Patent Description:
This invention was made with U. government support under contract or award No. DE-EE0006521 awarded by the Department of Energy. The Government has certain rights in the invention. The award subrecipient, John Deere Electronic Solutions, Inc. of the aforementioned contract or award, elects title to any and all subject inventions set forth in this disclosure for U. and any European patent filings.

The invention relates generally to an electrical transformer. More specifically, the invention relates to an isolation transformer with an integral inductor that can be used in a direct current (DC) to DC converter.

Touch-safe DC export power (24V to 56V) from a high-voltage bus (≥600V) can be realized using a dual active bridge (DAB)-based DC-to-DC converter. <FIG> depicts a typical single-phase DAB converter, which comprises a primary converter coupled to a secondary converter via a single-phase magnetic circuit, which includes an isolation transformer. For export power ≤<NUM> kW, a single-phase DAB converter is often used, whereas a three-phase DAB converter (<FIG>) can be used for power demands larger than <NUM> kW. For proper control of a DC-to-DC converter, a primary winding of the isolation transformer includes a series inductor. The magnetic circuit for this type of transformer is shown in <FIG>.

The series inductor occupies <NUM>% to <NUM>% of the overall volume of the magnetic circuit. As the power density of DC-to-DC converters increases, the transformers can become bulkier, heavier, and costlier than desired to meet the design targets of an electronic assembly. Therefore, it would be advantageous to develop a transformer with an integral inductor to eliminate the series inductor, thereby improving its power density and reducing its size and cost.

<CIT> discloses a transformer including a main core, formed with a magnetic circuit, on which the primary and secondary windings are wound with a given gap, and a plurality of auxiliary cores disposed in the given gap with a given distance in a circumferential direction of the primary winding, and wherein the reactor is formed of a leakage inductance of the transformer.

<NPL>, discloses a transformer with a primary winding formed around the center leg of the magnetic core, while the secondary winding overlaps the primary winding with a controlled distance. A magnetic slab is introduced in the window area to separate primary and secondary windings. This separation, and insertion of magnetic slab, is to allow more leakage inductance to satisfy the resonant inductor value required for the circuit operation.

<CIT> discloses a leakage transformer including: a primary coil wound around a core, a secondary coil wound around the core, a plurality of bypass cores, and at least one non-magnetic member arranged between the bypass cores. The bypass cores are arranged between the primary coil and the secondary coil, and are magnetic members in which a portion of the magnetic flux generated by the core is internally induced. The plurality of bypass cores are disposed with gaps therebetween in a direction of the internally induced magnetic flux. A total value of the gaps of the bypass cores is determined in accordance with a target value of leakage inductance.

<CIT> discloses a transformer-inductor assembly including a core, a primary winding, and a secondary winding. The primary winding is wrapped about the core. The secondary winding is wrapped about the primary winding. Two or more ferromagnetic bars are arranged between the secondary and primary windings to generate series inductance in the secondary winding.

<CIT> discloses a power supply transformer consisting of a primary winding and a secondary winding. Both the windings are wound mutually on the centre leg of an EI type iron core. An iron core piece is provided over the primary winding, over which the secondary winding is wound. A magnetic board is assembled over the outer surface of the secondary winding. A magnetic circuit is produced extending through the secondary winding.

Disclosed is a transformer having a core comprised of a perimeter portion and central intervening portion. The perimeter portion is separated from the intervening portion by a first opening and a second opening. A primary winding and a secondary winding are wound around the intervening portion of the core. The primary winding is capable of electromagnetic interaction with the secondary winding. A pair of ferrite members arranged outwardly from a central axis of the central intervening portion of the core can increase a series inductance with the primary winding. In accordance with another aspect of the disclosure, each ferrite member may have an air gap next to the core to facilitate heat dissipation from the transformer or its integral inductor.

The present invention provides a transformer as claimed.

The integral inductor eliminates the series inductor by raising leakage inductance and keeping the leakage inductance within a tight bound over the range of power output for the converter. As a result, the miniaturized magnetics enables easier integration with the converter due to more compact packaging and better thermal management.

In one example embodiment, as shown in <FIG>, is a transformer <NUM> with an integral inductor <NUM>. The transformer <NUM> comprises a core <NUM>, a primary winding <NUM>, a secondary winding <NUM>, and at least one ferrite member <NUM> disposed between the primary winding <NUM> and the secondary winding <NUM>. Referring again to <FIG>, the core <NUM> has a perimeter portion <NUM> and a central intervening portion <NUM>. Air gaps <NUM> separate the central intervening portion <NUM> from the perimeter portion <NUM>. Further shown in <FIG> is the primary winding <NUM> and the secondary winding <NUM>, which are wound around the central intervening portion <NUM> of the core <NUM>, interior to the perimeter portion <NUM>. Alternatively, the secondary winding <NUM> is wound about the perimeter portion <NUM> of the core <NUM>.

