WO2004040599A1

WO2004040599A1 - A circuit board with a planar magnetic element

Info

Publication number: WO2004040599A1
Application number: PCT/CH2002/000588
Authority: WO
Inventors: Nigel Springett
Original assignee: Delta Energy Systems (Switzerland) Ag
Priority date: 2002-10-31
Filing date: 2002-10-31
Publication date: 2004-05-13

Abstract

A circuit board (1) includes a magnetic transformer (24) with a magnetic core (21) and two windings (9, 10). The magnetic core (21) comprises a C-shaped part (22) which is inserted into corresponding cut-out areas (7, 8) in the circuit board (1) as well as an I-shaped part (21) that is fitted to the legs of the C-shaped part. The windings of the magnetic transformer (24) are implemented as traces (9, 10, 11, 12) on the circuit layers (3, 4) of the circuit board (1). In order to efficiently cool the circuit board (1), the circuit board (1) is provided with a metallic layer (2) that is an integral part of the circuit board (1) and that includes slits (25) for prohibiting the metallic layer (2) to act as a short-circuit. Moreover, the metallic layer (2) enables a good shielding either of the circuit board itself or of external electrical and/or electronic circuits.

Description

A circuit board with a planar magnetic element

Technical field

The invention relates to a layered device for forming an electric and/or electronic circuit including a cut-out area for insertion of a core of a magnetic element and a circuit layer ha- ving a trace that forms a winding of the magnetic element. Prior art

Ever since the introduction of planar magnetic elements in the design of electronic systems, the number of applications has considerably increased. Because planar magnetic elements, for example transformers or inductors, not only offer improved electrical charac- teristics but also smaller size, the trend towards planar magnetic elements still continues. The power supply industry is one example where a major trend towards planar transformers is recognisable.

But this technique has some drawbacks too. The smaller the circuits become, the more heat is generated per unit of area. This means that more heat has to be removed from a gi- ven area. Another problem with an increased number of electronic components mounted on a circuit board is that the effects of electromagnetic interference (EMI), particularly with highly sensitive electronic components, are increased as well.

Usually, additional heatsinks are used in order to remove the heat generated by a magnetic element. Such heatsinks are either mounted directly on the core of the magnetic element or on a surface of the circuit board for example above the traces that form the transformer windings. However, providing additional components results not only in higher manufacturing costs but the circuits also require more space thereby reducing the space which otherwise could be used to implement supplementary functions.

To lower the effects of EMI, additional shielding elements were added to the circuit board or the entire assembly was installed within a shielding box. Again, additional components increase the costs and reduce the available space.

Summary of the invention

It is therefore an object of the invention to provide a layered device of the kind mentioned at the beginning, particularly to provide a layered device for forming an electric and/or electronic circuit with enhanced characteristics with respect to cooling and shielding at lower costs and space requirements.

The object of the invention is achieved by the layered device defined in claim 1. The layered device is designed for forming an electric and/or electronic circuit by mounting elec- trie and/or electronic components on the surface of the device and then interconnecting the components with electrically conductive connections. Some examples of components to mount on the layered device are resistors, inductors or capacitors as well as assemblies such as magnetic elements or microchips etc. To implement a planar magnetic element, the layered device includes one or more cut-out areas where the leg or the legs of the core of the magnetic element can be inserted. Further, the layered device comprises a circuit layer that includes a trace which for example is coiled around one of the cut out areas. With the core being installed within the cut-out areas, this trace forms a winding around one of the legs of the core of the magnetic element.

According to the invention the layered device further includes a layer of a metallic material that is an integral part of the layered device and which includes one or more slits for prohibiting an electrical short-circuit in this metallic layer. If the metallic layer had no slit, it would act as a short circuit for a current which would be induced by the magnetic field in the core. This metallic layer is a very important part of the layered device as it provides for cooling of the device. Typically, it is the thickest layer of the device.

