EP4318510A1

EP4318510A1 - Compact electric transformer with controlled leakage

Info

Publication number: EP4318510A1
Application number: EP22306176.3A
Authority: EP
Inventors: Victor William QUINN; Benoit Bertrand Hugues KRAFFT
Original assignee: Exxelia SAS
Current assignee: Exxelia SAS
Priority date: 2022-08-03
Filing date: 2022-08-03
Publication date: 2024-02-07

Abstract

An electric transformer (1) comprising left and right primary coils (20a, 20b), left and right secondary coils (22a, 22b) and a first magnetic circuit (10), the first magnetic circuit comprising left and right limbs (11a, 11b) connected together by upper and lower yokes (13, 14) so as to define an inner space (15), the left primary and secondary coils being wound around the left limb and the right primary and secondary coils being wound around the right limb, the electric transformer further comprising left and right bypass cores (31a, 31b) located inside the inner space of the first magnetic circuit, upper and lower ends of each of the left and right bypass cores being in contact with the upper and lower yokes of the first magnetic circuit, the left bypass cores being positioned radially from the left limb to be between the left primary coil and the left secondary coil and the right bypass cores being positioned radially from the right limb to be between the right primary coil and the right secondary coil.

Description

BACKGROUND

1. Field

The present invention relates to the technical field of electric transformers.

2. Description of the related art

Document EP3846187A1 discloses a transformer with a controlled leakage impedance.
This transformer comprises a main core shaped as a closed ring, which comprises left and right limbs connected by upper and lower yokes.
A left bypass core is positioned along the left limb, but outside the main core, away from the left limb, on the side thereof opposite the side oriented toward the central plane of the transformer.
A primary left winding is wound around the left limb only, while a secondary left winding is wound around the left limb and the left bypass core.
Symmetrically through the central plane, a right bypass core is positioned along the right limb of the main core, but away from the right limb, on the side thereof opposite the side oriented toward the central plane of the transformer.
A primary right winding is wound around the right limb only, while a secondary right winding is wound around the right limb and the right bypass core.
With such an arrangement, in addition to a main magnetic loop formed along the main core, two additional magnetic loops are formed: one left magnetic loop along the left limb of the main core, a lower air gap between the left limb and the left bypass core, the left bypass core and an upper air gap between the left bypass core and the left limb of the main core; and one right magnetic loop along the right limb of the main core, a lower air gap between the right limb and the right bypass core, the right bypass core and an upper air gap between the right bypass core and the right limb of the main core.
The design of these two additional magnetic loops allows the precise setting of the value of the leakage impedance of the transformer.
However, this known transformer is relatively bulky, when there is a need to make transformers more compact to reduce their size (and increase their integrability) and improve their power density (and thus their efficiency).

SUMMARY OF THE INVENTION

The present invention addresses this problem by providing an electric transformer with a controlled leakage impedance exhibiting a particular compact design.
The subject matter of the invention is defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a front cross-section view of a preferred embodiment of an electric transformer according to the invention;
Figure 2 is a top view of the transformer according to Figure 1; and,
Figure 3 is a top cross-section view of another embodiment of an electric transformer according to the invention.

DETAILED DESCRIPTION OF THE PREFFERRED EMBODIMENT

Figures 1 and 2 illustrate a preferred embodiment of an electric transformer 1 according to the invention.
Axis X, Y and Z define a system of reference attached to the transformer 1.
Axis Z is oriented. The adjectives of "upper" or "lower" are defined relatively along axis Z.
In the embodiment disclosed here, the transformer is symmetrical relative to a central plane P (parallel to axis Y and Z). The parts of the transformer 1 located on the left side of the middle plane P are called "left", while the parts of the transformer 1 located on the right side of the middle plane P are called "right".
The transformer 1 comprises :

a main magnetic circuit 10;
left and right primary coils 20 a and 20b;
left and right secondary coils 22a and 22b; and,
left and right bypass cores 31a and 31b respectively.

