Patent Description:
A transformer is typically used to couple two sides of an electrical system, sometimes called a primary side and a secondary side. The two sides are often electromagnetically coupled and separated by a galvanic isolation barrier.

A transformer can be used to transfer information or energy across the galvanic isolation barrier, to form an isolated communication channel, or to supply power to a different portion of a circuit for safety and/or data integrity considerations. Both data flow and power flow can be uni- or bi-directional, depending on the application requirements.

Some isolated DC-DC converters include a driver that drives a primary winding of a transformer to transmit power to a secondary winding of the transformer across an isolation barrier. A rectifier converts the received voltage at the secondary winding of the transformer into an output DC voltage.

There are many construction technologies and topologies for transformers. Generally, transformers include conductive wires wound to form loops which generate and collect the magnetic field on the respective sides of the isolation barrier. Optionally, there may be a magnetic material provided in a transformer to help direct and contain the magnetic field.

Miniature transformers or planar transformers are transformers having conductors that are integrated in a semiconductor package, for example embedded in a substrate.

When a transformer is stimulated with an electric signal, the magnetic field coupling between the two sides may be represented in a simplified circuit model by coupled inductors, while the electric field across the isolation barrier may be represented by capacitors across the barrier. When a transformer operates with signals having a frequency in the MHz to GHz range or RF signals, the capacitive effect between both sides of the isolation barrier becomes more pronounced compared to low frequency operation. The design of a circuit system employing a RF transformer sometimes may model the transformer as a single electromagnetic device that is an inductive device with a parasitic capacitive effect. <CIT> relates to a dual core planar transformer that comprises: a core part having a pair of cores electromagnetically coupled to each other; and a substrate part located between the pair of cores, and including a plurality of substrates in which a first through hole and a second through hole are formed. The substrate can have a primary winding and a secondary winding formed in a double helical structure along circumferences of the first through hole and the second through hole. <CIT> relates to a switching supply such as a DC/DC converter, where primary side coils provided in two transformers are mutually connected in series. <CIT> relates to a semiconductor device that includes: an base substance having a ferromagnetic material; a first semiconductor chip and a second semiconductor chip installed on the base substance; a first coil installed on the base substance and electrically connected to the first semiconductor chip; a second coil installed on the first coil, electromagnetically connected to the first coil and electrically connected to the second semiconductor chip; a transformer assembly made of a ferromagnetic material and installed on the base substance, and a sealing body. <CIT> relates to a transformer that comprises a first magnetic core, a second magnetic core, at least two layers of wiring boards and coils, wherein the at least two layers of wiring boards are provided with at least one pair of magnetic core through holes, and each pair of magnetic core through holes include the first magnetic core through hole and the second magnetic core through hole; a closed magnetic circuit can be formed by the first magnetic core and the second magnetic core, and the closed magnetic core penetrates through the first magnetic core through holes and the second magnetic core through holes; one coil is fixed around each pair of magnetic core through holes in each layer of wiring board, and the coils are used for forming at least one first primary winding and at least one secondary winding. <CIT> relates to a centre-tapped transformer that comprises at least one first layer comprising a first portion of a primary winding interleaved with a first portion of a secondary winding and at least one second layer comprising a second portion of the primary winding interleaved with a second portion of the secondary winding. The second layer is positioned on top of the first layer. The primary winding of the first layer is connected to the primary winding of the second layer through the centre of the first and second layers and the secondary winding of the first layer is connected to the secondary winding of the second layer through the centre of the first and second layers. <CIT> relates to a transformer and switching power supply device having a core. A first winding is wound around the core in a first side of the core. A second winding is wound around the core in a second side of the core that faces the first side of the core. A third winding is wound around the core in the first side of the core. A fourth winding is wound around the core in the second side of the core. The first and second windings are connected in series or parallel to each other, and the third and fourth windings are connected in series or parallel to each other.

The invention relates to a transformer set forth in independent claim <NUM> for use in an isolated DC-DC converter as set forth in claim <NUM>. The dependent claims <NUM> to <NUM> refer to preferred embodiments.

Disclosed herein is a symmetric split planar transformer in the context of a DC-DC isolated converter. The symmetric split planar transformer reduces or eliminates asymmetry in the distribution of parasitic capacitance across the isolation barrier going from one end to another end of a primary coil, and as a result, undesirable electromagnetic interference (EMI) due to common mode dipole emission across the isolation barrier may be reduced.

The primary winding is split into at least a first coil and a second coil, each occupying a different area side-by-side on a substrate. The transformer is symmetric in the sense that a capacitive coupling of the first coil to a secondary winding is the same as a capacitive coupling of the second coil to the secondary winding, such that common mode EMI may be reduced. Each coil may include stacked spiral coil portions in multiple metal planes to increase inductive density across the isolation barrier. Furthermore, in some embodiments the first and second coils may have opposite spiral directions such that far field radiation effect from the transformer may be reduced.

