Patent Publication Number: US-7915992-B2

Title: Planar, high voltage embedded transformer for analog and digital data transmission

Description:
BACKGROUND 
     This invention relates generally to air-core transformers, and more particularly, to a planar air-core transformer design to replace a traditional parallel resonant balun that is comprised of a co-axial inductor and capacitor that is used in receiver coils in magnetic resonance imaging (MRI) systems to isolate coil elements. The transformer structure provides enhanced isolation at a specific frequency that can be used to replace a traditional transformer or to reduce common mode currents when used in high frequency switching power electronic converters and thereby reducing EMI generation and subsequently filtering requirements. 
       FIG. 1  illustrates a traditional balun  10  for MRI receiver coils that can be used multiple times, depending upon the location of the corresponding coil cable bundle exiting the corresponding structure, relative to the receive coil element. Balun  10  includes a common mode inductor  12  connected to a co-axial cable  14  at each end. Balun  10  further includes a common mode capacitor  16  external to a copper shield  18  that surrounds and shields the common mode inductor  12 . External copper shield  18  includes an end cap  20  at each end to fully encapsulate the common mode inductor  12  within its shielded environment. A hole (not shown) may be centered on one of the end caps to allow for a brass tuning screw in some embodiments. Traditional balun  10  is used to isolate coil elements in MRI system receiver coils. 
     Optical isolating devices are generally used to provide signal isolation in power converters such as that depicted in  FIG. 2  that illustrates a transformer employed in a medium voltage power electronic converter. Compact transformers are used in rare instances, but are limited to medium voltage (&lt;1500V) systems. 
     Ethernet transformers have been designed to provide common mode isolation with differential mode matching to ensure the best possible transmission of the data signals.  FIG. 3  illustrates an equivalent circuit for one Ethernet transformer that is known in the art. These transformers are limited in the isolation voltage that they can provide. 
     It would be desirable to provide a transformer that lends itself for integration in printed circuits, e.g., flexible printed, PCB, etc., and that provides higher isolation than traditional signal transformers at a particular and useful frequency such that the transformer is suitable to replace a traditional parallel resonant balun that is comprised of a co-axial inductor and capacitor such as the one depicted in  FIG. 1  or a simple transformer used for power supply isolation or data isolation in high frequency switching power electronic converters. 
     BRIEF DESCRIPTION 
     Briefly, in accordance with one embodiment, a transformer comprises: 
     a flex or printed circuit board comprising a substrate material having a desired permittivity; and 
     at least one primary winding and at least one secondary winding, each winding integrated with a corresponding flex or printed circuit board layer such that one or more respective transformer parasitic elements and the substrate permittivity between the primary and secondary windings together are tuned to a desired parallel resonant frequency. 
     According to another embodiment, a transformer is integrated with a flex or printed circuit board such that one or more transformer parasitic elements and the flex or printed circuit board permittivity between corresponding transformer primary and secondary windings together are tuned to a desired parallel resonant frequency. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawing in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates a traditional balun for MRI receiver coils; 
         FIG. 2  illustrates a transformer balun employed in a medium voltage power electronic converter for signal or power isolation from one potential to another; 
         FIG. 3  illustrates an equivalent circuit for one Ethernet transformer that is known in the art; 
         FIG. 4  is an exploded perspective view illustrating a transformer, according to one embodiment of the present invention; 
         FIG. 5  is an equivalent circuit transformer electrical model for a conventional core transformer; 
         FIG. 6  illustrates an air-core signal isolation transformer electrical model for the transformer depicted in  FIG. 4 ; 
         FIG. 7  illustrates an external H-field immune transformer winding, according to one embodiment of the present invention; 
         FIG. 8  illustrates a partial cross-sectional side view an air-core transformer embedded in a multi-layer flex circuit, according to one embodiment of the present invention; 
         FIG. 9  is a partial cross-sectional side view illustrating an air-core transformer embedded in a multi-layer circuit board, according to one embodiment of the present invention; 
         FIG. 10  illustrates an MR imaging system that employs an air-core transformer according to one embodiment of the present invention; 
         FIG. 11  is a simplified block diagram illustrating a high isolation voltage embedded transformer applied to a gate driver circuit for low and medium voltage devices, such as IGBTs and MOSFETs, and performs the functions of signal and power isolation according to one embodiment of the invention; 
         FIG. 12  is a simplified block diagram showing a similar application of the transformers depicted in  FIG. 11  where the transformer functions have been combined into a single transformer, according to one embodiment of the invention; and 
         FIG. 13  is a schematic diagram depicting another variation of the transformer application shown in  FIG. 12 . 
