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Title:
COIL AND METHOD FOR INTERCONNECTING PLANAR WINDINGS
Document Type and Number:
WIPO Patent Application WO/2008/003825
Kind Code:
A1
Abstract:
The invention relates to coils and the coil manufacturing methods used in power supply electronic. A coil comprises at least a foil, a core of the coil body made of ferromagnetic material, planar coils, and pins connected to the planar coils. Each planar coil is formed of at least one electrically conductive turn. The coil is implemented as follows: a) every second segment of the foil comprises two planar coils, one on each side of a segment, b) each second segment of the foil operates as an electrical insulator, and,c) the core of the coil body is pushed through the openings made in the segments of the foil. During the coil manufacturing the foil is preferably folded as a pile and a side of the pile, which includes at least one pin, is immersed into solder. Then the solder fills in the boles and openings formed into a rim of the foil and connect at least two planar coils to ether.

Inventors:
RIDAL, Toivo (Heinäkorventie 8 B, Vantaa, FI-01680, FI)
Application Number:
FI2007/050396
Publication Date:
January 10, 2008
Filing Date:
June 27, 2007
Export Citation:
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Assignee:
ZELTEQ OY (Heinäkorventie 8 B, Vantaa, FI-01680, FI)
RIDAL, Toivo (Heinäkorventie 8 B, Vantaa, FI-01680, FI)
International Classes:
H01F27/32; H01F5/00
Domestic Patent References:
WO2003036664A1
Foreign References:
EP0830053A2
US4517540A
EP0689214A1
JPH05304029A
JPS61270804A
US5521573A
Attorney, Agent or Firm:
PATENT AGENCY COMPATENT LTD. (Hitsaajankatu 6, Helsinki, FI-00810, FI)
Download PDF:
Claims:

Claims

1. A coil comprising a foil, a core of a coil body made of ferromagnetic material, and planar coils, each planar coil being formed of at least one electrically conductive turn, characterized in that, every second segment of the foil comprises two planar coils, a first planar coil on one side of a segment and a second planar coil on the opposite side of the segment, the foil functioning as an electrical insulator between the first planar coil and the second planar coil, every second segment of the foil functions as an electrically insulating intermediate plate for at least one planar coil, for connecting at least two planar coils together with a pin, holes are formed in segments which include two planar coils and openings are formed in segments functioning as intermediate plates, and the core of the coil body is pushed through the openings made for the core of the coil in segments of the foil.

2. The coil as in claim 1 , characterized in that the number of pins is two and said pins are attached to at least free ends of the first planar coil and the second planar coil for feeding electric current to the coil. 3. The coil as in claim 1, characterized in that the foil includes the holes and the openings for at least two pins.

4. The coil as in claim 1, characterized in that a pin is soldered by immersing a side of a pile, which includes the pin as well as the holes and openings made for the pin, in solder, the solder filling in the holes and the openings.

5. The coil as in claim 1 , characterized in that the foil comprises at least one electrically conductive layer and the first planar coil is formed by corroding an electrically conducive layer.

6. The coil as in claim 1 , characterized in that the first pla- nar coil is formed by attaching at least one electrically conductive layer to the foil.

7. A method for connecting planar coils together, wherein in each planar coil is formed of at least one electrically conductive turn and the planar

coils are placed on a ribbon-like, electrically insulating foil so that the foil separates the planar coils from each other, characterized by the steps of: forming holes in segments of the foil, each of the segments being intended for one planar coil, and openings in the other segments of the foil, folding the segments of the foil as a pile, pushing pins through the holes and the openings, and immersing a side of the pile including at least one pin in solder, the solder filling in the holes and the openings, and connecting by the pins at least two planar coils together.

8. The method as in claim 7, characterized in that the pin connects the planar coils in parallel.

9. The method as in claim 7, characterized in that the pin connects the planar coils in series. 10. The method as in claim 7, characterized in that at least one coil is formed on the foils by means of the method.

