Abstract:
A new packaging technology which improves the electrical and mechanical performance of the circuits using magnetic elements. High frequency current loops generate electromagnetic fields which are radiated or induce high frequency current in the rest of the circuit. To reduce the radiated field, these loops have been minimized by locating the high frequency switching components close to each other and very close to the magnetic elements. By separating the high frequency switching electronic components from the rest of the electronic components and locating them on the same multilayer PCB where the magnetic element is constructed, optimal results are obtained.

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 09/086,365, filed on May 28, 1999, now U.S. Pat. No. 5,173,923 and entitled “Packaging Power Converters.” 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to packaging electrical components for converters and power magnetics. 
     One approach to packaging electric components in power converters (FIG. 1) includes providing a structure  1  having a housing which encloses both the components and the means by which heat is extracted from the components. The housing includes a non-conductive casing  5  and an aluminum heat-sinking base. A printed circuit board (PCB)  3  is mounted next to the upper wall  5   a  of the casing. Conductive pins  7  are attached directly to the PCB  3  and extend up through the wall  5   a . Electronic components  9   a ,  9   c  are mounted to one or both sides of the PCB  3 . Larger size components such as the transformer  9   c  are mounted to the lower side where space is available. Power-dissipating devices such as  9   b  are mounted directly on the base-plate  6  for better heat transfer. Power components  9   b  are electrically connected to the PCB by leads  12 . Some of the power-dissipating devices  9   d  are attached to the base-plate by means of a thermally conductive insulator material  8 . Structure  1  may be filled with an encapsulant, which acts as a heat spreader and provides mechanical support. In the case where a hard epoxy encapsulant is used, a “buffer coating” material is used to protect some of the components. 
     However, there is a need for improved transformers. 
     SUMMARY OF THE INVENTION 
     The present invention provides for many improvements in the field of transformer layout and construction. 
     In one embodiment of the invention, a new packaging technology which improves the electrical and mechanical performance of circuits using magnetic elements is provided. In this embodiment, high frequency current loops generate electromagnetic fields, which are radiated, or induce high frequency current in the rest of the circuit. To reduce the radiated field, the extent of these loops is minimized by locating the high frequency switching components close to each other and very close to the magnetic elements. By separating the high frequency switching electronic components from the rest of the electronic components and locating them on the same multilayer PCB where the magnetic element is located, optimal results are obtained. 
     The invention provides a packaging technology for power converters and power magnetics that is compact, inexpensive, and easy to manufacture. The invention features a package for electrical components on a circuit board. In this packaging concept most of the power magnetic elements are integral to the multilayer PCB. The windings of the magnetic elements such as transformers, inductors, and in some cases signal transformers are incorporated in the multilayer PCB, with the top and bottom layers providing support for electronic components. In this way the footprint of the magnetic elements is reduced to the footprint of the transformer core. The power-dissipating devices are placed on pads, which have a multitude of copper plated vias to the other side of the PCB. The heat transferred to the other side of the PCB can be further spread using a larger pad, or transferred to a metallic base-plate attached to the PCB through an isolating material. Due to the limited surface of the heat spreader, an additional heat sink may be attached to the heat spreader to increase its cooling area. 
     The unique aspect of this packaging concept is the fact that the magnetic element&#39;s windings are incorporated on the multilayer PCB construction which also serves as a support for power-dissipating components and some of the control components. The heat from the power-dissipating components is extracted through copper plated vias which transfer the heat to the other side of the PCB. The heat is further transferred to a metal base-plate connected to the PCB by means of a thermally conductive insulator. For airflow cooling applications the heat spreader connected to the thermal vias can serve as a cooling surface. A heatsink can also be attached to the heat spreader to increased the heat dissipation area. 
     The new packaging technology of this invention improves the electrical and mechanical performance of circuits which include magnetic elements. 
     Switched mode power processing converters employ high frequency currents. High frequency current loops are created by high frequency switching electronic components and the interconnection paths between them. 
