Patent Publication Number: US-8536971-B1

Title: Magnetic component

Description:
FIELD OF THE INVENTION 
     This invention relates to a magnetic component of an electrical circuit that provides efficient coupling between multiple primary inductor windings and a secondary inductor winding, while minimizing coupling between each of the multiple primary inductor windings. In certain embodiments, the invention is directed to an apparatus and method for providing a magnetic component that may be incorporated into an electrical circuit arranged to extract power from a power source having a relatively high internal impedance. 
     BACKGROUND OF THE INVENTION 
     Many electrical power sources exist that have high internal resistances. The internal resistance of a power source measures the resistance within that power source to the flow of current through the power source. Internal resistance can be dependent upon many factors, including the construction of the power source, ambient temperature conditions, and changes in the internal chemistry of the power source. Although internal resistance is often associated with a power source comprising batteries, other types of power sources can have relatively high internal resistance. Examples of such power sources include solar cells and fuel cells. 
     When a power source has a relatively high internal resistance, it is difficult to extract electrical energy from the source in an efficient manner because the power source&#39;s internal resistance dissipates a relatively large portion of the electrical energy. That dissipated energy is therefore consumed within the power source and is never delivered to the load. Additionally, it is difficult to use a power source with high internal resistance to provide a desired voltage to a given electrical load, such as a particular electrical or electronic circuit, because the voltage supplied by the power source drops substantially as the load draws current from the power source. If a number of high internal resistance power sources are connected in series (for example, to generate a high output voltage), there are additional losses in terms of extracted power because the current flowing through each power source must pass through the internal resistances of the other power sources that connect to it. 
     In a circuit arranged to extract energy from a high impedance power source, a number of magnetic components may be utilized to facilitate energy extraction. To ensure efficient operation, it is important that those magnetic components efficiently couple primary and secondary windings to minimize losses within the extraction circuit. 
     SUMMARY OF THE INVENTION 
     In one implementation, the present invention is a magnetic component including a core. The core includes a first plate, a second plate, a secondary core post connected between the first plate and the second plate, and a plurality of primary core posts disposed between the first plate and the second plate. Each of the plurality of primary core posts includes a first section connected to the first plate and a second section connected to the second plate. The first and second section of each of the plurality of primary core posts is separated by a gap. The magnetic component includes a secondary winding formed about the secondary core post, and primary windings formed about each of the plurality of primary core posts. 
     In another implementation, the present invention is a magnetic component core. The magnetic component core includes a first plate, a second plate, a secondary core post connected between the first plate and the second plate, and a plurality of primary core posts disposed between the first plate and the second plate. Each of the plurality of primary core posts includes a first section connected to the first plate and a second section connected to the second plate. The first and second section of each of the plurality of primary core posts is separated by a gap. 
     In another implementation, the present invention is a method of manufacturing a magnetic component. The method includes forming a first plate of a magnetic component core. The first plate includes a first primary core post and a first secondary core post formed over a surface of the first plate. A height of the first primary core post above the surface of the first plate is less than a height of the first secondary core post above the surface of the first plate. The method includes forming a second plate of the magnetic component core. The second plate includes a second primary core post and a second secondary core post formed over a surface of the second plate. A height of the second primary core post above the surface of the second plate is less than a height of the second secondary core post above the surface of the second plate. The method includes connecting the first and second plate of the magnetic component core by joining the first secondary core post to the second secondary core post. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from a reading of the following detailed description taken in conjunction with the drawings in which like reference designators are used to designate like elements, and in which: 
         FIG. 1  is an illustration of an example core design for use within the present magnetic component or transformer. 