Description:

COIL AND METHOD FOR INTERCONNECTING PLANAR WINDINGS

Field of the invention

[001] The invention relates to coils to be used in power supplies and a method for manufacturing coils. Especially, the invention relates to different ways to interconnect planar coils.

Background of the invention

[002] Coils are used in a number of electronic devices such as transformers and inductors.

[003] A planar coil is considered to mean a part of a coil. One coil in- eludes at least one planar coil and each planar coil includes at least one turn. A turn begins and ends on a circuit board approximately at the same point. Most common form of the turn is a circle, but also other forms exist. Turns are made of electrically conductive material such as copper films.

[004] In addition to planar coils, a coil comprises connectors i.e. pins. A pin is made of electrically conductive material. For example, the pin is a copper wire coated with gold. The number of pins is an even number so that electrical current is lead through one pin into a coil and the electrical current is lead out from the coil through another pin. Usually, it is formed on a circuit board holes for pins and the pins are placed in the holes and soldered with tin to the circuit board. Soldering of the pins can be performed with a soldering iron.

[005] Some known methods to manufacture a coil are described in the following.

[006] In a first manufacturing method the planar coils and so-called in- termediate plates are made of different pieces. The intermediate plates function as electrical insulators between the planar coils. The planar coils and the intermediate plates form a pile in which one intermediate plate is placed between two planar coils. In addition, one intermediate plate is placed as the lowest layer of the pile and one intermediate plate is placed as the topmost layer of the pile. Typically, it is used in a transformer a coil in which planar coils and intermediate plates form a pile of 10-36 layers.

[007] The first manufacturing method is related to certain problems. First, manufacturing a coil is relatively time-consuming, because the coil is made of a number of pieces, i.e. planar coils and intermediate plates. The holes of the planar coils and intermediate plates must be placed carefully on top of one another for leading pins through them. The planar coils and the intermediate plates must be clued or attached by some other way together so that the holes can be placed accurately on top of one another. Secondly, making the coil requires handwork, which raises the manufacturing costs. Thirdly, the planar coils and the intermediate plates may be added to the pile in a wrong order at which time the coil does not operate in a correct way. In other words, the manufacturing method is error-prone.

[008] In a second manufacturing method the coil is integrated into an electrical device. Let us assume that the electronic device in which the coil is integrated is a transformer. Then the planar coils included in the coil are placed on the same printed circuit board together with the other wiring of the transformer.

[009] The second manufacturing method is involved with the following problems. First, making stripes in an economically reasonable way is difficult. The stripes are meant to be establishing copper strips on a printed circuit board. For example, making a coil of the transformer may require using a copper strip of thickness 0.1 mm in the stripes. In the same time, the wiring of other components of the transformer may require using a copper strip of thickness 0.018 mm in the stripes.

[010] Usually, a printed circuit board includes at least two copper layers, but there may as well be over ten copper layers. Wiring and stripes of an electronic device are located in the copper layers. In more detail, the wiring and stripes of the coil are made by corroding the printed circuit board on certain areas and in certain depths. The thicknesses of the copper layers of the printed circuit board are defined on the basis of the requirements of the wir- ing. These thicknesses are often inappropriate for the stripes of the coil.

[011] Secondly, it is difficult to find optimal site for the pins of the coil, especially when the pins penetrate a number of overlapping printed circuit cards. Badly located pins increase the leakage inductance of the transformer.

[012] Thirdly, manufacturing of the coil may require more layers in the printed circuit card than the other components of the transformer require. In other words, integrating the coil with the transformer (or with another device) is often an uneconomic manufacturing method. [013] In a third manufacturing method a coil is made of one thin and solid copper plate. The thickness of the copper plate corresponds to the thickness of the stripes. The planar coils are cut from the copper plate so that the planar coils are attached to each other. Different methods can be used in the cutting. Then the cut copper plate, which includes the planar coils, is folded together. After the folding, the planar coils form a pile. Because the planar coils are at partly connected to each other, none pin is needed in the manufacturing of the coil. The intermediate planes functioning as an electrical insulator are, however, needed between the planar coils.