The transformer comprises a substrate having a first surface and a second surface opposing the first surface; a primary winding having a first coil in contact with and defining a first enclosed area on the first surface and a second coil in contact with and defining a second enclosed area on the first surface. The first and second enclosed area are disposed side by side and separated from each other. The transformer further comprises a secondary winding having a third coil and a fourth coil each in contact with the second surface. A capacitive coupling between the first coil and the third coil equals a capacitive coupling between the second coil and the fourth coil.

In some embodiments, an isolated DC-DC converter is disclosed. The isolated DC-DC converter comprises a substrate having a first surface and a second surface opposing the first surface; a first transformer comprising a first coil in contact with and defining a first enclosed area on the first surface, and a third coil in contact with the second surface. At least a portion of the first coil is aligned with at least a portion of the third coil along a vertical direction perpendicular to the first surface. The isolated DC-DC converter further comprises a second transformer comprising a second coil in contact with and defining a second enclosed area on the second surface, and a fourth coil in contact with the second surface, at least a portion of the second coil is aligned with at least a portion of the fourth coil along the vertical direction. The first and second enclosed area are disposed side by side and separated from each other. A capacitive coupling between the first coil and the third coil equals a capacitive coupling between the second coil and the fourth coil.

In not claimed embodiments, a split planar transformer is disclosed. The split planar transformer comprises a substrate having a first surface and a second surface opposing the first surface; a first transformer comprising a first coil in contact with the first surface, and a third coil in contact with the second surface. The first coil is aligned with the third coil along a vertical direction perpendicular to the first surface. The split planar transformer further comprises a second transformer comprising a second coil in contact with the second surface, and a fourth coil in contact with the second surface. The second coil is aligned with the fourth coil along the vertical direction. The first and second coils are disposed side by side and separated from each other by a separation along a lateral direction along the first surface. A capacitive coupling between the first coil and the third coil equals a capacitive coupling between the second coil and the fourth coil.

Various aspects and embodiments of the disclosure will be described with reference to the following figures. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear.

Some aspects of the present disclosure are directed to a symmetric transformer design in the context of a DC-DC isolated converter. The symmetric transformer reduces or eliminates asymmetry in the distribution of parasitic capacitance across the isolation barrier going from one end to another end of a primary coil, and as a result, undesirable electromagnetic interference (EMI) due to common mode dipole emission across the isolation barrier may be reduced.

A first aspect relates to a split transformer design in which a primary winding is split into separate first and second coils, with a serial impedance connected in between the first and second coils. The transformer is symmetric in the sense that a capacitive coupling of the first coil to a secondary winding is the same as a capacitive coupling of the second coil to the secondary winding, such that common mode EMI may be reduced. The serial impedance is a single capacitor serving as capacitance in a resonant LLC network for a DC-DC converter, saving cost and reducing complexity compared to circuit designs having a pair of capacitors provided on the outside of the primary winding. In some embodiments, the transformer is a planar transformer spanning multiple metal layers, and the serial impedance may be connected to a pair of center-tapped terminals in the primary winding.

A second aspect relates to a split planar transformer design in which the primary winding of a transformer is split into at least a first coil and a second coil, each occupying a different area side-by-side on a substrate. The transformer is symmetric in the sense that a capacitive coupling of the first coil to a secondary winding is the same as a capacitive coupling of the second coil to the secondary winding, such that common mode EMI may be reduced. In some embodiments, each coil may include stacked spiral coil portions in more than one metal planes, such that upon a current flow magnetic field generated in the coil portions overlap to increase inductive density across the isolation barrier, leading to higher inductance for a given area. Furthermore, in some embodiments the first coil and the second coil may have opposite spiral directions such that their generated magnetic fields have opposite polarity. As a result, far field radiation effect from the transformer may be reduced.

<FIG> is a schematic diagram that illustrates an example of electromagnetic interference in an isolated DC-DC converter. <FIG> shows a DC-DC converter <NUM> that includes a primary side <NUM> having a driver <NUM> and a primary winding <NUM> of a transformer <NUM>. The DC-DC converter <NUM> further includes a rectifier <NUM> and a secondary winding <NUM> of transformer <NUM> in a secondary side <NUM>.

In the DC-DC converter <NUM>, driver <NUM> is powered by a DC voltage Vcc to drive the primary winding <NUM> of the transformer <NUM> between terminals P1 and P2. The primary winding <NUM> is electromagnetically coupled to the secondary winding <NUM> across an isolation barrier <NUM>, which may be formed of a dielectric. The secondary winding <NUM> is coupled to rectifier <NUM> via terminals S <NUM> and S2 to convert signals received in the secondary winding <NUM> into an output DC voltage VISOOUT. Driver <NUM> may take any suitable form, and in some embodiments may be a resonating circuit. In a preferred embodiment, driver <NUM> may be a full bridge driver.

