Techniques for a coupled inductor circuit

A space efficient planar transformer can include a coupled inductor circuit that can include a metallic core, a first planar winding comprising a conductive coil having an electrical path encircling a first post of the metallic core, and a second planar winding configured to magnetically couple with the first winding via the metallic core. The second planar winding can have multiple portions. A portion of the second winding can include a first sub-portion comprising a single U-shaped planar conductive trace wrapped about the first post and a second sub-portion comprising a single U-shaped planar conductive trace wrapped about the first post. A layout of the first sub-portion can be oriented opposite a layout of the second sub-portion.

TECHNICAL FIELD

This application provides techniques for coupled inductor circuits for DC-DC voltage converters or regulators.

BACKGROUND

DC-DC switching regulators may use high-frequency switching to generate a desired output voltage for an electronic device. In certain applications, the demand for low voltage electronics to accept relatively high supply voltages creates challenges for stepping down the supply voltage to a low supply voltage. Similar challenges can also be found in step-up applications where a high supply voltage is converted from a low input supply voltage.

SUMMARY OF THE DISCLOSURE

A multiple-layer planar transformer can have a multiple-turn first planar winding implemented on multiple layers of a substrate and a single-turn second winding implemented on multiple layers of the substrate. The first and second windings can be magnetically coupled such as via a core. The individual planar traces of the second winding can be electrically coupled in parallel such as to allow for an efficient layout of the transformer that can allow for an improved power density.

DETAILED DESCRIPTION

Planar transformers, planar inductors that are magnetically coupled, or coupled inductor circuits, can employ a u-shaped or horseshoe-shaped planar conductive trace to form a single turn of a transformer winding. An assembly of several layers of the planar windings can allow for a very compact transformer circuit that can include output inductors such as for filtering.

FIG.1illustrates generally a schematic of an example planar transformer system100according to the present subject matter. The planar transformer system100can include a planar transformer101, a first set of switches (Q1-Q4), a second set of switches (M5-M8), output inductors102,103,104,105and a controller107. one or more switches of the first set of switches (Q1-Q4) or the second set of switches (M5-M8) can be accompanied by an optional gate driver110. The planar transformer101can include a core108, a first winding106, and a second winding such as can include two portions111,112. The first winding106can include a number of turns and typically can include more than one turn. Each portion111,112of the second winding can have a single turn.

The first winding106can be either a primary winding of the planar transformer or a secondary winding of the planar transformer. The first winding106is referred to as a primary winding unless noted otherwise. The first set of switches (Q1-Q4) can be controlled by the controller107and can operate to recurrently or periodically connect and disconnect the primary winding106to voltage supply nodes and to establish a cyclical primary voltage across the first winding106and a cyclical primary current through the first winding106. The portions111,112of the second winding can be magnetically coupled to the first winding106via the core108. The core108can include an air core, a metal core or other materials capable of providing a flux link.

The controller107can control the first set of switches (Q1-Q4) and the second set of switches (M5-M8) such as to provide an output voltage (VOUT) different than the input voltage (VIN). In the illustrated planar transformer system100, the output voltage (VOUT) can be lower than the input voltage (VIN) but the subject matter is not so limited. When the planar transformer101is operated as a step-down transformer, the controller107can control the first set of switches (Q1-Q4) to oscillate or alternate a polarity of the input voltage (VIN) across the first winding106. When the planar transformer101is operated as a step-down transformer, the controller107can operate or synchronize the second set of switches (M5-M8) to extract power from the winding segments, or portions, of the second winding. The second set of switches (M5-M8) can be controlled with two phase signals (PH1, PH2), such as discussed below with respect toFIG.3. The output inductors102,103,104,105can filter or smooth the periodic voltage and switching spikes developed on the second windings such as to provide a relatively constant output voltage. In certain examples, the output inductors102,103,104,105can include but are not limited to, magnetic core inductors, crystalline core inductors, etc.

FIG.2illustrates generally plots of signal waveforms illustrating an example of the operation of the planar transformer system of100ofFIG.1. The plots include the logic level of the phase1(PH1) and phase2(PH2) signals that control the switches (M5-M8) of the second winding, the voltage (V1) across the first winding, the current (I1) in the first winding, voltages (VA, VB, VC, VD) at the nodes (A, B, C, D) of the second winding, and the output voltage (VOUT) of the step-down transformer.

