Patent Publication Number: US-9837194-B1

Title: Output transformer and resonant inductor in a combined magnetic structure

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of the following patent application which is hereby incorporated by reference: U.S. Provisional Patent App. No. 62/238,434 filed Oct. 7, 2015, entitled “Output Transformer and Resonant Inductor in a Combined Magnetic Structure.” 
    
    
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     In a typical DC-DC converter circuit  100  illustrated in  FIG. 1 , the converter circuit receives a voltage from a DC source  110 , which may be a DC voltage supply that produces a DC voltage from an AC source (not shown). In the illustrated converter circuit in  FIG. 1 , the DC source is illustrated as a conventional battery, and the voltage from the DC source is provided on a VRAIL supply line  112 . The voltage on the VRAIL supply line is referenced to an input ground reference  114 . A first semiconductor switch (e.g., a power metal oxide semiconductor field effect transistor (MOSFET) or bipolar junction transistor (BJT)  120  has a first terminal connected to the VRAIL supply line and has a second terminal connected to a common switching node  122 . A second semiconductor switch (MOSFET or BJT)  124  has a first terminal connected to the common switching node and has a second terminal connected to the input ground reference. Together, the two switches operate as a half-bridge circuit  126  to produce a switched DC voltage on the common switching node. 
     The control terminal (e.g., gate of a MOSFET or base of a BJT) of the first switch  120  is connected to a first output  132  of an integrated circuit controller  130 . The control terminal of the second switch  124  is connected to a second output  134  of the controller. The controller operates in a conventional manner to turn on the first switch to couple the common switching node  122  to the VRAIL supply line  112 ; and then turn on the second switch to couple the common switching node to the input ground reference  114 . When one of the switches is turned on, the other switch is turned off. The two switches are turned on and off at a selected repetition rate and with selected duty cycles to produce a voltage on the common switching node that alternates between the VRAIL voltage and ground. 
     The common switching node  122  of the half-bridge circuit  126  is connected to a resonant tank circuit  140  that includes a resonant circuit inductor  142 , a first clamping diode  144 , and a second clamping diode  146 . A first terminal of the resonant circuit inductor  142  is connected to the common switching node  122  of the half-bridge circuit. A second terminal of the resonant circuit inductor is connected to the anode of the first clamping diode and to the cathode of the second clamping diode at a resonant tank node  148 . The cathode of the first clamping diode  144  is connected to the VRAIL supply line  112 . The anode of the second clamping diode is connected to the input ground reference  114 . The two clamping diodes prevent the voltage on the resonant tank node from exceeding the VRAIL voltage by more than one diode forward voltage drop and from going below the input ground reference voltage by more than one diode forward voltage drop. 
     The resonant tank circuit  140  further includes a resonant circuit capacitor  150  and the primary winding  162  of an output transformer  160 . A first terminal of the primary winding is connected to the resonant tank node  148 . A second terminal of the primary winding is connected to a first terminal of the resonant circuit capacitor. A second terminal of the resonant circuit capacitor is connected to the input ground reference  114 . 
     The output transformer  160  includes a center-tapped secondary winding  170  having a first winding half  172 , a second winding half  174  and a center tap  176 . The first winding half  172  is connected between the center tap and a first secondary output terminal  180 . The second winding half  174  is connected between the center tap and a second secondary output terminal  182 . The center tap  176  is connected to an output ground reference  184 . The output ground reference is isolated from the input ground reference  114  by the output transformer. Accordingly, the output transformer may also be referred to as an isolation transformer. 
     The first secondary output terminal  180  of the output transformer  160  is connected to the anode of a first rectifier diode  190 . The second secondary output terminal  182  is connected to the anode of a second rectifier diode  192 . The cathodes of the two rectifier diodes are connected together at an output node  194 . A filter capacitor  196  is connected between the output node and the output ground reference  184 . A DC load (“LED LOAD”)  198  is connected across the filter capacitor between the output node and the output ground reference. In the illustrated embodiment, the DC load includes a plurality of light-emitting diodes (LEDs) connected in series or connected in a series-parallel combination. 
     In operation, the switched DC voltage on the common switching node  122  is AC-coupled to the primary winding  162  of the output transformer  160 . Accordingly, an AC voltage is produced on the secondary winding  170  of the output transformer. The AC output of the secondary winding is rectified by the two rectifier diodes  190 ,  192  to produce a DC voltage (V LED ) across the filter capacitor  196  to drive the LEDs of the DC load  198 . 
