EP1430491B1 - Planar transformer comprising plug-in secondary windings - Google Patents

Description

The invention relates to a planar transformer having a ferrite core, at least one primary coil and at least one secondary coil, which can be connected to a printed circuit board, and a coil body, which encloses a part of the ferrite core and carries at least one secondary coil.

DE 195 05 463 A1 discloses a power transformer with two E-shaped core parts that can be assembled into a core. A bobbin is shaped so that it can be inserted over the middle leg of the core, being closed to the outside and the primary winding formed by two spirally wound, flat wire windings and the secondary coil by two correspondingly round and flat-shaped secondary winding halves. In between there are insulating spacers. The transformer is assembled by inserting the various components axially in the correct order into the laterally open bobbin.

EP 0 435 461 A2 discloses a magnetic component with a ferrite core and a separate bobbin, which is elongate and has laterally projecting contact pins at the ends. On these pins a plurality of sheet-shaped copper windings (sheet winding patterns) attached, which for this purpose have trained, annular projections, via the contact pins, the individual turns to the primary as well as the secondary windings and these in turn connected to corresponding contact points on a circuit board , By placing the bobbin on the core and then on a circuit board, you get the finished component.

DE 28 23 779 1 discloses a transformer for putting a protection circuit. It has a bobbin with an outer winding chamber for winding a copper wire as a secondary winding. The primary winding is formed by a single, U-shaped bent conductor, which is surrounded except in the region of its ends by a thick insulating plastic body. The primary winding is inserted into the bobbin, which has an inner chamber for this purpose.

With increasing demands on the volume-related power density (VA / in ³ ) of a switched-mode power supply, the demands on their inductive components, in particular on the main transformer (s), also increase. For this reason, conductor card transformers in all imaginable designs, as a separate component or in the mainboard of a power supply, have been increasingly used for about 20 years.

An example of such a ladder-type transformer is known from US 5,010,314. Its primary and secondary coils are etched onto printed circuit boards, which have a recess in their middle, so that the printed circuit boards can be superposed on the ferrite core of the transformer, wherein an insulating layer is provided between adjacent printed circuit boards. The printed circuit boards are held together by a bobbin consisting of two halves, the printed circuit board with the primary coil being seated between the two halves and the secondary windings being arranged on the mutually remote sides of the halves of the bobbin. All printed circuit boards are covered by webs which rotate on both sides of the halves of the bobbin. The ferrite core consists of two E-shaped halves, wherein the conductor cards carrying bobbin on the central web of the Half of the ferrite core attached and the other half of the ferrite core from the other side of the bobbin is placed.

This type of printed circuit board technology is mainly used for signal transformers, storage chokes and transformers in the power range up to approx. 150 VA.

In the power range above 150 VA for outputs with low voltages (<12V) and correspondingly high output currents, considerable quality problems arise in the manufacture of PCB transformers. Thus, the copper thickness of the printed circuit boards at high currents must be correspondingly large and then no longer meets the standard of printed circuit board industry.

At high output power comparatively expensive printed circuit boards with special thicknesses are required, possibly existing standard copper thicknesses must be copper-plated. If printed circuit boards with special copper thicknesses are used, the etching gap between the printed conductors can only be guaranteed with optimum process setting. Even the smallest deviations in the process or impurities cause tiny copper bridges between the tracks. Such a bridge between two tracks results in too small a number of turns, a winding short circuit or, in the case of a conductive connection between the conductor track and the outer edge, even safety-relevant creepage distances between the windings or between the winding and the ferrite core. Such a conductive connection between two tracks can be detected during the printed circuit board production only after the respective process step by consuming measuring method, or it is only at the final functional test of the complete mounted transformer detected. However, the added value is negated and much of the material used can no longer be used.

Alternatively, several thin copper layers of multilayer printed circuit boards can be connected in parallel. However, the total thickness of such printed circuit board is comparatively high because of the insulating layers between the conductor layers. There is also the disadvantage that the exact connection of the parallel conductor layers in the printed circuit board is complicated and, in compliance with required safety standards, possible only with hidden penetrators.

