Patent Publication Number: US-2020286670-A1

Title: Transformers Having Integrated Magnetic Structures For Power Converters

Description:
FIELD 
     The present disclosure relates to transformers having integrated magnetic structures for power converters. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     Power converters convert electrical power between inputs and outputs. The power converters sometimes include multiple phases each having a transformer. In such examples, each transformer may include windings and its own core for the windings. In other examples, the windings of multiple transformers may be wound on inner legs of a magnetic core. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     According to one aspect of the present disclosure, a transformer for a multiphase interleaved power converter includes a magnetic structure, a first set of coils, and a second set of coils. The magnetic structure includes a top member, a bottom member, a plurality of legs extending between the top member and the bottom member, and a middle member positioned between the top member and the bottom. The plurality of legs include two outer legs, and the middle member extends between the two outer legs. The first set of coils is wound about the two outer legs of the magnetic structure and electrically coupled in series. The second set of coils is wound about the two outer legs of the magnetic structure and electrically coupled in series. 
     According to another aspect of the present disclosure, a transformer includes a magnetic structure and a set of coils. The magnetic structure includes a top member, a bottom member, and a plurality of legs extending between the top member and the bottom member. The plurality of legs include two outer legs. The set of coils is wound about the two outer legs of the magnetic structure and electrically coupled in series. 
     According to another aspect of the present disclosure, a multiphase interleaved power converter includes a plurality of phases and a transformer for the plurality of phases. The transformer includes a magnetic structure, a first set of coils, and a second set of coils. The magnetic structure includes a top member, a bottom member, a plurality of legs extending between the top member and the bottom member, and a middle member positioned between the top member and the bottom. The plurality of legs include two outer legs, and the middle member extends between the two outer legs. The first set of coils is wound about the two outer legs of the magnetic structure and electrically coupled in series. The second set of coils is wound about the two outer legs of the magnetic structure and electrically coupled in series. 
     Further aspects and areas of applicability will become apparent from the description provided herein. It should be understood that various aspects of this disclosure may be implemented individually or in combination with one or more other aspects. It should also be understood that the description and specific examples herein are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1A  is a transformer for a multiphase interleaved power converter including a magnetic structure having an E-I-E core configuration, and multiple sets of coils wound about outer legs of the magnetic structure, according to one example embodiment of the present disclosure. 
         FIG. 1B  is a transformer for a multiphase interleaved power converter including a magnetic structure similar to the magnetic structure of  FIG. 1A , but including air gaps, according to another example embodiment. 
         FIG. 2  is a two-phase interleaved LLC power converter where each phase includes primary windings coupled in series according to yet another example embodiment. 
         FIG. 3  is a circuit diagram of one phase of a multiphase interleaved LLC power converter including secondary windings coupled in series according to yet another example embodiment. 
         FIG. 4  is a graph showing voltages applied to two sets of primary windings in a two-phase interleaved power converter having a phase shift of ninety degrees according to another example embodiment. 
         FIG. 5  is a graph showing current flowing through the two sets of primary windings of  FIG. 4 . 
         FIG. 6  is a graph showing flux flowing in a portion of a magnetic structure in the two-phase interleaved power converter of  FIG. 4 . 
         FIG. 7-10  are block diagrams of the transformer of  FIG. 1C  with magnetic flux generated from the current of  FIG. 6 , according to other example embodiments. 
         FIG. 11A  is a transformer for a multiphase interleaved power converter including a magnetic structure having a U-I-U core configuration, and multiple sets of coils wound about outer legs of the magnetic structure, according to another example embodiment. 
         FIG. 11B  is a transformer for a multiphase interleaved power converter including a magnetic structure similar to the magnetic structure of  FIG. 11A , but including air gaps, according to another example embodiment. 
         FIG. 12-15  are block diagrams of the transformer of  FIG. 11B  with magnetic flux generated from the current of  FIG. 6 , according to other example embodiments. 
         FIG. 16  is a block diagram of a transformer for a multiphase interleaved power converter including a magnetic structure having an E-E-I-E-E core configuration, according to another example embodiment. 
         FIG. 17  is an isometric view of the transformer of  FIG. 16  according to yet another example embodiment. 
         FIG. 18A  is a transformer for a single-phase power converter including a magnetic structure having an E-E core configuration and a set of coils wound about outer legs of the magnetic structure, according to another example embodiment. 
