Abstract:
A system and method for manufacturing of a micro-transformer providing direct electrical isolation between a primary winding and a secondary winding while featuring tight magnetic coupling for a large possible step-up or step-down ratio. The micro-transformer may be implemented in an integrated circuit, and may include a magnetic core. A high stepping ratio, e.g. approximately 50 to 100, may be achieved by connecting multiple symmetric primary windings in parallel and multiple symmetric secondary windings in series, or vice-versa. A plurality of windings may be stacked vertically. The micro-transformer may be of particular utility in wireless sensor networks, thermal and vibrational energy harvesters, power converters, and signal isolators.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to U.S. application Ser. No. 13/273,726, entitled “A Small Size And Fully Integrated Power Converter With Magnetics On Chip” (Atty. Dkt. No. 13641/441001), filed on Oct. 14, 2011. This related application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to a micro-transformer that may provide galvanic isolation between a primary winding and a secondary winding while providing a large step-up or step-down ratio via tight magnetic coupling. The micro-transformer may be implemented in an integrated circuit. 
         [0003]    Transformers enable magnetic signal transfer between two or more circuit networks via mutual inductance, while providing direct electrical (i.e., galvanic) isolation. Such isolation may prevent extraneous transient signals, including common-mode transients, from being inadvertently processed as intended signals. Isolation may also protect equipment from shock hazards, or permit equipment on either side of an isolation barrier to operate at different supply voltages without necessarily sharing a common ground connection. Optical isolators are used to provide such isolation by converting input electrical signals to light signals, and then converting the light signals back into electrical signals again, but they have numerous known disadvantages. 
         [0004]    Transformers also enable alternating voltages and currents of the magnetically coupled circuit networks to be stepped up or down significantly, ideally with no overall power loss. The ratio of the number of turns in the primary winding to the number of turns in the secondary winding determines the stepping ratio for ideal transformers. Transformers are accordingly used in power supplies and power converters for a wide variety of applications. 
         [0005]    Small transformers are often manufactured from discrete components, versus components that can be made by planar processes like those used to manufacture integrated circuits. As used herein, a “micro-transformer” refers to a small transformer in which at least one winding is formed using planar fabrication methods, including but not limited to semiconductor fabrication techniques. At present, micro-transformers are quite limited in stepping ratio and power transfer efficiency. 
         [0006]    Accordingly, the inventor has identified a need in the art for an improved micro-transformer to provide the benefits of isolation but with improved stepping ratio and power transfer efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1A-1C  are diagrams depicting an exemplary micro-transformer according to one embodiment of the present invention. 
           [0008]      FIGS. 2A-2B  are diagrams depicting an exemplary micro-transformer according to another embodiment of the present invention. 
           [0009]      FIG. 3  is a diagram depicting an exemplary micro-transformer circuit according to a further embodiment of the present invention. 
           [0010]      FIG. 4  is a block diagram depicting an exemplary micro-transformer power converter according to another embodiment of the present invention. 
           [0011]      FIG. 5  is a flowchart depicting an exemplary micro-transformer manufacturing method according to a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Embodiments of the invention provide a micro-transformer apparatus, and a method for manufacturing a micro-transformer. The micro-transformer enables direct electrical isolation between a primary winding and a secondary winding, while featuring tight magnetic coupling for a large possible stepping ratio and improved power transfer efficiency. Each winding may be thought of as a magnetic field generating element coupled magnetically with a magnetic field receiving element. 
         [0013]    One difficulty in the creation of such a micro-transformer is the need to achieve close magnetic coupling between the primary and secondary windings. Another difficulty is in physically realizing designs with a high relative number of winding turns between the primary and secondary windings. Further, the minimum winding pitch and maximum integrated circuit size are limited, as are the number of different metal and insulating layers available in a given manufacturing process. The embodiments described address these difficulties and constraints. 
         [0014]    Referring now to  FIG. 1A , a diagram depicting an exemplary micro-transformer  100  according to one embodiment of the present invention is shown. This figure provides is a top view and side view of the micro-transformer, fabricated using planar processes that may include but are not limited to semiconductor fabrication technologies. Structures on an upper layer are shown in solid lines in the top view, while structures on at least one lower layer are shown in dashed lines in the top view. An optional region of magnetic core material is shown as item  102 ; this material may be positioned on one layer or on multiple layers, if available. 
         [0015]    Primary winding  104  may begin at terminal  106 , proceed laterally through conductive stripes on an upper layer, downward through substantially vertical interconnection structures or “vias” (shown as black circles), and laterally through conductive stripes on a lower layer. In this example, primary winding  104  may continue upward through via  116 , laterally through conductive stripes on an upper layer, downward through vias, and laterally through conductive stripes on a lower layer, altogether forming two complete turns that end at terminal  108 . Insulating layers (not shown) may be disposed between various conductive layers to provide electrical (i.e. galvanic) isolation. Insulating layers may comprise silicon dioxide, silicon nitride, or polyimide, for example. 