     The high frequency current loops generate electromagnetic fields, which are radiated and induce high frequency current in the rest of the circuit. To reduce the radiated field these loops should be as small as possible. As a result the high frequency switching components are located close to each other and very close to the magnetic elements. This is achieved more particularly by separating the high frequency switching electronic components from the rest of the electronic components and locating the high frequency switching electronic components on the same multilayer PCB carrying the magnetic elements. The multilayer PCB which incorporates the winding of the magnetic element contains more layers than most of the multilayer PCBs which serve as support and interconnection between electronic components. 
     To reduce the cost of the assembly it is desirable to reduce the cost of the multilayer PCB that incorporates the magnetic elements. This multilayer PCB that contains the magnetic element and the high frequency switching electronic components is herein referred to as the power PCB. The multilayer PCB that accommodates the rest of the electronic components is referred to as the mother PCB. The high frequency switching electronic components are located on the power PCB close to each other and close to the magnetic element. As a result the size of the high frequency current loop can be significantly reduced. The interconnection between the power PCB and the mother PCB will carry lower frequency currents. This package concept leads to better electrical performance while providing an economical utilization of expensive multilayer PCBs. 
     According to the invention, the electronic components are located very close to the magnetic elements in order to minimize the size of current loops through the magnetic elements. 
     The invention, together with various embodiments thereof, will be more fully explained by the accompanying drawings and the following descriptions thereof. 
    
    
     DRAWINGS IN BRIEF 
     FIG. 1 is a cross-sectional side view of prior art components packaging. 
     FIG. 2 is a perspective exploded view of component packaging according to the invention. 
     FIG. 3A is a top view of the packaging with a detailed section of the magnetic winding. 
     FIG. 3B is an enlarged view of a section of FIG.  3 A. 
     FIG. 4A is a top view of the packaging with a detailed section of cooling vias. 
     FIG. 4B is a section of the cooling vias of FIG.  4 C. 
     FIG. 4C is a section of the packaging through the cooling vias and through a magnetic element. 
     FIG. 4D is a section of the cooling vias of FIG. 4C wherein the insulator material penetrates the vias. 
     FIGS. 5A and 5B are views of the horizontal packaging with airflow cooling. 
     FIG. 6 is a top view of the packaging. 
     FIG. 6A is a cross-section of the package with cooling by airflow and cavities for magnetic cores. 
     FIG. 6B is a cross-section of the package with cooling by airflow and holes for magnetic cores. 
     FIG. 7A is a perspective view of the power packages for airflow cooling. 
     FIG. 7B is a perspective view of the power packages for airflow cooling and additional heat sinking applied to the multilayer circuit board. 
     FIG. 8 is a cross-section of the packaging connected to the motherboard. 
     FIG. 9 is another embodiment of the present invention. 
     FIG. 10A is a high power magnetics package according to this invention. 
     FIG. 10B is a cross-section of the magnetic package presented in FIG.  10 A. 
     FIG. 11 presents a multilayer PCB which contains the magnetic element and the high frequency switching electronic components of the electronic circuit. 
     FIG. 12 presents the criteria used to define the high frequency switching electronic components. 
     FIG. 13A presents the multilayer PCB structure that incorporates a magnetic element and high frequency electronic components. 
     FIG. 13B illustrates a method of attaching the power PCB to the mother PCB using a copper plated via and solder penetrating through the via. 
     FIG. 13C illustrates a method of attaching the power PCB to the mother PCB using through-hole pins. 
     FIG. 13D illustrates a method of attaching the power PCB to the mother PCB using a pressed insert. 
     FIG. 13E illustrates a method of attaching the power PCB to the mother PCB using surface-mounted pins. 
     FIG. 13F illustrates a method of attaching the power PCB to the mother PCB using two pressed inserts and a screw. 
     FIG. 14 shows the same structure depicted in FIG. 11 wherein surface-mounted components are placed on both sides of the mother PCB. 
     FIG. 15A shows the same structure depicted in FIG. 11 wherein a cooling plate is attached to the mother PCB. 