         FIG. 18B  is a transformer for a single-phase power converter including a magnetic structure and a set of coils wound about outer legs of the magnetic structure, according to another example embodiment. 
     
    
    
     Corresponding reference numerals indicate corresponding parts and/or features throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     A transformer for a multiphase interleaved power converter according to one example embodiment of the present disclosure is illustrated in  FIG. 1A  and indicated generally by reference number  100 . As shown in  FIG. 1A , the transformer  100  includes a magnetic structure  102  and coils  104 ,  106 ,  108 ,  110 . The magnetic structure  102  includes a top member  112 , a bottom member  114 , legs extending between the top member  112  and the bottom member  114 , and a middle member  116  positioned between the top member  112  and the bottom member  114 . The legs include two outer legs  118 ,  120 , and the middle member  116  extends between the outer legs  118 ,  120 . As shown in  FIG. 1A , the coils  104 ,  106 ,  108 ,  110  are wound about the outer legs  118 ,  120 . 
     As shown in  FIG. 1A , the coils  104 ,  106  are electrically coupled in series and form one set of coils, and the coils  108 ,  110  are electrically coupled in series and form another set of coils. As such, the same amount of current (e.g., substantially the same amount of current due to losses) flows through the series-coupled coils  104 ,  106 , and the same amount of current (e.g., substantially the same amount of current due to losses) flows through the series-coupled coils  108 ,  110 . 
     In the particular example of  FIG. 1A , the magnetic structure (e.g., a magnetic core)  102  includes a center leg  122  positioned between the two outer legs  118 ,  120 . The center leg  122  may extend between the top member  112  and the bottom member  114 . In some embodiments, the center leg  122  is optional and therefore may be removed, as further explained below. 
     When the magnetic structure  102  includes the center leg  122 , magnet flux (not shown) generated from the coils  104 ,  106 ,  108 ,  110  may be cancelled. For example, when the coils  104 ,  106  are energized, magnetic flux generated from the coils  104 ,  106  flows in opposite directions in the center leg  122 . As such, when current flowing through the coils  104 ,  106  is the same, the generated magnetic flux cancels (e.g. completely cancels) in the center leg  122  as explained herein. The same magnetic flux cancellation is true with respect to the coils  108 ,  110  and the center leg  122 . 
     By cancelling magnetic flux in the center leg  122 , the size of the magnetic structure  102  may be reduced as compared to conventional magnetic structure(s) employed in multiphase power converters. In some examples, the volume of the center leg  122  may be neglected in core configuration design and/or reduced in size due to the cancelling magnetic flux. Because of the reduced size of the magnetic structure  102 , the power density of a multiphase power converter employing the transformer  100  may be increased and core losses in the multiphase power converter may be decreased as compared to conventional converters. This reduction of core losses may lead to higher efficiency. 
     In some cases, if complete cancellation of magnetic flux is achieved such that no magnetic flux travels through the center leg  122 , the center leg  122  may magnetically decouple portions of the outer legs  118 ,  120  of the magnetic structure  102 . In such examples, one portion of the outer leg  118 , one portion of the outer leg  120 , and the coils  104 ,  106  wound about these portions in  FIG. 1A  may be used for one phase of a multiphase power converter. Likewise, another portion of the outer leg  118 , another portion of the outer leg  120 , and the coils  108 ,  110  may be used for another phase of the multiphase power converter. 
     As explained above, the transformer  100  includes the coils  104 ,  106 ,  108 ,  110  wound about the outer legs  118 ,  120 . In the particular example of  FIG. 1A , the coils  104 ,  106 ,  108 ,  110  are wound about only the outer legs  118 ,  120 . As such, the transformer  100  includes no coils wound about the center leg  122  or the middle member  116 . 
     In the particular example of  FIG. 1A , the magnetic structure  102  has an E-I-E core configuration. For example, the top member  112  and portions of the outer legs  118 ,  120  may form one E-shaped core, and the bottom member  114  and portions of the outer legs  118 ,  120  may form another E-shaped core. The middle member  116  and/or portions of the outer legs  118 ,  120  and center leg  122  may form an I-shaped core. 
     In some embodiments, the E-I-E magnetic structure  102  may have a monolithic construction as shown in  FIG. 1A . Alternatively, the E-I-E magnetic structure  102  may be formed with multiple members, legs, etc. mechanically coupled to one another (e.g., via adhesives or other suitable means). 
