Planar transformer

Abstract

The present disclosure relates to planar transformer (10)comprising a first PCB (1) comprising a first coil pattern surrounding a first opening (11), a second PCB (2) comprising a second coil pattern surrounding a second opening (21), and at least one elastically compressible element (5) sandwiched between the first and second PCB (1, 2). The planar transformer (10) further comprises a top core portion (3) and a bottom core portion (4), wherein the first and second PCB (1, 2) are arranged between the top and bottom core portions (3, 4) and, wherein the at least one elastically compressible element (5) is compressed by the first and second PCB (1, 2) so as to push the first and second PCB (1, 2) away from each other, towards the inner surfaces of the top and bottom core portions (3, 4) of the magnetic core assembly, respectively.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a planar transformer, specifically a planar transformer well suited for use in DC-DC converters in vehicles.BACKGROUND OF THE INVENTION

[0002] Electrical energy powers most parts of society including vehicles, heating systems and many types of consumer products such as computers and smartphones.

[0003] In many situations, it is necessary to convert electric power of a first voltage level (e.g. voltage level output by an energy source) to power of a second voltage level (e.g. a voltage level which is suitable for powering various electrical devices). For example, in electrical vehicles, EVs, the voltage of a high voltage battery is often converted with a DC-DC converter to a lower voltage suitable for powering low voltage devices onboard the EV, such as the headlights and the infotainment system. Therefore, a voltage converter is often arranged between the energy source and the electrical load, to convert the voltage output by the energy source to a voltage suitable for the load.

[0004] Many types of DC-DC converter topologies utilize a transformer. For example, in a dual active bridge, DAB, topology a transformer is arranged between two full-bridges and the direction of power flow, as well as the power level, can be adjusted by controlling the switches of the respective full-bridge circuits. In addition to the versatility of the DAB topology, another advantage with this topology is that the transformer introduces galvanic isolation between the input and output.

[0005] A transformer comprises two coils that are wound around a common magnetic core and the electric properties of the transformer depends on many factors such as the number of windings in each coil, the shape and size of the magnetic core and the relative arrangement of the coils around the core.

[0006] A drawback with many types of transformers is that they are large and bulky. To make the transformers more space efficient so called planar transformers have been introduced where the transformer windings are integrated into printed circuit boards, PCBs, and the magnetic core fitted around the PCBs. An advantage of planar transformers is therefore that they can be made more compact while achieving higher power volume densities and / or higher power mass densities.

[0007] Transformers are however also difficult to manufacture precisely so as to achieve the desired electrical properties with a small margin of error. Accordingly, external components are often added to tune the electrical characteristics of the transformer according to meet requirements. However, external components increase the cost of the final electrical system while also requiring extra space and adding to the total system weight.

[0008] Additionally, even if external components are added to tune the characteristics of the transformer there is still the problem of the transformer properties changing over time due to the transformer aging or being subject to mechanical stress. Especially within the field of e-mobility, where transformers are used in e.g. the DC-DC converters onboard EVs, the transformers may be subject to vastly varying temperatures, mechanical vibration and other external factors which overtime causes the transformer properties to change, leading to sub-optimal efficiency or even electrical failure. To this end, there is a need for an improved transformer design which offers both high efficiency and high reliability even in harsh environments.SUMMARY OF THE INVENTION

[0009] It is a purpose of the present invention to overcome at least some of the shortcomings of traditional transformers and provide a new transformer design which is reliable and efficient while being compact and easy to manufacture according to specifications.

[0010] According to a first aspect of the present invention there is provided a planar transformer, comprising a stacked coil assembly. The stacked coil assembly comprises a first PCB comprising a first coil pattern formed in a planar conducting layer of the first PCB, the first coil pattern surrounding a first opening extending through the first PCB, and a second PCB comprising a second coil pattern formed in a planar conducting layer of the second PCB, the second coil pattern surrounding a second opening extending through the second PCB. The stacked coil assembly further comprising at least one elastically compressible element sandwiched between the first and second PCB. The planar transformer further comprises a magnetic core assembly comprising a top core portion and a bottom core portion, wherein the first and second PCB are arranged between the top and bottom core portions and wherein the top and bottom core portions are configured to form a casing around the first and second PCB. Each of the top and bottom core portion comprises an inner surface, facing the first and second PCB, respectively. At least one of the top and bottom core portions comprises a protrusion protruding out from the inner surface so as to contact the other core portion through the central openings of the first and second PCB. Additionally, the at least one elastically compressible element is compressed by the first and second PCB so as to push the first and second PCB away from each other, towards the inner surfaces of the top and bottom core portions of the magnetic core assembly, respectively.

[0011] With the elastically compressible element sandwiched between the PCBs and pushing the PCBs away from each other towards respective inner surfaces of the top and bottom core portions. The PCBs will hereby be in thermal contact with the magnetic cores allowing heat to be dissipated via the core portions. Additionally, since the core portions can be manufactured with excellent manufacturing tolerances, the spacing between the two PCBs can be very accurate, allowing the transformer to be manufactured to exhibit desired electrical properties (e.g. meeting leakage inductance requirements) with low error margins while at the same time enabling these electrical properties to be retained even when the transformer ages or is used in a harsh environment. Notably, the leakage inductance is proportional to the separation distance between the PCBs, and with the planar transformer of the present invention this distance can be tuned accurately allowing for the desired leakage inductance to be realized with small margin of error which facilitates correct and efficient operation when the planar transformer is e.g. in a DAB DC-DC converter. This in turn allows physical series inductances typically connected in series with the transformer in some circuits (e.g. DAB converters) to be omitted and replaced entirely by the leakage inductance.

[0012] By comparison, in solutions where there is e.g. an air gap between each PCB and each core portion it is difficult to achieve consistent electrical properties across multiple transformer samples.as Additionally, as the transformer e.g. is subject to mechanical stress or temperature variations, there is a risk that transformer is damaged or breaks and the elastically compressible elements makes the transformer more reliable and robust. A further advantage of placing the elastically compressible element between the PCBs to push the PCBs against the inner surfaces of the core portions is that a very thin layer of thermal interface material (TIM) can be placed between each PCB and the respective core portion. Compared to fastening the PCBs against the inner surfaces of the transformer by glueing (e.g. using an adhesive TIM) the elastically compressible element allows a thinner TIM layer to be used, which increased thermal conductive, and enables non-adhesive TIMs to be used which may have improved thermal conduction properties compared to adhesive TIMs. Additionally, the elastically compressible element between the PCBs has a much improved lifetime, especially in harsh environments. If a transformer is subject to mechanical stress and / or temperature variations, there is a risk that the adhesive TIM degrades whereby the PCB may come loose which will degrade the thermal conductivity between the PCB and the core portions while also altering the electrical properties of the transformer.