In general, the transformer operates in one of three phases to capture an output voltage induced by the first winding during the transitions associated with the supply voltage being applied to, or isolated from the first winding. When the supply voltage (VIN) is applied to, or isolated from, the first winding, the change in current (I1) through the first winding can induce a voltage across each second winding segment. By switching the connections of the second winding segments to capture the voltage induced as current polarity of the first winding is changed, a stepped-down DC voltage can be captured at the output inductors. The plot of signals assumes that a logic high places each switch, or transistor, in a low impedance state (e.g., “on”) and a logic low places each switch in a high impedance state (e.g., “off”). However, switches or transistors responding to logic commands differently can be used.

InFIG.2, at to, the first winding circuit is in a first, free-wheeling state and the second winding circuit has all the switches (M5-M8) “on” (e.g., PH1=PH2=“high”), thus, each node (A, B, C, D) of the second winding is connected to ground. The “free-wheeling” state of the first winding allows any current in the first winding to continue to flow until terminated by the circuit losses. Any current in the output inductors (e.g.,FIG.1,102,103) can be discharged to the output terminal, or charges the output voltage node (VOUT).

At t1, the first winding circuit moves to the second state, and a supply voltage can be applied across the first winding with a first polarity (−VIN). Applying the supply voltage (VIN) can induce a change in current (I1) of the first winding and a voltage can be induced across the winding segments of the second windings. For example, at or in response to applying the supply voltage (−VIN) on the first winding, the switches (FIG.1; M6, M8) associated with the phase2control signal (PH2) can be turned “off”. The change in current (I1) of first winding can induce a voltage at the drains (B, D) of the switches (FIG.3; M6, M8) associated with the phase2control signal (PH2). The magnetic coupling of the planar first and second windings can be such that the induced voltage (VB, VD) of the segments of the second winding can match the sharp, pulse shape of the supply voltage (VIN) applied to the first winding. At t1, the phase2signal (PH2) is low and the associated switches (M6, M8) are “off”. The first winding106, or primary winding for this step-down application, sees the full change in voltage across the winding terminals (e.g., VIN). Voltages across the connected winding segments of each of the second windings is given by:

VB=VD=VINN,
Where N is the turns ratio of the primary winding to each individual secondary winding. Assuming a load at the output terminal of the planar transformer system, current in the output inductors102,103can increase due to positive voltage across them.

At t2, the controller can transition the first winding back to the first, free-wheeling state and the second winding circuit has all the switches (M5-M8) “on” (e.g., PH1=PH2=“high”), thus, coupling each node of the second windings to ground. As before, any current flowing in the first winding continues to flow because the first winding inductance resists a change in current flow. The current may fall slightly during the free-wheeling state due to losses in the circuit. Any current in the output inductors (e.g.,FIG.1,102,103) is discharged to the output terminal, or charges the output voltage node (VOUT).

At t3, the first winding circuit moves to the third state, and the supply voltage (VIN) can be applied across the first winding with a second polarity (+VIN). Applying the supply voltage (VIN) can induce a change in current (I1) of the first winding and voltage can be induced across segments of the second winding. For example, at or in response to applying the supply voltage (+VIN) on the first winding, the switches (M5, M7) associated with the phase1control signal (PH1) can be turned “off”. The change in current (I1) of first winding can induce a voltage at the drains (A, C) of the switches (FIG.3; M5, M7) associated with the phase1control signal (PH1). The magnetic coupling of the planar first and second windings can be such that the induced voltage (VA, VC) at the nodes of the second winding can match the sharp, pulse shape of the supply voltage (VIN) applied to the first winding. At t3, the phase1signal (PH1) is low and the associated switches (M5, M7) are “off”. The first winding106, or primary winding for this step-down application, sees the full change in voltage across the winding terminals. Voltages across the connected winding segments of each of the second windings can be given by:

VA=VC=VINN,
Where N is the turns ratio of the primary winding to each individual secondary winding. Assuming a load at the output terminal of the planar transformer system, current in the output inductors102,103can increase due to positive voltage across them.

FIG.3illustrates generally an example layout320of conductive traces of sub-portions of an example second winding according to the present subject matter. For example,FIG.3illustrates the overlay of two, single-turn, planar sub-portions321,322that can form a portion (e.g.,FIG.1,111,112) of the second winding as discussed above. In certain examples, the example layout320can include the core108, a first sub-portion321of a first portion of the second winding, a first sub-portion322of a second portion of the second winding, first and second inductors382,383, and first and second switches380,381. The layout320can use nearly identical layouts for each sub-portion321,322but rotated 180 degrees from each other. The rotated, mirrored, or complimentary nature of the layout of each single turn sub-portion321,322allows the inductors382,383and switches380,381to be compactly positioned with minimal, if any, additional routing of the conductive portion of each single turn sub-portion321,322. Such reduction in routing allows each sub-portion to reduce or minimize losses compared to conventional layouts of planar transformers. In addition, when current (I) is induced in each sub-portion322,321, the current (I) can flow in the same direction about the post of the core108thus reinforcing the magnetic coupling between the second winding and the first winding. Although a single pair of sub-portions include all the nodes for coupling to the switches and inductors and can provide a basic step-up or step-down voltage function, a second pair of sub-portions can allow higher output current with little if any increase in circuit footprint.