[014] The third manufacturing method is involved with the following problems. Placing the intermediate plates in their locations requires quite a lot of time and is an error-prone manufacturing method. In addition, usability of the manufacturing method is limited to the coils in which relatively thick stripes are used.

[015] In a fourth manufacturing method a coil is made of an electrical conductor and a ribbon functioning as an electrical insulator. This manufacturing method is described in the U.S. patent 4,517,540 in which each spiral coil is formed at least one turn of an electrical conductor. One end of the electrical conductor is placed to one end of the ribbon and the opposite end of the electrical conductor is placed to the opposite end of the ribbon. The ribbon is composed of segments so that each segment includes a slit and an aperture formed for the electrical conductor. The electrical conductor passes through the slit and aperture to the opposite side of the ribbon. Some segments of the ribbon include a spiral coil and some other segments include a part of the electrical conductor connecting two planar coils. The planar coils and the parts of the electrical conductors connecting them are placed on the opposite sides of the ribbon so that they don't touch themselves when the ribbon is folded segment by segment as a pile. Thus, the ribbon functions as an electrical insulator, i.e. as an "intermediate plate". In addition to the slit and aperture formed on the rim of a segment, another aperture, through

which the coil core is penetrated when the coil ribbon is folded as the pile, is formed in the middle of each segment.

[016] The fourth manufacturing method solves the problem involved with the intermediate plates. Placing the electrical conductor through the slits and apertures to the opposite sides of the ribbon seems to be a time-consuming and error-prone manufacturing phase. In addition, the fourth manufacturing method applies to coils whose planar coils are joined in series.

[017] A technical problem related to the invention especially concerns a manufacturing method and how a placement of planer coils on the ribbon functioning as the electrical insulator and how an electrical interconnection of the planar coils could be performed in a new, more versatile way than before, either in series or in parallel.

Summary of the invention

[018] One objective of the invention is a coil structure providing solutions to the above-mentioned four problems and the technical problem.

[019] A coil includes at least a film, a coil core made of magnetisable material, planar coils, and pins which are attached to the planar coils. The coil is preferably implemented as follows. [020] Each segment of the coil comprises two planar coils, a first planar coil locating on one side of a segment and a second planer coil locating on the opposite side of the segment. The film functions as an electrical insulator between the first and the second planar coils. Every second segment of the coil comprises an intermediate plate that functions as electrical insulator for at least one planar coil. [021] In addition, openings are formed in the segments functioning as intermediate plates of the film and holes are formed in the segments which include least two planar coils. Through the openings and the holes at least two planar coils are connectable together by a pin. [022] When the film comprising the planar coils is folded as a pile, the coil core is penetrable through openings formed in the segments.

[023] The invention enables that planer coils can be connected in series or in parallel, and more than one coil can be created on the same film. [024] In addition to the above-described coil, the invention comprises a method for connecting planar coils together by a new type of pin solder joints.

Brief description of the drawings

[025] The invention is described more closely with reference to the examples shown in the figures of the accompanying drawings, in which

FIG. 1 shows an example of a coil ribbon, FIG. 2 shows coil ribbon openings intended for pins of a coil,

FIG. 3A shows a together folded coil ribbon and a coil body,

FIG. 3B shows the together folded coil ribbon looked from the side,

FIG. 4 shows a planar coil, a turn, and a stripe,

FIG. 5A shows a perspective figure of a solder area, FIG. 5B illustrates soldering of a pin in the solder area,

FIG. 6 illustrates a coil comprising two pins,

FIG. 7 shows a coil manufacturing method,

FIG. 8 shows a method for connecting planar coils together,

FIG. 9A shows connecting planar coils in pairs and in parallel, FIG. 9B shows connecting planar coils in series.