         FIGS. 2A and 2B  are illustrations of an alternative core design for use within the present magnetic component or transformer having a rectangular geometry; 
         FIGS. 3A and 3B  are illustrations of an alternative core design for use within the present magnetic component or transformer having a circular geometry; 
         FIG. 4  is an illustration showing a top view of the core design illustrated in  FIGS. 3A and 3B  showing the direction of flux flow through the structure; 
         FIGS. 5A and 5B  are illustrations of an alternative core design for use within the present magnetic component or transformer; 
         FIG. 6  is an illustration showing an example Z-folded flex circuit structure for use in the present magnetic component; 
         FIG. 7  is an illustration of an electrical circuit including a power source having a relatively high internal resistance and a load, where the circuit incorporates a power extraction circuit configured in accordance with the present disclosure; 
         FIG. 8  is a graph showing an energy transfer rate from a power source to a capacitor as well as the corresponding voltage across the capacitor versus time; 
         FIG. 9  is a flowchart illustrating an example method for extracting power from a power source; 
         FIG. 10  is an illustration of an electrical circuit including two power sources connected in series and connected to power extraction circuitry; and 
         FIGS. 11A-11D  illustrate a number of potential interconnections between solar cells within a device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention is described in preferred embodiments in the following description with reference to the Figures, in which like numbers represent the same or similar elements. Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     The described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are recited to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
     Some of the functional units described in this specification have been labeled as modules in order to more particularly emphasize their implementation independence. For example, a module may be implemented in field programmable gate arrays, programmable array logic, programmable logic devices, or the like. Modules may also be implemented in software for execution by various types of processors. 
     The schematic flow chart diagrams included are generally set forth as logical flow-chart diagrams (e.g.,  FIG. 9 ). As such, the depicted order and labeled steps are indicative of one embodiment of the presented method. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the illustrated method. Additionally, the format and symbols employed are provided to explain the logical steps of the method and are understood not to limit the scope of the method. Although various arrow types and line types may be employed in the flow-chart diagrams, they are understood not to limit the scope of the corresponding method. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the method. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted method. Additionally, the order in which a particular method occurs may or may not strictly adhere to the order of the corresponding steps shown. 
     The invention will be described as embodied in an apparatus and method for a magnetic component of an electrical circuit that provides efficient coupling between multiple primary inductor windings and a secondary inductor winding, while minimizing coupling between each of the multiple primary inductor windings. In certain embodiments, the invention is directed to an apparatus and method for providing a magnetic component that may be incorporated into an electrical circuit arranged to extract power from a power source having a relatively high internal impedance. 
     In the present disclosure, the invention is described as being configured for incorporation into an electrical circuit arranged for extracting power from a power source, though a person of ordinary skill in the art would appreciate other uses for the magnetic component such as in applications calling for an efficient, multiple-winding transformer or applications calling for a device capable of transferring electrical energy from one circuit to another. As such, the present disclosure provides an example use of the present magnetic component, but other uses of the present magnetic component are conceivable. In certain embodiments, the invention is directed to a magnetic component for use in an electrical circuit for extracting power from a power source having a relatively high internal impedance. In one implementation, an enhancement factor of approximately 2 to 3.5 can be achieved using the present apparatus and method in such a circuit. 
     In one particular implementation, the present magnetic component is incorporated into an electrical circuit including a capacitor connected across the terminals of a high impedance power source. The capacitor is configured to operate as both a power extractor and a resonant component to transfer extracted power from the power source to an input of the magnetic component. The circuit is operated in a high frequency mode and power is transferred from the power source to the magnetic component in an efficient manner. The magnetic component is then used to deliver energy to an attached load. 
     In another implementation of the present system, the invention may be used in a circuit configured to reduce the internal power dissipation in solar cells (or other high internal resistance power sources) that are connected in series. The circuit is configured to provide an independent path for the current to flow from each power source to the extracting circuit. This implementation also facilitates obtaining a desired voltage from the series-connected power sources. 
     The present invention is a magnetic component for use in an electrical circuit. The magnetic component provides efficient coupling between multiple primary inductor windings and a secondary inductor winding formed about the magnetic component, while minimizing coupling between each of the multiple primary inductor windings. As described below, the magnetic component may be fabricated as a transformer having a core, a plurality of primary windings and a single secondary winding. The core posts of the primary windings incorporate air gaps, while the core of the secondary windings does not incorporate an air gap, as described below. In various other implementations, the present invention may be incorporated into flyback transformers or other coupled inductor systems, such as those where current flow through a second inductor does not occur while current flows through a first inductor. Throughout the present disclosure, all references to transformers should be considered equally applicable to these other types of magnetic components, as would be recognized by a person of ordinary skill in the art. 
     In such an arrangement, the magnetic component allows for near perfect coupling between each one of the primary inductor windings and the second inductor winding. That is, the coupling constant between each one of the primary inductor windings and the second inductor winding is near unity. As a result, all, or a majority of, the magnetic flux generated in the core of one of the primary inductor windings is coupled to the core of the secondary inductor winding. Additionally, this configured results in near-zero coupling of magnetic flux between each of the primary inductor windings with one another. These characteristics allow for efficient operation of the present magnetic component. 
