A method and a system for manufacturing a planar winding assembly for power applications

Abstract

The present invention concerns a method of manufacturing a modular planar winding assembly for power applications comprising a stack of planar members having electrically conductive windings on their sides for forming at least one coil, said method comprising the steps of providing a stack of planar members with electrically conductive bonding material on at least a portion of the windings and having a first and second terminal between the coil formed by the windings on the planar members; applying pressure to said stack of planar members for pressing the planar members together in the stack; providing an electric current between the first and second terminals for a period of time; and bonding the planar members in the stack together. The invention further concerns an arrangement for such manufacture and a planar member suitable for building such a stack.

Description

[0001] A METHOD AND A SYSTEM FOR MANUFACTURING A PLANAR WINDING ASSEMBLY

[0002] FOR POWER APPLICATIONS

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a method for manufacturing a modular planar winding assembly for power applications, such as a planar transformer or a planar inductor, a busbar and other power applications.

[0005] BACKGROUND OF THE INVENTION

[0006] Planar winding assemblies for high-power applications are used for power supplies and other applications. Planar transformers and inductors are components used in power supplies, such as operating at high or low frequency. Planar transformers and inductors usually contain winding turns made of thin copper sheets riveted together at the ends of turns in the case of high current windings, or windings etched on a Printed Copper Board (PCB) in a spiral form. In the development of such magnetic transformer and inductor modules efforts are being made to reduce the size and manufacturing complexities. Accordingly, planar magnetic components, such as transformers and inductors, have been provided with low profiles fabricated with circuits and multiple layer printed circuit boards (PCB). Examples of such planar applications are known from e.g. US 5,781,093 and US 5,010,314.

[0007] The drawbacks with the planar applications, such as high-power magnetics are that such high-power magnetics are difficult and costly to manufacture and are large in size and are arranged with terminals for connection. The conventional planar magnetics are preferably connected to different PCBs in parallel.

[0008] Conventional planar magnetics can be connected in series using a series of extra connector pins and through-holes. This is difficult and costly to produce when the number of PCB layers is large. Another limitation in conventional planar magnetics is that there is a high thermal stress which limits the maximum power which the planar magnetic can be subjected to. Thus, to improve the performance of the planar magnetics there has been provided a modular power magnetics for high-power applications that reduces the undesired effects of the conventional planar magnetics. This modular power magnetics assembly is subject to PCT / EP2023 / 084293 (not yet published), which is hereby incorporated by reference, and wherein this modular magnetics assembly comprises a stack of layers of printed circuit boards having planar windings thereon is provided. This modular concept is advantageous since the design and manufacturing of planar applications, for example planar inductors and transformers, thereby become highly scalable. The layers may be stacked with windings in series and / or in parallel or a mixture of series windings and parallel windings.

[0009] However, it is found that by the stacking of the multiple conducting layers the roughness of surfaces and bends of conductors will introduce significant contact resistance, which can generate huge heat resulting in a high temperature at and around the connection points, and potentially burn the system.

[0010] There are several known methods that could be considered for manufacturing such planar winding assembly. These methods include ultrasonic welding, electrical welding, conventional soldering using solder stations, soldering with a vapor phase machine, and soldering with an infrared heater.

[0011] Ultrasonic welding is a process of joining two metal parts by applying ultrasonic vibration in a shear (scrubbing) mode. The ultrasonic vibration displaces the surface oxides and contaminates at the interfaces of the two parts, thereby allowing intimate metal-to-metal contact from which the weld occurs. The weld temperature is relatively low and does not involve melting. However, this method is only applicable for small joint points, and not suitable for welding a large number of layers with a large area together.

[0012] By electrical welding the electrical current will give a very high temperature to melt the soldering material or the material at the joint points. Thus, the electrical welding involves a high risk of damaging by burning the assembly. By the conventional soldering using solder stations, the solder station cannot provide enough power to heat the copper, which the windings in the modular planar winding assembly is usually made of. Even if the soldering station might be powerful, this method requires to solder each layers at the joints individually, which is time consuming.

