Abstract:
A vertically packaged cellular power converter solves the problems associated with conventional designs and paves the way for a cellular circuit architecture with ultra-low interconnect resistance and inductance. The vertical packaging results in a power flow in the vertical direction (from the bottom to the top) with very short internal interconnects, thereby minimizing the associated conduction losses and permitting high conversion efficiency at high currents. The cellular architecture is ideally suited for generating multiple supply voltages.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to power converters and more specifically to a packaging architecture that provides for vertical power flow that is effective for providing lower supply voltages, dynamic voltage scaling, multiple supply voltages, and fast transient response and tight regulation.  
         [0003]     2. Description of the Related Art  
         [0004]     Power converters are key components in many military and commercial systems and they often govern size and performance. Power density, efficiency and reliability are key characteristics used to evaluate the characteristics of power converters. Transformers and inductors used within these power converters may be large and bulky and often limit their efficiency, power density and reliability. These deficiencies can be improved by using a high-frequency “switch-mode” architecture instead of a traditional low frequency transformer and by replacing conventional core-and-wire designs with “planar magnetics”. Planar magnetics offer several advantages, especially for low-power dc-dc converter applications, such as low converter profile, improved power density and reliability, reduced cost due to the elimination of discrete magnetic components, and close coupling between different windings.  
         [0005]     As shown in  FIG. 1 , a conventional switch-mode power converter  10  for transforming an input voltage V in , e.g. 48V, to one or more ultra low supply voltages V s , e.g., 1.5V, 3.3V, to drive a load  12  utilizes horizontal packaging in which components are mounted on the same multi-layer printed circuit board (PCB)  14 , and power flows sequentially over long interconnects from input to the output side. Moreover, multiple secondary windings and cross regulation are utilized for the generation and control of multiple supply voltages. Components such as an input filter  16 , primary switches  18 , a primary control IC  20 , a transformer  22 , secondary devices  24 , a secondary control IC  26 , an output inductor  28  and an output capacitor  30  are mounted on the board  14 , forming what will be referred to hereafter as a “horizontal package”. Arrow  32  indicates that the power flows through the different components in the horizontal direction from the input to the output and is coupled horizontally via traces on the PCB to load. Other internal layers may be used for interconnections, ground planes, or some active or passive devices in MCM-type (multi-chip module) or embedded packaging.  
         [0006]     A popular implementation of the dc/dc switch-mode converter  10  to supply a single regulated output voltage incorporates a drive circuit  34  having a double-ended, half-bridge topology and a current-doubler rectifier (CDR) circuit  36  shown in  FIG. 2  (U.S. Pat. No. 6,549,436 issued Apr. 15, 2003). Early CDR circuits used three separate magnetic components, namely, one transformer and two inductors. The illustrated CDR is based on an integrated magnetic implementation in which the transformer and inductors are combined into a single magnetic structure with one magnetic core. The integrated magnetic implementation is further refined to include an output inductor that increases the effective filtering inductance (See U.S. Pat. No. 6,549,436).  
         [0007]     The drive circuit  34  comprises first and second input filter capacitors  40  and  42  and first and second primary switches  44  and  46 , e.g. power MOSFETs. The capacitors  40  and  42  and switches  44  and  46  process power from a dc voltage source V in  at input terminals  48  and  50 . The drive circuit  34  provides a pulse width modulated voltage to the CDR&#39;s split-primary winding arrangement  52  and provides an ac voltage at the input terminals of the integrated magnetics.  
         [0008]     The CDR circuit  36  comprises a magnetic core  54 , the split-primary winding arrangement  52 , a secondary winding arrangement  56 , an output capacitor  58 , and first and second secondary switches  59  and  60 , and first and second rectifiers  61  and  62  connected in parallel across the respective switches. The switches  59  and  60  function as diodes, termed synchronous rectification, and can be replaced by diodes only. The magnetic core  54  comprises a center leg  64  and a first outer leg  66  and a second outer leg  68  disposed on opposite sides of the center leg  64 . A plate  67  on the outer legs forms an air gap  69  with the center leg to prevent saturation of the core.  
         [0009]     The split-primary winding arrangement  52  comprises a primary winding  70  that is wound around the outer leg  66  and a second primary winding  72  that is wound around the outer leg  68 . The secondary winding arrangement  56  comprises first, second and third secondary windings  74 ,  76 ,  78  that are wound around legs  66 ,  68  and  64 , respectively. The outer leg windings  74  and  76  provide both the secondary windings for the transformer and the output inductors. The center leg inductor winding  78  increases the filter inductance of the CDR circuit thereby reducing the voltage and current ripple and improving efficiency.  
