WO2008128913A1 - Electronic component - Google Patents

Abstract

Disclosed is an electronic component, in particular a planar transformer (1) or a planar inductor, having at least one core (2) and at least one main circuit board (4) equipped with electronic components. According to the invention, at least one secondary circuit board (6) having one and/or two-layers is assigned to the main circuit board (4) and is provided with at least one coil designed as a conductor path (28) and arranged in the area of the core (2).

Description

 description

Electronic component

Technical area

The invention relates to an electronic component, in particular a planar transformer or a planar inductance, with at least one core and at least one main circuit board equipped with electronic components.

PRIOR ART The electronic component according to the invention can be designed, for example, as a planar transformer or planar inductance. Due to the demand for ever smaller and more compact electronic devices, planar transformers in the field of power supplies but also in signal transmission are gaining in importance. Planar transformers have like conventional transformers on a primary winding and at least one Senkundärwicklung, which are inductively coupled by a core, such as a ferrite core, which consists of an upper and a lower part. Primary and secondary windings are arranged isolated from one another in the transformer. Compared to conventional transformers, the windings in planar transformers are arranged as correspondingly shaped printed conductors planar on the printed circuit board (printed circuit board). Conventional transformers usually have bobbins for receiving the windings. In the case of planar transformers eliminates the need for bobbin, since their function is taken over by the circuit board. Planar transformers are characterized by a high power density and very good efficiency. The transferable service area covers a few watts up to several kilowatts. The disadvantage of such planar transformers, in particular with a large transmission ratio, many windings, high voltage or current load and therefore large distances between the turns of a winding or wide interconnects is that complex and therefore expensive multilayer printed circuit boards are required, since only these the necessary effective PCB area in the area enclosed by the transformer.

Presentation of the invention

The invention has for its object to provide an electronic component that allows compared to conventional solutions a cost-effective and variable construction technology with low space requirement.

This object is achieved by an electronic component, in particular a planar transformer or a Planarinduktivität, with at least one core and at least one equipped with electronic components main circuit board, wherein the main circuit board is assigned at least one single- and / or two-layer secondary circuit board with at least one as a conductor provided winding and arranged in the region of the core. Particularly advantageous embodiments of the invention are described in the dependent claims.

In the solution according to the invention, a plurality of secondary printed circuit boards are preferably inserted into the free cross section of the core. Instead of a single, for example, eight-layer multilayer printed circuit board between the two core halves, for example, four two-layer Sekundärlei- terplatten are provided. This eliminates the need for costly, eight- or multi-layer circuit boards. The main circuit board is can be equipped to achieve different circuits with different secondary circuit boards, so that different variants of a circuit without changing the main circuit board can be realized. It is assumed that a single main board, but which is equipped with different or the same secondary circuit boards. As a result, different electronic ballasts (ECGs), for example, can be inexpensively manufactured in lamp technology based on the same circuit concept for different lamp types. One example is the electronic ballasts for Hg-containing and Hg-free discharge lamps in automotive engineering. The planar transformer consisting of inserted into the core secondary circuit boards is then populated like a conventional electrical component on the main circuit board.

The secondary printed circuit board or, in the case of several secondary printed circuit boards, the sum of the secondary printed circuit boards, for example, has a layer thickness ratio of electrically conductive material, for example copper and support material, of greater than or equal to 1, or has a layer thickness ratio of electrically conductive material and insulating material of greater than or equal to 1, or has with additional insulating layers, such as insulating paper or insulating film, a layer thickness ratio of electrically conductive material and insulating material of greater than or equal to 0.8.

The core preferably has an upper part which, with a lower part, forms a gap receiving the at least one secondary printed circuit board at least in sections. The core is preferably a ferrite core use. It may be advantageous to use multiple secondary circuit boards which all have congruent recesses for the core or core and a mounting bracket.

In a particularly preferred Ausführunsgbeispiel of the invention, the secondary circuit boards on both sides of at least one conductor track. Preferably, the tracks of the top and bottom of the secondary circuit boards are connected by means of a via.

It has proved to be particularly advantageous if the main printed circuit board are designed as two-layer printed circuit boards, since these are particularly inexpensive to produce.

In addition to the at least one single or double layered

Secondary circuit board is in a Ausführunsgbeispiel of the invention in the region of the core a multilayer

Secondary printed circuit board, in particular a 4-layer multilayer

Secondary circuit board arranged.