     In the conventional resonant tank circuit  140  illustrated in  FIG. 1 , the resonant circuit inductor  142  and the output transformer  160  are two entirely separate magnetic components. For example,  FIG. 2  illustrates a conventional resonant inductor  142 .  FIG. 3  illustrates an exploded view of the resonant inductor. As illustrated, the resonant inductor includes a bobbin  200  having a coil  202  wound around a central passage  204 . A first E-core  210  of the inductor has a center leg  212  inserted into the central passage from a first end of the bobbin. The first E-core has a first outer leg  214  and a second outer leg  216  positioned on opposed sides of the bobbin. A second E-core  220  of the inductor has a center leg  222  inserted into the central passage from a first end of the bobbin. The second E-core has a first outer leg  224  and a second outer leg  226  positioned on opposed sides of the bobbin. The bobbin further includes a first pin rail  230  and a second pin rail  232  at opposed ends of the bobbin. Each pin rail supports a plurality of pins  234 . The ends of the winding (not shown) of the coil are connected to selected pins on one or both of the pin rails. For example, in a conventional inductor having a single coil winding, a first end of the winding is connected to a pin on the first pin rail and a second end of the winding is connected to a pin on the second pin rail. Alternatively, both ends of the winding can be connected to respective pins on the same pin rail. 
       FIG. 4  illustrates a conventional output transformer  160 .  FIG. 5  illustrates an exploded view of the output transformer. As illustrated, the transformer includes a bobbin  240  having a coil  242  wound around a central passage  244 . A first E-core  250  of the transformer has a center leg  252  inserted into the central passage from a first end of the bobbin. The first E-core has a first outer leg  254  and a second outer leg  256  positioned on opposed sides of the bobbin. A second E-core  260  of the transformer has a center leg  262  inserted into the central passage from a first end of the bobbin. The second E-core has a first outer leg  264  and a second outer leg  266  positioned on opposed sides of the bobbin. The bobbin further includes a first pin rail  270  and a second pin rail  272  at opposed ends of the bobbin. Each pin rail supports a plurality of pins  274 . The ends of the windings (not shown) of the coil are connected to selected pins on one or both of the pin rails. For example, in a conventional transformer, the two ends of the primary winding may be connected to respective pins on the first pin rail, and the two end terminals and the center tap of the secondary winding may be connected to three pins on the second pin rail. 
     As shown in  FIGS. 2-5 , each of the inductor  140  and the output transformer  160  occupies a respective surface area defined by the spacing between the respective first and second pin rails, the widths of the pin rails and spacing required between adjacent components. For example,  FIG. 6  illustrates a first plan view of the inductor and transformer positioned longitudinally with respect to each other on a typical printed circuit board with a minimal spacing between the two components.  FIG. 7  illustrates the two components positioned laterally with respect to each other. In either configuration, the surface area occupied by the two components is considerably greater than the surface area occupied by the transformer alone or by the inductor alone. 
     SUMMARY OF THE INVENTION 
     The invention disclosed herein provides a solution to reduce the surface area for two magnetic components. One aspect of the invention is a magnetic assembly that includes a single bobbin structure that supports a first coil and a second coil. A first E-core has a center leg positioned within the first coil. A second E-core has a center leg positioned within the second coil. An I-core is positioned between the first E-core and the second E-core. The I-core completes a first set of magnetic paths between the center leg and outer legs of the first E-core and also completes a second set of magnetic paths between the center leg and outer legs of the second E-core. The two E-cores and the I-core are stacked in a vertical E-I-E configuration with respect to a common set of pin rails. In one embodiment, the first coil and the first E-core are configured as a transformer; and the second coil and the second E-core are configured as an inductor. 
     Another aspect of the invention is a magnetic assembly that combines two magnetic device functions in a single unified structure. The magnetic assembly includes a bobbin structure having a first core passage, a second core passage and a third core passage. The third core passage is positioned between the first core passage and the second core passage. A first coil at least partially surrounds the first core passage. A second coil at least partially surrounds the second core passage. An I-core has at least a central portion positioned in the third core passage. A first E-core has a center leg positioned in the first core passage and has a first outer leg and a second outer leg. The first and second outer legs of the first E-core have respective end surfaces contacting the I-core. A second E-core has a center leg positioned in the second core passage and has a first outer leg and a second outer leg. The first and second outer legs and the center leg of the second E-core have respective end surfaces positioned proximate to and spaced apart from the I-core. 
     In certain embodiments, the center leg of the first E-core is shorter than the outer legs of the first E-core such that the end surface of the center leg of the first E-core is spaced apart from the first planar surface of the I-core by a selected gap distance. 
     In certain embodiments, a gap spacer is positioned between the I-core and the end surfaces of the first outer leg, the second outer leg and the center leg of the second E-core. The gap spacer has a thickness selected to provide a predetermined gap spacing between the end surfaces of the first outer leg, the second outer leg and the center leg of the second E-core and the I-core. 
     In certain embodiments, the first coil, the first E-core and at least a first portion of the I-core comprise a first inductive device; and the second coil, the second E-core and at least a second portion of the I-core comprise a second inductive device. 
     In certain embodiments, the first inductive device is a transformer; and the second inductive device is an inductor. 
