Patent Publication Number: US-2022223336-A1

Title: Integrated quad-core transformer with asymmetric gap distribution for magnetic flux balancing

Description:
TECHNICAL FIELD 
     This disclosure relates to automotive vehicle power electronic components. 
     BACKGROUND 
     Power systems may include transformers that permit flow of current between various sources and loads. 
     SUMMARY 
     A magnetically integrated quad-core transformer system includes a pair of quad cores each having a set of legs arranged such that the legs face, and are spaced away from, each other to define four air gaps of two different widths. The system also includes primary and secondary windings wound around each of the legs. One of the quad cores and the primary and secondary windings wound around the legs thereof define a first transformer. The other of the quad cores and the primary and secondary windings wound around the legs thereof define a second transformer. The primary and secondary windings of the first transformer are in parallel with the primary and secondary windings of the second transformer. 
     A transformer system includes a first transformer having a first quad core with four first legs and first windings wound around each of the first legs, and a second transformer having a second quad core with four second legs and second windings wound around each of the second legs. The first four legs and second four legs are arranged adjacent to, but spaced away from, each other to define four gaps of two different widths. The first windings and second windings are in parallel. 
     A transformer system includes a first transformer having a first quad core with four first legs and first windings wound around each of the first legs such that a winding direction for diagonal ones of the first legs is same, and a second transformer having a second quad core with four second legs and second windings wound around each of the second legs such that a winding direction for diagonal ones of the second legs is same. The first four legs and second four legs are arranged adjacent to, but spaced away from, each other to define four gaps. The first windings and second windings are in parallel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A through 1D  are schematic diagrams of a proposed magnetic structure. 
         FIGS. 2A through 2D  are schematic diagrams of two paralleled quad-core transformers. 
         FIG. 3  is a circuit diagram of the transformers of  FIGS. 2A through 2D . 
         FIGS. 4A through 4D  are schematic diagrams of magnetically integrated two paralleled quad-core transformers. 
         FIG. 5  is a circuit diagram of the transformers of  FIGS. 4A through 4D . 
         FIG. 6  is a schematic diagram of a vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     Windings of a single-phase transformer carry the full current. At higher power levels, large currents may cause excessive losses in the printed circuit board (PCB) windings. Paralleling the PCB layers may be necessary to achieve acceptable efficiency. Eddy current losses attributed to proximity and skin effects increase with the increased number of stacked PCB layers. One method for reducing current stress in windings is realized by reducing the number of stacked layers through the implementation of matrix transformers. An array of transformers are intertwined so that their combined structure functions as a single transformer. Primary and secondary windings can either be connected in parallel or in series to obtain the desired turn ratio. 
     Flux cancellation techniques have been utilized in matrix transformers to reduce ferrite utilization. Previously introduced matrix transformers that realize flux cancellation have a uniform primary and secondary winding distribution in each core leg. While the magnetizing inductance can be controlled by adjusting the air gap length, leakage inductance is not controlled. 
     EI cores have been used for building integrated transformers. Through the implementation of unevenly distributed primary and secondary windings in the outer core legs, an unbalance in flux density is realized. By adjusting the air gap length along with the winding distribution ratio, the flux densities in the outer legs are mismatched. The leakage inductance is controlled by setting the flux ratios in the core legs. This operation requires a third leg to pass the residual flux from the two outer legs. 
     Integration of a large leakage inductance without substantially growing the core size requires incorporating a large air gap. The fringing flux increases with the increased air gap length; higher losses are expected in the PCB layers near the air gap. In order to avoid large winding losses due to the fringing magnetic fields, the window length is increased. This comes at the expense of increasing the ferrite utilization. Also, increasing the flux path will result in higher core loss. Due to the uneven distribution of primary and secondary windings, an unbalance in the magnetic flux density can occur in the core legs. Due to the nature of imbalanced flux, the overutilization of ferrite material is needed to keep the peak flux density below the magnetic saturation limit. 
     Since the EI core is geometrically unsymmetrical, paralleled layers have an unequal current distribution and, consequently, higher winding losses. Paralleling two EI transformers may result in a more equalized current distribution between the paralleled layers. Due to the tolerance in the air gaps however, current sharing may be degraded. 
     Ferrite utilization reduction is achieved through the implementation of matrix transformers. The integration of large leakage inductance in a matrix transformer is challenging due to the unsymmetrical flux density. Integrated EI cores offer the ability to integrate the leakage inductance with the price of increased ferrite utilization. There is a need for a new magnetic structure that combines the benefits of a matrix transformer through flux cancellation while integrating large leakage inductance. 
     A quad-core with asymmetric air gap distribution is designed to realize a balanced flux density in all core legs. An uneven winding distribution between the primary and secondary windings is implemented on two diagonal legs. A same winding arrangement is implemented on the other diagonal legs. Windings wound around the first diagonal leg are flipped with respect to the windings wound around the second diagonal leg. The two primary windings implemented on the two diagonal legs are connected in series. Similarly, the two secondary windings implemented on the two diagonal legs are connected in series. 
