EP2274753A1 - Power transformer for radiofrequency signals - Google Patents

Abstract

The present invention relates to a power transformer for radiofrequency signals. The transformer is made in a low-cost multilayer printed circuit card (201) comprising at least successively the following stacked layers: a first conducting layer, a first dielectric substrate layer, a second conducting layer, a second dielectric substrate layer, and a third conducting layer, the primary winding being formed by a turn printed in the second conducting layer, the secondary winding (103) being formed by a first turn printed in the first conducting layer, this first turn being linked to a second turn printed in the third conducting layer, the turns of the secondary winding being placed opposite the turn of the primary, the card being clamped above and below by two plates of ferromagnetic material. Capacitive components connected between winding(s) and an exposed conductive part can improve the performance of the proposed transformer.

Description

 Power transformer for radio frequency signals

The present invention relates to a power transformer for radiofrequency signals. The invention applies in particular to the realization of RF radio transmitting terminals.

Power transformers are generally made by wire windings on cores of ferromagnetic materials. This artisanal achievement has an impact on the cost of manufacturing the transformer. Also, other types of transformer, made with printed coils, have been proposed to facilitate their manufacture.

In particular, a planar transformer has been proposed by Motorola Inc in the US patent published under the reference US5015972. The planar transformer has two printed turns separated by a dielectric layer. Capacitors may also be provided to interconnect two printed turns to improve the performance of the transformer in the operating frequency band. However, the structure of this transformer uses a thermal conductive substrate to dissipate the heat generated during its operation, making the realization complex and expensive. In addition, the design of the proposed windings generates a non-optimal coupling between said windings, which leads to losses of magnetic energy.

An object of the invention is to provide a high-frequency power transformer that is efficient - that is to say, with low losses and by controlling the impedances in the frequency band considered - and whose integration into an electronic circuit is simple, this to provide an impedance transformation function at a lower cost. For this purpose, the subject of the invention is a high-frequency power transformer comprising a primary winding and a secondary winding, characterized in that it is produced in a low-cost multilayer printed circuit board, for example of the FR4 type (FIG. known to those skilled in the art), comprising at least successively the following stacked layers: a first conductive layer, a first dielectric substrate layer, a second conductive layer, a second layer; of a dielectric substrate, and a third conductive layer, the primary winding being formed by turns printed in the second conductive layer, the secondary winding being formed by a first half-winding printed in the first conductive layer, the first half-winding. being connected to a second half-winding printed in the third conductive layer, the turns of the secondary winding being placed opposite the turns of the primary, the widths of the turns of the windings being chosen as a function of the thicknesses of the dielectric substrate layers, the instantaneous frequency band and the power of the high frequency signal passing through the transformer in order to minimize the losses and to favor the impedance matching in the RF band considered, said card being clamped above and below by two plates of ferromagnetic material. Thus, the primary winding is clamped by the secondary winding, the choice of the width of the lines and the thickness of the substrate layers to optimize the capacitive coupling between the windings.

According to one embodiment, an orifice is formed in the center of the printed circuit board, the ferromagnetic block comprising two parts assembled to form a binocular block, each part of said block occupying one side of the printed circuit board, the first part of said block being formed of an extruded E, the central leg of the E being inserted into the hole formed in the center of the printed circuit, thereby forming a magnetic core in the center of the windings, the second portion of said block being formed of a substantially planar plate. The presence of a magnetic core makes it possible in particular to obtain a better concentration of the magnetic field.

According to one embodiment, at least one capacitor is connected between at least one turn of one of the windings, preferably the primary winding, and an electrical ground in order to improve the behavior of the transformer with respect to the impedances that it is present in the RF band under consideration. The presence of this (these) capacitance (s) makes it possible to better control the impedances presented by said winding in the RF band with respect to the surrounding components, for example power transistors connected to the input of the transformer, particularly in the high frequencies of the frequency band of operation. The capacitance value is chosen according to the amplifier setup using this RF transformer.

According to one embodiment, the ferromagnetic block parts are made of ferrite and have standard dimensions, thus enabling a low-cost transformer to be produced.

According to one embodiment, the transformer is able to operate for high frequency signals comprised in an instantaneous frequency band between 1 MHz and a few tens of MHz, preferably from 1 to 50 MHz. In addition, the transformer according to the invention is able to operate at high power, of the order of 1 Watt to a few tens of Watts.

Moreover, the widths of the turns of the windings are chosen according to the operating frequency band of the transformer.

The invention also relates to a radio transmitter power card comprising a transformer as described above.

Other characteristics will become apparent on reading the detailed description given by way of nonlimiting example, which follows, with reference to appended drawings which represent:

- Figure 1, an illustration of a winding structure used in the power transformer according to the invention;

- Figure 2, a top view of a printed circuit used in a transformer according to the invention;

FIGS. 3a and 3b, an example of a block of ferromagnetic material included in the transformer according to the invention; - Figure 4, a perspective view of a transformer according to the invention;

- Figure 5, a cross-sectional view of the transformer of Figure 4;

FIG. 6, an illustration of a second embodiment comprising compensation capacitors, FIG. 7, an illustration, through a graph, of the effect produced by the addition of compensation capacitors on the impedance matching.