     Although not shown in  FIG. 1A , the magnetic structure  102  may optionally include one or more gaps to physically separate portions of the magnetic structure  102 . The gaps may be formed of air and/or another suitable non-magnetic material. The gaps may assist in generating a magnetizing inductance in a multiphase power converter as further explained below. 
     For example,  FIG. 1B  illustrates a transformer  100 B substantially similar to the transformer  100  of  FIG. 1A , but including air gaps to physically separate portions of its magnetic structure  102 B. For instance, each leg  118 ,  120 ,  122  of  FIG. 1A  may be formed of two or more portions (e.g., legs  118 A,  118 B,  120 A,  120 B,  122 A,  122 B of  FIG. 1B ) to assist in creating the air gaps. Specifically, the magnetic structure  102 B includes the members  112 ,  114  of  FIG. 1A , outer legs  118 A,  118 B,  120 A,  120 B extending between the members  112 ,  114 , center legs  122 A,  122 B extending between the members  112 ,  114 , and a middle member  116 B positioned between the members  112 ,  114 . As shown in  FIG. 1B , the transformer  100 B includes an air gap between each outer leg  118 A,  118 B,  120 A,  120 B and the middle member  116 B. Placing the air gaps between the outer legs and the middle member  116 B maintains the reluctance of the center legs  122 A,  122 B at a low value. Alternatively, the air gaps may be positioned between the center legs  122 A,  122 B and the middle member  116 B if desired. 
     As shown in  FIG. 1B , the transformer  100 B includes the series-coupled coils  104 ,  106 , and the series-coupled coils  108 ,  110  of  FIG. 1A . In the example embodiment of  FIG. 1B , the coils  104 ,  106  are wound about the outer legs  118 A,  120 A, respectively, and the coils  108 ,  110  are wound about the outer legs  118 B,  120 B, respectively. As shown, no coils are wound about the center legs  122 A,  122 B or the middle member  116 B. 
     The transformers disclosed herein may be employed in power converters such as single-phase power converters or multiphase interleaved power converters. In such examples, the multiphase interleaved power converters may include multiphase interleaved resonant power converters. For instance, the series-coupled coils  104 ,  106  of  FIGS. 1A and 1B  may represent transformer windings in one phase of the multiphase interleaved resonant power converter, and the series-coupled coils  108 ,  110  of  FIGS. 1A and 1B  may represent transformer windings in another phase of the multiphase interleaved resonant power converter. 
     For example,  FIG. 2  illustrates a two-phase interleaved LLC power converter  200  including the transformer  100 B. Although  FIG. 2  illustrates the two-phase interleaved LLC power converter  200 , other suitable multiphase interleaved power converters may employ the teachings disclosed herein without departing from the scope of the disclosure. 
     As shown in  FIG. 2 , the power converter  200  includes an input for receiving an input voltage Vin+, Vin−, an output for providing an output voltage Vo+, Vo−, and two phases  202 ,  204  coupled between the input and the output. The phases  202 ,  204  electrically coupled in parallel. 
     In the particular example of  FIG. 2 , the phases  202 ,  204  include the same circuit configuration. For example, the phase  202  includes two series-coupled primary windings P 1 , P 2 , two secondary windings S 1 , S 2 , a switching circuit  206 A coupled between the primary windings P 1 , P 2  and the input, and two switching circuits  208 A,  210 A coupled between the secondary windings S 1 , S 2  and the output. Likewise, the phase  204  includes two series-coupled primary windings P 3 , P 4 , two secondary windings S 3 , S 4 , a switching circuit  206 B coupled between the primary windings P 3 , P 4  and the input, and two switching circuits  208 B,  210 B coupled between the secondary windings S 3 , S 4  and the output. As shown, the primary side switching circuits  206 A,  206 B include two MOSFET power switches Q 1 , Q 2 , Q 3 , Q 4  arranged in a half-bridge configuration, and the secondary side switching circuits  208 A,  208 B,  210 A,  210 B are rectifying circuits. 