       FIG. 1  is an illustration of an example core  10  for use within the present magnetic component or transformer. Core  10  includes three core posts  12 ,  14  and  16  configured to receive two primary inductor windings at core posts  12  and  14 , and a single secondary inductor winding at core post  16 . The two primary inductors or windings of the transformer are wound around the two outer core posts  12  and  14 . The secondary inductor winding is wound about central core post  16 . 
     As shown in  FIG. 1 , each of core posts  12  and  14  include air gaps  18  and  20 , respectively, while core post  16  does not. To construct core  10 , two E-shaped core sections are joined to one another. In that case, the two E-shaped sections are sized so that when joined together, equal air-gaps are formed in the outer legs of the core structure. In one implementation of core  10 , the dimension of core posts  12  and  14  are approximately the same, while the area of the cross-section of center post  16  is approximately twice the area of the cross-section of either of posts  12  or  14 . The actual dimensions of core posts  12 ,  14 , and  16  can be determined by many factors, which include the electrical characteristics of the input power source, the frequency of operation of the circuit, the value chosen for the input capacitors, and the magnetic material used for making the core. The size of the air-gaps  18  and  20  may also be determined by a number of factors such as the input power source electrical characteristics, the frequency of operation of the circuit, the primary capacitor value, the value of the inductance required, and magnetic materials making up core  10 . 
     To use core  10  within a transformer, windings are formed around each of core posts  12 ,  14 , and  16 . Two separate primary windings (illustrated by example windings W 12  and W 14 ) are formed around core posts  12  and  14 . A secondary winding (see, for example, winding W 16 ) is formed around core post  16 . Each of the windings may have any suitable construction. If constructed using wires or litz-wire, bobbin or bobbins with three legs may be used to wind the respective windings onto any of the three core posts. In that case, the bobbins, which are typically made from a plastic or other dielectric material, have three holes into which the core pieces can be inserted. If the windings are formed as z-folded windings, the windings are folded first and then the core pieces are inserted in the central holes within the windings. Barrel-wound windings can also use bobbins. The windings may be formed to lay over either of gaps  18  and  20 , either partially or completely coving the air-gap, or may be formed to lay over only the solid portions of core posts  12  and  14 . 
     With the windings formed around each of core posts  12 ,  14 , and  16 , each of the primary windings (formed around core posts  12  and  14 ) can be driven by different excitation currents to generate magnetic flux that is communicated to the secondary winding through core post  16 . In  FIG. 1  example fluxes are indicated by arrows  11  and  13 . 
     As illustrated in  FIG. 1 , the cross-sectional area of the central core post  16  is greater than the cross-sectional area of both core posts  12  and  14 . In one implementation, the ratio of cross-sectional area between the outer core posts and the inner core post is approximately 1:2. By sizing core posts  12 ,  14 , and  16  in this manner, fluxes produced by the currents in the primary windings (e.g., in core posts  12  and  14 ) both pass through the central core post  16 . Due to the introduction of air gaps  18  and  20  into core posts  12  and  14 , respectively, and the fact that central core post  16  is sized for extremely low reluctance, there will be nearly zero coupling between the two primary inductors windings around core posts  12  and  14 . Additionally, there will be very good coupling between each primary inductor winding (wound about core posts  12  and  14 ) with the secondary inductor winding (wound about core post  16 ). 
     To ease the construction of the transformer or the magnetic component, the air-gap formed in the outer posts can be filled with any dielectric material, which has a relative magnetic permeability of 1. In other words, the magnetic properties of the material within air-gap should correspond to air or vacuum. Examples of such material include most of the plastics, a resin, and paper. 
     In general, core posts  12 ,  14 , and  16  may include any soft magnetic materials having high magnetic permeability. These materials include soft ferrites, soft iron and steels, amorphous metals, and nano-magnetic materials. The materials may be selected in order to minimize high frequency core losses. 
