Patent Publication Number: US-2023162904-A1

Title: Micro-scale planar-coil transformer with shield

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation claiming the benefit under 35 U.S.C. § 120 of U.S. Application Serial No. 16/396,585, filed Apr. 26, 2019, under Attorney Docket No. G0766.70259US00, and entitled “MICRO-SCALE PLANAR-COIL TRANSFORMER WITH SHIELD,” which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present application relates to micro-scale planar-coil transformers. 
     BACKGROUND 
     For micro-scale planar-coil transformers, a large component of electromagnetic interference (EMI) can come from dipole radiation between sides of the transformer. EMI may cause undesirable effects on electronic components. 
     SUMMARY OF THE DISCLOSURE 
     Micro-scale planar-coil transformers including one or more shields are disclosed. The one or more shields can block most common-mode current from crossing from one side to another side of a transformer. Reduction of common-mode current crossing the transformer results in reduction of dipole radiation, which is the main component of electromagnetic interference (EMI) in some transformers. 
     According to aspects of the present disclosure, there is a micro-scale planar-coil transformer arranged on a substrate having an upper surface. The micro-scale planar-coil transformer comprises a first planar coil arranged in a first layer occupying a first area in a plane parallel to the upper surface, a second planar coil arranged in a second layer, and a shield arranged in an intermediate layer, the intermediate layer being between the first and second layers. A projection onto the first planar coil of a region enclosed by a perimeter of the shield covers a second area of the coil that is less than the first area. 
     According to aspects of the present application, there is a micro-scale planar-coil transformer arranged on a substrate having an upper surface. The micro-scale planar-coil transformer comprises a first side of a transformer comprising a first planar coil arranged in a first layer and configured to couple to a first electrical ground, a second side of the transformer comprising a second planar coil arranged in a second layer and configured to couple to a second electrical ground, a first shield arranged in a first intermediate layer and configured to couple to the first electrical ground, the first intermediate layer being between the first and second layers, and a second shield arranged in a second intermediate layer and configured to couple to the second electrical ground, the second intermediate layer being between the first and second layers. 
     According to aspects of the present disclosure, there is a micro-scale planar-coil transformer arranged on a substrate having an upper surface. The micro-scale planar-coil transformer comprises a first planar coil arranged in a first layer occupying a first area in a plane parallel to the upper surface, a second planar coil arranged in a second layer, and a shield arranged in an intermediate layer, the intermediate layer being between the first and second layer. A projection onto the first planar coil of a region enclosed by a perimeter of the shield encloses a second area that is equal to the first area. 
     According to aspects of the present application, there is a micro-scale planar-coil transformer, comprising a first planar coil arranged in a first layer, a second planar coil arranged in a second layer, and means for reducing dipole radiation between the first and second planar coils. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and embodiments of the application will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear. 
         FIG.  1    is a cross-sectional side view of a micro-scale planar-coil transformer; 
         FIG.  2    is an exploded top perspective view of a micro-scale planar-coil transformer; 
         FIG.  3    is a schematic of a circuit comprising a transformer; 
         FIG.  4    is a top view of some components of a micro-scale planar-coil transformer; 
         FIG.  5    is a top view of some components of a micro-scale planar-coil transformer; 
         FIG.  6    is a top view of some components of a micro-scale planar-coil transformer; 
         FIG.  7    is a cross-sectional side view of a micro-scale planar-coil transformer; 
         FIG.  8    is a schematic of a circuit comprising a transformer; 
         FIG.  9    is a cross-sectional side view of a micro-scale planar-coil transformer; 
         FIG.  10    is a top view of some components of a micro-scale planar-coil transformer; and 
         FIG.  11    is a schematic of a circuit comprising a transformer. 
     
    
    
     DETAILED DESCRIPTION 
     According to aspects of the present application, a micro-scale planar-coil transformer includes a first planar coil arranged on a primary side of the transformer electromagnetically coupled to a second planar coil arranged on a secondary side of the transformer. The micro-scale planar-coil transformer is configured to exhibit reduced dipole radiation between the primary and secondary sides of the transformer. One or more shields are included between the first and second planar coils to reduce the dipole radiation. The shield(s) may be a Faraday shield. 
     The present application relates to micro-scale devices. One type of micro-scale device is a planar-coil transformer. Transformers may be used to transform a first voltage level at a first portion of an electrical circuit to a second voltage level at a second portion of an electrical circuit. Transformers may transfer data and/or power signals. Transformers may be used to isolate signals between two portions of a circuit, and thus may form part of an isolator in some embodiments. Transformers typically transfer AC signals. In some scenarios, DC-DC power converters may utilize a transformer, such as a micro-transformer. 
     According to aspects of the present application, a transformer is arranged having a primary side and a secondary side. The primary side transmits data and/or power signals and the secondary side receives the data and/or power signals. In some embodiments, data and/or power signals may be sent both directions across the transformer. 
     In some embodiments, the primary side of the transformer is coupled to circuity which generates the signals that are to be transmitted. In some embodiments, the secondary side of the transformer is coupled to an output. In DC-DC converters utilizing transformers, a rectifier may couple the secondary side to the output such that a signal from the secondary side is rectified prior to being output. 
     A micro-scale planar-coil transformer may include two or more micro-scale planar coils. A first coil may be arranged on a primary side of the transformer and a second coil may be arranged on a secondary side of the transformer. Each of the first coil and the second coil may be arranged on a substrate in a substantially planar configuration. A planar coil may be patterned on a substrate in a substantially spiral shape about a coil axis. 
     The primary side may transfer electromagnetic energy to the secondary side. A varying current applied to a first spiral-shaped coil induces a varying magnetic field arranged substantially along a coil axis of the first spiral-shaped coil, and the coils are arranged and electromagnetically coupled such that the varying magnetic field is oriented through the second spiral-shaped coil substantially along a coil axis of the second spiral-shaped coil, which induces a changing current in the second spiral shaped coil on the secondary side. 
     Some configurations of transformers produce electromagnetic interference (EMI). High levels of EMI can interfere with the performance of electrical devices and may cause undesired effects on components. EMI may come from different source components, and various electrical phenomena may contribute to EMI, such as dipole radiation. In some micro-scale transformers, the main component of EMI is dipole radiation. This is true of some integrated isolated DC-DC converters. 