[0013] Furthermore, planar transformers can be made thin and flat enabling more efficient cooling which contributes to achieving higher energy densities. In planar transformers the heat generated by the coil patterns is conveyed to the top and bottom core portions, whereby the heat can be dissipated effectively via the core portions. As a consequence of higher achievable energy densities, the planar transformer may be realized with a smaller volume and / or mass compared to regular transformers. Smaller and / or lighter transformers are in general more resilient to mechanical stress, such as impacts or vibrations.

[0014] A general benefit of incorporating the coils as conductive traces in PCBs is that the windings may be made in a more accurate and repeatable reputable fashion compared to windings of regular transformers. This also contributes to achieving the desired electrical properties with very good tolerances which could minimize the use of external components.

[0015] In some implementations, at least one of the top and bottom core portions which comprises the protrusion forms an E core, and the protrusion forms a central leg of the E core.

[0016] The other core portion may also be an E-core, or it is envisaged that the other core portions is an I-core. In the alternative, one of the core portions is T-core and the other core portion is a U-core. These different core types (shapes) all enable the formation of a casing surrounding the PCBs and the elastically compressible element sandwiched there between.

[0017] In some implementations, the planar transformer further comprises a separation module comprising a separation plate sandwiched between the first PCB and the second PCB. The separation plate comprises an opening allowing the central protrusion of at least one of the top and bottom core portion to pass through and the at least one elastically compressible element is at least two elastically compressible elements, a first elastically compressible element located between the first PCB and a first side of the separation plate, and second elastically compressible element located between the second PCB and a second side of the separation plate, opposite to the first side.

[0018] The separation plate further enhances reliable retention of the PCBs at their desired positions in contact with the inner surface of the core portions. Additionally, the separation plate allows the separation distance between the PCBs to be made larger and also facilitates distributing the pressure exerted by the compressed elastically compressible elements more evenly across the surface of the respective PCB.

[0019] The separation plate may be made of non-conductive material such as plastic.

[0020] In some implementations, at least one of the first and second side of the separation plate is provided with a recess, and wherein at least one of the first and second elastically compressible element is placed in the recess.

[0021] The recess enables the elastically compressible elements to be held in place between the PCB and separation plate without physically attaching (e.g. adhering) the elastically compressible elements to the separation plate.

[0022] Optionally, the recess forms a grid of grooves, and wherein the at least one of the first and second elastically compressible element placed in the recess is grid shaped, the grid shape matching the grid of grooves.

[0023] A grid shaped elastically compressible element arranged in a matching recess on the separation further facilitates retention of the elastically compressible element with respect to the separation plate and may facilitate a homogenous distribution of the pressure exerted on the PCBs.

[0024] In some implementations, the separation module further comprises a first and second side portion extending perpendicular to the separation plate at opposite sides of the separation plate. The first side portion comprises a first slot for receiving the first PCB on the first side of the separation plate, and the second side portion comprises a second slot for receiving the second PCB on the second side of the separation plate.

[0025] Hereby, the side portions will also contribute to the retention of the PCBs on opposite sides of the separation plate. The side portions are either mechanically coupled to the separation plate or made a single piece with the separation plate whereby the separation module acts a backbone for the transformer providing structural stability.

[0026] In some implementations, each side portion comprises two connectors, and each side portion further houses two conductors connecting each connector to a respective PCB terminal on the respective PCB.

[0027] That is, the side portions may also be used to house and protect the terminals of the PCBs while also offering connectivity to external devices. By integrating the connectors into the side portions it is possible to make the planar transformer very compact while also making the transformer easier to install since access to the PCB terminals is not required due to these already being connected to connectors, wherein the connectors may be standard type connectors.

[0028] In some implementations, each side portion comprises one or more protrusions, configured to engage an alignment recess formed in the top and / or bottom core portion.

[0029] With side portions mechanically coupled to the core portions the structural stability of the transformer is even further enhanced, making it less prone to breakage when e.g. used in harsh environments with mechanical stress.

[0030] In some implementations, a TIM layer is provided between each PCB and the respective inner surface of the top and bottom core portion.

[0031] As stated above, a TIM material layer will facilitate thermal transfer from the PCBs to the core allowing the transformer to be more effectively cooled. For example, the TIM layer is less than X micrometers thick. In the alternative, no TIM is provided between the PCBs and the inner surface of the core portions.

[0032] In some implementations, the transformer has a width along a width axis, a length along a length axis, and a height along a height axis. The width, length and height axes are perpendicular to each other and the top and bottom core portions of the magnetic core surrounds at least a portion of the first and second PCB along the width axis and along the height axis.

[0033] That is, the casing formed by the core portions substantially surrounds the PCBs and the elastically compressible element along at least two axes. While the core portions may leave the PCBs exposed along the third length axis, the side portions described above may substantially cover the PCBs along this direction whereby the PCBs according to some implementations is substantially wholly covered and protected by the core portions and the side portions of the separation module.

[0034] In some implementations, at least one of the top and bottom core portions comprises two separate core pieces separated along the length direction.

[0035] Instead of using a single piece top core portion or single piece bottom core portion the provision of two-piece core portions further improves the reliability of the transformer and reduces the risk for breakage. Since the core portions are typically made of a rigid and solid material (such as a ferrite material) these may be subject to cracking or breakage if they are made too large. To this end, by splitting the top and / or bottom core portion into two pieces the pieces can move slightly relative to each other thereby reducing the risk for breakage.

[0036] A top and bottom core portion which is split into two or more pieces does not impede the transformer operation if the splitting plane (i.e. an imaginary plane delimiting the two pieces) is defined appropriately. Advantageously, the splitting plane is perpendicular to the plane of the PCBs. For example in a width-height plane if the PCBs extend in a respective width-length plane.

[0037] To further improve reliability, the two separate core pieces may interface at width-height plane, and wherein a spacer element is arranged at the interface between the two separate core pieces. The spacer element may be elastically compressible. Advantageously, the spacer material has a high thermal conductivity, and it is envisaged that a thin TIM layer may be used as the spacer element.

[0038] In some implementations, a spacer element is arranged between the top and bottom portions of the magnetic core.

[0039] The spacer element placed between the core portions separates the core portions which will mainly influence the magnetization inductance of the planar transformer whereby this spacer element can be used to tune the electrical properties of the transformer by providing a thicker or thinner spacer element between the top and bottom core portions. When the planar transformer is used in a DC-DC converter, e.g. in a DAB, the top and bottom core portions are advantageously in direct contact with each other. One the other hand, it is envisaged that the planar transformer can be used in other implementations as well, e.g. as a flyback transformer or in resonant topologies such as LLC topologies, where it can be advantageous to tune the magnetization inductance of the planar transformer by adapting the size of the spacer element and thereby the spacing between the top and bottom core portion. It is understood that even though the spacing between the core portions mainly influences the magnetization inductance it could also influence other electrical parameters of the transformer. In addition to tuning the magnetization inductance, the planar transformer also allows the leakage inductance to be tuned by setting the separation distance between the PCBs.