         [0010]     The inductor winding  78  is connected in series with the output capacitor  58 . The output capacitor has first and second terminals  80  and  82 , which form the output terminals of the integrated current-doubler rectifier  36  and the dc/dc converter circuit  10  shown in  FIG. 1  for connection to the load. The secondary switch  59  and rectifier  61  are connected in parallel between the output capacitor  58  and the winding  74 . The secondary switch  60  and rectifier  62  are connected in parallel between the output capacitor  58  and the winding  76 .  
         [0011]     In operation, a dc voltage is applied to the capacitors  40  and  42  and the primary switches  44  and  46  via input terminals  48  and  50 . A primary control IC  84  controls the primary switches such that at most only one switch is on at a time and synthesizes a high frequency AC voltage that is applied to the primary windings  70  and  72 . This causes a current to flow in the secondary windings  74 ,  76  and  78 . A current i 1  flows in the switch-diode pair  59 - 61 , a current i 2  flows in the switch-diode pair  60 - 62 , and a current i 3  to flow in the secondary winding  78  (where i 1 +i 2 =i 3 ), though ordinarily not all at the same time. One of the currents i 1  or i 2  is zero during power transfer periods, while in the free wheeling periods the load current to node  80  is shared among them. A secondary control IC  86  controls secondary switches  59  and  60  so that current i 1  flowing through winding  74  is rectified by the switch-diode pair  59 - 61  and the current i 2  flowing through the winding  76  is rectified by the switch-diode pair  60 - 62 . Current i 3  charges the output capacitor  58  to produce a DC output voltage across output nodes  80  and  82  so that a regulated power is delivered to the load. Power flows from the input terminals horizontally through the primary switches, the transformer plus inductors, secondary switches to the output terminals for connection to a load on the same board.  
         [0012]     As shown in  FIGS. 3   a  and  3   b , the primary and secondary winding arrangements are implemented with a multi-layer printed circuit (PCB)  90  having copper traces that form the various horizontal windings in the plane of the PCB. E-core  54  is positioned underneath the PCB so that its outer legs  66  and  68  extend through holes in the PCB that coincide with the edges of primary and secondary windings  70  and  74  and  72  and  76 , respectively, and its center leg  64  extends through a hole that allows inductor winding  78  to be wound around it. Required creepage distance is maintained between the windings and the core during fabrication. Plate  67  rests on the outer legs forming an air gap  69  with the center leg. Vias  92  in the PCB are used to connect the primary windings in series to form a multi-turn primary and to connect the secondary windings in parallel to form a single-turn secondary with reduced resistance. The windings are terminated in the plane of the PCB so that power flows horizontally from the primary side to the secondary side.  
         [0013]     Among the various power reduction and power management requirements for developing systems, the needs for lower supply voltages, dynamic voltage scaling, multiple supply voltages, and fast transient response with tight regulation will have the most dramatic effects on power converter design. While each individual requirement represents a challenge for the power converter design and packaging, it is the combination of them all together that is pushing the existing power conversion technology to its limit.  
         [0014]     The conventional horizontal package has fundamental limitations that will render it ineffective for these developing applications, including a) inherently low efficiency, especially at sub-1V output, due to the long internal interconnects and the associated high conduction losses, b) a difficult 1-D interface with the load, c) inability to supply tightly regulated multiple outputs, and (d) switching frequency limitation due to the inductive and capacitive parasitics inherent in long interconnects. The needs for coordination among multiple supply voltages, such as sequencing, also makes it difficult to use multiple, individually controlled single-output converters. In addition, conventional control design focuses on constant output regulation with steady-state load, which cannot meet the future needs for dynamic voltage scaling and fast transient responses.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention provides a package design for a power converter that addresses the need for multiple, low supply voltages with tight regulation and fast transient responses.  
         [0016]     This is accomplished with a vertical package that incorporates a magnetic core and winding arrangement that allow power to flow vertically from an input module to an output module. The vertical package provides very short internal interconnects and a 2-D interface to the load, which reduce losses and parasitics. The vertical package may be implemented with vertical winding arrangements that are inherently more efficient than conventional planar integrated magnetics. The use of a matrix integrated magnetics (MIM) core creates a cellular structure that can be used to provide multiple output voltages and/or interleaving to provide output voltages with very low ripple and faster transient response.  