If the magnetic element is a transformer, its core comprises for example a C-shaped part and an l-shaped part. The cut-out areas of the layered device correspond to the size and arrangement of the legs of the C-shaped part. To install the core, the legs of the C-shaped part are inserted into the cut-out areas from a first side of the device. Then, from the other side of the device, the l-shaped part is mounted on top of the legs of the C-shaped part in order to create closed paths for the magnetic flux within the core.

By providing the layered device with a layer of a metallic material, a better shielding and cooling of the electric and/or electronic circuit can be achieved. Furthermore, if the layered device has to be provided with a heatsink, there is no need for a separate manufactu- ring and mounting process, because the manufacturing of the metallic layer is a part of the manufacturing of the layered device. In fact, the layered device is fabricated on the basis of the metallic layer.

As already explained above, the term "layered device", is to be understood to define a thin or flat plate or base on which chips and other electric and/or electronic components are placed. As this definition is almost identical to the definition of circuit boards, which are commonly used for forming electric and/or electronic circuits, the term "circuit board" is used as a synonym for "layered device" hereafter.

The metallic layer preferably includes aluminium. However, other metals with a good ther- mal and/or electrical conductivity can be utilised as well. The metallic layer does not have to include a pure metal, the utilisation of compounds is possible too.

In order to prevent unwanted current flow from the circuit layer to the metallic layer or vice versa, an insulating layer is provided between the metallic layer and the circuit layer. It does not matter how this insulating layer is produced, either by providing the circuit board with a separate layer of an insulating material, by a certain treatment of the surface of the metallic or the circuit layer or by any other suitable technique.

The invention can be applied to different kinds of circuit boards. It can be applied to circuit boards where the circuit layer is an outer layer, i. e. a layer that forms the surface of the board as well as to circuit boards where the circuit layer is an inner layer of the board, such as for example a multilayer circuit board.

It is also possible to implement different kinds of magnetic elements such as for example inductors or transformers. As transformers, for example transformers that are used in power conversion and distribution systems, generate a lot of heat and EMI, the trace on the circuit layer in a preferred embodiment of the invention forms a winding of a transformer.

One trace on the circuit layer is sufficient to employ the invention. However, if more than one trace is provided on the circuit layer it is possible to implement much more complex circuits with the utilisation of only one single circuit layer. So in another preferred embodi-. ment of the invention the circuit layer includes a second trace which forms a second winding of the magnetic element.

A further useful possibility to implement complex magnetic structures is to provide a cir- cuit board with a plurality of circuit layers where two adjacent circuit layers are separated by an insulating layer. In this case at least two traces are provided on two different circuit layers where each trace forms a winding of the magnetic element.

While it is possible that both traces are either part of the same coil or of different coils of the magnetic element, it is preferred that at least two traces on different circuit layers are electrically conductively connected to form a single winding of the magnetic element, thereby enabling large windings and even more complex magnetic structures.

The invention can be used to implement different types of circuits. It is for example possible to realize a circuit board with just one or more magnetic elements such as transformers and to use such a circuit board together with other circuit boards to perform a speci- fied function. In a more convenient embodiment of the invention other electric and/or electronic components are mounted on the same circuit board as the magnetic element or elements. One could for example mount additional components on the circuit board in order to add filters, rectifiers or means to control the performance of a transformer. Such additional components typically are mounted on a surface of the circuit board and are inter- connected with each other and/or with the magnetic elements by means of traces on one or more circuit layers of the circuit board.

From the following detailed description and from the entirety of the claims it will be clear to a person skilled in the art, that there are more advantageous embodiments and feature combinations of the invention. Short description of the drawings

The drawings used for illustration of the examples show:

Fig. 1 A circuit board according to the invention in a perspective view;

Fig. 2 the circuit board as shown in fig. 1 in a front view;

Fig. 3 the first circuit layer of the circuit board as shown in fig. 1 ;

Fig. 4 the second circuit layer of the circuit board as shown in fig. 1;

Fig. 5 an electrical circuit with the circuit board as shown in fig. 1;

Fig. ό the electrical circuit as shown in fig. 5 in a front view;

Fig. 7 an electrical circuit with a circuit board according to the invention including two magnetic cores and further components mounted on its surface;

Fig. 8 another electrical circuit with a circuit board according to the invention including an E-shaped core;

Fig. 9 a metallic layer for the electrical circuit as shown in fig. 8;

Fig. 10 a further example of a metallic layer for the electrical circuit as shown in fig. 8;

Fig. 1 1 another example of a metallic layer for the electrical circuit as shown in fig. 8 and

Fig. 12 yet another example of a metallic layer for the electrical circuit as shown in fig. 8.