Another problem is, especially in stationary printed circuit transformers, the mechanically stable and current-resistant contacting the circuit board with all the required inner layers of the circuit board, for example, a motherboard of the power supply, is.

The object of the invention is to provide a flat transformer in which the aforementioned disadvantages do not exist.

This object is achieved with a flat transformer of the type mentioned above in that each of the bobbin worn secondary coils is formed by at least one open on one side winding plate, which can be plugged onto the bobbin and connected to the circuit board.

A basic idea of the invention is to completely dispense with printed circuit boards and their limitation with respect to the thickness of the conductor layer, and instead to use a conductor plate that as a winding is formed and can be plugged onto the ferrite core. The winding plate is then connected directly to the circuit board, for example to the mainboard of the power supply unit. As a result, the winding carries itself due to a sufficient rigidity of the conductor plate itself, while conductor card transformers, the windings are all applied to a substrate and held by this, the substrate itself must be contacted with the motherboard in addition, for example by angle connectors or headers, and the pins must be mechanically stabilized.

The use of simple winding plates as secondary windings, which are connected directly via a printed circuit board, instead of printed circuit boards, in the conductor layers of which one or more windings are etched and connected via terminal strips and connected to the circuit board, there are unexpectedly many advantages over the above described conductor card transformers.

First, the design and production of such flat transformers is independent of standardized printed circuit boards and their copper thicknesses. By eliminating the need for PCBs with special copper thicknesses, flat transformer production costs can be significantly reduced, up to a quarter of the cost of comparable PCB transformers, or even less. For the same reason, there is no longer a problem with the availability of high-quality printed circuit boards.

Also, the production of such printed circuit transformers is simplified insofar as they can be manufactured almost everywhere in series with respect to conductor card transformers with relatively little effort, and in particular There are no single-source dependencies of manufacturers for printed circuit boards with special conductor layer thickness.

On the other hand eliminates all disadvantages in terms of possible qualitative degradation of PCBs in a non-exact manufacturing. Also safety-related risks, for example, no sufficient separation of the primary and secondary coils from each other due to possible creepage distances and clearances due to air inclusions or impurities, as they exist in printed circuit boards, can be safely excluded.

Another significant advantage is that the plugged and / or soldered to the circuit board of a device connections of the winding or sheets simultaneously serve as a mechanical fixation, so that an additional gluing, stapling, screwing the flat transformer on the device or on the circuit board is not required is.

Furthermore, the flat transformer according to the invention has significant advantages in terms of its environmental performance compared to conductor card transformers. Thus, in the method of manufacturing circuit boards, unlike in the manufacture of coil sheets, waste of a considerable amount is generated and a large amount of energy is required. In addition, in the production of printed circuit boards in special thickness, the rejection rate is high due to quality defects, while the sheet metal elements are extremely easy to manufacture, for example, in which they are punched out of a full-surface conductor material, so that the reject rate in the production of winding sheets is comparatively low , Furthermore, the planar transformer according to the invention can be recycled better, since it is easy to dismantling and less composites are used, which is particularly important in view of upcoming electronic scrap regulations where manufacturers are expected to be required to take back shipped equipment.

As a result, with the planar transformer according to the invention, a solution which is technically comparable but substantially cheaper than conductor card transformers is made available which can be used, in particular, for use in the power range of approximately 150-400 VA.

In a particular embodiment of the planar transformer according to the invention at least two winding plates are joined together via the circuit board to form a secondary coil. It is possible to equip the flat transformer according to the invention with a plurality of individual winding plates, which are interconnected via the circuit board optionally to a high-current winding or to a plurality of high-current windings with the same or different number of windings on the circuit board. If the interconnection of the individual winding plates is controlled by a driver or one or more relays, so that optionally individual winding plates of the or one of the secondary coils can be switched on or off, it is even possible to flexibly use a planar transformer provided with a plurality of winding plates. Also, based on the principle of the flat transformer according to the invention over comparable printed circuit transformers short-term pattern, prototypes and small series can be realized with changed or adapted number of turns, so the development times can be shortened.