         [0017]     In a first embodiment, the MIM core can be viewed as consisting of multiple pairs of small E cores that are arranged in two dimensions in the horizontal plane to define a plurality of legs that lie in the plane and a plurality of windows through the plane. The windings are formed by patterned copper on the input and output modules on both sides of the core and conductors (pins) that extend through the core windows to connect the copper structure.  
         [0018]     In a second embodiment, the MIM core is positioned so that its legs extend through holes in a PCB that coincide with the edges of windings formed on the PCB. The base plate and top plate are formed with vias for terminating the horizontal windings with, for example, pins and providing electrical connection to the input and output modules.  
         [0019]     In a third embodiment, vertical windings are wound around the legs of the MIM core. The windings are extended laterally outside the core window and then turned vertically, either up or down, to extend through slits in the output or input module. The windings are terminated on winding pads on the underside of the input module and top side of the output module.  
         [0020]     These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1 , as described above, is a block diagram of a conventional horizontal package for a switch-mode power converter;  
         [0022]      FIG. 2 , as described above, is a simplified schematic diagram of a known current-doubler rectifier (CDR) circuit;  
         [0023]      FIGS. 3   a  and  3   b , as described above, are perspective and section views of a planar magnetics implementation of the winding structure;  
         [0024]      FIG. 4  is a simplified diagram of a vertical package for planar magnetic power converters in accordance with the present invention;  
         [0025]      FIG. 5  is a diagram illustrating the cellular architecture of the vertical package;  
         [0026]      FIGS. 6   a  and  6   b  are exploded and integrated views of the input module, matrix integrated magnetics (MIM) core and output module that make up the vertical package;  
         [0027]      FIGS. 7   a  through  7   d  are diagrams illustrating the construction of the vertical windings in the package;  
         [0028]      FIG. 8  is an exploded view of an alternate MIM core for transferring power vertically using horizontal windings on a PCB; and  
         [0029]      FIG. 9  is an exploded view of another MIM core with an alternate vertical winding structure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     The present invention provides a package design for a power converter that addresses the need for multiple, dynamically scalable low supply voltages with tight regulation and fast transient responses. This is accomplished with a vertical package that incorporates a magnetic core and winding arrangement that allows power to flow vertically from an input module to an output module. The vertical package provides very short internal interconnects and a 2-D interface with the load, which reduce losses and parasitics. The vertical package may be implemented with vertical winding arrangements that are inherently more efficient than convention planar magnetics. The use of a matrix magnetics core creates a cellular structure that can be used to provide multiple output voltages and/or interleaving of currents to provide output voltages with very low ripple and faster transient response. The vertical package design can be implemented with both isolated and non-isolated CDR circuits as well as boost and buck converters.  
         [0031]     As shown in  FIG. 4 , a vertical package  100  for a switch-mode power converter includes an input module  102 , a magnetic core and winding assembly  104  and an output module  106  stacked on top. As indicated by arrow  108 , power is provided at the bottom of the input module and flows vertically through the package to supply power to a load  110  mounted on top of the vertical package. This configuration provides very short internal interconnects and a 2-D interface between the power converter and the load. The vertical package can be used to implement isolated and non-isolated CDR, and boost and buck converters including interleaved buck as well as other switch-mode power converters.  
         [0032]     Input module  102  is suitably a double-sided or multi-layer printed circuit board (PCB)  112 . All of the primary side circuitry including input filtering capacitors  114 , primary switches  115 , e.g. MOSFETS or other power devices, a primary control IC  116  and a pair of pins  118  for receiving a DC input voltage are mounted on the underside of the PCB  112 . The magnetic core and winding assembly  104  is mounted on the top side of the PCB  112 . Depending on the specific implementation of the magnetic core and winding assembly  104 , copper traces on the top side, as well as those in the inner layers of a multi-layer PCB, may be etched to form portions of the windings or winding terminations for connection to the primary side circuitry. Vias are provided in the PCB to connect the traces on the top side and inner layers to the circuitry on the bottom side.  