Referring back to <FIG>, which shows distributed parasitic capacitances C1 and C2 across the isolation barrier <NUM>. It should be appreciated that while two capacitor symbols C1 and c2 are used, they are a simplified representation of distributed capacitive coupling from one end P1 of the primary winding <NUM> to another end P2 of the primary winding, with C1 representing the capacitive coupling between the portion of the primary winding adjacent terminal P1 and the secondary winding, and vice versa for C2.

As shown in <FIG>, when transformer <NUM> is stimulated by alternative current (AC) driving signals <NUM> from driver <NUM>, a common mode AC current <NUM> is injected across the isolation barrier <NUM>. Common mode current <NUM> does not have a physical return path, and will create a dipole antenna between the voltage domain in primary side <NUM> and the voltage domain in secondary side <NUM>, which can radiate and generate electromagnetic interference (EMI) emissions <NUM>.

The inventors recognized and appreciated that the net capacitive current across the isolation barrier is related to asymmetries in the DC-DC converter system. For example, if the driving signals are fully differential and the capacitances across the barrier are balanced (e.g. C1 is equal to C2), then capacitive currents injected closer to terminal P1 cancels out capacitive currents injected closer to terminal P2, and the net sum is <NUM>.

It is recognized that asymmetries that may lead to poor EMI performance may include but not limited to: asymmetry of the active switches in the transformer driver and rectifier; asymmetry of the timing of driver and rectifier; asymmetry of the transformer and asymmetry of the impedance connecting the power stage (driver and rectifier) to the transformer. Aspects of the present disclosure is related to reducing asymmetry of the transformer, namely, asymmetry in the capacitance distribution (e.g. a C1 that is different from C2 in <FIG>) to improve EMI performance by reducing common mode dipole emissions.

<FIG> is a simplified circuit diagram of an exemplary symmetric transformer, according to a first aspect of the present disclosure. <FIG> shows a transformer <NUM> having a primary winding <NUM> and a secondary side <NUM>. The primary winding is split into two serially-connected coils <NUM>, <NUM>. First coil <NUM> is coupled between terminals P1, P2. Second coil <NUM> is coupled between terminals P3, P4. The secondary winding <NUM> is coupled between terminals P5, P6. According to some embodiments, transformer <NUM> is symmetric in that a capacitance C1 between the first coil <NUM> and secondary winding <NUM> equals a capacitance C2 between the second coil <NUM> and secondary winding <NUM>.

While <FIG> only depicts a secondary winding having a single coil, it should be appreciated that the split coil design in primary winding <NUM> may be applied to the secondary winding as well to provide a symmetric transformer.

<FIG> is a simplified circuit diagram showing a symmetric transformer that is a variation of the transformer in <FIG>. In <FIG>, transformer <NUM> has a primary winding <NUM> and a secondary winding <NUM>. Primary winding <NUM> is split into two separate coils <NUM>, <NUM> that each has an equal capacitance to the secondary winding <NUM>. Transformer <NUM> differs from transformer <NUM> in <FIG> in that instead of connected in series, the two coils <NUM> and <NUM> are connected in parallel, such that each of the coils is coupled between terminals P1 and P2. According to an aspect, connecting coils within the primary winding in parallel may increase the magnetic field and hence the inductive coupling in the transformer.

<FIG> is a simplified circuit diagram showing a DC-DC converter having a symmetric transformer, in accordance with some embodiments. Shown in <FIG> is a DC-DC converter <NUM> having a driver <NUM>, a transformer <NUM>, and a rectifier <NUM>. Driver <NUM> is coupled to terminals P1, P4 of a primary winding <NUM> of transformer <NUM> to drive a RF signal through the primary winding. Rectifier <NUM> is coupled to terminals P5, P6 of a secondary winding <NUM> of transformer <NUM> to convert signals received in the secondary winding <NUM> into an output DC voltage.

Still referring to <FIG>, transformer <NUM> is a symmetric transformer having a split primary winding <NUM> that includes first coil <NUM>, second coil <NUM>, and an impedance <NUM> that are in series along one current path <NUM>. Transformer <NUM> is symmetric in that a capacitance C1 between the first coil <NUM> and secondary winding <NUM> equals a capacitance C2 between the second coil <NUM> and secondary winding <NUM>.

In <FIG>, first coil <NUM> is coupled between terminals P1, P2. The serial impedance <NUM> is coupled between terminals P2, P3. Second coil <NUM> is coupled between terminals P3, P4. Serial impedance <NUM> may comprise a capacitor. In some embodiments, DC-DC converter <NUM> may be a resonant DC-DC converter, and serial capacitance <NUM> when combined with the inductance L of the transformer <NUM> form a symmetric LLC resonant network that is driven by driver <NUM>. An example of DC-DC converter that has an LLC resonant network based on a transformer is described in <CIT> to Zhao et. It should be appreciated that aspects of the present disclosure are not limited to providing a serial capacitance, and that serial impedance <NUM> may comprise other reactive components.

<FIG> are a cutaway view diagram and a top view diagram, respectively, of a planar transformer that can be used to implement the symmetric transformer in the DC-DC converters of <FIG>, in accordance with some embodiments.