     Referring to  FIGS. 1A, 1B , IC, and ID, a transformer  10  includes a core  11  with four legs  12 ,  14 ,  16 ,  18 . The legs  12 ,  18  form one diagonal pair, and the legs  14 ,  16  form another diagonal pair. Additionally, each of the legs  12 ,  14 ,  16 ,  18  has a respective primary winding  20 ,  22 ,  24 ,  26  wound therearound, and a secondary winding  28 ,  30 ,  32 ,  34  wound therearound. The number of turns of the primary winding  20  is less than the number of turns of the primary winding  26 . That is, the number of turns for each of the primary windings  20 ,  26  is different. Similarly, the number of turns for each of the secondary windings  28 ,  34  is different. Likewise, the number of turns for each of the primary windings  22 ,  24  is different, and the number of turns for each of the secondary windings  30 ,  32  is different. An imaginary center leg  36  is added for explanation purposes. Air gaps  38  are implemented at the legs  12 ,  14 ,  16 ,  18  while the imaginary center leg  36  has no air gap. 
     For each of the diagonal pairs of legs  12 ,  18  and  14 ,  16 , the flux generated by the primary and secondary windings on the left leg is decoupled from the flux generated from the primary and secondary windings on the right leg. The air gaps  38  and windings turn ratio on both sides are adjusted to realize the desired magnetizing and leakage inductances. If two EI cores are implemented, the imaginary center leg  36  must be used as a return path for the flux. In the proposed transformer, two EI cores are positioned in a crossed fashion allowing for an imaginary center leg to be shared. Since the imaginary center leg  36  has no air gap, the flux generated by the windings  20 ,  22 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34  on the legs  12 ,  14 ,  16 ,  18  is decoupled. By applying opposite winding arrangements (shown by arrows in  FIGS. 1A and 1C ) between the two diagonal legs  12 ,  18  and  14 ,  16 , the flux in the imaginary center leg  36  is cancelled, thus the center leg can be removed. That is, the flux density in the imaginary center leg  36  with reference to  FIG. 1B  is not zero due to the unevenly distributed windings. Since the winding direction is flipped in  FIG. 1D  however, the net flux density in the imaginary center leg  36  has the same magnitude as in  FIG. 1B , but opposite polarity. 
     For high power applications, connecting multiple windings in parallel may be necessary for current sharing and loss optimization. Due to the unsymmetrical geometry of the QI core, the winding near the air gap will have higher losses. Hence, paralleling two transformers rather than two windings may be the better option. Current sharing problems, however, may arise due to the tolerances of the air gaps. To solve this problem, two of the transformers  10  are arranged as shown in  FIGS. 2A, 2B, 2C, and 2D , with the primary coils  20 ,  22 ,  24 ,  26  and secondary coils  28 ,  30 ,  32 ,  34  of each of the transformers  10  being connected as shown in  FIG. 3 . That is for each of the transformers  10 , the series connected primary coils  20 ,  26  are connected in series with the series connected primary coils  22 ,  24 . Similarly, the series connected secondary coils  28 , 34  are connected in series with the series connected secondary coils  30 ,  32 . Also, the primary and second coils of the transformers  10  are in parallel with each other. 
     If the “I” sections of the cores  11  are shared between the two transformers, a net-zero flux is expected to flow in it due to the opposing flux directions. By eliminating the “I” sections, the symmetric structure in  FIGS. 4A, 4B, 4C, and 4D  is realized. Thus, a pair of transformers  110  each includes a quad core  111  with four legs  112 ,  114 ,  116 ,  118 , and are arranged such that the legs  112 ,  114 ,  116 ,  118  face, and are spaced away from, each other to define four air gaps  113 ,  115 ,  117 ,  119  of at least two different widths, w, W. 
     The legs  112 ,  118  form one diagonal pair, and the legs  114 ,  116  form another diagonal pair. Additionally, each of the legs  112 ,  114 ,  116 ,  118  has a respective primary winding  120 ,  122 ,  124 ,  126  wound therearound, and a secondary winding  128 ,  130 ,  132 ,  134  wound therearound. The number of turns of the primary winding  120  is less than the number of turns of the primary winding  126 . That is, the number of turns for each of the primary windings  120 ,  126  is different. Similarly, the number of turns for each of the secondary windings  128 ,  134  is different. Likewise, the number of turns for each of the primary windings  122 ,  124  is different, and the number of turns for each of the secondary windings  130 ,  132  is different. 