     Dipole radiation may occur between the primary and secondary sides of a transformer. Dipole radiation may result from common-mode current crossing an isolation barrier, such as an isolation layer, that is arranged between the primary and secondary coils of a transformer. The common-mode current may cross between the primary and the secondary sides due to a parasitic capacitance between two coils on the different sides of a transformer. For example, if the common-mode current passes from the primary side to the secondary side due to the parasitic capacitance, it may flow back from the secondary side to the primary side in the form of dipole radiation, therefore forming a current loop across the isolation layer. When a first coil and a second coil are arranged on two different substrates, for example, two different printed circuit boards (PCBs), dipole radiation may pass between electrical ground/power planes of the two different substrates. 
     The common-mode current may be induced by differences in common-mode voltage ripple amplitudes between sides of a transformer. In one exemplary embodiment, a coil of a transformer has two terminals, T1 and T2. In the exemplary embodiment, a first voltage signal, V1, at the terminal T1 and a second voltage signal, V2, at the terminal T2 are each half-sinusoidal and have a same sign. In the exemplary embodiment, the first voltage signal and the second voltage signal are 180 degrees out-of-phase. Due to their half-sinusoidal nature, the first voltage signal and the second voltage signal are not differential. The common-mode voltage, Vcom, may be found as an average of the two signals, where Vcom=(V1+V2)/2. In the exemplary embodiment, a common-mode voltage ripple amplitude of the two terminals T1 and T2 of the coil is greater than substantially zero. The common-mode voltage fluctuates at even harmonics of an applied oscillation frequency. 
     Shields described herein may be applied to transformers that have one or more coils with a common-mode voltage ripple amplitude greater than substantially zero. However, it should be appreciated that the signals described above are merely one example of a set of voltage signals applied a transformer that result in a common-mode voltage ripple amplitude greater than substantially zero. Other sets of signals resulting in similar common-mode voltages and similar ripples of common-mode voltages are possible. Further, shields described herein are not limited in application to transformers with such signals. In various embodiments, one or more shields are applied to a transformer that exhibits a common-mode current between sides of the transformer, a transformer that exhibits dipole radiation between sides of the transformer, and/or a transformer which exhibits EMI. 
     A common-mode voltage of two terminals, TC1 and TC2, of a primary side coil, Vcom1, may be found as the average of the voltage of the first terminal, VTC1, and the voltage of the of the second terminal, VTC2, where Vcom1=(VTC1+VTC2)/2. In the exemplary embodiment, the signals applied to the primary side coil are the half-sinusoidal, 180 degree out-of-phase signals with the same sign described above, and the common-mode voltage ripple amplitude of the primary side coil is greater than substantially zero. 
     A common-mode voltage of two terminals, RC1 and RC2, of a secondary side coil, Vcom2, may be found as the average of the voltage of the first terminal, VRC1, and the voltage of the of the second terminal, VRC2, where Vcom2=(VRC1+VRC2)/2. In the exemplary embodiment, the common-mode voltage ripple amplitude of secondary side coil is substantially zero. 
     In some embodiments, a coil, for example the primary coil, may be center-tapped in addition to having two terminals, VT1 and VT2. The coil may comprise two coil portions separated by the center tap. A voltage signal applied to the center tap may be a constant voltage, VDD. In some embodiments, the center tap is held to electrical ground. The total common-mode voltage, Vcom, of the center-tapped coil may be found as the average of the common-mode voltage of each of the two coil portions, therefore Vcom=(VT1+VT2)/4+VDD/2. 
     In some embodiments, there is parasitic capacitance between the two coils on different sides of the transformer. Because of the parasitic capacitance, differences between the common-mode voltage ripple amplitude may introduce a common-mode current crossing the isolation layer from the primary side to the secondary side of the transformer. Typically, the current is small enough that it will not cause safety issues, however, the current flows back to primary side by dipole radiation, which induces high levels of EMI. 
     As described above, in the exemplary embodiment, the common-mode voltage ripple amplitude of the primary side coil is greater than substantially zero, and the common-mode voltage ripple amplitude of secondary side coil is substantially zero. The difference in ripple amplitudes results in common-mode current passing between sides of the transformer, and therefore also results in dipole radiation between sides of the transformer. 
     The inventors have recognized that a shield, such as a Faraday shield, can reduce or substantially eliminate the dipole radiation encountered in micro-scale transformers. Thus, aspects of the present application provide a micro-scale transformer having one or more shields between the primary and secondary coils of the transformer. 
       FIG.  1    shows a cross-sectional side view of a micro-scale planar-coil transformer  100 . Micro-scale planar-coil transformer  100  is arranged on a substrate  110  having an upper surface  112 , and includes a first coil layer  120 , an intermediate layer  130 , a first coil  140 , a second coil  150 , a shield  160 , and a via  170 . 
     The micro-scale planar-coil transformer  100  includes a first coil  140 . In the illustrative embodiment of  FIG.  1   , the first coil  140  is arranged in a first coil layer  120 . In  FIG.  1   , the first coil  140  is arranged above the substrate  110 . In some embodiments, the first coil  140  is arranged on a primary side of the transformer. 
     The micro-scale planar-coil transformer  100  includes a second coil  150 . The second coil  150  may be arranged in a second coil layer (not illustrated in  FIG.  1   ). The second coil  150  is arranged over and above the first coil  140 . In some embodiments, the second coil  150  is arranged on a secondary side of the transformer. 
     The first coil  140  and the second coil  150  are electromagnetically coupled. In an exemplary embodiment where the first coil  140  is arranged on a primary side of the micro-scale planar-coil transformer  100 , and the second coil  150  is arranged on a secondary side of the micro-scale planar-coil transformer  100 , the first coil  140  may be a transmit coil and the second coil  150  may be a receive coil. The first coil  140  may transmit signals via electromagnetic energy, which are received by the second coil  150 . However, each coil may comprise either a transmit coil arranged on a primary side or a receive coil arranged on a secondary side. For example, in some embodiments, a coil arranged in an upper layer, such as second coil  150 , may be arranged on a primary side as a transmit coil and a coil arranged in a lower layer, such as first coil  140 , may be arranged on a secondary side as a receive coil. 
     In  FIG.  1   , the first coil  140  and the second coil  150  are respectively arranged about a first coil axis and a second coil axis. The coils are patterned in their respective layers such that the coil axes are arranged substantially perpendicular to the upper surface  112 . The coil axes are substantially aligned with each other. 
     A transmit coil is configured to transmit signals to a receive coil. When a changing current is applied to a transmit coil, it induces a changing magnetic field. At close distances to a coil, a changing current flowing through a coil induces a changing magnetic field that is substantially aligned with the axis of the coil. Coil axes of a coil pair may be substantially aligned, and the coils may be arranged at a distance such that an acceptable portion of the changing magnetic field induced by a transmit coil passes through a receive coil. The changing magnetic field applied through the receive coil induces a changing current in the receive coil which facilitates the transfer of power/data signals. 
