Patent Publication Number: US-2023134367-A1

Title: Power transformer of the symmetric-asymmetric type with a fully-balanced topology

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. Patent Application No. 16/275,091, filed Feb. 13, 2019, which is a continuation application of U.S. Patent Application No. 15/223,148, filed Jul. 29, 2016, now issued as U.S. Patent No. 10,249,427, which is a translation of and claims the priority benefit of French patent application number 1652713, filed on Mar. 30, 2016, and entitled “Balanced-To-Unbalanced Transformer For Power Application With Fully Balanced Topology,” which are hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to integrated transformers of the symmetric-asymmetric type, commonly denoted by the term BALUN (BALanced to UNbalanced). The disclosure is, for example, applicable in mobile telephony and motor vehicle radar systems. 
     BACKGROUND 
     The fabrication of integrated systems made of silicon, whether power or processing systems, is increasingly implemented using differential structures and variable reference impedances for analog parts. On the other hand, most everything else remains essentially a system of the asymmetric mode type with 50 Ohms reference impedances. 
     The link between a symmetric transmission line and an asymmetric transmission line cannot be implemented without a matched electrical circuit. This transition is provided by a transformer of the symmetric-asymmetric type called a balun. 
     A balun converts, for example, a signal of the asymmetric mode type (or single-ended according to terminology widely used by those skilled in the art) into a signal of the differential mode type, and vice-versa. The balun also ensures the transformation of impedances. 
     One of the main electrical characteristics of a balun is its insertion loss, which is advantageously as low as possible. Indeed, the insertion loss is the result in loss of the transformation applied. The loss may be due to an impedance mismatch, an imbalance in amplitude and/or phase between the two channels, a resistive loss, and/or all of these factors combined. This loss causes a reduction in the overall performance of the system employing this device. 
     Furthermore, the performance characteristics of a balun are mainly expressed in terms of amplitude and phase symmetries. There is a difference in amplitude and a phase shift between the input and output signals which are advantageously minimized. 
     Baluns may furthermore be used, for example, in receiver and transmission circuits of wireless communications systems. In particular, for the design of differential circuits such as amplifiers, mixers, oscillators and antenna systems. 
     In the transmission and receiver circuits of wireless communications systems, the impedance on the differential side may be low, typically on the order of 10 to 20 Ohms for a low-noise amplifier. The impedance on the single-ended side, in other words on the side of the antenna, as indicated above, is generally around 50 Ohms. This, therefore, means that a high transformation ratio is necessary, which can be particularly complicated to achieve. 
     Furthermore, notably in transmission, the power amplifier is to be supplied with a high current, on the order of a few hundred milliamps. Then, if it is desired to supply the power amplifier by means of the transformer (balun), this will have an impact on the performance of the balun. 
     For example, the high currents require a very wide metal track, which introduces an increase of the series resistance which is to the detriment of the insertion loss. Consequently, the design of baluns is usually limited to one turn per loop on the secondary circuit for high-power circuits. This has the consequence that the coupling between the differential and single-ended channels is generally unequal and poorly distributed. This leads to poor performance characteristics, such as phase-shifts and amplitude mismatches. 
     SUMMARY 
     According to one embodiment, an integrated architecture is provided for a transformer of the symmetric-asymmetric type that is totally balanced, which allows signals to be obtained that are in phase and with corresponding amplitudes. This may notably be for power amplifier applications. 
     A transformer of the symmetric-asymmetric type may comprise an inductive primary circuit and an inductive secondary circuit formed in the same plane by respective interleaved and stacked metal tracks. The tracks may comprise at least a first crossing region in which two connection plates facing one another take the form of rectangular plates, wider than the metal tracks, and may each be diagonally connected to tracks of the secondary circuit. 
     The plane shapes facing one another of the crossing regions may offer a large crossing surface area. This may increase the coupling capacitance between all the turns of the transformer. Advantageously, notably in regards to noise signals, the widened portions may be the same size and may be aligned along an axis perpendicular to the plane. 
