Patent Publication Number: US-8988182-B2

Title: Transformers and methods for constructing transformers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/465,632 filed Mar. 22, 2011, the entire disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     This disclosure generally relates to transformers and, more specifically, methods for constructing transformers having a relatively low leakage inductance. 
     BACKGROUND 
     Solar panels, also referred to herein as photovoltaic (PV) modules, generally output direct current (DC) electrical power. To properly couple such solar panels to an electrical grid, or otherwise provide alternating current (AC) power, the electrical power received from the solar panels is converted from DC to AC power. At least some known solar power systems use a single stage or a two-stage power converter to convert DC power to AC power. Some systems are controlled by a control system to maximize the power received from the solar panels and to convert the received DC power into AC power that complies with utility grid requirements. 
     However, at least some known solar power converters are relatively inefficient and/or unreliable. It is desirable for a solar power converter to operate at relatively high efficiency to capture as much energy from a PV module as possible. At least some solar power converters utilize an isolated DC/DC converter including a transformer. One of the loss factors in such converters is the energy loss associated with the leakage inductance of the converter&#39;s transformer. In some converters, the losses are proportional to the leakage inductance of the transformer. A greater leakage inductance leads to greater losses and, accordingly, to a lower total conversion efficiency. Some known designs attempt to recover the energy stored in the leakage inductance. These recovery mechanisms, however, are generally not satisfactory. 
     This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     BRIEF SUMMARY 
     One aspect of the present disclosure is a method of constructing a transformer. The method includes wrapping a first primary winding around a core, wrapping a secondary winding around the core, and wrapping a second primary winding around the core. The first primary winding traverses substantially an entire circumference of the core in a first circumferential direction. The secondary winding includes a first half and a second half. The first half traverses substantially the entire circumference of the core in the first circumferential direction, and the second half traverses substantially the entire circumference of the core in a second circumferential direction opposite the first circumferential direction. The second primary winding traverses substantially the entire circumference of the core in the second circumferential direction. 
     Another aspect of the present disclosure is a transformer. The transformer includes a core having an outer circumference, a first primary winding including a plurality of turns wound on the core, a secondary winding including a first portion and a second portion, and a second primary winding including a plurality of turns wound on the core. The plurality of turns of the first primary winding traverse in a first circumferential direction around the core. The first portion of the secondary winding includes a plurality of turns traversing in the first circumferential direction around the core. The second portion of the secondary winding includes a plurality of turns traversing in the second circumferential direction around the core. The plurality of turns of the second primary winding traverse in the second circumferential direction around the core. 
     Yet another aspect of the present disclosure is a power conversion system. The system includes a power converter configured to convert an input power to an output power. The power converter includes a controller, at least one switch, and a transformer. The transformer includes a core, a first primary winding, a secondary winding, and a second primary winding. The first primary winding includes a plurality of turns wound on the core. The plurality of turns traverses in a first circumferential direction around the core. The secondary winding includes a first portion and a second portion. The first portion includes a plurality of turns traversing in the first circumferential direction around the core. The second portion includes a plurality of turns traversing in a second circumferential direction around the core. The second primary winding includes a plurality of turns wound on the core. The plurality of turns traverses in the second circumferential direction around the core. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an embodiment of a power conversion system. 
         FIG. 2  is a schematic diagram of a converter for use in the system shown in  FIG. 1 . 
         FIG. 3  is a schematic diagram of a transformer for use in the converter shown in  FIG. 2 . 
         FIGS. 4-9  illustrate construction of the transformer for use in the converter shown in  FIG. 2 . 
         FIG. 10  shows a magnetic core with a single conductor defining one turn in air. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     The embodiments described herein generally relate to transformer. More specifically embodiments described herein relate to methods for constructing transformers having relatively low leakage inductance. Moreover, some embodiments described herein relate to transformers and methods of constructing transformers for power converters for use with a photovoltaic (PV) power source. 
     Although described herein with reference to power converters in general and for use with a PV source, the teachings of this disclosure may be utilized to construct relatively low leakage inductance transformers for any suitable use. 
       FIG. 1  is a schematic block diagram of this power conversion system  100 . A power source  102  is coupled to power conversion system  100  to supply electrical current to system  100 . In this embodiment, power source  102  is a photovoltaic or “solar” array that includes at least one photovoltaic panel. Alternatively or additionally, power source  102  includes at least one fuel cell, a direct current (DC) generator, and/or any other electric power source that enables power conversion system  100  to function as described herein. 