     For each of the diagonal pairs of legs  112 ,  118  and  114 ,  116 , the flux generated by the primary and secondary windings on one of the legs is decoupled from the flux generated from the primary and secondary windings on the other of the legs. The air gaps  113 ,  115 ,  117 ,  119  and windings turn ratio on both sides are adjusted to realize the desired magnetizing and leakage inductances. Since the windings share the same flux path and the core has a symmetrical shape, perfect current sharing between the paralleled windings is accomplished. 
     The proposed integrated transformers achieve a 30% reduction in ferrite utilization as compared to other arrangements. The integration of a large resonant inductor is achieved for transformers with an uneven distribution of primary and secondary windings. Through the implementation of uneven air gap lengths between the diagonal core legs, an equalized flux density is realized in all core branches without compromising the ability to realize flux cancellation. 
     The transformers contemplated herein can be used within the context of vehicle, such as the vehicle  200  of  FIG. 6 . The vehicle  200  includes one or more electric machines  202  mechanically coupled to a hybrid transmission  204 . The electric machines  202  may operate as a motor or generator. In addition, the hybrid transmission  204  is mechanically coupled to an engine  206  and drive shaft  208 . The drive shaft  208  is mechanically coupled to wheels  210 . The electric machines  202  can provide propulsion and slowing capability when the engine  206  is turned on or off. The electric machines  202  may also act as generators and can provide fuel economy benefits by recovering energy that would normally be lost as heat in a friction braking system. The electric machines  202  may also reduce vehicle emissions by allowing the engine  206  to operate at more efficient speeds and allowing the vehicle  200  to be operated in electric mode with the engine  206  off under certain conditions. The vehicle  200  may also be a battery electric vehicle. In such a configuration, the engine  206  may not be present. Other arrangements are also contemplated. 
     A traction battery or battery pack  212  stores energy that can be used by the electric machines  202 . The vehicle battery pack  212  may provide a high voltage direct current (DC) output. The traction battery  212  may be electrically coupled to one or more power electronics modules  214 , which may include the transformers contemplated herein. One or more relays  216  may isolate the traction battery  212  from other components when opened and connect the traction battery  212  to other components when closed. The power electronics module  214  is also electrically coupled to the electric machines  202  and provides the ability to bi-directionally transfer energy between the traction battery  212  and the electric machines  202 . For example, the traction battery  212  may provide a DC voltage while the electric machines  202  may operate with a three-phase alternating current (AC). The power electronics module  214  may convert the DC voltage to a three-phase AC current to operate the electric machines  202 . In a regenerative mode, the power electronics module  214  may convert the three-phase AC current from the electric machines  202  acting as generators to the DC voltage compatible with the traction battery  212 . 
     In addition to providing energy for propulsion, the traction battery  202  may provide energy for other vehicle electrical systems. The vehicle  200  may include a DC/DC converter module  218  that converts the high voltage DC output of the traction battery  212  to a low voltage DC supply that is compatible with low-voltage vehicle loads. The DC/DC converter module  218  may include the transformers contemplated herein. An output of the DC/DC converter module  218  may be electrically coupled to an auxiliary battery  220  (e.g., 12 V battery) for charging the auxiliary battery  220 . Low-voltage systems  222  may be electrically coupled to the auxiliary battery  220 . One or more electrical loads  224  may be coupled to the high-voltage bus. The electrical loads  224  may have an associated controller that operates and controls the electrical loads  224  when appropriate. Examples of the electrical loads  224  include a fan, electric heating element, air-conditioning compressor, and other heating, ventilating, and air conditioning components. 
     The engine  206  may also provide energy for other vehicle electrical systems. The engine  206  via the transmission  204  may drive the electric machines  202  to generate power for the power electronics module  214  and electrical loads  224 , etc. In plug-in configurations, the electrified vehicle  200  may be configured to recharge the traction battery  212  as well as power the electrical loads  224  from an external power source. 
     Controllers/interfaces/modules  226  in the vehicle  200  may communicate via one or more vehicle networks, and exert control over the components shown. The vehicle network may include a plurality of channels for communication. One channel may be a serial bus such as a CAN. Another channel may include an Ethernet network defined by the Institute of Electrical and Electronics Engineers  802  family of standards. Additional channels may include discrete connections between modules and may include power signals from the auxiliary battery  220 . Different signals may be transferred over different channels. For example, video signals may be transferred over a high-speed channel (e.g., Ethernet) while control signals may be transferred over CAN. 
     The processes, methods, or algorithms disclosed herein can be deliverable to/implemented by a processing device, controller, or computer, which can include any existing programmable electronic control unit or dedicated electronic control unit. Similarly, the processes, methods, or algorithms can be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on non-writable storage media such as Read Only Memory (ROM) devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, Compact Discs (CDs), Random Access Memory (RAM) devices, and other magnetic and optical media. The processes, methods, or algorithms can also be implemented in a software executable object. Alternatively, the processes, methods, or algorithms can be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software and firmware components. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. 
     As previously described, the features of various embodiments can be combined to form further embodiments that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes may include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.