According to a preferred embodiment of the invention, a plurality of secondary circuit boards provided with an insulation forms a stack arranged at least in sections in the core. The insulation may preferably have a non-conductive lacquer layer, an insulating paper and / or an insulating film.

When using an insulating film as insulation, it has proved to be advantageous if on both sides of the insulating film, an adhesive surface for mechanical fixing of the secondary circuit boards is provided with each other. The mechanical fixation of the printed circuit boards with each other facilitates in particular the assembly.

The connection of the secondary circuit boards with each other takes place in a simple production engineering form of the invention by soldering. Preferably, the secondary circuit board has at least one solder pad for soldering to other secondary circuit boards. Particularly advantageous is a compound in which the secondary circuit boards have solder pads on the top and bottom and are soldered over them in an SMD soldering process. At these points, the insulation of the individual circuit boards is interrupted against each other.

The secondary circuit boards are connected in a preferred embodiment of the invention with each other and / or with the main circuit board by soldered wire pins. It has proved to be particularly advantageous if the secondary printed circuit boards have cover-like recesses, in particular bores, for receiving vias, pins and / or bridges. The bores are preferably surrounded by a copper edge (pad) for electrical contacting. In a corresponding embodiment of the holes, no wire is required to connect the individual circuit boards with each other, since the solder is pulled into the entire stack due to the capillary action. Advantageously, the holes are made as plated-through holes (plated holes) so that the copper in the hole connects the pad on the top with the electrically on the bottom.

According to an alternative embodiment, the secondary circuit boards are connected to each other and / or to the main circuit board by means of a bonding or riveting connection.

In a variant of the invention, the secondary circuit boards have surfaces of different sizes, so that intermediate secondary circuit boards can be easily contacted. For contacting intermediate secondary circuit boards, according to one embodiment of the invention, at least one secondary circuit board has at least one recess.

Due to its particularly compact design, the planar transformer is particularly suitable for vehicle technology applications, for example for electronic ballasts of high-pressure discharge lamps.

The solution according to the invention can also be used for planar inductors. In this variant, the winding of the inductance is preferably formed from partial windings of the main circuit board and the at least one secondary circuit board. That is, the winding of the inductance is divided into a plurality of circuit boards, wherein the main circuit board is used with.

Brief description of the drawings

The invention will be explained in more detail below with reference to preferred embodiments. Show it:

1 shows a longitudinal section of a planar transformer according to a first embodiment of the invention;

Figure 2 is the top of a board benefit for an E18 core;

Figure 3 shows the underside of the PCB benefit of Figure 2;

FIG. 4 shows an individual view of the upper side of the secondary printed circuit board labeled EILp7 from FIG. 2;

FIG. 5 shows the underside of the secondary printed circuit board from FIG. 4;

Figure 6 is a detail view of the top of the secondary circuit board labeled EILpβ of Figure 2; FIG. 7 shows the underside of the printed circuit board from FIG. 6;

Figure 8 shows the top of a PCB benefit for a planar transformer with ER23 core and

FIG. 9 shows the underside of the printed circuit board from FIG. 8.

Preferred embodiments of the invention

FIG. 1 shows a longitudinal section of a planar transformer 1 which essentially comprises a core 2 and a main printed circuit board 4 for mounting with electronic components (not illustrated) and four secondary printed circuit boards 6, the secondary printed circuit boards 6 being surrounded by the core 2 in sections. The core 2 is designed as a ferrite core with an upper part 8, which complements with a lower part 10 a the secondary circuit boards 6 receiving in sections intermediate space 12. The upper part 8 is glued to the lower part 10 by means of an adhesive suitable for ferrite material. The therefore considered as a mechanical unit core 2 sits loosely around the stack formed of secondary circuit boards 6 and can rest on the main circuit board 4. Upper and lower parts 8, 10 need not be identical as shown. For example, instead of two E-core halves, the core 2 may be composed of an E-core and an I-core half. As a result, it is possible to dispense with the use of cost-intensive multilayer circuit boards. Instead of a single, for example, eight-layer multilayer printed circuit board four two-layered secondary circuit boards 6 are provided. In an embodiment of the invention, not shown, cost-effective multilayer printed circuit boards with relatively low number of layers use. For example, instead of an 8-layer multilayer printed circuit board, a 4-layer multilayer secondary printed circuit board and two two-layered secondary printed circuit boards 6 are used. The planar transformer 1 is due to the particularly compact design, in particular for vehicle technology applications, for example, suitable for electronic ballasts of high pressure discharge lamps. Since the main circuit board 4 can be equipped to achieve different circuits with different secondary circuit boards 6, different variants of a circuit without changing the main circuit board 4 can be realized. As a result, different electronic ballasts based on the same circuit concept for different lamp types can be produced inexpensively.