FIG.4illustrates a stack configuration for an example planar transformer400according to the present subject matter. In this illustrative example, the planar transformer400can include a 6:1 turn ratio between the first winding and the second winding, but the present subject matter is not so limited. The planar transformer400can include a single post core but the present subject matter is not so limited. The stack can include 9 layers including five layers for routing the six turns of the first winding and 4 layers for routing two pairs (SECOND WINDING A, SECOND WINDING B) of sub-portions of the second winding.

FIGS.5A-5Eillustrate generally layouts of conductive portions of the first winding within each of the five layers of the example planar transformer400ofFIG.4.FIG.5Aillustrates a first layer551of a portion of the first winding of the example planar transformer that includes a first conductive portion531routed to provide a single turn of the first winding. The first conductive portion531can have a first node541or first set of vias for coupling to a voltage source, or load, and a second set of vias542for coupling to a second layer of the first winding. In certain examples, the first node541can correspond to a first node of the intermediate nodes of the first set of switches (e.g.,FIG.1, Q1-Q4).FIG.5Billustrates generally a second layout552of a portion of the first winding of the example planar transformer ofFIG.4. The second layer552can include a second conductive portion532of the first winding routed to provide a single turn or more of the first winding. A first end of the second conductive portion532can electrically connect with the first conductive portion531of the first winding using the second set of vias542. A second end of the second conductive portion532can couple to a third set of vias543.

FIG.5Cillustrates generally a third layout553of a portion of the first winding of the example planar transformer400ofFIG.4. The third layer553can include a third conductive portion533of the first winding routed to provide a single turn or more of the first winding. A first end of the third conductive portion533can electrically connect with the second conductive portion532of the first winding using the third set of vias543. A second end of the third conductive portion533can couple to a fourth set of vias544.

FIG.5Dillustrates generally a fourth layout554of a portion of the first winding of the example planar transformer400ofFIG.4. The fourth layer554can include a fourth conductive portion534of the first winding routed to provide a single turn of the first winding. A first end of the fourth conductive portion534can electrically connect with the third conductive portion533of the first winding using the fourth set of vias544. A second end of the fourth conductive portion534can couple to a fifth set of vias545.

FIG.5Eillustrates generally a fifth layer555of a portion of the first winding of the example planar transformer ofFIG.4. The fifth layer555can include a fifth conductive portion535of the first winding routed to provide about a turn and one half of the first winding. A first end of the fifth conductive portion535can electrically connect with the fourth conductive portion534of the first winding using the fifth set of vias545. A second end of the fifth conductive portion535can couple to a sixth set of vias546. In certain examples, the sixth set of vias546can be at least part of a second node of the first winding and can correspond to a second node of the intermediate nodes of the first set of switches (e.g.,FIG.1, Q1-Q4).

For the planar transformer400ofFIG.4, and increasingly numbering the nine layers from the bottom to the top, the first layout551of the first winding can be part of layer4, the second layout552of the first winding can be part of layer2, the third layout553of the first winding can be part of layer7, The fourth layout554of the first winding can be part of layer9, and the fifth layout555of the first winding can be part of layer5. In the illustrated example, the serial connection of each of the five layouts (551-555) of the first winding forms six turns around the center post of the core108. It is understood that other configurations of portions of the first winding that include the same or a different number of turns are possible without departing from the scope of the present subject matter.

FIGS.6A and6Billustrate generally layouts660,670of sub-portions of the second winding of the example planar transformer400ofFIG.4.FIG.6Aillustrates generally an example layout660of a first sub-portion of the second winding. The layout660can include the core105, a first conductive portion661routed to form a single turn of the second winding. The single-turn planar conductor can substantially enclose a void with a small opening to the void to isolate the ends of the conductor. For example, the single-turn planar conductor can be in the form of a “horseshoe”, or other U-shape that is mostly closed about a center void but includes a gap to prevent full closure about the center void. Some single-turn planar conductors can have a “hook-shape”. The layout660can also include first and second sets of vias671,672. The first set of vias671can be used to connect the first sub-portion with a second, parallel-connected, first sub-portion, one or more output inductors, one or more switches, or combinations thereof to form one of the two portions (111,112) of the example planar transformer400ofFIG.4. In certain examples, the layout660can be used for two layers of the nine layers of the example planar transformer400ofFIG.4.

