Patent Publication Number: US-2021175002-A1

Title: Low profile high current composite transformer

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 13/750,762, filed Jan. 25, 2013, the entirety of which is incorporated by reference as if fully set forth herein. 
    
    
     FIELD OF INVENTION 
     The embodiments of the present invention described herein relate to an improved low profile, high current composite transformer. 
     BACKGROUND 
     Transformers, as the name implies, are generally used to convert voltage or current from one level to another. With the acceleration of the use of all different types of electronics in a vast array of applications, the performance requirements of transformers have greatly increased. 
     There has also been an increase in the types of specialized converters. For example, many different types of DC-to-DC converters exist. Each of these converters has a particular use. 
     A buck converter is a step-down DC-to-DC converter. That is, in a buck converter the output voltage is less than the input voltage. Buck converters may be used, for example, in charging cell phones in a car using a car charger. In doing so, it is necessary to convert the DC power from the car battery to a lower voltage that can be used to charge the cell phone battery. Buck converters run into problems maintaining the desired output voltage when the input voltage falls below the desired output voltage. 
     A boost converter is a DC-to-DC converter that generates an output voltage greater than the input voltage. A boost converter may used, for example, within a cell phone to convert the cell phone battery voltage to an increased voltage for operating screen displays and the like. Boost converters run into problems maintaining a higher output voltage when the input voltage fluctuates to a voltage that is greater than the desired output voltage. 
     Most prior art inductive components, such as inductors and transformers, comprise a magnetic core component having a particular shape, depending upon the application, such as an E, U or I shape, a toroidal shape, or other shapes and configurations. Conductive wire windings are then wound around the magnetic core components to create the inductor or transformer. These types of inductors and transformers require numerous separate parts, including the core, the windings, and a structure to hold the parts together. As a result, there are many air spaces in the inductor which affect its operation and which prevent the maximization of space, and this assembled construction generally causes the component sizes to be larger and reduces efficiency. 
     Since transformers are being used in a greater array of applications, many of which require small footprints, there is a great need for small transformers that provide superior efficiency. 
     SUMMARY 
     A low profile high current composite transformer is disclosed. Some embodiments of the transformer include a first conductive winding having a first start lead, a first finish lead, a first plurality of winding turns, and a first hollow core; a second conductive winding having a second start lead, a second finish lead, a second plurality of turns, and a second hollow core; and a soft magnetic composite compressed surrounding the first and second windings. The soft magnetic composite with distributed gap provides for a near linear saturation curve. 
     Multiple uses for the transformer are also disclosed. In some embodiments, the transformer operates as a flyback converter, a single-ended primary-inductor converter, and a Cuk converter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding may be had from the following description, given by way of example in conjunction with the accompanying drawings, wherein: 
         FIG. 1  illustrates the windings of a low profile high current composite transformer; 
         FIG. 2  illustrates an alternate configuration of the windings of a low profile high current composite transformer; 
         FIG. 3  illustrates an alternate configuration of the windings of a low profile high current composite transformer; 
         FIGS. 4 and 4A  illustrate alternate configurations of the windings of a low profile high current composite transformer; 
         FIG. 5  illustrates an alternate configuration of the windings of a low profile high current composite transformer; 
         FIG. 6  illustrates a transformer constructed in accordance with some embodiments; 
         FIG. 7  illustrates a transformer constructed in accordance with some embodiments; 
         FIG. 8  illustrates a transformer constructed in accordance with some embodiments; 
         FIG. 9  illustrates a linear saturation curve for a transformer using pressed powder technology as compared to a transformer using ferrite technology; 
         FIG. 10  illustrates a block diagram of a converter using embodiments of the transformer described above; 
         FIG. 11  illustrates a block functional diagram of a converter using the transformer; 
         FIG. 12  illustrates an effective circuit diagram for the use of a converter using the transformer and operating as a SEPIC; 
         FIG. 13  illustrates an effective circuit diagram for the use of a converter using the transformer and operating as a flyback converter; and 
         FIG. 14  illustrates an effective circuit diagram for the use of a converter using the transformer and operating as a Cuk converter. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, many other elements found in inductor and transformer designs. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art. 
