Centre-tapped transformer

Field

The present invention is concerned with transformers. More particularly, the invention relates to centre-tapped micro-transformers.

Background

Integrated transformers are generally constructed using spiral coils for windings due to their high inductance density. These coils can be either fabricated on silicon or on a printed circuit board (PCB). There are two typical primary and secondary windings arrangements used in such integrated micro-transformers. One arrangement is known as interleaved primary and secondary windings, and is shown in the air core transformer of Figure 1 . This arrangement is typically realized using a single layer of metal. However, this results in a transformer having poor coupling and low efficiency, due to the size of the single layer footprint when compared with transformers which use double layers of metal.

The second typical transformer winding arrangement is where the primary and secondary windings of the transformer are stacked using multiple layers of metal, with the primary winding on one layer and the secondary winding on another layer. The cross section of such a typical stacked arrangement using two layers of metal is shown in Figure 2.

Numerous publications in the art of transformers exist. For example US201 1 /0032065, Raczkowski, discloses a centre tapped transformer where the design emphasis is on symmetry. The transformer is called a Balun and in the primary and secondary windings ideally should have identical impedance. This conventional transformer design relies on critical symmetry of the windings. US 2006/0077028 A1 (Huang) discloses another transformer design as shown in Fig 5A and 5B. In Fig 5A and Fig 5B, the via plugs connects two layers of the conductors are different from the concept of a centre tap transformer. US2007/0001794 (Alford et al) discloses a transformer design for air-core application for replacing ferrite core structure and made using a PCB process for large feature sizes. Alford provides a substantially two-dimensional solution for performing the DSL transformer function which comprises of a planar structure characterized by the absence of a ferromagnetic element.

EP1 6381 18, Alps Electric, discloses an inexpensive manufacturing process of a transformer by using an inexpensive insulating substrate, by forming a primary coil and a secondary coil on both surfaces of the insulating substrate. The coils are concentrically arranged and based on micro-fabrication techniques with all the layers deposited in a sequential order. Other patent publications in the art of transformers include JP2012169 410 and JP2000277354.

In the field of transformers a centre tap is a contact made to a point normally halfway along a winding of a transformer or inductor. The centre tap provides an extra connection along with the usual connections at the two ends of each of two windings.

It is commonly understood that a centre tapped transformer cannot be realized using spiral coils by adopting either the interleaved winding arrangement or the stacked winding arrangement described above. This is due to space constraints, as well as the resulting asymmetric primary and secondary windings. However, having a centre tap is critical for transformers in certain applications, such as in power converter topologies, for example a push pull converter.

Accordingly, it is an object of the present invention to provide an improved centre tapped transformer. Summary of the Invention

The present invention, as set out in the appended claims, provides a centre- tapped transformer comprising:

at least one first layer comprising a first portion of a primary winding interleaved with a first portion of a secondary winding; and

at least one second layer comprising a second portion of the primary winding interleaved with a second portion of the secondary winding;

wherein the second layer is positioned on top of the first layer; and wherein the primary winding of the first layer is connected to the primary winding of the second layer through the centre of the first and second layers and the secondary winding of the first layer is connected to the secondary winding of the second layer through the centre of the first and second layers. By providing a centre tapped transformer of this structure, the transformer provides high inductance density and high coupling factor when compared to conventional centre tapped transformers. In one embodiment the invention provides a magnetic core. The magnetic core can provide higher inductance. Hence, the symmetry of windings is not critical since the air-core inductance is much smaller compared to the magnetic core inductance. The transformer of the invention allows minimum leakage inductance and the convenience of constructing centre taps for the ease of interconnects.

In one embodiment the first and/or second layers are dimensioned in the form of an elongated spiral or 'racetrack' shape. The difference is that the new approach used round shape in the ends rather than 90 degree bends as with prior art transformers.

In one embodiment there is provided a centre tapped transformer which can be realized using elongated spiral coils with magnetic core.

In one embodiment the centre taps are also used as via plugs connecting the two layers of conductors in the transformer. In one embodiment multilayer interleaved windings can be used to reduce the leakage inductance of a transformer with a magnetic core. In one embodiment there is provided at least one first layer of magnetic material comprising the bottom portion of the magnetic cores of a transformer. The bottom portion of the magnetic cores at least is separated into two parts.

In one embodiment there is provided at least one second layer of magnetic material comprising the top portion of the magnetic cores of a transformer. The top portion of the magnetic cores at least is separated into two parts.

In one embodiment there is provided at least one insulating layer between the first layer of magnetic material and the first layer of windings.

In one embodiment there is provided at least one insulating layer between the second layer of magnetic material and the second layer of windings.

The primary winding of the first layer may be connected to the primary winding of the second layer by primary winding centre pads provided in the first and the second layers and the secondary winding of the first layer may be connected to the secondary winding of the second layer by secondary winding centre pads provided in the first and the second layers.