The main printed circuit board 4 and secondary printed circuit boards 6 are each provided with windings (not shown) designed as printed conductors, which form primary and secondary windings of the planar transformer 1. The uppermost secondary printed circuit board 6 and the secondary printed circuit board 6 a arranged therebelow form a first and a second part of a secondary winding of the planar transformer 1. The underlying secondary circuit boards 6b and 6c form a first and second part of a primary winding of the planar transformer 1, wherein the tracks of the secondary circuit board 6 and 6a in series and the tracks of the secondary circuit boards 6b and 6c are connected in parallel. The secondary printed circuit boards 6, 6a are insulated from the secondary printed circuit boards 6b, 6c by means of an approximately 120 μm thick insulating film 14. It has proven to be advantageous if on both sides of the insulating film 14, an adhesive layer for mechanical fixing of the adjacent secondary circuit boards 6a and 6b is provided. The insulation between the secondary circuit boards can be done in addition to the insulating alternatively or additionally by a non-conductive lacquer layer or an insulating paper. The secondary side plates 6, 6a, 6b, 6c have a symmetrical structure in the vertical (this dimension is referred to as "thickness" hereinafter) consisting of

20 μm insulating varnish 70 μm copper layer

100 μm FR4 printed circuit board material 70 μm copper layer 20 μm insulating varnish.

Thickness ratios greater than about 1 between the copper layer and enclosed FR4 printed circuit board material can be achieved by the specified stacking technique. The FR4 printed circuit board material serves as a carrier material for the two copper layers. During the manufacturing process, the carrier material coated on both sides with copper is processed and partial copper is removed. This ratio of greater than about 1 also applies to the thickness ratio of all the secondary circuit boards 6 between the copper layer and the insulating material (insulating varnish and FR4 printed circuit board material). As a result, planar transformers 1 with a high copper ratio, which are thus suitable for particularly high power densities, can be achieved. In addition, particularly high power densities can be achieved with a thickness ratio of electrically conductive to electrically insulating material in the intermediate space 12 of 0.8 or greater, wherein the trapped air is not taken into account. The considered embodiment has a thickness ratio of about 0.82, considering the total thicknesses of 560 μm copper and 680 μm insulating material resulting from the four secondary printed circuit boards and the insulating film.

The connection of the secondary circuit boards 6, 6a, 6b, 6c with each other as well as with the main circuit board 4 takes place in the illustrated embodiment by means of wire pins 16, which are soldered into the circuit boards 4, 6, 6a, 6b, 6c. In this case, all stacked secondary circuit boards 6 have holes 18 at the same locations, so that the wire pins 16 can be pushed through the entire stack. These holes 18 are surrounded by a copper rim (pad) to make a corresponding electrical contact. In this embodiment of the bores 18, no wire is necessary for connecting the individual circuit boards to one another, since the solder is drawn into the entire stack due to capillary action. The wire pins 16 may be formed as vias 20, pins 22 or bridges 24. Vias 20 connect several secondary circuit boards 6 with each other. Pins 22 and bridges 24 also perform this function. However, pins 22 also contact the main circuit board 4 and, in addition to the electrical connection, also serve to mechanically fasten the secondary circuit boards 6 to the main circuit board 4. Bridges 24 allow connection of secondary circuit boards 6, the connection points not being in the same level. As an alternative to soldering the wire pins 18 on the circuit boards 4, 6, a press-in technique can be used. In this case, elastic or rigid pins are pressed into closely tolerated and metallized holes in the circuit boards 4, 6. Due to the plastic deformation of the metals involved secure electrical connections without soldering result.

In an embodiment of the invention not shown, the secondary circuit boards 6 on the top and bottom solder pads and are thus soldered together in the SMD soldering process as conventional SMD components. If an SMD soldering system is available, the secondary printed circuit boards 6 can first be printed with solder paste, stacked and then the transformer stack can be soldered. the. Finally, the wire pins 16 are soldered, which serve as pins 22.