     In the particular example of  FIG. 2 , the multiphase interleaved LLC power converter  200  includes the transformer  100 B. As such, the coils  104 ,  106  of  FIG. 1B  may be equivalent to the primary windings P 1 , P 2 , respectively, and the coils  108 ,  110  of  FIG. 1B  may be equivalent to the primary windings P 3 , P 4 , respectively. In such examples, the magnetic structure  102 B of  FIG. 1B  may be shared between the phases  202 ,  204 . Therefore, in the particular example of  FIG. 2 , the magnetic structure, the primary windings P 1 , P 2 , P 3 , P 4  and the secondary windings S 1 , S 2 , S 3 , S 4  form the transformer 
     As shown in  FIG. 2 , the power converter  200  includes various resonant components. For example, and as shown in  FIG. 2 , the phases  202 ,  204  includes capacitors C 1 , C 2 , C 4 , C 6  coupled to the switching circuits  206 A,  206 B, inductors L 1 , L 2  coupled between the capacitors C 1 , C 2 , C 4 , C 6  and the primary windings P 1 , P 2 , P 3 , P 4 , and output filter capacitors C 3 , C 5  coupled across the outputs of the secondary side switching circuits. The capacitors C 1 , C 2 , the inductor L 1  and a magnetizing inductance generated by the transformer form an LLC resonant tank circuit in the phase  202 . Additionally, the capacitors C 4 , C 6 , the inductor L 2  and the magnetizing inductance generated by the transformer form another LLC resonant tank circuit in the phase  204 . 
     The secondary windings S 1 , S 2 , S 3 , S 4  of each phase  202 ,  204  may be coupled in parallel. For example, in the particular example of  FIG. 2 , the secondary windings S 1 , S 2  are coupled in parallel via the switching circuits  208 A,  210 A. Specifically, the secondary windings S 1 , S 2  are coupled separately to inputs of the switching circuits  208 A,  210 A, respectively. Outputs of the switching circuits  208 A,  210 A are coupled in parallel. In other examples, the secondary windings S 1 , S 2  may be coupled directly in parallel. The secondary windings S 3 , S 4  of the phase  204  may have a similar configuration with its switching circuits  208 B,  210 B. 
     As shown in  FIG. 2 , each set of the primary windings P 1 , P 2 , P 3 , P 4  are coupled in series. For example, one end of the primary winding P 1  is coupled between the power switches Q 1 , Q 2  and the other end of the primary winding P 1  is coupled to one end of the primary winding P 2 . The other end of the primary winding P 2  is coupled between the capacitors C 1 , C 2  via the resonant inductor L 1 . The secondary windings P 3 , P 4  of the phase  204  may have a similar configuration with its switching circuit  206 A, capacitors C 4 , C 6 , and resonant inductor L 2 . The series-coupled primary windings P 1 , P 2 , P 3 , P 4  may assist in current sharing in the parallel-coupled connected secondary windings S 1 , S 2 , S 3 , S 4 . 
     Additionally, because each set of the primary windings are coupled in series, the same amount of current passes through the primary windings P 1 , P 2  and the same amount of current passes through the primary windings P 3 , P 4 . As such, the same amount of magnetic flux is generated in the magnetic structure from the current passing through the primary windings. In such examples, if the magnetic structure includes a center leg (e.g.,  122 ,  122 A,  122 B of  FIGS. 1A and 1B ), complete magnetic flux cancellation may be achieved in the center leg as further explained below. 
     In other embodiments, one or both sets of series-coupled coils may represent secondary windings. For example,  FIG. 3  illustrates one phase  300  of a multiphase interleaved LLC power converter in which a set of coils represent two series-coupled secondary transformer windings. 
     As shown in  FIG. 3 , the phase  300  includes an input for receiving an input voltage Vin+, Vin−, an output for providing an output voltage Vo+, Vo−, two primary windings P 1 , P 2 , two series-coupled secondary windings S 1 , S 2 , and switching circuits  304 ,  306 ,  308 . The switching circuit  304  is coupled between the primary winding P 1  and the input, and the switching circuit  306  is coupled between the primary winding P 2  and the input. As shown, the switching circuits  304 ,  306  receive the same input voltage Vin+, Vin−. As such, inputs of the switching circuits  304 ,  306  are coupled in parallel. The switching circuit  308  (e.g., a rectifying circuit) is coupled between the secondary windings S 1 , S 2  and the output. 
     In the particular example of  FIG. 3 , the primary side switching circuits  304 ,  306  have half-bridge topographies. For example, the switching circuit  304  includes two MOSFET power switches Q 1 , Q 2  arranged in a half bridge configuration, and the switching circuit  306  includes two MOSFET power switches Q 3 , Q 4  arranged in a half bridge configuration. 