       FIGS. 2A and 2B  are illustrations of an alternative core  30  for use within the present magnetic component or transformer.  FIG. 2A  shows core  30  as comprising separate top and bottom sections  31  and  33 , respectively.  FIG. 2B  shows core design  30  after the top and bottom sections  31  and  33  have been joined to one another. The top and bottom sections  31  and  33  may be held together by an adhesive tape, adhesives, glues, mechanical fasteners, or one or more clamp-like devices. The clamp-like device can be made of a metallic material or a hard plastic, for example. Alternatively, the top and bottom section may be held together by the windings formed around each of posts  32  and core structure  34 . Core design  30  includes a number of primary core posts  32  configured to receive six separate primary inductor windings. In another implementation, though, a single winding may be wrapped about more than one core post using, for example, a Z-fold winding structure, as discussed below. For example, core  30  may be used with three primary windings, where each winding is wrapped about two core posts. Core design  30  also includes a secondary core structure  34  configured to receive a secondary inductor winding. The primary inductor windings are wound about each of core structures  32 , while the secondary inductor winding is wound about core structure  34 . 
     In one implementation, the area of the cross-section of the center (secondary) post  34  is approximately equal to the sum of the areas of the core posts  32 , though in other implementation the area of cross section of center post  34  may be larger. 
     As illustrated in  FIG. 2B , core posts  32  are sized so that when top section  31  and bottom section  33  of core design  30  are connected to one another, the top and bottom section of core post  34  mate to one another, while air gap  35  is formed between each of core posts  32 . As such, each core post  32  includes an air gap  35 , while core post  34  includes no such air gap. The size of the air-gap will be determined, primarily, by the inductance requirement for the primary inductor. The inductance value of the primary inductor will depend upon the power source, frequency of operation, and the resonant frequency. 
     Because air gaps  35  are formed within each core post  32 , and because core post  34  includes no such air gap and is sized for extremely low reluctance, there will be nearly zero coupling between the two primary inductors windings around each of core posts  32  (see example winding W 32  formed about one of core posts  32 ), but there will be very good coupling between the each primary inductor winding (wound about core posts  32 ) with the secondary inductor winding (wound about core post  34 —see, for example, winding W 34 ). 
     In other implementations, core  30  can be modified to accept any number of primary windings. For example, additional core posts may be positioned at either end of core  30  at, for example, ends  36  or  38  for the addition of windings. Additionally, the overall length of core design  30  may be increased so as to increase the number of windings structures that may be positioned along a length of core design  30 . Although core  30  is illustrated as a rectangle, core  30  may be formed in the shape of a square, depending upon the number of primary windings. 
       FIGS. 3A and 3B  are illustrations of an alternative core design  40  for use within the present magnetic component or transformer.  FIG. 3A  shows core structure  40  as comprising separate top and bottom sections  41  and  43 , respectively.  FIG. 3B  shows core design  40  after to the top and bottom sections  41  and  43  have been joined to one another. Top and bottom sections  41  and  43  may be held together by an adhesive tape, adhesives, glues, mechanical fasteners, or one or more clamp-like devices. The clamp-like device can be made of a metallic material or a hard plastic, for example. Alternatively, the top and bottom section may be held together by the windings formed around each of posts  42  and core structure  44 . As illustrated, top section  41  and bottom section  43  include rounded disks ( 45  and  47 , respectively) to which the components of core design  40  are mounted. In alternative implementations, though, the disks may have different shapes, such as that of ellipses. 
     Core design  40  includes a number of primary core posts  42  configured to receive four separate primary inductor windings (see example winding W 42 ). Core design  40  also includes a secondary core structure  44  configured to receive a secondary inductor winding (see example winding W 44 ). The primary inductor windings are wound about each of core posts  42 , while the secondary inductor winding is wound about core post  44 . Although the geometry of both core posts  42  and  44  are shown in  FIGS. 3A and 3B  as being cylindrical, in other implementations, core posts  42  and/or  44  can have different shapes, such as elliptical cylinders, other generalized cylinders, or cuboids. 
     As illustrated in  FIG. 3B , core posts  42  are sized so that when top section  41  and bottom section  43  of core design  40  are connected to one another, the top and bottom sections of core post  44  mate to one another, while air gap  46  is formed within each of core posts  42 . As such, each core post  42  includes an air gap  46 , while core post  44  includes no such air gap. 
     Because air gaps  46  are formed within each core post  42 , and because core post  44  includes no such air gap and is sized for extremely low reluctance, there will be nearly zero coupling between the two primary inductors windings around each of core posts  42 , but there will be very good coupling between the each primary inductor winding (wound about core posts  42 ) with the secondary inductor winding (wound about core post  44 ). 