If a planar transformer according to the invention is designed with two winding plates, these can be arranged on both sides of the primary coil, wherein sufficient insulation must be provided between the secondary windings and the primary coil. If a plurality of winding plates arranged side by side, they can either be coated in each case with an insulating layer, or it is preferably arranged between two adjacent coil sheets, an insulating intermediate layer. The latter embodiment is advantageous in that the respective winding plate consists exclusively of a conductor material and can be recycled more easily.

Preference is given to using coil plates as punched or eroded copper sheets. Copper is a preferred conductor material that can be easily processed. Also, the winding plates are preferably - especially in the region of their terminal ends - galvanically tin-plated, so that the sheets can be soldered easier and better stored.

Furthermore, the bobbin of the planar transformer according to the invention has a guide for at least one of the winding plates into which the winding plate is inserted. As a result, the winding plate is fixed in its position to the ferrite core, so that no quality or security losses due to obliquely inserted winding plates arise. For the same purpose, at least one of the winding plates and / or at least one of the insulating intermediate layers may have a recess which cooperates with a latching nose of the bobbin. Another way of fixing winding sheets is, for example, that the circuit board has slot-shaped receptacles into which the winding plates can be used and are thus fixed in the same way.

In a further preferred embodiment of the planar transformer, the bobbin has a winding chamber for the primary coil, wherein the primary coil may be formed from one or more wound conductor wires. Although it is basically possible, similar to the conductor card transformers to arrange the primary winding on a printed circuit board and to arrange between two halves of a bobbin. But if you want to completely dispense with printed circuit boards, this preferred embodiment offers, wherein the bobbin can then be integrally formed, for example as an injection molded part of a suitable insulating plastic. In this case, the bobbin on a jacket which surrounds a part of the ferrite core, and two to the central axis of the shell vertically outwardly projecting, circumferential walls. The conductor wire can then be wound between the walls on the mantle, while the winding plates are attached for the secondary winding on the side facing away from the winding chamber of the walls. The width and height of the winding chamber formed by the shell and the walls can be adjusted so that for a given wire diameter uniform winding structure with constant number of turns per layer and - in series-produced transformers - a consistent number of layers is achieved and the winding chamber is optimally filled.

A particular advantage of this design is that a secure primary-secondary separation is always guaranteed, as provided by the construction of the bobbin, with proper mounting, the required distance between the primary and secondary coil (s) never can be fallen below. The required for various approvals creepage distances and air gaps between the primary and secondary windings (usually> 6.4mm) are far exceeded depending on the design of the Spritzugßkörpers (thickness of the walls). Another significant advantage is that when using a wound conductor wire as a primary winding can be dispensed entirely with printed circuit boards within the flat transformer, so that higher operating temperatures are possible depending on the material used for the bobbin. In contrast, in conductor card transformers, the max. Operating temperature limited by the Tg value (glass transition point) of the carrier material and a corresponding approval of conductor card transformers to about 130 ° C.

In a further embodiment of this embodiment of the bobbin per primary coil at least two receptacles for pins, to which the beginning and the end of at least one conductor wire of a primary coil winding are connected, is provided. The advantage is a simple manufacture, wherein the ends of the primary coils can first be soldered to the pins before the flat transformer is simply placed with the rigid pins inside the circuit board and the pins are soldered to the circuit board.

The bobbin can also be advantageously designed with at least one of the bottom of the winding chamber to one of the pins, obliquely to the axis of this pin extending Drahtführungsnut. This on the one hand ensures that the windings of a layer can lie completely flat and parallel to each other at the bottom of the winding chamber without these windings around the tail of the conductor wire have to be guided around or one of the windings rests on this end. Thus, a pressure relief of the windings of all winding layers is achieved because each winding rests exactly on the winding of the underlying layer. On the other hand, with the Drahtführungsnut a strain relief of the tail of the conductor wire at the pin during winding of the primary coil sure guaranteed.