     FIG. 15B shows the same structure depicted in FIG. 15A wherein the cooling plate is replaced by an isolated metal substrate, wherein surface-mounted components are placed in the space between the power PCB and the isolated metal substrate. 
     FIG. 15C shows an embodiment wherein the magnetic element is attached to an isolated metal substrate composed of a dielectric material, copper foil, and metal plate. 
     FIG. 16 shows another embodiment of this invention wherein the magnetic core extends through both the power PCB and the mother PCB. 
     FIG. 17 shows another embodiment of this invention wherein the magnetic core extends through several PCBs. 
     FIG. 18 shows another embodiment of this invention wherein the magnetic core extends through several power PCBs which contain high frequency switching electronic components, and through the mother PCB. 
     FIG. 19 shows another embodiment of this invention wherein a metal plate is attached to the power PCB. 
     FIG. 20 shows another embodiment of this invention wherein a metal plate with cavities to accommodate some of the high frequency switching electronic components is attached to the power PCB. 
     FIG. 21 shows another embodiment of this invention wherein some of the high heat dissipation components are located on an alumina substrate placed on a metal plate. 
     FIG. 22 shows another embodiment of this invention wherein the high heat dissipation components are located on top of a thermally conductive insert further soldered to a copper foil edged on an isolated metal substrate board. 
     FIG. 23 shows another embodiment of the invention wherein multiple power PCBs contain high frequency switching electronic components for power processing cells. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 2 showing a packaging structure  7  according to the invention, power-dissipating electronic components  22  are located on a multilayer PCB  28  on top of a heat spreader pad  48  (FIG.  4 B). The heat spreader pad is connected to one or more copper plated vias  42 . A heat spreader  74  (FIG. 4B) is connected to the bottom of the PCB through copper plated vias. The copper plated vias can be filled with solder or can be empty in which case the heat will be transferred through the metalization on the walls of the vias. The metalization is a copper deposit formed in a plating process associated with the manufacturing process of the PCB. The vias can also be filled with a thermally conductive material  30   a  as shown in FIG.  4 D. The material  30   a  is introduced into the vias under pressure to fill the vias. An insulator material  30  with good thermal conductivity characteristics is placed between PCB  28   a  and a metal base-plate  32 . Heat from the power-dissipating components  22  is thereby transferred though the heat spreader pad  48 , on which the power-dissipating device is mounted, through the copper plated vias  42  to the other side of PCB  28   a , to the heat spreader  74 . The heat is further transferred through the thermally conductive insulator material  30  to the base-plate  32 . Where the insulator material  30   a  penetrates through the copper plated vias  42 , surface contact increases, and as a result the thermal transfer from the heat spreader pad  48  to the metal plate  32  is improved. 
     An outstanding feature of the packaging  7  is the incorporation of magnetic elements implementation in the multilayer PCB and the means by which heat is transferred to the base-plate  32  from the power-dissipating devices, the magnetic winding  50 , (FIGS.  3 A and  3 B), the magnetic cores  26   a  and  26   b , and the low power dissipation devices  20 . To increase power density, some components  88  (FIG. 3A) are mounted on top of the multilayer PCB  28   a , above the windings  50  embedded in the inner layers of the multilayer PCB  28 . In this way the footprint of the magnetic element is reduced to the footprint of the magnetic core  26   a.    
     Prior art magnetic elements such as transformers and inductors are discrete devices which are connected to the PCB by means of through-hole or surface-mounted pins. The presence of the connecting pins increases the cost of the magnetic element and reduces the reliability of the magnetic device due to mechanical failure of the pins. The interconnection pins can be bent or broken easily. The presence of the interconnection pins adds supplementary stray inductance in series with the transformer, which negatively impacts the electrical performance of the circuit. In most applications the energy contained in this parasitic inductance is dissipated. However, reducing the parasitic inductance can increase voltage or current stress on the electrical components. 