     The micro-scale planar-coil transformer  100  includes a shield  160 . The shield  160  is arranged between the first coil  140  and the second coil  150 , along the direction of the coil axes. The shield may further be arranged at least partially between the first coil  140  and the second coil  150  along directions substantially perpendicular to the coil axes. 
     In  FIG.  1   , the shield  160  is arranged in the intermediate layer  130 . The intermediate layer  130  is arranged between the first coil  140  and the second coil  150 . The intermediate layer may comprise an isolation layer, which may be formed of an insulating or dielectric material. In some embodiments, the intermediate layer  130  may comprise polyimide. 
     In  FIG.  1   , the shield  160  is arranged on a side of the intermediate layer  130  that is proximate the first coil  140 . In such an arrangement, the shield  160  and the first coil  140  may be configured to couple to a same ground. 
     Dipole radiation may pass from one side of the micro-scale planar-coil transformer  100  to the other side of the transformer. The dipole radiation may be as a result of common-mode current flowing between the first coil  140  and the second coil  150 . The shield  160  may be configured to reduce the dipole radiation between the primary side and the secondary side of the transformer. The shield  160  may reduce the dipole radiation by reducing or substantially eliminating the common-mode current flowing between the first coil  140  and the second coil  150 . The shield  160  may be configured to couple to a ground and the common-mode current may be absorbed through the ground coupling. 
     The micro-scale planar-coil transformer  100  includes a via  170  configured to electrically couple between two layers of the micro-scale planar-coil transformer  100 . In  FIG.  1   , the via  170  couples between the first coil layer  120  and the shield  160  arranged in the intermediate layer  130 . The via  170  may couple the shield  160  to a ground that is coupled to elements on the primary side of the transformer or to elements in the first coil layer  120 , such as a ground coupled to the first coil  140 . 
     Coils, shields, vias, and other elements may comprise various materials. In some embodiments, coils or shields comprise conductive materials, for example, metals such as copper, gold, or aluminum, or may comprise semiconductor materials, such as doped semiconductor materials. In some embodiments, each coil or shield is arranged in a metallization layer, or is arranged in more than one metallization layer. 
     The substrate  110  has an upper surface  112 . The upper surface  112  may be substantially planar. The upper surface of the substrate  110  is arranged between the substrate  110  and the first coil layer  120 . 
     According to aspects of the present application, a substrate, such as substrate  110 , may comprise various materials. In some embodiments, a substrate may comprise a semiconductor material. For example, a substrate may comprise a bulk or monocrystalline semiconductor substrate, such as a bulk or monocrystalline silicon substrate. In some embodiments, a substrate may comprise a deposited semiconductor substrate, such as polycrystalline silicon. In some embodiments, a substrate may comprise a silicon-on-insulator substrate or may comprise a buried oxide layer. Other semiconductor materials may be used as substrates. In some embodiments, a substrate, such as substrate  110 , may comprise a PCB. 
       FIG.  2    shows an exploded top perspective view of a micro-scale planar-coil transformer  200 . The micro-scale planar-coil transformer  200  may be a more detailed implementation of the micro-scale planar-coil transformer  100 . Micro-scale planar-coil transformer  200  is arranged on substrate  210 , and includes an intermediate layer  230 , a first coil  240 , a second coil  250 , and a shield  260 . 
     The first coil  240  includes a first coil portion  242   a , a second coil portion  242   b , a first pad  244   a , a second pad  244   b , center tap pad  246 , and an intermediate portion  248 . In  FIG.  2   , the two coil portions, first coil portion  242   a  and second coil portion  242   b  are arranged in series. 
     The first coil portion  242   a  and the second coil portion  242   b  are each arranged as planar coils. The first coil portion  242   a  and the second coil portion  242   b  are patterned as planar spirals having a plurality of turns. In some embodiments, a coil or a coil portion may comprise only a single turn. The first coil portion  242   a  and the second coil portion  242   b  are each arranged about coil axes that are substantially perpendicular to an upper surface  212  of the substrate  210 . 
     The first coil  240  is coupled to input and output pads. A first end of the first coil portion  242   a  is coupled to the first pad  244   a , which is arranged outside the first coil portion  242   a . A first end of the second coil portion is coupled to a second pad  244   b , which is arranged outside the second coil portion  242   b . Signals may be applied to the first coil  240  via the first pad  244   a  and the second pad  244   b . 
     The first coil portion  242   a  and the second coil portion are coupled by the intermediate portion  248 . A second end of the first coil portion  242   a  is coupled to the intermediate portion  248  of the first coil  240 . A second end of the second coil portion  242   b  is coupled to the intermediate portion  248 . As such, the first coil portion  242   a  is coupled to the second coil portion  242   b  through the intermediate portion  248 . 
     The intermediate portion  248  may comprise a jumper, such as an overpass portion. An overpass portion couples from an end of a coil located inside the coil portion to a location outside the coil portion. The overpass portion may pass above or below at least one turn of a spiral shaped coil, and may therefore be arranged in a layer above or below the layer of the first coil  240 . An overpass portion may be arranged at least partially in a same layer as the first portion and may be arranged at least partially in a different layer as the coil portion. 
     The intermediate portion  248  is coupled to a center tap pad  246 . Signals may be applied to the first coil  240  via the center tap pad  246 . For example, the center tap pad may be held to a constant voltage or may be held to ground. 
     In  FIG.  2   , the first coil  240  is arranged in an S-coil shape. Other configurations are possible, for example, C-coil shapes. An S-coil shape is arranged such that a current flowing through a first coil portion will spiral in a first rotation direction and current flowing through the second coil portion will spiral in a second rotation direction. The opposite rotation directions of the two portions of the S-coil induce changing magnetic fields which are arranged in substantially anti-parallel directions. 
     Alternatively, a C-coil shape is arranged such that a current flowing through a first coil portion and current flowing through the second coil portion will spiral in a same rotation direction. The C-coil induces changing magnetic fields which are arranged in substantially parallel directions. 
     The second coil  250  includes a first coil portion  252   a , a second coil portion  252   b , a first pad  254   a , a second pad  254   b , and an intermediate portion  248 . 
     The second coil  250  differs from the first coil  240  in that the first pad  254   a  is arranged within the first coil portion  252   a  and the second pad  254   b  is arranged within the second coil portion  242   b . In some embodiments, where the second coil  250  is arranged in a second coil layer, the first pad  254   a  and the second pad  254   b  may be left exposed or coupled to a via so that the ends of the second coil  250  may be coupled to other circuit elements. This arrangement of the pads may reduce manufacturing complexity or cost compared to manufacturing the second coil  250  with one or more overpass portions. 