     Another aspect of the invention is a bobbin for a magnetic assembly. The bobbin includes a first flange, a second flange, a third flange and a fourth flange. A first winding portion is positioned between the first flange and the second flange. A second winding surface is positioned between the third flange and the fourth flange. An I-core receiving passage is positioned between the second flange and the third flange. A first core leg receiving passage extends from the first flange to the second flange. A second core leg receiving passage extends from the fourth flange to the third flange. 
     In certain embodiments, the first core passage and the second core are aligned along a first passage direction; and the third core passage is oriented orthogonally to the first core passage and the second core passage. 
     Another aspect of the invention is a magnetic assembly. The magnetic assembly includes a bobbin structure. The bobbin structure includes a first flange and a second flange. The second flange is parallel to the first flange and is displaced away from the first flange. A first core passage extends from the first flange to the second flange. The first core passage is perpendicular to the first flange and the second flange. A first winding surface surrounds the first core passage between first flange and the second flange. A third flange is parallel to the second flange and is displaced away from the second flange. The third flange is coupled to the second flange by at least one spacer wall to define an I-core receiving slot between the second flange and the third flange. A fourth flange is parallel to the third flange and is displaced away from the third flange. A second core passage extends from the third flange to the fourth flange. The second core passage is perpendicular to the third flange and the fourth flange. A second winding surface surrounds the second core passage between the third flange and the fourth flange. A least a first coil is wound around the first winding surface. At least a second coil is wound around the second winding surface. A first E-core has a respective first outer leg, a respective second outer leg and a respective center leg. Each leg of the first E-core has a respective end surface. The center leg of the first E-core inserted into the first core passage with the end surface of the center leg positioned proximate to the second flange. A second E-core has a respective first outer leg, a respective second outer leg and a respective center leg. Each leg has a respective end surface. The center leg of the second E-core is inserted into the second core passage with the end surface of the center leg positioned proximate to the third flange. An I-core is positioned in the I-core receiving slot. The I-core has a first planar side positioned against the end surfaces of the first outer leg and the second outer leg of the first E-core. The I-core has a second planar side positioned proximate to and spaced apart from the end surfaces of the first outer leg, the second outer leg and the center leg of the second E-core. 
     In certain embodiments, the magnetic assembly further includes a gap spacer positioned between the second planar side of the I-core and the end surfaces of the first outer leg, the second outer leg and the center leg of the second E-core. The gap spacer has a thickness selected to provide a predetermined gap spacing between the end surfaces of the first outer leg, the second outer leg and the center leg of the second E-core and the second planar side of the I-core. 
     In certain embodiments, the gap spacer includes a polyester film material having a thickness selected to provide the predetermined gap. For example, the thickness of the gap spacer may be in a range between 0.0025 inch and 0.0200 inch. In certain embodiments, the gap spacer includes a polyethylene terephthalate (PET) film. 
     In certain embodiments, the center leg of the first E-core is shorter than the outer legs of the first E-core such that the end surface of the center leg of the first E-core is spaced apart from the first planar surface of the I-core by a selected gap distance. 
     In certain embodiments, the first coil, the first E-core and at least a first portion of the I-core comprise a first inductive device; and the second coil, the second E-core and at least a second portion of the I-core comprise a second inductive device. 
     In certain embodiments, the first inductive device is a transformer; and the second inductive device is an inductor. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  illustrates a circuit diagram showing a DC-DC converter as conventionally known in the art. 
         FIG. 2  illustrates a perspective view of a conventional inductor. 
         FIG. 3  illustrates an exploded perspective view of the inductor of  FIG. 2 . 
         FIG. 4  illustrates a perspective view of a conventional output transformer. 
         FIG. 5  illustrates an exploded perspective view of the transformer of  FIG. 2 . 
         FIG. 6  illustrates a plan view of the inductor of  FIG. 2  and the output transformer of  FIG. 4  positioned on a printed circuit board in a longitudinal relationship. 
         FIG. 7  illustrates a plan view of the inductor of  FIG. 2  and the output transformer of  FIG. 4  positioned on a printed circuit board in a lateral relationship. 
         FIG. 8  illustrates a perspective view of a combined magnetic assembly with an inductor and an output transformer sharing a single bobbin, the view referenced to X-, Y-, and Z-axes. 
         FIG. 9  illustrates the combined magnetic assembly of  FIG. 8 ; the view flipped 180 degrees about the X-axis and then rotated 90 degrees about the Y-axis to show the surfaces hidden in  FIG. 8 . 
         FIG. 10  illustrates a front cross-sectional view of the combined magnetic assembly of  FIG. 8  taken along the line  10 - 10  in  FIG. 8 . 
         FIG. 11  illustrates an exploded perspective view of the combined magnetic assembly of  FIG. 8 . 
         FIG. 12  illustrates a perspective view of the inductor E-core of the magnetic assembly as oriented in  FIG. 8 , the view referenced to the X-, Y-, and Z-axes of  FIG. 8 . 