[0013] Soldering with Vapor phase machine can only solder 2 -3 layers. An attempt to solder more layers, e.g. 14 layers, it is found that the power provided by the vapor phase change is not enough. It is also expensive to run vapor phase soldering machines.

[0014] Soldering with an infrared heater is not reliable. The copper of the windings needs to extract the heat from the ambient air. This means the heating process will take a long time, normally above 30 mins depending on the size. During the process, the dielectric insulating material of the PCBs will also be heated for a long time and will be aged and non-reliable.

[0015] Hence, by the present disclosure, it is found that the known conventional methods are not suitable for the present application.

[0016] As the conventional methods for manufacturing such planar winding assembly are not suitable for manufacturing the modular planar winding assembly for power applications, it is an object of the present invention to reduce or even eliminate the contact resistance and provide a suitable method for manufacturing a modular planar winding assembly for power applications, such as a planar transformer or a planar inductor.

[0017] SUMMARY OF THE INVENTION

[0018] This object is achieved in a first aspect of the invention by providing a method of manufacturing a modular planar winding assembly for power applications comprising a stack of planar members having electrically conductive windings on their sides for forming at least one coil, said method comprising the steps of providing a stack of planar members with electrically conductive bonding material on at least a portion of the windings and having a first and second terminal between the coil formed by the windings on the planar members; applying pressure to said stack of planar members for pressing the planar members together in the stack; providing an electric current between the first and second terminals for a period of time; and bonding the planar members in the stack together.

[0019] Advantageously, the method according to the first aspect of the invention is a self-heated soldering / sintering solution, where the solution to bond multiple layers of windings with a large contact surface area by soldering or sintering includes applying an electrically conductive bonding material, such as solder paste material or sintering paste material, between different planar members in the stack. It is flexible to apply solder / sintering paste material since the following steps ensure a qualified bonding result. Often, the PCB itself may have a thin layer of soldering material, which is commonly used for the purpose of protecting against oxidation.

[0020] By applying enough pressure, such as at least 0.1N / cm2, on the multiple layers of planar windings, the pressure will drive the solder material to the voids between two surfaces under ambient temperature or during the process of self-heating. It is found advantageous that the bonding material is a solder paste material, since less pressure is needed than for instance using sintering, where much more pressure will be needed. The sintering technologies can withstand a higher temperature and can provide a much thinner intermediate layer compared to soldering.

[0021] By providing an electric current between the terminals of the winding formed and by the rule of Ohm's law, the current will generate heat inside the planar windings, mainly inside the conductive winding material and intermediate bonding material.

[0022] Advantageously, the place with high contact resistance will naturally generate more heat. Such heat will melt the solder / sintering paste resulting in a more fluidity status. Driving by the force and more fluidity status, the solder / sinter material is forced by the applied pressure flooding to the places with voids. Furthermore, the method can provide strong and reliable connections. The full process of heating in the proposed solution can happen within a few seconds.

[0023] Compared to the known solutions, the present solution is very unique, especially for a multiple layer planar structure with large interface areas and large volume of metal. It is found that the method according to the present invention is very efficient and simple to execute in practice.

[0024] In an embodiment the electric current is provided for a time period, for example, between 1 second to 15 minutes, preferably 5-60 seconds. The duration here is very case by case, depending on the size of component, and also on the magnitude of current. The current may be constant or pulsed, the current type can be either direct-current or alternative-current. The current may have a Voltage range of IV- 5000V and a Current range of 10A-5000A.

[0025] In one embodiment, the electric current applied is a direct current (DC), for instance voltage of 5-10V de voltage and 110-500 A. Alternatively, the electric current applied may be an alternating current (AC), such as 50Hz- 30MHz, depending on the circumstances.