     Another outstanding feature of packaging  7  is that it allows the use of more complex winding arrangements and more magnetic elements in the same multilayer PCB construction. The interconnections of these magnetic elements are also made within the multilayer PCB. The packaging  7  structure may contain a number of smaller magnetic elements achieving a low profile package. The magnetic cores  26   a  and  26   b  penetrate through the multilayer PCB by means of the cutouts  78   a  and  78   b  adapted to receive outer legs  80   a  and inner legs  80   b  (FIG.  2 ). The magnetic cores  26   a  and  26   b  can be glued together or attached by means of spring clips  82 . To accommodate the spring clips additional cutouts in the PCB  28  are provided. Cutouts  86  are made through the insulator material  30  to accommodate magnetic cores  26   b . In most applications the thickness of the insulator material is less than the height of the magnetic core. To accommodate the magnetic core  26   b , cavities  56  (FIG. 4C) are produced in the base-plate  32 . Due to fragility of the magnetic cores, a soft pad  34  (FIG. 4C) with low thermal impedance is placed under the magnetic core  26   b  in the cavity. The pad dampens the vibration of the magnetic core, The low thermal impedance of the pad  34  also offers a cooling path for the magnetic core. In applications where electrical isolation from the base plate is required, the pad preferably has insulation properties. 
     The entire structure  7  is pressed together so that the magnetic core  26   b  is placed on top of the soft pad  34 . The thickness of the pad is chosen so that the metallic plate  32  makes good contact with the insulator  30 . Permanent attachment of the insulator material  30  can be made in several ways. In the preferred embodiment the insulator material  30  has adhesive properties resulting from a curing process at high temperature, thereby adhering the insulator to the PCB  28  and base-plate  32 . 
     A section  90 , including a power-dissipating device on top of the heat spreader  48  and copper plated cooling vias  42  is shown in FIG.  4 C. The vias  42  transfer heat to the heat spreader  74 . The heat is further transferred by means of the thermally conductive insulator  30  to the metal plate  32 . 
     Also shown in FIG. 4C, a structure  92  includes the upper section of the magnetic core  26   a , the bottom section of the magnetic core  26   b , and a pad  34  under the magnetic core  26   b , in the cavity  56 . The heat generated in the magnetic cores  26  is transferred to the base-plate through the pad  34 . For components which require maintaining a temperature close to that of the base-plate, one or more copper plated vias are placed under the components or in thermal connection to the traces or pads connected to the components. In this way low thermal impedance to the base-plate is achieved. 
     Screws, clips, or different means of applying pressure to the structure  7  can also be used to attach the PCB  28  to the insulator  30  and the base-plate  32 . In some applications the cavities  56  in the base-plate  32  can penetrate through the plate to become cutouts. For protecting the magnetic cores  26   b , soft epoxy material can be used to cover the remaining cavity in between the magnetic core and the surface of the base plate  32 . In some applications the cavity can be left open. 
     FIG. 8 shows a structure  9  wherein a motherboard  96  is attached to the package  7  of FIG.  4 C. The attachment is made through power connectors  24   a  and  24   b , and screws  98 . A signal connector  106 , located on the structure  7 , is adapted for connection to a matching signal connector  104  located on the motherboard  96 . More than one structure  7  can be connected to the same motherboard  96 . There may be additional components  100  and  102 . The structure is advantageous for systems in which only the power train and some control functions are located on the structure  7 , whereas other control function components, as well as supplementary logic circuits and EMI filters, are located on the motherboard. Noise sensitive components are located on the motherboard, whereas the power-dissipating components, some control components and the magnetics are located on the structure  7 . The bottom layer of the motherboard  96  may contain copper shields to further protect the noise sensitive components. 
     FIGS. 5 a  and  5   b  show a packaging structure  11 . In this structure the magnetic element has its winding  50  embedded within the multilayer PCB  28  as in the structure  7 . The components are preferably located on both sides of the multi layers PCB. This packaging structure is advantageous in low power dissipation applications where airflow is available. The entire surface of multilayer PCB  28  functions as a heatsink. The structure  11  is connected to other circuitry by means of pins  52 . 