     In this embodiment, power conversion system  100  includes a power converter  104  to convert DC power received from power source  102 , via an input capacitor  105 , to an alternating current (AC) output. In other embodiments, power converter  104  may output DC power. This power converter  104  is a two stage power converter including a first stage  106  and a second stage  108 . First stage  106  is a DC to DC power converter that receives a DC power input from power source  102  and outputs DC power to second stage  108 . Second stage  108  is a DC to AC power converter (sometimes referred to as an inverter) that converts DC power received from first stage  106  to an AC power output. In other embodiments, power converter  104  may include more or fewer stages. More particularly, in some embodiments power converter  104  includes only second stage  108 . 
     Power conversion system  100  also includes a filter  110 , and a control system  112  that controls the operation of first stage  106  and second stage  108 . An output  114  of power converter  104  is coupled to filter  110 . In this embodiment, filter  110  is coupled to an electrical distribution network  116 , such as a power grid of a utility company. Accordingly, power converter  104  may be referred to as a grid tied inverter. In other embodiments, power converter  104  may be coupled to any other suitable load. 
     During operation, power source  102  generates a substantially direct current (DC), and a DC voltage is generated across input capacitor  105 . The DC voltage and current are supplied to power converter  104 . In this embodiment, control system  112  controls first stage  106  to convert the DC voltage and current to a substantially rectified DC voltage and current. The DC voltage and current output by first stage  106  may have different characteristics than the DC voltage and current received by first stage  106 . For example, the magnitude of the voltage and/or current may be different. Moreover, in this embodiment, first stage  106  is an isolated converter, which operates, among other things, to isolate power source  102  from the remainder of power conversion system  100  and electrical distribution network  116 . More specifically, in this embodiment, first stage  106  is a flyback converter. The DC voltage and current output by first stage  106  are input to second stage  108 , and control system  112  controls second stage  108  to produce AC voltage and current, and to adjust a frequency, a phase, an amplitude, and/or any other characteristic of the AC voltage and current to match the electrical distribution network  116  characteristics. The adjusted AC voltage and current are transmitted to filter  110  for removing one or more undesired characteristics from the AC voltage and current, such as undesired frequency components and/or undesired voltage and/or current ripples. The filtered AC voltage and current are then supplied to electrical distribution network  116 . 
       FIG. 2  is a simplified schematic diagram of this converter  200  for use as first stage  106 . Converter  200  is an isolated converter. More specifically, converter  200  is a flyback converter. Converter  200  is operable to receive DC power at an input  202  and output DC power at an output  204 . In this embodiment, converter  200  is operated by control system  112  to output DC power to electrical second stage  108 . Generally, the peak output voltage of converter  200  is greater than the input voltage to converter  200 . 
     Converter  200  includes a transformer  206  having a first primary winding  208 , a second primary winding  210 , and a secondary winding  212 . Primary windings  208  and  210  are magnetically coupled to, but electrically isolated from, secondary winding  212 . Primary windings  208  and  210  are connected to input  202  in parallel with each other. First primary winding  208  and second primary winding  210  are coupled to a switch  214 . In this embodiment, switch  214  is a MOSFET. In other embodiments, switch  214  may be any other suitable switch. 
     Converter  200  is generally operated as a flyback converter as known in the art. In general, switch  214  is switched on and off to store and release energy in transformer  206 . More specifically, when switch  214  is closed (also referred to as switched on), current flows through first and second primary windings  208  and  210  and energy is stored in the core (not shown in  FIG. 2 ) of transformer  206 . When switch  214  is opened (also referred to as switched off), current ceases flowing through first and second primary windings  208  and  210 . Current flow is induced in secondary winding  212 , releasing the energy stored in the core of transformer  206  to the output  204 . The output of transformer  206 , and more specifically the output of secondary winding  212 , is rectified by a diode  218 . Thus, DC output power is provided at output  204  of converter  200 . 
       FIG. 3  is a schematic diagram of this embodiment of transformer  206  for use in converter  200 . Each primary winding  208  and  210  includes three windings  300  connected in parallel. As will be described in more detail below, each primary winding  208  and  210  includes three conductors, one for each winding  300 , connected together in parallel. 
     In this embodiment, transformer  206  is a low leakage inductance transformer. As described above, losses in an isolated converter, such as converter  200 , are proportional to the leakage inductance of the transformer. Accordingly, by reducing the inductance of transformer  206  over some known designs, the losses in converter  200  may be reduced. 