Detailed description of the invention

[026] A coil comprising at least two planar coils is implemented by means of the invention. A film with the planar coils is termed a "coil ribbon".

[027] FIG. 1 shows an example of the coil ribbon. The coil ribbon com- prises a film to which folds and openings are formed. The film should be made of material whose surfaces operate as an electrical insulator.

[028] For example, a film can be used as the foil of the coil ribbon. A polyamide foil enduring the heat of 300 Celsius degrees is one appropriate material. The foil may be throughout the same material, such as polyamid. It is, however, possible that the foil includes different materials in layers. When the foil includes electrical conductor layers and electrical insulator layers, the foil can be termed a printed circuit board. When choosing a foil material, it should be noticed that the folds and openings must be formed into the foil. In other words, the foil should fold without getting broken. [029] Planar coils, such as planar coil 102, are placed on the foil and the foil includes the openings, for example, a part of a solder area, i.e. hole 101. It is essential in a manufacturing the foil in accordance with the invention that in addition to the planar coils the coil ribbon comprises the intermediate plates, because this remarkably facilitates the manufacturing of the coil. The

required openings, such as opening 103, are formed into the intermediate plates.

[030] The foil to be used in the coil ribbon must be usually cut from a wider panel. Dashed lines 104 illustrate cutting the foil from the panel. The width of the panel may sometimes be the appropriate one at which time it is enough that a piece of the appropriate length is cut from the panel for the coil.

[031] In addition to planar coil 102, the coil comprises other planar coils, such as planar coil 105. We may consider the coil ribbon to be composed of segments so that each second segment 106 includes an intermediate plate and each second segment 107 includes a planar coil.

[032] Folds are made between the segments. For example, folds 108 are involved with the segment including the planar coil 105. The coil ribbon is folded as a pile so that on top of each planar coil and under it, is folded a segment including an intermediate plate. The folds are located so that the planar coils and the intermediate plates form a pile. In addition, the locations of the folds in the coil ribbon are arranged so that the openings, such as openings 110 intended for the coil core, get to be placed in the pile one on the other. [033] In FIG. 1 another of the long sides of the coil ribbon has neither holes nor openings, but if required, those could also be made into that other long side.

[034] The coil ribbon is implemented so that the opposite sides of it are in principle of the same kind. In more detail, in FIG. 1 the opposite sides of segments 106 and 109 are of the same kind, i.e. they include a pure foil and thus they operate as intermediate plates. Segments 107 include two coil plates, i.e. one coil plate on each side of them.

[035] In FIG. 1 it can be seen two planar coils placed on the foil, and two other planar coils are placed on the opposite side of the foil, thus there are altogether four planar coils.

[036] FIG. 2 shows the coil ribbon openings intended for the pins of a coil. When the coil ribbon of FIG. 1 is folded as a pile, the openings are placed one on the other. FIG. 2 illustrates those four solder areas that are

located on one side of the pile. Solder area 214 is located at hole 101. Each solder area extends through the pile.

[037] The pile is therefore composed of the segments of the coil ribbon.

The openings can be formed in different ways. The openings intended for the pins are preferably implemented so that a slit, i.e. opening 215, is formed into each second segment of the coil ribbon, and a hole is formed into each second segment. In more detail, in the segment including an intermediate plate the solder area has the slit, i.e. opening 215, and in segment 216 which includes a planar coil the solder area has the hole. Then a number of pins can be simultaneously soldered by immersing the side of the pile into solder.

[038] FIG. 3A shows a together folded coil ribbon and a coil body. Coil body 311 and the coil ribbon, for which space 312 is reserved between the upper part and the lower part of the coil body, can be considered to be the main parts of the coil. Coil body 311 is made of ferrite, iron, or some other magnetisable material.

