WO2016005537A2

WO2016005537A2 - Inductor structure for multi-phase power conversion

Info

Publication number: WO2016005537A2
Application number: PCT/EP2015/065776
Authority: WO
Inventors: Santosh Kulkarni; Ningning Wang; Cian O'MATHUNA
Original assignee: University College Cork - National University Of Ireland, Cork
Priority date: 2014-07-09
Filing date: 2015-07-09
Publication date: 2016-01-14
Also published as: WO2016005537A3; GB201415065D0

Abstract

The invention provides a multi-phase inductor assembly comprising a plurality of coupled inductors, wherein the windings of each inductor are stacked in a vertical arrangement. The invention provides a way to realize stacked coupled inductor for power conversion application, such as Buck converters.

Description

Title

Inductor structure for multi-phase power conversion Field

The present invention relates to inductors, and more particularly to multi-phase coupled inductors or inductor assembly.

Background

Coupled inductors are an essential component in a variety of electronic circuits, such as for example power conversion circuits. A coupled inductor structure provides the advantages of DC flux cancellation, low ac ripple, fast transient response and higher efficiency at light loads for high power applications.

In the past, coupled inductor structures were typically fabricated using discrete ferrite cores, with the inductor coils wrapped around the core. A typical such structure is described in US Patent No. 7,864,016 in the name of Volterra. This patent describes a multi-phase in-plane coupled inductor. However, as such a structure requires sub millimetre magnetic material, it results in a relatively large inductor profile. Therefore, this inductor is not suitable for implementation on silicon.

US Patent No. 7936244 in the name of Vishay discloses a coupled inductor which uses a conducting electromagnetic shield close to the bottom magnetic plate to improve the coupling and reduce the leakage inductance. The Vishay US patent is about constructing a multiphase coupled inductor on the same planar with shielding. The coupling is provided by the top and bottom magnetic plates and the coupling is between two adjacent phases.

US Patent Publication No. US2004/0127036 in the name of Micron discloses a stacked spiral inductor structure which is electrically coupled. US 2004/0127036 is about providing, on semiconductor devices, coupled inductors with high coupling coefficients and quality factors. It is acting a transformer to provide a way to efficiently add or subtract RF or other time-varying signals in a semiconductor circuit.

More recently, there have been developments in the fabrication of multi-phase coupled inductors on a silicon chip. For example, Columbia University, IBM research and Duke University have fabricated a switched inductor integrated voltage regulator using 2.5D chip stacking for inductor integration. INTEL has also designed a 250 A voltage regulator using an on-chip coupled inductor. In the INTEL design, the coupled inductor uses two-phase stripline windings with laminated NiFe cores wrapped around the winding strips. This device worked for frequencies higher than 60 MHz. Columbia University have also reported the design and fabrication of 4-phase coupled inductors with a current density of 1 1 A/mm2 which can be used in a voltage regulator, with a switching frequency of 100 MHz. The coupled inductors from INTEL only provide coupling between two adjacent phases, and they are coupled in plane. Coupled inductors from IBM is similar to Intel's case. The 4 phase coupled inductor from Columbia is different from Intel and IBM work in the sense that all phases are coupled each other. However, the coupling in Columbia structure is achieved through one layer of magnetic material.

Other patent publications in the art include US5,884,990 (Burghartz et al) which teaches a schematic, cross-section and pad layout of a toroidal planar transformer. This construction is for a 2-phase transformer application, good coupling between primary and secondary windings in a normal transformers. (primary and secondary windings both have two terminals). Good coupling (coupling factor close to 1 ) is desirable for such a conventional transformer.

US4,969,032 (Scheitlin et al) discloses a structure with the vertical stacking of different components on GaAs substrate.

US2008/0284552 (Lim et al) describes a construction of an interleaved 2-phase transformer with a high coupling factor. US2001 /0030591 (Gardner) discloses the manufacture of a standard single phase inductors with copper windings sandwiched between two magnetic layers.