In general, the same objects in different drawings are given the same reference numerals. Ways of carrying out the invention

Fig. 1 and 2 show a perspective and a front view respectively of a layered device according to the invention. The layered device is for example a circuit board 1. The circuit board 1 includes a first circuit layer 3, a first insulation layer 5, a second circuit layer 4, a second insulation layer 6 and a metallic layer 2 (from top to bottom as shown in the figure). In this connection, the term "metallic layer" 2 means a layer which is substantially made of a metal or a metal compound such as for example aluminium in order to achieve a high thermal conductivity. Therefore, metallic layer 2 allows an efficient cooling of the circuit board when in operation as well as a good shielding of the components mounted on the circuit board 1.

For example the circuit layers 3, 4 include a thin copper foil and the insulation layers are made of FR4, a commonly utilised substrate material in circuit board design. The thickness of each layer extremely depends on the application requirements and can be varied in a wide range. While typical values vary from a few micrometers to some millimetres, there are other applications with even smaller or larger layer thicknesses.

As shown in fig. 1, the circuit board 1 includes two cut-out areas 7, 8 for insertion of the core of a magnetic element (not shown in fig. 1). The windings of the magnetic element are formed by traces 9, 10, 1 1, 12 which are provided on (or in) the circuit layers 3 and 4 around each cut-out area 7, 8 respectively.

The metallic layer 2 further includes three slits (25) which are arranged such that they interrupt any circular currents which would flow in the metallic layer 2 around the cut-out areas 7, 8 when a magnetic field flows in a core that is inserted into the cut-out areas 7, 8. Two of the slits (the outer ones) lead from an outer edge of the metallic layer 2 to an edge of the cut-out areas 7, 8 respectively and the middle slit separates the windings on each core half, when a core is inserted in the cut-out areas 7, 8. The cut-out areas 7, 8 can also be seen in fig. 3 which shows a top view of circuit layer 3. In order to connect the traces 9, 10 with other traces or with other electrical or electronic components, each trace 9, 10 starts and ends with a pad (13, 14, 15, 16). Trace 9 starts with pad 13 and ends with pad 14 (or vice versa) and trace 10 starts with pad 15 and ends with pad 16 (or vice versa).

Fig. 4 shows a top view of circuit layer 4, which is very similar to circuit layer 3. Fig. 4 shows the cut-out areas 7 and 8 as well. Further, circuit layer 4 includes two traces 1 1 and 12 with pads 17 and 18 or 19 and 20 respectively.

The circuit layers 3, 4 with their traces 9, 10, 1 1 , 12 can be manufactured with methods as known in the art such as for example with an etching process where those areas of the copper foil which are not needed within a circuit layer, are etched away. It is to mention that, although not shown, circuit layers 3, 4 may include additional traces or conductive areas to perform other functions such as for example power distribution or signal transmission within the circuit board.

The circuit layers 3,4 are interconnected by vias (not shown) as known in the art. Such connections may include through-hole vias, blind vias, buried vias or any other suitable kind of connection. In the example as shown in fig. 1 , the circuit layers are at least interconnected as follows: pad 13 is interconnected to pad 17, pad 14 is interconnected to pad 18, pad 15 is interconnected to pad 1 and pad 16 is interconnected to pad 20.

Thus, the traces 9 and 1 1 form one single coil wound around the cut-out area 7 and the traces 10 and 12 form one singie coil wound around the cut-out area 8. Although both coils are shown as having the same number of turns, it is obvious for a person skilled in the art that coils with different numbers of turns can be realised.

