Abstract:
A circuit board has apertures. Separate magnetic flux paths each form a closed loop that passes through at least one of the apertures and surrounds an interior space. The flux paths comprising portions that lie within magnetically permeable core pieces. At least two of the flux paths are oriented so that there is a straight line in the circuit board that passes through the interior spaces of the two flux paths without passing through any of the apertures that are included in the paths. An electrically conductive primary winding having a first segment that passes through the interior spaces of the permeable paths and a second segment located outside of the interior spaces. There are two or more electrically conductive secondary windings.

Description:
BACKGROUND 
   This description relates to printed circuit transformers. 
   A DC-to-DC transformer apparatus, called a Sine Amplitude Converter (“SAC”), is described in U.S. patent application Ser. No. 10/066,418, filed Jan. 31, 2002, and U.S. patent application Ser. No. 10/264,327, filed Oct. 1, 2002 (the “Factorized Applications”, incorporated in their entirety by reference). One embodiment of a SAC, shown in  FIG. 1 , comprises a series-resonant full-bridge converter operated at a fixed frequency slightly below a characteristic resonant frequency of the converter to provide for switching at times of zero-voltage and zero-current. 
   Some embodiments of the SAC operate in a “low-Q” configuration, where the “quality factor,” Q, of a series resonant converter operating at resonance is defined in the Factorized Applications, as Q=Z L /R eq , where Z L =2π*f R *L R  is the total inductive impedance of the resonant circuit at the resonant frequency, f R ; where the inductance L R  includes all discrete, leakage and circuit parasitic inductances, reflected into the transformer primary and in series with the resonant circuit; and where R eq  is the total equivalent series resistance of the circuit, reflected to the transformer primary and including, resistances of windings, ON-state resistances of switches, rectifiers and resonant capacitors. 
   A SAC uses principles of resonant charge transfer so the quality factor, Q, does not directly reflect cycle-by-cycle losses in a SAC. Rather, the losses in the resonant tank of a SAC are proportional to the equivalent series resistance R eq . In some SACs, the equivalent series resistance R eq  is minimized and the transformer is designed to minimize leakage inductance. Use of a low-Q resonant circuit generally provides higher bandwidth and shorter transient response time, together with greater inherent stability. 
   Herbert, U.S. Pat. No. 4,665,357, “Flat Matrix Transformer” describes a “matrix transformer” that has interdependent magnetic circuits, arranged in a matrix, between and among which electrical conductors are interwired, the circuits and conductors cooperating to behave as a transformer. Matrix transformers generally have magnetically decoupled magnetic cores, each having one or more coupled windings, the windings on different cores being interconnected to form the transformer structure. See, for example, Hibbits, U.S. Pat. No. 3,323,091, “Multicore Transformer Including Integral Mounting Assembly”; in Papaleonidas, U.S. Pat. No. 3,477,016, “Transformer System Including a Large Number of Magnetically Independent Transformer Elements”; and in three papers by Ngo, Alpizar and Watson: “Development and Characterization of a Low-Profile Matrix Transformer”, HFPC May 1990 Proceedings; “Modeling of Magnetizing Inductance and Leakage Inductance in a Matrix Transformer”, IEEE Transactions on Power Electronics, Vol. 8, No. 2, April 1993; “Modeling of Losses in a Sandwiched-Winding Matrix Transformer”, IEEE Transactions on Power Electronics, Vol. 10, No. 4, July 1995. 
     FIG. 2  shows an exploded perspective view of a portion of a matrix transformer having a serpentine winding that includes a pair of conductive patterns  10   a ,  10   b  (e.g., copper etch) on surfaces of one or more substrates  12   a ,  12   b  (e.g., printed circuit boards (“PCBs”)). Current enters conductive pattern  10   b  at the location marked “A” and exits conductive pattern  10   a  at the location marked “B.” The conductive patterns  10   a ,  10   b  are connected in series by, e.g., conductive interconnections, not shown. The patterns carry a current in the direction indicated in the Figure by the arrows. Magnetic core pieces  16   a – 16   c ,  17   a – 17   c  pass through holes  14   a – 14   f ,  15   a – 15   f  in the substrates  12   a ,  12   b  so that opposing pairs of core pieces (e.g., core pieces  16   a  and  17   a ) form an essentially closed permeable flux path. The term “flux path”, as used herein, refers to the principal path followed by the flux that links a pair of transformer windings, as distinguished from minor flux paths, such as those associated with, e.g., leakage flux or fringing fields). In the apparatus of  FIG. 2 , each flux path formed by each core pair is coupled by two turns formed by the conductive patterns  10   a ,  10   b  and each core pair creates a magnetic flux path that is essentially independent of the magnetic flux path formed by the other core pairs. 