     When using core  40  of  FIGS. 3A and 3B  within a transformer, the windings are formed so that the fluxes of core posts  42  travel in opposite directions. As an illustration,  FIG. 4  shows a top view of core  40  illustrates the direction of flux flow through each of core posts  42  and core post  44 . In  FIG. 4 , flux flowing through core posts  42  enters through the top of core  40  (indicated by the dots in the center shown in each core post  42 ). Flux flowing through core post  44  flows upwards out of core  40 , as indicated by the ‘X’ drawn over post  44 . 
       FIGS. 5A and 5B  are illustrations of an alternative core  50  for use within the present magnetic component or transformer.  FIG. 5A  shows core  50  as comprising separate top and bottom sections  51  and  53 , respectively.  FIG. 5B  shows core  50  after to the top and bottom sections  51  and  53  have been joined to one another. As illustrated, top section  51  and bottom section  53  include rounded disks ( 55  and  57 , respectively) over which the components of core  50  are mounted. In an alternative implementation, though, the disks may have different shapes, such as that of ellipses. Core  50  includes a number of primary core posts  52  configured to receive a number of separate primary inductor windings (see example winding W 52 ). Depending upon the system requirements, the number of core posts  52  can be increased by increasing the size of the top and bottom disks  55  and  57  of core  50  and then locating additional core posts  52  thereon. Core  50  also includes a secondary core post  54  configured to receive a secondary inductor winding (see example winding W 54 ). The primary inductor windings are wound about each of core posts  52 , while the secondary inductor winding is wound about core post  54 . Although the geometry of both core posts  52  and  54  are shown in  FIGS. 5A and 5B  as being cylindrical, in other implementations, core posts  52  and/or  54  can have different shapes, such as elliptical cylinders, other generalized cylinders, or cuboids. 
     As illustrated in  FIG. 5B , core posts  52  are sized so that when top section  51  and bottom section  53  of core  50  are connected to one another, the top and bottom section of core  54  mate to one another, while air gap  56  is formed between each of core posts  52 . As such, each core post  52  includes an air gap  56 , while core post  54  includes no such air gap. 
     Because air gaps  56  are formed within each core post  52 , and because core post  54  includes no such air gap and is sized for extremely low reluctance, there will be nearly zero coupling between the two primary inductors windings around each of core posts  52 , but there will be very good coupling between the each primary inductor winding (wound about core posts  52 ) with the secondary inductor winding (wound about core post  54 ). 
     Each of the core designs illustrated in  FIGS. 1-5B  include any soft magnetic materials having high magnetic permeability. These materials include soft ferrites, soft iron and steels, amorphous metals, and nano-magnetic materials. The materials may be selected in order to minimize high frequency core losses. 
     The magnetic components or transformer device described above can be utilized in a number of electronic circuits. One such circuit includes an electrical circuit configured to extract electrical energy from a power source having a high impedance, as described below. 
     Additionally, the material and construction types for various primary and secondary windings may include combinations of insulated metallic wire, such as copper or aluminum, high frequency Litz wire, sheet copper with insulating tape, Z-fold flex circuit windings, or barrel wound flex circuit, such as that illustrated in U.S. Pat. No. 5,570,074. 
       FIG. 6  is an illustration showing an example Z-folded flex circuit structure for use in the present magnetic component. As illustrated, the winding material  70  is weaved between a number of core posts  72 . At certain points within the weave (indicated by dashed lines  74 , winding material  70  is folded in a particular direction according to arrows  76 . As illustrated, this winding structure is useful when preparing a winding that is formed around a number of core posts  72 . 
       FIG. 7  is an illustration of an electrical circuit including a power source having a relatively high internal resistance and a load, the circuit incorporates a power extraction circuit  130  connected between the power source and the load, where the power extraction circuit is configured in accordance with the present disclosure. As shown in  FIG. 7 , the circuit includes power source  110 . Power source  110  includes a voltage source  112  and an internal resistance  114 . Power source  110  may include a power source having a relatively high internal resistance  114 , such as a solar cell or a fuel cell. A typical solar cell can comprise a power source having an open circuit voltage of 0.6 volts and an internal resistance of approximately 3 ohms. In such a supply, the short circuit current is typically 0.2 amperes. 