[039] The coil body is composed of two separated parts each of which includes a piece of coil core 313. The coil ribbon is placed in space 312 between the lower and the upper part of the coil body 311 so that the core 313 of the coil body penetrates the pile formed of the coil ribbon. [040] Let us note that FIG. 3A shows only one possible coil body. The coil body can be shaped in different ways. For example, the core of the coil body as a whole can be placed inside either part of the coil body. In principle, the coil body may include only the core. The coil ribbon of FIG. 3A can also be shaped in different ways. For example, a number of the segments and openings of the coil ribbon may vary.

[041] The coil ribbon in accordance with the invention comprises at least three segments so that two planar coils are placed into one segment of the coil ribbon, two other segments of the coil ribbon functioning as intermediate plates. [042] FIG. 3B shows the together folded coil ribbon shown in FIG. 1 looked from the side. The coil ribbon has been folded in the following way as a pile. Segment 314 which includes an intermediate plate is the topmost layer of the pile. In addition, the segment (segment 315) including an intermediate plate is the bottommost layer of the pile.

[043] Each second segment of the coil ribbon includes a planar coil (316-319) on the both sides of the coil ribbon. The foil included in the coil ribbon is located between planar coils 316 and 317, thus there is the required electrical insulator between the planar coils. Correspondingly, the foil which functions as an electrical insulator is located between planar coils 318 and 319.

[044] Each second segment of the ribbon coil, i.e. segments 314, 320, and 315, include only the foil. Thus, these segments are termed intermediate plates. [045] Let us note that the ribbon coil shown in FIG. 1 includes altogether five segments (106, 107, and 109). However, a principle linking the all ribbon coils of different lengths is that the core of the coil body penetrates through openings made for the core the segments of the coil ribbon. As an example of the openings we may mention openings 110 of FIG 1. [046] FIG. 4 shows a planar coil, a turn, and a stripe. The planar coil comprises at least one turn. FIG. 4 shows the planar coils comprising three turns 401, 402, and 403. Turns 401-403 are attached to (a piece of the foil) foil 404, for example, by clueing. Alternatively, the turns can be corroded into foil 404 in accordance with the printed circuit board technique. The stripe is meant to be the whole planar coil or some part of it. For example, the part of the coil starting from the point 405 and ending to the point 406 is termed a stripe.

[047] A planar coil can be attached to one or more pins, for example, by the following way: the outermost turn 401 of the planar coil is attached from its free end 407 to a pin and innermost turn 403 of the planar coil is attached from its free end 408 to another pin or to another planar coil.

[048] FIG. 5A shows a perspective figure of the solder area. The solder area includes three pieces so that piece 501 is part of the segment operating as an intermediate plate, the midmost piece, 502, is part of the segment comprising a planar coil, and piece 503 is part of another segment operating as an intermediate plate.

[049] The midmost piece 502 may be part of the segment of the ribbon coil shown in FIG. 1 that includes planar coil 102. In that case, pieces 501

and 503 are parts of those segments (106) locating on the both sides of the concerned segment, and hole 504 corresponds to hole 101 in FIG. 1.

[050] Piece 502 includes hole 504 in which the pin is soldered. Usually, solder containing mostly tin and at least one other metal is used in soldering. Instead of tin it is possible to use, for example, lead. In more detail, the pin is soldered so that pieces 501-503 are immersed down to line 505 in melted, liquid solder. The hole 504 is located under the liquid level and may be filled only for this reason. Probably also the known capillary phenomenon which lifts the liquid solder to the direction shown by arrow 506 effects to the filling of hole 504 with the solder. When the temperature of the solder decreases the solder becomes solid and fastens in hole 504.

[051] Pieces 501-503 and the capillary phenomenon are observed in the following figure from below, i.e. from the direction shown by arrow 506.