[0026] In an advantageous embodiment, there is provided a further step of measuring the electrical resistance while the current is applied. This is advantageous since this can control the bonding process, so that the electric current is switched off when the measured resistance drops to a predetermined value and thereby complete the bonding of the planar members in the stack. Thus, it is realised that when the electrical resistance is sufficiently low the windings on the layers are bonded together in a satisfactory manner where the electrical resistance is very low.

[0027] Since the electric current has generated heat in the stack, it is found suitable that when the electric current is switched off again the stack is cooled before the stack is released from the pressure. Hereby, any imperfections in the stack due to thermal expansion and shrinking may be avoided. In one embodiment, the bonding material is provided on a portion of at least some the windings on the planar members, for instance such that only one or two windings abutting each other are provided with bonding material. Furthermore, in order to avoid creating short circuits in the winding in the stack during the heating process, it is found advantageous that said portion is between 1 and 50% of the area of a particular winding provided with bonding material.

[0028] In a preferred embodiment, planar members are printed circuit boards (PCBs) and the windings thereon are copper windings. This ensures a superior electrical conductivity and also a fast heat build-up and thereby a fast bonding process of the stack.

[0029] In another embodiment, the progress of the soldering process may be monitored by detecting light between the planer members during the application of the electric current. Once the light cannot penetrate through the stack, the bonding process is completed.

[0030] Alternatively, the method could also comprise a step of measuring the height of the stack before the application of the electric current and during the application of the electric current. The height will decrease as the solder material is melted and distributed onto the surfaces of the winding sections on the layers of the stack. Accordingly, the method could then also comprise a step of switching off the current to complete the bonding of the planar members in the stack once a predetermined drop in the measured height is detected.

[0031] In a second aspect of the disclosure, there is provided an arrangement for manufacturing a modular planar winding assembly for power applications and suitable for performing the method of the first aspect, said apparatus comprising: a power supply for proving either AC or DC current through two terminals of the planar coil to a modular planar winding assembly comprising a stack of planar members having electrically or partial-electrically conductive windings on their sides for forming at least one coil, and having two terminals on each end of said coil; and a support for accommodating said stack of planar members and means for providing a predetermined pressure on said stack while being accommodated in the support. Hereby, a system or arrangement is provided, which is adapted for performing the manufacturing method according to the first aspect described above.

[0032] In a preferred embodiment of the system, there is provided a control unit is provided in relation to the power supply, which is preferably adapted to providing the electric current for a time period of between 1 second to 15 minutes, preferably 15-60 seconds depending on the size of components and magnitude of currents and the frequency if AC current is used.

[0033] The power supply may be adapted to providing the electric current as direct current (DC), for instance voltage of 1-500 volts. Alternatively, the power supply may be adapted to providing the electric current as alternating current (AC), such as 1-500 volts at 50-30MHz.

[0034] In accordance with an embodiment of the method of the first aspect, the control unit may preferably be provided with means for measuring the electrical resistance while the current is applied. The control unit may further be provided with means for switching off the electric current when the measured resistance drops to a predetermined value and thereby complete the bonding of the planar members in the stack.

[0035] The arrangement may preferably be provided with means for cooling the stack is provided, which is adapted for cooling the stack once the electric current is switched off and before the stack is released from the pressure.

[0036] In a third aspect of this disclosure, there is provided a planar member for a modular planar winding assembly for power applications comprising a stack of planar members having electrically conductive windings on their sides for forming at least one coil, said planar member comprising an insulating planar sheet of an electrically insulating material and having a first and a second side, and a first conductive winding on said first side and / or a second winding on said second side, wherein at least a portion of said at least one winding is provided with an electrical bonding material, which is reactive to heat. Hereby, a planar member, such as a printed circuit board or PCB, suitable for use in a modular planar winding assembly, which can be manufactured by performing a method according to the first aspect of the invention.

[0037] Preferably, the bonding material is a solder paste material. This material has enough electrical conductivity and is particularly suitable for use in relation to electronic components. The bonding material is provided on a portion of the windings; and said portion of bonding material is preferably between 1 and 50% of the area covered by the at least one winding.

