Low-profile fractional planar transformer

Abstract

A winding for a low-profile planar transformer is provided that includes a printed circuit board having a first surface and a second surface, the printed circuit board has a plurality of through openings therein. A current path is also provided that includes a first portion that is positioned on the first surface of the printed circuit board and a second portion that is positioned on the second surface of the printed circuit board, the current path winding around the plurality of through openings and crossing between the first surface and second surface of the printed circuit board via the through openings. The current path having a first terminal end and a second terminal end both positioned on the first surface of the printed circuit board. A low-profile planar transformer is also provided that includes the winding along with a multiple limb magnetic core.

Description

LOW-PROFILE FRACTIONAL PLANAR TRANSFORMERFIELD OF THE INVENTION

[0001] The present invention relates to a high power density and high frequency converter, and more particularly to a low-profile fractional planar transformer for a high power density and high frequency dual active bridge converter.BACKGROUND OF THE INVENTION

[0002] Power density and efficiency are crucial considerations in energy storage applications, and magnetic components play a significant role in improving these factors. The pursuit of high power density and high frequency converters has driven advancements in passive components. Planar transformers have emerged as crucial elements in high power density designs due to their low profile and ease of manufacturing. However, in higher voltage applications, ferrite cores with substantial effective core area and higher number of turns are necessary to prevent core saturation. High current, high-frequency (HF) applications present unique challenges in magnetic design, necessitating the use of litz wires with a high number of strands or multiple parallel copper layers to enhance current handling capabilities [l]-[3]. However, ensuring equal current sharing among these layers remains a major hurdle in such applications [4], Planar transformers have emerged as a promising solution to address this challenge by interleaving copper layers to balance the magnetic field (H) distribution [5]-[7], In low-voltage planar transformers, utilizing singleturn windings offers the advantage of reducing the number of layers, lower termination loss and minimizing inter-winding capacitance [8], To prevent high inter-winding capacitance complex printed circuit board (PCB) designs are presented in [9],

[0010] that result in manufacturing complexity and higher cost. In high current 48V applications, other than multiple number of turns and PCBs, customdesigned ferrite cores are widely used to avoid core saturation and provide high current capability

[0011] —

[0014] ,

[0003] One of the challenges in planar transformer design lies in balancing the minimization of intra-winding and inter-winding capacitance against winding resistance. Several interleaving methods have been proposed to reduce AC resistance, albeit at the cost of increased interwinding capacitance. The root of this issue stems from the manner in which multi-turn windings are wound around the cores and how the PCBs are stacked. Achieving a high turns ratio entails either placing the loops for multiple turns side by side and twisting them around the core or stacking them on the top and bottom sides of the PCB. Both methods have their limitations. Placing consecutive turns side by side without overlapping results in inefficient use of the window area, as one side of the PCB remains unused to avoid overlapping areas with non-zero voltage gradients between turns. Conversely, placing consecutive turns across both sides of the PCB efficiently utilizes the window area but leads to high energy stored between the two copper layers with non-zero voltage gradients across the FR4 material, which has a permittivity 4.7 times that of air, resulting in high intra-winding capacitance. To mitigate this issue and reduce capacitance, each winding should be split into two identical copper layers across both sides of a PCB and connected in parallel. Then, separate PCBs can be connected in series to form a complete winding. In this configuration, all copper layers which are placed across FR4 material have zero voltage gradient due to parallel connection and it minimizes intra-winding capacitance, while PCBs with non-zero voltage gradients are separated by air to reduce inter-winding capacitance. However, this results in a tall stacked planar structure for a high number of turns, which is typically not suitable for high-power-density power converters and provides a disproportionate height-to-surface ratio, inefficiently using the volume.

[0004] Thus, there exists a need for a new infinite-shape winding transformer to meet the demands of high-power applications and that achieves balanced magnetic field exposure andequal current distribution for the parallel windings to effectively reduce ac resistance, particularly an alternative expandable winding configuration that addresses all the aforementioned challenges to achieve low-profde planar transformers, crucial for high-power- density applications.SUMMARY OF THE INVENTON

[0005] A winding for a low-profile planar transformer is provided that includes a printed circuit board having a first surface and a second surface, the printed circuit board has a plurality of through openings therein. A current path is also provided that includes a first portion that is positioned on the first surface of the printed circuit board and a second portion that is positioned on the second surface of the printed circuit board, the current path transiting around the plurality of through openings and crossing between the first surface and second surface of the printed circuit board via the through openings. The current path having a first terminal end and a second terminal end both positioned on the first surface of the printed circuit board.

[0006] The printed circuit board can be a two layer structure, formed of FR4, and in still other embodiments has a permittivity of 4.7±10%.