   Matrix transformers comprising serpentine windings are described in the three papers by Ngo, Alpizar and Watson, ibid; in Leno, U.S. Pat. No. 2,943,966, “Printed Electrical Circuits”; in Yerman et al, U.S. Pat. No. 4,959,630, “High-Frequency Transformer” and U.S. Pat. No. 5,017,902, “Conductive Film Magnetic Components”; and in Roshen, U.S. Pat. No. 5,381,124, “Multi-Turn Z-Foldable Secondary Winding for a Low-Profile Conductive Film Transformer.” 
   Walters, U.S. Pat. No. 5,300,911, “Monolithic Magnetic Device With Printed-Circuit Interconnections” describes a monolithic magnetic device having transformer elements having single turn primaries and single turn secondaries fabricated on a plate of ferrite which has the outline of a ceramic leadless chip carrier. Each of the magnetic elements has a primary winding formed from a copper via plated on the ferrite. Each element&#39;s secondary is another copper via plated over an insulating layer formed over the first layer of copper. The elements&#39; primaries are interconnected on the first copper layer and the elements&#39; secondaries are interconnected on the second copper layer. The configuration and turns ratio of the transformer are determined by the series and or parallel interconnections of the primary and secondaries. 
   SUMMARY 
   In general, in one aspect, the invention features an apparatus comprising: a circuit board having apertures; separate magnetic flux paths each forming a closed loop that passes through at least one of the apertures and surrounds an interior space, the flux paths comprising portions that lie within magnetically permeable core pieces, at least two of the flux paths being oriented so that there is a straight line in the circuit board that passes through the interior spaces of the two flux paths without passing through any of the apertures that are included in the paths, an electrically conductive primary winding having a first segment that passes through the interior spaces of the permeable paths and a second segment located outside of the interior spaces; and two or more electrically conductive secondary windings. 
   Implementations of the invention include one or more of the following features. The two flux paths are adjacent on the board. The primary winding comprises a loop. The loop comprises two longer parallel straight segments and two shorter bridging segments that connect the two longer parallel straight segments. At least one of the secondary windings is on a second layer of the circuit board. Each of the secondary windings comprises a first segment that passes through fewer interior spaces than does the primary winding. Each of the secondary windings comprises a first segment that overlays a first segment of the primary winding and a second segment that overlays a second segment of the primary winding. The flux path comprises no gaps between permeable core pieces. The flux path comprises gaps between permeable core pieces. The magnetically permeable core pieces include flat pieces and pieces with legs. At least two of the secondary windings are connected in parallel. The connections to the primary winding are made along an edge of the circuit board that is on a different side of the apparatus from an edge along which connections are made to at least one of the secondary windings. The core pieces include flat pieces and pieces that each have one leg on one side of an interior space and two legs on another side of the interior space. The secondary windings are connected to form a single center-tapped winding. The secondary windings lie on two different layers of the board. The secondary windings are configured to produce different turns ratios. 
   In general, in another aspect, the invention features an apparatus comprising: a circuit board having apertures; separate magnetic flux paths each forming a closed loop that passes through at least one of the apertures and surrounds an interior space, the flux paths comprising portions that lie within magnetically permeable core pieces, at least two adjacent ones of the flux paths being oriented so that there is a straight line in the circuit board that passes through the interior spaces of the two flux paths without passing through any of the apertures that are included in the paths, an electrically conductive primary winding in the form of a loop having a first segment that passes through the interior spaces of the permeable paths and a second segment located outside of the interior spaces; and two or more electrically conductive secondary windings on at least a second layer of the circuit board, each of the secondary windings comprising a first segment that passes through fewer interior spaces than does the primary winding, each of the secondary windings comprises a first segment that overlays a first segment of the primary winding and a second segment that overlays a second segment of the primary winding, at least two of the secondary windings connected in parallel, connections to the primary winding being made along an edge of the circuit board that is on a different side of the apparatus from an edge along which connections are made to at least one of the secondary windings. 