     The connection plate passing over the other connection plate may comprise two wings each respectively situated on one end of two opposing sides of the rectangular plate. The ends may be diagonally opposite and the metal tracks of the secondary circuit may be connected to the lower surface of the wings. Advantageously, the wings may each have a bevel at its connection with the rectangular plate. This configuration is notably advantageous in regards to current flow, such as in the case of a high intensity current flow. 
     The primary and secondary inductive circuits may each comprise a loop describing at least two turns and have an architecture that is symmetrical with respect to an axis of the plane. A geometrically symmetrical and balanced architecture with respect to coupling minimizes or reduces the phase and amplitude imbalances of the signals present on the primary and secondary circuits. 
     Generally speaking, one terminal of the primary circuit may be connected to a load and the other terminal to ground. Consequently, the coupling between the primary and secondary circuits does not take place in the same way between the tracks at positions close to the load terminal and at positions close to the ground terminal. 
     The primary and secondary inductive circuits may be configured such that, over all of the positions of the secondary circuit at which a coupling with the primary circuit may take place, the sum of the distances from one terminal of the primary circuit to the corresponding coupled positions of the primary circuit may be equal to the sum of the distances from the other terminal of the primary circuit to the same coupled positions. 
     In this configuration, the secondary circuit may be coupled with the primary circuit in equal proportions at positions of the primary circuit close to one terminal and at positions of the primary circuit close to the other terminal. In other words, the signal in the secondary circuit sees the ground terminal as much as the load terminal of the primary circuit. 
     Thus, during the flow of a signal over the secondary circuit, this signal may be coupled in a uniform manner with the whole of the primary circuit, offering good phase and amplitude symmetries. This allows excellent behaviors to be obtained with regard to the balance of phases and the balance of amplitudes. This is notable for power amplifier applications. 
     In one embodiment, the at least a first crossing region may comprises first metal tracks for connecting tracks of the primary circuit crossing each other under the connection plates. 
     According to one embodiment, the interleaved loops may comprise at least a second crossing region, in which second metal tracks for connecting the primary circuit cross each other on either side of a biasing terminal. One of the second connection tracks may pass above the biasing terminal and the other underneath. 
     Thus, the symmetry of the architecture and the balance of the couplings between the primary and secondary circuits may also be optimized at the crossing regions. Advantageously, the biasing terminal takes the form of a rectangular plate connected to a mid-point of the secondary circuit and is situated in the neighborhood of the terminals of the secondary circuit. This allows decoupling capacitors to be connected between the biasing terminal and the ground of the differential circuit in an optimized manner with regard to space and performance. 
     According to one embodiment, the metal tracks of the primary circuit may be narrower than the metal tracks of the secondary circuit, over at least a portion of the primary circuit. This may allow, aside from an advantageous reduction in the surface area occupied by the transformer, a stray capacitance between the primary circuit and ground of the substrate on which the transformer is fabricated to be limited. 
     According to one embodiment, the transformer may be fabricated in an integrated manner on top of a semiconductor substrate. 
     A circuit may also be provided that comprises an antenna, processor or processing circuit and a transformer previously described, connected between the antenna and the processor. Furthermore, a telecommunications system may be provided, for example, of the cellular mobile telephone type, or tablet or equivalent, comprising such a circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the disclosure will become apparent from examining the detailed description of embodiments and their implementation, which are in no way limiting, and from the appended drawings in which: 
         FIG.  1    shows a transformer according to the disclosure in a plan view; 
         FIGS.  2  and  3    show the crossing regions of the transformer in perspective views; 
         FIG.  4    shows an input or output stage of a radio frequency telecommunications system comprising a transformer according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG.  1    shows a view from above of one embodiment of a symmetric-asymmetric transformer, or balun BLN. The balun BLN belongs to a plane P comprising an axis X forming an axis of symmetry for the whole architecture of this embodiment, and is fabricated on a semiconductor substrate SC. 