     The invention relates to a low profile high current composite transformer. The transformer includes a first wire winding having a start lead and a finish lead. In addition, the device includes a second wire winding. A magnetic material completely surrounds the wire windings to form an inductor body. Pressure molding is used to mold the magnetic material around the wire windings. 
     Applications for the present device include, but are not limited to, a Cuk converter, flyback converter, single-ended primary-inductance converter (SEPIC), and coupled inductors. For SEPIC and Cuk converters, the leakage inductance between the two windings of the transformer improves efficiency of the converter by lowering loss with the soft magnetic composite. 
     Referring now to  FIG. 1 , there is shown the windings of a low profile high current composite transformer  10  that may be used in a converter as described below. A winding, also referred to as a coil in some embodiments, may include one or more turns of an electrical conductor of any shape on a common axis where the inside perimeter or diameter is equal or variable. Each turn may be any shape, including circular, rectangular, and square. The conductor cross-section may be any shape including circular, square or rectangular. Transformer  10  includes two individual windings, a first winding  20  and a second winding  30 . First winding  20  includes a plurality of turns  22  and includes a start lead  24  and a finish lead  26 . Second winding  30  includes a plurality of turns  32  and includes a start lead  34  and a finish lead  36 . 
     First winding  20  may have any number of turns. Second winding  30  may also have any number of turns. The ratio of the turns of first winding  20  and second winding  30  may be in the range of 1/10 to 10. Specifically, first winding  20  may include a number of turns approximately in the range of 4 to 40, and more specifically approximately 10 turns. Similarly, second winding  30  may include a number of turns approximately in the range of 4 to 40, and more specifically approximately 10 turns. 
     First winding  20  may be wound in a first direction and second winding  30 , while maintaining the same center of rotation, may be wound in the opposite direction. Alternatively, the second winding  30  may be wound in the same direction as the first winding  20 , while again maintaining the same center of rotation. Further, second winding  30  may be concurrently wound side-by-side with first winding  20 . First winding  20  and second winding  30  may be wound simultaneously in an interleaved winding, which is also known as a bifilar winding. This enables both first winding  20  and second winding  30  to maintain a low profile for the transformer  10 . Transformer  10  may be sized with dimensions of 10×10×4 mm or other suitable dimensions that are larger or smaller. 
     Another configuration for the windings is shown in  FIG. 2 . This configuration illustrates a flat wire for forming transformer  10 . This illustration shows an exaggerated spacing between the first  20  and second windings  30 . Transformer  10  includes a wire winding  20 ,  30  from a flat wire having a rectangular cross section. An example of a wire for windings  20 ,  30  is an enameled copper flat wire made from copper with a polymide enamel coating for insulation. While a flat wire configuration is shown and described, the present invention can use Litz wire, and/or braided wire configurations as well. Similar to the round configuration above, windings  20 ,  30  in the flat wire configuration include a plurality of turns  22 ,  32 . First winding  20  includes a start lead  24  and a finish lead  26 . Second winding  30  includes a start lead  34  and a finish lead  36 . Start lead  24  is interconnected to a first lead  16  and finish lead  26  is interconnected to a second lead  17 . Start lead  34  is interconnected to a third lead  18  and finish lead  34  is interconnected to a fourth lead  19 . 
     Other configurations of the windings may also be used. For example, as shown in  FIG. 3 , gapped windings may be used to form transformer  10 . In  FIG. 3 , there are two windings shown, although any number may be used. Gapped windings may include a first winding  20  where the center of winding is displaced laterally from the center of winding of the second winding  30 . This displacement may be in the horizontal and/or vertical direction within the confines of the transformer body. 