[0040] In some implementations, the top and bottom core portions comprise retention features, such as an indentation configured to receive a clip for holding the top and bottom core portions together.

[0041] The retention features may e.g. be indentations or protrusions provided on the outer surface of the core portions configured to allow engagement of a clip. Hereby, it becomes easy to assemble the magnetic core portions to form a housing around the PCBs and the elastically compressible element. Advantageously, the elastically compressible element is arranged such that it becomes compressed when the core portions are assembled to form the casing around the PCBs.

[0042] According to a second aspect of the invention there is provided a dual active bridge (DAB) voltage converter, comprising a planar transformer according to the first aspect.

[0043] DAB converters are frequently used in vehicles such as cars, trucks, boats or airplanes where the converter is exposed to temperature variations and mechanical stress. Accordingly, a DAB converter with the planar transformer described herein may be more reliable, robust and compact compared to previous DAB converters utilizing traditional transformers.BREIF DESCRIPTION OF THE DRAWINGS

[0044] Aspects of the present invention will be described in more detail with reference to the appended drawings, showing currently preferred embodiments. Figure 1 is an exploded view of a planar transformer according to some implementations. Figure 2a shows a cross-section of a separation module according to some implementations. Figure 2b shows a cross-section of a separation module with the PCBs installed. Figure 2c shows a cross-section of a planar transformer. Figure 3a shows a perspective view of a side portion provided with an inner protrusion, for retaining the magnetic core portions and / or a PCB. Figure 3b shows a perspective view of a side portion provided with an inner protrusion together with a PCB. Figure 4 shows a perspective view of an assembled planar transformer according to some implementations. Figure 5a shows an exploded view of two grid-shaped elastically compressible elements and a separation plate with a textured surface according to some implementations. Figure 5b shows a side-by-side comparison of a grid-shaped elastically compressible element and textured separation plate. Figure 5c shows an exploded view of two alternative elastically compressible elements and an associated alternative separation plate, according to some implementations. Figure 5d shows a side-by-side comparison of the alternative elastically compressible element and the associated separation plate. Figures 6a-c show different types of magnetic core portions that could be used to form the planar transformer according to some implementations. Figure 7 is a cross-section of a planar transformer according to some implementations. DETAILED DESCRIPTION

[0045] The present disclosure relates to a planar transformer with high reliability, structural stability and which offers a reduced performance decrease as the planar transformer ages. The planar transformer is therefore highly suitable for use in environments with high mechanical stress, such as in DC-DC converters onboard vehicles.

[0046] In some implementations, the planar transformer is used in a dual active bridge (DAB) DC-DC converter. The DAB DC-DC converter topology is as such known in the art and widely used due to its versatility, efficiency and galvanic isolation between the input and output. However, the planar transformer of the present invention may of course be used in other DC-DC topologies that require a transformer or generally used in any electrical system, not limited to voltage converters.

[0047] Fig. 1 shows an exploded view of the planar transformer 10 according to some implementations. The planar transformer 10 comprises a first printed circuit board, PCB, 1 and a second PCB 2. Each PCB 1, 2 is substantially flat and arranged to extend in parallel planes. In fig. 1 the PCBs 1, 2 extend in planes spanned by the width axis W and the length axis L whereby the PCBs 1, 2 are offset from each other along the height axis H.

[0048] The first PCB 1 comprises a first conductive coil surrounding a first opening 11 extending through the PCB 1. The first conductive coil may be formed by a conductive trace of at least one conductive layer of the first PCB 1. The first PCB 1 comprises at least a substrate layer and one conductive layer arranged on the substrate layer. However, there are many types of multi-layer PCBs that can be used to form the first PCB 1. For example, the PCB 1 may be realized with two substrate layers wherein the conductive layer forming the first coil is sandwiched between the two substrate layers. Furthermore, it is envisaged that multiple conductive layers can be used, each conductive layer separated by at least one substrate layer.

[0049] The first PCB 1 further comprises at least two terminals 13, 14, a first terminal 13 and a second terminal 14. The first terminal 13 is connected to a first end of the conductive trace that forms the first coil and the second terminal 14 is connected to the other end of the conductive trace that forms the first coil. The terminals 13, 14 may be arranged arbitrarily on the first PCB 1 but advantageously the terminals 13, 14 are arranged close to a same edge of the PCB 1, and out of the way of the coil, to make it easy to connect to the terminals 13, 14.

[0050] The second PCB 2 is generally the same as the first PCB 1. The second PCB 2 comprises a second coil pattern formed in a conducting layer of the second PCB 2 and the second coil pattern surrounds a second opening 21 extending through the second PCB 2. The second PCB 2 also comprises at least two terminals 23, 24, allowing electric connection to the second coil pattern of the second PCB 2.

[0051] Generally, to provide voltage conversion between different voltage levels the number of windings n 2 (i.e. laps around the second opening 21) formed by the second coil pattern will be different from the number of windings n 1 (i.e. laps around the first opening 11) formed by the first coil pattern. The voltage conversion ratio achieved by a voltage transformer incorporating the planar transformer 10 will be proportional to n 1 / n 2 , or n 2 / n 1 , depending on the direction of power flow. It is also noted that bidirectional power flow can also be achieved when the first and second number of windings n 1 , n 2 are the same. While no voltage conversion is achieved in this case the planar transformer still provides control of the power flow level, power flow direction and galvanic isolation.

[0052] At least one elastically compressible element 5 is sandwiched between the first and second PCB 1, 2 whereby the first PCB 1, the second PCB 2 and the elastically compressible element 5 forms a stacked coil assembly.

[0053] In the stacked coil assembly, the first and second opening 11, 21 are at least partially overlapping forming a thoroughgoing channel extending through the first and second PCB 1, 2. The elastically compressible element 5 is advantageous arranged so as to not obstruct the channel extending through the first and second PCB 1,2.

[0054] The elastically compressible element 5 may be a piece of foam material which has a relaxed uncompressed state. When the elastically compressible element 5 is arranged in the planar transformer it is at least partially compressed by the first and second PCB 1, 2 whereby the elastic properties of the elastically compressible element 5 as a consequence of being compressed exerts a force on the PCBs 1, 2, pushing them apart.

[0055] As another alternative, the elastically compressible element 5 is a spring (e.g. a coil spring, wave spring or linear wave spring) which is sandwiched between the first and second PCB 1, 2 and at least partially compressed by the PCBs 1, 2 to exert a force on the PCBs 1, 2 forcing the PCBs 1, 2 apart.