FIG.6Billustrates generally an example layout670of a second sub-portion of the second winding. The layout670can include the core108, a second conductive portion662routed to form a single turn of the second winding. The single turn can be in the form of a “horseshoe” or “U-shaped”. The orientation of the second conductive portion662can complement the orientation of the first conductive portion661, or can generally mirror or reflect the orientation of the first conductive portion. In general, the orientation of the first and second conductive portions661,662is opposite each other such that the direction of the ends of one conductive portion is opposite the direction of the corresponding ends of the other conductive portion to allow for compact arrangement of other components of the second winding. As another example, the arrangement of the second conductive portion662may be referred to as being rotated about 180 degrees when stacked with and compared to a corresponding first sub-portion (e.g., layout660) such as to support an over-all, compact planar transformer. The layout670can also include the first and second sets of vias671,672. The second set of vias672can be used to connect the second sub-portion622with a second, parallel-connected second sub-portion, one or more inductors, one or more switches, or combinations thereof to form one of the two portions (111,112) of the example planar transformer400ofFIG.4. The first set of vias671can be isolated from the second conductive portion662of the second winding and can be used to couple together components of a second portion of the second winding. In certain examples, each portion of the second winding can include more or less numbers of planar conductive portions (e.g.,661,662) to assist in more closely coupling the second winding with the first winding.

With regard to the example planar transformer400ofFIG.4, and increasingly numbering the nine layers from the top to the bottom, the example layout660of a first sub-portion of the second winding can be part of layers1and8, and the example layout670of a second sub-portion of the second winding can be part of layers3and6. In the illustrated example, a first portion of the second winding can be formed by the parallel connection of layers1and8and the second portion of the second winding can be formed by the parallel connection of layers3and6. It is understood that other configurations of sub-portions of the second winding are possible without departing from the scope of the present subject matter. In certain examples, each conductive portion661,662can include additional conductive areas663,664to assist with coupling the second winding with the first winding.

FIGS.7A and7Billustrate generally various views of a physical specimen of an example planar transformer package according to the present subject matter.FIG.7Aillustrates a top view of the example planar transformer. The top view shows the multiple layer substrate, the core, four inductors coupled to the second winding, a controller and other control components such as drivers for the second set of switches.FIG.7Billustrates a bottom view of the example planar transformer package. The bottom view shows the multiple layer substrate, the core, the second set of switches, and additional ancillary control components. In an example, the power density for a planar transformer according to the present subject matter can be greater than 50 kilowatts per liter (kW/L). Such power density is provided by the complementary orientation of the various sub-portions of the second winding as discussed above. In an example, a planar transformer with greater than 50 kW/L power density can provide an output voltage of 3.3 volts at 140 amperes from a 54 volt supply voltage. The overall dimensions of such an example can be 24.5 mm (L)×24 mm (W)×15 mm.

FIG.8illustrates a conductive trace880of a layer having a portion of a second winding for a center-tap configuration of an example planar transformer. In certain examples, the conductive trace880can have a question mark type shape or a hook shape that approximates a single turn of the second winding. The shape allows for control components such as switches881and inductors882associated with the portion of the second winding to be compactly arranged about a core883.

FIG.9illustrates generally a circuit for an example planar transformer system900having a center-tap configuration according to the present subject matter. The planar transformer system900can include a planar transformer901, a first set of switches (Q1-Q4), a second set of switches (M5-M8), output inductors902,903and a controller904. In some examples, one or more switches of the first set of switches and the second set of switches can be accompanied by a gate driver910. The planar transformer901can include a core905, a first winding906, and one or more second windings907,908. The first winding906can include a number of turns and typically can include more than one turn. Each second winding907,908can include a number of winding segments911,912,913,914. Each winding segment911,912,913,914can form single turn of a second winding907,908. As used herein, a winding segment includes the primary conductive portion of a turn of a second winding and does not include ancillary parts of the second winding such as an external terminal, a fuse, a switch, etc.