     
    
    
     While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
     DETAILED DESCRIPTION 
       FIG. 4  is an exploded perspective view illustrating a transformer  30 , according to one embodiment of the present invention. Transformer  30  comprises a planar spiral primary winding  32  and a planar spiral secondary winding  34 . The planar spiral windings  32 ,  34  are each integrated with a corresponding layer of a common multi-layer flexible circuit or printed circuit board. The multi-layer flexible circuit board may further include a primary side shield layer  36  and a secondary side shield layer  38 . Transformer shield layers  36 ,  38  are located externally such that the primary and secondary windings  32 ,  34  can still couple magnetically. Planar spiral windings  32 ,  34  are positioned with a common central axis perpendicular to the planar surfaces of the planar spiral windings  32 ,  34  with respect to one another to provide differential mode signal transmission capabilities. 
     The air-core transformer design was recognized by the present inventors to provide advantages over traditional parallel resonant baluns that employ a co-axial inductor and capacitor such as those used in receiver coils in MRI systems to isolate coil elements by providing a broader frequency range of isolation between the primary and secondary with enhanced isolation at the tuned resonant frequency. It is important to note that the enhanced isolation at the resonant frequency does not increase the isolation withstand voltage but rather will increase the common mode impedance between the primary and secondary. 
     With continued reference to  FIG. 4 , each planar spiral winding  32 ,  34 , is spirally wound about an axis that is substantially common to both windings  32 ,  34 . Thus, the windings are concentric and have an increasing diameter. Further, each winding  32 ,  34  is planar such that the width of each winding  32 ,  34  in a direction parallel to the planar surface of the flex or printed circuit board is substantially greater than the height of each winding  32 ,  34  in a direction perpendicular to the planar surface of the flex or printed circuit board. According to one aspect, the windings are wound inwards on one layer and outwards on the next to ensure external connection to the windings  32 ,  34 . 
       FIG. 5  is an equivalent circuit electrical model  40  for a conventional transformer. Equivalent circuit transformer electrical model  40  comprises a primary winding  54 , a secondary winding  56 , and inherent lumped circuit parasitic elements. These inherent lumped circuit parasitic elements include a primary leakage inductance  44 , a primary magnetizing inductance  46 , a secondary leakage inductance  48  and a secondary magnetizing inductance  50 . Transformer electrical model  40  further includes a primary-secondary capacitance  52 . Although not technically correct, a transformer core  42  is shown to clearly show the difference between inductances  44 ,  46 ,  48 ,  50  as the mutual coupling for the transformer. 
       FIG. 6  illustrates an air-core signal isolation transformer electrical model  60  for the transformer  30  depicted in  FIG. 4 . Similar to equivalent circuit electrical model  40  depicted in  FIG. 5 , air-core signal isolation transformer model  60  comprises inherent lumped circuit parasitic elements including primary magnetizing inductance  62 , secondary magnetizing inductance  64 , primary-secondary (common mode) capacitance  66 , and primary-secondary (common mode) inductance  68 . A transformer core is again shown only to clearly show the difference between inductances  62 ,  64  and the mutual coupling effect of the transformer. 
     Transformer  30  can be used to enhance data transmission for both analog and digital signals. Applications may include, without limitation, use in medium voltage power electronic circuits such as shown in  FIG. 2 , where digital communications are required for gating transmission and digital sensor signals, and where very high voltages and high transient voltages are imposed on the transformer. In such cases, the dominant digital data frequency is used to tune the differential mode parameters; and the dominant Fourier component of the switching transition is used to tune the common mode parameters. The tuned common mode impedance helps reduce the generation and propagation of common mode currents in power circuits. The transformer  30  can use both common mode and differential mode or either of the tuned parameter features. MRI signals and digital signals will, for example, benefit from both, whereas power supplies may only benefit from tuning the common mode parameters. 