         FIG. 13  illustrates the inductor E-core of  FIG. 12  flipped 180 degrees about the X-axis and then rotated 90 degrees about the Z-axis to show the surfaces hidden in  FIG. 12 . 
         FIG. 14  illustrates a perspective view of the transformer E-core of the magnetic assembly as oriented in  FIG. 8 , the view referenced to the X-, Y-, and Z-axes of  FIG. 8 . 
         FIG. 15  illustrates the transformer E-core of  FIG. 14  flipped 180 degrees about the X-axis and then rotated 90 degrees about the Z-axis to show the surfaces hidden in  FIG. 14 . 
         FIG. 16  illustrates a perspective view of the I-core of the magnetic assembly as oriented in  FIG. 8 , the view referenced to the X-, Y-, and Z-axes of  FIG. 8 . 
         FIG. 17  illustrates the I-core of  FIG. 21  flipped 180 degrees about the X-axis and then rotated 90 degrees about the Z-axis to show the surfaces hidden in  FIG. 16 . 
         FIG. 18  illustrates a perspective view of the bobbin of the combined magnetic assembly of  FIG. 8  prior to installation of the coils and prior to insertion of the cores, the view referenced to the X-, Y-, and Z-axes of  FIG. 8 . 
         FIG. 19  illustrates a perspective view of the bobbin of  FIG. 18  flipped 180 degrees about the X-axis and then rotated 180 degrees about the Z-axis with respect to the view in  FIG. 18 . 
         FIG. 20  illustrates a front elevational view of the bobbin of  FIG. 18 . 
         FIG. 21  illustrates a right elevational view of the bobbin of  FIG. 18 . 
         FIG. 22  illustrates a top plan view of the bobbin of  FIG. 18 . 
         FIG. 23  illustrates a bottom plan view of the bobbin of  FIG. 18 . 
         FIG. 24  illustrates a schematic representation of the front cross-sectional view of the combined magnetic assembly in accordance with  FIG. 11 , the view annotated to illustrate the flux pattern in the resonant inductor E-core in dash-dot lines, the flux pattern in the output transformer E-core in dash-dot-dot lines, and the shared flux pattern in the common I-core in dashed lines. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary invention to the problem disclosed in  FIGS. 1-7  is illustrated by a combined magnetic assembly  500  in  FIGS. 8-24 . 
       FIGS. 8-11  illustrate views of the combined magnetic assembly  500  with a resonant inductor  510  and an output transformer  520  sharing a single bobbin  530 . In the drawings, the X-axis, Y-axis and Z-axis are shown for reference to assist in visualizing the orientations of the structures. 
     The inductor  510  and the transformer  520  are installed in the bobbin  530  with the inductor “stacked” on top of the transformer. The inductor includes an inductor coil  540  positioned around a first (upper) portion of the bobbin. The inductor further includes an inductor E-core  542  positioned with respect to the inductor coil. The transformer includes a transformer coil  550  positioned around a second (lower) portion of the bobbin. The transformer further includes a transformer E-core  552  positioned with respect to the transformer coil. The inductor and the transformer share a common I-core  560  positioned between the two E-cores as described below. The two E-cores and the I-core comprise a suitable ferromagnetic material such as, for example, silicon steel, a ferrite material, or other suitable material. In the illustrated embodiment, a gap spacer  570  is positioned between the I-core and the inductor E-core. 
     As shown in  FIGS. 12 and 13 , the E-core  542  of the inductor  510  includes a main body  600 , a first outer leg  602 , a second outer leg  604  and a center leg  606 . The main body has an outer surface  610  and an inner surface  612 . As illustrated, the outer surface has a length L (in a direction parallel to the Z-axis) and a width W 1  (in a direction parallel to the X-axis). The main body has a thickness T (in a direction parallel to the Y-axis) between the inner surface and the outer surface. The three legs extend perpendicularly from the inner surface. The main body and the three legs have a first (front) common side surface  614  and a second (rear) common side surface  616 . 
     The first outer leg  602  of the inductor E-core  542  extends from the inner surface  612  of the main body  600  of the E-core to a first outer leg end surface  620 . The first outer leg has an outer lateral surface  622  and an inner lateral surface  624 . The first outer leg has a width W 2  between the two lateral surfaces. 
     The second outer leg  604  of the inductor E-core  542  extends from the inner surface  612  of the main body  600  of the E-core to a second outer leg end surface  630 . The second outer leg has an outer lateral surface  632  and an inner lateral surface  634 . In the illustrated embodiment, the second outer leg also has the width W 2  between the two lateral surfaces corresponding to the width of the first outer leg  602 . 
     The center leg  606  of the inductor E-core  542  extends from the inner surface  612  of the main body  600  of the E-core to a center leg end surface  640 . The center leg has a first lateral surface  642  that faces the inner lateral surface  624  of the first outer leg  602  and has a second lateral surface  644  that faces the inner lateral surface  634  of the second outer leg  604 . The center leg has a width W 3  between the two lateral surfaces. In the illustrated embodiment, the width W 3  of the center leg may be approximately twice the width W 2  of the two outer legs. 