The first winding can be either a primary winding of the planar transformer or a secondary winding of the planar transformer. InFIG.9, the first winding906is referenced as a primary winding unless noted otherwise. The first set of switches (Q1-Q4) are controlled by the controller904and operate to recurrently or periodically connect and disconnect the primary winding906to voltage supply rails and to establish a cyclical primary voltage across the first winding906and a cyclical primary current through the first winding906. The second windings907,908can be magnetically coupled to the first winding906via the core905. The core905can be an air core, a magnetic core with an air gap, or any materials and structures capable of providing a flux link.

The controller904can control the first set of switches (Q1-Q4) and the second set of switches (M5-M8) to provide an output voltage (VOUT) different than the input voltage (VIN). In the illustrated planar transformer system900, the output voltage (VOUT) is lower than the input voltage (VIN) but the subject matter is not so limited. When the planar transformer901is operated as a step-down transformer, the controller904can control the first set of switches (Q1-Q4) to oscillate or alternate a polarity of the input voltage (VIN) across the first winding906. When the planar transformer901is operated as a step-down transformer, the controller904can operate or synchronize the second set of switches (M5-M8) to extract power from the winding segments of the second winding. The second set of switches can be controlled with two phase signals (PH1, PH2). In certain examples, each second winding907,908can be configured to include one or more taps (E, F) between connected winding segments (911/912and913/914). In certain examples, the tap (E, F) between winding segments can allow the output voltage (VOUT) to include a step-up or step-down multiplier of the input voltage (VIN). In addition, compared to conventional methods and even recent planar techniques, the tap (E, F) between the connected winding segments (911/912and913/914) can help reduce the complexity or the cost of the overall planar transformer design by using fewer switches. Furthermore, the tap (E, F) between the winding segments also allows for use of output inductors902,903with much lower inductance to smooth the output voltage (VOUT). The lower inductance of the output inductors902,903results from the circuit implementation using parasitic inductance of the other planar transformer components to smooth output ripple in the output voltage (VOUT). The output inductors902,903can be coupled between a corresponding tap (E, F) and an output voltage terminal of the planar transformer system900. In certain examples, the output inductors902,903can include air-core inductors.

FIG.10illustrates generally an example compact layout1090of the second winding of the example circuit ofFIG.9, including the output inductors902,903and the second set of switches (M5-M8). In certain examples, the conductive traces or winding segments911,912,913,914of each portion of the second winding can include respective conductive traces that can be located on multiple layers of the substrate of an example center-tap, planar transformer. Conductive traces associated with a particular winding segment of the second winding can be coupled together in parallel such as to handle higher currents as additional layers of the substrate add a small vertical dimension to the transformer without adding to the larger planar footprint of the transformer.

FIG.11illustrates generally an example portion of planar transformer1100configured, for example, to provide multiple voltage outputs. The example portion illustrates generally a core1105including multiple posts, and sub-portions1191,1192,1193,1194of a second winding about each post. The portion of the planar transformer1100also shows a compact layout for filter inductors (L) and switches (S) for electrically coupling the sub-portions of the second winding as discussed above, for example to ensure current (I) flow on each sub-portion is in the same direction about the respective post of the core1105. Using the techniques discussed above, the planar transformer1100can provide multiple output voltages or a single output voltage with more output current capacity from a multiple-layer substrate package having a very small height.

FIG.12illustrates generally a flowchart of an example step-down method1200for operating a planar transformer according to the present subject matter. At1201, a supply voltage can be applied, with alternating polarity, across a first planar winding of the planar transformer. The first planar winding can have multiple turns about a post of a metallic core to induce a current in a second planar winding of the planar transformer. At1203, a first node of a first, single turn of the second winding can be coupled to ground in response to a first polarity of the supply voltage via a first second winding switch. The ground connection of the second winding can provide a reference potential for an induced voltage across the second winding. At1205, a second node of the first single turn can be isolated from ground in response to the first polarity of supply voltage via a second switch. Isolating the second node allows current induced in the second winding to provide an output voltage of the planar transformer. At1207, the output voltage can be provided at a first node of a first inductor coupled to the first single turn. The first inductor can be directly coupled in series with the first single turn at a second node of the first single turn. The first inductor, for the step-down configuration can provide filtering of the output voltage. In certain examples, current stored in the output inductor can assist in smoothing the output voltage level as the alternating polarity of the supply voltage is switched across the first planar winding.

In certain examples, the single turn can include multiple, parallel-connected conductive traces. Via a multiple layer substrate, the conductive traces can be stacked to provide a very space efficient planar transformer. In addition, by orienting some conductive traces 180 degrees with respect to other conductive traces, the transformer power density can be increased without increasing the major footprint of the planar transformer.