The main magnetic circuit 10 is made of a main core comprising two limbs, respectively a left limb 11a and right limb 11b, connected by two yokes, respectively an upper yoke 13 and a lower yoke 14.
The main core is for example made of a ferrite material. Alternatively it is made of lamination, wound ribbon, agglomerated magnetic powder or may be composed of various assembled discrete magnetic component materials selected as preferred for the anticipated operating conditions for each component volume.
The main core forms a closed ring. For example, the core results of the assembly of two "C" shaped pieces, associated along a middle horizontal plane (plane XY) of the transformer 1.
The cross-section of the main core is roughly rectangular. It may have preferred pronounced radii at selected corners to facilitate wrapping of large diameter conductors or bundles of twisted strands. The cross section of the main core may not be constant as it will be explained below.
The main core delimits an internal space 15.
Focusing more specifically on the left side of the transformer 1, the left primary coil 20a is made of an inner left primary winding 21a and an outer left primary winding 23a connected in series one with the other.
The inner left primary winding 21a is for example made of a wire wound around the left limb 11a.
The inner left primary winding 21a forms a first column of radius R1 around the left limb 11a.
The outer left primary winding 23a is for example made of a wire wound around the left limb 11a.
The outer left primary winding 23a forms a third column of radius R3 around the left limb 11a.
The wires of the inner and outer left primary coils are wound in the same direction around the left limb.
The same left primary current I1a circulates in the inner and outer left primary winding.
The left secondary coil 22a is made of one coil.
The left secondary coil 22a is for example made of a wire wound around the left limb 11a.
The left secondary coil 22a forms a second column of radius R2 around the left limb 11a.
A left secondary current I2a circulates in the secondary coil corresponding to the load current that the transformer generates from the induced secondary voltage.
The exciting magnetic flux in the main transformer core limbs 11a and 11b is given by the applied input voltage applied to the primary winding and application of Faraday's Law. In normal operation. the induced ampere-turn contribution of primary current I1a counters the ampere-turn contribution of load current I2a so that the exciting magnetic flux remains as given by the applied input voltage.
Therefore the left primary and secondary induced load currents, I1a and I2a, flow in opposite direction around the left limb 11a. Further, the effective total current in the inner and outer primary coils is formed from two components of current: an exciting current which results from the applied input voltage and a load current that counters the induced secondary load current.
The left primary windings 21a and 23a and left secondary coil 22a are radially positioned around the left leg 11a so that the second column is received inside the third column, and the first column is received inside the second column. In other words, radius R1 is smaller than radius R2, that is in turn lower than radius R3.
Preferably, two columns are separated one from the other by a thin film or coating of an insulation material.
The left bypass core 31a is received inside the inner space 15 of the main magnetic circuit 10.
It is arranged to be parallel to the left limb 11a.
It has a roughly rectangular outer shape. It may be provided with pronounced radii at selected corners to facilitate wrapping of large diameter conductors or bundles of twisted strands.
Its height is preferably equal to the distance between the upper and lower yokes 13 and 14 of the main magnetic circuit 10. Consequently, the upper and lower ends of the left bypass core 31a is preferably in contact with the main core of the first magnetic circuit 10.
The left bypass core 31a is located outside the second column made by the left secondary coil 22a but inside the third column made by the outer left primary winding 23a.
Thus, if the exciting current and load induced currents circulating in the left inner primary winding 21a, the left outer primary winding 23a and the load induced current in the left secondary coil 22a contribute together to create the magnetic flux B1 in the left limb 11a, only the current circulating in the outer primary winding 23a contributes to create the magnetic flux B2 in the left bypass core 31a.
Since a portion of the primary has been selected for the outer winding to contribute the magnetic flux in the bypass core, the bypass flux in this example results from the combined effect of exciting current and load induced current. The magnitude of the primary load current is usually significantly larger than the primary exciting current for most transformers so the bypass flux B2 largely results from induced load current and thereby causes an inherent contribution to leakage impedance as related to load current. Should an alternate winding configuration have been selected that resulted in an outer winding that conducted secondary load current, then the bypass flux B2 would result from load current exclusively.
Such winding configuration exemplifies a first level of primary and secondary winding interleave to mitigate eddy current losses from magnetic field proximity effect. This invention may also be applied to increasing levels of primary and secondary interleaves as may be preferred to mitigate magnetic field proximity effect. This invention can therefore provide a resultant increased and controlled leakage impedance even when primary and secondary interleaves are necessary to reduce magnetic field proximity effect losses.
A similar description could be done for the right part of transformer 1. A component on the right of central plane P identical to a component on the left is referenced by the same numeral replacing the index "a" by the index "b".
Consequently, the right part of the transformer 1 comprises :

a right primary coil 20b made of an inner right primary winding 21b (forming a first column of radius R1 around the right limb 11b) and an outer right primary winding 23b (forming a third column of radius R3 around the right limb 11b);
a right secondary coil 22b forming a second column of radius R2 around the right limb 11b;
a right bypass core 31b located outside the second column made by the right secondary coil 22b, but inside the outermost third column made by the outer right primary winding 23b.