US Patent Number US6, 144,269 (Okamoto et al) discloses a device construction with two spiral windings without a magnetic core and configured to reduce noise in the device. However, in all of the above mentioned multi-phase designs, the coupling between phases is done in-plane, i.e. with the phases next to each other. Therefore, due to the higher phase count, the overall footprint of such devices is increased when compared to a single phase inductor. In addition, all of these known devices are designed to work at frequencies higher than 60 MHz.

Accordingly, it is an object to provide a multi-phase inductor which overcomes at least one of the above mentioned problems.

Summary

According to the invention there is provided as set out in the appended claims, a multi-phase inductor comprising:

a plurality of coupled inductors, wherein the windings of each inductor are stacked in a vertical arrangement. In one embodiment the advantage of the invention is that there is provided a 4- phase coupled inductor structure or assembly with stacked windings, where each phase contributes to the flux stored across each magnetic core providing significant flux cancellation when all phases are operated together. The invention is configured to evenly distribute the flux generated by one phase (one winding) to other phases. In other words, this is to avoid perfect coupling between any two of the four phases. The present invention allows for the construction of a coupled inductor structure with individual phases vertically stack to optimize the power density and also improve the flux cancellation allowing for increased current handling capability and significant ripple reduction for smaller output capacitors and higher efficiency of the power supply.

In one embodiment the invention provides multi-phase inductor structure with single turn windings which share single magnetic layer and the construction is so designed for the flux to average along all the other magnetic layers which increases the flux cancellation in individual magnetic layers and reduces the output ripple from the inductor.

In one embodiment the invention alternates the deposition of conductors and core materials to create a stacked structure with all windings coupled to each other.

In one embodiment the windings of each inductor are wrapped around a magnetic core. In one embodiment each magnetic core comprises a thin film magnetic layer.

In one embodiment the phases of the inductor are adapted to maximise the magnetic flux averaging between phases. In one embodiment the windings comprise planar coils.

In one embodiment the inductor is fabricated on silicon.

In one embodiment the inductor is adapted to operate at switching frequencies of less than 20MHz.

In another embodiment there is provided a dc-dc converter, such as a buck or boost converter, comprising a multi-phase inductor having a plurality of coupled inductors, wherein the windings of each inductor are stacked in a vertical arrangement.

In one embodiment there is provided an inductor assembly comprising a plurality of coupled inductors, each inductor comprising a winding, wherein the windings of each inductor are stacked in a vertical arrangement with respect to a substrate layer and each winding is coupled with a magnetic core such that flux is averaged and distributed across the magnetic cores.

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 a & 1 b illustrates a cross-section of the coupled inductor structure of the present invention and a number of process flow steps to make the inductor structure; and

Figure 2 illustrates a 4-phase coupled inductor structure or assembly with stacked windings according to one embodiment of the invention. Detailed Description of the Drawings