     In the illustrated embodiment, the center leg  606  of the inductor E-core  542  has substantially the same length as the lengths of the two outer legs  602 ,  604  of the inductor E-core with respect to the inner surface  612  of the main body  600  of the inductor E core such that the respective end surfaces of the three legs are aligned. The alignment is indicated by a dashed line  646  that passes through the edges of the three end surfaces. The end surfaces of the three legs are spaced apart from the inner surface of the main body of the inductor E-core by a first height H 1 . 
     As shown in  FIGS. 14 and 15 , the E-core  552  of the transformer  520  includes a main body  650 , a first outer leg  652 , a second outer leg  654  and a center leg  656 . The main body has an outer surface  660  and an inner surface  662 . As illustrated, the outer surface has the length L and the width W 1  corresponding to the length and width of the outer surface  610  of the main body  600  of the inductor E-core  542 . In the illustrated embodiment, the main body of the transformer E-core may have the same thickness T between the inner surface and the outer surface as the thickness of the main body of the inductor E-core. The three legs extend perpendicularly from the inner surface. The main body and the three legs of the transformer E-core have a first (front) common side surface  664  and a second (rear) common side surface  666 . 
     The first outer leg  652  of the transformer E-core  552  extends from the inner surface  662  of the main body  650  to a first outer leg end surface  670 . The first outer leg has an outer lateral surface  672  and an inner lateral surface  674 . In the illustrated embodiment, the first outer leg has the width W 2  between the two lateral surfaces corresponding to the width of the first outer leg  602  of the inductor E-core  510 . 
     The second outer leg  654  of the transformer E-core  552  extends from the inner surface  662  of the main body  650  of the E-core to a second outer leg end surface  680 . The second outer leg has an outer lateral surface  682  and an inner lateral surface  684 . In the illustrated embodiment, the second outer leg also has the width W 2  between the two lateral surfaces corresponding to the width of the first outer leg  652 . 
     The center leg  656  of the transformer E-core  552  extends from the inner surface  662  of the main body  650  of the E-core to a center leg end surface  690 . The center leg has a first lateral surface  692  that faces the inner lateral surface  674  of the first outer leg  652  and has a second lateral surface  694  that faces the inner lateral surface  684  of the second outer leg  654 . In the illustrated embodiment, the center leg has the width W 3  between the two lateral surfaces corresponding to the width of the center leg  606  of the inductor E-core  510 . 
     In the illustrated embodiment, the lengths of the two outer legs  652 ,  654  of the transformer E-core  552  with respect to the inner surface  662  of the main body  650  of the transformer E-core are substantially the same such that the respective end surfaces of the two outer legs are aligned as indicated by a first dashed line  696  that passes through the edges of the two end surfaces. Thus, the end surfaces of the two outer legs are spaced apart from the inner surface of the main body of the transformer E-core by a second height H 2 . 
     In the illustrated embodiment, the center leg  656  of the transformer E-core  552  is shorter than the two outer legs  652 ,  654  with respect to the inner surface  662  of the main body  650  of the transformer E-core as illustrated by a second dashed line  698  that passes through the edge of the end surface of the center leg. The second dashed line is offset from the first dashed line by a gap G. The difference in length (e.g., the gap G) may be exaggerated in  FIGS. 14 and 15  for the purpose of illustrating the difference. For example, the gap may range from 0.001 inch to 0.05 inch). 
     In the illustrated embodiment, the outer legs  602 ,  604  of the inductor E-core  542  are shorter than the outer legs  652 ,  654  of the transformer E-core  552 ; however, in other embodiments, the outer legs of the inductor E-core may be as long as or may be longer than the outer legs of the transformer E-core. The lengths of the outer legs of the E-cores of the inductor and the transformer are determined in part by the lengths of the respective coil windings. 
     In the illustrated embodiment, the center legs  606 ,  656  of the two E-cores  542 ,  552  have rectangular cross sections; however, in other embodiments, the center legs may be configured to have other cross sections (e.g., circular cross sections). 
     As illustrated in  FIGS. 16 and 17 , the I-core  560  is a regular parallelepiped having a first (upper) major planar surface  700  and a second (lower) major planar surface  702 . The two major surfaces have the length L and the width W 1  corresponding to the lengths and the widths of the outer surfaces  610 ,  660 , respectively, of the inductor E-core  542  and the transformer E-core  552 . The two major surfaces are spaced apart by a height H 3  to form a first (front) lateral surface  704  and a second (rear) lateral surface  706  along the width of the two major surfaces and to form a first end surface  710  and a second end surface  712  along the length of the two major surfaces. In the illustrated embodiment, the height H 3  of the I-core is similar to the thickness T of the main bodies  600 ,  650  of the two E-cores; however, the thickness of the I-core may be greater or less than the main body thicknesses in other embodiments. 