     Other configurations of the windings shown in  FIGS. 4 and 4A  are gapped windings with a shared inner diameter. Again, while showing two windings, any number of windings may be used in this configuration. Gapped windings with a shared inner diameter may include a first winding  20 , a second winding  30  with an air gap in between the first winding  20  and second winding  30 . 
     Another configuration of the windings is shown in  FIG. 5 . This configuration includes three windings. As shown, the first winding  20  is configured with the same center of winding as second winding  30  and third winding  40 . Other configurations may be used for a three winding transformer. As shown, first winding is wound about a center of winding, second winding  30  shares the same center of winding and has a larger inner diameter than the outer diameter of first winding  20 . Third winding shares the same center of winding and has a larger inner diameter than the outer diameter of second winding  30 . 
     The windings of  FIG. 1-5  may have a transformer body formed thereon or around. The transformer body may include a soft magnetic composite comprised of insulated magnetic particles with a distributed gap. The use of the term soft in defining the soft magnetic composite refers to the composite being magnetically soft, such as where the HC, or coercive force, is less than or equal to 5 oersteds. The soft magnetic composite may comprise an alloy powder, an iron powder or a combination of powders. The powder may also include a filler, a resin, and a lubricant. The soft magnetic composite has electrical characteristics that allow the device to have a high inductance, yet low core losses so as to maximize its efficiency. 
     The soft magnetic composite has high resistivity (exceeding 1 MΩ) that enables the transformer as it is manufactured to perform without a conductive path between the surface mount leads. The magnetic material also allows efficient operation up to 40 MHz depending on the inductance value. The force exerted on the soft magnetic material may be approximately 15 tons per square inch to 60 tons per square inch. This pressure causes the soft magnetic material to be compressed and molded tightly and completely around the windings so as to form the transformer body including in between the windings. Compression and molding tightly and completely around the windings may, in some embodiments, include around and/or in between each turn of the windings. 
     Transformer  10  is shown in  FIG. 6  as constructed to be mounted such as on a circuit board (not shown) or for installation with first and second windings  20 ,  30  formed inside the body  14 . Transformer  10  includes a body  14  with a first lead  16  and a second lead  17  extending outwardly therefrom. Body also has a third lead  18  and fourth lead  19  (not visible) extending outwardly therefrom. The leads  16 ,  17 ,  18  and  19  are bent and folded under the bottom of body  14  and may be soldered to a pad or pads as needed to connect to a circuit. Once connected to the circuit board, the leads  16 ,  17 ,  18  and  19  may be interconnected as desired to enable and affect performance of the transformer  10 . In a similar manner, any number of coils or leads may be added as required. 
     As shown in  FIG. 7 , transformer  10  includes a two winding configuration to be mounted such as on a circuit board (not shown) or for installation. Transformer  10  includes a body  14  that may be cylindrical as shown or any other shape, such as square or hexagonal, with first and second windings  20 ,  30  (not visible) formed inside the body  14  and with a first lead  16  and a second lead  17  extending outwardly therefrom. Body also has a third lead  18  and fourth lead  19  extending outwardly therefrom. The leads  16 ,  17 ,  18  and  19  extend from the underside of the body and may be soldered to a PCB as needed. Once connected to the circuit board, the leads  16 ,  17 ,  18  and  19  may be interconnected as desired to enable and affect performance of the transformer  10 . 
     As shown in  FIG. 8 , transformer  10  includes a three winding configuration to be mounted such as on a circuit board (not shown) or for installation. Transformer  10  includes a body  14  with first and second windings  20 ,  30  (not visible) formed inside the body  14  and with a first lead  16  and a second lead  17  extending outwardly therefrom. Body also has a third lead  18  and fourth lead  19  extending outwardly therefrom. Body also has a fifth lead  12  and sixth lead  13  extending outwardly therefrom. The leads  12 ,  13 ,  16 ,  17 ,  18 , and  19  extend from the underside of the body and may be soldered to a PCB as needed. Once connected to the circuit board, the leads  12 ,  13 ,  16 ,  17 ,  18 ,  19  may be interconnected as desired to enable and affect performance of the transformer  10 . In a similar manner, any number of coils or leads may be added as required. 