Alternatively, a bond connection or a riveted connection is conceivable. Usually all secondary printed circuit boards 6 have the same geometric dimensions. This can be deviated from, for example, if it is bonded to a printed circuit board 6, which is not the topmost but the second uppermost circuit board 6 in the stack, for example. Then, the uppermost printed circuit board 6 is designed with a recess, for example at the edge as a cut corner, so that the second lowermost printed circuit board is accessible. Different shapes of the printed circuit boards 6 can also be advantageous for reasons of dielectric strength or for preventing flashovers between individual printed circuit boards (larger creepage distances).

FIG. 2 shows the upper side of a printed circuit board useful element 26 for a planar transformer 1 with an E18 core consisting of 32 secondary circuit boards 6 each having ten holes 18. For the sake of simplicity, only one secondary circuit board is shown in detail in FIG. The remaining 31 secondary circuit boards on the benefit have at the same locations holes and recesses as the illustrated secondary circuit board 6, and differ only by the shape and design of the conductors. It is a two-layer PCB benefit 26. The ten holes 18 are arranged with respect to their distances from each other so that they correspond to the ten terminal pins of the bobbin of a conventional, non-planar designed transformer with EFD25 core. The main circuit board 4 can be optionally equipped with a conventional or planar transformer. The utility 26 is cut apart at the perforated locations along the grid lines to form the 32 secondary circuit boards 6 to separate. All secondary circuit boards 6 have the same size and an identical hole pattern, which allows a particularly simple stacking and handling. The hole pattern, ie the locations at which all material was removed in the vertical structure during the manufacturing process of the secondary printed circuit board, is formed by the holes 18 and the recess (cutout, recess) for the core 2. In one or more clamping regions 17 of the recess of the core 2, a fastening clip could be formed so as to enclose the upper part 8 and the lower part 10 of the core 2 and, alternatively to the above-mentioned adhesive, to mechanically fix the upper and lower parts. All pads 18 are plated-through (plated holes). Depending on the desired structure of the piano transformer 1, for example the desired transmission ratio and the required current carrying capacity, the individual secondary circuit boards 6 can be combined with each other. However, only one of the 32 secondary circuit boards 6 is shown in detail in FIGS. 2 and 3. Of the other secondary circuit boards 6 only the outlines are shown in Figures 2 and 3.

The secondary circuit boards 6 each have a symmetrical structure in the vertical (with respect to Figure 1) consisting of

10 μm insulating varnish

70 μm copper layer

80 μm FR4 printed circuit board material

70 μm copper layer

10 μm insulating varnish.

As can be seen from FIG. 3, which shows the underside of the printed circuit board usefulness 26 from FIG. 2, for the sake of simplicity, only one secondary conductor plate was used in analogy to FIG. te shown in detail, each of the secondary circuit boards 6 has at least two terminals, ie at least two of each 10 pads of the secondary circuit boards 6 are contacted by a conductor 28 (elementary winding). In the following, the term "elementary winding" is understood to mean the connection of two terminals of a secondary printed circuit board 6 through a printed conductor 28. A secondary printed circuit board 6 with three terminals is also possible by tapping the elementary winding 28, which ultimately corresponds to two EI elementarwicklungen 28 connected in series. By tying each elementary winding 28 to pads 18, each elementary winding 28 within the board stack can be accessed so that by appropriate choice of board layout and placement of bridges 24, pins 22 and vias 20 (see FIG ), the desired configuration of the planar transformer 1 can be achieved. This will be explained in more detail below with reference to Figure 4, which shows a single representation of the top of the secondary circuit board 6 with the name EILp7 of Figure 2.

According to FIG. 4, the secondary printed circuit board 6 with the designation EILp7 has, for example, an elementary winding 28 with only one turn. The elementary winding 28 connects the terminals 7 and 5 of the secondary circuit board 6.

As can be seen from FIG. 5, which shows the underside of the secondary printed circuit board 6 from FIG. 4, an elementary winding 28 with one turn is likewise formed on the underside of the printed circuit board. The elementary winding 28 connects the terminals 7 and 5 of the secondary circuit board 6. This is the double copper cross-section and thus a higher current carrying capacity available.

If an elementary winding 28 has only a single turn, as in the case under consideration, then the embodiment of this is Elementarwicklung 28 sufficient in one layer of the circuit board and other layers can be used for other elementary windings 28 or removed altogether. The waiver of the higher current carrying capacity results in a reduced overall height and optionally in a smaller number of required secondary circuit boards 6.