[0007] In some inventive embodiments, the current path is formed of copper. In other inventive embodiments, the first portion of the current path is connected in parallel to the second portion of the current path.

[0008] A low-profile planar transformer is also provided that includes a winding as detailed above along with a first magnetic core having a plurality of limbs each of the plurality of limbs having an area, the plurality of limbs configured to align with and be received within the plurality of through openings in the printed circuit board of the winding.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention is further detailed with respect to the following figures that depict various aspects of the present invention.

[0010] FIG. 1 is a schematic diagram of a parallel output Dual Active Bridge topology according to embodiments of the present invention;

[0011] FIG. 2A shows a conventional, prior art two turn winding on center limb;

[0012] FIG. 2B shows a conventional, prior art single PCB with two turns on a top and bottom layer that fully overlap resulting in 2-effective turns;

[0013] FIG. 2C shows a conventional, prior art two PCBs series connection with Top and Bottom layer in parallel resulting in 4-effective turns;

[0014] FIG. 2D shows the magnetic flux path and the equivalent magnetic circuit of the , prior art two turn winding around center limb of FIG. 2B;

[0015] FIG. 3A shows an infinite-shape winding on an E core according to embodiments of the present invention;

[0016] FIG. 3B shows a two layer PCB to implement the infinite winding according to embodiments of the present invention;

[0017] FIG. 3C shows the current path of the inventive winding flowing in top and bottom layers of the PCB according to embodiments of the present invention;

[0018] FIG. 3D shows the magnetic flux path and the equivalent magnetic circuit of the inventive winding according to embodiments of the present invention;

[0019] FIG. 4A shows a transformer with series and parallel connection of the winding according to embodiments of the present invention;

[0020] FIG. 4B shows a 3 -winding transformer for a Dual Active Bridge converter according to embodiments of the present invention;

[0021] FIG. 5A shows a magnetic field distribution of a transformer having a SSSPPPPPPSSS PCB placement according to embodiments of the present invention;

[0022] FIG. 5B shows an exploded view of the SSSPPPPPPSSS structure of FIG. 5 A;

[0023] FIG. 6A shows a magnetic field distribution for a transformer having an interleavedPSSPPSSPPSSP structure according to embodiments of the present invention;

[0024] FIG. 6B shows a magnetic field distribution for a transformer having an interleaved PSPSPSPSPSPS structure according to embodiments of the present invention;

[0025] FIG. 6C shows a magnetic field distribution for a transformer having an interleaved P / 2 SPSPSPSPSPS P / 2 structure according to embodiments of the present invention;

[0026] FIG. 7A shows a Maxwell 3D simulation showing loss and current distribution for a transformer having a PSSPPSSPPSSP structure according to embodiments of the present invention;

[0027] FIG. 7B shows a Maxwell 3D simulation showing loss and current distribution for a transformer having a PSPSPSPSPSPS structure according to embodiments of the present invention;

[0028] FIG. 7C shows a Maxwell 3D simulation showing loss and current distribution for a transformer having a P / 2 SPSPSPSPSPS P / 2 structure according to embodiments of the present invention;

[0029] FIG. 8A shows the current distribution in a P / 2 SPSPSPSPSPS P / 2 structure for the top layer of all PCBs except Pl / 2;

[0030] FIG. 8B shows the current distribution in a P / 2 SPSPSPSPSPS P / 2 structure for the bottom layer of all PCBs except Pl / 2;

[0031] FIG. 8C shows the current distribution in a P / 2 SPSPSPSPSPS P / 2 structure for the top layer of P 1 / 2;

[0032] FIG. 8D shows the current distribution in a P / 2 SPSPSPSPSPS P / 2 structure for the bottom layer of P 1 / 2;

[0033] FIG. 9A shows an exploded view of primary PCBs series connections according to embodiments of the present invention;

[0034] FIG. 9B shows an exploded view of secondary and tertiary winding parallel connections according to embodiments of the present invention;

[0035] FIG. 9C show individual PCBs of a planar transformer according to embodiments of the present invention;

[0036] FIG. 10A is a photograph of a planar transformer according to embodiments of the present invention;

[0037] FIG. 1 OB is an exploded view of a DAB converter according to embodiments of the present invention; and

[0038] FIG. IOC is a photograph of a DAB converter using the planar transformer according to embodiments of the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0039] The present invention has utility as a new infinite-shape winding transformer to meet the demands of high-power applications and that achieves balanced magnetic field exposure and equal current distribution for the parallel windings to effectively reduce ac resistance, particularly an alternative expandable winding configuration that addresses all the aforementioned challenges to achieve low -profile planar transformers, crucial for high-power- density applications.