[052] FIG. 5B illustrates soldering a pin in the solder area. FIG. 5B in- eludes the same pieces 501-503 as FIG. 5A. In addition, FIG. 5B includes two other pieces 507 and 508. Consequently, there are altogether five pieces so we may consider that the pieces belong to of the coil ribbon shown in FIG. 1. The five pieces form one solder area, for example, the solder area 214 shown in FIG. 2. [053] Let us assume that the coil ribbon of FIG. 1 is folded as a pile and one side of the pile, which is illustrated in FIG. 2, is immersed into solder. Then the solder simultaneously fills four solder areas. In more detail, each of the four solder areas are filled so that the solder penetrates through openings 509-511 made into the intermediate plates in holes 512 and 504. Hole 504 is thus the same hole, which is shown in FIG. 5A.

[054] Before soldering pin 513 must be, of course, pushed through openings 509-511 and holes 512 and 504 so that one end of pin 513 extends up to opening 509 or over it, and the opposite end extends up to opening 511 or over it. [055] FIG. 6 illustrates a coil comprising two pins. It is possible to place more turns/planar coils between two pins by making the turns/planar coils of thinner stripes and by compensating the thinness of the stripes by a number of identical planar coils. In this way, the impedance of the coil decreases. The connection of two identical planar coils in parallel enables a double number

of turns between two pins without an increase of impedance between these pins. Correspondingly, the connection of four identical planar coils in parallel enables a quadruple number of turns.

[056] The coil illustrated in FIG. 6 comprises two planar coils 601 and 602 whose free ends are connected with two pins 603 and 604.

[057] Preferably, there are two pins 603, 604 and they are attached to the free ends of the planar coils of a coil, such as the free ends of planar coils 601 and 602.

[058] FIG. 6 presents such a problem that pin 604 is located near the coil core making difficult to solder pin 604. Later on, in FIG. 9A and 9B it is shown more accurately than in FIG. 6 how planar coils can be connected in parallel or in series in the coil in accordance with the invention.

[059] It is obvious for a person skilled in the art that impedance must be minimize, because it causes heating of a coil and the device using it. As well known, thin stripes increases resistance. However, when connecting the identical planar coils in parallel, as shown in FIG. 6, it is possible to minimize by means of the invention the resistance caused by the thin stripes

[060] The person skilled in the art knows a physical phenomenon due to which electricity (high frequency current) runs on the surface of a cable. Therefore, two thin stripes of a coil corresponds to one thicker stripe even if the quantity of copper related to the total number of the thin stripes would be less than the quantity of copper related to one thicker stripe. In other words, the thicker stripe can be replaced with the thin stripes when the thin stripes are connected in parallel. [061] FIG. 7 shows a coil manufacturing method. The method comprises the following steps.

[062] First, each second segment of a foil is formed 701 two planar coils: a first planar coil on one side of a segment and a second planar coil on the opposite side of the segment, the foil operating as an electrical insulator between the first planar coil and the second planar coil.

[063] Each second segment of the foil is left 702 as an intermediate plate which functions as the electrical insulator for at least one planar coil, each planar coil of the coil including at least one electrically conductive turn.

[064] The segments of the foil are folded 703 as a pile and at least two pins are soldered 704 into the pile, the pins being attached to at least two planar coils.

[065] Finally, a core of the coil made of magnetic material is pushed 705 through openings formed in the segments of the foil.

[066] The method is involved with the following details.

[067] Let us assume that holes are formed in each segment comprising a planar coil, and openings for the pins are formed in the other segments of the foil. Then a side of the pile, which contains a pin and the holes and the openings formed for the pin, is immersed into solder.

[068] A first planar coil is formable by corroding the first planar coil into an electrical conductor layer.

[069] Alternative, the first planar coil is formable by attaching at least one electrically conductive turn to the foil. [070] FIG. 8 shows a method for connecting planar coils together. The method is intended for connecting the planar coils when each planar coil is formed of at least one electrically conductive turn and the planar coils are placed into a ribbon-like, electrically insulating foil so that the foil separates the planar coils from each other. The planar coils are formable into the foil by at least the above-mentioned implementation ways

[071] The method comprises the following steps.