In use, a right primary current 11b circulates in the right primary coil 20b and a right secondary current I2b circulates in the secondary coil 22b.
Thus, if the exciting current and load induced currents circulating in the right inner primary winding 21b, the right outer primary winding 23b and the load induced current in the right secondary coil 22b contribute together to create the magnetic flux B1 in the right limb 11b, only the current circulating in the right outer primary winding 23b contributes to create the magnetic flux B2 in the right bypass core 31b.
As can be seen from the figures, the fist magnetic field B1 circulates along a first magnetic loop inside the first magnetic circuit 10 and the second magnetic field B2 circulates along a second magnetic loop inside a second magnetic circuit made of the left bypass core 31a, the central part of the lower yoke 14, the right bypass core 31b and the central part of the upper yoke 13.
There is only one second magnetic loop in transformer 1, that is situated in the vicinity of the middle plane P.
The parameters defining the properties of this second magnetic loop are adjusted to set the value of the leakage impedance of the transformer 1.
Since the second magnetic loop is common to both left and right outer primary windings 23a and 23b, wherein currents I1a and I1b circulate, the corresponding ampere turn contribution combined in a single magnetic circuit increases the effective inductance generated by the second magnetic flux. On the contrary, in the transformer according to the state of the art, the left and right additional magnetic fluxes each result from the respective ampere turn contributions independently and therefore significantly reduced the total effective inductance of the transformer.
In order for the transformer 1 to be particularly efficient, the amplitude of the first magnetic field B1 may be selected so that the material of the main core of the first magnetic circuit 10 is working near its saturation point. Consequently, without specific arrangements, the addition of the magnetic flux of the second magnetic field B2 in the central part of the yokes may lead to the saturation of the material. To avoid this behavior, it may be advantageous to increase the cross section of the central part of each yoke to accommodate the fluxes of the magnetic fields B1 and B2 in a way that the magnetic material does not reach its saturation threshold. For example, as shown in figure 2, the width (along axis Y) of the central part of each yoke 13 and 14 is increased, while maintaining their thickness (along axis Z).
In terms of compactness, to position the bypass cores inside the main core reduces the overall length (along axis X) of the transformer compared to the transformer according to the state of the art. Such configuration can minimize the volume of the portion of the magnetic circuit 10 that operates in a condition of superposition of B1 and B2 fluxes. Moreover, the relative phase relationship of B1 and B2 fluxes depends largely on the phase relationship of exciting current and load current and will therefore depend on the circuit application conditions for the transformer. In the general case the evaluation of required increased cross sectional area for the central part of each yoke should be based on appropriate vector addition of B1 and B2 fluxes accounting for expected phase relationship.
However, in addition to positioning the bypass cores inside the main core, to create one second magnetic loop in the center of transformer 1, it is necessary to select carefully the relative directions of the currents circulating in the primary and secondary coils on each left and right parts of the transformer 1.
Furthermore, to avoid the saturation of the magnetic material, the central portion of the yokes of the main core are advantageously provided with a suitable cross section compared to the other portions of the main core. To widen the central portion of the yokes has no consequences on the overall width of the transformer 1, since it is defined by the dimensions of the outermost coil.
Even, the central portions of the yokes 13 and 14, that conducts first and second magnetic fluxes, may be widened to nearly approach the extent of the overall coil dimension, so that, without modifying the area of that cross-section of the bypass cores 31a and 31b, the thickness (along axis X) of the bypass cores 31a and 31b may be reduced to benefit the overall thickness of the transformer.
Rather than along the yokes, gaps could be provided along limbs 11a and 11b so that these gaps may be set as preferred for the inductance of the primary magnetic circuit (magnetic field B1), independently of the inductance of the secondary magnetic circuit (magnetic field B2).