[0038] In some embodiments, the windings are provided on both sides of the planar sheet. However, by the invention it is also realised that some planar members, such as the top or the bottom member of the stack my only be provided with a winding path on one of their sides. In such cases, the vias should either be mentalized or be filled with enough solder / sintering paste materials.

[0039] In an embodiment, the first conductive winding on said first side of the planar member and the second winding on said second side are identical or at least similar in shape, and wherein the first winding is slightly larger in width. This ensures that a small peripheral gap is created between two neighbouring PCBs or a PCB and a copper winding block - if such an element is inserted in the stack. This gap ensures that any excess solder material will be retained in this gap, which is advantageous since this reduces or even eliminates the risk that any excess solder flows uncontrolled away from the area on winding and thereby could cause a short circuit within the winding.

[0040] DETAILED DESCRIPTION

[0041] In the following, the invention is described in more detail with reference to the embodiments shown in the accompanying drawings, in which:

[0042] Fig. 1 is a perspective view of a modular planar winding assembly of the kind to which the invention pertains; Fig. 2 is a perspective exploded view of an embodiment of the modular planar winding assembly;

[0043] Fig. 3 is a schematic view of the system or arrangement according to the invention for manufacturing a modular planar winding assembly;

[0044] Fig. 4 is a table of resistance measurements;

[0045] Figures 5a-5d are schematic sketches illustrating the self-heating process according to the invention;

[0046] Fig. 6 is a schematic illustration of one embodiment for simultaneous manufacturing of a plurality of modular planar winding assemblies;

[0047] Fig. 7 is a schematic illustration of another embodiment for simultaneous manufacturing of a plurality of modular planar winding assemblies; and Figures 8-15 are flow diagrams of different embodiments of the manufacturing method according to the invention.

[0048] With reference to the 1 and 2, a planar winding assembly for high-power applications is shown and to which the invention pertains. The assembly comprises a frame 1 accommodating a stack 2 of a plurality of printed circuit boards (PCBs) 21, 22, 23. The PCBs comprise conductive windings 31, 32 on one or both sides of the PCBs as three different types of PCBs are provided. The PCBs 21, 22, 23 are tightly held together against each other in the stacked configuration by the frame 1 for forming a coil winding through the planar magnetic winding assembly.

[0049] In the stack 2, an uppermost first PCB 21 is provided. This first PCB 21 comprises a ceramic layer having a top surface and a bottom surface, where the bottom surface is provided with a conductive winding 31 having a first winding pattern. Centrally, a rectangular central window 4 is provided in the ceramic layer. This central window is formed to tightly fit around the central legs 13 of the upper and lower frame portions 11, 12 and the winding 31 is formed around this central window 4. Preferably, also a cut-away is provided in the first PCB 21 so that an external connection point is established to the end of the coil winding in the finished planar winding assembly.

[0050] In the stack 2, below the first PCB 21, a second PCB 22 is provided. This second

[0051] PCB 22 also comprises a ceramic layer having a top surface and a bottom surface with conductive windings 31, 32 thereon. The top surface is provided with the first winding pattern 31 and the bottom surface is provided with a second winding pattern 32 mirroring the first winding pattern 31. Centrally, a rectangular central window 4 is provided in the ceramic layer. This central window 4 is formed to tightly fit around the central legs 13 of the upper and lower frame portions 11, 12 and the windings 31, 32 are formed around this central window 4. The windings 31, 32 both have a first end 25 and a second end 24, where the first end 25 is the innermost end and the second end 24 is the outermost end. The innermost first ends 25 of the top and bottom windings 31, 32 on the second PCB 22 are connected in series through the ceramic layer of the second PCB 22 by a via connection.