         [0016]    Lower layer portions of the primary winding  104  on the left side of the diagram connecting to via  116  are shown offset to the left slightly for clarity; an actual fabricated micro-transformer winding may not be so offset, to provide better winding symmetry. Similarly, via  116  may be placed more centrally between the two primary winding turns, or may be placed elsewhere that a winding “tap” may be desired. 
         [0017]    Each primary winding turn may be split into (in this example) four conductive stripes, to allow the secondary winding turns to intertwine to provide better magnetic coupling. Although in this example the primary winding  104  has two turns, any number of turns may be employed. 
         [0018]    The secondary winding  110  may begin at terminal  112 , on a lower layer in this example, proceed laterally through a conductive stripe on a lower layer, upward through a via, and laterally through a conductive stripe on an upper layer to complete one winding turn. In this example, secondary winding  110  may comprise eight such repeated winding turns in series, and may end on an upper layer at terminal  114 , but other configurations are possible. For example, each winding may have its terminals on the same or different layers. 
         [0019]      FIG. 1B  depicts the primary winding  104  of the  FIG. 1A  micro-transformer  100  alone for clarity. Vias  118 A- 118 D may transport current in the first turn substantially vertically between an upper layer and a lower layer. Vias  120 A- 120 D may similarly transport current in the second turn substantially vertically between an upper layer and a lower layer. Conductive stripes  122 A- 122 D and  124 A- 124 D conduct current substantially horizontally on an upper layer, while conductive stripes  126 A- 126 D and  128 A- 128 D conduct current substantially horizontally on a lower layer. 
         [0020]    Similarly,  FIG. 1C  depicts the secondary winding  110  of  FIG. 1A  micro-transformer  100  alone for clarity. Vias  132 A- 132 H and  134 A- 134 G may transport current in the secondary winding  110  substantially vertically between an upper layer and a lower layer. Conductive stripes  136 A- 136 H conduct current substantially horizontally on an upper layer, while conductive stripes  138 A- 138 H conduct current substantially horizontally on a lower layer. In this exemplary micro-transformer  100 , only two layers, an upper layer and a lower layer, are depicted for clarity but an actual fabricated micro-transformer may employ any number of layers to produce as many windings as needed. Any of the layers may be selectively interconnected by vias or other conductive structures. Multiple micro-transformers may also be interconnected. 
         [0021]    The embodiment of  FIGS. 1A-1C  may use symmetric and alternately spaced intertwined conductive stripes for both the primary and secondary windings. This feature may be optimized to pack the windings as closely together as the design rules for a given process will allow. Such packing minimizes overall circuit size while helping to ensure a close magnetic coupling between the windings. Although multiple-turn windings are shown for both the primary and secondary windings of the exemplary micro-transformer  100 , single turns may be used if appropriate for the application for which the micro-transformer  100  will be used. Micro-transformers having a one-to-one stepping ratio may be of particular use in isolator applications. 
         [0022]    A variety of conductive materials may be used to form the windings, including but not limited to metals and doped semiconductor regions. The conductive materials may include those already used to fabricate metal traces in integrated circuits, such as aluminum and copper. Non-process metals (e.g., gold) may also be deposited after a substrate has been processed to include circuit elements. This approach may allow the windings to be thicker than typical metal layers used in an integrated circuit process, providing a higher inductance to resistance ratio. 
         [0023]    The windings may have portions placed within a substrate, on a substrate, or deposited onto an electrically insulating film that covers a substrate for example, so the various upper and lower portions of the windings may be oriented substantially parallel to the substrate surface. The capacitance between the substrate and the lower portions of the windings may be reduced by placing the windings above a relatively thick electrically insulating film. Vertical portions of the windings (e.g., the vias of  FIGS. 1A-1C ) may connect the upper portions of the windings to the lower portions of the windings through openings cut through selected intervening layers. The overall magnetic field resulting from currents flowing through the conductive windings may be oriented substantially parallel to the substrate. 
         [0024]    Further, this embodiment may allow one set of windings (e.g., the stripes and vias of  FIGS. 1A-1C ) to be effectively connected in parallel to increase the magnetic flux generated by a given input. Other winding sets may be connected in series to increase the induced output voltage. The combination of these features may enable quite a large stepping ratio. For example, if the primary winding is connected in parallel while the secondary winding is connected in series, the secondary voltage may be stepped up by a large factor, e.g. 50 to 100. Similarly, if the primary winding is connected in series while the secondary winding is connected in parallel, a large step-down voltage ratio may be achieved. A large step-down voltage ratio may be of particular utility for sensing large voltages. A large step-up voltage ratio may be of particular utility in energy harvesting applications, to be described. 