     In the example of  FIG. 3 , the multiphase interleaved LLC power converter includes a magnetic structure that is shared between the phase  300  and at least one additional phase (not shown) of the multiphase interleaved LLC power converter. The magnetic structure, the primary and secondary windings P 1 , P 2 , S 1 , S 2  of  FIG. 3 , and primary and secondary windings of the other phase may form a transformer as explained above. In such examples, the primary windings P 1 , P 2  and the secondary windings S 1 , S 2  of  FIG. 3  may be wound on one or more portions of the magnetic structure as explained above. 
     Like the phases  202 ,  204  of  FIG. 2 , the phase  300  of  FIG. 3  may include various resonant components. For example, and as shown in  FIG. 3 , the phase  300  includes capacitors C 1 , C 2  coupled to the switching circuit  304 , capacitors C 3 , C 4  coupled to the switching circuit  306 , an inductor L 1  coupled between the capacitors C 1 , C 2  and the primary winding P 1 , another inductor L 2  coupled between the capacitors C 3 , C 4  and the primary winding P 2 , and an output filter capacitor C 5  coupled across the output. The capacitors C 1 , C 2 , the inductor L 1  and a magnetizing inductance generated by the transformer form one LLC resonant tank circuit. Additionally, the capacitors C 3 , C 4 , the inductor L 2  and the magnetizing inductance generated by the transformer form another LLC resonant tank circuit. 
     The primary windings P 1 , P 2  of  FIG. 3  may be coupled in parallel. For example, the primary windings P 1 , P 2  may be coupled in parallel via the switching circuits  304 ,  306  as shown in  FIG. 3 . For instance, the primary windings P 1 , P 2  are coupled to the switching circuits  304 ,  306 , respectively, and the inputs of the switching circuits  304 ,  306  are coupled in parallel. In other examples, the primary windings P 1 , P 2  may be coupled directly in parallel. 
     As explained above, the secondary windings S 1 , S 2  are coupled in series. For example, one end of the secondary winding S 1  is coupled to the switching circuit  308  and the other end of the secondary winding S 1  is coupled to one end of the secondary winding S 2 . The other end of the secondary winding S 2  is coupled to the switching circuit  308 . As such, the same amount of current passes through each secondary winding S 1 , S 2 . The series-coupled secondary windings S 1 , S 2  may assist in current sharing of the parallel-connected primary windings P 1 , P 2 , and cause complete magnetic flux cancellation as further explained herein. 
     The interleaved power converters disclosed herein may experience a phase shift between each phase. In some examples, one phase may operate with a ninety (90) degree phase shift, a one hundred eighty (180) degree phase shift, etc. relative to another phase. For example,  FIG. 4  illustrate waveforms  400 ,  402  of phase voltages a phase shift of ninety (90) degree therebetween over time T 0  through time T 4  (e.g., one period (T)).  FIG. 5  illustrates waveforms  500 ,  502  of currents corresponding to the phase voltages of  FIG. 4 . Specifically, the waveform  400  of  FIG. 4  represents a voltage applied to a set of series-coupled primary windings (e.g., the coils  104 ,  106  of  FIG. 1B ) in one phase (Phase 1), and the waveform  402  of  FIG. 4  represents a voltage applied to another set of series-coupled primary windings (e.g., the coils  108 ,  110  of  FIG. 1B ) in another phase (Phase 2). The waveform  500  of  FIG. 5  represents a current flowing through the series-coupled primary windings (e.g., the coils  104 ,  106  of  FIG. 1B ) in Phase 1, and the waveform  502  of  FIG. 5  represents a current flowing through the series-coupled primary windings (e.g., the coils  108 ,  110  of  FIG. 1B ) in Phase 2.  FIG. 6  illustrates a waveform  600  of generated flux when the series-coupled primary windings are energized with the current waveforms  500 ,  502  of  FIG. 5 . 
     This current flow through the primary windings generates magnetic flux in the magnetic structure as explained above. For example,  FIGS. 7-10  illustrate the transformer  100 B of  FIG. 1B  in which the coils  104 ,  106 ,  108 ,  110  are energized thereby generating magnetic flux in the magnetic structure  102 B based on the current flow shown in  FIG. 5 . Specifically,  FIG. 7  illustrates the generated magnetic flux from time T 0  to time T 1 ,  FIG. 8  illustrates the generated magnetic flux from time T 1  to time T 2 ,  FIG. 9  illustrates the generated magnetic flux from time T 2  to time T 3 , and  FIG. 10  illustrates the generated magnetic flux from time T 3  to time T 4 .  FIG. 6  illustrates the generated flux in the middle member  116 B of the transformer  100 B. 