     In one embodiment, a method of constructing a transformer, such as transformer  206 , includes wrapping a first primary winding around a toroidal core. The first primary winding traverses substantially the entire circumference of the toroidal core in a first circumferential direction. A secondary winding is wrapped around the toroidal core. The secondary winding includes a first half and a second half. The first half of the secondary winding traverses substantially the entire circumference of the toroidal core in the first circumferential direction and the second half of the secondary winding traverses substantially the entire circumference of the toroidal core in a second circumferential direction. The second circumferential direction is opposite the first circumferential direction. The method includes wrapping a second primary winding around the toroidal core. The second primary winding traverses substantially the entire circumference of the toroidal core in the second circumferential direction. 
     With reference to  FIGS. 4-8 , construction of an embodiment of transformer  206  according to the method will be described. The transformer  206  includes a first primary winding  208  and a second primary winding  210  each having twenty-one turns, and a secondary winding having one hundred and sixty turns. The method described above and transformer  206  are not limited, however, to a transformer as described hereinafter. In other embodiments, transformer  206  may have a different numbers of turns, a different turns ratios, different numbers of windings, and/or may be constructed of different materials. 
       FIGS. 4-9  illustrate construction of an embodiment of transformer  206  that may be used in a power converter, such as power converter  200 . Transformer  206  includes a magnetic core  400  visible in  FIGS. 4 and 5 . In this embodiment, core  400  is a toroidal core. In other embodiments, core  400  may have any other suitable shape. The core  400  is a distributed air gap toroidal core made from powder core material. The powder core material may be any suitable magnetic powder core material or combination of materials including, for example, nickel, iron, molybdenum. Moreover, in other embodiments, core  400  may be made of any other suitable materials and/or constructions including for example, iron, laminated silicon steel, carbonyl iron, ferrite, tape wound, etc. 
     In  FIG. 4 , first primary winding  208  has been wound onto core  400 . In this embodiment, three conductors  402 ,  404 , and  406  are wrapped around core  400  in a counterclockwise direction  408 . Each one of conductors  402 - 406  will become one winding  300  of first primary winding  208 . In this embodiment, conductors  402 - 406  are No. 22 AWG magnet wire. In other embodiments, any other suitable conductive material may be used. Conductors  402 - 406  are wrapped around core  400  twenty-one times beginning at point  410  to form a twenty-one turn primary winding  208 . Each turn of conductors  402 - 406  is spaced apart from each other turn such that the completed twenty-one turns traverse substantially the entire circumference of core  400  to form first primary winding  208 . 
     Secondary winding  212  is wound on core  400  in  FIGS. 5-8 . A single conductor  500  is wound around core  400  beginning at point  502  to form secondary winding  212 . Point  502  is located approximately 180 degrees around the circumference of core  400  from point  410  at which first primary winding  208  began and ended. In this embodiment, conductor  500  is 0.4 millimeter (mm) triple insulated wire (TIW). In other embodiments, any other suitable conductive material may be used. Conductor  500  is wrapped around core  400  forty times beginning at point  502  and ending at point  410  in  FIG. 5 . An additional forty turns are wrapped around core  400  ending substantially at point  502  in  FIG. 6 . In  FIG. 6 , conductor  500  has been wrapped a total of eighty turns around core  400  traversing substantially the entire circumference of core  400  in counterclockwise direction  408  beginning and ending at point  502 . These first eighty turns form a first portion of secondary winding  212 . 
     A second portion of secondary winding  212 , e.g., the remaining eighty turns, is wound on core  400  in a clockwise direction  700  in  FIGS. 7 and 8  using the same conductor  500 . In the embodiment shown in  FIG. 7 , approximately forty turns of the second portion of secondary winding  212  have been wound around core  400  beginning point  502  and ending at point  410 . An additional forty turns are wrapped around core  400  in clockwise direction  700  ending substantially at point  502  in  FIG. 8 . In the embodiment of  FIG. 8 , conductor  500  has been wrapped eighty turns around core  400  in counterclockwise direction  408  and eighty turns around core  400  in clockwise direction  700 . 
     In  FIG. 9 , second primary winding  210  has been wound onto core  400 . In this embodiment, three conductors  900 ,  902 , and  904  are wrapped around core  400  in clockwise direction  700 . Each one of conductors  900 - 904  will become one winding  300  of second primary winding  210 . In this embodiment, conductors  900 - 904  are No. 22 AWG magnet wire. In other embodiments, any other suitable conductive material may be used. In this embodiment, conductors  900 - 904  are wrapped around core  400  twenty-one times beginning at point  410  to form a twenty-one turn primary winding  210 . Each turn of conductors  900 - 904  is spaced apart from each other turn such that the completed twenty-one turns traverse substantially the entire circumference of core  400  to form second primary winding  210 . 