     As shown in the exploded view of  FIG. 11 , the gap spacer  570  includes a relatively thin material having planar surfaces (one shown) generally corresponding to the shape and size of the two planar surfaces  700 ,  702  of the I-core  560 . For example, in one embodiment, the gap spacer comprise a polyester film material (e.g., a Mylar® polyethylene terephthalate (PET) film) having a thickness of between 0.0025 inch to 0.0200 inch). As discussed below, the thickness of the gap spacer is selected to provide a desired gap distance between the legs  602 ,  604 ,  606  of the inductor E-core  542  and the upper planar surface  700  of the I-core. 
     As shown in  FIGS. 19-23 , the bobbin  530  includes a first (lower) flange  800 . The first flange has a lowermost surface  802  and an uppermost surface  804 . The first flange supports a first pin rail  810  and a second pin rail  812 . Each pin rail has a plurality of pins  814  extending downward from the respective pin rail perpendicular to the first flange. The first pin rail includes a first lower channel wall  816  that extends from the lowermost surface of the first flange. The second pin rail includes a second lower channel wall  818  that also extends from the lowermost surface of the first flange. In the illustrated embodiment, each lower channel wall extends downward from the lowermost surface of the first flange by a height H 4 . The height is selected to be at least as great as the thickness T of the main body  650  of the transformer E-core  552 . The channel walls also serve as standoffs. When the magnetic assembly  500  is installed on a printed circuit board (not shown), only a lowermost portion of each pin is inserted into through-holes in the printed circuit board. Thus, an uppermost portion of each pin is available to receive wiring without interfering with the later installation of the magnetic assembly. 
     For the purposes of the following discussion the first pin rail  610  is located at the front of the bobbin  530  and extends from the left side to the right side of the bobbin as positioned in  FIGS. 18 and 20 . The second pin rail  612  is located at the rear of the bobbin and is parallel to the first pin rail. 
     A second flange  820  is spaced apart vertically from the first flange  800 . The second flange has a lowermost surface  822  that faces the first flange and has an uppermost surface  824  that faces away from the first flange (e.g., upward when the bobbin is positioned as shown in the drawings). A first winding portion  830  is defined between the uppermost surface  804  of the first flange and the lowermost surface of the second flange. In the illustrated embodiment, the first winding portion is defined by four rectangular outer surfaces  832 . Corresponding inner surfaces  834  define a first passage  840  ( FIG. 19 ) between the lowermost surface of the first flange and the uppermost surface of the second flange. The outer surfaces define the base of the first winding portion onto which the first layer of the transformer coil  550  is wound. In the illustrated embodiment, the first passage has a rectangular profile to match the profile of the center leg  656  of the transformer E-core  552  described above; however, in other embodiments, the passage may have a different profile (e.g., circular) to match a different profile of the center leg of the transformer E-core. In the illustrated embodiment, the first passage has a width slightly greater than the width W 3  of the center leg of the transformer E-core and has a length slightly greater than the length L of the transformer E-core such that the center leg of the transformer E-core fits snugly within the passage when inserted into the passage. 
     The first passage  840  has a height between the lowermost surface  802  of the first flange  800  to the uppermost surface  824  of the second flange  820  that is substantially the same as the height H 2  of the outer legs  652 ,  654  of the transformer E-core  552  such that when the inner surface  662  of the main body  660  of the transformer E-core is positioned against the lowermost surface of the first flange, the exposed ends of the outer legs are substantially coincident with the uppermost surface of the second flange. To assure a continuous magnetic path between the end surfaces  670 ,  680  of the first and second outer legs of the transformer E-core and the second (lower) planar surface  702  of the I-core  560 , the height of the passage may be slightly shorter than the height H 2  of the two outer legs so that at least a short length of each outer leg extends beyond the uppermost surface of the second flange. 
     A first spacer wall  850  ( FIGS. 20 and 21 ) extends perpendicularly upward from the uppermost surface  824  of second flange  820  along the front of the bobbin  530 . A second spacer wall  852  ( FIG. 21 ) also extends perpendicularly upward from the uppermost surface of the second flange along the rear of the bobbin. The first and second spacer walls are parallel to each other and are spaced apart from each other across the length of the first passage  840  such that respective inner surfaces of the spacer walls are coincident with the inner surfaces  834  that define the passage. In the illustrated embodiment, each spacer wall has a width approximately the same as the width of the first passage. Each spacer wall has a common spacer wall height. 
     The first spacer wall  850  and the second spacer wall  852  extend to a lowermost surface  862  of a third flange  860 . The third flange has an uppermost surface  864 . The sides of the bobbin between the first spacer wall and the second spacer wall are open to define a horizontally disposed I-core receiving passage (or slot)  866  ( FIG. 2 ) between the two spacer walls and between the second flange  840  and the third flange. The spacer wall height between the second flange and the third flange is slightly greater than the height H 3  of the I-core  560  such that the I-core receiving passage accommodates the combined height of the I-core and thickness of the gap spacer  570 . 