[0040] The present invention provides a new, infinite-shape winding uses a simple 2-layer printed circuit board (PCB), effectively exciting all three limbs of E cores and enabling higher step-up turn ratios for low-voltage and high current primary applications with minimized intra-winding capacitance as the top and bottom layers are connected in parallel on both sides of the PCB. In addition, a novel interleaving method specifically tailored for the proposed planar transformer, serves two primary objectives: a) achieving equal current distribution among parallel PCBs while ensuring similar magnetic field (H) exposure, and b) enabling equal current sharing in dual output transformers. The results for different configurations are validated through Maxwell 3D simulations and experimentation to validate the performance with an exemplary 5kW setup. According to some inventive embodiments, the planar transformer is designed for a 5kW Dual Active Bridge (DAB) converter, which utilizes parallel low-voltage bridges to significantly enhance current capability for a portable 5kW energy storage application (ESS). To achieve perfect copper utilization in the winding area and equal current sharing in parallel layers, an interleaving method is also provided. FEA simulation results validate the effectiveness of the inventive method.

[0041] The present invention will now be described with reference to the following embodiments. As is apparent by these descriptions, this invention can be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. For example, features illustrated with respect to one embodiment can be incorporated into other embodiments, and features illustrated with respect to a particular embodiment may be deleted from the embodiment. In addition, numerous variations and additions to the embodiments suggested herein will be apparent to those skilled in the art in light of the instant disclosure, which do not depart from the instant invention. Hence, the following specification is intended to illustrate some particular embodiments of the invention, and not to exhaustively specify all permutations, combinations, and variations thereof.

[0042] It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0044] Unless indicated otherwise, explicitly or by context, the following terms are used herein as set forth below.

[0045] As used in the description of the invention and the appended claims, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0046] Also as used herein, “and / or” refers to and encompasses any and all possible combinations of one or more of the associated listed items, as well as the lack of combinations when interpreted in the alternative (“or”).

[0047] FIG. 1 shows the parallel output power circuit of a dual active bridge (DAB) converter that is used, for example, in a 5kW, high current ESS application. The LV port is connected to the battery changing between 45-55Vdc and the HV side is in the range of 375- 425 Vdc. As the current at the LV terminal is very high, the total current is shared among two full bridges using parallel MOSFETs to reduce the conduction loss. However, the frequency is set at 120kHz and the turns ratio resulting in a reasonable zvs range and full load efficiency is considered as N=6 for the HF transformer. As shown in FIG. 1, the LV side is directly appliedon the transformer. Hence, the Vbat contribute to flux density in the transformer core. Considering a square wave voltage applied to the transformer, V = 4NfBAeis the equation which determines the transformer flux density, B. The core effective area Ae, turns ratio, N, frequency, f and V are the circuit parameters that should be elaborately selected to avoid saturation in the transformer.

[0048] In planar transformers E cores are commonly used for obtaining the lowest profile transformer. Planar transformers using E cores are commonly exciting all of the primary and secondary windings on the center leg. As shown in FIG. 2A, the flux density of the center and side limbs are made by the turns around the center leg. Based on the number of turns around the center leg, the magnetic flux will be excited into the core and the flux return path is closed from both side limbs. The conventional approach for two turns winding around the center limb utilizes a top and a bottom layer of a PCB using the full width of the transformer window area, as shown in FIG. 2B. FR4 is a common material used in PCB manufacturing having a permittivity of about 4.7±10% higher than air. FR4 is a flame resistant, glass-reinforced epoxy resin. By placing two turns on top and bottom layer with maximum overlap, the interwinding capacitance of this two-turn PCB is maximum as two turns are facing each other with maximum overlap across the FR4 material. High inter-winding capacitance is considered as one of the most critical parameters in planar transformer design and it affects the power circuit performance causing excessive losses and oscillations. One common approach is to always avoid placing two coppers with differential voltage, across FR4 due to its high permittivity. For example, to achieve two turn winding, the top and bottom layer are connected in parallel at all times across two sides of PCB. Having this configuration with the same width results in lower copper utilization since double copper is being used. To keep the constant current density, two turns are connected in two loops around the center limb of the core while having top and bottom layer in parallel for the whole path, as shown in FIG. 2C. It should be consideredthat in FIG. 2B, the two termination points of the winding are accessible while in FIG. 2C, the two loops of each PCB end in the middle point close to core. This will cause some limitation whenever specific number of turns are needed since there is a necessity for another PCB to continue the current path and take the middle point to an accessible terminal which results in 4 loops (4 tons). In this case, 4 effective turns are achieved using two PCBs while keeping the top and bottom layer of PCB facing zero voltage difference for the entire path. The voltage difference of the 4 ton is applied between two boards which is separated by air as dielectric. FIG. 2D shows the magnetic flux path and the equivalent circuit showing the effective magnetomotive force (MMF) of 2NI is generated on the center limb using the two turn winding shown in FIG. 2B. As the effective area of the two side limbs of the E cores are designed to be half of the center limb, the flux generated in the center limb(< >ci) is equally shared between two side limbs (<pc2,<pc3).