[072] First, it is formed 801 holes in segments of the foil, each of the segments being intended for one planar coil, and in addition it is formed openings in the other segments of the foil. Then the segments of the foil are folded 802 as a pile and pins are pushed 803 through the holes and the openings.

Finally, a side of the pile including at least one pin is immersed

804 in solder, the solder filling in the holes and the openings and connecting by the pins at least two planar coils together. As known, electrical components can be connected in parallel or in series. Therefore, the above-mentioned pin is related to two options: the pin connects the planar coils in parallel, or the pin connects the planar coils in series.

[073] At least one coil is formed on the foil by means of the method. Thus, it is possible that two or even more coils are formed in the same pile.

[074] As mentioned in the above, two pins are needed for each coil.

[075] If the coil is composed of the planar coils connected in parallel, there are exactly two pins.

[076] If the coil is composed of the planar coils connected in series and there are only two planar coils, the concerned planar coils are placed on the opposite sides of the same segment and exactly two pins are needed.

[077] If the coil is composed of the planar coils connected in series and there are more than two planar coils, more than two pins are needed.

[078] FIG. 9A shows connecting planar coils in pairs and in parallel.

[079] Planar coils 901 and 902 are placed on the opposite sides of the same segment. The ends of planar coils 901 and 902 are connected through the concerned segment to each other, which is illustrated by a dashed line. An end of planar coil 901 is connected to pin 903 and an end of planar coil 902 is connected to pin 904.

[080] Correspondingly, planar coils 905 and 906 are placed on the opposite sides of the same segment. The ends of planar coils 905 and 906 connected through the concerned segment to each other, which is illustrated by a dashed line. The other end of planar coil 905 is connected to pin 903 and the other end of planar coil 906 is connected to pin 904.

[081] Arrow 907 describes a direction from which electric current comes to the coil composed of planar coils 901 , 902, 905, and 906. Arrow 908 describes a direction to which the electric current runs from the coil. [082] Planar coils 901 and 902 connected in series form one pair and planar coils 905 and 906 connected in parallel form another pair. Therefore, planar coils 901 , 902, 905, and 906 are connected in pairs in parallel.

[083] Generally speaking, all the planar coils included in a pile can be connected in parallel when each planar coil comprises only one turn. [084] FlG. 9B shows connecting planar coils in series.

[085] Planar coils 911 and 912 are placed on a foil on opposite sides of the same segment. Ends of planar coils 911 and 912 are connected through

the concerned segment together, which is illustrated with a dashed line. One end of planar coil 911 is connected to pin 913 and an end of planar coil 912 is connected to pin 917.

[086] Correspondingly, planar coils 915 and 916 are placed on a foil on the opposite sides of the same segment. The ends of planar coils 915 and 916 are connected through the concerned segment to each other, which is illustrated with a dashed line. The other end of planar coil 915 is connected to pin 917 and the other end of planar coil 916 is connected to a third pin, i.e. pin 914. [087] Arrow 918 describes a direction from which electric current comes to the coil composed of planar coils 911, 912, 915, and 916. Arrow 919 describes a direction to which the electric current runs from the coil.

[088] As mentioned above, planar coils can be connected in a new, more versatile way together. An implementation in which the planar coils shown in FIGs. 9A and 9B are placed in the same pile is mentioned as one example. In other words, planar coils 901, 902, 905, 906, 911 , 912, 915, and 916 are placed in the pile one on the other. Then the pile comprises two coils and altogether five pins are needed for these two coils.

[089] In addition to the above shown descriptions and examples, the coil and the method in accordance with the invention can be implemented by some other ways that, however, are obvious for the person skilled in the art, because of his/her professional ability and the teachings presented by this patent application.

[090] The invention is defined in the attached patent claims.