     A fourth flange  870  is spaced apart vertically from the third flange  860 . The fourth flange has a lowermost surface  872  that faces the third flange and has an uppermost surface  874  that faces away from the third flange (e.g., upward when the bobbin is positioned as shown in  FIG. 18 ). A second winding portion  880  is defined between the uppermost surface of the third flange and the lowermost surface of the fourth flange. In the illustrated embodiment, the second winding portion is defined by four rectangular outer surfaces  882  (one outer surface is shown in each of  FIGS. 20 and 21 ). Corresponding rectangular inner surfaces  884  define a second passage  890  between the lowermost surface of the third flange and the uppermost surface of the fourth flange. The outer surfaces define the base of the second winding portion onto which the first layer of the inductor coil  540  is wound. 
     The second passage  890  extends through the third flange  860  and the fourth flange  870  from the lowermost surface  862  of the third flange to the uppermost surface  874  of the fourth flange. The second passage is sized to receive the center leg  606  of the inductor E-core  542 . In the illustrated embodiment, the second passage has a length in the direction from the front of the bobbin  530  to the rear of the bobbin that is slightly greater than the length L of the center leg of the inductor E-core. The second passage has width in the direction from the left side to the right side of the bobbin that is slightly greater than the width W 3  of the center leg of the inductor E-core. If the center leg of the inductor E-core has a different cross section (e.g., a circular cross section), the second passage may be configured to have a corresponding different cross section. If, for example, the second passage has a circular cross section, the second winding surface may be configured to be cylindrical. 
     The second passage  890  has a height between the lowermost surface  862  of the third flange  860  to the uppermost surface  874  of the fourth flange  870  that is about the same as the height H 1  of the three legs  602 ,  604 ,  606  of the inductor E-core  542  such that when the inner surface  612  of the main body  600  of the inductor E-core is positioned against the uppermost surface of the fourth flange, the exposed end surfaces  620 ,  630 ,  640  of the three legs of the inductor E-core are substantially coincident with the lowermost surface of the third flange. The height of the passage may be slightly less than the height of the three legs to assure that the end surfaces of the three legs are positioned against the gap spacer  570  when the center leg  606  is fully inserted into the passage. 
     A first inductor core channel wall  900  and a second inductor core channel wall  902  extend perpendicularly from the uppermost surface  874  of the fourth flange  870 . In the illustrated embodiment, the first inductor core channel wall is aligned with the first spacer wall  850 . The second inductor core channel wall is aligned with the second spacer wall  852 . Accordingly, the two inductor core channel walls are mutually parallel with each other and are spaced apart across the length of the second passage  890  by a length generally corresponding to the common length L of the main body  600  and the legs  602 ,  604 ,  606  of the inductor E-core. In the illustrated embodiment, the two inductor core channels have a common height. As illustrated, the common height of the two inductor core channels may be approximately half the thickness T of the inductor core main body; however, the height may be varied. 
     Each of the third flange  860  and the fourth flange  870  includes a plurality of wiring slots  920  (e.g., four in each flange). Two of the wiring slots of each flange are positioned at the front of the bobbin  530 , and two of the wiring slots are positioned at the rear of the bobbin. The fourth flange further includes a plurality of wiring guide posts  922  (e.g., four posts) that extend vertically upward from the uppermost surface  874  of the fourth flange. One of the posts is positioned at each of the four corners of the fourth flange. The third flange further includes a plurality of wiring notches  924  (e.g., four notches) positioned near the four corners of the third flange. The second flange  820  also includes a plurality of wiring notches  926  (e.g., four notches) positioned near the four corners of the second flange. In the illustrated embodiment, the wiring notches of the second flange are in substantial alignment with the wiring notches of the third flange. 
     The first flange  800  includes a plurality of wiring notches  930  (e.g., six notches). Three of the wiring notches are positioned along the front of the bobbin  530 ; and three of the wiring notches are positioned along the rear of the bobbin. The three front wiring notches extend vertically from the uppermost surface  804  of the first flange through the first pin rail  810 . The three rear wiring notches extend vertically from the uppermost surface of the first flange through the second pin rail  812 . 
     The transformer coil  550  is wound around the first winding portion  830  in a conventional manner. The coil includes the primary and secondary windings of the transformer  520 . As shown in  FIG. 9 , terminal ends  950  of the windings of the transformer coil pass through selected notches  930  and are connected to selected pins  814  of the first (front) pin rail  810 , the second (rear) pin rail  812 , or both pin rails. In the illustrated example, four of the terminal ends are connected to pins in the second pin rail and two of the terminal ends are connected to pins in the first pin rail. 