     Capacitor  116  is connected across the terminals of power source  110 . Capacitor  116  may be selected to have a low equivalent series resistance (ESR). Inductor  118  is also connected across the terminals of power source  110 . Switch  120  is connected between a first terminal of inductor  118  and a first terminal of power source  110 . Switch  120  is configured to optionally connect or disconnect the first terminal of inductor  118  to or from the first terminal of power source  110 . The open or closed status of switch  120  is controlled by processor  122  that is connected to switch  120 . Processor  122  is also configured to monitor energy flow into capacitor  116  from power source  110  and out of capacitor  116  into inductor  118 . In one implementation, switch  120  includes a switch having low high-frequency switching losses, capacitance, and low turn-on resistances. Example switches include metal oxide semiconducting field effect transistors (MOSFETs), bi-polar Junction transistors (BJTs), and silicon-controlled rectifiers (SCRs). 
     In one implementation, energy flow into and out of the capacitor is determined by analyzing the voltage capacitor using, for example, a voltage sensor connected across the capacitor. Energy flow into and out of the capacitor is related to the voltage across the capacitor. This energy flow can be determined by measuring a voltage across the capacitor and then using that voltage measurement to identify an energy transfer rate using, for example the transfer rate curve of  FIG. 8 . 
     Inductor  118  is coupled to inductor  124  so that a change in current flow through one inductor induces a voltage across the second inductor. This allows electrical energy to be transferred from inductor  118  to inductor  124 . In one implementation, inductors  118  and  124  include inductors having relatively low high-frequency losses and are each wound around the same core to facilitate magnetic coupling. Alternatively, inductors  118  and  124  may each be replaced by the primary and secondary windings of a transformer. For example, inductors  118  and  124  may form the primary and second winding on a transformer comprising a core structure as described above. In one implementation, inductor  118  may be wrapped around core  12  and/or core  14  of core design  10  shown in  FIG. 1 . In that case, inductor  124  may be wrapped around core  16  of core design  10 . In other implementations, the core structures shown in  FIGS. 2A ,  2 B,  3 A,  3 B,  5 A and  5 B could be incorporated into a transformer where inductor  118  operates as the transformer&#39;s primary winding and inductor  124  operates as the transformers secondary winding. 
     Inductor  124  is connected across load  126  in order to deliver electrical energy thereto. Diode  128  (e.g., a diode having a low forward voltage drop) is disposed between a first terminal of inductor  124  and load  126  to limit current flow between inductor  124  and load  126  to a single direction. In one implementation, an output capacitor (not shown) may be coupled across load  126 , wherein the output capacitor is selected to meet the required output ripple voltage requirement. 
     The combination of capacitor  116 , switch  120 , and inductor  118  form power extraction circuit  130  of the present invention. 
     During operation of the circuit shown in  FIG. 1 , capacitor  116  initially draws current from power source  110  (for example, when power source  110  is first connected to capacitor  116 ). As the voltage of capacitor  116  increases, an amount of current (and, thereby, energy) flowing from power source  110  to capacitor  116  begins to decrease (see, for example,  FIG. 8 , described below). Because the current flow to capacitor  116  is varied (and, consequently, the voltage of capacitor  116  is varied), the rate at which energy is transferred from power source  110  to capacitor  116  is not constant. Instead, the rate of energy transfer varies with time. This variation in energy transfer is illustrated in  FIG. 8 . 
       FIG. 8  is a graph showing energy transfer rate from power source  110  to capacitor  116  as well as the corresponding voltage across capacitor  116  versus time. As seen in  FIG. 8 , initially, the energy transfer to the capacitor is relatively high (see, for example, point  150  of the transfer curve). As such, there is a period of time after power source  110  initially begins transferring energy to capacitor  116  in which energy is transferred most efficiently. This period of time is illustrated by shaded region  152  of  FIG. 8 . 
     As time passes, however, the energy transfer to capacitor  116  become less and less efficient. Referring to  FIG. 8 , for example, after the initial period of efficient energy transfer, the energy transfer efficiency diminishes relatively rapidly. As the voltage of capacitor  116  approaches that of power source  110  minus the voltage drop across power source  110 &#39;s internal resistance, energy transfer is very inefficient. This period of inefficient transfer is illustrated by shaded area  154  on  FIG. 8 . 