     The second coil  250  differs from the first coil  240  in that the intermediate portion  258  does not include an overpass portion. Because the first pad  254   a  and the second pad  254   b  are arranged within the second coil  250 , the intermediate portion may be coupled to the outermost turn of coil  250  and therefore is not required to pass over any turns of the second coil  250  above or below a layer of the second coil  250 . This may reduce complexity and cost of manufacture. 
     The second coil  250  differs from the first coil  240  in that the intermediate portion  258  is not coupled to a center tap pad. In some embodiments, the intermediate portion may be center-tapped, for example, by a via. In some embodiments, the second coil  250  may include a center tap pad coupled to the intermediate portion  258 . 
     The first coil  240  and the second coil  250  are electromagnetically coupled. In some embodiments, the first coil  240  is arranged on a primary side of the micro-scale planar-coil transformer  200 , and the second coil  250  is arranged on a secondary side of the micro-scale planar-coil transformer  200 . In some embodiments, the first coil  240  is a transmit coil and the second coil  250  is a receive coil. 
     The first coil portion  242   a  of the first coil  240  is electromagnetically coupled to the first coil portion  252   a  of the second coil  250 . Similarly, the second coil portion  242   b  of the first coil  240  is electromagnetically coupled to the second coil portion  252   b  of the second coil  250 . In some embodiments, each of the coil portion couplings may act as an individual transformer coupling, forming two transformers arranged in series. In  FIG.  2   , each of the portions  242   a ,  242   b ,  252   a , and  252   b  are respectively arranged about coil axes the coil axes being substantially perpendicular to upper surface  212  of the substrate  210 . The coil axes of each pair of coupled coil portions are substantially aligned. 
     A coil occupies an area in a plane parallel to the upper surface of the substrate upon which the coil is arranged. The coil area may include the area of any intermediate portions. Similarly, a coil portion occupies an area. The coils making up a pair of coupled coils may occupy a substantially same area. Coil portions making up a pair of coupled coil portions may occupy a substantially same area. 
     The shield  260  includes first shield portion  262   a , second shield portion  262   b , shield pad  264 , intermediate portion  266 , first overpass opening  268   a , and second overpass opening  268   b . In some embodiments, where coil portions comprise planar spirals, shield portions may have substantially circular perimeters. Shield portions may have perimeters forming other shapes. 
     Each shield portion may be arranged between a pair of coupled coil portions. In  FIG.  2   , first shield portion  262   a  is arranged intermediate the coupled first coil portion  242   a  of first coil  240  and first coil portion  252   a  of second coil  250 . Second shield portion  262   b  is arranged intermediate the coupled second coil portion  242   b  of first coil  240  and second coil portion  252   b  of second coil  250 . 
     In some embodiments, shield portions are coupled together. The intermediate portion  266  couples the first shield portion  262   a  to the second shield portion  262   b . In some embodiments, the shield  260  does not include an intermediate portion  266 , and the first shield portion  262   a  is not coupled to the second shield portion  262   b  by an intermediate portion. 
     According to aspects of the present application, shield  260  has external couplings. In  FIG.  2   , the intermediate portion  266  is coupled to the shield pad  264 . The shield pad may be coupled to power or ground. In  FIG.  2   , the shield pad  264  is coupled to the intermediate portion  266  and is therefore arranged in a substantially symmetric location of the shield  260 . Other arrangements are possible. The shield  260  may be arranged such that the shield  260  is not arranged substantially symmetrically. For example, the shield pad  264  may be arranged at an outer location of the shield  260 , distal from the intermediate portion  266 . 
     Shield couplings may be symmetric or asymmetric. Each portion of a transformer itself has significant inductance at high frequencies, therefore at these higher frequencies, a symmetric arrangement can achieve equal shield effectiveness for each of the pairs of coupled coil portions and EMI may be reduced at odd harmonics of an applied oscillation frequency. Symmetric connections may also reduce the resistance of a shield component, as the maximum distance from any one point on the shield to the shield pad is reduced. Reduced shield resistance may result in more effective routing of common mode current to ground. 
     In some embodiments, other arrangements of shield couplings are used. In some embodiments, a shield is coupled to a first shield pad at a first edge of the shield and is coupled to a second shield pad at a corresponding symmetric second edge of the shield, across the intermediate portion, such that symmetry is maintained. In some embodiments, an asymmetric connection may be used, for example to reduce complexity or cost, with the tradeoff of reduction in shield effectiveness. 
     The first overpass opening  268   a  and the second overpass opening  268   b  are cutouts in the shield  260  configured to accommodate overpass portions of the intermediate portion  248  of first coil  240  within a perimeter of the shield  260 . 
     Overpass openings in shields may be configured to reduce the number of layers in a transformer. When an overpass opening is arranged in a shield, the overpass may be arranged in same layer as the shield. This may allow two coils and a shield to be arranged in a total of three layers. If overpass openings are not arranged in the shield, the same two coils and shield require a total of at least four layers. 
     In some embodiments, the intermediate portion  258  of second coil  250  includes overpass portions and the intermediate portion  248  of first coil  240  does not. In such an arrangement, the first overpass opening  268   a  and the second overpass opening  268   b  of the shield  260  are configured to accommodate the overpass portions of the intermediate portion  258  of second coil  250  instead. In such an embodiment, because second coil  250  is an upper coil, the overpass portions of the second coil  250  may be arranged to pass under turns of the second coil  250 . Because the shield  260  is arranged between the coils, the overpass portions of the second coil  250  are arranged in a same layer that the shield  260  is arranged in. Accordingly, this transformer may also comprise only three metallization layers. In this alternative arrangement, the shield  260  may be arranged proximate to second coil  250 , and the isolation layer  230  may be arranged between shield  260  and first coil  240 . Shield  260  may be coupled to a same ground or power connection that the second coil  250  is coupled to in this arrangement. A perimeter of a shield encloses a region having a shield area. The perimeter of the shield surrounds any shield portions and any intermediate portions of the shield. The region enclosed by the perimeter of the shield includes the area occupied by the physical elements of the shield as well as the area of any gaps between physical elements of the shield that are within the perimeter. Gaps include overpass openings and other gaps, for example, gaps configured to reduce eddy current, described below. 