[0052] Below the second PCB 22, there is provided a third PCB 23. The third PCB 23 comprises a ceramic layer having a top surface and a bottom surface with conductive windings 31, 32 thereon. The top surface is provided with the second winding pattern 32 and the bottom surface is provided with the first winding pattern 31 mirroring the second winding pattern 32. Centrally, a rectangular central window 4 is provided in the ceramic layer. This central window 4 is formed to tightly fit around the central legs 13 of the upper and lower frame portions 11, 12 and the windings 31, 32 are formed around this central window 4. The windings 31, 32 both have a first end 25 and a second end 24, where the first end 25 is the innermost end and the second end 24 is the outermost end. The outermost second ends 24 of the top and bottom windings 31, 32 on the third PCB 23 are connected in series through the ceramic layer of the third PCB 23 by a via connection.

[0053] In the assembly the stack comprises a plurality of PCBs 21, 22, 23 as shown in fig. 2. One uppermost first PCB 21 and a series of alternating second PCBs 22 and third PCBs 23 and finally a lowermost PCB 21, which is the same type as the uppermost first PCB 21 but flipped around. Hereby, the copper windings 31, 32 on the PCBs together form a coil winding through the planar assembly. This is achieved since the uppermost first PCB 21 on its bottom side 21b is abutted by the upper side of the second PCB 22 connecting the two windings 31 in parallel, which in turn on its bottom side is abutted by the upper side of the third PCB 23, which then further is abutted on its bottom side by a further second PCB 22, etc. Thus, the abutment of the aligned windings 31 or 32 of two neighbouring PCBs 21, 22, 23 means that the windings can share the same current and therefore creates a parallel connection.

[0054] In the preferred embodiments of the planar assembly disclosed and shown in the figures, on the PCBs 21, 22, 23 the conductive windings are copper windings. The PCBs 21, 22, 23 are two-layered PCBs with direct bonded copper (DBC) windings 31, 32 on each side.

[0055] As shown in figures 1 and 2, the frame 1 comprises a set of core members 11, 12 made of a magnetic material. The set of core members comprises an upper core member 11 and a lower core member 12 each having a central leg 13 extending through the central windows 4 of the PCBs 21, 22, 23 and two side legs 14. Thus, the core members 11, 12 are E-shaped. The stack 2 of PCBs is encompassed by the frame core members 11, 12, which are assembled around the PCB stack 2 where the central legs 13 of each core member 11, 12 abut each other and the side legs 14 abut each other on each side of the PCB stack 2 to form the planar winding assembly as shown in figure 1.

[0056] The central legs 13 of the core members 11, 12 are provided with a rectangular cross-sectional shape snugly fitting into the correspondingly shaped central windows 4 of the PCBs 21, 22, 23.

[0057] In order to assemble the PCBs, the invention provides a self-heating process. In fig. 3 there is shown an embodiment of an arrangement for carrying out the selfheating process. The PCBs are aligned in the stack 2. The windings 31, 32 on the PCBs are provided with a thin layer of solder material 33. The stack 1 is subjected to a pressure as indicated by the arrows P in fig. 3. At the top and bottom of the stack the two terminals tl and t2 are provided. These terminals tl, t2 are connected to an electric power supply PS. The electric power may be alternating current (AC) or direct current (DC).

[0058] When power is applied, the current generates heat at the contact point and around between the abutting windings on the PCBs in the stack 2. This heat causes the solder material 33 to melt so it becomes more fluid. In additional to the pressure P applied, this causes the fluidity solder material to the gaps between the abutting winding surfaces and thereby soldering the windings together.

[0059] As shown in fig. 4, this self-heating soldering process is found advantageous since the DC resistance between the terminals tl, t2 decreases significantly. In an example it is found that the resistance dropped from 20.9 mQ before soldering to 16.4 mQ when pressure was applied and drops significantly more to 3.9 mQ after soldering, which is very close to the theoretical value which is calculated to be 3.7 mQ.

[0060] In the following and with reference to figures 5a to 5d, the contact resistance and the self-heating process is explained in more detail:

[0061] It is a nature to have contact resistance between two plates, due to the non-flat and non-ideal surface of the conductor. In Fig.5a, it can be seen that the efficient contacts between two plates are limited, and therefore, the contact resistance comes.