         [0033]     Output module  106  is also a double-sided or multi-layer printed circuit board (PCB)  122 . All of the secondary side circuitry including secondary switches  124 , e.g. MOSFETS or other power devices, a secondary control IC  126  and an array of pins  131  for supplying the output voltage are mounted on the top side of the PCB  122 . The output filtering capacitors  130  can either be formed integrally with the load  110  that is mounted on an array of output pins  131  or on the top side of the PCB. For clarity, only a few output pins are shown in  FIG. 4 . The bottom side of PCB  122  contains secondary winding terminations and is mounted on the magnetic core and winding assembly  104 . Depending on the specific implementation of the magnetic core and winding assembly  104 , copper traces on the bottom side, as well as those in the inner layers of a multi-layer PCB, may be etched to form portions of the windings or winding terminations for connection to the primary and/or secondary side circuitry. Vias are provided in the PCB to connect the traces on the bottom side and inner layers to the circuitry on the top side.  
         [0034]     The magnetic core and winding assembly  104  is the centerpiece of the vertical power converter package. The assembly processes power vertically from the input module to the output module and maintains a close coupling between the primary and secondary windings in isolated converters. In non-isolated converters, the input module is configured to provide a phase-shifted input excitation to the magnetics, which is then rectified by the switch-diode combination, to realize an interleaved converter with reduced output ripple and faster transient response. The magnetics in this case consists of interleaved coupled inductors integrated in a matrix magnetic core (matrix integrated magnetics, MIM). In isolated converters, the MIM core also provides isolation via integration of transformer and inductors into a single core. The assembly may be configured to generate a single output voltage, or multiple output voltages using a matrix integrated magnetics (MIM) core in which multiple functionally identical power converter cells can be defined. The assembly includes a magnetic core  140 , windings  142 , primary and/or secondary (shown schematically), primary side winding terminations  143  on the input module and secondary side winding terminations  144  on the output module.  
         [0035]     The primary side circuitry converts the DC input voltage to a high frequency AC voltage that energizes the windings to produce currents in the windings. In the isolated case, a galvanic isolation between primary and secondary sides of the magnetics is maintained, while in the non-isolated converter, two or more inductor currents may be interleaved by creating a phase-shifted ac voltage to the magnetic assembly. The secondary side circuitry rectifies the currents to charge the output capacitor and supply a DC output voltage(s) at the output terminals. The DC output voltages can be distributed to respective pins to provide multiple supply voltages or interleaved to provide a single supply voltage with low ripple and with or without galvanic isolation from the input voltage.  
         [0036]     The vertical package is, in particular, effective for providing lower supply voltages, dynamic voltage scaling, multiple supply voltages, fast transient response, and tight regulation at high efficiency and power density. More specifically, the short internal interconnects and 2-D interface will reduce conduction losses, which is critical to achieving sub-1V outputs at high currents. The MIM core also provides a low profile for the overall converter which is desirable in certain systems. Configuration of the preferred cellular structure in series, parallel or interleaving provides the flexibility to generate single or multiple output voltages. Lastly, the short interconnects and interleaving allows the output voltage to be dynamically changed depending on the load requirements with short transition times. This is accomplished by changing the duty cycle of the input or primary-side switches.  
         [0037]     As shown in  FIG. 5 , the cellular circuit architecture  150  refers to the use of multiple converter cells  152 , all within the same package. The inputs  154  and outputs  156  of the cells are connected either in series or parallel, or a combination of both, to provide the required input and output voltage and current ratings. In the illustrated eight-cell, two-output design, the inputs are all connected in parallel to receive a single DC input voltage V in . A single DC input is typical but not required. One group of four cells is connected to produce a first output voltage v 01  and a second group of four cells is connected to produce a second output voltage v 02 . This is accomplished by controlling the duty cycle of the input or primary side switches. Each output can be separately regulated through the duty cycle of the respective primary side switches. Multiple output voltages that are an integer multiple of each other may also be generated by providing different turns ratios in the magnetics of the two sets of cells. In some cases, the magnetic element used for generating two or multiple sets of output voltages may require a low permeability material between the corresponding cores to control the amount of magnetic coupling between them since the load variations at one output may affect the other output(s).  
         [0038]     Operation of the interconnected cells can also be either synchronized or interleaved. Additional benefits of the proposed cellular architecture include easy scalability to suit applications requiring different power levels, fault tolerance due to parallel operation of multiple cells, as well as fast transient response and low output voltage ripple, especially under interleaved operation, which will be further elaborated in the next section.  