[0056] The planar transformer 10 further comprises a magnetic core assembly comprising a top core portion 3 and a bottom core portion 4. Each core portion 3, 4 is made of a magnetic material. For example, each core portion 3, 4 is made of a ferromagnetic material to act as a permanent magnet. In fig. 1 the top and bottom core portions 3, 4 are both E-cores but this combination of core portions 3, 4 is merely one example and combinations of core portions can be used, as discussed in connection with figs. 6a-c.

[0057] The stacked coil assembly (i.e. the PCBs 1, 2 and the elastically compressible element 5 arranged therebetween) is arranged between the top and bottom core portions 3, 4. The top and bottom core portions 3, 4 are configured to form a casing around the stacked coil assembly. The top and bottom core portions 3, 4 may be configured to be held together around the stacked coil assembly by one or more retention elements, such as clamps or screws.

[0058] Each of the top and bottom core portion 3, 4 hereby comprises an inner surface 3a, 4a, facing the first and second PCB 1, 2, respectively, and an opposite outer surface 3b, 4b facing away from the first and second PCB 1, 2, respectively.

[0059] The elastically compressible element 5 of the stacked coil assembly forces the PCBs 1, 2 away from each other and against the respective inner surfaces 3a, 4a of the top and bottom core portions 3, 4. In this way, the PCBs 1, 2 will remain at predetermined separation distance from each other and in contact with the inner surfaces 3a, 4a of the core portions 3, 4 even as the planar transformer ages or exposed to mechanical stress (e.g. vibrations) or exposed to varying temperatures. Compared to a solution where each PCB 1, 2 is only adhered directly to the inner surfaces 3a, 4a of the core portions 3, 4 with an adhesive the provision of an elastically compressible element has the benefit of being more reliable. At the same time, adhering the PCBs 1, 2 to the inner surfaces 3a, 4a of the core portions 3, 4 requires a comparatively thick layer of adhesive to obtain sufficient retention. However, a thick layer of adhesive will impede the thermal coupling between the core portions 3, 4 and the PCBs 1, 2 which may make it challenging to properly cool the PCBs 1, 2.

[0060] Each magnetic core portion 3, 4 comprises a central protrusion 33, 43 protruding out from the respective inner surface 3a, 4a so as to contact the other core portion 3, 4 (specifically a central protrusion 33, 43 thereof) through the openings 11, 21 of the first and second PCB 1, 2. In this way, the magnetic core portions 3,4 form a magnetic transformer core that magnetically couples coils of the first and second PCB 1, 2 to each other.

[0061] Each magnetic core portion 3, 4 further comprises side protrusions 34, 44 (sidewalls) that contact each other outside the first and second PCB 1, 2 to form the casing which at least partially surrounds the stacked core assembly. However, as discussed in connection with figs. 6a-c this arrangement is merely exemplary and it is envisaged that in some implementations only one of the core portions 3, 4 comprises a central protrusion 33, 43 and / or only one of the core portions 3,4 comprises side protrusions 34, 44.

[0062] Each core portion 3, 4 may further comprise two or more individual core pieces 31, 32, 41, 42. As seen in fig. 1, the top core portion 3 comprises two individual core pieces 31, 32 and the bottom core portion 3 comprises two individual core pieces 41, 42.

[0063] The planar transformer 10 has a width along a width axis W, a length along a length axis, L, and a height along a height axis, H, wherein the width, length and height axes are perpendicular to each other. The core portions 3, 4 are split in a splitting plane spanned by the width and height axes to form the core pieces 31, 32, 41, 42. Splitting along other splitting planes than the width-height plane is also envisaged, such as a plane spanned by the length-height axis instead. Generally, it is advantageous if the splitting plane does not intersect the magnetic field generated by the coils, whereby the splitting plane should be substantially perpendicular to the planes in which the PCBs 1, 2 extend.

[0064] With core portions 3, 4 realized by multiple pieces 31, 32, 41, 42 it is possible to make a planar transformer which is more reliable and tolerant to mechanical stress, such as vibrations. For example, two pieces 31, 32 making up the top core portion 3 can move slightly relative to each other allowing the planar transformer to better withstand mechanical stress compared to if the top core portion is made a single rigid piece.

[0065] To dampen vibrations a spacer element 8 may be placed at the interface between two core pieces 31, 32, 41, 42. The spacer element 8 may e.g. be an elastically compressible element. In some implementations, to facilitate heat transfer from one piece 31, 41 to the other piece 32, 42 a thermal interface material (TIM) is used to realize the spacer element 8.

[0066] The TIM may be a thermal paste, a thermal adhesive or one or more thermally conductive pads which may provide some dampening for mechanical stresses and allow the core pieces 31, 32, 41, 42 to move slightly relative to each other.

[0067] Each core piece 31, 32, 41, 42 may further comprise one or more indentations 36 configured to receive a clip for holding the core pieces together, over the PCBs 1, 2, the separation plate 61 and the elastically compressible elements 5.

[0068] The stacked coil assembly further comprises a separation module 6 in some implementations. The separation module 6 comprises a separation plate 61 sandwiched between the first and second PCB 1, 2 whereby the at least one elastically compressible element 5 is arranged between the separation plate 61 and at least one of the PCBs 1, 2. Advantageously, at least two elastically compressible elements 5 are provided, with one elastically compressible element 5 located on either side of the separation plate 61 such that a first elastically compressible element 5 is sandwiched between the first PCB 1 and the separation plate 61 and a second elastically compressible element 5 is located between the second PCB 2 and the separation plate 61.

[0069] The separation plate 61 comprises a throughgoing opening 62 overlapping the openings 11, 21 of the PCBs 1, 2 to allow the central protrusion(s) 33, 43 of the core portions 3, 4 to pass through also the separation plate 61.

[0070] Hereby, the two elastically compressible elements 5 will serve to press the PCBs 1, 2 away from each other, away from the separation plate 61, and against the inner surfaces 3a, 4a of the core portions 3, 4.

[0071] While the separation plate 61, and the entire separation module 6, may facilitate more homogenous pressure distribution against the PCBs 1, 2 and generally contribute to the structural reliability of the planar transformer 10 it is envisaged that the planar transformer can be realized without these components. For example, the at least one elastically compressible element 5 may be provided directly between the first and second PCB 1, 2 in some embodiments.

[0072] Turning to fig. 2a, a cross-sectional view of the separation module 6 is illustrated schematically according to some implementations.

[0073] The separation module 6 may be made of a non-conductive material such as a non-metal material. For example, the separation module 6 is made of a plastic material, rubber material, ceramic material or composite material. Advantageously, the entire separation module 6 is made as a single piece, for example in a 3D printing process or injection molding process. However, it is envisaged that the parts of the separation module 6 are made separately and then assembled to form a structurally stable complete separation module 6.