As already mentioned above, the bobbin is preferably formed in one piece, in particular as an injection molded part.

The flat transformer according to the invention is preferably formed with a ferrite core, which is composed of two E-shaped core halves, wherein the coil body is seated on the middle of the three mutually parallel core webs. In particular, it may be formed with an ETD, EFD, ELP or PQ core. It is also possible to form the flat transformer instead of with such a double-closed ferrite core, with a single-core ferrite core (U-core), in which the primary coil (s) on one leg and the plug-in winding plates of the secondary coil (s) on the other leg to sit. However, in principle embodiments are also conceivable in which the planar transformer is designed with a toroidal core. In this case, it would be, for example, form the bobbin in two parts such that each part comprises a shell half, wherein the shell halves are assembled to form a jacket around the toroidal core.

In the following the invention will be explained in more detail with reference to figures which show preferred embodiments of the planar transformer according to the invention.

Fig. 1

eine Explosionszeichnung einer ersten bevorzugten Ausfüh rungsform des Flachtransformators,

Fig. 2

eine isometrische Unteransicht des in Figur 1 dargestellten Flachtransformators mit eingesteckten Wicklungsblechen,

Fig. 3

eine Explosionszeichnung einer zweiten bevorzugten Ausführungsform des Flachtransformators, und

Fig. 4

eine isometrische Unteransicht des in Figur 3 dargestellten Flachtransformators mit eingesteckten Wicklungsblechen.

Show it

Fig. 1

an exploded view of a first preferred Ausfüh tion form of the flat transformer,

Fig. 2

an isometric bottom view of the flat transformer shown in Figure 1 with inserted winding plates,

Fig. 3

an exploded view of a second preferred embodiment of the planar transformer, and

Fig. 4

an isometric bottom view of the flat transformer shown in Figure 3 with inserted winding plates.

Figure 1 shows essential components of an embodiment of the flat transformer according to the invention, namely consisting of two halves 1a, 1b, three-limbed ferrite core, two secondary coils forming a coil plates 2 and a bobbin 3. The primary winding is not shown for clarity.

The winding plates 2 are made of a conductor material and are preferably stamped or eroded from a copper sheet and are tinned. They have a substantially U-shaped, so to one side open, profile. The upper web 4 of the U-shaped profile has at the center of the outer edge of a small, substantially rectangular notch 5. At both ends of the web, free legs 6, 7 join.

The thickness of the winding plates 2 is compared to the width of their webs 4 and the legs 6, 7 low. The width of a major part of the legs 6, 7 corresponds substantially to the width of the web 4 in the region of the notch 5. The free ends of the legs 6, 7 are as soldering or plug contacts 8, 9, whose width is slightly less than half as large as that of the majority of the legs 6, 7. The ends could also be formed as a cutting contacts by being chamfered.

The bobbin 3 is a one-piece injection molded part with a lateral surface 10, which surrounds the middle leg of the ferrite core in the assembled state of the flat transformer. The lateral surface 10 is adjoined by two walls 11, 12 extending perpendicularly thereto and in the circumferential direction, which together with the lateral surface 10 form a winding chamber 13 which is open outwardly in the circumferential direction for the primary coil. The width and height of this winding chamber are coordinated so that at a selected wire diameter of the conductor wire for the primary coil a uniform winding structure with constant number of conductors per layer achieved and the winding chamber can be optimally filled. As a result, the winding structure of the primary coil can be optimized in electrical and magnetic terms, in particular with regard to skin and proximity effects.