         [0025]    Referring now to  FIG. 2A , a diagram depicting an exemplary micro-transformer  200  according to another embodiment of the present invention is shown. This micro-transformer  200  may use stacked spiral windings that may be layered substantially parallel with a substrate, with insulating layers (not shown) between the windings providing direct electrical (i.e. galvanic) isolation. 
         [0026]    In advanced integrated circuit fabrication processes, there may be many layers of metal available to form the spiral windings. Connections to each spiral winding may be made on a given vertical layer at the outside edge of the spiral, and through an inter-layer connection or “via” at the central region of the spiral winding to allow current flow through an intervening insulating layer to a neighboring or other conductive layer. 
         [0027]    A first spiral winding  204  may be positioned on a first layer  230 , and have a first terminal  206  on that layer. A second terminal  208  may be positioned on another layer  240 , with a via  210  providing a conductive interconnection between the layers  230  and  240 . The second terminal  208  may be connected to a point on the same layer  230  as first terminal  206  by another via  212 . 
         [0028]    A second spiral winding  216  may be positioned on another layer  250 , and have a first terminal  212  on that layer. A second terminal  214  may be positioned on yet another layer  260 , with a via  220  providing a conductive interconnection between the layers  250  and  260 . The second terminal  214  may be connected to a point on the same layer  250  as first terminal  212  by another via  222 . Although the windings are depicted as round spirals, all other winding shapes (e.g., rectangular spirals, hexagonal spirals, etc.) may be used. Further, although the first spiral winding  204  is depicted as having approximately 1.5 turns and the second spiral winding  216  is depicted as having approximately 4.5 turns for clarity, the number of turns in each winding may be tailored to suit individual application needs. 
         [0029]    Micro-transformer  200  may pack the windings as closely together vertically as the design rules for a given process will allow, to minimize overall circuit size, while helping ensure a close magnetic coupling between the windings. The winding layers may alternate, so that a primary winding is proximate to a secondary winding. Similarly, as shown in a variation depicted in  FIG. 2B , concentrically wound primary and secondary windings may be placed proximately on a common layer. Vias may connect their inner terminals to another layer for example as previously described. The overall magnetic field resulting from currents flowing through the conductive windings may be oriented substantially perpendicular to the substrate. 
         [0030]    The diameter of the individual spiral windings may be made relatively large compared to the separation between the individual spiral windings, to achieve tighter magnetic coupling. Similarly, symmetry between the primary windings and the secondary windings, however many spirals each may comprise, may be maximized to avoid degradation of the magnetic coupling. In practice, magnetic coupling ratios of 0.8 may be achieved with reasonable spiral sizes. Such a micro-transformer may have an exemplary power transfer efficiency of ten to fifteen percent. Alternating currents that switch at a frequency from ten MHz to 20 MHz for example may be used by such a micro-transformer, though operation at over 100 MHz may be feasible. 
         [0031]    As previously described, the different windings of this embodiment may also be connected in parallel or serially as needed to yield a high stepping ratio. For example, when stepping up a voltage from the primary side to the secondary side of the micro-transformer, the primary spiral windings may be connected in parallel and the secondary spiral windings may be connected in series. The turning direction of a winding may also be selected to yield an induced voltage of desired relative polarity. 
         [0032]    Referring now to  FIG. 3 , a diagram depicting an exemplary micro-transformer circuit  300  according to a further embodiment of the present invention is shown. This micro-transformer  300  also may use stacked spiral windings that may be layered substantially parallel with a substrate, with insulating layers (not shown) between the windings providing direct electrical (i.e. galvanic) isolation. A single spiral winding may be vertically surrounded by one or more turns of the other winding on other layers above and/or below the single spiral winding. 
         [0033]    In this example, two secondary windings  310  and  320  may be positioned on layers  312  and  322  above a primary winding  330  on layers  332  and  334 , and two secondary windings  340  and  350  may be positioned on layers  342  and  352  below primary winding  330 . The primary side of micro-transformer  300  may include various contact pads and connections as well as a spiral winding. Thus the primary side may begin at contact pad  336  on layer  312 , proceed downward through intervening layers (e.g.,  322 ) by a via, laterally to an exterior edge of spiral winding  330 , downward again through another via to layer  334 , and up to contact pad  338 . The vias providing electrical interconnection to contact pads  336  and  338  may allow external connection of a primary voltage source (not shown), but the invention is not so limited. A primary voltage source may be available on any interior layers or on substrate  360 . Contact pads such as  336  and  338  may be separated from each other and from the windings to the extent possible, to decrease capacitive coupling. 