[0074] The separation module 6 comprises the separation plate 61 and two side portions 65, 66, a first side portion 65 and a second side portion 66. The side portions 65, 66 serve to protect and / or retain the PCBs at their respective opposite sides of the separation plate 61.

[0075] Each side portion 65, 66 may comprise a respective slot 67, 68 wherein each slot 67, 68 is sized and adapted to receive a respective PCB on opposite sides of the separation plate 61. Each side portion 65, 66 may further comprise at least one electrical connector 651, 661 configured to be connected to one of the terminals on each PCB. Optionally, each side portion 65, 66 comprises two connectors (not shown), one for each PCB terminal.

[0076] Hereby each side portion 65, 66 may further serve to house and protect the electrical connection between the terminals on the PCBs and the connector(s) 651, 661 provided in each side portion 65, 66.

[0077] In some implementations, each side portion 65, 66 comprises two respective protrusions 69a-d configured to facilitate retention of the core pieces and / or the PCBs. As seen in fig. 2a, the protrusions 69a-d protrude inwards so as to overhang the separation plate 61. Each protrusion 69a-d may be configured to engage a respective alignment recess formed in each of the core pieces. Specifically, the alignment recesses are formed in a part of the core pieces which face the side portions 65, 66 when the core pieces are arranged over the separation plate 61 and PCBs 1, 2. For example, the alignment recesses 36, 46 of fig. 1 may be configured to engage a respective protrusion of the side portions 65, 66 so as to retain the core pieces 31, 32, 41, 42 in at least the width-length plane.

[0078] In fig. 2b, the first and second PCB 1, 2 have been arranged in their respective slots 67, 68 on opposite sides of the separation plate 61. A plurality of elastically compressible elements 5 are placed between the separation plate 61 and the first PCB 1 and likewise a plurality of elastically compressible elements 5 are placed between the separation plate 61 and the second PCB 2.

[0079] In some implementations, the protrusions 69a-d also contribute to retaining the PCBs 1, 2 when the transformer is exposed to physical stress such as vibrations and bending forces. For example, protrusion 69c helps keeping the first PCB 1 towards the elastically compressible element 5 and separation plate as the PCB is sandwiched under protrusion 69c and retained in the associated slot 67 on the opposite side of the separation module 6.

[0080] The terminal(s) 13 of the first PCB 1 may be connected to the electrical connector(s) 651 of the first side portion 65 using conductors 653 and terminal(s) 23 of the second PCB 2 may be connected to the electrical connector(s) 661 of the second side portion 66 using conductors 663. The connection between the terminal(s) 13, 23 and the electrical connector 651, 661 may hereby be protected and housed fully inside each side portion 65, 66. Accordingly, the terminals 13, 23 of each PCB 1, 2 are advantageously provided on a same edge portion of each PCB 1, 2 to allow both terminals to be housed inside the respective side portion 65, 66.

[0081] In fig. 2c the magnetic core portions 3, 4 each comprising two core pieces 31, 32, 41, 42 have been fitted over the separation plate 61, the elastically compressible elements 5, and the PCBs 1, 2. The magnetic core portions 3, 4 are adapted to fit between the first and second side portions 65, 66 of the separation module 6 whereby the side portions 65, 66 may serve as side walls which, together with the magnetic core portions 3, 4 substantially fully surrounds and protects the PCBs 1, 2. When the magnetic core portions 3, 4 are fitted over the PCBs 1, 2 the elastically compressible elements 5 are compressed so as to push the PCBs 1, 2 against the inner surface of the core portions 3, 4.

[0082] As described above, the magnetic core portions 3, 4 (e.g. each magnetic core piece 31, 32, 41, 42) may comprise an alignment recess configured to engage the protrusions 69a-d of each side portion 65, 66.

[0083] It is also envisaged that the magnetic core pieces 31, 32, 41, 42 may instead be provided with a protrusion and the side portions 65, 66 are provided with alignment recesses. It is understood that this arrangement of the protrusions and alignment recesses may provide retention of the core pieces in an analogous manner.

[0084] Fig. 3a shows a close-up view of a side portion 65 with a protrusion 69a arranged above the slot 67 configured to receive the first PCB. In fig. 3b, the first PCB 1 has been arranged on top of the separation plate 61 so as to extend through the slot 67. The protrusion 69a is still free above the PCB 1 and can engage a corresponding alignment recess provided on the core piece, so as to contribute to retaining the core piece.

[0085] Fig. 4 is a perspective view of the planar transformer 10 in an assembled state. The planar transformer 10 comprises a top core portion 3 comprising two core pieces 31, 32 and a bottom core portion 4 comprising two core pieces 41, 42. The separation plate, at least one elastically compressible element and the first and second PCBs are sandwiched together inside the casing formed by the top and bottom core portions 3, 4.

[0086] To reliantly hold the top core portion 3 against the bottom core portion 4 one or more clips 7 may be provided for holding the core portions 3, 4 together. Here, each core piece 31, 32, 41, 42 is provided with two indentations 36, 46 and four clips 7 configured to engage the indentations 36, 46 are used to pairwise hold the core pieces 31, 32, 41, 42 together over the PCBs. Of course, this specific type of clips 7 is merely exemplary, and many other types of retention mechanisms to hold the magnetic core portions 3, 4 together can be used. For example, it is envisaged that different types of clamps, such as band clamps, or even cable ties can be used to reliantly hold the magnetic core portions 3, 4 or pieces 31, 32, 41, 42 together.

[0087] Advantageously, the retention of magnetic core portions 3, 4 is elastic, allowing some displacement of the core portions 3, 4 relative to each other since this may reduce the risk of the magnetic core portions 3, 4 breaking or cracking when the planar transformer is exposed to mechanical stress.

[0088] As seen in fig. 4, the terminals of the second PCB 2 are accessible inside the second side part 66 of the separation module. A respective electric conductor 663, 664 connects to each terminal of the second PCB 2 at one end and the corresponding connector 661, 662 at the other end. A similar arrangement is present also in the first side portion 65 with electric conductors connecting each terminal of the first PCB to the respective connectors 651, 652. Optionally, a lid or similar is used to close each side portions 65, 66 to protect the connection between the respective PCB 2 and the connectors 651, 652, 661, 662.

[0089] Hereby, when the planar transformer 10 is assembled the elastically compressible element(s), the PCBs and the separation plate are covered from the outside by the magnetic core portions 3, 4 and the side portions 65, 66 resulting in a compact planar transformer 10. However, connection to the transformer from the outside is still possible via the connectors 651, 652, 661, 662. Furthermore, due to at least one of the elastically compressible elements and the core portions 3, 4 being split into multiple pieces 31, 32, 41, 42 the planar transformer 10 can withstand high levels of mechanical stress and will be less influenced by aging compared to other planar transformers.