On each of the walls 11, 12 are on its side facing away from the winding chamber 13 side for each free leg 6, 7 of the winding plates 2 each two lateral guide slots 14a, 14b, 14c, 14d provided, which arranged on the outer edge of the walls 11, 12 Guide slots 14a, 14d for the outer edges of the legs 6, 7 of the winding plates over the entire edge length of the walls 11, 12 extend and the guide slots 14b, 14c for the inner edges of the free legs 6, 7 from the upper lateral surface 10 to the lower edge of the bobbin 3 extend. In addition, on the upper lateral surface 10 on both outer sides of the walls 11, 12 is a joint edge 15 for the inside of the webs 4 of the winding plates 2 and at the upper edges of the walls 11, 12 centered a latching lug 16a, 16b formed so that the inserted on the outer sides of the walls 11, 12 in the bobbin 3 winding plates 2 through the guide slots 14a, 14b, 14c, 14d, the abutting edges 15 and cooperating with the notches 5 locking lugs 16a, 16b are completely fixed, the solder or plug contacts 8, 9 of the winding plates 2 protrude beyond the lower edge of the bobbin 3. With this fixation ensures that there is always a defined distance to the later inserted ferrite core 1a, 1b, which is imperative for compliance with existing safety and approval requirements. At the same time a sufficiently large surface portion of the winding plates 2 is detected directly from the forced air flow of the device, so that a sufficient cooling of the transformer can be ensured.

As can be seen in particular also Figure 2, in which the flat transformer is shown with inserted winding plates 7 in view from below, the walls 11, 12 thickened in its lower region between the guide slots 14a, 14b, 14c, 14d for the inner leg edges formed and each have at least one downwardly open bore as a receptacle for pins 17a, 17b, which have a square cross-section for connecting the ends of the primary windings. The diameter of the holes is slightly smaller than the cross-sectional diagonal of the pins 17a, 17b, so that the pins 17a, 17b must be pressed into the holes and are sufficiently fixed due to the interference fit. The pressed into the holes pin 17a, 17b are about the same extent over the lower edge of the bobbin 3 over as the solder or plug contacts 8, 9th

In one of the thickened trained areas of the walls 11, 12 an obliquely to the axis of the pins 17a, 17b, downwardly open Drahtführungsnut 18 from the pin 17b to the winding chamber 13 is provided. Through this Drahtführungsnut 18 unnecessary mechanical pressure on the wire of the winding start is avoided by the subsequent turns, which could possibly lead to rollovers and Windungs Kurzschlüssen in the winding in operation at possibly applied high primary voltages.

To assemble the flat transformer shown, the bobbin 3 is first equipped with the pins 17. After pressing the pins 17, the desired number of turns of the primary winding is wound in a conventional manner with a winding machine in the winding chamber 13 of the bobbin 3. Depending on the insulation requirement of the device, the conductor wire for the primary winding can be designed, for example, as a single or multi-insulated copper round wire or as a nylon-wound high-frequency stranded wire. For winding, the beginning of the conductor wire for the primary coil is stripped to the required length and wound around one of the terminal pins 17. From this pin 17 of the conductor wire is guided by the oblique Drahtführungsnut to the bottom of the winding chamber 13, wound in the winding chamber to the primary coil and then passed the corresponding stripped end of the conductor wire to the other pin and wound around it. Thereafter, the pins 17 with the stripped wire ends, for example, in the immersion Schwallötbecken soldered.

After the soldering, the winding plates 2 are inserted as secondary windings in the guide slots 14a, 14b, 14c, 14d on both sides of the winding chamber 13. When inserting the winding plates 2 must engage in the detents 16a, 16b of the bobbin 3, to prevent later sliding back of the winding plates 2, such as during transport or during assembly of the entire transformer on a circuit board. Finally, the two ferrite core halves 1a, 1b are inserted with their middle legs on both sides in the bobbin 3 and glued together. Alternatively, the ferrite core halves 1a, 1b may be held together with staples or an adhesive tape wrapped around the entire ferrite core.