<FIG> shows a transformer <NUM> having a primary winding <NUM> and a secondary winding <NUM> on a substrate <NUM>. Substrate <NUM> may be a printed circuit board (PCB), and additional circuit components, while not shown, may be provided on the PCB. In some embodiments, substrate <NUM> may be integrated in a same package as a DC-DC converter using semiconductor manufacturing techniques known in the field.

<FIG> is a cutaway view of substrate <NUM> along A-A' in <FIG>, and shows that primary winding <NUM> and secondary winding <NUM> each comprises a plurality of coil portions <NUM>, <NUM> that are disposed on opposing surfaces of a core layer <NUM> of substrate <NUM>. Core layer <NUM> comprises an insulative dielectric material, and serves as galvanic isolation between the primary and secondary windings.

Coil portions <NUM> in the primary winding <NUM> comprise conductors that are coplanar in a first plane <NUM>. Each of the coil portions <NUM> may comprise a conductive material such as metal that are be fabricated on a top surface of core layer <NUM> by any suitable fabrication method known in the field, such as but not limited to masked deposition, etching, or combinations thereof. In such embodiments, plane <NUM> may be a metal layer of the process flow. As viewed in <FIG>, coil portions <NUM> may be elongated conductors of substantially the same width, although uniform width is not a requirement and any suitable dimension may be used.

Still referring to <FIG>, conductors in primary winding <NUM> are wound within plane <NUM> and extend from terminal P1 toward P4 (see e.g. the circuit diagram in <FIG>). The primary winding may be shaped as a spiral that extends in a clockwise fashion beginning at terminal P1, with conductors in coil portion <NUM> that initially shrink in enclosed area to avoid shorting to itself. When the primary winding reaches a smallest enclosed area, it continues to extend clockwise while extending outward via coil portions <NUM> with increasing enclosed area towards terminal P4. To avoid shorting of coil portion <NUM> with coil portions <NUM>, vias <NUM> and bridges <NUM> are provided such that coil portions <NUM> may crisscross coil portion <NUM> at a different plane from plane <NUM> (not shown in <FIG>). Details of the vias <NUM> and bridges <NUM> will be discussed below with respect to the examples in <FIG>.

Referring back to <FIG>, which shows that secondary winding <NUM> coupled between terminals P5, P6 and having coil portions <NUM> that are coplanar in a second plane <NUM>. In some embodiments, the second plane <NUM> may be another metal layer in the substrate <NUM>. As shown in <FIG>, coil portions <NUM> in the secondary winding are aligned vertically with coil portions <NUM> in the primary winding <NUM>. Because capacitance is proportional to areas and distance between two adjacent conductive objects, when the distances between pairs of coil portions are maintained to be a uniform spacing based on the thickness of the core layer <NUM>, and when the lateral dimensions of coil portions <NUM> and <NUM> are matched to each other, the distributed capacitive coupling between conductors in coil portions <NUM> and <NUM> may be maintained to be uniform throughout the length of the primary winding <NUM>. It should be appreciated that while obfuscated in <FIG>, secondary winding <NUM> may additionally comprise vias and bridges that extend out to a plane different from plane <NUM> to allow winding coil portions <NUM> to crisscross each other without shorting in a similar way to the primary winding <NUM>.

Referring back to <FIG>, which shows that the primary winding <NUM> has a break <NUM> splitting the primary winding into two coils. A first coil <NUM> comprises coil portion <NUM> that is coupled to terminal P1, and further to terminal P2 via conductive structure 446a. A second coil <NUM> comprises coil portions <NUM> that is coupled to terminal P4, and further to terminal P3 via conductive structure 446b. In this embodiment, the break <NUM> is selected such that a capacitance between the first coil <NUM> to the secondary winding <NUM> equals a capacitance between the second coil <NUM> to the secondary winding <NUM>, and the transformer <NUM> is symmetric in this regard.

It should be appreciated that break <NUM> is not necessarily situated at the geometric center of the primary winding <NUM>, even though break <NUM> appears to be situated at the bottom center of primary winding <NUM> along the x-direction. The exact geometry of where primary winding <NUM> is split into first and second coils may be determined during the design phase of a symmetric transformer. By way of example, using simulation methods known in the field, a difference in distributed capacitance between the first coil to the secondary winding, and between the second coil to the secondary winding may be iteratively calculated while the coil geometry is adjusted, until the transformer becomes symmetric. Besides the location of a break in the primary winding, the size, dimension, dielectric isolation barrier thickness are all among exemplary parameters of the coils that can be adjusted to achieve symmetry, as is known by a person skilled in the field of RF transformers.

While not shown in <FIG>, in a preferred embodiment an impedance such as impedance <NUM> in <FIG> may be coupled to terminals P2, P3, and be serially connected to the first coil <NUM> and second coil <NUM> in a current path from terminal P1 to terminal P4.