[0062] To eliminate the contact resistance, pressure is applied, where copper is soft enough to be bended, where more contact points of copper of copper may occur, as shown in Fig. 5b, where contact resistance will be reduced.

[0063] However, it is not easy to apply enough and even-distributed pressure. Therefore, a further step in the process is to solder the conductor windings together. Some soldering (or sinter) material is applied to create more joints by such paste material, as shown in Fig. 5c. However, such material normally comprise epoxy, and therefore the resistivity is high, but as some pressure is also applied, then some contact points are established. Although it may still be weak contacts, at least an electric loop is established, and therefore the current can flow.

[0064] Ruled by ohm's law, the current generates heat at the contact point and around. And such heat is enough to melt the solder / sintering material so it becomes more fluid. In additional to the pressure applied, it will drive the fluidity solder / sintering material to the gaps (see fig. 5d). During this self-heating process, the epoxy or similar material used in solder / sintering paste, will be evaporated. Epoxy should have a very high resistivity, where the rest part of solder / sintering paste material should have much lower resistivity compared to before. Then this process can help to almost eliminate the contact resistance completely (see the table of fig. 4), since the solder / sinter layer is thin enough due to the pressure we applied. Driven by the force, some solder material may flow into the vias connecting the upper and lower windings on the PCBs.

[0065] With reference to fig. 6, another embodiment of the self-heating process according to the invention is shown, where a plurality of winding assemblies 2', 2", ... , 2ncan be produced simultaneously simply by putting multiple windings 2', 2", ... , 2nin series connection, so that the current can flow though all windings 2', 2", ... , 2nwith using a common power supply source (not shown in fig. 6) connected to the terminals tl and t2.

[0066] With reference to fig. 7, another embodiment for producing a plurality of winding assemblies is shown. According to this embodiment, an AC current is applied to the terminals tl, t2 of the first winding 2', and the 2nd-nth windings 2", 2"', ... , 2nare individually shorted on their own terminals (ts). Then ruled by the electromagnetic field laws, the 2nd- nth windings 2", 2"', ... , 2nwill generate an antidirection magnetic field to against the filed from the 1stwindings 2' and excited current is generated in each of the windings 2", 2"', ... , 2ndue to the shorted terminals. Therefore, all the windings 2', 2", 2"', ... , 2ncan be heated by the current and resistance due to the only applied AC voltage / current signal in the first winding 2' from the power supply unit (not shown in fig. 7).

[0067] In the flowcharts of figures 8 to 15, various embodiments of the self-heating process according to the invention are shown.

[0068] The process requires: 1) To apply enough solder paste material between different layers. It is flexible to apply solder paste material since the following steps are more important to ensure a qualified soldering result.

[0069] 2) To apply enough pressure on the multiple layers of planar windings. The pressure will drive the solder material to the voids between two surfaces.

[0070] 3) To conduct dc / ac current between the terminals. The current could be constant or pulsed.

[0071] 4) Rule by Ohm law, the current will generate heat inside the planar windings, mainly inside the mental material and intermediate materials.

[0072] 5) Since the object is to eliminate the contact resistance, and the place with high contact resistance will naturally generate more heat. Such heat will melt the solder paste to provide strong and reliable connections.

[0073] 6) Cool the planar windings, when the self-heating process has generated the reliable connections.

[0074] The full process of heating in the proposed solution can happen within a few seconds. The additional cost of electricity and solder paste is very limited. Compared to the state-of-the-art solutions, our solution is very unique for a multiple layer planar structure with large interface areas and large volume of metal. It is very efficient and simple to execute in practice.

[0075] The process requires applying pressure, applying soldering / sintering material, and conducting current.

[0076] As described in flowcharts, automated control solutions to solder the planar magnetic components are provided.