[0049] FIG. 3A shows an infinite-shape winding of the present invention as an example used in E magnetic core. It utilizes all limbs of the magnetic core to achieve the maximum number of tons without stacking them on top of each other across two sides of a PCB. This is expandable in any core geometry and can be implemented on a well-known ELP planar core. As in standard E cores, the side legs have half of the area of the center leg, each full turn on the side legs counts as 0.5 ton. This results in two effective tons by using the inventive winding on atypical E core. As shown in FIG. 3B, the inventive infinite-shape winding is made of simple two layer PCB. FIG. 3C illustrates the current path in the top and bottom layers. As the current path has some cross over due to its infinite shape structure, the current goes from top to bottom and bottom to top in the crossover region using vias to make a complete loop and having two end points as terminals on one side of the PCB. To avoid leaving unused space in the transformer winding area, the inventive winding utilizes two parallel copper traces around each limb. Hence, the copper planes are connected in parallel, except in the overlap area wherethe top and bottom current paths cross over. In this overlap area, to form the infinite-shape of the single turn, at the cross over area the two stacked parallel copper twists permute such that both copper twists join in one single layer but with double of width. The width of the crossover planes is designed to be double to have equal current density. This leaves half of the height of the stacked copper twists unused for the infinite shape return path. After the cross over area, the joint copper twists are split into two similar stacked copper planes on the top and bottom layer with their original width which loops and returns to the same cross over area again. This time, the top and bottom copper layers join and use the unused space of the cross over area and return to complete the infinite shape. This results in extremely small intra-winding capacitance since the surface area of the cross over region is so limited. The equivalent magnetic circuit shown in FIG. 3D verifies that by using the inventive infinite-shape winding and looping around each of the side limbs and one turn on the center limb, the flux intensity of the core in all 3 limbs of the infinite shape winding is similar to conventional winding on the center limb.

[0050] In order to achieve the desired transformer turns ratio, the PCB layers are connected in series and to increase the current capability parallel copper layers are used. Using parallel layers in planar transformers is quite challenging as the current can be distributed unevenly among parallel layers due to Proximity effect at high frequency. To mitigate the proximity effect, interleaved structures are used to reduce the magnetic field intensity and improve ac resistance. However, interleaved structures increase inter-winding capacitance between primary and secondary winding. Assuming two copper planes with overlapping area of A, separation distance of d, separated by a material with permittivity of er, the static capacitance is equal to Cstatic= eoeA / where eois the permittivity of air. Using interleaved structure, the primary and secondary inter sections increase and as the layers are interleaved like parallel copper layers, the capacitance will be multiplied by the number of intersections.

[0051] Hence,where k is the number of intersections between primary and secondary. The permittivity of FR4 which is the PCB material commonly used by PCB manufacturer is 4.7±10% that can magnify the inter- winding capacitance if two conductive layers placed across the PCB. According to embodiments, the planar transformer, by using the infinite shape winding, the material between all primary and secondary PCBs is air and hence the top and bottom coppers on two sides of PCB are mostly connected in parallel. FIG. 4A shows a series connection of two primary boards in green color and the parallel connection of the two secondary board is shown in pink. The design of the three port transformer shown in FIG. 4B is discussed in detail for the aforementioned DAB converter.

[0052] Transformer turns ratio and core selection: According to embodiments, the transformer turns ratio (N=6) is selected based on the converter operating frequency (120kHz) and optimized efficiency for the entire power range. With the advantage of the proposed infinite-shape winding, a single PCB of the proposed infinite-shape winding is directly applied to the low voltage winding, exciting the transformer with the maximum flux density in the magnetic core at the maximum voltage at LV terminal. Having the maximum flux density,557, ELP64 / 10 / 50 ferrite core is selected resulting in 3.25W core loss using N97 core material.

[0053] Series connected Primary turns: To achieve the required transformer turns ratio (N=6), the high voltage winding is constructed by 6 PCBs of the infinite-shape winding connected in series to provide an equivalent of N=6 as the effective transformer turns ratio. The series connection of the required boards is similar to FIG. 4A which can be extended to 6 series-connected PCBs. Hence, this transformer is constructed by 6 Infinite-shape PCBs for the primary (High voltage) winding and one single Infinite-shape PCB for each of the secondary and tertiary winding.