         [0034]    The secondary side of exemplary micro-transformer circuit  300  may begin at node  364 , shown in this example as being within integrated circuitry  362  on substrate  360 . A via may interconnect node  364  to an exterior edge of secondary winding  310  through intervening layers (e.g.,  322 ,  332 ,  334 ,  342 , and  352 ). Another via may interconnect secondary winding  310  to secondary winding  320 , forming a series winding pair. Secondary windings  340  and  350  are similarly interconnected, and attached to the upper secondary winding pair through another via. Secondary winding  350  may connect by a via to node  366 , shown here as being within exemplary integrated circuit  362 . 
         [0035]    The total number of turns in the secondary side of micro-transformer  300  may thus significantly exceed the number of turns in any of the particular secondary windings. Further, each secondary winding may have many more turns than the single primary winding shown. For example, if the primary winding has two turns while each of the secondary windings has twenty-five turns, an exemplary overall turns ratio of 100:2 or 50 may be achieved, with a seven metal layer process. 
         [0036]    In this embodiment, a primary winding voltage applied to contact pins  336  and  338  may cause an electrical current to flow through primary winding  330 , which generates a magnetic field. The magnetic field may couple to all of the series-connected secondary windings, inducing a significantly stepped-up overall secondary voltage between nodes  364  and  366 . The secondary voltage may be conditioned into a supply voltage for use by integrated circuit  362 , to be described. 
         [0037]    The embodiments described above may employ a variety of magnetic materials available in the given fabrication process for use as a magnetic core. For example, a mixture or alloy of nickel and iron (nickel ferrite or NiFe) may be deposited on at least one layer of a planar process to serve as a magnetic core. Other magnetic materials of high permeability may include CoTaZr (cobalt tantalum, zirconium), and FeCo (ferrite cobalt)-based alloys. 
         [0038]    Further, in the embodiments the number of turns employed in each winding may be configurable by a user. Transistors or other switches (not shown) may selectively connect portions of each winding or a varying number of complete turns of each winding to enable a particular stepping ratio to be achieved. Similarly, transistors or other switches may selectively connect multiple distinct windings, enabling any number of separate primary windings to be magnetically coupled to any number of separate secondary windings. 
         [0039]    Referring now to  FIG. 4 , a block diagram depicting an exemplary micro-transformer power converter  400  according to another embodiment of the present invention is shown. An energy harvester, depicted as item  402 , may generate output power at a very low alternating voltage (e.g., one mV to 20 mV, depicted as voltage  410 ). Such a voltage may be generally inadequate to serve as a supply voltage for even low-power integrated circuitry. A micro-transformer  404  may convert the voltage  410  provided by energy harvester  402  into a stepped up output voltage, depicted as voltage  412 . A micro-transformer with a stepping ratio of 50 to 100 may for example yield between 50 mV to 2 V. A voltage regulator  406  may then rectify the stepped up output voltage  412  and control a resulting dc voltage  414  that is applied to a load  408 . The load may comprise an integrated circuit that may include at least one of the energy harvester  402 , micro-transformer  404 , and voltage regulator  406 . The efficiency of such conversion circuitry may vary considerably with applied load, so good regulation may be required. 
         [0040]    Referring now to  FIG. 5 , a flowchart depicting an exemplary micro-transformer manufacturing method according to a further embodiment of the present invention is shown. Exemplary steps may proceed from a lower-most layer upward. Step  502  comprises fabricating a winding portion on a selected layer. Step  504  comprises fabricating an insulating layer. Step  506  comprises fabricating a magnetic layer, if that is feasible in and is required of a given process. Step  508  comprises fabricating interconnecting structures, such as vias, to electrically link winding portions through intervening layers. If additional layers are required, as determined in exemplary step  510 , the previous method steps may be repeated. 
         [0041]    The fabricating steps may each be performed photolithographically. The lowest layer may comprise a substrate or an insulating layer, and the upper layer(s) may comprise an insulating layer or a conductive layer. The substrate may comprise a printed circuit board or semiconductor wafer that may include integrated circuitry. The insulating layers may comprise silicon dioxide, silicon nitride, or polyimide, for example, or other known passivating materials. The winding portions may comprise process metals, such as aluminum or copper for example. The interconnections may comprise vias formed from process metals, or from non-process metals, such as gold for example. 
         [0042]    As described above, one aspect of the present invention relates to a micro-transformer. The provided description is presented to enable any person skilled in the art to make and use the invention. For purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the present invention. Description of specific applications and methods are provided only as examples. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and steps disclosed herein.