     According to one aspect, the inherent lumped circuit parasitic elements  62 ,  64  are tuned to match the frequency of data transmission and to further enhance the isolation between the primary and secondary transformer windings by tuning the transformer elements  66 ,  68  to a specific frequency of interest such as the imaging frequency of a respective MRI system. External tuning capacitors  70 ,  72 ,  74 ,  76  are added to the inherent lumped circuit parasitic elements  62 ,  64 ,  66 ,  68  as shown in  FIG. 6  such that together, the external tuning capacitors  70 - 76  and the inherent lumped circuit parasitic elements  62 - 68  can be tuned to a desired working frequency and/or a tuned impedance. The present invention is not so limited however, and it is noted that tuning can also be implemented through the use of embedded passive tuning elements, printed passive tuning elements, and the like, including without limitation, one or more tuning capacitors. 
       FIG. 7  illustrates an external H-field immune transformer winding  80 , according to one embodiment of the present invention. Transformer winding  80  allows improved signal integrity by providing greater immunity to external magnetic fields that could corrupt data. The structure lends itself for integration in printed circuit boards. According to one aspect, this alternating coil arrangement may be replicated over larger larger areas to further improve H-field insensitivity. The windings shown in  FIG. 7  could be repeated, for example, in a square pattern such that the nearest neighbors of each coil are coils that are oppositely wound. 
     Transformer winding  80  can be seen to include a dual planar primary spiral winding  82  and a dual planar secondary spiral winding  84 . Dual planar primary spiral winding  82  comprises a first planar primary spiral winding  84  and a second planar primary spiral winding  86 . Dual planar secondary spiral winding  88  comprises a first planar secondary spiral winding  90  and a second planar secondary spiral winding  92 . Each planar primary spiral winding  84 ,  86  shares a common winding axis with a corresponding planar secondary winding  90 ,  92 . Planar primary spiral winding  84 , for example, shares winding axis with planar secondary spiral winding  90 ; while planar primary spiral winding  86  shares winding axis with planar secondary spiral winding  92 . 
     Planar, high isolation voltage embedded transformers  30 ,  80  advantageously provide for improved manufacturability over traditional balun structures, while simultaneously providing a planar structure having reduced volume. The corresponding planar balun  30 ,  80  structures are suitable for use with commercial flex or printed circuit board technology and printed circuit processes. Other advantages include a reduction in balun tuning and test times. Device costs are reduced over traditional structures due to reduced component count requirements and embedded structure capabilities. The planar structure embodiments allow more consistent performance and are more stable than traditional balun structures. The planar structure embodiments further allow for easy isolation of a whole range of frequencies including DC with enhanced isolation at a selected frequency and provide enhanced data transmission due to matching impedance of transformer parameters to differential mode transmission impedance. 
     In summary explanation, a transformer structure comprises at least one planar spiral primary transformer winding integrated with a first layer of a flex or printed circuit board and at least one planar spiral secondary transformer winding integrated with a second layer of the flex or printed circuit board. A desired signal can be decoupled using the resultant transformer/balun in contradistinction with a traditional design that ensures direct coupling of the signal to the system. The parasitic parameters of the transformer are utilized to enhance the performance of the design and to ensure desired signal integrity. 
     The profile of the transformer or balun is dependent on ancillary components or the overall design. According to one aspect, the transformer is, basically, embedded or co-planar with the PCB, etc. so it achieves the lowest possible profile. The resultant structure provides several advantages over traditional parallel resonant baluns that employ a co-axial inductor and capacitor such as those used in receiver coils in MRI systems to isolate coil elements. These advantages include, without limitation, 1) low profile transformer/balun, 2) wide band isolation voltage, 3) enhanced common-mode isolation at selectable/tunable frequency by tuning the parasitic elements of the transformer(s) equivalent lumped circuit model(s), 4) integration with flex or printed circuit board technology, 5) shielding can be provided to prevent cross communications with other balun(s) or signal(s) in close proximity, 6) can be used in very high dv/dt environments such as gate drivers, 7) an external capacitor (or fixed or tunable embedded passive) can be placed across the primary-secondary common terminals to enhance the tenability of the parallel resonant frequency, 8) overlap between primary and secondary windings can be used to program a desired parallel resonant frequency, 9 distance between primary and secondary windings can be used to program a desired parallel resonant frequency, 10) permittivity of the substrate between primary and secondary windings can be used to program a desired parallel resonant frequency, 11) conductors of the windings are patterned to enhance the quality factor (QF) of the magnetizing inductances, 12 figure and shape can be used to make the design highly immune to external magnetic field influences, and 13) no magnetic core is required for the transformer. 