     To complete transformer  206  as schematically shown in  FIG. 3 , first primary winding  208  and second primary winding  210  are connected together in parallel. More specifically, one end of each of conductors  402 - 406  is connected to the corresponding end of conductors  900 - 904 . Because there is a single secondary winding  212 , conductor  500  does not need to be connected to anything to complete construction of transformer  206 . If desired, the ends of conductor  500  may be cut to a desired length, insulation may be removed from the cut ends, and the ends may be tinned with solder to prepare transformer  206  for installation in a circuit. Similarly, the ends of conductors  402 - 406  and  900 - 904  may be cut to length, stripped and tinned with solder. In some embodiments, additional windings may be added for low power auxiliary circuits, sensing, etc. 
     When completed, transformer  206  includes a first primary winding  208  and a second primary winding  210 , each of which includes three windings, having a same number of turns. Moreover, first primary winding  208  and secondary winding  210  are wound in opposite directions around core  400 . Similarly, secondary winding  212  includes a first portion and a second portion, having a same number of turns. The first portion and the second portion are wound in opposite directions around core  400 . As will be explained in more detail below, these winding techniques may produce transformers having improved characteristics, including reduced leakage inductance, over some known transformers. 
     A prototype transformer was built as described herein. Specifically, the prototype included a powdered material toroidal core. The primary windings were wound from No. 22 AWG magnet wire, and the secondary winding was wound with 0.4 mm TIW in the manner shown in  FIGS. 4-9 . The two primary windings were connected in parallel, as also described above. The resulting transformer exhibited a primary magnetizing inductance (Lm) of 26.1 microhenries (uH), a primary self resonant frequency of 471 kilohertz (kHz), a leakage inductance (Le) of 0.112 uH, and a DC resistance of 0.010 ohms. The secondary winding of the prototype transformer had a DC resistance of 0.872 ohms. The turns ratio of the prototype was 7.62 and the leakage ratio, i.e. Le/Lm, was 0.0043. 
     A second transformer was constructed using techniques other than those described herein. In the second transformer, the first primary windings were wound around the core, the secondary winding was wound over the first primary winding, and the second primary windings was wound over the secondary winding. All of the windings were wound with the same direction around the circumference of the toroidal core, e.g., all clockwise or all counterclockwise. Furthermore, the second transformer did not match the number of turns on the first and second primary windings that were then paralleled. The second transformer had a leakage ratio three times the leakage ratio of the prototype transformer constructed using the methods described herein. 
     A third transformer was constructed exactly the same as the second transformer, but matching the number of turns in the first and second primary windings. The primary windings and the secondary winding were all wound in the same direction around the circumference of the toroidal core, e.g., all clockwise or all counterclockwise. The third transformer had a leakage ratio two times the leakage ratio of the prototype transformer constructed using the methods described herein. 
     The reduced leakage inductance, and hence the reduced leakage ratio, of the prototype transformer constructed according to an embodiment of this disclosure as compared to, for example, the third prototype may be explained with reference to  FIG. 10 .  FIG. 10  shows a toroidal core  1000  with a single conductor  1002  wrapped in fourteen turns around core  1000  in a counterclockwise direction. Each turn of conductor  1002  forms a loop around core  1000 . As will be understood by one skilled in the art, current flowing through conductor  1002  generates a magnetic flux through core  1000 . However, conductor  1002  also describes a turn around the circumference of core  1000 , indicated by path  1004 , sometimes referred to as a turn in air or air turn. Because of this turn in air, current through conductor  1002  also creates a magnetic flux that is not coupled to core  1000 . This magnetic flux coupled through the air results in an increased leakage inductance. By winding first primary winding  208  and second primary winding  210  in opposite directions around the circumference of core  400  according to this disclosure, the turn in air from each winding  208  and  210  is canceled out, resulting in a zero net turn in air. Similarly, by winding a first portion of secondary winding  212  in a first direction around the core  400  and the second portion of the secondary winding  212  in a second direction, the turn in air of each portion is canceled out by the other, also resulting in a zero net turn in air. The leakage inductance may be further reduced by matching the number of turns in the opposite direction of the windings, e.g. matching the number of turns of first primary winding  208  and second primary winding  210 . 
     Transformers constructed in accordance with this disclosure have a lower leakage inductance, and when such transformers are used in power converters, the lower leakage inductance results in reduced losses and higher efficiency. 
     When introducing elements of the present invention or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.