The same references in different figures designate the same elements.

Figure 1 shows a winding structure used in the power transformer according to the invention. In this example, the power transformer 100 comprises a winding 101 for the primary circuit framed by two half-windings 103, 103 'belonging to the secondary circuit. Each winding 101, 103, 103 'is a printed track in a multilayer printed circuit layer 201, two neighboring layers comprising a winding being separated by at least one thickness E1, E2 of dielectric substrate. To clarify Figure 1, the substrate separating the conductive layers is not shown. The secondary half-windings 103, 103 'are connected by one or more metallized vias 107. The turns of the primary winding 101 are placed substantially parallel to and opposite the turns of the half-windings 103, 103' of the secondary winding. to obtain an efficient coupling between the primary circuit and the secondary circuit of the power transformer 100. In addition, since the layer C2 in which the winding 101 of the primary circuit is printed is framed by two layers C1, C3 on which are printed the secondary half-windings 103, 103 ', magnetic leakage is minimized. Figure 2 shows a perspective view of a printed circuit used in a transformer according to the invention.

The transformer according to the invention comprises a multilayer printed circuit 201 comprising the windings 101, 103, 103 'as shown in FIG. The printed circuit 201 comprises a central orifice 202 around which the windings 101, 103, 103 'are printed. In FIG. 2, only the first half-winding 103 of the secondary circuit is visible, the winding 101 of the primary circuit and the second half-winding 103 'of the secondary circuit being printed in inner layers of the printed circuit 201. FIG. 3a shows an example of a block of ferromagnetic material 203 forming part of the transformer according to the invention, the block being in two parts 203a, 203b shown separately in the figure.

In the example, the ferromagnetic block 203 has a first portion 203a E-shaped extruded and a second portion 203b parallelepiped whose width 11 and length 12 are substantially equal to those of the first portion 203a. In other words, the first part 203a of the block 203 is a plate whose flank F1 is provided with three protuberances 204, 204 ', 204 "substantially parallelepipedal and of the same dimensions in addition to the thickness of the plate. first protrusion 204 being placed, in the example, in the width 11 on a first edge B1 of the plate, a second protrusion 204 'being placed substantially in the center, the third protrusion 204 "being placed on the edge B2 opposite that B1 of the first protrusion 204. The second portion 203b of the ferromagnetic block 203 is a plate having, in the example, substantially the same dimensions 11, 12 as the plate of the first portion 203a. In addition, unlike the plate of the first portion 203a, the second portion 203b of the ferromagnetic block 203 is devoid of growths. Thus, as illustrated in FIG. 3b, when a flank F3 of the second portion 203b is contiguous with the projections 204, 204 ', 204 "of the first portion 203a, the block of ferromagnetic material 203 takes the form of a block binocular, that is to say, in the example, a substantially parallelepiped block in which two orifices 207, 207 'are distinct.The binocular form of the ferromagnetic block 203 makes it possible to concentrate the magnetic field lines in order to extend the band In fact, this block form 203 also makes it possible to improve the electromagnetic shielding of the transformer 100. This shielding can also be improved by increasing the efficiency of the transformer 100 to the lowest frequencies (on the order of 1 MHz). insertion of vias around the perimeter of the printed circuit 201 and / or ground pads disposed on either side of the windings 101, 103, 103 '.

The two parts 203a, 203b of the ferromagnetic block 203 consist of ferrite, whose permeability μ is quite high (of the order of 700-1000) to ensure proper operation for frequencies bass. In the example, the two parts 203a, 203b of the ferromagnetic block 203 are held together simply by means of a metal rod 205 enclosing the two parts 203a, 203b. According to another embodiment, a bonding is performed to hold the two parts 203a, 203b together.

Figure 4 shows a perspective view of a transformer according to the invention comprising the printed circuit of Figure 2a wherein the first portion 203a of the ferromagnetic block 203 is embedded. Figure 5 shows a view of this transformer in cross section. The second protrusion 204 'of the plate is inserted into the central orifice 202 of the printed circuit 201 and the first 204 and third 204 "protuberances frame the windings of the printed circuit 201 on its sides, so that when the second portion 203b of the block 203 is contiguous with the first part 203a, the ferromagnetic block 203 composed of the two contiguous parts envelops the printed circuit 201 and forms a magnetic core in the center of said circuit 201.

According to one embodiment of the transformer according to the invention, a thermal interface is plated on the ferromagnetic block 203 to dissipate the calories from the magnetic losses inside said block

203. Unlike a conventional planar power transformer, the RF transformer according to the invention exploits the capacitive coupling between the primary 101 and secondary windings 103, 103 '(Figure 1). Indeed, at operating frequencies, that is to say radiofrequency, a capacitive coupling occurs between the turns of the windings placed opposite 101, 103 and 101, 103 ', this capacitive coupling to improve the behavior of the transformer (impedances of the windings) in particular vis-à-vis the power transistors possibly connected to the input of the transformer according to the invention, particularly at the highest frequencies. The widths of the turns (ie printed tracks) are chosen in particular as a function of the thicknesses E1, E2 (FIG. 1) of the dielectric substrate layers, the greater the thickness E1, E2 of the substrate, the larger the turns of the layers. windings 101, 103, 103 'to be enlarged to promote capacitive coupling between the windings 101, 103 and 101, 1 03'. The power of Current flowing through the transformer is also a variable taken into account in choosing the width of the turns.