ALTERNATIVE EMBODIMENTS

In an alternative embodiment, the transformer is no more symmetrical relative to the central plane. An asymmetric construction may be selected for manufacturability, from application requirement or from other preferred consideration. As an example, the left and right coils may incorporate non integer turns that when connected in series provide a preferred transformer final turns ratio. Such series connection of non-integer turns may result in asymmetric contributions of ampere turns from left and right coils respectively to the induced flux(es) in related magnetic flux path(s).
In another alternative embodiment, the transformer comprises only one set of primary and secondary coils wound around one limb of the main core, the bypass core being located between all or part of the primary and all or part of the secondary coil to generate a controlled leakage of magnetic flux.
Similarly, the transformer may comprises more than two sets of primary and secondary coils wound around the limbs of the main core.
The main core may be provided with more than two limbs (in particular three limbs for a main cores resulting of the assembly of two "E" shaped parts).
In another alternative embodiment, the primary coil is made of only one primary winding. It is wound to enclose the limb of the main core and the bypass core, while the secondary coil is wound to enclose solely the limb of the main core.
In further another embodiment, the primary coil, that is made up of only one winding, is received between two windings making up the secondary coil. In this embodiment, the bypass core is located outside the primary coil but inside the outermost winding of the secondary coil.
In further another embodiment wherein the primary coil is made up of two windings and the secondary coil of one winding only, the bypass core is located outside the innermost winding making up the primary coil, but inside the secondary coil.
In another embodiment, the two primary and secondary coils are made up of more than one windings each. In this embodiment, the columns formed by the windings of the primary and secondary coils alternate radially around the limb of the main core, i.e. primary and secondary interleaving to achieve acceptable losses or operating efficiency. The bypass core is inserted in this layer structure to create a leakage of magnetic flux between the primary and secondary coils.
An important advantage of the transformer according to the invention is to maximize the effective total inductance associated with the second magnetic flux with minimal volume penalty related to the additional bypass cores. At higher operating frequency, as common for power electronics applications, increasing degrees of primary and secondary interleaves are preferably used to reduce eddy current losses from proximity effects. As the number of primary and secondary interleaves increase to manage eddy current losses, the ampere turn contribution of the current involved in the second magnetic loop is accordingly reduced. This invention therefore provides means to maximize inductance from second magnetic flux with increasing extent of primary and secondary interleaves as it becomes necessary at higher frequency or from some other concern that limits magnetic field proximity effect.
In another embodiment, the transformer 101 is a matrix electric transformer made up of a plurality of elementary electric transformers, each of these elementary electric transformers being an electric transformer according to the embodiment presented above (or its alternatives).
More specifically, transformer 101 comprises common primary and secondary coils associated with a matrix structure of magnetic components. Figure 3 is a top cross-section view of the matrix structure only, the primary and secondary coils being omitted for clarity reasons.
The matrix structure comprises a common magnetic circuit 110, whose core comprises multiple limbs (111, 112, 113, 121, 122, 123, 131, 132, and 133) between a common upper yoke (not shown in Figure 3) and a common lower yoke (104 in Figure 3).
The upper and lower yokes may be constructed from more simpler rectangular plate for reasons of manufacturability or to increase magnetic coupling between selected coils on related limbs as may be beneficial for cases of multiple orthogonal input or output phases applied to corresponding multiple core limbs.
The matrix structure further comprises multiple bypass cores (113, 114, 123, 124, 133, 134,125, 126, 143, 144, 145, and 146).
Thus a set of two neighboring limbs and two bypass cores positioned between these two neighboring limbs corresponds to the left and right limbs and associated left and right bypass cores of a compact elementary electric transformer with controlled leakage impedance.
In figure 3 for example, the matrix electric transformer corresponds to the association of six elementary transformers incorporating bypass cores to control leakage impedance in a compact configuration.
The common primary coil, respectively the common secondary coil, results of the association of the primary coils (left and right), respectively of the secondary coils (left and right) of the elementary electric transformers.
Some primary coils, respectively secondary coils, may be electrically connected (in parallel or in series). Some primary coils, secondary coils, may be shared between two elementary electric transformers. For example the primary and secondary coils around limb 112 may be shared between a first elementary electric transformer involving limbs 111 and 112 and bypass core 113 and 114 and a second elementary electric transformer involving limbs 112 and 122 and bypass core 143 and 144.
By incorporating multiple bypass cores, the matrix increases effective leakage impedance by providing magnetic coupling of selected winding portions between adjacent coils. Such matrix structure can be implemented to a selected degree to yield an overall preferred effective leakage impedance and to create a preferred resultant transformation ratio.
The primary coils, respectively the secondary coils, of each of the plurality of elementary electric transformers operate in a multiple phase configuration including the configuration of orthogonal input phases or non-orthogonal input phases.
An additional flux return limb may be added for preferred residual flux circulation.
The primary and secondary winding portions of each coil may be connected in series or parallel configurations, or combination thereof, as desired to provide the preferred transformation ratio and leakage impedance.
Such matrix structure may be implemented using various conductors for the primary and secondary windings including conventional magnet wire, twisted strands, conductor ribbon, printed circuit board(s), flex circuits or other conductors and possibly including methods of fabrication and integration using compact micro circuits as known by engineers who are experienced in design of conventional transformers and large scale integrated circuits.
Experienced engineers will recognize this matrix structure may be expanded to an arbitrary extent in the x-y plane of figure 3 with preferred number of layers in the z direction as required.