<FIG> also shows an insulative layer <NUM> disposed above core layer <NUM>, and an insulative layer <NUM> disposed below core layer <NUM>. Insulative layers <NUM>, <NUM> encapsulates primary and secondary windings <NUM> and <NUM>, respectively, and provides mechanical support and electrical isolation to conductors disposed on the core layer <NUM>. Insulative layers <NUM>, <NUM> may comprise any suitable dielectric materials known in the field of PCB manufacturing, such as but not limited to oxide, nitride, ceramics, polymers and mixtures thereof. In a preferred embodiment, insulative layers <NUM>, <NUM> comprise a prepreg layer.

<FIG> additional shows an insulative layer <NUM> in contact with a surface of insulative layer <NUM>, and an insulative layer <NUM> in contact with a surface of insulative layer <NUM>. Layers <NUM>, <NUM> may comprise a polymer material, for example a solder mask.

It should be appreciated that substrate <NUM> is illustrated as comprising a composition of five layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> for illustrative purpose only, and that additional materials and layers may be present. It is also not a requirement that core layer <NUM> and insulative layer <NUM>, <NUM> be of different compositions. Furthermore, while <FIG> only illustrates one metal layer on either surface of core layer <NUM>, additional metal layers may be provided in the substrate <NUM>.

<FIG> are top view and perspective view diagrams of an interleaved planar transformer, in accordance with some embodiments. <FIG> show a planar transformer <NUM> having a primary winding <NUM> coupled between terminals P1 and P4, and a secondary winding <NUM> separated and isolated from the primary winding <NUM> by a core layer (not shown for simplicity). Secondary winding <NUM> is coupled between terminals P5 and P6.

<FIG> shows that primary winding <NUM> is split into a first coil <NUM> and a second coil <NUM>. The first coil <NUM> is coupled to terminals P1 and P3 via traces <NUM> and pads <NUM>. The second coil <NUM> is coupled to terminals P3 and P4 via traces <NUM> and pads <NUM>. Traces <NUM> are elongated conductors that fan-out in an x-y plane to coupled the respective coils to pads <NUM> serving as terminals in order to accommodate the different pitch and sizing between the pads and conductors in the coils.

As shown in <FIG>, the primary winding <NUM> comprise coil portions primarily in a first metal layer <NUM>, while traces <NUM> and pads <NUM> are disposed in a second metal layer <NUM> that is offset and above the first metal layer <NUM>. Vias <NUM> are provided to vertically interconnect a trace <NUM> with corresponding coil portions in the metal layer below.

Transformer <NUM> is a symmetric transformer, with a break <NUM> in the primary winding such that a capacitance between the first coil <NUM> to the secondary winding <NUM> equals a capacitance between the second coil <NUM> to the secondary winding <NUM>. While not shown in <FIG>, in a preferred embodiment a serial impedance such as impedance <NUM> in <FIG> is provided and coupled to terminals P2, P3, and be serially connected to the first coil <NUM> and second coil <NUM> in a current path from terminal P1 to terminal P4.

As shown in <FIG>, second coil <NUM> comprises a plurality of coil portions such as coil portions 532a, 532b. Second coil <NUM> crisscrosses first coil <NUM> using a bridge <NUM> that is connected to adjacent coil portions 532a, 532b using a pair of vertical vias <NUM>. Bridge <NUM> is disposed in a different plane than first metal layer <NUM>. In a preferred embodiment, bridge <NUM> comprises conductors in the same second metal layer <NUM> as traces <NUM> and pads <NUM>. In the embodiment shown, at least one bridge <NUM> is enclosed on at least two sides by a pair of traces 546a, 546b that connect to terminals P3 and P2.

<FIG> show that secondary winding <NUM> have coil portions <NUM> that are vertically aligned with coil portions in the primary winding <NUM>. <FIG> illustrates a flipped view point of transformer <NUM> compared to the perspective in <FIG>, and shows that coil portions in the secondary winding <NUM> are disposed in a third plane or third metal layer <NUM>. Bridges <NUM> are provided in the secondary winding <NUM> to allow crisscross between coil portions <NUM> without shorting to each other. In some embodiments, bridges <NUM> comprise conductors disposed in a fourth metal layer <NUM> that is offset from metal layer <NUM> and away from the metal layers <NUM> and <NUM> of the primary winding.

As shown in <FIG>, the two terminals of the secondary winding P5 and P6 are provided at the top of transformer <NUM>, opposite terminals P1 and P4 for the primary winding along the y-direction. In a preferred embodiment, terminals P5, P6 are implemented as pads <NUM> that are disposed in the second metal layer <NUM>, the same layer as pads <NUM> for terminals P1~P4, as well as traces <NUM>. <FIG> and <FIG> show that a pair of vertical interconnects <NUM> passes through the insulative materials and connects coil portions in the third metal layer <NUM> of the secondary winding to respective pads <NUM> serving as terminals P5, P6. In such an embodiment, all six terminals are disposed in the same metal plane on one side of the substrate to make the pads more accessible for electrical connections to other components.