     Moving now to  FIG. 8 , a partial cross-sectional side view illustrates air-core transformer windings  108 ,  110  embedded in a multi-layer flex circuit  100 , according to one embodiment of the present invention. Multi-layer flex circuit  100  includes a top layer  102 , a primary winding layer  108 , a middle layer  104 , a secondary winding layer  100 , and a bottom layer  106 . The number of flex circuit layers is exemplary and may otherwise include any suitable number of layers depending on the particular application. Any one or more of flex circuit board layers  102 ,  104 ,  106  may be a shield layer configured to prevent cross coupling of communications with other balun(s) or signal(s) in close proximity to the primary and secondary transformer windings  108 ,  110 . 
       FIG. 9  is a side cross-section view illustrating an air-core transformer  128 ,  130  embedded in a multi-layer circuit board  120 , according to one embodiment of the present invention. Multi-layer flex circuit  120  includes a top layer  122 , a primary winding layer  128 , a middle layer  124 , a secondary winding layer  130 , and a bottom layer  126 . Again, the number of circuit board layers is exemplary and may otherwise include any suitable number of layers depending on the particular application. Any one or more of circuit board layers  122 ,  124 ,  126  may be a shield layer configured to prevent cross coupling of communications with other balun(s) or signal(s) in close proximity to the primary and secondary transformer windings  128 ,  130 . Although  FIGS. 8 and 9  are described with reference to flexible and printed circuits, the present invention is not so limited, and it shall be understood that these are only illustrative of particular embodiments. Other embodiments may include circuit construction techniques including without limitation, photolithographic, printed/deposited/additive, transfer, punched, excised, and the like. 
       FIG. 10  illustrates an MR imaging system  150  that employs at least one air-core transformer  30 ,  80  according to one embodiment of the present invention. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly  152  to produce magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  152  forms part of a magnet assembly  143  which includes a polarizing magnet  156  and a whole-body RF coil assembly  158  configured to sense signals emitted by excited nuclei in a patient. RF coil assembly  158  employs one or more air-core transformers  30 ,  80  to replace the traditional parallel resonant balun(s) used in RF coil assembly  158  to isolate designated coil elements. 
       FIG. 11  is a simplified block diagram  200  illustrating a high isolation voltage embedded transformer applied as signal and power isolation means to a gate driver circuit for low and medium voltage devices, such as IGBTs and MOSFETs  206 . These types of devices typically switch between 1 and 20 kV/μs, and any capacitance between the primary and secondary circuits  202 ,  204  will have a corresponding current flowing through it during the switching transitions. In this application power and signal isolation is provided by separate transformers  208 ,  210 . Both these transformers  208 ,  210  have a common-mode tuned frequency that is set to the same frequency since they will both be exposed to the same switching conditions. The differential tuning of the power supply transformer  208  will match the power supply&#39;s switching frequency  212 . The differential tuning of the signal transformer  210  will match the gating signal (PWM)  214  main frequency component. The design of these transformers  208 ,  210  can be identical or different and depends on the power requirements and data through put. 
       FIG. 12  is a simplified block diagram  220  showing a similar application of the transformers  208 ,  210  depicted in  FIG. 1  where the transformer functions have been combined into a single transformer  222 . Single transformer  222  is used for both power and signal isolation. In this case, the common-mode isolation is tuned to the PWM switching frequency  224  and the differential mode parameters are tuned to the carrier frequency (power supply switching frequency (oscillator))  226 . 
       FIG. 13  is a schematic diagram  240  depicting another variation of the transformer application shown in  FIG. 12 . The power and signals are combined in this application without a carrier frequency. This application makes use of pulse signals  242  to drive the signal and power through a transformer  244 , according to one embodiment of the present invention. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.