[0054] Parallel connected Secondary turns: In high current applications, connecting the copper layers in parallel is a common method to increase the current capacity. By choosing the proper thickness due to effective copper considering the skin effect, the required number of parallel PCBs can be determined to pass the required current within a reasonable current density. However, it is quite challenging to evenly distribute the current among parallel layers as the magnetic field intensity (H) varies in the window area and determines how the current is shared between parallel copper layers. Considering the skin effect at 120kHz, the maximum usable copper thickness is around 0.2mm. Hence, 6 ounce copper is the maximum thickness that can be used in each PCB layer. For the selected core, the width of the copper plane is designed to be 9mm and as in the proposed infinite-winding structure both top and bottom layer are connected in parallel, the effective utilized copper can be considered as 0.4, assuming that the current can be shared equally between top and bottom layer of the PCB. This leads to the maximum current rating of each 2-layer PCB to be 18A rms having J=5 A / mm2 current density. In this application, the required current on each of the low voltage windings (secondary and tertiary) is designed to be 55A rms. To maintain 5A / mm2 current density to properly manage the dissipated heat due to copper losses, for each of the secondary and tertiary windings, three PCBs of the infinite-shape winding are required to be connected in parallel assuming all the current is evenly distributed among parallel PCBs.

[0055] For implementing the dual output transformer according to embodiments of the present invention, there are two main challenges: (a) To efficiently utilize the parallel full bridges on the low voltage side, equal current sharing among secondary and tertiary is achieved by equal leakage inductance on LV windings, (b), For each winding, current sharing among parallel PCBs also plays an important role to utilize all the copper used in transformer winding. Assuming exactly equal applied voltage on two secondary windings of the transformer, the leakage inductance will determine the current sharing between the two parallel bridges. Bysymmetrically placing secondaries on the sides of the primary as shown in FIG. 5, any geometry mismatch between two secondaries is eliminated resulting in perfectly matched leakage inductances verified by FEA results. FIG. 5 A shows the magnetic field distribution of the SSSPPPPPPSSS structure illustrated in FIG. 5B. It is clear that a high magnetic field is constructed in the region that the primary is adjacent to the secondary and tertiary winding as the turns of each winding are stacked on top of each other. Hence, the magnetic flux is built up and reaches its maximum intensity at the adjacent region to the other windings that is going to compensate the magnetic flux with reverse direction. In such non-interleaved structure, the maximum amplitude of MMF is achieved in regions between two different windings. This phenomenon directly determines the current sharing of the parallel turns. The current is distributed based on the magnetic field exposure of the parallel turns. In case the parallel turns or layers are exposed to unequal magnetic field, the layer or turn that is exposed to higher magnetic field will absorb higher current and vice versa. So, in the non-interleaved structure shown in FIG. 5, the 3 parallel turns on the secondary and tertiary winding are exposed to core as zero MMF on one side and the maximum MMF of 31 on the other side. Hence, the current displaces and tend strongly to the higher magnetic field. This results in the total current passing through the adjacent turns to the primary which are S3 and S4. Due to unequal current distribution, SI, S2, S5, and S6 are unused resulting in excessive amount of current passing through S3 and S4 which significantly increase the copper loss and is also not acceptable from the thermal perspective. This example shows the interleaving structure to achieves a balanced current distribution and maximum copper utilization.