         [0039]     The cellular circuit architecture necessitates the use of multiple magnetic components such as inductors and transformers. Instead of using discrete magnetic components each built on a separate magnetic core, the proposed design uses matrix integrated magnetics (MIM) in which all magnetic components form a matrix and are constructed on a single MIM core of the type show in  FIG. 4 . Three different embodiments for the matrix magnetics using different MIM core structures and winding configurations are illustrated in  FIGS. 6 through 9 .  
         [0000]     MIM Core and Winding Structure 1:  
         [0040]     As shown in  FIGS. 6 and 7 , a vertical package  200  for a switch-mode power converter includes an input module  202 , a MIM core  204  and an output module  206  stacked on top so that power flows vertically from the input module to the output module to a load.  
         [0041]     MIM core  204  can be viewed as consisting of multiple pairs of small E cores  208  that are arranged in two dimensions in the horizontal plane to define a plurality of legs  210  that lie in the plane and a plurality of windows  212  through the plane. The basic requirements for the core material for transformer and inductor applications are high saturation field, high permeability, and low loss at high frequency. Ferrite is a mature magnetic material for high frequency applications. However, its low saturation field necessitates the insertion of air gap in the magnetic path when used for inductors, which is undesirable for the matrix structure due to its close proximity to the conductor windings. Alternately, the core can be formed with a composition of a high permeability material such as ferrite and a high saturation material such as powdered iron in place of the air gap as detailed in copending application “Composite Magnetic Core for Switch-Mode Power Converters” filed on Aug. 19, 2004. Alternative magnetic materials suitable for this application are also possible. In cases where multiple outputs are desired, the amount of magnetic coupling between the cells must be controlled. This can be accomplished by interposing a low permeability material between the integrated magnetic elements used for generating the multiple outputs.  
         [0042]     The winding and associated interconnect designs are another key aspect of matrix magnetics critical for achieving high efficiency and high power density. In this approach, a winding  220  for a magnetic cell  221  is formed by patterned copper conductors  222  and  224  on both sides of the core, as well as conductors  226  through the core windows that connect them. The thickness and number of the conductors  226  is dependent on the current per cell and the total number of cells connected in parallel. The basic cell design can be repeated for any number of cells to form the windings for the entire matrix. The regular shape and repeating pattern of the copper conductor allows them to be constructed on the printed circuit boards (PCBs)  228  and  230  on which the input and output modules are formed. This result is very short interconnects with low resistance and inductance from input to output. The interconnections between the winding PCBs are shown to use conductors  226  and vias  232  in the PCBs. Other interconnection techniques are possible as well.  
         [0043]     The basic winding design for forming a single turn, multiple turns and multiple windings on a single leg is shown in  FIGS. 7   a - 7   c . The PCB is not shown in this figure for clarity. As shown in  FIG. 7   a , a single turn between points A and B is formed by etching away an insulating region  234  in the copper conductor  224  so that A and B lie on opposite sides. Current flows from A, down pins  226 , across copper conductor  222  and up pins  226  to point B. As shown in  FIG. 7   b , two series-connected turns between points A and B are formed by etching away first and second regions  236  and  238  of copper conductor  224  and a first region  240  of copper conductor  222 . Current flows from A, down one pin  226 , across the back part of conductor  222 , up one pin  226 , across the diagonal part of conductor  224 , down one pin  226 , across the front part of plate  222 , up one pin  226  to point B completing the two series connected turns around leg  210 . As shown in  FIG. 7   c , two separate single-turn windings between points A and B and C and D are formed by etching away a region  242  of conductor  224  and a region  244  of conductor  222 . Current flows from A, down pins  226 , across conductor  222  and up pins  226  to point B. Current flows similarly from C to D. Any arbitrary number of windings and number of turns for a given winding can therefore be implemented using the arrangement of conductor and etched patterns. Also, as illustrated, these designs can be used as secondary side windings terminated at points on the output module. Primary side windings are similarly formed by arranging the conductor  250  and etched regions  252  and terminated at points P and Q at the input module ( FIG. 7   d ). This arrangement allows proper separation between the primary and secondary terminations to meet voltage isolation requirements.  
         [0000]     MIM Core and Winding Structure 2:  
         [0044]     As shown in  FIG. 8 , a MIM core  300  and winding arrangement  302  that utilizes conventional planar windings formed on a multi-layer PCB can be sandwiched between the input and output modules so that power flows vertically to a load.  