Generally, every kind of layered device with a metallic layer, independently of its manufac- turing process, can be utilised to employ the invention. However, there are several known methods to fabricate circuit boards with a metallic layer. One of these known circuit boards are called IMS (insulated metal substrate) boards. An IMS board for example com- prises an aluminium base layer, a circuit layer made of copper and a dielectric layer between the base and the circuit layer. The dielectric layer is for example made of FR4. Multilayer circuit boards can be produced by adding additional dielectric and circuit layers alter- natingly. Another known method to produce a circuit board with a metallic layer is called DCB (direct copper bonded) or DBC (direct bonded copper). Here, the circuit layer (typically a thin copper foil) is eutectically bonded to the base (typically an Al₂0₃ ceramic substrate), which yields a strong mechanical connection between the copper and the base with a good thermal conductivity. In a further known method a thin adhesive foil is utilised to bond the circuit layer to the metallic base (cold adhesion). After the bonding, the adhesive foil serves as the insulation layer.

Fig. 5 shows an assembled electrical and/or electronic circuit including the circuit board 1 as shown in fig. 1. Further, the circuit includes a magnetic core 21. As best seen in fig. 6, the magnetic core 21 includes a C-shaped part 22 with two legs 22.1 and 22.2 as well as an l-shaped part 23. To assemble the magnetic core 21 and the circuit board 1 , the legs 22.1, 22.2 of the C-shaped part 22 are inserted in the cut-out areas 7, 8 respectively. Then, the C-shaped part 22 and the l-shaped part 23 are fitted together as shown in fig. 6 with any known fastening means, including for example glueing or clamping.

The magnetic core 21 and the windings that are formed by the traces 9, 10, 1 1, 12 build up a transformer 24. The primary winding of the transformer 24 is for example formed by the two traces 9 and 1 1 and is wound around leg 22.1 of the magnetic core 21, while the traces 10 and 12 form the secondary winding wound around leg 22.2. That is, a current flow through the traces 9 and 1 1 induces a magnetic flow within the magnetic core 21, the direction of which depending on the direction of the current flow. The magnetic flow within the magnetic core 21 in turn induces a voltage in the secondary winding.

In order to prohibit an electrical short-circuit in the metallic layer 2, three slits 25 are provided in the metallic layer 2 as explained in connection with fig. 1. Regarding the number of slits 25 it is to say that the metallic layer 2 may comprise more or less than three slits 25 depending on the actual application and its implementation. Fig. 7 shows a further electrical and/or electronic circuit where a circuit board according to the invention, i. e. a circuit board 1.1 with a metallic layer 2.1 and at least one circuit layer 3.1 is used. Two magnetic cores 21.1, 21.2 are mounted on the circuit board 1.1 in the same manner as explained above. Together with the traces 9.1, 10.1 and 1 1.1 , 12.1 respectively, they form two transformers 24.1 and 24.2. Furthermore, the metallic layer 2.1 includes seven slits 25 for the purpose explained above, one for each cut-out area.

Apart from the fact that the circuit as shown in fig. 7 includes two transformers 24.1 , 24.2 and not only one as shown in the assembly according to fig. 5, the circuit board 1.1 includes a number of electrical and/or electronic components 31, 32, 33, 34 mounted on its surface. Some of the components 31 , 32, 33 are for example mounted directly on the circuit layer 3.1 (COB, chip on board) where the electrical connections between the components 31 , 32, 33 and the circuit layer 3.1 are formed by corresponding pads at the bottom of the component 31, 32, 33 and the circuit layer 3.1. Another component 34 has two kinked leads 36.1, 36.2 which jut out of the housing of the component 34 and are inserted in the holes of two corresponding pads 35.1 , 35.2 on the circuit layer 3.1. While the electrical connections to lead 35.2 of component 34 and the terminals of the components 31, 32, 33 include traces on an inner circuit layer of the circuit board 1.1, lead 35.1 of component 34 is interconnected to pad 16.2 of trace 10.2 by a further trace 16.2 on circuit layer 3.1.