While the embodiment shown in <FIG> involve conductors in four metal layers <NUM>, <NUM>, <NUM>, <NUM>, it should be appreciated that aspects of the present application are not so limited as one or more components may be disposed in additional metal layers. It should also be appreciated that while the embodiments show a bridge or a trace/pad that is routed above or below a coil portion to avoid shorting, both underpass and overpass relative to the coil portion may be used for such routing and aspects of the present disclosure is not so limited.

<FIG> are top view and perspective view diagrams of another exemplary interleaved planar transformer <NUM> that is a variation of the planar transformer <NUM> shown in <FIG>. Transformer <NUM> is similar in many aspects to transformer <NUM>, with like components marked with identical reference numbers. Transformer <NUM> differs from transformer <NUM> in that instead of using traces to couple a serial impedance in the primary winding <NUM>, a pair of bond wires 649a, 649b are provided.

As shown in <FIG>, an end of the first coil <NUM> is coupled to a bond pad 647b, and the bond pad 647b is connected to bond pad 647d acting as terminal P4 via bond wire 649b. Similarly, an end of the second coil <NUM> is coupled to a bond pad 647a, and the bond pad 647a is connected to bond pad 647c acting as terminal P3 via bond wire 649a. In some embodiments, the two terminals P1 and P4 may be directly connected to ends of coil portions in the first coil and the second coil without using traces. This is shown in the example in <FIG>, where bond pads 647e, 647f act as terminals P1, P4, respectively and are vertically interconnected using vias to conductors in respective first coil <NUM> and second coil <NUM> in the first metal layer <NUM>. In some embodiments, pads 647a, 647b, 647c, 647d, 647e and 647f are disposed in the second metal layer <NUM>.

<FIG> is a top view diagram of an exemplary interleaved planar transformer <NUM> that is a variation of the planar transformer <NUM> shown in <FIG>. Transformer <NUM> is similar in many aspects to transformer <NUM>, with like components marked with identical reference numbers. Transformer <NUM> differs from transformer <NUM> in that instead of using bond wires to couple a serial impedance with bond pads 6147a, 617b, a discreet component <NUM> may be directly mounted to the bond pads, as shown in <FIG>. Discreet component <NUM> may be a passive component such as a capacitor, and when mounted on bond pads 617a, 617b, is serially connected within the primary winding <NUM>. In a preferred embodiment, component <NUM> is a surface-mountable component and the mounting may be performed using soldering.

While embodiments discussed thus far in relation to <FIG> are based on planar transformers integrated in a substrate, aspects of the present disclosure are not so limited. For example, a symmetric split transformer with a serial impedance may be implemented in a non-planar coil topology, such as but not limited to a toroid or solenoid.

<FIG> is a schematic diagram of a symmetric split transformer implemented in a solenoid structure, in accordance with some embodiments. In <FIG>, solenoid transformer <NUM> has a primary winding <NUM>, a secondary winding <NUM> both wound up around a magnetic core <NUM>. Because conductors of the primary winding <NUM> and secondary winding <NUM> are intertwined but isolated from each other, there is a distributed capacitance between the primary winding and the secondary winding along the length of the primary winding <NUM> from terminal P1 to terminal P4.

The primary winding <NUM> is split into a first coil <NUM> coupled between terminals P1 and P2 and a second coil <NUM> coupled between terminals P3 and P4, such that a capacitance between the first coil <NUM> to the secondary winding <NUM> equals a capacitance between the second coil <NUM> to the secondary winding <NUM>. An impedance <NUM> is coupled to terminals P2 and P3, such that the first coil <NUM>, second coil <NUM>, and the impedance <NUM> are in series with each other and that a current path <NUM> flows through each of the first coil <NUM>, impedance <NUM> and second coil <NUM>.

A second aspect of the present disclosure relates to a planar transformer design having a primary winding that is split into a first coil and a second coil each enclosing a different area side-by-side on a substrate. In some embodiments, the secondary winding is also split into a third coil and a fourth coil that each is aligned underneath respective first and second coils of the primary winding, thereby splitting the planar transformer into two half-transformers that are connected in series.

<FIG> is a simplified circuit diagram showing a DC-DC converter having a symmetric transformer, in accordance with the second aspect of the present disclosure. Shown in <FIG> is a DC-DC converter <NUM> having a driver <NUM>, a transformer <NUM>, and a rectifier <NUM>. Driver <NUM> is coupled to terminals P1, P6 of a primary winding <NUM> of transformer <NUM> to drive a RF signal through the primary winding. Rectifier <NUM> is coupled to terminals P3, P8 of a secondary winding <NUM> of transformer <NUM> to convert signals received in the secondary winding <NUM> into an output DC voltage.