     The balun BLN comprises a primary inductive circuit L 1  formed by metal tracks whose disposition forms an octagonal loop, which is wound and unwound while making three complete rotations, or three turns. The primary circuit L 1  comprises two terminals SE and GND designed to be connected in asymmetric, or single-ended, mode respectively to a load and to ground. For example, the load may be a transmitting or receiving antenna. 
     The terminals SE and GND of the primary circuit L 1  are disposed side-by-side in a symmetrical manner with respect to the axis X, on an external side of the balun BLN. The balun BLN also comprises a secondary inductive circuit L 2 , formed by metal tracks whose disposition forms an octagonal loop which is wound and unwound while making two turns, in an interleaved manner with the turns of the loop of the primary circuit L 1 . 
     The metal tracks P 11 -P 15 , P 21 -P 25  forming the turns of the primary L 1  and secondary L 2  circuits are situated in the same metallization level. Furthermore, the octagonal geometries of the loops of the primary and secondary circuits are given by way of a non-limiting example, and may take another polygonal or circular form. 
     The secondary circuit L 2  comprises two terminals PA 1  and PA 2  designed to be connected in a symmetric, or differential, mode to transistors of a power amplifier circuit, for example. A biasing terminal VCC is connected to a mid-point of the secondary circuit L 2  and is designed to receive a common-mode DC voltage. 
     The terminals PA 1 , VCC and PA 2  of the secondary circuit L 2  are respectively disposed side-by-side in a symmetrical manner with respect to the axis X. This is on an external side of the balun BLN, opposite to the side comprising the terminals SE, GND of the primary circuit L 1 . 
     Thus, the interleaved nature of the primary L 1  and secondary L 2  inductive circuits provides an arrangement in which the metal tracks of the turns of the primary circuit L 1  are disposed on either side of, and directly next to, the track of each turn of the secondary circuit L 2 . The winding and unwinding of the turns of the primary and secondary circuits introduce crossing points for metal tracks. Thus, the metal tracks are stacked, notably in the crossing regions, passing over and under the metallization level of the turns, in respectively higher and lower levels of metallization. 
     It is nevertheless considered that the balun BLN is included within a plane P and that the symmetry with respect to the axis X does not take into account the differences in height of the levels of metallization. This is commonly admitted in microelectronics due to the very small vertical dimensions of the architecture. 
     Thus, the balun BLN comprises two crossing regions CR 1  and CR 2  in which the metal tracks cross one another, via metal tracks referred to as connection tracks. 
     The first crossing region CR 1  is situated in the turns on the side of the terminals SE, GND of the primary circuit and comprises a crossing of the primary circuit L 1  and a crossing of the secondary circuit L 2 . The second crossing region CR 2  is situated in the turns on the side of the terminals of the secondary circuit L 2  and comprises a crossing of the primary circuit, passing vertically on either side of the biasing terminal VCC. 
     The primary circuit L 1  runs from the terminal SE to the terminal GND via a track P 11  which arrives at the second crossing region CR 2 . A metal connection track PL 6  directs the turn towards the interior of the loop and connects the track P 11  to a track P 23  which runs to the first crossing region CR 1 . In the crossing region CR 1 , a connection track PL 4  directs the turn towards the interior and connects the track P 23  to a track P 15 . 
     The primary circuit L 1  has described a first turn (one complete circuit). The circuit then describes a second turn according to two half-turns formed by the tracks P 15  and P 25  connected together at a mid-point. The loop of the primary circuit has so far been wound and then starts to unwind. The track P 25  arrives at the first crossing region CR 1 , in which a connection track PL 3  directs the turn towards the exterior and connects the track P 25  to a track P 13 . The track P 13  runs to the second crossing region CR 2 , in which the connection track PL 5  directs the turn towards the exterior and connects the track P 13  to a track P 21 . The track P 21  then arrives at the ground terminal GND. The tracks of the primary circuit L 1  have thus formed a loop of three turns which is wound and unwound. 