     In some embodiments, regions enclosed by shield perimeters may occupy various areas relative to coils, or may cover various areas of coils. In some embodiments, for example, shield  260  in  FIG.  2   , when the region enclosed by the perimeter of a shield is projected onto an underlying and/or overlying coil, such as first coil  240  or second coil  250 , the projection of the region enclosed by the perimeter of the shield covers an area of the coil that is substantially equal to the area occupied by the coil. This may be referred to as a “full shield” arrangement. Other arrangements are possible. In some embodiments, the projection of the region enclosed by the perimeter of the shield covers an area of the coil that is less than the area occupied by the coil. This may be referred to as a “partial shield” arrangement. 
     Similarly, a perimeter of a shield portion may enclose a region having shield portion area. The perimeter of the shield portion separates the shield portion from any intermediate portions of the shield. In  FIG.  2   , the perimeter of the first shield portion  262   a  and the perimeter of the second shield portion  262   b  are substantially circular. In some embodiments, such as in shield portion  262   a  in  FIG.  2   , when the region enclosed by the perimeter of a shield portion is projected onto an underlying and/or overlying coil portion, such as first coil portion  242   a  of first coil  240  or first coil portion  252   a  of second coil  250 , the projection of the region enclosed by the perimeter of the shield portion covers an area of the coil portion that is substantially equal to the area occupied by the coil portion. Other arrangements are possible. In some embodiments, the projection of the region enclosed by the perimeter of the shield portion covers an area of the coil portion that is less than the area occupied by the coil portion. 
       FIG.  3    shows an exemplary schematic of a circuit  300 . Circuit  300  comprises a transformer The circuit  300  may be a circuit schematic implementation of the micro-scale planar-coil transformer  200 . Circuit  300  includes a primary side  310 , a secondary side  320 , a first coil  330 , a second coil  340 , a shield  350 , and an isolation layer  360 . 
     The primary side  310  is coupled to a primary side ground  312 , and the secondary side  320  is coupled to a secondary side ground  322 . In  FIG.  3   , the primary side ground  312  and the secondary side ground  322  are not coupled. 
     The secondary side  320  includes output  324 . The output  324  supplies the output signal from the circuit  300 . 
     The first coil  330  includes a first coil portion  332   a  and a second coil portion  332   b , and the first coil  330  is coupled to a first terminal  334   a , a second terminal  344   b , and a center tap  336 . The first terminal  332   a  and the second terminal  332   b  comprise input terminals of the first coil  330 . In some embodiments, each of the first terminal  332   a  and the second terminal  332   b  are directly or indirectly coupled to the primary side ground  312 . 
     The first coil is coupled to a center tap  336 . The center tap  336  is arranged intermediate the first coil portion  332   a  and the second coil portion  332   b . 
     The second coil  340  includes a first coil portion  342   a  and a second coil portion  342   b , and the second coil is coupled to a first terminal  344   a  and a second terminal  344   b . The first terminal  342   a  and the second terminal  342   b  comprise output terminals of the second coil  340 . In some embodiments, each of the first terminal  342   a  and the second terminal  342   b  are directly or indirectly coupled to the secondary side ground  322  and the output  324 . In some embodiments, a transformer comprises a rectifier that is arranged intermediate a second coil and one or more of an output and a secondary side ground. 
     In  FIG.  3   , the second coil is not coupled to a center tap. In some embodiments, the second coil  340  is coupled to a center tap, such as a center tap arranged intermediate the first coil portion  342   a  and the second coil portion  342   b . 
     Input signals may be applied to each of the first terminal  332   a  and the second terminal  332   b . In some embodiments, the signals are independent or non-differential, for example, the half-sinusoidal, 180 degree out-of-phase signals with the same sign described above. When such signals are applied to the first terminal  332   a  and the second terminal  332   b , a common-mode voltage ripple amplitude of first coil  330  is greater than substantially zero. The center tap  336  may be held at a constant voltage or held to ground. 
     In some embodiments, the common-mode voltage ripple amplitude of the second coil  340  is substantially zero. In some embodiments, the common-mode voltage ripple amplitude of the second coil  340  is greater than substantially zero and less than the common-mode voltage ripple amplitude of the first coil  330 . 
     Due to parasitic capacitance between the first coil  330  and the second coil  340  transformer, the ripple amplitude differences between the first coil  330  and the second coil  340  would introduce a common-mode current crossing the from the primary side  310  to the secondary side  320  of the circuit  300 , which would flow back to primary side  310  by dipole radiation, potentially inducing unacceptable levels of EMI. 
     However, the shield  350 , arranged in the isolation layer  360 , is configured to reduce the dipole radiation between the primary side  310  and the secondary side  320 . The shield  350  may comprise a Faraday shield, and is arranged between the first coil  330  and the second coil  340 . 
     The shield  350  is configured to block the electric field between the first coil  330  and the second coil  340 . Accordingly, most or substantially all of the common-mode current is blocked by the shield, and absorbed by a power or ground coupled to the shield  350 , for example, primary side ground  312 . As such, most or substantially all of the common-mode current only flows in loop  370  on the primary side  310  and will not cross the isolation layer  360 . The loop  370  is entirely on primary side  310 , and therefore substantially zero common-mode current from loop  370  will cross the isolation layer  360 , and will not cause dipole radiation and induce EMI. 
     In some embodiments, a common-mode voltage ripple amplitude of the second coil  340  is greater than substantially zero. Accordingly, some common-mode current may the isolation layer  360  cross from the secondary side  320  to the shield  350  via loop  380 . Some dipole radiation may occur from loop  370 , but the dipole radiation is reduced or substantially eliminated. 
       FIG.  4    shows a top view of some components of a micro-scale planar-coil transformer  400 . The components of a micro-scale planar-coil transformer  400  comprise a first coil  440  and shield  460 . The first coil  440  and the shield  460  may be arranged primarily in different layers of micro-scale planar-coil transformer  400 . 
     The first coil  440  includes a first coil portion  442   a , a second coil portion  442   b , a first pad  444   a , a second pad  444   b , center tap pad  446 , and an intermediate portion  448  comprising first overpass portion  448   a  and second overpass portion  448   b . 
     In  FIG.  4   , overpass portions cross over turns of the coil portions. First overpass portion  448   a  is arranged in a same layer as shield  460  and couples an end of the first coil portion  442   a  that is within the first coil portion  442   a  to the second coil portion  442   b , crossing above the turns of the first coil portion  442   a . Similarly, second overpass portion  448   b  is arranged in the same layer as shield  460  and couples an end of the second coil portion  442   b  that is within the second coil portion  442   b  to the first coil portion  442   a , crossing over turns of the second coil portion  442   b . 