Figure 6 shows an illustration of a second embodiment including compensation capabilities. In the example, the first connection of a capacitance 601a, 601b, 601c is connected to each turn of the winding 101 of the primary circuit. The second connection of each of these capacitors 601a, 601b, 601c is connected to an electrical ground 602. The addition of these capacitors 601a, 601b, 601c makes it possible to improve the adaptation of the transformer to its environment , for example the adaptation to power transistors connected to the transformer, particularly in the high frequencies, in the example in a frequency band of 15 MHz to 50 MHz. These capacitors 601a, 601b, 601c connected in parallel between the winding considered and the ground compensate in an original way the imperfections of the transformer by creating a transmission line with the inductive component of the considered winding. They may possibly be replaced by different capacitive elements, or even more complex components (for example LC networks) in order to modify more favorably the behavior of the transformer in the band, in particular at the highest frequencies.

According to another embodiment, to improve the impedance control, additional capacitive elements can be connected in parallel with the primary winding or, as proposed by US5015972 mentioned above in preamble, between the turns of the same winding.

FIG. 7 illustrates, through a graph, the effect produced by the addition of capacitors 601a, 601b, 601c of compensation connected between the turns of the primary winding 101 and the electrical earth 602 (FIG. 6). A first curve 701 shows the evolution of the reflection coefficient S1 1 of a transformer without compensation capacity as a function of the frequency of the signal entering the transformer. A second curve 702 shows the evolution of the reflection coefficient S1 1 of a transformer comprising compensation capacity as a function of the frequency of the signal entering the transformer. The adaptation of the transformer is improved, especially at high frequencies.

An advantage of the transformer according to the invention is its low manufacturing cost, in particular because the multilayer printed circuit used can be a standard circuit at low cost. In addition, the structure of the transformer according to the invention simplifies the connection of components to the primary circuit and the secondary circuit. Indeed, the transformer being formed from a multilayer printed circuit, no transfer manipulation (ie a manual soldering) is required to integrate the transformer in an existing circuit.

Moreover, the RF power transformer according to the invention limits the losses of magnetic coupling with respect to the structure proposed in US5015972 cited in the preamble, in particular because of its winding structure in which the primary circuit is surrounded by above and below by the secondary circuit.

Claims

1. A high frequency power transformer comprising a primary winding (101) and a secondary winding (103, 103 '), characterized in that it is realized in a multilayer printed circuit board (201) comprising at least successively the following stacked layers: a first conductive layer (C1), a first dielectric substrate layer (E1), a second conductive layer (C2), a second dielectric substrate layer (E2), and a third conductive layer (C3), the primary winding ( 101) being formed by turns printed in the second conductive layer (C2), the secondary winding (103, 103 ') being formed by a first half-winding (103) printed in the first conductive layer (C1), this first half-winding being connected to a second half-winding (103 ') printed in the third conductive layer, the turns of the secondary winding (103, 103') being placed facing the turns of the primary (101), the widths of the turns of the windings (101, 103, 103 ') being chosen as a function of the thicknesses of the dielectric substrate layers, the instantaneous frequency band and the power of the high frequency signal passing through the transformer, said card (201) being clamped above and below by two plates (203a, 203b) of ferromagnetic material assembled to form a binocular block, the first portion (203a) of said block being formed of an extruded E, the central branch of E (204 ') being inserted into the hole (202) formed in the center of the printed circuit board, thereby forming a magnetic core in the center of the windings, the second portion (203b) of said block being formed of a substantially planar plate. 2. Radio frequency power transformer according to claim 1, characterized in that at least one capacitive element (601a, 601b, 601c) is connected between at least one turn of one of the windings (101, 103, 103 '). ) and an electrical ground (602). 3. radiofrequency power transformer according to claim 2, characterized in that at least one capacitive element (601 a, 601 b, 601 c) is connected between at least one turn of the primary winding (101) and an electrical mass. (602). Radio frequency power transformer according to any one of the preceding claims, characterized in that an orifice (202) is formed in the center of the printed circuit board (201), the ferromagnetic block comprising at least two parts (203a, 203b). assembled to form a binocular block, each portion of said block occupying one side of the printed board, the first portion (203a) of said block being formed of an extruded E, the central branch of the E (204 ') being inserted into the hole (202) formed at the center of the printed circuit board, thereby forming a magnetic core in the center of the windings, the second portion (203b) of said block being formed of a substantially planar plate. 5. radiofrequency power transformer according to any one of the preceding claims, characterized in that it is able to operate for instantaneous frequency band signals between 1 MHz and 50 MHz. 6. Radiofrequency power transmitter card, characterized in that it comprises a transformer according to one of the preceding claims.