FIG. 6 shows a detail view of the top of the secondary printed circuit board 6 labeled EILpβ of FIG. 2, in the case of more than one turn within an elementary winding 28. The elementary winding 28 begins at a land 18 (terminal 2) on top of the secondary printed circuit board 6 The trace 28 extends from outside to inside, around the recess for the center leg 34 and terminates in a via 30. The via 30 is in the form of 6 small copper sleeves through the FR4 printed circuit board material from the copper layer on top to the copper layer on the Bottom realized.

According to FIG. 7, which shows the underside of the secondary printed circuit board 6 from FIG. 6, the elementary winding 28 is continued on the underside, which now extends from the inside, ie from the through-hole 30, to the outside in another land 18 (terminal 7) end up. A parallel connection of elementary windings 28 takes place through the multiple use of identical secondary printed circuit boards 6. The electrical parallel connection is obtained, for example, by the contacting wire pins 16 (see FIG. 1). If several elementary windings 28 are to be connected in series, this has to be considered in the layout. In this case, identical winding arrangements are to be provided, albeit these must end at different pads 18. For example, the secondary circuit boards 6 with the designation EILpβ (see FIGS. 6 and 7), with an elementary winding 28 between the terminals 2 and 7, can also be used the secondary printed circuit board 6 with the designation EILp7 (see FIGS. 4 and 5), with an elementary winding 28 between the terminals 7 and 5, are connected in series through the connection via the connection 7. Further combination possibilities result from turning and turning over (the upper side of at least one secondary printed circuit board 6 lies oriented downwards in the stack of the remaining secondary printed circuit boards 6 whose upper sides are oriented upward) of secondary printed circuit boards. For example, the secondary printed circuit board 6 with the designation EILp7 connects the connections 7 and 5 in the illustrated orientation, whereas the connections 9 and 10 are rotated by 180 ° in the plane of the drawing. By interconnecting different secondary circuit boards 6, it is also possible to use windings 28 with the same number of turns achieve a different geometric structure. For example, three different secondary circuit boards 6, each connected to a winding 28 in series or three identical secondary circuit boards 6, each with three windings 28 are connected in parallel. In both cases, the DC resistance is almost equal, but the two arrangements have different high-frequency behavior. Thus, the parasitic capacitances, inductances and also the losses occurring due to skin and proximity effect differ.

FIG. 8 shows the upper side of a printed circuit board useful element 32 for a planar transformer 1 with an ER23 core 2 consisting of 14 secondary printed circuit boards 6, likewise each having ten solder bumps 18. For the sake of simplicity, only one secondary circuit board is shown in detail in FIG. 8 analogous to FIGS. 2 and 3. The benefit 32 is cut apart at the perforated locations to singulate the 14 secondary circuit boards 6. The winding 28 extends from outside to inside, around the schematically indicated round center leg 34 of the ER23 core 2 and ends in a through-hole. contacting 30. The arrangement and contacting of the secondary circuit boards 6 takes place, for example, analogously as described with reference to FIGS 2 to 7. The vertical structure of the PCB benefit corresponds to that of the secondary printed circuit boards 6 described in FIG.

According to Figure 9, the winding 28 is continued on the bottom, which now extends from the inside out to end in another pad 18 (connection). For the sake of simplicity, only one secondary printed circuit board is shown in detail in FIG. 9, analogous to FIG. Of the 14 secondary printed circuit boards 6, only one is shown in detail in FIGS. 8 and 9. Of the other secondary circuit boards 6, only the outlines are shown in FIGS. 8 and 9.

The electronic component according to the invention is not limited to the described embodiment as a planar transformer 1, but rather different electronic components can be realized in the inventive design. In particular, the solution according to the invention is also applicable to planar inductors. In this variant, the winding of the inductance is preferably formed of partial windings of the main circuit board 4 (air coil) and the at least one secondary circuit board 6 (series connection of an air coil and a coil with magnetizable core). Furthermore, the invention is not limited to the illustrated core geometries, but rather different core shapes or core combinations known from the general state of the art can be used.

Disclosed is an electronic component, in particular a planar transformer 1 or a planar inductance, with at least one core 2 and at least one main circuit board 4 equipped with electronic components. According to the main circuit board 4 is associated with at least one single- and / or two-layer secondary circuit board 6, which is provided with at least one designed as a conductor 28 winding and disposed in the region of the core 2.