         [0045]     The MIM core  300  includes a magnetic base plate  304 , a plurality of magnetic legs  306  on the base plate and a magnetic top plate  308 . To prevent saturation of the core some of the legs may be gapped using conventional techniques or portion of the core in high flux areas may be formed from a magnetic material of high saturation field. The winding arrangement  302  is implemented with a multi-layer printed circuit board (PCB)  310  having copper traces that form the various horizontal windings  311  in the plane of the PCB. MIM core  300  is positioned underneath the PCB so that its legs  306  extend through holes  312  in the PCB that coincide with the edges of the windings  311  with proper creepage distances maintained between the core and the conductor. The base and top core plates or sheets are formed with vias  314  and  316 , respectively, for terminating the horizontal windings  311  with, for example, conductor pins  318  with an outside insulation layer and providing electrical connection to the input and output modules. Pins for the primary side module extend out through the bottom plate  314  while those for the secondary side module extend out through the plate  316 . The core plates  314  and  316  may be formed of a sheet of magnetic material.  
         [0000]     MIM Core and Winding Structure 3:  
         [0046]     As shown in  FIG. 9 , a vertical package  400  for a switch-mode power converter includes an input module  402 , a MIM core  404  and winding arrangement  406  that utilizes vertical windings, and an output module  408  stacked on top so that power flows vertically from the input to output modules.  
         [0047]     MIM core  404  includes at least first, second and third outer legs  412 ,  414  and  416 , respectively, disposed on a base  420  and separated along a first outer edge to define first, second, etc. windows there between. A fourth outer leg  426  and window are also included in this embodiment and this construction can be used to add legs as dictated by the design. A center leg  428  is disposed on base  420  along a second outer edge and separated from the first, second and third (or more) legs to define a center window. A plate  434  is disposed on the first, second and third (or more) legs opposite the base. If the core is formed from a single high permeability material such as ferrite as is conventional, an air gap is formed between the plate  434  and center leg  428  to avoid core saturation. If a composite core comprising high permeability material for the outer legs ( 412 ,  414 ,  416 ,  426  etc.) and high saturation field material for the center leg  428  are used, no air gap is necessary. Alternately, the outer legs may also be arranged at the four corners of the base and the center leg formed in the shape of a cross and positioned at the center of the base.  
         [0048]     Winding arrangement  406  is implemented with vertical conductors that are wrapped around the legs orthogonal to the plane of the core and input and output modules. The vertical conductors may be formed from, for example, a copper foil insulated on the outside, which provides electrical isolation between the windings and core, as well as between the windings themselves. The insulation is removed at the terminations to provide an electrical contact to copper pads made on PCB  408  and  402 . In the example shown in  FIG. 9 , the winding arrangement provides a split-primary winding, secondary windings and an additional inductor winding around the center leg  428 . The windings are made in accordance with  FIG. 2  (U.S. Pat. No. 6,549,436). For clarity only a two-turn primary winding is shown in leg  416  while a single turn secondary is shown in leg  414 . Outer leg  416  is wound with two turns of an insulated copper foil  442  to form one side of the two-turn split-primary winding. Outer leg  414  is wound with a single turn of copper foil  444  to form one side of the single-turn secondary. The split primary and secondary windings shown are repeated on each of the legs  412 ,  414 ,  416  and  426  in accordance with the arrangement in  FIG. 2  and a co-pending application “Extended E Matrix Integrated Magnetics (MIM) Core” filed on Aug. 19, 2004. The primary turns are serially connected while the secondary turns may be parallel connected for a single turn. Following the same method and arranging the terminations can be used to achieve larger number of turns for primary or secondary windings. Center leg  428  is wound with a single turn of copper foil  446  to form the single turn inductor winding.  
         [0049]     To avoid the formation of vias in the magnetic core in this illustration, the copper foils  442 ,  444  and  446  are extended laterally outside the core window and turned vertically to form winding terminations  448 ,  450  and  452 , respectively. The primary winding termination  448  extends down through slots  454  in the input module  402  and is turned laterally to terminate on the primary winding pads  456  on the underside of the input module. The secondary winding termination  450  extends up through slots  458  in the output module  408  and is also turned laterally to terminate on secondary winding pads  460  on the top side of the output module. Similarly, the inductor winding  452  on the center leg extends through slots  462  in the output module  408  and is terminated on center leg pads  464  on the top side of the output module. This arrangement provides voltage isolation between the primary and secondary sides.  
         [0050]     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.