected to each other in parallel, and in that the active turns are in even number (2P) and are connected in series of two turns in the same winding direction, two turns, the first of odd order K, the second of even order 2P-K + 1, being connected in series (K, 2P-K + 1), the P series of 2 turns being connected in the form,

the input of the K + 1 series being connected to the output of the K series. 6. Transformer according to claim 4, characterized in that the secondary turn adjacent to the screen turn of the primary consists of a wide, almost closed conductive track, the two access terminals 24, 25 of which are arranged on the opposite side of the secondary turn relative to the side of the access terminals of the screen turns. 7. A transformer according to claim 6, characterized in that the secondary turn adjacent to the screen turn is obtained by cutting a track initially closed, the cut comprising at least two straight non-aligned parts. 8. Transformer according to claim 4, characterized in that the screen turn consists of two concentric turns oriented in opposite directions to each other, almost closed, and as close to each other as possible. 9. Transformer according to claim 8, characterized in that the inner turn (29) has one end (30, 33 or 40) free unconnected and a second end (30a, 32 or 38) connected as close to a terminal (26) of the external turn (28, 33 or 43) disposed at a fixed potential whose value is as close as possible to that of the next turn. 10. Transformer according to claim 9, characterized in that the second end (30a) of the inner turn (29) is connected in full width to the outer turn (28) near the terminal (26) with fixed potential. 11. Transformer according to claim 9, characterized in that the two ends of the internal turn (31) are arranged face to face on the side opposite to the pair of terminals (26, 27) of the external turn (34), the potential of the fixed terminal (26) being brought back to the terminal (32) of the internal turn by a thin track (35) traced between the two internal (31) and external (34) turns so that the directions of travel of the electric current in the two turns are opposite. 12. Transformer according to claim 11, characterized in that the internal turn (36) is split into two parts equal to the height of the terminals (26, 27) of the external turn, the part (36b) carrying the end (40 ) free from the internal turn having a second end (38) created by the splitting of the internal turn electrically connected to the corresponding end (37) of the second part (36a) by a narrow track (41) which makes a complete turn around the common central region of the two turns (36, 43). 13. Transformer according to one of claims 4 to 12, characterized in that it comprises two identical stacks of printed circuits (on the basis of a printed circuit by turn), each stack comprising a semi-primary surrounded by two halves d '' a high school with screen turns, each printed circuit comprising terminals (A, B, C, D) assigned to the connections of the ends of secondary turns on a first side of the printed circuits, and terminals (1 to 8) assigned to the connections of the primary turns on a second side of the printed circuits, each of the terminals (1 to 8, A to D) being provided with a bore so as to allow two turns to be connected on two printed circuits perpendicular to the plane of the turns when the end to be connected is connected to the terminal (1 to 8, A to D). 14. Transformer according to one of the preceding claims, characterized in that two layers of copper (100, 101) of the printed circuit facing each other, are drawn so that two edges (105, 106) of the two surfaces of etched copper (102, 103) are never in coincidence so as to improve the coupling between primary and secondary by reducing the thickness of the insulator (104) like prepreg while avoiding the risks of shearing thereof during pressing the printed circuit. 15. Transformer according to one of the preceding claims, characterized in that it comprises a magnetic circuit (63) produced in two symmetrical halves relative to the median plane (64) of the transformer and comprising a central core (62) around which are stacked the different layers of the transformer windings, and in that the windings are distributed in two halves (56, 57) on either side of the median plane (64) so as to provide a free space (60) between them, the height is determined according to the chosen cooling and fixed by studs (58, 59) each having a first function of electrical connection between at least two layers of coils and a second function of spacer making it possible to fix the spacing between the two halves (56, 57) of the windings, this spacer being produced by a shoulder (65) of the stud. 16. Switching power supply circuit comprising a transformer according to one of the preceding claims. 17. A switching power supply circuit according to claim 16, characterized in that the end (26) of a screen turn and the end (24) of the adjacent secondary turn are brought to fixed potentials and decoupled by a capacitor of value determined by the switching frequency.

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