The coupled inductor structure of the present invention comprises a multi-phase inductor having vertical stacked windings. Figure 1 a discloses a cross-section of one embodiment of the inductor structure. The vertically stacked windings can comprise planar coils which are electrodeposited onto a substrate material 2, for example Silicon. The windings 1 are wrapped around a number of layers of magnetic cores 3a, 3b. The core material comprises thin film magnetic material. Figure 1 b shows a number of process steps are repeated to make up the stacked inductors, according to one embodiment of the invention. By having the inductor windings 1 stacked in a vertical arrangement, reduction in the magnetic path length between phase windings is achieved. This greatly improves the coupling effect between the phases when using thin film magnetic material. By coupling the flux between the phases, a significant portion of the AC flux in the magnetic structure can be cancelled out, while also significantly reducing the phase ripple current. The phases are arranged to maximize the flux/voltage averaging between phases. Figure 2 illustrates an inductor assembly according to one embodiment where 1 , 2, 3, 4 are the number of phases of each winding, and +/- indicates the polarity. As a result, the required magnetizing inductance per phase is reduced. The magnetic flux generated by winding 1 + and winding 1 - flows along with a horizontal part of core 200 from left to right. The flux travels along the magnetic material and split into two, one of which travels from right to left along the horizontal part of core 205 with the rest of magnetic flux travelling further downwards. The flux travelling in 205 generated by winding 1 + and 1 - couples winding 2 comprising 2+ and 2-. Similarly, the downward travelling flux is further split into two, one of which travels from right to left along core part 210. This results in coupling between winding 1 and winding 3. The rest of the magnetic flux travels along core part 215 from right to left, which provides coupling between winding 1 and winding 4. Hence, winding 1 is coupled to all other 3 windings. Similarly, the magnetic flux generated by winding 2 is split into 3 strands with each strand travelling in the corresponding magnetic core part 200, 210 and 215 from right to left, which couples winding 1 , winding 3 and winding, respectively. In part 200, the flux generated by winding 2, is opposite to the one generated by winding 1 . Hence, winding 1 and winding 2 are negatively coupled. In other words, phase 1 is negatively coupled to phase 2. The same methodology can be applied to analyze the distribution of flux generated by each winding. In the structure given in Fig 2, each phase is negatively coupled to any of the other 3 phases. It will be appreciated that the phase can be in the opposite order (so phase 1 is bottom layer & phase 4 on top), in other words phases 1 & 4 , 2 &3 can be exchanged depending on the application required. A manufacturing process, to make the inductor assembly as described in Fig 2, involves deposition of metal layers (conductor and magnetic alternately) along with the metal via (conductor via and magnetic via) to complete the connection for the individual phases. The process can be used for both PCB based and semiconductor based processing. The construction of the structure starts with deposition on the first patterned conductor layer on the processing substrate (PCB or semiconductor), along with the first via. This layer in insulated with a dielectric, followed by deposition of the second metal layer (magnetic) along the surface and sidewalls of the dielectric. Followed by a second insulation layer and then deposition of second conductor layer including via. This process is repeated till all four phases are realised.

This structure also allows significant reduction in output capacitance, due to its reduced output inductance and high operating frequency. The energy is stored as leakage inductance which is of small value, and thus results in improved transient response without increasing the phase current ripple.

The stacked approach also provides an advantage of lower device footprint for high phase count when compared to the footprint required for existing multiphase coupled inductor structures on silicon, where the coupling is done in- plane. In addition, as the inductor is fabricated using thin film magnetic material, the coupled inductor structure may be fabricated on silicon using standard Complimentary Metal-Oxide Semiconductor (CMOS) and Back End-Of-Line (BEOL) processing technologies. The device also be fabricated on PCB as well as Silicon.

Furthermore, the coupled inductor structure of the present invention can operate at switching frequencies of less than 20MHz. Therefore, the coupled inductor structure is suitable for integration into integrated circuits such as switches and drivers. This is mainly due to the fact that the ripple can be reduced by using multi-phase coupled inductor. This can either reduce the switching frequency while keeping the ripple current the same, or can reduce the inductance (or the size of the device) to keep the ripple current the same .One such device where the structure could be used is a multi-phase dc-dc converter device for high current applications, such as for example microprocessor power delivery. 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.

Claims

1 . An inductor assembly comprising:

a plurality of coupled inductors, each inductor comprising a winding, wherein the windings of each inductor are stacked in a vertical arrangement with respect to a substrate layer.

2. The inductor assembly of claim 1 , wherein the windings of each inductor is coupled with a magnetic core.

3. The inductor assembly of claim 1 or claim 2, wherein each magnetic core comprises a thin film magnetic layer.

4. The inductor assembly of any of the preceding claims, wherein the phases of each inductor are adapted to maximise the magnetic flux averaging between phases during operation.

5. The inductor assembly of any of the preceding claims, wherein the windings comprise planar coils.

6. The inductor assembly of any of the preceding claims, wherein the substrate layer comprises silicon.

7. The inductor assembly of any of the preceding claims, wherein the inductor is adapted to operate at switching frequencies of less than 20MHz.

8. A dc-dc converter comprising the inductor assembly as claimed in any of the preceding claims.