The primary winding <NUM> is capable of electromagnetic interaction with the secondary winding <NUM>. Additionally, the primary winding <NUM> may be capable of coupling an alternating current frequency (e.g., radio frequencies) with the secondary winding <NUM>. In some alternate embodiments of transformer <NUM>, windings <NUM>/<NUM> are placed on the central intervening portion <NUM> and the perimeter portions <NUM>, particularly for a transformer <NUM> for a three-phase DC-to-DC converter <NUM>. The core <NUM> comprises one or more of the following: a ferrous material, a laminated iron core, a powdered iron core matrix in a resin, or a ferrite material.

A pair of ferrite members <NUM> are arranged outward from a central axis of the central intervening portion <NUM> of the core <NUM> to increase a series inductance with the primary winding <NUM>. As shown in <FIG>, a first ferrite member <NUM> is disposed on the front side of the intervening portion <NUM> and a second ferrite member <NUM> is disposed on the back of the intervening portion <NUM>. Stated differently, the pair of ferrite members <NUM> are disposed on opposing sides of the core <NUM> relative to the central axis. In this configuration, the ferrite members <NUM> may concentrate or interact with the magnetic flux that emanates from the primary winding <NUM>, the secondary winding <NUM>, or both. Accordingly, the ferrite members <NUM> may attenuate or block DC energy paths, attenuate induced current (e.g., Eddy current) in the windings <NUM>/<NUM>, or facilitate maintenance of a target inductance at a minimum threshold level, such as greater than or equal to <NUM> micro Henries (µH). Each ferrite member <NUM> can have a substantially rectangular shape, a generally polyhedron shape, or a block shape. In the example embodiment shown in <FIG>, each ferrite member <NUM> has a width along the X-axis, a height along the Z-axis, and a depth along the Y-axis, forming a plate-shaped member <NUM>. <FIG> shows the ferrite members <NUM> in isolation from the additional components of the transformer <NUM> to show their position relative to each other. As shown in <FIG>, the pair of ferrite members <NUM> are substantially parallel and open.

In one embodiment, each of the ferrite members <NUM> has an air gap about its perimeter that can pass ambient air or flowing air with respect to one or more openings <NUM> in the core <NUM> to support or enhance heat dissipation from the transformer <NUM> or its integral inductor <NUM>. For example, each ferrite member <NUM> can be centered with respect to the vertical dimension of the core <NUM> or with respect the openings <NUM> in the core <NUM> along the Z-axis. In <FIG>, the height of the ferrite member <NUM> is generally less than or equal to the height of the core opening <NUM> to leave a perimeter air gap, such as a lower air gap <NUM> and an upper air gap <NUM>, permitting dissipation of thermal energy from the transformer <NUM> or its integral inductor <NUM>. In some embodiments, air communication means that the air can flow between the air gap <NUM>/<NUM> that is exposed to the ambient air and one or more openings <NUM> in the core <NUM>.

Each ferrite member <NUM> can be centered with respect to the horizontal dimension of the core <NUM> or with respect to the outermost width of the openings <NUM> of the core <NUM> along the X-axis. As shown <FIG>, the width of the ferrite member <NUM> is generally less than or equal to the outermost portion of the core opening <NUM> width (in this example, the entire span of the two core openings <NUM> in <FIG>) to leave a perimeter air gap, such as a left-side air gap <NUM> and a right-side air gap <NUM>, for dissipation of thermal energy from the transformer <NUM> or its integral inductor <NUM>. The air gaps <NUM>/<NUM> allow the ambient air to circulate within the openings <NUM> and around the primary winding <NUM>, the secondary winding <NUM>, or both to promote heat dissipation from the transformer <NUM> and the integral inductor <NUM>.