Still referring to <FIG>, primary winding <NUM> of transformer <NUM> is split into a first coil <NUM>, second coil <NUM>, and an impedance <NUM> that are in series along current path <NUM>. Secondary winding <NUM> of transformer <NUM> is split into a third coil <NUM>, and a fourth coil <NUM> connected in series. Transformer <NUM> is symmetric in that a capacitance C1 between the first coil <NUM> and third coil <NUM> equals a capacitance C2 between the second coil <NUM> and the fourth coil <NUM>.

In <FIG>, first coil <NUM> is coupled between terminals P1, P2. The serial impedance <NUM> may be similar to impedance <NUM> as shown in <FIG>, and is coupled between terminals P2, P5. Second coil <NUM> is coupled between terminals P5, P4. Third coil <NUM> is coupled between terminals P3, P4, while fourth coil <NUM> is coupled between terminals P7, P8. Second coil <NUM> is coupled between terminals P5, P4. In a preferred embodiment, serial impedance <NUM> comprises a capacitor.

Still referring to <FIG>. As first coil <NUM> is inductively coupled to third coil <NUM>, the pair forms a half-transformer or transformer A. Similarly, second coil <NUM> forms a transformer B with fourth coil <NUM>.

<FIG> are a cutaway view diagram and a top view diagram, respectively, of a split planar transformer that can be used to implement the transformer as shown in <FIG>, in accordance with some embodiments.

<FIG> is a cutaway view of a substrate <NUM> along B-B' in <FIG> shows a transformer <NUM> having a split primary winding that comprises a first coil <NUM> and a second coil <NUM>. Transformer <NUM> also have a secondary winding that comprises a third coil <NUM> and a fourth coil <NUM>. Each of the first coil <NUM> and the second coil <NUM> comprise conductors disposed in two metal planes M1 and M2. Each of the third coil <NUM> and the fourth coil <NUM> comprise conductors disposed in two metal planes M3 and M4.

Still referring to <FIG>. The coils are disposed in a substrate <NUM>, which may be similar to substrate <NUM> as shown in <FIG>. In the example shown, substrate <NUM> includes an insulative core layer <NUM>. Core layer <NUM> comprises an insulative dielectric material, and serves as galvanic isolation between the primary and secondary windings. First coil <NUM> and second coil <NUM> of the primary winding are disposed on a first surface 404a of the core layer <NUM>. Third coil <NUM> and fourth coil <NUM> are disposed on a second surface 404b of the core layer <NUM>.

Turning now to the top view diagram in <FIG>, which shows that the first coil <NUM> encloses an area S1 on the first surface 404a of the core layer, while the second coil <NUM> encloses an area S2 on the first surface 404a that is side-by-side to but separated from the area S1. Thus the transformer <NUM> is split into two half-transformers 1000A and 1000B that each occupy a different area S1, S2 on a surface of the semiconductor substrate <NUM>. Within transformer 1000A, at least a portion of first coil <NUM> is vertically aligned with and overlapped with at least a portion of the third coil <NUM>. Similarly, within transformer 1000B, at least a portion of second coil <NUM> is vertically aligned with and overlapped with at least a portion of the fourth coil <NUM>.

In <FIG>, the two half-transformers 1000A, 1000B are disposed side-by-side, and each shaped like an elongated racetrack, with a longer leg along the y-direction than an leg along the x-direction. While there are no limitation as to the shape and size of the primary winding, secondary winding, and each half-transformer, having the two half-transformers 1000A, 1000B each shaped like a half-width transformer as shown in <FIG> takes up less footprint on the substrate <NUM>.

The operation and connection between terminals within transformer <NUM> will be explained with reference to <FIG> is a perspective view diagram showing coil portions in different metal planes that are included in each coil of the transformer <NUM> in <FIG>. <FIG> is a series of cutaway view diagrams showing coil portions in the four different metal planes of <FIG>.

On the primary side of half-transformer 1000A, <FIG> shows that first coil <NUM> comprises at least two coil portions <NUM>, <NUM> each disposed in a respective metal layer M1, M2. Because the first coil <NUM> spans two different metal layers, a via <NUM> is used to vertically interconnect the coil portions <NUM>, <NUM> such that first coil <NUM> comprises a spiral having two turns. While coil portions in two metal layers are shown in <FIG>, first coil <NUM> may comprise more than two layers of coil portions, as more coil portions means higher number of turns within the first coil and higher inductance values.

When a clockwise current I flows from terminal P1 through the first coil <NUM> towards terminal P2, a magnetic field is generated within area S1 that is pointing downward, with the strength of the magnetic field proportional to the number of turns in the first coil. As a result, compared to single-metal layer coils, transformer <NUM> may generate multiple times higher magnetic field within the same unit area on the substrate. One benefit of the multiple metal plane coil design in transformer <NUM> is that each of a plurality of metal planes may be used to host spiral coils without the need to utilize a metal plane to form bridges as underpass or overpass, such as bridge <NUM> shown in <FIG>. As a result, a high inductance can be implemented within a relatively small footprint on a substrate, given the quadratic impact of the number of turns on the amount of transformer inductance.