[0077] As described in fig. 8, a simple pre-set timer automatically turns off the power after a predetermined duration time. With reference to fig. 9, when applying the ac / dc current the resistance is measured online. By applying any shape of current, when the system detects the resistance of planar windings dropping to a certain value, the power supply may automatically stop. In fig. 10, a variant of the process control by measuring the resistance is shown.

[0078] Another way of controlling the process is shown in fig. 11. This process control involves measuring the temperature of the winding for instance by using a thermal camera or a thermal sensor. When the temperature exceeds a certain value, the power is turned off bringing the process to an end.

[0079] In fig. 12, it is realised that the power supply controller controlling the current / voltage may also control the self-heating process using pre-set algorithms.

[0080] With reference to fig. 13, it is realised that the process may also be controlled by measuring the height of the planar windings. It is found that the total height of the planar windings will be reduced by lOum-O. lmm after soldering.

[0081] With reference to fig. 14, a control solution is provided using a light / image-based camera solution across the planar windings to detect the soldering process has been finished. It is noticed that when the soldering process is finished, the planar windings can block the light across their structures.

[0082] In fig. 15, a process control is provided using a noise detector detecting the noise generated by the heating of the soldering material.

[0083] By the invention it is realised that these different process control methods may be used separately or used in combination.

[0084] In a particular embodiment of the invention, the copper pattern on the PCB should be slightly larger than the pattern of the winding abutting the PCB or a conductor block (can be copper or other conductor). It can also be, the copper pattern on the PCB, should be slightly smaller than the pattern of the conductor block (can be copper or any other conductor). These embodiments has the benefit that the extra soldering paste material will be held by the additional conductor pattern, and therefore the extra soldering paste material will be more difficult to flow out of the conductor pattern area. This phenomenon is well-observed by experiments.

[0085] By the invention it is realised that the method can also be used with soldering material that are not electrically conducted. For example, some soldering material may have very poor conductivity or almost no conductivity. By applying the pressure, conductivity between two copper plates can be established and the weak point can contribute to heat to melt the non-conductive soldering material, where the epoxy material is dried and the soldering material become a good conducting material after drying.

[0086] In general, when directional terms like "upper" and "lower" or similar directional references are used in the present disclosure, these terms are meant to be understood as relative terms e.g. where the term "upper" refers to a direction essentially opposite to the "lower", but it is realised that the planar assembly when in use can be positioned with any orientation.

[0087] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

CLAIMS1. A method of manufacturing a modular planar winding assembly for power applications comprising a stack of planar members having electrically conductive windings on their sides for forming at least one coil, said method comprising the steps of providing a stack of planar members with electrically conductive bonding material on at least a portion of the windings and having a first and second terminal between the coil formed by the windings on the planar members; applying pressure to said stack of planar members for pressing the planar members together in the stack; providing an electric current between the first and second terminals for a period of time; and bonding the planar members in the stack together.

2. A method according to claim 1, whereby the electric current is provided for a time period of between 1 second to 15 minutes, preferably 5-120 seconds.

3. A method according to claim 1 or 2, whereby the electric current applied is a direct current (DC), for instance voltage of 1-500 volts.

4. A method according to claim 1 or 2, whereby the electric current applied is an alternating current (AC), such as from 1 V-500 V at 50-60 Hz, from 1 V-1000 V at 100 Hz- 1 kHz.

5. A method according to any of the preceding claims, whereby there is provided a further step of either online or offline measuring the electrical resistance while the current is applied or after the current is applied.

6. A method according to claim 5, whereby the electric current is switched off when the measured resistance drops to a predetermined value and thereby complete the bonding of the planar members in the stack.

7. A method according to any of the preceding claims, whereby when the electric current is switched off again the stack is cooled before the stack is released from the pressure.

8. A method according to any of the preceding claims, whereby the bonding material is a solder paste material or sintering material.

9. A method according to any of the preceding claims, whereby the bonding material is provided on a portion of at least some the windings on the planar members.

10. A method according to claim 9, whereby said portion is between 1 and 50% of the area of the windings provided with bonding material.