[0056] A common approach to decrease the magnetic field intensity is to resort to an interleaved winding structure. By splitting the primary, secondary and tertiary windings and placing them in different interleaving structures shown in FIG. 6, the MMF on the boundaries of each of the secondary or tertiary winding is reduced which can improve the current sharingand the copper utilization. In FIG. 6, each conductive layer is a two layer PCB made by the inventive infinite-shape winding. FIGs. 6A and 6B show that although interleaved structure reduces the peak MMF to +1,-1, each PCB is exposed by I and zero magnetic field on both sides. In PSSPPSSPPSSP, the interleaving method shown in FIG. 6A, the maximum MMF is reduced to +1 and -I by starting with a single primary PCB and placing two similar secondary or tertiary turns on top of each other to compensate and limit the MMF to I at all layers. In PSPSPSPSPSPS, the method shown in FIG. 6B, although the windings are shuffled between primary turns which will increase inter-winding capacitance due to higher number of adjacent layers to the primary, the exposed MMF to each turn is similar to PSSPPSSPPSSP and is limited to I. As in the inventive infinite-shape winding, 2 layers are connected in parallel for the most of the current path, having an unequal MMF distribution on the both sides of the PCB leads to unequal current in parallel layers. To achieve an equal current distribution in a single PCB, there should be an equal MMF exposed to top and bottom layer. It can be seen in FIGs. 6A and 6B, that all PCBs are exposed to +1 or -I on one side and zero MMF on the other side. This induces all the current to pass through the layer adjacent to the higher MMF. The interleaved structure is shown in FIG. 6C. In this method, one turn of the primary winding (Pl), is made by connecting two similar PCBs in parallel and one is placed on the top and the other on the bottom of all stacked PCBs. Using this structure results in not only lower magnetic field equals to 1 / 2, but also having all PCBs exposed to equal MMF +1 / 2 and -1 / 2 from top and bottom. This balances the MMF across each PCB resulting in the current to be spread equally in parallel layers and utilizing the total copper in the window area. FIG. 7 shows the FEA simulation results using Maxwell 3D. In all three simulations, the core loss is constant as the flux density inside the ferrite core has not changed in all the interleaving methods. Using N97 as the core material (with a base material of MnZn), 3.25W core loss is achieved in all cases.FIGs. 7A and 7B show that as long as each PCB is exposed to unbalanced MMF on top andbottom layer, the current will displace and move to one layer and the current density is thus higher than expected. This unequal current distribution results in 14.9W and 10.5W losses for PSSPPSSPPSSP and PSPSPSPSPSPS accordingly. Using the proposed novel interleaving method by reducing the MMF to 1 / 2 on both sides of each PCB, a balanced MMF exposure to all turns and equal current distribution is achieved for all the windings. FIG. 7C shows that using the inventive interleaving method, a total loss of 8.04W is achieved for the full power excitation of the proposed transformer. The current injected into secondary and tertiary winding is 80A peak which should be shared in 3 parallel PCBs and it results in 18.85 A rms on the high voltage winding which is the nominal current at the maximum power. The current density of the top and bottom layer of the proposed interleaving method is shown in FIGs. 8A-8D. The current distribution in the top and bottom layer of all the PCBs except the first and the last one is shown in FIGs. 8A and 8B. This clearly shows that both top and bottom layers share equal current for almost the entire path and a slightly higher current density is achieved at the cross over region which is due to the proposed infinite-shape winding structure and the required vias to switch between the layers. FIGs. 8C and 8D show the current distribution for the two PCBs that construct Pl by parallel connection of two similar infinite-shape winding. Only one layer of these two PCBs are used effectively as these are the only PCBs that are exposed to zero and 1 / 2 MMF and the current is displaced to higher MMF as expected. However, since two PCBs are connected in parallel to construct Pl, the current density is still in the desired range of 5 A / mm2 same as the other layers. The FEA simulation results verify the effectiveness of the proposed interleaving structure to perfectly utilize the total used copper for such low profile, high current application.

[0057] The series and parallel connections of the inventive interleaved planar transformer is shown in FIGs. 9A and 9B. The PCBs are stacked on top of each other by a narrow 0.1mm thin KAPTON® tape as spacer and the spaces between two adjacent PCBs is mostly air. Thecurrent path for the series and parallel connections is shown for each winding for further clarification. FIG. 9C shows all the PCBs used in the proposed transformer. As shown in Table I, each PCB is made of two simple layers having 2oz copper thickness which makes it pretty simple and cost effective for manufacturing.

[0058] Table I- Experimental Setup Specification armne Li2 is valuesBatten VisLlaInput Voltage,Sv. itching frequency - 120kHzTi tlsfoi ltier turns rntiu K 6 ! 1 ! L LV w indings 3 x parallel infinite -shape | t11 v winding ? x infinite- shape pebInfinite-shape w inding peb 2 lav er boxTotal I nJ u dunce ]l). ul lBloc king capacitor i 'lf4. SubPeak eilitieney W

[0059] The performance of an inventive transformer is verified in a 5kW, 48V-400V, 120kHz DAB converter with the specification shown in Table. I. The power density of the low- profile planar transformer is 43 kW / L which is impressive for such high current application. Using the proposed infinite-shape winding, the planar transformer height is as low as 20 mm which makes this transformer suitable for low profile applications. Power density of 3.7 kW / L is achieved for the entire converter shown in FIG. 10C using the proposed transformer including all the heatsinks for the cooling. To achieve equal current distribution in all parallel copper layers and maximize copper utilization in the proposed transformer, a novel interleaving method is disclosed herein and validated by FEA results using Maxwell 3D. The real size of the proposed transformer is shown in FIG. 10A and the exploded view of the mechanical designof the converter is shown in FIG. 10B. The proposed infinite-shape winding uses the side limbs of the magnetic core to obtain the required effective turns ratio maintaining low inter-winding capacitance. This provides the opportunity of using simple two layer PCBs with off-the-shelf planar cores for wider range of applications.

[0060] Unlike the conventional winding structures where two turns are achieved by stacking two copper layers on top of each other and connected in series around one limb of the magnetic core, embodiments of the present invention use a novel winding structure wherein a two turn PCB winding is made by a two layer PCB in such a way that the top and bottom layer are identical and connected in parallel to make a single turn loop. At the end of each turn (loop) around each limb of a core, the top and bottom layer flip 180 degrees on an adjacent limb to make another turn(loop) and generate a flux that adds to the flux generated by the first turn. This results in a permuting infinite-shape winding structure for a planar transformer core, wherein the winding utilizes all limbs of the core to maximize effective turns.