On the primary side of half-transformer 1000B, <FIG> that second coil <NUM> comprises at least two coil portions <NUM>, <NUM> that spirals in the opposite direction compared to the first coil <NUM>, such that when current I flows through interconnect <NUM> toward terminal P6 via terminal P5, there is a counterclockwise current generating a magnetic field in the upward direction within area S2. Thus when a current flows serially through the primary coils <NUM>, <NUM> of transformer <NUM>, magnetic fields generated within half-transformers 1000A and 1000B point to opposing directions. As a result of the two magnetic field canceling each other, far field radiation from transformer <NUM> may be reduced.

Returning to the secondary side of half-transformer 1000A. <FIG> show that third coil <NUM> comprises at least two coil portions <NUM>, <NUM> each disposed in a respective metal layer M3, M4. While first coil <NUM> is isolated from third coil <NUM> by the core layer <NUM>, there is high capacitive coupling between coil portion <NUM> on layer M2 (connected to P2) and coil portion <NUM> on M3 (connected to P4) compared to a low capacitive coupling between coil portion <NUM> on M1 and coil portion <NUM> on M4 which are spaced farther apart vertically. In some embodiments, such nodes will be driven by equal and opposite voltages, hence cancelling the impact of such capacitors on common mode currents. In some embodiments, transformer <NUM> can be designed to be symmetric such that a capacitance between first coil <NUM> and third coil <NUM> equals a capacitance between second coil <NUM> and fourth coil <NUM>.

In some embodiments, because of symmetry in the transformer <NUM>, terminal pairs P1/ P6, P3/P8, P2/P5 and P4/P7 will be driven by equal and opposite voltages, hence cancelling the impact of such capacitors on common mode currents across the isolation barrier and reducing EMI emissions.

Referring back to <FIG>. The various terminals P1~P8 of transformer <NUM> may be implemented in any suitable ways, such as but not limited to pads, traces, or other conductive structures within substrate <NUM> as shown in <FIG>. The number of accessible terminals makes transformer <NUM> a flexible design. For example, in the embodiment shown in <FIG>, first coil <NUM> and second coil <NUM> may be serially connected by an interconnect <NUM>. Interconnect 1046a may further couple an impedance <NUM> serially in between terminals P2 and P5. In the same embodiment, an interconnect 1046b may serially couple third coil <NUM> to fourth coil <NUM> by connecting terminal P4 to terminal P7.

In an alternative embodiment, the primary side coils of transformer <NUM> may be reconfigured to have parallel connection between the two half-coils <NUM> and <NUM>, for example in a configuration similar to the primary side of <FIG>.

<FIG> is a plan view schematic diagram of a split planar transformer that is a variation of the transformer in <FIG> with an added magnetic material, in accordance with some embodiments. <FIG> shows two halves 1000A, 1000B of transformer <NUM> disposed in a substrate <NUM>. The two halves 1000A, 1000B are magnetically coupled via a magnetic material <NUM> disposed on the substrate <NUM> to enhance the overall inductance of the primary and secondary side, hence increasing the amount of coil quality factors. In some embodiments, magnetic material <NUM> may comprise one or more magnetic material cores shared between the two transformer halves 1000A, 1000B.

In the embodiment shown in <FIG>, a single magnetic core <NUM> is disposed through a first aperture 1170a and a second aperture 1170b in the substrate <NUM>. The first and second apertures each passes through an area enclosed by the coils within respective half transformers 1000A, 1000B, such that the magnetic core <NUM> couples the two half transformer magnetically.

Having thus described several aspects of at least one embodiment of this technology, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art.

For example, while aspects of a symmetric transformer are discussed in the context of application in a DC-DC converter, the disclosed transformers may be used in any suitable RF transformer applications and are not limited to be used in a DC-DC converter, or with a soft-switching scheme.

Various aspects of the technology may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments.

Also, aspects of the technology may be embodied as a method, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

Claim 1:
A radio-frequency, RF, transformer (<NUM>) for use in an isolated DC-DC converter, comprising:
a substrate (<NUM>) having a first surface (404a) and a second surface (404b) opposing the first surface;
a primary winding having a first coil (<NUM>) in contact with and defining a first enclosed area (S1) on the first surface, a second coil (<NUM>) in contact with and defining a second enclosed area (S2) on the first surface, and a capacitor (<NUM>) disposed between and connected in series with the first and second coils, wherein the first and second enclosed area are disposed side by side and separated from each other, wherein the first coil comprises:
a first coil portion (<NUM>) disposed in a first plane (M2);
a second coil portion (<NUM>) disposed in a second plane (M1) parallel to and offset from the first plane; and
at least one via (<NUM>) connecting the first coil portion and the second coil portion to form the first coil;
a secondary winding having a third coil (<NUM>) and a fourth coil (<NUM>) each in contact with the second surface, wherein
a capacitive coupling between the first coil and the third coil equals a capacitive coupling between the second coil and the fourth coil.