     To ensure that energy is being transferred efficiently to capacitor  116  (and, therefore, out of power source  110 ), the present system is configured to ensure that capacitor  116  and power source  110  are operating so that the energy transfer characteristics fall within shaded area  152  of  FIG. 8 . To accomplish this, switch  120  is periodically closed to move energy out of capacitor  116  into inductor  118  and, from there, into load  126 . By periodically discharging capacitor  116  into inductor  118 , the voltage across capacitor  116  can be maintained below a threshold value allowing for energy to be efficiently transferred out of power source  110 , even when power source  110  has a high internal resistance. 
     To provide for efficient transfer of energy out of power source  110  (and into capacitor  116 ), processor  122  is configured to implement the method illustrated in  FIG. 9 . In step  160 , processor  122  monitors energy delivery from power source  110  to capacitor  116 . In step  162 , processor  122  compares the measured energy flow to a predetermined threshold in step  162 . When the energy flow into capacitor  116  from power source  110  falls above the predetermined threshold (e.g., within shaded region  152  (shown in FIG.  8 )), processor  122  causes switch  120  to be open allowing energy to continue to flow from power source  110  to capacitor  116  in step  164 . 
     In a typical installation (e.g., using a solar cell), optimal energy transfer occurs when the capacitor voltage is between 20% and 75% of the open circuit voltage of the solar cell (or other high internal resistance power source). In other words, for a typical solar cell with an open circuit voltage of 0.6 volts and an internal resistance of 3 ohms, the capacitor voltage for which the switch will remain open is between 0.12 volts and 0.45 volts. For all other values of capacitor voltage the switch will be closed. This condition allows for efficient energy transfer from power source  110  directly to capacitor  116 . 
     As capacitor  116  charges, however, the energy transfer from power source  110  becomes less efficient (see, for example,  FIG. 8 ). If processor  122  detects that the energy transfer rate from power source  110  to capacitor  116  falls below the threshold (e.g., outside of shaded region  152  shown in  FIG. 8 ), processor  122  causes switch  120  to close in step  166 . The various thresholds at which switch  120  opens and closes will be adjusted based upon the characteristics of power source  110 . With switch  120  closed, capacitor  116  is connected to inductor  118 . Capacitor  116  will then resonate with inductor  118  causing energy to be transferred from capacitor  116  to inductor  118 , where it is eventually transferred through inductor  124  into load  126 . 
     After closing switch  120 , processor  122  monitors the amount of energy remaining within capacitor  116  in step  168  and compares that amount of energy to a threshold value. As the voltage across capacitor  116  is directly related to the energy stored therein, the remaining energy of capacitor  116  may be determined by detecting and monitoring a voltage across capacitor  116 . Until sufficient energy has been transferred out of capacitor  116 , processor  122  holds switch  120  in a closed condition. After sufficient energy has been dissipated from capacitor  116  through switch  120  and into inductor  118  so that power source  110  may again efficiently transfer energy into capacitor  116 , processor  122  opens switch  120  in step  170 . The method is shown in  FIG. 9  then repeats with processor  122  continually monitoring energy transfer from power source  110  to capacitor  116 . 
     With switch  120  open, capacitor  116  begins the next cycle of energy transfer from power source  110  to the capacitor  116  and, eventually, to inductor  118 . In one implementation, the period of time in which switch  120  is closed is typically smaller than the period of time that switch  120  is open. This results because the circuit is operating in a mode in which most of the time maximum power is transferred to capacitor  116 . 
     In one specific implementation of the electrical circuit illustrated in  FIG. 7 , power source  110  includes a solar cell having an open circuit voltage of 0.6 volts, an internal resistance of 3 ohms, and a short-circuit current of 0.2 amperes. In conjunction with such a power source, capacitor  116  may be selected to have a capacitor of approximately 5 micro-Farads. The frequency of sampling the capacitor will depend on the time constant of the circuit, which is given by the product of capacitance value and the internal resistance of the solar cell or panel. In one implementation, the time period between samples is approximately 50-100 times shorter than that time constant. 
     The present system differs greatly from conventional power supply circuits that incorporate input capacitors. In those circuits, the input capacitor is selected to be as large as possible in order to provide a constant voltage to the remaining circuit. In other words, the input capacitor operates as a buffer to compensate for temporary disruptions in power consumption of the connected load. In contrast, in the present system, a relatively small capacitor is used because the capacitor does not operate as an input capacitor and is instead used to only extract energy from the high internal resistance power source. Additionally, the capacitor should be capable of resonating with the inductor to quickly transfer accumulated energy to the circuit&#39;s load. If a large, conventional input capacitor were to be used in conjunction with the present system, the rate of transfer of energy to the capacitor (e.g., capacitor  116  of  FIG. 7 ) would be diminished, as indicated by shaded area  154  of  FIG. 8 . 