   Implementations of the invention may include one or more of the following features. The secondary windings are connected to form a single center-tapped winding. The secondary windings lie on two different layers of the board. The secondary windings are configured to produce different turns ratios. 
   In general, in another aspect, the invention features a circuit comprising: electrical elements forming a series-resonant full-bridge converter, the elements including a transformer comprising a circuit board having apertures; separate magnetic flux paths each forming a closed loop that passes through at least one of the apertures and surrounds an interior space, the flux paths comprising portions that lie within magnetically permeable core pieces, at least two of the flux paths being oriented so that there is a straight line in the circuit board that passes through the interior spaces of the two flux paths without passing through any of the apertures that are included in the paths, an electrically conductive primary winding having a first segment that passes through the interior spaces of the permeable paths and a second segment located outside of the interior spaces; and two or more electrically conductive secondary windings. 
   Other advantages and features of the invention will become apparent from the following description and from the claims. 

   
     DESCRIPTION 
     We first briefly describe the drawings: 
       FIG. 1  shows a schematic of a sine amplitude converter. 
       FIG. 2  shows a portion of a transformer. 
       FIG. 3  shows an exploded view of a transformer. 
       FIG. 4  shows a sectional side view of the transformer of  FIG. 3 . 
       FIG. 5  shows an exploded schematic view of current flows in the transformer of  FIG. 3 . 
       FIG. 6  shows an exploded view of another transformer. 
       FIG. 7  shows a schematic of a circuit for the transformer of  FIG. 6 . 
       FIG. 8  shows an exploded schematic view of currents flowing in the transformer of  FIG. 6 . 
       FIGS. 9 through 21  show metal etch layers comprising windings for a transformer. 
       FIG. 22  is a schematic diagram of interconnections of windings. 
       FIG. 23  is a diagram of circuit connections. 
   

     FIG. 3  shows an exploded view of a example transformer  18 . The transformer comprises secondary windings  20 ,  21 , a primary winding  24 , and two pairs of permeable first and second magnetic core pieces  25 ,  27 , respectively.  FIG. 4  shows a sectional side view (through the section labeled AA in  FIG. 3 ) of the constructed transformer. The windings  20 ,  21 ,  24  may be etch patterns on surfaces of a printed circuit board (“PCB”) and the windings  20  and  21  may be separated from the winding  24  by non-conductive substrate material  40  ( FIG. 4 ). For clarity, the non-conductive substrate  40  is not shown in  FIG. 3 . 
   Referring to  FIGS. 3 and 4 , each pair of top and bottom core pieces  25  and  27  defines a closed magnetic flux path that surrounds a single interior space  30  ( FIG. 4 ). The closed magnetic flux path may (but need not) include a gap (e.g., at locations  29 ,  FIG. 4 ). A portion of the primary winding (i.e., portion  24   a ) and a portion of each of the secondary windings (i.e., portions  20   b ,  21   b ) are located within the interior space  30 . Legs  25   a  of core pieces  25  pass through apertures  34   a ,  34   b ,  35  formed by the windings and through apertures  37  in the non-conductive substrate  40  (one aperture is shown in  FIG. 4 ). A portion of the primary winding  24   a  and portions of the secondary windings  20   a ,  21   a  are located outside of the interior space  30 . The conductive primary winding terminations  24   c ,  24   d  and the conductive secondary winding terminations  20   c ,  20   d ,  21   c ,  21   d  are located on opposite sides of the transformer. The conductive primary winding terminations  24   c ,  24   d  are brought out of the transformer  18  in a “primary region”  33  (in  FIG. 4 , the region to the right of core legs  25   b ; the terminations  24   c  and  24   d  are located on surfaces  41   b  of non-conductive substrate  40 , but do not appear in  FIG. 4  owing to the location of section AA), whereas the conductive secondary winding terminations  20   c ,  20   d ,  21   c ,  21   d  are brought out of the transformer in a “secondary region”  33  (in  FIG. 4 , the region to the left of core legs  25   a ). The windings are located to overlap, both in the interior spaces  30  between the core legs  25   a ,  25   b  and in the region adjacent to core leg  25   a , outside of interior region  30 , in secondary region  31 . 