     The secondary circuit runs from the terminal PA 1  to the terminal PA 2  passing under the track P 11  to join with a track P 12  which arrives at the first crossing region CR 1 . In the crossing region CR 1 , a connection plate PL 1  directs the turn towards the interior and connects the track P 12  to a track P 24 . The track P 24  follows a half-turn up to a mid-point position connected to the biasing terminal VCC. Here, the secondary circuit L 2  has formed a first turn by winding and starts to unwind. A track P 14  starts from the mid-point and arrives at the first crossing region CR 1  in which a connection plate PL 2  directs the turn towards the exterior and connects the track P 14  to a track P 22 . The track P 22  arrives at the terminal PA 2  after passing under the track P 21 . 
     The tracks of the secondary circuit are disposed between the tracks of the primary circuit. In particular, the track P 12  is situated between the track P 11  and P 13 , the track P 14  is situated between the tracks P 13  and P 15 , the track P 22  is situated between the track P 21  and P 23 , and the track P 24  is situated between the tracks P 23  and P 25 . A constant gap separates, from edge to edge, the tracks of the primary circuit and the tracks of the secondary circuit. 
     Such a configuration forms a structure such that, over all of the positions of the secondary circuit at which a coupling with the primary circuit takes place, the sum of the distances from one terminal of the primary circuit to the corresponding coupled positions of the primary circuit is equal to the sum of the distances from the other terminal of the primary circuit to the same coupled positions. 
     In this configuration, the secondary circuit is coupled with the primary circuit in equal proportions at positions of the primary circuit close to one terminal and positions of the primary circuit close to the other terminal. In other words, the signal on the secondary circuit sees the ground terminal GND as much as the load terminal SE of the primary circuit. 
     Thus, when a signal travels over the secondary circuit, this signal is coupled in a uniform manner with the whole of the primary circuit, providing good phase and amplitude symmetries. This allows excellent behaviors with regard to balance of phases and balance of amplitudes to be obtained, and notably for power amplifier applications. 
     Moreover, the tracks P 11 , P 21 , P 15  and P 25  of the primary circuit L 1  are narrower than the other tracks. Their width is approximately half of the width of a track of the secondary circuit L 2 . Narrower metal tracks notably allow the stray capacitance existing between the metal tracks and the substrate to be reduced. The current flowing in the primary circuit is usually lower than that flowing in the secondary circuit. Thus, an advantageous decrease in the width of the tracks over certain parts of the primary circuit is not detrimental with respect to current flow. 
     It is also possible to form each of the tracks P 13  and P 23  in the form of two narrow parallel tracks. Each narrow parallel track may be separated from the edge of the tracks of the secondary circuit by the same constant separation. In this embodiment, the tracks for connecting the primary circuit can have the same thickness as the tracks of the secondary circuit. This is advantageous with regard to noise signals. 
       FIG.  2    shows a perspective view of the first crossing region CR 1  in which the interleaved and stacked metal tracks are shown in transparency for a better understanding of the architecture of this embodiment. In the first crossing region CR 1 , the metal track of the secondary circuit P 14  is connected to the metal track P 22  via a connection plate PL 2 . The metal track of the secondary circuit P 24  is connected to the track P 12  via another connection plate PL 1 . 
     The connection plate PL 2  is formed at the same level of metal as the metal tracks forming the turns of the primary and secondary inductive circuits, and takes the form of a rectangular plate. The tracks P 14  and P 22  are connected to the connection plate PL 2  on two opposing sides of the rectangular plate, each on one respective end of the side, with the ends being diagonally opposite. 
     The connection plate PL 1  is formed on a level of metal that is higher than the level of the metal tracks of the primary and secondary inductive circuits. The connection plate PL 1  also takes the form of a rectangular plate additionally comprising two wings respectively on two opposing sides of the rectangular plate. Each wing is on one end of the respective side, and with the ends being diagonally opposite. 