The ferrite member <NUM> may comprise a metal oxide, such as an oxide of iron, or other high-mu material to produce an isotropic (e.g., uniform properties along orthogonal axis or in different spatial dimensions) dielectric and magnetic material. Ferrite members <NUM> may be used to enclose the transformer <NUM>, the primary winding <NUM>, the secondary winding <NUM>, or portions of transformer <NUM> and its windings. Alternatively, the ferrite members <NUM> may be composed of layers with different magnetic permeabilities. For example, the ferrite member may have a first layer composed of a lower permeability composition such as a nickel-zinc ferrite, whereas a second layer may be composed of a higher permeability composition such as a manganese-zinc ferrite composition. Further, each one of the ferrite members <NUM> may comprises a ferrite block or ferrite polyhedron.

In an alternative configuration of the transformer <NUM>, the plurality of ferrite members <NUM> are U-shaped, C-shaped, box-portion-shaped, or substantially rectangular and can be connected to form a unified structure that is hollow to receive the central intervening portion <NUM>, the primary winding <NUM>, and the secondary winding <NUM>. For example, the pair of ferrite members <NUM> may have facing edges that are configured with snap-fit connectors to mechanically and magnetically connect the ferrite members <NUM>. Alternatively, the ferrite members <NUM> may be interconnected by mechanical fasteners, adhesives, elastomers, or bonding agents.

The transformer <NUM> disclosed herein can be used in power electronics, electronic assemblies, DC-to-DC converters <NUM>, motor controls, power supplies, and other industrial applications. The series inductance of the integral inductor <NUM> of the transformer <NUM> supports power or energy transfer from the primary winding <NUM> to the secondary winding <NUM> of the transformer <NUM>. In one example configuration of the transformer <NUM>, the series inductance of an integral inductor <NUM> comprises a leakage inductance of equal to or greater than <NUM>µH.

The ferrite members <NUM> ensure that leakage inductance does not decline below a threshold, such as <NUM>µH, resulting in a simplified control method. Otherwise, high bandwidth control may not be possible due to passage of DC current that could saturate the inductor. Further, the transformer <NUM> offers better protection of power devices on the primary side of DAB-based DC-to-DC converters <NUM> because the inductance will ensure that the bandwidth of control systems and the protection circuit is sufficient to respond to abnormal operating conditions.

The transformer windings <NUM>/<NUM> can be configured to meet various design and technical requirements. For DC-to-DC converters <NUM> that feature a transformer <NUM>, the primary winding <NUM> may comprise a first winding and a second winding that are coupled in series to a first winding a node. The first winding node represents the interface between the primary winding <NUM> and the integral inductor <NUM>. In some configurations, the primary winding <NUM> can be interleaved with the secondary winding <NUM>. The primary terminals <NUM> are associated with the primary winding <NUM>. The secondary terminals <NUM> are associated with the secondary winding <NUM>. The ratio of turns between the primary winding <NUM> and the secondary winding <NUM>, or vice versa, is known and may be defined as N. The ratio of turns, N, may depend upon whether the transformer <NUM> is a step-up configuration for input/output voltages or a step-down configuration for input/output voltages, for instance.

The transformer <NUM> is adapted to be used with a DC-to-DC power converter <NUM>/<NUM> comprising a first power converter <NUM> coupled to the primary winding <NUM> and a second power converter <NUM> coupled to the secondary winding <NUM>, wherein the series inductance supports power or energy transfer from the primary winding <NUM> to the secondary winding <NUM> of the transformer <NUM> (single-phase configuration) or transformers <NUM> (three-phase configuration).

<FIG> shows a single-phase DAB-based DC-to-DC converter <NUM> with the transformer <NUM> disclosed herein. <FIG> show two different configurations of a three-phase DAB-based DC-to-DC converter <NUM>. As shown in <FIG>, three single-phase transformers <NUM> are shown. <FIG> shows a unified core <NUM> with three intervening sections <NUM>.

Referring again to <FIG>, the single-phase DAB-based DC-to-DC converter comprises a primary converter <NUM> coupled to a secondary converter <NUM> via the transformer <NUM>. The primary converter <NUM> comprises first pair of primary switches and a second pair of primary switches. The primary switches are coupled between direct current terminals <NUM> of the primary converter <NUM>. A primary capacitor <NUM> may be placed between the direct current terminals <NUM> to filter ripple current or unwanted alternating current components from the direct current bus. The primary direct current terminals <NUM> may be coupled to an energy source, such as a battery or another DC voltage source <NUM>. The secondary converter <NUM> comprises secondary switches. The secondary switches are coupled between direct current terminals <NUM> of the secondary converter <NUM>. A secondary capacitor <NUM> may be placed between the direct current terminals <NUM> to filter ripple current or unwanted alternating current components from the direct current bus. The secondary direct current terminals <NUM> may be coupled to an energy source, such as a battery or another DC voltage source <NUM>.