Preferably, the first and second layers are metal.

Preferably the pads are provided external to the primary and secondary windings.

The centre-tapped transformer has an elongated spiral shape to take advantage of anisotropic properties of magnetic material with hard axis aligned and parallel to the straight portion of windings. The centre-tapped transformer has at least two magnetic cores surrounding the two straight portions of windings. The two parts of magnetic core are separated to each other with non-conductive materials or air.

In one embodiment there is provided a winding for use in a transformer, said winding comprising an elongated spiral shape. In one embodiment the winding comprises two substantially straight opposing sides and rounded or arcuate opposing sides to define the 'racetrack' shape.

The centre-tapped transformer may comprise a plurality of first and second layers, wherein the windings of each first layer are connected to the corresponding windings of each second layer through centre pads provided in the first and the second layers.

The present invention also provides a method of fabricating a centre-tapped transformer having a primary and a secondary winding comprising the steps of: interleaving a first portion of the primary winding and a first portion of the secondary winding on a first layer of metal ;

interleaving a second portion of the primary winding and a second portion of the secondary winding on a second layer of metal ;

positioning the second layer of metal on top of the first layer of metal ;

connecting the primary winding of the first layer to the primary winding of the second layer through the centre of the first and second layers; and

connecting the secondary winding of the first layer to the secondary winding of the second layer through the centre of the first and second layers.

The method may further comprise connecting the primary winding of the first layer to the primary winding of the second layer by primary winding centre pads provided in the first and the second layers and connecting the secondary winding of the first layer to the secondary winding of the second layer by secondary winding centre pads provided in the first and the second layers. Brief Description of the Drawings

The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which :-

Figure 1 shows a cross section of a prior art transformer with interleaved primary and secondary windings;

Figure 2 shows a cross section of a prior art transformer with stacked primary and secondary windings;

Figure 3 shows a cross section of the centre-tapped transformer of the present invention;

Figure 4 shows a top view of the first layer of the centre-tapped transformer of the present invention;

Figure 5 shows a top view of the first and second layers of the centre- tapped transformer of the present invention;

Figure 6 shows the magnetic field distribution over the cross section of the centre-tapped transformer of the present invention;

Figure 7 shows the magnetic field distribution of a prior art transformer having interleaved primary and secondary windings;

Figure 8 shows the magnetic field distribution of a prior art transformer having stacked primary and secondary windings

Figure 9 a one exemplar application of a centre-tapped transformer with centre taps, T1 c and T2c, connected to external circuitry; and

Figure 10 is a conventional centre-tapped transformer using the same winding arrangement as shown in Fig. 2.

Detailed Description of the Drawings

The present invention discloses a centre tapped transformer fabricated with spiral coils.

Figure 3 shows a cross section of one embodiment of the transformer structure of the present invention. In this structure, there are two layers of magnetic material surrounding the winding layers. The bottom layer of magnetic core is separated into two portions, 300 and 305, to avoid excess eddy current losses induced by the magnetic flux perpendicular to the magnetic layer. There is at least one layer of insulating material between bottom magnetic layer and the first winding layer. The top layer of magnetic core is also separated into two portions, 310 and 315, to avoid excess eddy current losses induced by the magnetic flux perpendicular to the magnetic layer. For the winding structure, two layers of metal stacked on top of one another are used to realize the primary and secondary windings of the transformer, and the windings are interleaved on each layer. Each layer contains a portion of each of the respective primary and secondary windings. Therefore, the first layer comprises a first portion of the primary winding interleaved with a first portion of the secondary winding, and the second layer comprises a second portion of the primary winding interleaved with a second portion of the secondary winding. In this embodiment of the transformer structure, the windings are shaped to form an elongated spiral, also known as a "racetrack". This is achieved by stretching half of a circular spiral by some distance to create an elongated spiral shape.

The transformer winding structure can be fabricated through a number of steps. In step 1 , a pair of interleaved primary and secondary windings is realized on one layer. Figure 4 illustrates a first layer windings laid as an elongated spiral, or racetrack configuration, and a plurality of pads. In Figure 4, pad 400 is the primary winding start pad, while pad 405 is the primary winding end pad. Pad 405 also is the primary winding centre pad, being the pad which provides the connection between the portion of the primary winding on the first layer and the portion of the primary winding on the second layer. Similarly, pad 410 is the secondary winding start pad, while pad 415 is the secondary winding end pad, as well as the secondary winding centre pad, being the pad which provides the connection between the portion of the secondary winding on the first layer and the portion of the secondary winding on the second layer. In step 2, another pair of interleaved primary and secondary windings is realized on a second layer. This second layer is then stacked or positioned on top of the first layer, as shown in Figure 5. In Figure 5, pad 520 is the primary winding centre pad, being the pad that provides for the connection between the primary winding on the second layer and the primary winding on the first layer, by connecting to centre pad 405 of the first layer, while pad 525 is the secondary winding centre pad, being the pad that provides for the connection between the secondary winding on the second layer and the secondary winding on the first layer, by connecting to centre pad 415 of the first layer.