Claims

An electric transformer (1) comprising left and right primary coils (20a, 20b), left and right secondary coils (22a, 22b) and a first magnetic circuit (10), the first magnetic circuit comprising left and right limbs (11a, 11b) connected together by upper and lower yokes (13, 14) so as to define an inner space (15), the left primary and secondary coils being wound around the left limb and the right primary and secondary coils being wound around the right limb,
characterized in that the electric transformer further comprises left and right bypass cores (31a, 31b) located inside the inner space of the first magnetic circuit, upper and lower ends of each of the left and right bypass cores being in contact with the upper and lower yokes of the first magnetic circuit, the left bypass cores being positioned radially from the left limb to be between the left primary coil and the left secondary coil and the right bypass cores being positioned radially from the right limb to be between the right primary coil and the right secondary coil.
The electric transformer according to claim 1, wherein left currents (I1a, I2a) are circulating in opposite directions in the left primary coil (20a) and the left secondary coil (22a) and right currents (I1b, I2b) are circulating in opposite directions in the right primary coil (20b) and the right secondary coil (22b), so as to create a first magnetic loop along the first magnetic circuit (10) and a second magnetic loop along a second magnetic circuit, the second magnetic circuit comprising the left bypass core (31a), a central portion of the upper yoke (13) between the left and right bypass cores, the right bypass core (31b) and a central portion of the lower yoke (14) between the left and right bypass cores.
The electric transformer according to claim 2, wherein the second magnetic circuit acts as a component for setting a total impedance of the electric transformer at a predefined value.
The electric transformer according to claim 2 or claim 3, wherein a cross section of the central portion of the upper yoke, respectively of the central portion of the lower yoke, is increased compared to the cross section of other portions of the first magnetic circuit (10).
The electric transformer according to claim 4, wherein the cross section of the central portion of the upper yoke, respectively of the central portion of the lower yoke, is wider than the cross section of other portions of the first magnetic circuit (10).
The electric transformer according to claim 5, wherein the width of the cross section of the central portion of the upper yoke (13), respectively of the central portion of the lower yoke (14), is equal to the overall radius of the left and right primary and secondary coils, to minimize a thickness of the left and right bypass cores (31a, 32b).
The electric transformer according to any one of the claims 1 to 6, wherein the left primary and secondary coils, respectively the right primary and secondary coils, are interleaving, the left primary coil and/or the left secondary coil, respectively the right primary coil and/or the right secondary coil, being made up of a plurality of windings, one winding of the plurality of windings being connected in parallel or in series with another winding of the plurality of windings.
The electric transformer according to claims 7, wherein a first coil among the left, respectively the right, primary coil and the left, respectively the right, secondary coil is made up of at least two windings, respectively an inner winding and an outer winding, that are wound around the left, respectively the right, limb of the first magnetic circuit (10), so that the second coil among left, respectively the right, primary coil and the left, respectively the right, secondary coil is received between the inner winding and the outer winding, the left, respectively the right, bypass core being located either between the inner winding and the second coil or between the second coil and the outer winding.
The electric transformer according to any one of claims 1 to 8, wherein a coil among the left, respectively the right, primary coil and the left, respectively the right, secondary coil is made of at least one wire of a conductive material insulated with a dielectric coating or one or several insulating layer(s) wrapped around the conductive material.
The electric transformer according to any one of claims 1 to 9, wherein air gaps are provided along the left limb (11a) and/or the right limb (11b).
A matrix electric transformer (101), comprising a plurality of elementary electric transformers, each elementary electric transformer being an electric transformer according to any one of claims 1 to 9,
the upper yokes, respectively the lower yokes, of each of the plurality of electric transformers making up a common upper yoke, respectively a common lower yoke, to associate the first magnetic circuit of each of the plurality of elementary electric transformers into a first common magnetic circuit (110),

the primary coils, respectively the secondary coils, of each of the plurality of elementary electric transformers, make up a common primary coil, respectively a common secondary coil.
The matrix electric transformer (101) according to claim 11, wherein the primary coils, respectively the secondary coils, of each of the plurality of elementary electric transformers operate in a multiple phase configuration including the configuration of orthogonal input phases or non-orthogonal input phases.