     FIG. 7A shows a power system which contains several packaging structures  15  connected to a motherboard  64 . As in the structure  7 , the structure  15  includes magnetic elements  26 , power-dissipating components  22 , and low power dissipation components  20 . Unlike the structure  7 , however, the structure  15  includes neither an insulator  30 , nor a base-plate  32 . The cooling is accomplished by airflow across which flows in between the packaging structures  15 . This maximizes the effective surface available for sinking heat. The structures  15  are connected to a motherboard  64  through signal connectors  70   a  and power connectors  70   b . Supplementary components are located on the motherboard  64 . 
     FIG. 7B shows two packaging structures  17  connected to the motherboard  64 . These packaging structures contain the same components as structure  15  with an additional heat sink  58  attached to the multilayer PCBs  28  through the insulator  30 . 
     Two types of heatsink construction are shown in FIGS. 6A and 6B. In FIG. 6A a heatsink  58  has cooling fins  60  and cavities  68 . A pad  34 , formed of a soft compressible material with low thermal impedance, is placed in the cavities. The insulator  30 , which has low thermal impedance, is placed between the multilayer PCB  28  and the heat sink  58 . Heat is conducted from magnetic core  26   b  through the pad  34 , and from power-dissipating devices through copper plated vias, as in the structure  7 . 
     In FIG. 6B the heatsink  58  with cooling fins  60  has cutouts  64  to accommodate the magnetic core  26   b . The cooling of the magnetic cores  26   a  and  26   b  is accomplished by airflow across the cooling fins  60 . Heat is conducted from the power-dissipating devices to the cooling fins through one or more copper plated vias  42 , as in FIG.  4 C. 
     FIG. 9 shows a packaging structure wherein the base-plate  32  does not contain cavities. Instead, elevated sections  104  are provided which make contact with the thermally conductive insulator  30  placed under the multilayer PCB  28 . The elevated sections  104  are preferably placed under the power-dissipating devices  22  and other low dissipation components  20   c  which require maintaining a temperature close to that of the base-plate. The pad  34  is placed on the base-plate  32  and supports the magnetic cores  26   a  and  26   b . The height of the elevated section  104  is a function of the height of the magnetic core  26   b  and the compression ratio of the pad  34 . An advantage of the packaging structure is that more components can be mounted on the bottom side  20   b  of the multilayer PCB. This structure is particularly advantageous for power converters which contain all the control and signal interface functions. The power connectors  24   a  and  24   b  provide access to power and signal connections. A cover  107  contains holes  110  to accommodate the power connectors  24   a  and  24   b . Teeth  112  are formed along the lower edge of the cover  107  for attaching it to the base-plate  32  (FIG.  9 ). A matching groove  108  is undercut into the base-plate  32 . 
     FIG. 10A shows a high power magnetic structure  19  wherein the magnetic core comprises several small magnetic cores  26 . A cross-section through the structure  19  is shown in FIG.  10 B. The windings  50  of the magnetic structure are embedded in the multilayer PCB  28 . A cutout  56  in the multilayer PCB  28  is provided to accommodate the magnetic cores  26 . Power connectors  24   a  and  24   b  are inserted in the multilayer PCB  28  and are connected to the windings  50 . The cores  26   a  and  26   b  are attached together by means of clips  82 . The multilayer PCB  28  also provides support for the magnetic cores  26 . A cavity  56  is placed in the base-plate  32 . A soft compressible thermally conductive pad  34  is placed between the magnetic core  26   b  and the base plate  32 . 
     FIG. 11 shows another embodiment of the invention, wherein a multilayer PCB  2  incorporates a winding  18  of a magnetic element  16 . The multilayer PCB  2  also supports high frequency switching electronic components  8 . The interconnection between the magnetic element  16  and the high frequency switching electronic components  8  is made to minimize both parasitic inductance and insertion impedance. Other electronic components  6  of the electronic circuit are disposed on a mother PCB  4  that can contain a reduced number of layers, reducing cost. 