     Alternatively, overpass portions may connect ends of a coil to coil pads. In such an arrangement, the overpass portions may cross from within first coil portion  442   a  to the first pad  444   a  and from within the second coil portion  442   b  to the second pad  444   b . In such an arrangement, the intermediate portion  448  do not include an overpass portion, and are be arranged entirely in the layer of the first coil. Overpass openings in the shield are arranged to accommodate the alternative location of overpass portions. 
     The shield  460  includes first shield portion  462   a , second shield portion  462   b , shield pad  464 , intermediate portion  466 , first overpass opening  468   a , and second overpass opening  468   b . The first overpass portion  448   a  is arranged within the first overpass opening  468   a  and the second overpass portion  448   b  is arranged within the second overpass opening  468   b . 
     Overpass portions and their corresponding overpass openings in shields may provide reduced shield effectiveness. When an overpass overlies a portion of the coil, that portion of the coil is therefore not covered by a shield, which may reduce shield effectiveness or provide a noise source. An overpass or an underpass in a non-shield layer may increase shield effectiveness at the cost of requiring additional layers. In some partial shield arrangements, an overpass portion may alternatively pass over a portion of a coil not covered by a shield perimeter. In some embodiments, shield area may include area occupied by overpass portions. 
     In some embodiments, alternative shield couplings may be used. For example, in some embodiments, a shield may be coupled to a center tap. In such an embodiment, portions of the shield can act as the overpass portion of the coil. Accordingly, a separate shield pad and center tap pad are not required. Such an arrangement can reduce manufacturing cost and complexity but may reduce shield effectiveness in some embodiments. 
     The inventors have recognized that in some transformer configurations, a shield covering less than the full area of a coil may further reduce or substantially eliminate dipole radiation. For example, when common-mode voltage ripple amplitude of coils on both a primary side and secondary side are greater than substantially zero, shields that do not fully cover the coils may be used. In such a configuration, only a single shield may be used. 
     In  FIG.  4   , when the region enclosed by the perimeter of a shield  460  is projected onto the underlying or overlying coils, for example first coil  440 , the projection of the region enclosed by the perimeter of the shield  460  covers an area of the first coil  440  that is less than the area occupied by the first coil  440 . 
     Similarly, when a region enclosed by the perimeter of a shield portion such as shield portion  462   a , is projected onto the underlying and/or overlying first coil portion, such as first coil portion  462   a , the projection of the region enclosed by the perimeter of the shield portion  462   a  covers an area of the coil portion  442   a  that is less than the area occupied by the coil portion  442   a . 
     In some embodiments, the common-mode voltage ripple amplitude of a first coil is greater than substantially zero, and the common-mode voltage ripple amplitude of a second coil is greater than substantially zero and less than the common-mode voltage ripple amplitude of the first coil. 
     The inventors have recognized that the dipole radiation may be reduced if the common-mode current is reduced or substantially eliminated by matching common mode voltage ripple amplitudes. As discussed above, there is a parasitic capacitance between first and second coils. If voltages applied to plates of a capacitor are the same, then there will be no current crossing the capacitor. Similarly, if the common-mode voltage ripple amplitude is matched on the first and second coil, common-mode current will not flow. Accordingly, the common-mode current may be eliminated, and therefore no dipole radiation will cross between the sides. 
     The inventors have recognized that the area of a partial shield covering a coil may be configured so that the exposed common-mode voltage ripple amplitude of a primary side and the exposed common-mode voltage ripple amplitude of a secondary side more closely match or substantially match each other. A partial shield may covering a percentage A of a transmit coil may absorb percentage A of the common-mode voltage ripple amplitude and expose the receive coil to a percentage B of the transmit coil’s common-mode voltage ripple amplitude, where the percentage B is substantially equal to percentage A subtracted from 100 percent. 
     The first coil may occupy a first area in a plane parallel to an upper surface of a substrate. The coupled second coil may occupy substantially the first area. A projection onto the first coil of a region enclosed by a perimeter of a shield, in the plane, may cover a second area of the coil that is less than the first area. 
     In some embodiments, an area ratio comprising a ratio of the second area to the first area is related to or at least partially based on a ripple ratio that is a ratio of the second common-mode voltage ripple amplitude of the second coil to the first common-mode voltage ripple amplitude of the first coil. For example, if the ripple ratio is less than 1, a shield and a coil may be patterned to have an area ratio that is less than 1. In some embodiments, the area ratio and the ripple ratio comprise substantially complementary percentages. Complementary percentages add up to a value of 1, or to 100 percent. Therefore, the first ratio and the second ratio substantially add up to a value of 1, or to 100 percent. Accordingly a shield may be patterned to have an area ratio in relation to the coils that is substantially equal to the ripple ratio subtracted from a value of 1, or from 100 percent. 
     The inventors have recognized that, for some transformers, the ripple ratio is related to the coupling factor, k, of the first coil and the second coil. Accordingly, in some embodiments, the area ratio may be related to or at least partially based on the coupling factor of the coils. In some embodiments, the ripple ratio is directly proportional to the coupling factor of the first coil and the second coil. In some embodiments, the ripple ratio may substantially equal the coupling factor. Accordingly, in some embodiments, the area ratio and the coupling factor substantially add up to a value of 1, or to 100 percent. A shield may therefore be configured to have an area ratio that is substantially equal to the coupling factor subtracted from a value of 1, or from 100 percent. 
     The coupling factor, k, of a transformer is affected by coil diameter, distance between coils, coil alignment, and bonding wire inductance, among other factors. The coupling factor k is not substantially affected by the circuits coupled to the transformers, such as driving or rectifying circuits. 
     The same principles may be additionally or alternatively applied to shield portions and coil portions as they are applied shields and coils herein. 
     The inventors have recognized that is beneficial to block only the electric field and to allow the magnetic field to substantially pass through a shield. In some embodiments, such as shield  460  in  FIG.  4   , a shield may be configured to substantially block an electric field from passing through the shield. The shield may be configured to reduce or substantially eliminate the amount of the magnetic field that is blocked by the shield, so that the magnetic field is substantially entirely passed through the shield. 
     A shield applied to a spiral-shaped coil may be arranged having an overall circular perimeter and covering the entirety of the coils, or may be arranged having other perimeter shapes which only partially cover the coils. A shield which is configured to substantially block the electric field and substantially the pass magnetic field is not arranged purely as a circular shape. A shield, for example shield  460  or shield  1060  described below, may be divided into many narrow stripes. Such a shield may be comprised of a plurality of high aspect ratio segments configured to reduce eddy currents in the shield. The plurality of high aspect ratio segments may be patterned to form the overall circular or other shape of the shield. 