11. A method according to any of the preceding claims, whereby planar members are printed circuit boards (PCBs) and the windings thereon are copper windings, Direct-bond-copper (DBC) or Active Metal Brazed (AMB) with another type of substrate, e.g. ceramics, glass or the like.

12. A method according to any of the preceding claims, whereby the method further comprises a step of detecting light between the planer members during the application of the electric current.

13. A method according to any of the preceding claims, whereby the method further comprises a step of measuring the height of the stack before the application of the electric current and during the application of the electric current.

14. A method according to any of the preceding claims, whereby the method further comprises a step of measuring the time or duration of the provision of an electric current.

15. A method according to claim 13 or 14, whereby the method further comprises a step of switching off the current to complete the bonding of the planar members in the stack once a predetermined drop in the measured height is detected.

16. A method according to any of the preceding claims, whereby a plurality (n) of modular planar winding assemblies are provided connected in series at their first and second terminals and whereby the electric current is applied to the first terminal of the first winding and the second terminal of the n-th winding, whereby the planar members in each of the planar winding assemblies are bonded together in the same process.

17. A method according to any of claims 1 to 15, whereby a plurality (n) of modular planar winding assemblies are provided aligned in a common stack where the electric current is applied to the first and second terminals of the first winding and the second to n-th windings are individually shorted between their two terminals and positioned stacked on the first winding, and wherein the electrical current is an AC current.

18. An arrangement for manufacturing a modular planar winding assembly for power applications, said apparatus comprising: a power supply for proving an AC or DC current through two terminals to a modular planar winding assembly comprising a stack of planar members having electrically conductive windings on their sides for forming at least one coil, and having two terminals on each end of said coil; and a support for accommodating said stack of planar members and means for providing a predetermined pressure on said stack while being accommodated in the support.

19. An arrangement according to claim 18, wherein a control unit is provided in relation to the power supply, which is preferably adapted to providing the electric current for a time period of between 1 second to 15 minutes, preferably 5-120 seconds.

20. An arrangement according to claim 18 or 19, wherein the power supply is adapted to providing the electric current as direct current (DC), for instance voltage of 50A-1000A / 1-500 volts.

21. An arrangement according to claim 18 or 19, wherein the power supply is adapted to providing the electric current as alternating current (AC), such 50A- 1000A and 1-500 volts at 50-60 Hz, or 200A and 500V at 30MHz22. An arrangement according to any of claims 18 to 21, wherein the control unit is provided with means for measuring the electrical resistance while the current is applied.

23. An arrangement according to claim 22, wherein the control unit is provided with means for switching off the electric current when the measured resistance drops to a predetermined value and thereby complete the bonding of the planar members in the stack.

24. An arrangement according to any of claims 18 to 23, wherein means for cooling the stack is provided, which is adapted for cooling the stack once the electric current is switched off and before the stack is released from the pressure.

25. An arrangement according to any of claims 18 to 24, which is adapted for performing a method according to any of claims 1 to 17.

26. A planar member for a modular planar winding assembly for power applications comprising a stack of planar members having electrically conductive windings on their sides for forming at least one coil, said planar member comprising an insulating planar sheet of an electrically insulating material and having a fist and a second side, and a first conductive winding on said first side and / or a second winding on said second side, wherein at least a portion of said at least one winding is provided with an electrical bonding material, which is reactive to heat.

27. A planar member according to claim 26, wherein the bonding material is a solder paste material or a sintering material.

28. A planar member according to claim 26 or 27, wherein the windings are provided on both sides of the planar sheet.

29. A planar member according to claim 28, wherein the first conductive winding on said first side and the second winding on said second side are identical or similar in shape, and wherein the first winding is slightly smaller or larger in width.

30. A planar member according to any of claims 26 to 29, wherein said portion of bonding material is between 1 and 50% of the area covered by the at least one winding.

31. A plurality of planar members of any of claims 26 to 30, which is arranged in a stack of planar members for the performance of a method according to any of the claims 1-17.

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