The flat transformer assembled in this way can then be placed on a printed circuit board, not shown here, and soldered thereto. The circuit board is formed so that the winding plates 2 are then interconnected as a secondary coil.

Finally, the functional and safety testing of the complete transformer takes place.

FIG. 3 shows essential components of another preferred embodiment of the planar transformer according to the invention. It has a three-limbed ferrite core consisting of two halves 21a, 21b, four winding plates 22, which can be connected together to form one or more secondary windings via a printed circuit board, not shown, and a bobbin 23. Printed circuit board and primary winding are not shown here for clarity.

The winding plates 22 differ from those of the previously described embodiment in that each winding plate 22 is formed of four mutually perpendicular webs 24, 25, 26, 27 of equal width, wherein the lower web 27 is not continuous, but is broken to one side , On both sides of the opening 28 of the lower web 27 close to the lower web 27 down solder or plug contacts 29, 30, wherein one of the solder or plug contacts 29 in the middle of the lower edge of the winding plate 22 is arranged.

In addition, two insulating layers 31 are provided, whose profiles are formed of four circumferential webs which are slightly wider than the webs of the winding plates 22, so that two winding plates 22, between which such an insulating layer 31 is disposed, are completely electrically isolated from each other. At their upper edge, the insulating layers each have a notch 32.

Also in this embodiment, the bobbin 23 is a one-piece injection molded part with a lateral surface 33, which encloses the middle leg of the ferrite core in the assembled state of the flat transformer. On the lateral surface 33, two perpendicular thereto and extend in the circumferential direction walls 34, 35, which together with the lateral surface form a circumferentially outwardly open winding chamber 36. Width and height of this winding chamber 36 are coordinated so that at a selected wire diameter, a uniform winding structure with constant number of conductors per layer achieved and the winding chamber 36 can be filled optimally.

On each of the walls 34, 35, a guide frame for the winding plates 22 is provided on its side facing away from the winding chamber 36, the guide slots 37a, 37b for the outer edges of the lateral webs 24, 26 of the winding plates 22, extending over the entire edge length of the walls 34, 35 extend, and a lower web 38, which forms a joint edge for the lower edge of the inserted into the guide slots 37 a, 37 b winding plates 22. The guide slots 37a, 37b are dimensioned so that two winding plates 22, between which an insulating layer 31 is disposed, can be inserted. The lower web 38 of the guide frame has openings 39, 40, 41 for inserting the solder or plug contacts 29, 30 of the winding plates 22, wherein a central opening 40 is provided through which the two central soldering or plug contacts 29 both side by side lying winding plates can be pushed through, and two further openings 39, 41 are provided for each of the other plug contact 30 of the winding plates 22 on both sides of the central opening.

As in the embodiment described above, a latching lug 42a, 42b is formed centrally on the upper edges of the walls 34, 35. If two winding plates 22 stacked together so with an intermediate insulating layer 31 that the central solder or plug contacts 29 side by side and the lateral solder or plug contacts 30 are each to different sides of the central solder or plug contacts 29, they can the guide frame are inserted so that they are completely fixed by the guide frame and the locking lug 42a, 42b in their position on the bobbin 23.

On the outer edge of each one of the guide slots 37a of the guide frame extend - as can be seen in particular in Figure 4 - respectively in the direction of the winding chamber away receiving blocks 43a, 43b with holes for receiving two pins 44a, 45a, 44b, 45b for two separate primary coil windings. The underside of these blocks 43 terminates with the lower edge of the bobbin 23. The pins inserted into the holes 44a, 45a, 44b, 45b are about as far beyond the lower edge of the bobbin 23 as through the openings 39, 40, 41 of the guide frame plugged through solder or plug contacts 29, 30th

From the bottom view of the flat transformer with inserted windings shown in FIG. 4 it follows that wire guide grooves 46a, 46b also extend from the winding chamber 36 in the direction of the underside of the wall. The ends of the conductor wire (s) of one or more primary coils may thereover pass from the bottom of the winding chamber 36 across the bottoms of the receiving blocks 43a, 43b to one of the terminal pins 44a, 45a, 44b, 45b or both pins 44a, 45a, 44b, 45b of a receiving block 43a, 43b are guided.