[0090] In fig. 5a and fig. 5b an exemplary type of elastically compressible element 5a, 5b is shown. The elastically compressible element 5a provided between the separation plate 61 and the first PCB is shaped like a grid. Likewise, the elastically compressible element 5b provided between the separation plate and the second PCB is also shaped like a grid.

[0091] The grid is formed by a plurality of bar elements 51 made of an elastically compressible material and connected together to form the grid structure (which also may be referred to as a net structure or mesh structure). The bar elements 51 defines a plurality of grid apertures 53 and hereby the elastically compressible element 5a, 5b can be made as a single piece which spans substantially the whole surface of the separation plate 61 and / or spans substantially the whole surface of the first PCB. An elastically compressible element 5a, 5b which spans substantially the whole separation plate 61 and / or first PCB has the advantage of exerting an even pressure on the first PCB so as to press it evenly against the inner surface of the top core portion. With an even pressure the risk of the PCB bending or flexing is reduced, while at the same time the contact area against the inner surface of the top core portion is maximized which enhanced thermal transfer.

[0092] The grid shaped elastically compressible element 5a, 5b achieves these beneficial properties while using less material compared to using a slab without multiple grid openings as the elastically compressible element 5a, 5b. However, it is envisaged that a slab of elastically compressible material can be used as element 5a, 5b in some implementations. In any implementation, the elastically compressible element 5a, 5b is advantageously shaped and arranged so as to not obscure the opening 62 through the separation plate and / or the openings through either PCB.

[0093] In some implementations, the support plate 61 comprises a textured surface surrounding the opening 62 wherein the textured surface faces the grid shaped first elastically compressible element 5a and wherein the textured surface comprises a network of recesses 63 forming grooves. The recesses 63 are delimited by protrusions 64 forming a grid structure with grooves matching the grid shape of the elastically compressible element 5a. Hereby, the elastically compressible element 5a can be placed in the matching grid structure on the separation plate 61 whereby the elastically compressible element 5a will be kept in place by the textured surface such that the elastically compressible element 5a is prevented from sliding relative to the separation plate 61.

[0094] The bar elements 51 forming the grid shaped elastically compressible element 5a may further be provided with sections 52 of increased cross-sectional diameter, forming cylindrical sections of increased size or spheres or ellipsoids with increased diameter integrated into the bar elements 51. By providing multiple such enlarged sections 52 it is possible to tailor at what locations the elastically compressible element 5a will press against the first PCB and by distributing the enlarged sections evenly across the grid-shape it is possible to achieve highly homogenously distributed pressure.

[0095] Additionally, compared to placing multiple individual elastically compressible elements between the first PCB and the first side of the separation plate the provision of a grid shaped elastically compressible element 5a makes the assembly process of the planar transformer more efficient and less labor intensive since a single elastically compressible element 5a is placed between the separation plate and the first PCB instead of multiple individual elements.

[0096] The second elastically compressible element 5b located on the opposite side of the separation plate 61 may be identical to the first elastically compressible element 5a. Similarly, the second surface of the separation plate 61 facing the second elastically compressible element 5b and the second PCB may be identical to the opposite first surface and may thereby also be provided with a textured surface forming a grid of recesses 63 and protrusions 64 matching the grid shape of the second elastically compressible element 5b.

[0097] It is envisaged, however, that different grid shapes (and matching textured surfaces) can be used on the two opposite sides of the separation plate 61. In general, the type of elastically compressible element provided on either side of the separation plate may be different, and it is also envisaged that both sides, only one side, or neither side of the separation plate 61 are textured. Furthermore, it is possible to arrange a single elastically compressible element on one side of the separation plate while multiple elastically compressible elements are arranged on the other side.

[0098] In fig. 5c and fig. 5d show an alternative elastically compressible element 5a', 5b' and associated alternative separation plate 61'. It is understood that the elastically compressible element may be of various designs while being capable of exerting an even pressure across the PCB and the respective surfaces 61a of the separation plate that are intended to contact the elastically compressible element may feature a complementary design, configured to retain the elastically compressible element.

[0099] The alternative elastically compressible element 5a', 5b' is generally shaped like sheet, and comprises an opening 57 configured to allow the central protrusion(s) of the magnetic core portions to pass therethrough, so as to enable the core portions to contact each other through the opening 62 in the alternative separation plate 61'. The sheet forming the alternative elastically compressible element 5a', 5b' is further deflected at a plurality of locations so as to form rounded protrusions 55 extending away from a first surface of the sheet. The alternative elastically compressible element 5a', 5b' is arranged on the alternative separation plate 61' with the rounded protrusions extending away from the alternative separation plate to contact the PCB.

[0100] The material used to realize the sheet is an elastically compressible or elastically deformable material such as a rubber material or polymer material. For example, the sheet is made of a plastic material sufficiently thin so as to allow the rounded protrusions 55 to elastically deform when pressed against the PCB. The rounded protrusions 55 will hereby exert an evenly distributed pressure against the PCB.

[0101] In some implementations, the alternative elastically compressible element 5a', 5b' further comprises one or more alignment openings 56a, 56b and the associated alternative support plate comprises corresponding alignment protrusions 64a, 64b configured to receive the alignment openings 56a, 56b. In this way, the alternative elastically compressible element 5a', 5b' may be retained relative the separation plate 61' in the width-length plane.

[0102] In some implementations, the surface of the alternative elastically compressible element 5a', 5b' facing the alternative separation plate is provided with a plurality of grooves 57 which e.g. extend parallel to the width direction. The grooves 57 form a local weakening of the sheet forming the alternative elastically compressible element 5a', 5b' and enables the sheet bend more easily. This further facilitates exerting an even pressure on the PCBs and allows the alternative elastically compressible element 5a', 5b' to follow the alternative separation plate 61 even if the separation bends or deforms.

[0103] Fig. 6a-c depicts cross-sections of various shapes of magnetic core portions 3, 4 which could be used in embodiments of the present invention.

[0104] In fig. 6a both core portions 3, 4 are E-cores having a central protrusion 33, 34 and two side protrusions 34, 44 similar to the core portions 3, 4 of figs. 1, 2a-c and 4. Each E-type core comprises a respective central protrusion 33, 43 wherein the central protrusions 33, 43 contact each other via the central opening of the PCBs and the separation plate when the transformer is assembled. Each E-core further comprises two side protrusions 34, 44 which contact each other outside the PCBs and separation plate to form the casing when the transformer is assembled. A benefit of using two E-cores is that the two core portions can be made with a same tool. Another benefit of using two E-cores is that the side protrusions of each E-core can help hold the TIM in in place during manufacturing. The TIM may be liquid, whereby there is a risk that the TIM leaks if it is not stopped by the side protrusions.