     FIG. 12 illustrates criteria used to define the high frequency switching electronic components is presented. It is desirable to minimize the size of current loops Lp 1   44  and Lp 2   46  to improve electrical performance. As a result the components  38 ,  36 ,  40  and  42  are preferably located as close as possible to the magnetic element  16 . All these elements are disposed on the power PCB  2  (FIG.  11 ). The rest of the electronic components which process lower frequency signals, such as C 2   100 , Lin  102 , Lo  104  and Co 2   106 , can be disposed on the mother PCB  4  (FIG.  11 ). 
     FIG. 13A shows the magnetic element  16 , the high frequency switching electronic components  8 , and the interconnection pads  51 . 
     FIG. 13B shows another embodiment of the invention, illustrating a method for attaching the power PCB  2 , which supports the magnetic element  16 , to the mother PCB  4 . The power PCB  2  includes copper plated vias  42 , which allow penetration of the solder  23 . The solder  23  creates a medium for current flow and also provides a mechanical connection between power PCB  2  and the mother PCB  4 . A copper pad  52  is provided on the top of the mother PCB  4 , and a copper pad  54  is provided on the bottom of the power PCB  2 . During the soldering process, the melted solder  23  spreads between the pads  52  and  54  and further penetrates through the vias  42 , creating a mechanical bond and a path for current and heat flow. 
     FIG. 13C shows another embodiment of the invention wherein the interconnection between the power PCB  2  and the mother PCB  4  is accomplished with a pin  27  which is pressed into vias  28   a  and  28   b  respectively of the power PCB  2  and the mother PCB  4 , and is further soldered to the mother PCB  4 . 
     FIG. 13D shows the interconnection between the power PCB  2  and the mother PCB  4  accomplished with the use of a pressed insert  30 . The insert  30  is formed of a conductive material to create a path for current and heat, in addition to a mechanical connection. 
     FIG. 13E shows another embodiment of the invention wherein a surface-mounted pin  111  is used for interconnecting the power PCB  2  and the mother PCB  4 . The pin  111  is electrically and thermally conductive to allow an efficient current and heat flow between the power PCB  2  and the mother PCB  4 . 
     FIG. 13F shows another embodiment of the invention wherein the interconnection between the power PCB  2  and the mother PCB  4  is provided by pressed connectors  160  and  180  and a screw  182 . The connector  160  is press fit into the mother PCB  4  and the connector  180  is press fit into the power PCB  2 . The screw  182  ensures mechanical interconnection between the connectors  160  and  180 . The connectors  160  and  180  provide very good conduction of current and heat, creating a very low electrical and thermal impedance path between the power PCB  2  and the mother PCB  4 . 
     FIG. 14 shows an embodiment of the invention wherein components  120  are disposed on the bottom side of the mother PCB  4 . The power density of the power-processing device formed by the power PCB, mother PCB, high frequency switching electronic components, and the rest of the electronic components, may thereby be increased. 
     FIG. 15A shows an additional metal plate  126  attached to the mother PCB  4  through a dielectric material  124 . Heat flows from the mother PCB to the metal plate  126  which functions as a heatsink. 
     FIG. 15B shows another embodiment of the invention wherein a metal substrate element element comprising a metal plate  134 , a dielectric  132 , and a copper foil  130 , is attached to the mother PCB  4 . Additional electronic components  128  can be attached to copper pads on the copper foil  130 . This packaging method allows heat from the power PCB  2  and the mother PCB  4  to flow to the metal plate  134 . Providing for the additional components  128  leads to increased power density. 
     FIG. 15C shows an embodiment of the invention in which a magnetic element  16  is attached to an isolated metal substrate comprising the dielectric material  132 , copper foil  52 , and the metal plate  134 . A cutout  200  in the metal substrate is provided to accommodate the magnetic core  10 . The interconnection between the metal substrate and the magnetic element  16  is made with the copper foil  52  and a via  125 . In the soldering process, melted solder  23  penetrates through the via  125 , creating an electrical and mechanical bond. The advantage of this embodiment is that high power dissipation devices are disposed directly on the metal substrate, resulting in very low thermal impedance to the metal plate  134 . 