     FIG. 5  is a schematic view of the transformer of  FIGS. 3 and 4 . As shown in the Figure, a time varying voltage source, Vp  42 , connected to the primary winding  24  induces a time-varying flux in the core pieces, as illustrated schematically in  FIG. 5  by flux paths  43   a ,  43   b  (for clarity, the core pieces  25 ,  27  are not shown). The time varying flux induces voltages in the two secondary windings  20 ,  21 . Because each secondary winding is linked by half of the total primary flux, the voltage induced in each secondary winding, Vs 1  and Vs 2 , will be half of the primary voltage, Vp. Secondary currents, Is 1  and Is 2 , will flow in each of the secondary windings  20 ,  21 , the value of each secondary current depending on the size of the load  44 ,  45  connected to the winding. 
   Of particular interest is the case in which the secondary windings are connected in parallel. In  FIG. 5 , this would correspond to secondary winding ends marked “A” being connected together and secondary winding ends marked “B” being connected together. In this case, the voltage Vs 1 =Vs 2  across each secondary winding will equal Vp/2; the total current in the secondaries, Is 1 +Is 2  will equal 2*Ip; and each of the currents, Is 1  and Is 2 , will be equal to Ip. Given that the primary and secondary windings physically overlap each other along almost their entire lengths (except for the short sections  46   a  and  46   b  of the secondary windings that do not overlap the primary winding) and provide “eddy current” shielding, and that the overlapping currents flowing in the primary and secondary windings are equal and flow in opposite directions, the amount of leakage flux generated by the transformer will be very small. Thus, the transformer will have relatively low leakage inductance. 
   Another feature of the transformer  18  is that the interior spaces ( 30 ,  FIG. 4 ) of adjacent magnetic flux paths are aligned with each other so that the windings  20 ,  21 ,  24  can be routed straight through the interior spaces of the adjacent magnetic paths. Said another way, the interior spaces of each adjacent pair of magnetic flux paths are aligned so that a straight line may be drawn that will pass through the interior spaces without intersecting any portion of either magnetic flux path. This is in contrast to the serpentine transformer structure of  FIG. 2 , in which the alignment of the interior spaces of adjacent flux paths (e.g., in  FIG. 2 , the interior spaces defined by core pairs  16   a ,  17   a  and  16   b ,  17   b ) is such that a straight line cannot be drawn that passes through the adjacent interior spaces without intersecting one of the magnetic flux paths. In the latter case, the winding is made serpentine so that it can pass through the interior spaces defined by the plurality of core sets. As a result, for a given transformer surface area, the windings of a transformer may be made shorter and wider than those in a serpentine transformer, and the winding resistance in a transformer will be lower. 
   The combination of low leakage inductance and low resistance of a transformer provides the “low-Q” characteristic that is desirable in transformers used in contemporary high-frequency power converters, such as, e.g., the SAC converters. 
   Using the approach explained by example above, a transformer may be readily scaled to accommodate a wide variety of turns ratios and applications while retaining its desirable features. For example,  FIG. 6  shows an exploded view of a transformer comprising four sets of core pairs, each pair comprising a top core piece  48  and a bottom core piece  50 , each bottom core piece  50  comprising two narrow legs  61   a ,  61   b  and a wide leg  63 ; a primary winding  52 ; and a total of nine secondary windings  54   a – 54   i . The windings  52 ,  54   a – 54   i  may be etch patterns on surfaces of a PCB and may be separated from each other by non-conductive substrate material (not shown). In the example shown, the secondary windings may be connected to form a single, center-tapped, winding, as illustrated in  FIG. 7 , by connecting all of the points marked “C” ( FIG. 6 ); all of the points marked “D” ( FIG. 6 ); and all of the points marked “E” ( FIG. 6 ) (e.g., by use of plated vias and etch, not shown, on the PCB). Connected in this way, the turns ratio between the primary winding  52  and each half  55   a ,  55   b  ( FIG. 7 ) of the center-tapped secondary winding will be 8:1 and the turns ratio between the primary winding and the entire secondary winding  54 , comprising the two halves  55   a ,  55   b  of the winding connected in series, will be 4:1. Owing to the relative locations of the windings, the transformer in  FIG. 7  exhibits a very low value of leakage inductance. 