[0105] In fig. 6b the bottom core portion 4 is an I-core, i.e. shaped like a substantially rectangular slab and the top core portion 3 is an E-core. Here, the top core portion 3 will house both PCBs, the at least one elastically compressible element and the separation plate whereby the bottom I-core portion 4 will act like a lid. The central protrusion 33 of the top core portion 3 will extend through the openings of both PCBs and through the opening of the separation plate to contact the bottom I-core portion 4 and the two side protrusions 34 of the top E-core portion 3 contacts the bottom core portion 4 outside the PCBs and the separation plate.

[0106] In fig. 6c the top core portion 3 is a U-core and the bottom core portion 4 is a T-core. The bottom core portion 4 has a central protrusion 43 which extends through the openings of the PCBs and the separation plate to contact the inner surface of the top core portion 3. The top core portion 3 is devoid of a central protrusion and features two side protrusions 34 which extend outside the PCBs and separation plate to contact the bottom core portion 4.

[0107] The various core portion designs of figs. 6a-c are merely exemplary and the person skilled in the art will appreciate that other core shapes could be used as well. Additionally, irrespective of the type of core portions 3, 4 used each core portion can be divided into two or more individual core pieces as shown in fig. 1, figs. 2a-c and fig. 4 whereby the resulting planar transformer becomes less prone to breakage due to mechanical stress.

[0108] Fig. 7 shows a cross-sectional view of the planar transformer 10 in the assembled state. In this example, the top and bottom core portions 3, 4 are both E-cores that form a casing around the separation plate 61, the elastically compressible elements 5 and the PCBs 1, 2. The elastically compressible elements 5 will press each PCB 1, 2 away from the separation plate 61 against the inner surfaces of the core portions 3, 4.

[0109] To facilitate heat transfer from the PCBs 1, 2 to the core portions 3, 4 a TIM material 9 may be placed at the interface between each PCB 1, 2 and the inner surface of the core portions. The TIM material may e.g. be a thermally conductive grease, glue or paste or thermally conductive pads.

[0110] Since the PCBs 1, 2 are pressed against core portions 3, 4 by the elastically compressible elements 5 the TIM material 9 can be realized as a very thin layer since the TIM material 9 does not need to retain the PCBs 1, 2 by adhesion or similar. The TIM material 9 may hereby be a non-adhesive TIM.

[0111] Generally, the elastically compressible elements 5 allow a thinner layer of TIM material 9 to be used which will enhance thermal conductivity allowing heat to be more efficiently transferred from the PCBs 1, 2 to the core portions 3, 4 which facilitates better cooling via the core portions 3, 4. For example, the TIM material 9 has a thickness below 1000 µm or below 700 µm such as about 500 µm. The thickness may be at least 30 µm or at least 40 µm. Hereby, the TIM thickness may be between 40 µm and 700 µm.

[0112] In some cases, and especially when the TIM 9 is realized with thermal pads, the thermal pads may have a thickness exceeding 1 mm, e.g. the thermal pads have a thickness between 1 mm and 2 mm.

[0113] To achieve improved manufacturing tolerances the spacing between the PCBs 1, 2 is much larger than the sum of the thicknesses of the two layers of TIM material 9 arranged between the first PCB 1 and the top core portion and between the second PCB 2 and the bottom core portion respectively. For example, the separation distance between the PCBs 1, 2 is at least five times or at least ten times the sum of the thicknesses of the two layers of TIM material 9. Generally, the larger the separation distance between the PCBs 1, 2 is in relation to the TIM material 9 thickness the more the manufacturing tolerances are improved.

[0114] In some implementations, each PCB 1, 2 has a thickness of between 1 mm and 5 mm, such as about 2 mm. The separation distance between the PCBs 1, 2 is between 5 mm and 20 mm, such as between 10 mm and 15 mm. As an example, with PCBs 1, 2 having a 2 mm thickness, a PCB separation distance of 10 mm and 0.5 mm thick TIM layers the total separation distance between the inner surfaces of the top and bottom core portions is about 10 mm + 2 x 2 mm + 2 x 0.5 mm = 15 mm.

[0115] Additionally, the elastically compressible elements 5 will continue to exert a pressure on the PCBs 1, 2 even as the planar transformer ages. If the PCBs 1, 2 instead where to be kept in place by a thermally conductive adhesive there is a problem that as the planar transformer ages, the adhesive may deteriorate and may come loose, whereby the PCBs 1, 2 move relative to each other and the electrical properties of the transformer 10 changes. With the elastically compressible elements 5 (and optionally the separation plate 61) this problem is avoided.

[0116] The PCBs 1, 2 are separated with a PCB separation distance S P and the separation distance S P will influence the so called leakage inductance of the transformer. In general, the leakage inductance will depend on the volume enclosed between the PCBs falling inside the casing formed by the core portions and the windings of the PCBs. Hereby, tuning of the PCB separation distance S P will influence the leakage inductance which enables leakage inductance to be tuned with the separation distance S P .

[0117] Since the distance between the PCBs 1, 2 is achieved by pressing them against the inner surfaces of the core portions 3, 4 the planar transformer of the present invention enables provision of a large leakage inductance with excellent repeatability and manufacturing tolerances. The magnitude of the leakage inductance can be controlled by shaping the core portions 3, 4 accordingly and by using larger (deeper) core portions 3, 4 that allow a larger PCB separation distance S P the leakage inductance can be increased and by using smaller (shallower) core portions 3, 4 that allow a smaller PCB separation distance S P the leakage inductance can be reduced.

[0118] In practice, the leakage inductance of transformers is often taken into account and by addition of further circuit components (such as external inductors) it is possible to achieve a desired series inductance as a sum of the leakage inductance and the external inductor. However, it has traditionally been challenging to provide sufficiently large leakage inductance whereby the present invention solves this by providing a transformer design which can be used to obtain large PCB separation distances S P and leakage inductance. Hereby, with the planar transformer 10 of the present invention it is possible to completely eliminate the need for some external inductors components by relying solely on the leakage inductance inherent to the planar transformer to form the desired series inductance.

[0119] Another reason as to why external circuit components (e.g. series inductors) are added is that transformers typically suffer from high tolerance levels when it comes to leakage inductance. That is, transformers (even of the same make and model) may feature varying levels of leakage inductance that necessitates fine tuning by addition of external circuit components to get the transformer to operate as intended and exhibit the design leakage inductance. It has been found that with the planar transformer 10 of the present invention the tolerance levels of the leakage inductance (i.e. the spread of the leakage inductance levels between transformer samples) is severely reduced. Since the magnetic core portions 3, 4 may be made with high precision and the PCBs 1, 2 are pressed against the magnetic core portions (with optionally only a thin layer of TIM material 9 therebetween) the PCB separation distance S P is precisely defined, and the resulting leakage inductance is very similar from one transformer to another. This enables the planar transformer 10 of the present invention to be used without additional external circuit components for tuning, which makes the manufacturing process simpler, makes the final product smaller and lighter and may also improve overall system efficiency.