     In some implementations of the present system, a number of power sources, each having relatively high internal resistances, are connected together in series to serve a particular load. In conventional systems, this would ordinarily result in poor performance as any current flowing from one power source must pass through the high internal resistance of the other power sources that are each connected together. The present system, in contrast, allows for current drawn from each power source to flow to an inductor supplying the load without also flowing through the internal resistances of the other power sources. 
       FIG. 10  is an illustration of an electrical circuit including two power sources, each having a relatively high internal resistance, connected in series to a load, wherein the circuit incorporates power extraction circuitry connected to each power source. As shown in  FIG. 10 , the circuit comprises two power sources  200  and  202 . Each power source includes a voltage supply  204  and  206  and a relatively high internal resistance  208  and  210 . Capacitor  212  is connected across the terminals of power source  200 . Capacitor  214  is connected across the terminals of power source  202 . Inductor  216  is connected across the terminals of power source  200  via switch  220 . Inductor  218  is connected across the terminals of power source  200  via switch  222 . Switch  220  is connected between a first terminal of inductor  216  and a first terminal of power source  200 . Switch  222  is connected between a first terminal of inductor  218  and a first terminal of power source  202 . Each of switches  220  and  222  are configured to optionally connect or disconnect the first terminal of inductors  216  and  218  to their respective power sources. The open or closed status of switches  220  and  222  are controlled by one or more connected processors (not shown). 
     In one implementation of the circuit shown in  FIG. 10 , the capacitances of capacitors  212  and  214  are approximately equal, while the inductances of inductors  216  and  218  are also approximately equal. In that case, with the components connected to each power source being matched, both switches  220  and  222  may be operated synchronously, with each of the switches being opened and closed at the same in accordance with the method illustrated in  FIG. 9 . 
     Each of inductors  216  and  218  are wound about core  224  and are, thereby, coupled to inductor  226 . Because each of inductors  216  and  218  are wound about the same core, their flux is additive, causing inductor  226  to see a summed voltage for each inductor. In one implementation, inductors  216  and  218  and inductor  226  form the primary and secondary windings, respectively, in a transformer that incorporates a core structure as described above. In one implementation, inductors  216  and  218  may be wrapped around cores  12  and  14 , respectively of core design  10  shown in  FIG. 7  to form the primary windings for the transformer. In that case, inductor  226  may be wrapped around core  16  of core design  10  to form the secondary winding of the transformer. In other implementations, the core structures shown in  FIGS. 2A ,  2 B,  3 A,  3 B,  5 A and  5 B could be incorporated into a transformer by replacing core  224  of the circuit illustrated in  FIG. 10 . 
     Through core  224 , inductors  216  and  218  transfer energy to inductor  226  which is, in turn, connected across load  228 . Diode  230  is disposed between a first terminal of inductor  226  and load  228  to limit current flow between inductor  226  and load  228  to a single direction. In one implementation, an output capacitor may be coupled across load  228 , wherein the output capacitor is selected to meet the required output ripple voltage requirement. 
     Although the circuit of  FIG. 9  shows only two series-connected power sources, more power sources may be connected in series. 
       FIG. 10 , therefore, shows two separate power sources  200  and  202 , each independently configured in accordance with the single power source of  FIG. 7 . 
     Accordingly, each power source is connected to a power extraction circuit to improve the efficiency of energy transfer from each power source to the target load. Each power extraction circuit in  FIG. 10  includes a switch (e.g., switches  220  and  222 ) that can be periodically opened and closed to transmit energy from the power source to the attached capacitor (e.g., one of capacitors  212  and  214 ) in accordance with the method shown in  FIG. 9 . 
     In one implementation, the present system may be used to facilitate the retrieval of energy from one or more solar cells. Solar cells can be connected in series or in parallel or a combination of series and parallel connections. In any configuration, the power extraction circuit of the present system can be coupled to each solar cell to facilitate energy extraction therefrom.  FIGS. 11A-11D  illustrate a number of potential interconnections between solar cells within a device. In other implementations, though, the present system may be used to facilitate energy retrieval from any power source having a high internal resistance such as a solar cell or fuel cell. 
     While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.