     The tracks P 12  and P 24  are connected to the connection plate PL 1  on the lower surface of the respective wings. Furthermore, the connection plates PL 1  and PL 2  are the same size and are aligned in a vertical axis perpendicular to the plane. The diagonals along which the tracks of the secondary circuit are connected to one connection plate or another opposite to each other. 
     Moreover, in this non-limiting representation, the wings of the connection plate PL 1  each have a bevel 1 and 2 at their attachment with the rectangular plate PL 1 . This configuration is advantageous with regard to current flow and is not detrimental to the balanced aspect of the couplings implemented by the disclosure. Indeed, although not being strictly geometrically symmetric with respect to the axis X, this configuration is balanced with regard to coupling between the primary and secondary circuits. 
       FIG.  3    shows a perspective view of the second crossing region CR 2  in which the interleaved and stacked metal tracks are also shown in transparency for a better understanding of the architecture provided for this embodiment. In the second crossing region CR 2 , the metal track of the primary circuit P 11  is connected to the metal track P 23  via a connection track PL 6 . The connection track PL 6  is at a lower level than the metallization level of the tracks forming the turns of the circuit, passing under the biasing terminal VCC. 
     The metal track P 13  is connected to the track P 21  via a connection track PL 5 , passing over the biasing terminal VCC, in a higher metallization level than the metallization level of the tracks forming the turns of the circuit. 
     The biasing terminal VCC takes the form of a rectangular plate and is connected along one of its widths in such a manner as to be centered on the mid-point of the secondary circuit. The width of the rectangular plate of the biasing terminal measures around twice the width of a metal track due to the high current flowing on the biasing terminal. 
     Thus, the connection tracks PL 5  and PL 6  cross each other on either side of the biasing terminal VCC in a symmetrical manner with respect to the axis X. This provides good performance characteristics with regard to phase and amplitude symmetries. 
     The connection tracks PL 5  and PL 6  may take the form of rectangular plates of identical size to the plate of the biasing terminal VCC, superposed over each other and with the biasing terminal. All three are aligned along a vertical axis perpendicular to the plane P. 
     The disclosure may advantageously be employed for any power application in radio frequency (RF) telecommunications systems, and  FIG.  4    shows one example of an input or output stage of such a system SYS. For example, the system is of the cellular mobile telephone or tablet type, and comprises a balun BLN according to the disclosure. 
     The terminal SE of the primary circuit L 1  of the balun BLN is connected to an antenna ANT, typically with an impedance of 50 Ohms, and the terminal GND is connected to an external ground. The antenna may be used both as a transmitter and a receiver. 
     The terminals PA 1  and PA 2  of the secondary circuit L 2  are, on the other hand, connected to processing circuit or a processor MTD in differential mode. This may comprise, for example, a low-noise amplifier LNA. The mid-point of the secondary circuit L 2  is connected to a decoupling capacitor Cap connected to the ground GND_PA associated with the differential-mode circuit connected to the terminals of the secondary circuit L 2 . 
     The balun BLN thus supplies an output signal in a differential mode (or in single-ended mode) starting from an input signal received in a single-ended mode (or in differential mode) with very little losses, excellent phase and amplitude symmetries, while at the same time allowing the passage of a current of high intensity. Such performance characteristics allow the efficiency of power amplifiers combined with the transformer BLN according to the disclosure to be optimized. 
     Furthermore, the disclosure is not limited to the embodiments that have just been described but encompasses all their variations. Thus, a balun comprising a primary circuit with three turns and a secondary circuit with two turns has been described, but it is possible, notably in order to design the impedance transformation ratio of the balun BLN, for the primary circuit to comprise N+1 turns and the secondary circuit to comprise N turns. N is an integer number greater than or equal to 2. The number of first crossing regions and of second crossing regions comprising the features previously described may vary as a function of the number of turns on the primary and secondary circuits.