Pad 505 is the primary winding end pad, while pad 515 is the secondary winding end pad. As the two layers are connected through their centre pads (i.e. the two middle pads) as vias, this structure results in a centre-tapped transformer.

In the described embodiment, two layers of metal are used in the fabrication of the windings in a centre-tapped transformer. However it should be understood that the same structure could equally well be employed to realize multiple layers of winding structures (such as for example 4, 6, 8, or more layers), by repeatedly stacking the two-layer structure shown in Figure 5. For example, a four layer structure could be realized by the following steps:

In step 1 , a pair of interleaved primary and secondary windings is realized on one layer, in the same manner as was previously explained with reference to Figure 4.

In step 2, another pair of interleaved primary and secondary windings is realized on a second layer and connected to the first layer in the same manner as was previously described with reference to Figure 5.

However, in accordance with the four layer structure of this embodiment, pad 505 is now also used to provide for the connection between that portion of the primary winding on the second layer and that portion of the primary winding on the third layer. In addition, pad 515 is now also used to provide for the connection between that portion of the secondary winding on the second layer and that portion of the secondary winding on the third layer. In step 3, another pair of interleaved primary and secondary windings is realized on a third layer and connected to the second layer. This third layer will have the same pattern of pads as that shown in Figure 4, with the exception of there being two additional pads on the third layer in order to provide the connection between the portion of the primary winding on the second layer and the portion of the primary winding on the third layer and the connection between the portion of the secondary winding on the second layer and the portion of the secondary winding on the third layer. In addition, it will be understood that pads 400 and pad 410 are not needed.

In the final step, another pair of interleaved primary and secondary windings is realized on a fourth layer and connected to the third layer. This fourth layer will have the same pattern of pads as that shown in Figure 5, with pad 505 corresponding to the primary end pad and pad 515 corresponding to the secondary end pad.

Figures 6 to 8 enable the magnetic field distribution of the centre-tapped transformer of the present invention to be compared with the two typical prior art primary and secondary arrangements of transformer windings described in the background to the invention section. Figure 6 shows a simulation of the magnetic field distribution of the centre-tapped transformer of the present invention. In Figure 7, a simulation of the field distribution of a transformer having interleaved primary and secondary windings is shown, while Figure 8 shows a simulation of a transformer having stacked primary and secondary windings. It will be appreciated that Finite Element Analysis can be directly transferred into leakage inductance, since the leakage inductance is proportional to the energy stored within the magnetic field. Accordingly, the energy of the simulated transformers in Figures 6, 7 and 8 are 7.81 x10 ^"6 J/m, 4.14 x10 ^"6 J/m, and 2.43 x10 ^"6 J/m, respectively. The simulation therefore clearly shows that the leakage inductance can be reduced by up to approx. 70% through the use of the transformer structure of the present invention. Fig. 9 is a one exemplar application of a centre-tapped transformer with centre taps, T1 c and T2c, connected to external circuitry. T1 a and T1 b are the two end terminals for primary winding. T2a and T2b are the two end terminals for secondary winding.

Fig. 10 is a conventional centre-tapped transformer using the same winding arrangement as shown in Fig. 2. Pad 900 corresponds to one primary end pad and pad 905 corresponding to the other primary end pad with 910 corresponds to the centre tap of primary winding. Pad 920 corresponds to one secondary end pad and pad 925 corresponds to the other secondary end pad with 930 corresponding to the centre tap of secondary winding. The occupied footprint area of such a structure will be considerably larger than that of the present invention. Hence, the present invention will result in a much higher inductance density.

There are a number of advantages associated with the centre tapped transformer of the present invention. Firstly, by fabricating a centre tapped transformer with an elongated spiral winding, the high inductance density associated with spiral coils can be exploited. The elongated shape is to take advantage of anistropic properties of magnetic material. Furthermore, the coupling factor of transformers, which is a critical parameter in evaluating the performance of a transformer, can be greatly improved through the use of this transformer structure. This is due to the fact that centre-tapped transformers have a much smaller leakage inductance when compared to conventional integrated transformers, as has been illustrated clearly with respect to Figures 6 to 8 and Fig. 10 above. Therefore, it will be understood that the transformer of the present invention provides ultra-low leakage inductance and excellent coupling. In addition, as the contact pads are located externally on the transformer, it enables micro-transformer arrays to be implemented with no extra interconnection requirement, and thus leads to an improvement in the flexibility in transformer design both at circuit level and component level, as well as efficiency.

The transformer of the present invention can also be easily realized using several standard fabrication processes, such as PCB, CMOS or MEMS. In addition, the transformer can be further extended to realize a multi-layer structure through stacking.

In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.