     FIG. 7  shows a schematic of the transformer of  FIG. 7  connected in a full-wave rectifier circuit comprising rectifiers  57 ,  58  (which may be diodes or synchronous rectifiers comprising switches (not shown)).  FIG. 8  shows currents flowing in the windings when the secondaries are wired as shown in  FIG. 7  and the primary excitation is such that rectifier  58 , connected to the points marked “E” is conducting. Under this circumstance, all of the secondary currents, Is, are nominally equal to each other and to the primary current Ip, resulting in a total secondary current equal to 8*Is. With the windings arranged as shown, the flow of currents in the winding overlap in a way that reduces leakage flux. For example, the current flowing into terminal “D” of winding  54   a  overlaps an equal current flowing in the opposite direction out of terminal “E” of winding  54   e ; the current flowing along the rear portion  59  of winding  54   a  overlaps an equal current flowing in the opposite direction in the left rear leg  60  of the primary winding  52 ; and the current flowing into terminal “D” of winding  54   f  overlaps an equal current flowing in the opposite direction out of terminal “E” of winding  54   a . Likewise, opposing and overlapping currents flow in all of the remaining windings. The relative directions and locations of the currents flowing in the windings and the “eddy current” shielding characteristic of the overlapping windings results in a substantial reduction in leakage flux and leakage inductance. 
     FIGS. 9 through 21  show thirteen etch layers of a printed circuit board comprising primary and secondary windings that form part of a transformer. The printed circuit board is designed for use with four sets of core pairs of the kind shown in  FIG. 6 , each pair comprising a top core piece  48  and a bottom core piece  50 . As illustrated in  FIGS. 9–21 , holes  62   a ,  62   b  in the printed circuit board accommodate narrow legs  61   a ,  61   b  ( FIG. 6 ) and slots  64  accommodate wide legs  63 . The etch layers in  FIGS. 9 and 15  comprise essentially identical sets of four secondary windings  70 ,  71 ,  72 ,  73  each winding in each layer being connected to its numerical counterpart in the other layer by means of the via connections between the layers (e.g., via connections marked “A”, “B”, “C”, and “D”). The etch layers in  FIGS. 11 and 17  comprise essentially identical sets of four secondary windings  74 ,  75 ,  76 ,  77  each winding in each layer being connected to its numerical counterpart in the other layer by means of the via connections between the layers (e.g., via connections  66 ,  68 ,  90 ,  92  and those marked “A”, “B”, “C”, and “D”). The etch layers in  FIGS. 12 and 18  comprise essentially identical sets of four secondary windings  78 ,  79 ,  80 ,  81  each winding in each layer being connected to its numerical counterpart in the other layer by means of the via connections between the layers (e.g., via connections  67 ,  69 ,  91 ,  93  and those marked “A”, “B”, “C”, and “D”). The etch layers in  FIGS. 14 and 20  comprise essentially identical sets of four secondary windings  82 ,  83 ,  84 ,  85  each winding in each layer being connected to its numerical counterpart in the other layer by means of the via connections between the layers (e.g., via connections  66 ,  68 ,  90 ,  92  and those marked “A”, “B”, “C”, and “D”). 
   Secondary windings  70  through  77  surround holes  62   b  and are linked by flux in core legs  61   b ; secondary windings  78  through  85  surround holes  62   a  and are linked by flux in core legs  61   a . All of the locations connected by vias having the same numerical designator are connected together. Thus, e.g., windings  70  are connected in parallel with each other and with the two windings  78  ( FIGS. 12 and 18 ) by means of vias “A” and via connections  67 . The vias numbered  66 – 69  and  90 – 93  in  FIG. 9  connect to pad locations having the same numeric designators in  FIGS. 10–20 . The latter pad locations ( 66 – 69 ;  90 – 93 ,  FIG. 9 ) are brought out to another layer (not shown) for connection to ball-grid arrays of contacts on synchronous rectifier switches (not shown). A schematic of the connections between all of the thirty-two secondary windings (two each of secondaries  70 – 85 ) of  FIGS. 9 through 21 , along with their corresponding pad locations, via interconnection designators and synchronous rectifiers (shown as diodes  100   a – 100   h ), is shown in  FIG. 22 . The polarity dot on each winding reflects the relative polarity of the winding voltage for the same polarity of flux in each of the legs  61   a ,  61   b  and corresponding holes  62   a ,  62   b.    