Any available type of electric or electronic component, i. e. any type of active, passive, analog or digital component can be mounted on the circuit board 1.1 to form the required circuit.

Fig. 8 shows another electrical and/or electronic circuit according to the invention. It comprises a circuit board 1.2 with an E-shaped magnetic core 21.3 that includes for example an E-shaped part 26 and an l-shaped part 23 fitted together around the circuit board 1.2. The magnetic core 21.3 comprises three legs 26.1, 26.2, 26.3 which are inserted in three corresponding cut-out areas 27.1, 27.2, 27.3 in the circuit board 1.2. A winding, that is formed by the trace 28 in the circuit layer 3, is wound around the center leg 26.1 of the magnetic core 21.3. In order to prohibit an electrical short-circuit in the metallic layer 2, a slit 25 is provided in the metallic layer 2 leading from the center cut-out area 27.1 to an edge of the metallic layer 2, as can be seen best in fig. 9.

Fig. 10 to 12 show metallic layers 2 for usage in the circuit as shown in fig. 8 with different arrangements of one or more slits 25. Fig. 10 for example shows a metallic layer 2 with two slits 25 which are arranged such that a first slit 25 interconnects cut-out area 27.1 with cut-out area 27.2 and that a second slit 25 interconnects cut-out area 27.1 with cutout area 27.3.

Fig. 10 for example shows a metallic layer 2 with two slits 25 which are arranged such that a first slit 25 interconnects cut-out area 27.1 with cut-out area 27.2 and that a second slit 25 interconnects cut-out area 27.1 with cut-out area 27.3. Both slits 25 prohibit leakage currents around the cut-out area 27.1 induced by the magnetic field within the middle leg 26.1.

Fig. 1 1 shows a metallic layer 2 with only one slit 25 interconnecting cut-out area 27.1 with cut-out area 27.2 and thereby prohibiting leakage currents around the cut-out area 27.1.

A further metallic layer 2 is shown in fig. 12. It shows a slit arrangement which is substantially the same as the one shown in fig. 10, but with the slits 25 being of the same width as the cut-out areas 27.1 , 27.2, 27.3. That is the cut-out areas 27.1, 27.2, 27.3 together with the slits 25 form a big rectangular cut-out area. For a better understanding, the positions of the cut-out areas 27.1 , 27.2, 27.3 are shown with dotted lines.

To summarise it can be stated that the invention enables the creation of very compact electric and or electronic circuit arrangements with integrated cooling and shielding means. Particularly with circuit arrangements including power circuits with magnetic elements, the magnetic layer according to the invention allows an efficient cogling of the cir- cuit while other components are effectively shielded. In most cases, no additional heatsinks or shields are necessary.

Claims

1. A layered device for forming an electric and/or electronic circuit including a cut-out area for insertion of a core of a magnetic element and a circuit layer having a trace that forms a winding of the magnetic element, characterised in that the layered device in- eludes a layer of a metallic material for shielding and cooling said electric and/or electronic circuit, said layer of metallic material being an integral part of the layered device and having a slit for prohibiting a short-circuit within the layer of metallic material.

2. A layered device according to claim 1, characterised in that it includes an insulation layer between the circuit layer and said layer of metallic material, with said metallic material preferably including aluminium.

3. A layered device according to any of claims 1 or 2, characterised in that the trace forms a winding of a transformer.

4. A layered device according to any of claims 1 to 3, characterised in that the circuit layer includes a second trace that forms a second winding of the magnetic element.

5. A layered device according to any of claims 1 to 4, characterised in that the layered device includes a plurality of circuit layers where two adjacent circuit layers are separated by an insulating layer and where at least two traces are provided on two different circuit layers and where each trace forms a winding of the magnetic element.

6. A layered device according to claim 5, characterised in that at least two traces on dif- ferent circuit layers are electrically conductively connected to form a single winding of the magnetic element. A layered device according to any of claims 1 to 6, characterised in that an electronic component is mounted on a surface of the layered device.