On the underside of the bobbin, four positioning feet 47a, 47b, 47c, 47d protrude, which can be used for positioning the fully assembled planar transformer on a printed circuit board, if provided on this corresponding recesses.

This embodiment of the planar transformer according to the invention is assembled in the same way as the previously described embodiment, with the exception of the other type of plugging of the winding plates 22 together with insulating layer 31 in the guide frame and the ability to wind two primary windings in the winding chamber 22 and connect to the pins 44, 45.

For both embodiments, the connections of the sheets on the motherboard of the device must be interconnected by interconnects in order to obtain the desired number of turns for the particular topology of the circuit, for example, secondary turns a turn number of 1 or 2 in a Zweiblechvariante or second or 4 in a four-sheet variants of this invention possible.

The due to the high secondary currents wide and thick interconnects to the coil plates also also provide heat removal from the transformer. In addition, results from the 4 or 8 solder joints (beginning and end per winding plate) an extremely stable connection between the transformer and the main circuit board of the device. Other fixings are not required.

Claims (10)

1. A planar transformer comprising a ferrite core (1a, 1b, 21a, 21b), at least one primary coil and at least one secondary coil, which can be connected on a printed circuit board, and a coil former (3, 23) with a casing surface (10, 33), the coil former enclosing part of the ferrite core (1a, 1b, 21a, 21b) and carrying at least one secondary coil, characterized in that each of the secondary coils carried by the coil former (3, 23) is formed by at least one metal winding sheet (2, 22), which is open on one side, and the primary coil comprising at least one wound conductor wire, the coil former having two walls (11, 12), which perpendicularly adjoin the casing surface, run in the peripheral direction and together with the casing surface form a winding chamber (13, 36) for the primary coil that is open outward, and in that at least one of the walls (11, 12) has on a side averted from the winding chamber (13, 36) in each case two guiding slots (14a, 14d), it being possible for at least one of the winding sheets (2, 22) to be inserted into the guiding slots and to be connected to the printed circuit board.
2. The planar transformer as claimed in claim 1, characterized in that at least two winding sheets (2, 22) are connected together via the printed circuit board to form a secondary coil.
3. The planar transformer as claimed in claim 2, characterized in that an insulating intermediate layer (31) is arranged between two winding sheets (2, 22) arranged next to one another on the coil former (3, 23).
4. The planar transformer as claimed in one of claims 1 to 3, characterized in that the winding sheets (2, 22) are punched or eroded copper sheets.
5. The planar transformer as claimed in claim 4, characterized in that the winding sheets (2, 22) are electro-tin-plated.
6. The planar transformer as claimed in claim 5, characterized in that at least one of the winding sheets (2, 22) and/or at least one of the insulating intermediate layers (31) has a notch (5, 32), which interacts with a detent (16a, 16b, 42a, 42b) of the coil former (3, 23).
7. The planar transformer as claimed in claim 6, characterized in that the coil former (3, 23) has at least two receptacles for terminal pins (17, 44, 45), to which the beginning and the end of at least one conductor wire are connected.
8. The planar transformer as claimed in claim 7, characterized by at least one wire-guiding groove, running from the bottom of the winding chamber (13, 36) to at least one of the terminal pins (17, 44, 45) and obliquely with respect to the axis of this terminal pin (17, 44, 45).
9. The planar transformer as claimed in one of claims 1 to 8, characterized in that the coil former (3, 23) is in one part.
10. The planar transformer as claimed in one of claims 1 to 9, characterized in that the ferrite core is put together from two E-shaped core halves (1a, 1b, 21a, 21b) and the coil former (3, 23) is disposed on the middle of the three core legs that are parallel to one another.

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