     With reference to  FIGS. 5 and 7 , the current flowing through the coils  104 ,  106  (Phase 1) is positive (+Im) between times T 0 , T 1 , and the current flowing through the coils  108 ,  110  (Phase 2) is negative (−Im) between times T 0 , T 1 . As a result, magnetic flux in the center legs  122 A,  122 B is cancelled, and magnetic flux in the middle member  116 B ramps up as shown in  FIG. 6 . 
     With reference to  FIGS. 5 and 8 , the current flowing through the coils  104 ,  106 ,  108 ,  110  (Phases 1, 2) is positive (+Im) between times T 1 , T 2 . This causes magnetic flux in the center legs  122 A,  122 B to cancel. Additionally, and as shown in  FIGS. 5, 6 and 8 , magnetic flux in the middle member  116 B adds together and remains substantially constant because the current waveform  500  is ramping down and the current waveform  502  is ramping up. 
     With reference to  FIGS. 5 and 9 , the current flowing through the coils  104 ,  106  (Phase 1) is negative (−Im), and the current flowing through the coils  108 ,  110  (Phase 2) is positive (+Im) between times T 2 , T 3 . As a result, magnetic flux in the center legs  122 A,  122 B is cancelled, and magnetic flux in the middle member  116 B ramps down as shown in  FIG. 6 . 
     With reference to  FIGS. 5 and 10 , the current flowing through the coils  104 ,  106 ,  108 ,  110  (Phases 1, 2) is negative (−Im) between times T 3 , T 4 . This causes magnetic flux in the center legs  122 A,  122 B to cancel. Additionally, and as shown in  FIGS. 5, 6 and 10 , magnetic flux in the middle member  116 B adds together and remains substantially constant because the current waveform  500  is ramping up and the current waveform  502  is ramping down. 
     In some examples, the center legs  122 ,  122 A,  122 B of  FIGS. 1A, 1B and 7-10  may be removed if magnetic flux in the leg(s) is completely cancelled as explained above. For example,  FIG. 11A  illustrates a transformer  1100  substantially is similar to the transformer  100  of  FIG. 1A , but where its magnetic structure  1102  includes no center leg. Specifically, the magnetic structure  1102  of  FIG. 1A  includes the top member  112 , the bottom member  114 , the outer legs  118 ,  120 , and the middle member  116  of  FIG. 1A . As shown in  FIG. 11A , the magnetic structure  1102  does not include a center leg positioned between the outer legs  118 ,  120 . 
     In the particular example of  FIG. 11A , the magnetic structure  1102  has a monolithic U-I-U core configuration. For example, the top member  112  and portions of the outer legs  118 ,  120  may form one U-shaped core, and the bottom member  114  and portions of the outer legs  118 ,  120  may form another U-shaped core. Additionally, the middle member  116  and/or portions of the outer legs  118 ,  120  may form an I-shaped core as explained above. 
     As shown, the transformer  1100  of  FIG. 11A  further includes the coils  104 ,  106 ,  108 ,  110  of  FIG. 1A  wound about the outer legs  118 ,  120  as explained above. In such examples, when the coils  104 ,  106  are energized, magnetic flux travels through the bottom member  114 , a portion of the outer leg  118 , the middle member  116 , and a portion of the outer leg  120 . Additionally, when the coils  108 ,  110  are energized, magnetic flux travels through the top member  112 , a portion of the outer leg  118 , the middle member  116 , and a portion of the outer leg  120 . 
     Although not shown in  FIG. 11A , the magnetic structure  1102  may optionally include one or more gaps (e.g., air gaps, etc.) to physically separate portions of the magnetic structure  1102 . For example,  FIG. 11B  illustrates a transformer  1100 B substantially similar to the transformers  100 B,  1100  of  FIGS. 1B and 11A , but having air gaps to physically separate portions of its magnetic structure  1102 B and no center leg. Specifically, the transformer  1100 B includes the members  112 ,  114 ,  116 B and the outer legs  118 A,  118 B,  120 A,  120 B of  FIG. 1B . As shown in  FIG. 11B , the transformer  1100 B includes an air gap between each outer leg  118 A,  118 B,  120 A,  120 B and the middle member  116 B. 