[0061] To implement the 180-degree flipping in a single two-layer PCB, a crossover area is necessary where the top and bottom layers are swapped. In this crossover region, the two stacked parallel copper layers permute so that they merge into a single layer with double the width, leaving half of the height of the stacked copper layers unused for the return path. Subsequently, the joint copper layers are split into two identical stacked layers with their original width, looping and returning to the crossover area. This arrangement enables efficient utilization of the winding area without leaving unused space, thereby facilitating optimal utilization of the winding area.

[0062] To enhance current capability in high-current applications, parallel permuting infinite-shape windings are utilized in embodiments of the planar transformers. Equal current distribution among the parallel connected turns are used to have thermally stable copper planes and avoid any hotspots.

[0063] The present disclosure further involves an interleaving method to achieve balanced magnetic field exposure and equal current distribution for the parallel windings, effectively reducing AC resistance due to current displacement. As an example a planar transformer is investigated where the low voltage windings (secondary and tertiary) are single turn windings that need to be constructed by 3 windings (3 of the proposed PCB winding) in parallel to meet the required current capacity, and the primary side consists of 6 turns connected in series for a total turns ratio of 6: 1:1. Conventional interleaving methods only shuffle the primary, secondary, and tertiary windings, which have opposite currents in the window area, to reduce the built-up magnetic field intensity (H) that can displace current. In a fully interleaved PSPSPSPTPTPT structure, where P represents the primary winding, S represents the secondary winding, and T represents the tertiary winding, the maximum magnetic field intensity (H) is equal to NI (N=l), which is I in this case. So each PCB faces 0 MMF on one side and +1 or -I on the other side. Due to the parallel connection of the top and bottom layers of the inventive PCB, all the current is shifted toward the layer that faces nonzero MMF. However, embodiments of the present invention use a new interleaving method, such that one of the primary turns is split into two identical PCBs, similar to the other windings. Assuming this new split winding is called P / 2, one of them can be placed on the very top layer of the conventional stacking and the other P / 2 at the very bottom, resulting in P / 2SPSPSPTPTPTP / 2 placement of the windings in the window area. With this invention, the maximum magnetic field intensity across each PCB is +1 / 2 and -1 / 2, which results in equal MMF facing both top and bottom layers for all PCBs. In this proposed interleaving structure, fully equal current distribution in the top and bottom layers of all windings is achieved, and the total copper is effectively utilized.

[0064] The inventive winding structure can be expanded, comprising placing another similar core adjacent to each other, maintaining a constant profile while achieving a higher number of turns as needed. Unlike conventional approaches wherein an increase in the number of turnsresults in a proportional increase in the height of the magnetic component, this method utilizes the proposed expandable winding to achieve higher power capacity with existing core structures and eliminates the need for custom-made magnetic cores. By stacking a second core side by side, not only the voltage can be doubled, but also twice the power can be delivered. Accordingly, the present invention provides scalability, making this winding structure more versatile for different planar design applications with different voltage and power ranges.

[0065] An example, an embodiment of the present invention is applied to a planar transformer using an ELP64 / 10 / 50 core for a 5kW 48V-400V Dual Active Bridge (DAB) converter operating at 120kHz. By employing a single infinite-shape winding on the 48V side, equivalent to two effective turns, the flux intensity of 107mT is achieved at 55V voltage excitation, which avoids saturation of the core. The 110 amp is shared among two windings, each made of 3 parallel PCBs of the winding, and all copper layers do not exceed 5 A / mm2 current density. The performance of the proposed transformer is verified by both a 5kW experimental setup at full power and FEA simulation results, and the core and PCB windings temperature at steady state with natural cooling is less than 70 degrees Celsius at 25°C ambient temperature.

[0066] Patent documents and publications mentioned in the specification are indicative of the levels of those skilled in the art to which the invention pertains. These documents and publications are incorporated herein by reference to the same extent as if each individual document or publication was specifically and individually incorporated herein by reference.

[0067] While at least one exemplary embodiment has been presented in the foregoing description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the described embodiments in any way. Rather, the foregoing description and incorporated references will provide those skilled in the art with a convenient roadmap for implementing the exemplary embodiment or exemplary embodiments.It should be understood that various changes may be made in the function and arrangement of elements without departing from the scope as set forth in the appended claims and the legal equivalents thereof.

[0068] References

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[0071] [3] A. KhakparvarYazdi, N. Mazloum, M. Mahdavifard, and S. A. Khajehoddin,“Design and magnetic optimization of dual active bridge converters for energy storage application,” in 2022 IEEE Applied Power Electronics Conference and Exposition (APEC), March 2022, pp. 382- 389.