     When compared to other inductive components, embodiments of transformer  10  have several unique attributes. The conductive winding, with or without a lead frame, magnetic core material, and protective enclosure are molded as a single integral low profile unitized body that has termination leads suitable for surface or thru hole mounting. The construction allows for maximum utilization of available space for magnetic performance and is magnetically self-shielding. The unitary construction eliminates the need for multiple core bodies, as was the case with prior art E cores or other core shapes, and also eliminates the associated assembly labor. The unique conductor winding of some embodiments allows for high current operation and also optimizes magnetic parameters within the transformer&#39;s footprint. The transformer described herein is a low cost, high performance package without the dependence on expensive, tight tolerance core materials and special winding techniques. The pressed powder technology provides a minimum particle size in an insulated ferrous material resulting in low core losses and a high saturation without sacrificing magnetic permeability to achieve a target inductance. 
     Transformer  10  may realize energy storage as defined in Equation 1. 
       Energy storage=½* L*I   2   (Equation 1)
 
     Energy storage is maximized by the selection of the particle composition and size along with the gap created around the particle by the insulation, binder and lubricant. The pressed powder technology provides for superior saturation characteristics which keep the inductance high for the associated applied current to maximize storage energy. 
       FIG. 9  illustrates a near linear saturation curve for a transformer using pressed powder technology for forming the soft magnetic composite as compared to a transformer using ferrite technology. The pressed powder technology provides for a near linear saturation curve, shown in  FIG. 9 . The pressed powder curve  90  while rolling down below an inductance of 1 μH still remains over 0.9 μH at higher currents. On the other hand, the ferrite curve is a stepped or hard saturation curve. The ferrite curve  95  does not rise over 1 μH at any current, and has a steep rolloff between 12-15 A. At higher currents, the ferrite achieves less than 0.2 μH. The pressed powder curve allows higher current density in a smaller package with the ability to handle current spikes without a drastic drop in inductance. This improves the performance and stability of the circuit. 
     Referring now to  FIG. 10 , there is shown a block diagram of a converter utilizing transformer  10 . Converter  200  may have an input A and one or more outputs B. In converter  200 , the voltage level of input A may be greater than, less than, or equal to the voltage level of output B. 
     When operating as a SEPIC, for example, converter  200  is a type of DC-to-DC converter that allows the electrical input voltage to be greater than, equal to, or less than the output voltage, and the output voltage has the same polarity as the input voltage. The output of converter  200  is controlled by the duty cycle of the control transistor as described hereinafter. Converter  200  is useful where the battery voltage can be above or below that of the intended output voltage. For example, converter  200  may be useful when a 13.2 volt battery discharges 6 volts (at the converter  200  input), and the system components require 12 volts (at the converter  200  output). In such an example, the input voltage is both above and below the output voltage. 
     When operating as a Cuk converter, for example, converter  200  is a type of DC-to-DC converter that allows the electrical output voltage to be greater than, equal to, or less than the input voltage, and has the opposite polarity as the input voltage. 
       FIG. 11  illustrates a block functional diagram of a converter. Converter  200  includes an input  210 , an output  230 , a transformer  10  and a control unit  220 . Converter  200  may also include a feedback loop (not shown) from the output  230  to control unit  220 . Input  210  may optionally include voltage regulation and conditioning as desired. Input  210  may include input capacitor(s) to regulate the input voltage. Input  210 , after conditioning or regulating the input voltage as desired, provides a signal to transformer  10 . Transformer  10  may charge based on the provided signal. For example, a first side of transformer  10  may charge to the value of the input voltage. Based on control  220 , this charge in transformer  10  is then delivered to output  230 . Output  230  may optionally include conditioning and regulation of an output voltage as desired to provide a more usable voltage from converter  200 . 
