GB2529635A

GB2529635A - Centre-tapped transformer

Info

Publication number: GB2529635A
Application number: GB1415074.2A
Authority: GB
Inventors: Ningning Wang; Cian O'mathuna
Original assignee: University College Cork
Current assignee: University College Cork
Priority date: 2014-08-26
Filing date: 2014-08-26
Publication date: 2016-03-02
Also published as: GB201415074D0; WO2016005526A1

Abstract

A transformer, or a method of making a transformer, comprises: interleaving a first portion of a primary winding with a first portion of a secondary winding in a first layer; interleaving a second portion of the primary winding with a second portion of the secondary winding in a second layer and arranging the second layer on top of the first layer and interconnecting the primary and secondary portions, respectively though connections through the central region of the windings. The said interconnections may involve contact pads 525, 520, located in the centre of the windings, which may also be used for centre-tap connections for the primary and/or secondary windings of the transformer. Further contact pads 500, 505, 510, 515, external to the windings, may be provided for the respective terminals of the primary and secondary windings. A plurality of first and second layers may be stacked together and interconnected to form a transformer. The transformer may be formed as a compact PCB or integrated circuit micro-transformer component which is suitable for use in power converter arrangements such as a push-pull power converter system.

Description

Centre-tapped transformer

Field

s 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 is 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.

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 a centre tapped transformer which can be realized using spiral coils.

Summary of the Invention

s 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 is 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.

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

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, 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 "race track". This was achieved by stretching half of a circular spiral by some distance to create an elongated spiral shape.

The transformer structure is fabricated through a number of steps. In step 1, a pair of interleaved primary and secondary windings is realized on one layer, as shown in Figure 4, wherein the colour blue represents the first layer windings and 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 s 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, where the colour blue represents the first layer and the colour green represents the second layer. 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 is 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 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 is 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 xl 0-6 JIm, 4.14 x1O6J/m, and 2.43 xl 0-6 JIm, 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.

There are a number of advantages associated with the centre tapped transformer of the present invention. Firstly, by fabricating a centre tapped transformer with a spiral winding, the high inductance density associated with spiral coils can be exploited. 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 S 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. IC)

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