     As explained above, current flow through the coils  104 ,  106 ,  108 ,  110  generates magnetic flux in the magnetic structure. For example,  FIGS. 12-15  illustrate the transformer  110 B of  FIG. 11B  in which the coils  104 ,  106 ,  108 ,  110  are energized thereby generating magnetic flux in the magnetic structure  1102 B based on the current flow show in  FIG. 5 . Specifically,  FIG. 12  illustrates the generated magnetic flux from time T 0  to time T 1 ,  FIG. 13  illustrates the generated magnetic flux from time T 1  to time T 2 ,  FIG. 14  illustrates the generated magnetic flux from time T 2  to time T 3 , and  FIG. 15  illustrates the generated magnetic flux from time T 3  to time T 4 . 
     With reference to  FIGS. 5 and 12 , the current flowing through the coils  104 ,  106  (Phase 1) is positive (+Im) between times T 0 , T 1 , and the current flowing through the coils  108 ,  110  (Phase 2) is negative (−Im) between times T 0 , T 1 . As a result, magnetic flux in the middle member  116 B ramps up as shown in  FIG. 6 . 
     The current flowing through the coils  104 ,  106 ,  108 ,  110  (Phases 1, 2) is positive (+Im) between times T 1 , T 2 . This causes magnetic flux in the middle member  116 B to add together and remain substantially constant because the current waveform  500  is ramping down and the current waveform  502  is ramping up, as shown in  FIGS. 5, 6 and 13 . 
     With reference to  FIGS. 5 and 14 , the current flowing through the coils  104 ,  106  (Phase 1) is negative (−Im), and the current flowing through the coils  108 ,  110  (Phase 2) is positive (+Im) between times T 2 , T 3 . As a result, magnetic flux in the middle member  116 B ramps down as shown in  FIG. 6 . 
     The current flowing through the coils  104 ,  106 ,  108 ,  110  (Phases 1, 2) is negative (−Im) between times T 3 , T 4 . This causes magnetic in the middle member  116 B to add together and remain substantially constant because the current waveform  500  is ramping up and the current waveform  502  is ramping down, as shown in  FIGS. 5, 6 and 15 . 
     In some examples, the magnetic structures disclosed herein may include members, legs, etc. for an inductor coil. For example,  FIG. 16  illustrates a transformer  1600  similar to the transformer  100 B of  FIG. 1B , but including additional members, legs, etc. and coils for two resonant chokes. As shown in  FIG. 16 , the transformer  1600  includes a magnetic structure  1602  auxiliary members  1630 ,  1632 , and auxiliary legs  1634 ,  1636 ,  1638 ,  1640 ,  1642 ,  1644  extending between the auxiliary members  1630 ,  1632  and the members  114 ,  112  of  FIG. 1B . The auxiliary legs  1634 ,  1636 ,  1638 ,  1640  are outer legs, and the auxiliary legs  1642 ,  1644  are center legs. The magnetic structure  1602  of  FIG. 16  forms an E-E-I-E-E core configuration. 
     The transformer  1600  further includes coils  1646 ,  1648 ,  1650 ,  1652  wound about the outer auxiliary legs  1634 ,  1636 ,  1638 ,  1640 . The coils  1646 ,  1648 ,  1650 ,  1652  represent two resonant chokes. Specifically, the coils  1646 ,  1648  represent one resonant choke in one phase, and the coils  1650 ,  1652  represent another resonant choke in another phase. 
     As shown in  FIG. 16 , the coil  1646  is coupled to the coil  104 , the coil  1648  is coupled to the coil  106 , the coil  1650  is coupled to the coil  108 , and the coil  1652  is coupled to the coil  110 . With this configuration, the coils  1646 ,  104 ,  106 ,  1648  are electrically coupled in series, and the coils  1650 ,  108 ,  110 ,  1652  are electrically coupled in series. This ensures the same amount of current passes through the coils  1646 ,  104 ,  106 ,  1648 , and the same amount of current passes through the coils  1650 ,  108 ,  110 ,  1652 . As a result, magnetic flux in the center auxiliary legs  1642 ,  1644  cancels as explained above. Additionally, magnetic flux may cancel in portions of the members  112 ,  114  as shown in  FIG. 16 . 