     As further shown in  FIGS. 9 and 11 , the inductor coil  540  is wound around the second winding portion  880  in a conventional manner. In the illustrated embodiment, each of two terminal ends  960  of the winding in the inductor coil passes through a respective winding slot  920  of the fourth flanges  870 ; wraps around a respective winding post  922 ; passes through a respective wiring notch  924  of the third flange; passes through a respective wiring notch  926  of the second flange  820 ; passes through a respective wiring notch  930  of the first flange  800 ; and is connected to a respective pin  814  of the first pin rail  810 . Other wire routing configurations and pin selections can also be used. 
     The combined magnetic assembly  500  is completed by inserting the transformer E-core  552 , the inductor E-core  542 , the I-core  560  and the gap spacer  570  into the bobbin  530 . The I-core and the gap spacer are inserted into the I-core receiving passage  866  with the second (lower) planar surface  702  of the I-core facing downward toward the upper surface  824  of the second flange  820 . The gap spacer is positioned between the first (upper) planar surface  700  of the I-core and the lowermost surface  862  of the third flange  860 . 
     The center leg  656  of the transformer E-core  552  is inserted into the first passage  840  of the bobbin  530  from the bottom of the bobbin. The transformer E-core is fully inserted with the inner surface  662  of the main body  650  of the transformer E-core adjacent to the lowermost surface  822  of the first flange  800 . The end surfaces  670 ,  680  of the outer legs  652 ,  654  of the transformer E-core extend so that the end surfaces are at least flush with the uppermost surface  624  of the second flange  620  and may extend slightly beyond the uppermost surface to assure solid contact with second (lower) planar surface  702  of the I-core  570 . The main body  650  of the transformer E-core fits snugly between the inner surface of the channel wall  816  of the first pin rail  810  and the inner surface of the channel wall  818  of the second pin rail  812 . As described above, the heights of the pin rails with respect to the lowermost surface  802  of the first flange are selected to be at least as great as thickness T of the main body of the E-core such that the main body of the E-core does not extend below the channel walls. Thus, the mail body of the E-core does not interfere with the insertion of the pins  814  into a printed circuit board (not shown). 
     The center leg  606  of the inductor E-core  542  is inserted into the second passage  890  of the bobbin  530  from the top of the bobbin. The inductor E-core is fully inserted with the inner surface  612  of the main body  600  of the inductor E-core adjacent to the uppermost surface  874  of the fourth flange  870 . When fully inserted, the end surface  640  of the center leg and the end surfaces  620 ,  630  of the two outer legs  602 ,  604  of the inductor E-core are at least substantially flush with the lowermost surface  862  of the third flange  860 . The end surfaces may extend slightly beyond the lowermost surface to assure that the end surfaces contact the gap spacer  570  and are thus spaced apart from the I-core by no more than the gap distance provided by the gap spacer. The main body of the inductor E-core fits snugly between the inner surfaces of the first inductor core channel  900  and the second inductor core channel  902 . 
     The gap spacer  570  provides an equal gap between the first planar surface  700  of the I-core  560  and the end surfaces  620 ,  630 ,  640  of the three legs  602 ,  604 ,  606  of the inductor E-core  542 . The equal gap reduces the flux interference between the transformer  520  and the inductor  510 . The gap spacer also aids in forcing the second (lower) planar side  702  of the I-core against the end surfaces  670 ,  680  of the outer legs  652 ,  654  of the transformer E-core  552  so that no gap is formed between the outer legs of the transformer E-core and the I-core. As discussed above, the center leg  656  of the transformer E-core is shorter than the outer legs of the transformer E-core such that a single gap (G) is formed between the end surface  690  of the center leg of the transformer E-core and the second planar surface of the I-core. 
     The effect of the foregoing construction is illustrated in  FIG. 24 , which is a schematic representation of the front elevational view of the combined magnetic assembly  500 . The schematic representation shows a flux pattern  990  in the inductor E-core in dash-dot lines; shows a flux pattern  992  in the transformer E-core in dash-dot-dot lines; and shows a flux pattern  994  in the common I-core in dashed lines. As illustrated, the inductor and the transformer produce a shared flux pattern in the I-core. 
     As described above, the combined magnetic assembly  500  integrates the functions of the resonant inductor  510  and the output transformer  520  into a single assembly with a smaller printed circuit footprint than two separate devices. The resonant inductor and the output transformer are stacked to provide the reduced footprint. A single bobbin  530  is used for both the resonant inductor and the output transformer. The combined magnetic assembly shares the common I-core  560  positioned between the ends of the legs of the two E-cores  542 ,  552  within the I-core receiving passage  866  in the bobbin. The two E-cores and the common I-core form an “E-I-E” structure with the I-core providing a shared flux path for each of the E-cores. The combined magnetic assembly reduces the printed circuit board area needed for the two functions (resonant inductor and output transformer). By requiring only two E-cores and one shared I-core, the overall volume of core material is reduced. By using only a single bobbin for both functions, the overall costs of material are reduced. 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of the present invention of a new and useful “Output Transformer and Resonant Inductor in a Combined Magnetic Structure,” it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.