     Referring now additionally to  FIG. 12 , an effective circuit diagram for the use of transformer  10  as a SEPIC is shown. SEPICs generally provide a positive regulated output voltage regardless of whether the input voltage is above or below the output voltage. SEPICs are particularly useful in applications that require voltage conversion from an unregulated power supply. SEPIC  700  may include transformer  10  having two windings  702 ,  704 . Each winding may be supplied the same voltage during the switching cycle. Leakage inductance between the two windings may improve the efficiency of SEPIC  700  by lowering AC loss. As illustrated in  FIG. 12 , transformer  10  has a first lead  760  coupled to ground. A second lead  770  is interconnected with a diode  710  which is coupled to Vout and capacitor  720 . In addition, second lead  770  and a third lead  780  are interconnected via capacitor  730  with third lead  780  connected to the drain of transistor  750 . A fourth lead  790  of transformer  10  is coupled to Vin and capacitor  740 . The source of transistor  750  may be coupled to ground. 
     The effective inductance of the two windings of transformer  10  wired in series is shown in Equation 2. 
         L=L   1   +L   2 ±2* K *( L   1   *L   2 ) 0.5   (Equation 2)
 
     The + or − depends on whether the coupling is cumulative or differential. L 1  and L 2  represent the inductance of the first and second windings and K is the coefficient of coupling. Therefore, transformer  10  may provide 4 L inductance if the inductance of the first and second winding are both L and the coupling was perfect and cumulative. 
     In analyzing the circuit of  FIG. 12 , Vin is conditioned by capacitor  740 . The first winding  702  of transformer  10  charges and may eventually be equal to Vin. Depending on the control transistor  750 , the charge of the first winding may be propagated through circuit  700  to Vout. That is, the charge of first winding of transformer  10  may be conveyed to the second winding of transformer  10 . This charge is then coupled to Vout, based on control transistor  750 . Capacitor  720  may condition the output voltage from the charge of the second winding of transformer  10 . Diode  710  may prevent leakage from capacitor  720  into the remainder of circuit  700 . 
       FIG. 13  illustrates an effective circuit diagram for the use of a converter using the transformer and operating as a flyback converter. A flyback converter may be used in either AC/DC (requiring rectification) or DC/DC conversion. A flyback converter is a buck-boost converter with a transformer providing isolation. 
     In  FIG. 13 , circuit  800  includes an input voltage source  840  electrically coupled to a switch  810  and the primary winding  802  of the transformer. The secondary winding  804  of the transformer is electrically connected to a diode  820  with a capacitor  850  and load  830  coupled in parallel. In operation, when switch  810  is closed, the primary winding  802  is connected to the input voltage source  840 . The flux in the transformer increases, storing energy in the transformer. The voltage induced in the secondary winding  804  causes the diode to be reversed biased, and the capacitor  850  supplies energy to the load  830 . 
     When switch  810  is open, the secondary voltage causes the diode  820  to be forward biased. The energy from the transformer recharges the capacitor  850  and supplies the load  830 . 
       FIG. 14  illustrates an effective circuit diagram for the use of a converter using the transformer and operating as a Cuk converter. A Cuk converter is a DC/DC converter where the output voltage is greater or less than the input voltage while having opposite polarity between input and output voltages. 
     In  FIG. 14  circuit  900  includes an input voltage source  940  electrically coupled to a switch  910  and the primary winding  902  of the transformer. The secondary winding  904  of the transformer is electrically connected to a diode  920 , capacitor  950 , and load  930  coupled in parallel. In operation, when the switch  910  is open, capacitor  960  may be charged by the input source  940  through the first winding  902 . Current flows to the load  930  from the secondary winding  904  through diode  920 . When the switch  910  is closed, capacitor  960  and second winding  904  transfer energy to the load  930  through switch  910 . 
     Although the features and elements of the present invention are described in the example embodiments in particular combinations, each feature may be used alone without the other features and elements of the example embodiments or in various combinations with or without other features and elements of the present invention.