     FIG. 16 shows a magnetic core  10  penetrating through the power PCB  2  and the mother PCB  4 . Windings  18  in the layers of the power PCB  2  and windings  190  in the mother PCB  4  are magnetically coupled. This increases the number of electronic components that can be attached to the mother PCB  4 . 
     FIG. 17 shows two power PCBs  2 a and  2 b linked magnetically through a magnetic core  10  to the mother PCB  4 . This symmetrical structure offers some advantages such as interleaving the primary and secondary windings of the magnetic structure using the magnetic core  10 . 
     FIG. 18 shows high frequency switching electronic components disposed on both power PCBs  2   a  and  2   b . The primary winding of the transformer element using the magnetic core  10  can be disposed on the mother PCB  4 , and the secondary winding can be disposed in the power PCBs  2   a  and  2   b . Additional high frequency switching electronic components such as rectifiers can be surface-mounted on the PCBs. 
     FIG. 19 shows an additional metal plate  126  attached to the power PCB  2  by way of an intermediating dielectric material  124 . Heat produced by the magnetic element  16  and by the high frequency switching electronic components  8  can be transferred to the metal plate  126  and further transferred to the air or an additional heatsink. 
     FIG. 20 shows a similar concept to that of FIG. 19, where some of the high frequency switching electronic components are located under a metal plate  126 B. The metal plate  126 B includes cavities to accommodate components  8 A disposed on top of the power PCB  2 . 
     FIG. 21 shows high power dissipation electronic components  180  disposed on an alumina substrate  182 . Interconnection between the alumina substrate  182  and the power PCB  2  is provided by pins  188 . A thermally conductive compressible pad  184  is placed between the magnetic core  10  and the metal plate  186 . The module comprising the power PCB  2 , the high frequency switching electronic components  8  and  180 , and the metal plate  186 , is attached to the mother PCB  4 . The advantage of this structure is that a good thermal path from the high heat dissipation components to the metal plate heatsink  186  is provided. 
     FIG. 22 shows high heat dissipation components  240  disposed on top of a heat conductive insert  204 . The insert  204  penetrates into a copper interface  202 . The copper interface  202  is soldered to copper foil  210  disposed on a dielectric  206 , which is attached to a metal plate  208 . The structure comprising the heat conductive insert  204 , copper interface  202 , and the copper foil  210  can efficiently transfer heat from the components  240  to the metal plate  208 . This structure also provides an efficient path for current flow. A power processing module comprising the power PCB  2 , high frequency switching electronic components such as  8  and  240 , the heat conductive insert  204 , the copper interface  202 , copper foil  210 , the dielectric layer  206 , and the metal plate  208 , can be connected to the mother PCB  4 . 
     FIG. 23 shows another embodiment of the invention wherein multiple power PCBs  2 A . . .  2 Z are provided. Each power PCB includes at least one magnetic element along with high frequency switching electronic components and high power dissipation components. Additional electronic components, such as components for signal control, are disposed on the mother PCB  4 . Cooling plates  126 A . . .  126 Z can be further attached to the respective power PCBs for cooling. The high power dissipation components, which in most power converters are also high frequency switching components, are thereby separated from the noise and heat sensitive components located on the mother PCB. 
     Power PCBs are relatively costly compared to the mother PCB because of their typically large number of layers. According to the invention, some of the electronic components that would ordinarily be supported by the power PCBs are moved to the mother PCB, allowing for a reduction in the size of the PCBs, reducing overall cost. Further, separating the high frequency switching and high power dissipation electronic components from the rest of the components leads to improved electrical-thermal characteristics. The number of power PCBs provided can be increased to increase the output power, the PCBs functioning as standardized power-processing cells that constitute the building blocks for a power system having any desired power output. 
     It is clear that the present invention provides for a highly improved transformer.