     Eddy currents are closed loops of electrical current which may be induced in the shield or another electrical conductor perpendicular to a changing magnetic field passing through. The physical space through which the changing magnetic field passes determines the size of the eddy current loop. The magnitude of the current is proportional to the size of the loop. A larger current corresponds to a larger loss in energy from the magnetic field, which decreases the efficiency of the transformer. Therefore, reducing the size of the loop, for example by using the high aspect ratio segments, will allow more of the magnetic field to pass through the shield and will reduce or substantially eliminate the effect of reduced efficiency from eddy current. 
     In some embodiments, a non-shield technique may be used to at least partially cancel the ripple of one or more sides of a transformer. Accordingly, a single full shield may be used to block common-mode voltage ripple amplitude of another side of the transformer, such that the ripples of each side are separately removed. 
       FIG.  5    shows a top view of some components of a micro-scale planar-coil transformer  500 . The components of a micro-scale planar-coil transformer  500  comprises a first coil  540  and a second coil  570 . First coil  540  comprises first coil portion  542   a  and second coil portion  542   b  coupled together by intermediate portion  548 . The first coil  540  is coupled to pads  544   a  and  544   b . Intermediate portion  548  is coupled to center tap pad  546 . Second coil  570  comprises a first coil portion  572   a  and a second coil portion  572   b , which are not coupled by an intermediate portion. First coil portion  572   a  is coupled to pads  574   a  and  574   b . Second coil portion  572   b  is coupled to pads  574   c  and  574   d . The first coil  540  and the second coil  570  may each be arranged on the primary side of a transformer and may be arranged in a lower layer. 
     Separate signals are applied to first coil  540  and second coil  570  by their respective pads. An oscillation direction of the applied signals may be substantially adverse, such that the common-mode ripple of the first coil  540  and the second coil  570  is also adverse. Accordingly, the total common-mode ripple of the two primary side coils may substantially match and substantially cancel each other, and as a result, the common-mode voltage ripple amplitude of the primary side may be substantially zero. 
     In some embodiments, center tap pad  546  comprises two separate pads. The first pad is coupled to first coil portion  542   a , and the second pad is coupled to second coil portion  542   b , with no connection between the first pad and the second pad via any intermediate portion. Such an arrangement can result in an increased match between the first coil  540  and the second coil  570 . The increased match can occur in the two pad configuration due to the shapes of each coil and pad arrangement for first coil  540  and second coil  570  being more similar than in the single pad configuration. In some embodiments, in the two pad configuration, the shapes of each coil and pad arrangement for first coil  540  and second coil  570  are substantially the same. The arrangement requires an additional bonding wire connection for the second pad. 
       FIG.  6    shows a top view of some components of a micro-scale planar-coil transformer  600 . The components of a micro-scale planar-coil transformer  500  and the components of a micro-scale planar-coil transformer  600  may form a same transformer. The components of a micro-scale planar-coil transformer  600  comprises a third coil  650  and a shield  660 . 
     The third coil  650  includes a first coil portion  652   a , a second coil portion  652   b , a first pad  654   a , a second pad  654   b , and an intermediate portion  658  including first overpass portion  658   a  and second overpass portion  658   b . The third coil  650  may be arranged on the secondary side of a transformer, and may be arranged in an upper layer. 
     The shield  660  includes first shield portion  662   a , second shield portion  662   b , shield pad  664 , intermediate portion  666 , first overpass opening  668   a , and second overpass opening  668   b . 
     In some embodiments, the third coil  650  may be electromagnetically coupled to the first coil  540  and the second coil  570 . As described above, the common-mode voltage ripple amplitude of the primary side including coils  540  and  570  may be substantially zero from a non-shield technique. Accordingly, shield  660  is a single full shield used to block common-mode voltage ripple amplitude of the secondary side that includes third coil  650 , such that the ripple of both sides ripples is removed. 
     Shields may be applied to back to back transformers.  FIG.  7    shows a cross-sectional side view of a micro-scale planar-coil transformer  700 . Micro-scale planar-coil transformer  700  comprises a back to back transformer. Micro-scale planar-coil transformer  700  is arranged on a substrate  710  having an upper surface  712 , and includes an underlying layer  714 , lower coil layer  720 , intermediate layers  730 ,  732 ,  734 , and  736 , upper coil layer  722 , a first coil  740 , a second coil  750 , coil coupling  752 , third coil  760 , fourth coil  770 , first shield  780 , second shield  790 , first via  782  and second via  792 . Portions  784  and  794  of the coils may be exposed to provide coupling points. 
     The shields may be arranged more proximate first coil  740  and fourth coil  770  so that the common-mode current does not cross the isolation layer. The first shield  780  and the second shield  790  may be arranged above the intermediate layers  730 ,  732 , and  734 , and under the intermediate layer  736  so that the first shield and the second shield are arranged closer to the first coil  740  and the fourth coil  770  than to the second coil  750  and the third coil  760 . Having the shields  780  and  790  arranged closer to the coils  740  and  770  may reduce the distance into the isolation layers that any current loop from the coils  740  and  770  travels. 
       FIG.  8    shows an exemplary schematic of a circuit  800 . Circuit  800  comprises a back to back transformer. The circuit  800  may be a circuit schematic implementation of the micro-scale planar-coil transformer  700 . Circuit  800  includes a first transformer  810  having primary side  812   a  and secondary side  812   b , a second transformer  820  having primary side  822   a  and secondary side  822   b . Circuit  800  includes a first coil  840  having first terminal  844   a  and second terminal  844   b , a second coil  850  having first terminal  854   a  and second terminal  854   b , a third coil  860  having first terminal  864   a  and second terminal  864   b , and a fourth coil  870  having first terminal  874   a  and second terminal  874   b . Circuit  800  includes a first shield  880  coupled to primary side ground  814  and a second shield coupled to secondary side ground  824 . 
     In a back to back transformer, to prevent the flow of common-mode current, each transformer pair of coupled coils may have a shield arranged therebetween. In some embodiments, each shield comprises a full shield, and the shields are arranged in a same layer. In some embodiments, the shields may comprise a same shield. 