[0072] [4] G. C. Knabben, J. Schafer, L. Peluso, J. W. Kolar, M. J. Kasper, " and G. Deboy,“New PCB winding ”snake-core” matrix transformer for ultra-compact wide de input voltage range hybrid b+dem resonant server power supply,” in 2018 IEEE International Power Electronics and Application Conference and Exposition (PEAC), Nov 2018, pp. 1-6.

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[0074] [6] Z. Ouyang, O. C. Thomsen, and M. A. E. Andersen, “Optimal design and tradeoff analysis of planar transformer in high-power dc-dc converters,” IEEE Transactions on Industrial Electronics, vol. 59, no. 7, pp. 2800- 2810, July 2012.

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[0010] M. Pahlevaninezhad, D. Hamza, and P. K. Jain, “An improved layout strategy for common-mode emi suppression applicable to high-frequency planar transformers in high- power dc / dc converters used for electric vehicles,” IEEE Transactions on Power Electronics, vol. 29, no. 3, pp. 1211-1228, March 2014.

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[0011] Z. Zhang, J. Huang, and Y. Xiao, “Gan-based 1-mhz partial parallel dual active bridge converter with integrated magnetics,” IEEE Transactions on Industrial Electronics, vol. 68, no. 8, pp. 6729-6738, Aug 2021.

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[0012] Y. Cai, M. H. Ahmed, Q. Li, and F. C. Lee, “Optimal design of megahertz lie converter for 48-v bus converter application,” IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 8, no. 1, pp. 495-505, March 2020.

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Claims

CLAIMS1. A winding for a low-profile planar transformer, the winding comprising: a printed circuit board having a first surface and a second surface, the printed circuit board defining a plurality of through openings therein; a current path including a fust portion that is positioned on the fust surface of the printed circuit board and a second portion that is positioned on the second surface of the printed circuit board, the current path transiting around the plurality of through openings defining in the printed circuit board and crossing between the first surface and second surface of the printed circuit board via the through openings, the current path having a first terminal end and a second terminal end both positioned on the fust surface of the printed circuit board.

2. The winding of claim 1 wherein the printed circuit board is a two layer printed circuit board.

3. The winding of claim 1 wherein the printed circuit board is formed of FR4.

4. The winding of any one of claims 1 to 3 wherein the printed circuit board is formed of a material having a permittivity of 4.7±10% greater than air.

5. The winding of any one of claims 1 to 3 wherein the current path is formed of copper.

6. The winding of any one of claims 1 to 3 wherein the first portion of the current path is connected in parallel to the second portion of the current path.

7. A low-profile planar transformer comprising: a first winding of claim 1 ; and a first magnetic core having a plurality of limbs each of the plurality of limbs having an area, the plurality of limbs configured to align with and be received within the plurality of through openings in the printed circuit board of the winding.

8. The low-profile planar transformer of claim 7 wherein two of the plurality of limbs are positioned at a first outer edge and a second outer edge of the first magnetic core, respectively.

9. The low-profile planar transformer of claim 7 wherein the area of the two limbs positioned at the first outer edge and the second outer edge of the first magnetic core is half of the area of any limb of the plurality of limbs that is positioned between the two limbs positioned at the first outer edge and the second outer edge of the first magnetic core.

10. The low -profile planar transformer of claim 7 wherein the first magnetic core is formed of ferrite.

11. The low -profile planar transformer of claim 7 wherein the first magnetic core is an E- shaped magnetic core.

12. The low-profile planar transformer of claim 7 wherein the plurality of limbs is at least three limbs.

13. The low-profile planar transformer of claim 7 wherein each of the plurality of limbs are parallel to one another.

14. The low-profile planar transformer of claim 7 further comprising a one to six additional windings connected in series with the first winding, the first winding and the one to six additional windings forming a first primary board.

15. The low-profile planar transformer of claim 14 further comprising at least one additional primary board formed of at least two additional windings connected in series.

16. The low -profile planar transformer of claim 14 wherein further comprising at least one secondary board formed of at least two additional windings connected in parallel.

17. The low-profile planar transformer of claim 16 wherein the first primary board, the at least one additional primary board, and the at least one secondary board are interleaved with air therebetween.

18. The low -profile planar transformer of claim 17 wherein the interleaving achieves a balanced current sharing within the planar transformer.

19. The low-profile planar transformer of any one of claims 7 to 18 further comprising a second magnetic core positioned opposite to the first magnetic core with at least the first winding therebetween.

20. The low -profile planar transformer of any one of claims 7 to 18 wherein the planar transformer is configured for use in a dual active bridge converter.