[0120] In some implementations, a spacer element 8 is placed between the top and bottom core portions 3, 4. For example, a spacer element 8 is placed between the central protrusion and side protrusion of the core portions 3, 4, as shown in fig. 7. However, the spacer element 8 is generally not needed nor desired when the planar transformer is to be used in a DC-to-DC converter (such as in a DAB converter). In such implementations, the core portions 3, 4 are advantageously arranged so as to directly contact each other.

[0121] On the other hand, in some implementations, such as when the planar transformer is to be used as a flyback transformer the spacer element 8 may be useful for tuning the electric performance of the planar transformer. The spacer element 8 may be a thin layer of a TIM material which will define a core-to-core separation distance S C . The separation distance S C will mainly influence the magnetization inductance of the transformer and may also influence the leakage inductance to a lesser extent. The magnetization inductance and leakage inductance are central design parameters when designing e.g. transformers and hereby the planar transformer of the present invention is versatile and allows the leakage inductance as well as the magnetization inductance to be adjusted by adjusting the separation distances S P and S C . The leakage inductance and magnetization inductance will in turn influence the mutual inductance of the transformer, which is another important design parameter when implementing transformers.

[0122] The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, while the central protrusion of the top and / or bottom core portion has a rectangular cross-section in fig. 1 this is merely exemplary and it its envisaged that the central protrusion may have a rounded (e.g. circular cross-section) whereby one or both of the core portions may be a pot core.

[0123] In the claims, the word "comprising" does not exclude the presence of other elements or steps than those listed in the claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

1. A planar transformer (10), comprising: a stacked coil assembly comprising: - a first PCB (1) comprising a first coil pattern formed in a planar conducting layer of the first PCB (1), the first coil pattern surrounding a first opening (11) extending through the first PCB (1), - a second PCB (2) comprising a second coil pattern formed in a planar conducting layer of the second PCB (2), the second coil pattern surrounding a second opening (21) extending through the second PCB (2), - at least one elastically compressible element (5) sandwiched between the first and second PCB (1, 2), and a magnetic core assembly comprising a top core portion (3) and a bottom core portion (4), wherein the first and second PCB (1, 2) are arranged between the top and bottom core portions (3, 4) and wherein the top and bottom core portions (3, 4) are configured to form a casing around the first and second PCB (1, 2), each of the top and bottom core portion (3, 4) comprises an inner surface, facing the first and second PCB (1, 2), respectively, wherein at least one of the top and bottom core portion (3, 4) comprises a protrusion (43, 44) protruding out from the inner surface so as to contact the other core portion through the central openings (11, 21) of the first and second PCB (1, 2), and wherein the at least one elastically compressible element (5) is compressed by the first and second PCB (1, 2) so as to push the first and second PCB (1, 2) away from each other, towards the inner surfaces of the top and bottom core portions (3, 4) of the magnetic core assembly, respectively.

2. The planar transformer (10) according to claim 1, wherein the at least one of the top and bottom core portions (3, 4) which comprises the protrusion (43, 44) forms an E core, and the protrusion forms a central leg of the E core.

3. The planar transformer (10) according to claim 1 or claim 2, further comprising: a separation module (6) comprising a separation plate (61) sandwiched between the first PCB (1) and the second PCB (2), wherein the separation plate (61) comprises an opening (62) allowing the central protrusion (43, 44) of at least one of the top and bottom core portion (3, 4) to pass through, wherein said at least one elastically compressible element (5) is at least two elastically compressible elements (5a, 5b), a first elastically compressible element (5a) located between the first PCB (1) and a first side of the separation plate (61), and second elastically compressible element (5b) located between the second PCB (2) and a second side of the separation plate (61), opposite to the first side.

4. The planar transformer (10) according to claim 3, wherein at least one of the first and second side of the separation plate (61) is provided with a recess (63), and wherein at least one of the first and second elastically compressible element (5a, 5b) is placed in said recess.

5. The planar transformer (10) according to claim 4, wherein the recess (63) forms a grid of grooves, and wherein said at least one of the first and second elastically compressible element (5a, 5b) placed in said recess (63) is grid shaped, the grid shape matching the grid of grooves.

6. The planar transformer (10) according to any of claims 3 - 5, wherein the separation module (6) further comprises a first and second side portion (65, 66) extending perpendicular to the separation plate (61) at opposite sides of the separation plate (61), wherein the first side portion (65) comprises a first slot (67) for receiving the first PCB (1) on the first side of the separation plate (61), and wherein the second side portion (66) comprises a second slot (68) for receiving the second PCB (2) on the second side of the separation plate (61).

7. The planar transformer (10) according to claim 6, wherein each side portion (65, 66) comprises two connectors (651, 652, 661, 662), and wherein each side portion (65, 66) further houses two conductors (653, 663) connecting each connector (651, 652, 661, 662) to a respective PCB terminal on the respective PCB (1, 2).

8. The planar transformer (10) according to any of claims 6-7, wherein each side portion (65, 66) comprises a protrusion (69a-d), configured to engage an alignment recess (35, 45) formed in the top and / or bottom core portion (3, 4).

9. The planar transformer (10) according to any of the preceding claims, wherein a TIM layer (9) is provided between each PCB (1, 2) and the respective inner surface of the top and bottom core portion (3, 4).

10. The planar transformer (10) according to any of the preceding claims, wherein the transformer has a width along a width axis (W), a length along a length axis (L), and a height along a height axis (H), wherein the width, length and height axes (L, W, H) are perpendicular to each other, and wherein the top and bottom core portions (3, 4) of the magnetic core surround at least a portion of the first and second PCB (1, 2) along the width axis and along the height axis (W, H).

11. The planar transformer (10) according to claim 10, wherein at least one of the top and bottom core portions (3, 4) comprises two separate core pieces (31, 32, 41, 42) separated along the length direction (L).

12. The planar transformer (10) according to claim 11, wherein the two separate core pieces (31, 32, 41, 42) interface at width-height plane, and wherein a spacer element (8) is arranged at the interface between the two separate core pieces (31, 32, 41, 42).

13. The planar transformer (10) according to any of the preceding claims, wherein a spacer element (8) is arranged between the top and bottom portions (3, 4) of the magnetic core.

14. The planar transformer (10) according to any of the preceding claims, wherein the top and bottom core portions (3, 4) comprise retention features configured to receive a clip for holding the top and bottom core portions (3, 4) together.

15. A dual active bridge voltage converter, comprising a planar transformer (10) according to any of the preceding claims.

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