     In some embodiments, a back to back transformer may comprise one or more partial shields. For example, a back to back micro-scale planar-coil transformer such as micro-scale planar-coil transformer  700  may comprise a partial shield. In  FIG.  7   , the micro-scale planar-coil transformer  700  comprises first shield  780  and second shield  790 , which may each comprise a full shield. However, a back to back micro-scale planar-coil transformer may alternatively comprise a single shield, which may comprise a partial shield. For example, in some embodiments, the micro-scale planar-coil transformer  700  includes only one of the first shield  780  and the second shield  790 . The one shield included in the back to back micro-scale planar-coil transformer may be arranged according to partial shield arrangement techniques described above with respect to  FIG.  4   . For example, the area of the partial shield may be patterned related to or at least partially based on the area of the first coil  740 . Such an arrangement can reduce dipole radiation in a similar manner as described with respect to  FIG.  4   . This arrangement may require the same number of layers as a two full shield arrangement of micro-scale planar-coil transformer  700 . 
     Each of the coils of a back to back transformer may include one or more coil portions as described above with respect to  FIG.  2   . 
     Some embodiments may include two shields.  FIG.  9    shows a cross-sectional side view of a micro-scale planar-coil transformer  900 . Micro-scale planar-coil transformer  900  includes two shields. Micro-scale planar-coil transformer  900  is arranged on a substrate  910  having an upper surface  912 , and includes an underlying layer  914 , lower coil layer  920 , intermediate layers  930 ,  932 ,  934 , and  936 , upper coil layer  922 , a first coil  940 , a second coil  950 , first shield  960 , second shield  970 , and vias  982 . Portions  984  and  994  of the coils may be left exposed to provide coupling points. 
       FIG.  10    shows some components of a micro-scale planar-coil transformer  1000 . The components of a micro-scale planar-coil transformer  1000  may comprise components of micro-scale planar-coil transformer  900 . The components of a micro-scale planar-coil transformer  1000  comprises a first coil  1040  and shield  1060 . 
     The first coil  1040  includes a first coil portion  1042   a , a second coil portion  1042   b , a first pad  1044   a , a second pad  1044   b , center tap pad  1046 , and an intermediate portion  1048 . 
     The shield  1060  includes first shield portion  1062   a , second shield portion  1062   b , shield pad  1064 , intermediate portion  1066 , first overpass opening  1068   a , and second overpass opening  1068   b . The shield  1060  may comprise each of the first shield  960  and second shield  970  in  FIG.  9   . A transformer having two shields may have two full shields. A first full shield may block common-mode current from crossing the isolation layer from the primary side, and a second full shield may block common-mode current from crossing the isolation layer from the secondary side. 
       FIG.  11    shows an exemplary schematic of a circuit  1100 . Circuit  1100  comprises a transformer comprising double shields. The circuit  1100  may be a circuit schematic implementation of the micro-scale planar-coil transformer  900 . Circuit  1100  includes a primary side  1110  including a first coil  1130 , and a first shield  1150  coupled to primary side ground  1112 , a secondary side  1120  including a second coil  1140 , and a second shield  1160  coupled to secondary side ground  1160 , and an isolation layer  1170 . First coil  1130  has a first terminal  1134   a , a second terminal  1134   b , and a center tap  1136 . Second coil  1140  has a first terminal  1144   a  and a second terminal  1144   b  which are coupled to an output  1124 . 
     Circuit  1100  comprises two full shields. The first shield  1150  and the second shield  1160  are configured to block the electric field between the first coil  1130  and the second coil  1140 . Accordingly, most or substantially all of the common-mode current is blocked by the shields, and absorbed by a the primary side ground  1112  and the secondary side ground  1122 . As such, most or substantially all of the common-mode current only flows in the loop  1180  and the loop  1190 . Current in the loop  1080  flows only on the primary side  1110 , and may be absorbed by the primary side ground. Current in the loop  1190  flows only on the secondary side  1120 , and may be absorbed by the secondary side ground  1122 . The current in each of the loops will not cross the isolation layer  1170 . Because the loop  1180  is entirely on primary side  1110 , and the loop  1190  is entirely on secondary side  1110 , substantially zero common-mode current will cross the isolation layer and will not cause dipole radiation or induce EMI. 
     A double shield arrangement requires additional layers compared to single shield arrangements. For example, if overpass portions are arranged in shield layers, at least 4 layers are required. Some arrangements of double shields require 5 or more layers. 
     Each of the coils of a double shield transformer may include one or more coil portions as described above with respect to  FIG.  2   . 
     A method of manufacturing a micro-scale planar-coil transformer may comprise the steps of forming each of the layers described with respect to any of  FIGS.  1 ,  7 , and  9   , and patterning each of the coils and shields. For example, a substrate may be provided. An underlying layer may be formed on the substrate. A first coil layer including a first coil may be formed on the underlying layer or on the substrate. The first coil may be patterned in the first coil layer in the arrangements described above. One or more intermediate layers including one or more shields may be formed on the first coil layer. The one or more shields may be pattered in the one or more intermediate layers in the arrangements described above. A second coil may be patterned above the intermediate layer in the arrangements described above. The second coil may be formed in a second coil layer. 
     A method of operating a micro-scale planar coil transformer may comprise the steps of applying a set of signals to the terminals of a first coil arranged on a primary side of the transformer such that the first coil has a first common-mode voltage ripple amplitude greater than substantially zero and a second coil arranged on a secondary side of the transformer that is electromagnetically coupled to the first coil has a second common-mode voltage ripple amplitude greater than or equal to zero and less than the first common-mode voltage ripple amplitude. The method may comprise reducing the dipole radiation between the first and second coils. The method may comprise blocking common-mode current between the first and second coils with a shield. The method may comprise blocking substantially all of the common-mode current with a full shield. The method may comprise exposing the second coil to a portion of the first common-mode voltage ripple amplitude where the portion is substantially equal to the second common-mode voltage ripple amplitude, using a partial shield. In some embodiments, a ratio of the portion to the first common-mode voltage ripple amplitude is substantially equal to a coupling factor of the first and second coils. 
     While shields in the present application may generally be described with respect to micro-scale planar-coil transformers that comprise center tapped coils made up of two coil portions, aspects of the present application may be applied to other transformers. For example, in some embodiments, shields described in the present application may be applied to other transformers which exhibit dipole radiation. In some embodiments, shields may be applied to other transformers which exhibit common-mode current crossing between a primary side and a secondary side. In some embodiments, shields may be applied to transformers which exhibit common-mode voltage ripple amplitudes differences between primary and secondary sides. 
     Having thus described several aspects and embodiments of the technology of this application, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those of ordinary skill in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described in the application. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described. In addition, any combination of two or more features, systems, articles, materials, and/or methods described herein, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present disclosure. 
     Also, as described, some aspects may be embodied as one or more methods. 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. 
     The terms “approximately,” “substantially,” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value.