       FIG. 17  illustrates a transformer  1700  including the magnetic core  1602  and the coils  104 ,  106 ,  108 ,  110 ,  1646 ,  1648 ,  1650 ,  1652  of  FIG. 16 . Additionally, the transformer  1700  includes coils  1750 ,  1752 ,  1754 ,  1756  wound about similar outer legs as the coils  104 ,  106 ,  108 ,  110 . In the particular example of  FIG. 17 , the coils  104 ,  106 ,  108 ,  110  represent two sets of primary windings for two different phases in a two-phase interleaved LLC power converter, the coils  1750 ,  1752 ,  1754 ,  1756  represent two sets of secondary windings for the different phases, and the coils  1646 ,  1648 ,  1650 ,  1652  represent two resonant chokes for the different phases. 
     In the particular example of  FIG. 17 , the coils are plate windings. In other examples, other suitable types of coils may be employed such as conductive wire, etc. 
     In some examples, the teachings disclosed herein may be applied to single-phase power converters as explained above. For example,  FIG. 18A  illustrates a transformer  1800 A for a single-phase power converter. As shown in  FIG. 18A , the transformer  1800 A includes a magnetic structure  1802  and coils  1804 ,  1806  electrically coupled in series. The magnetic structure  1802  includes a top member  1812 , a bottom member  1814 , and legs extending between the top member  1812  and the bottom member  1814 . The legs include two outer legs  1818 ,  1820  and a center leg  1822 . In the some embodiments, the top member  1812 , the bottom member  1814 , and the legs  1818 ,  1820 ,  1822  may form an E-E core configuration. 
     As shown in  FIG. 18A , the coils  1804 ,  1806  wound about the outer legs  1818 ,  1820 . No coils are wound on the center leg  1822 . 
     When the coils  1804 ,  1806  are energized, magnetic flux is generated in the magnetic structure  1802 . Specifically, magnetic flux generated by the coil  1804  circulates in portions of the members  1812 ,  1814 , the outer leg  1818  and the center leg  1822 . Additionally, magnetic flux generated by the coil  1806  circulates in portions of the members  1812 ,  1814 , the outer leg  1820  and the center leg  1822 . 
     As shown in  FIG. 18A , the magnetic flux from the coils  1804 ,  1806  may cancel in the center leg  1822 . For example, the magnetic flux from the coil  1804  flows in the opposite direction as the magnetic flux from the coil  1806  in the center leg  1822  causing the fluxes to cancel out. 
     In some embodiments, the magnetic flux in the center leg  1822  may completely cancel out due to, for example, the series connection between the coils  1804 ,  1806  as explained herein. In such examples, the center leg  1822  may be removed. For example,  FIG. 18B  illustrates a transformer  1800 B substantially similar to the transformer  1800 A of  FIG. 18A , but including a magnetic structure  1802 B not having a center leg between its members  1812 ,  1814 . In such examples, magnetic flux generated in the magnetic structure  1802 B circulates in the members  1812 ,  1814  and the outer legs  1818 ,  1820 , as shown by the dashed line in  FIG. 18B . 
     The secondary side switching circuits disclosed herein may include any suitable rectification component and/or topography. For example, the rectifying circuits may include diode rectifying circuits as shown in  FIGS. 3 and 4 . The diode rectifying circuits each may be a full-wave bridge rectifying circuit, a half-wave bridge rectifying circuits or another suitable rectifying circuit. In other examples, the rectifying circuits may include power switches (e.g. MOSFETs, etc.) in addition to and/or alternative to diodes. 
     Additionally, the primary side switching circuits may include one or more switching devices arranged in any suitable topography. For example, the primary side switching circuits may include two power switches (MOSFETs) arranged in a half-bridge topography as shown in  FIGS. 3 and 4 . In other examples, the switching circuits may include another suitable switching device arranged in a half-bridge topography, a full-bridge topography, etc. Further, the switching devices may include MOSFETs other suitable transistors, diodes, etc. In some examples, gallium nitride (GaN) semiconductor switching devices may be employed. 
     As explained above, the magnetic structures disclosed herein achieve cancellation of magnetic flux in portions of the structures. As a result, power converters such as single-phase power converters or multiphase phase power converters (e.g., two-phase interleave power converters) employing the magnetic structures may have a reduced core volume as compared to conventional power converters. In turn, core losses may be less and efficiency in the power converters may be higher as compared to conventional power converters. For example, testing has shown that some two-phase interleaved power converters may reach an efficiency of about 99% when the magnetic structures disclosed herein are employed. Additionally, costs associated with the two-phase interleaved power converters is less than conventional power converters. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.