LIST OF REFERENCE NUMBERS

1 planar transformer

2 core

4 main circuit board

6 secondary circuit board

8 upper part

10 lower part

12 space

14 insulating film

16 wire pin

17 bracket area

18 hole

20 Via

22 pin

24 bridge

26 board benefits

28 trace or elementary winding

30 via

32 PCB benefits

34 Mittelschenke!

Claims

claims 1. Electronic component, in particular planar transformer (1) or planar inductance, with at least one core (2), and at least one main circuit board (4) equipped with electronic components, characterized in that the main circuit board (4) has at least one and / or two-layered secondary printed circuit board (6), which is provided with at least one winding designed as a conductor track (28) and arranged in the region of the core (2). 2. Electronic component according to claim 1, wherein the secondary printed circuit board (6) or in the case of several secondary printed circuit boards the sum of the secondary printed circuit boards (6) has a layer thickness ratio of electrically conductive material of greater than or equal to 1. 3. Electronic component according to claim 1, wherein the secondary circuit board (6) or in the case of several secondary circuit boards, the sum of the secondary circuit boards (6) has a layer thickness ratio of electrically conductive material and insulating material of greater than or equal to 1. 4. The electronic component according to claim 1, wherein the secondary printed circuit board (6) or in the case of several secondary printed circuit boards, the sum of the secondary printed circuit boards (6) and additional insulating layers has a layer thickness ratio of electrically conductive material and insulating material greater than or equal to 0.8. 5. Electronic component according to claim 1, wherein the core (2) has an upper part (8), which with a lower part (10) forms an at least one secondary printed circuit board (6) at least partially receiving intermediate space (12). 6. Electronic component according to one of the preceding claims, wherein a plurality of secondary printed circuit boards (6) are used, all having congruent recesses for the core (2) or core and a mounting bracket. 7. Electronic component according to one of the preceding claims, wherein the secondary printed circuit board (6) has on both sides at least one conductor track (28). 8. Electronic component according to claim 7, wherein the conductor tracks (28) of the top and bottom of the secondary printed circuit board (6) by means of a through-connection (30) are connected. 9. Electronic component according to one of the preceding claims, wherein the main circuit board (4) is designed as a two-layer printed circuit board. 10. Electronic component according to one of the preceding claims, wherein in the region of the core (2) a multi-layer secondary printed circuit board, in particular a 4-layer MuI- tilayer secondary printed circuit board, is arranged. 11. Electronic component according to one of the preceding claims, wherein a plurality of secondary circuit boards (6) provided with an insulation form a stack arranged at least in sections in the core (2). 12. Electronic component according to claim 11, wherein the insulation has a non-conductive lacquer layer and / or an insulating paper and / or an insulating film (14). 13. Electronic component according to claim 12, wherein the insulating film (14) on both sides of an adhesive surface for mechanically fixing the secondary circuit boards (6) with each other. 14. Electronic component according to one of the preceding claims, wherein the secondary circuit board (6) has at least one solder pad for soldering on additional secondary circuit boards (6). 15. Electronic component according to one of the preceding claims, wherein the secondary circuit boards (6) with each other and / or with the main circuit board (4) by means of soldered wire pins (16) are connected. 16. Electronic component according to claim 15, wherein the secondary printed circuit boards (6) have cover-like recesses, in particular bores (18), for receiving vias (20), pins (22) and / or bridges (24). 17. An electronic component according to claim 16, wherein the bores (18) are surrounded for making electrical contact with a copper edge. 18. Electronic component according to one of the preceding claims, wherein the secondary printed circuit boards (6) with each other and / or with the main circuit board (4) are connected by means of a bond connection. 19. Electronic component according to one of the preceding claims, wherein the secondary circuit boards (6) with each other and / or with the main circuit board (4) are connected by means of a rivet connection. 20. Electronic component according to one of the preceding claims, wherein the secondary circuit boards (6) have different sized surfaces. 21. Electronic component according to one of the preceding claims, wherein the secondary circuit board (6) has at least one recess for contacting an underlying secondary circuit board (6). 22. Electronic component according to one of the preceding claims, wherein the planar transformer (1) for vehicle technology applications, in particular for electronic ballasts of high pressure discharge lamps, is used. 23. Electronic component according to one of claims 1 to 22, wherein the winding of the inductance of partial windings of the main circuit board (4) and the at least one secondary circuit board is formed.