     FIGS. 10 ,  13 ,  16  and  19  show etch layers comprising primary windings  86 ,  87 ,  88  and  89 . Each primary winding passes through all four core pairs and current flowing in a winding induces essentially equal amounts of flux in each core pair. The regions mark “N” and “S” in, respectively,  FIGS. 13 and 16 , represent the ends of the complete primary winding. Region “N”, in  FIG. 13 , connects to one end of winding  87 ; the other end of winding  87  connects to an end of winding  86  ( FIG. 10 ) by means of vias “R”; the other end of winding  86  connects to vias “P” which connects to the pad marked “P”, in  FIG. 21 . Region “S”, in  FIG. 16 , connects to one end of winding  88 ; the other end of winding  88  connects to an end of winding  89  ( FIG. 19 ) by means of vias “Q”; the other end of winding  89  connects to vias “T” which connects to the pads marked “T” in  FIG. 21 . In  FIG. 21  the pads “P” and “T” are surface-mount pads for connection of two resonant capacitors (not shown). One such capacitor is connected between a pad “T” and pad “P” and another capacitor is connected between the other pad “T” and pad “P.” By this means the two capacitors are connected in parallel with each other and in series with all of the windings. A schematic of the connections between the four primary windings ( 86 – 89 ) of  FIGS. 10 ,  13 ,  16  and  19 , along with their corresponding pad locations, via interconnection designators and resonant capacitors (marked C R1  and C R2  in the Figure), is shown in  FIG. 23 . The polarity dot on each winding reflects the relative polarity of the winding voltage for the same polarity of flux in each of the legs  61   a ,  61   b  and corresponding holes  62   a ,  62   b.    
   With the secondary and primary windings configured as described above in  FIGS. 9 through 23 , and assuming that all of the cathodes of the synchronous rectifiers ( 100   a – 100   h ,  FIG. 22 ) are connected together, the primary-to-secondary turns ratio of the transformer will be 32:1. 
   An example of the transformer of  FIGS. 9 through 23  comprises a fourteen layer PCB. Each layer in the PCB is of nominal dimensions H=1.26 inches (32 mm) and W=0.85 inch (21.5 mm), where H and W are shown in  FIG. 21 . In the remainder of this paragraph, numerical references of “etch layers” will correspond to the Figure in which the etch layer is shown (e.g. “etch layer  9 ” refers to the etch layer corresponding that shown in  FIG. 9 ). Etch layers  9 ,  20  and  21  comprise 1.5 ounce copper etch; etch layers  10  through  19  comprise 2 ounce copper etch. Etch layers  9  and  10  and etch layers  19  and  20  are separated by 0.0041 inch (0.103 mm); etch layers  20  and  21  are separated by 0.0043 inch (0.109 mm); etch layers  10  and  11 , etch layers  12  and  13 , etch layers  14  and  15 , etch layers  16  and  17  and etch layers  18  and  19  are separated by 0.0035 inch (0.089 mm); etch layers  11  and  12 , etch layers  13  and  14 , etch layers  15  and  16  and etch layers  17  and  18  are separated by 0.0044 inch (0.110 mm). In all cases the separation medium comprises Nelco 13 substrate material, manufactured by Park Industries, Anaheim, Calif., USA. At a frequency of 1.5 Megahertz, the described transformer has a primary-referenced leakage inductance of 165 nanohenries and a primary-referenced equivalent resistance of 0.57 ohm; at a frequency of 0.5 Megahertz, the described transformer has a primary-referenced leakage inductance of 182 nanohenries and a primary-referenced equivalent resistance of 0.44 ohm. 
   Other implementations are within the scope of the following claims. For example, the numbers of primaries and secondaries, their configurations, the relationships between them, the configurations of the substrates on which they are formed, and other aspects of the transformer can vary from the examples given.