An inductor <NUM> is associated with at least one output terminal <NUM> of the primary switches. There are two output terminals <NUM> for a single phase full-bridge converter <NUM>. The inductor <NUM> may be integral with a primary winding <NUM> of the transformer <NUM> or may comprise a separate, discrete inductor <NUM>. Multiple inductors <NUM> may be used in some embodiments. The primary winding <NUM> of the transformer <NUM> is coupled to output terminals <NUM> of the primary switches via the inductor <NUM>. A secondary winding <NUM> of the transformer <NUM> is coupled to output terminal <NUM> of the secondary switches.

The DC-to-DC converters <NUM> shown in <FIG> are similar to the single-phase converters <NUM>, but use three-phases for the primary converter <NUM> and the secondary converter <NUM>.

A plurality of inductors <NUM> are associated with output terminals <NUM> of the primary switches. The inductors <NUM> may be integral with the primary winding <NUM> of each of the transformers <NUM> or may comprise separate, discrete inductors <NUM>. Each transformer <NUM> is coupled between the primary converter <NUM> and the secondary converter <NUM>. The primary winding <NUM> of the transformer <NUM> is coupled to output terminals <NUM> of the primary switches via the inductors <NUM>. A secondary winding <NUM> of each transformer <NUM> is coupled to output terminals <NUM> of the secondary switches.

The unified body transformer <NUM> depicted in <FIG> has a volume of <NUM>, as opposed to a volume of <NUM> for the three separate transformers <NUM> shown in <FIG>. For the transformer <NUM> with a unified-body core <NUM>, the flux density in the ferrite members <NUM> remains below <NUM>. Further, the retention elevation of leakage inductance by the ferrite members <NUM> ensure that the effective turn ratio of the transformer <NUM> remains close to the turn ratio of an ideal transformer. Magnetizing inductance remains > 10x of leakage inductance, ensuring that there is minimal distortion in current supplied by the primary converter <NUM> in single-phase and three-phase operation in DAB-based and resonant LLC mode. The ability to keep distortion to a minimal level will reduce on-loss in SiC power devices used in the primary converter <NUM>. This characteristic can lead to improved converters <NUM>/<NUM> if the hardware is repurposed for an on-board battery charging application.

As previously discussed, the series inductor typically used with the primary winding <NUM> of the isolation transformer <NUM> adds to the size of the magnetic circuit. If the series inductor is simply removed, leakage inductance is not sufficient for proper control of DAB-based DC-to-DC converters <NUM>/<NUM>. For example, an isolation transformer <NUM> without a series inductor may have a size of <NUM> x <NUM> x <NUM> (i.e. <NUM>), but would have a leakage inductance of about <NUM>µH. To raise the inductance, a transformer without a series inductor would have to grow in size to <NUM> x <NUM> x <NUM> (i.e. <NUM>), nearly triple the volume. Inserting the ferrite members <NUM> between the primary winding <NUM> and secondary winding <NUM> of the isolation transformer <NUM> permits a proper leakage inductance while minimizing the size of the magnetic circuit. As discussed, the ferrite members <NUM> can take various shapes and sizes. For a closed form / circuit ferrite member <NUM> the flux density across the XY, XZ, and YZ planes is above <NUM> T, which could lead to core loss in the ferrite member <NUM> and excessive flux density based on saturation of the ferrite member <NUM>. Excessive heat can cause the ferrite member <NUM> to experience deviation in its magnetic permeability. As a result, it is desirable to keep the leakage inductance of the ferrite member <NUM> below <NUM> T. By using two ferrite members <NUM> disposed on opposing sides of the intervening central portion <NUM> of the core <NUM>, the flux density in the XZ plain is absent as there is no magnetic material to divert or guide leakage flux between the primary winding <NUM> and secondary winding <NUM>. This configuration eliminates saturation and overheating issues while maintaining a uniform field in the ferrite members <NUM> disposed in the XY plane.

<FIG> shows a simplified electrical diagram of the transformer <NUM> with taps in the primary <NUM> and secondary windings <NUM>. If the primary winding <NUM> and secondary winding <NUM> are tapped, series and parallel operation of SiC and GaN-based power converters <NUM>/<NUM> becomes feasible, as these types of converters <NUM>/<NUM> may require the windings of the transformer <NUM> to operate at voltage and power values that differ from designed values.

The invention may also broadly consist in the parts, elements, steps, examples and/or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and/or features, within the scope of the claims. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein, within the scope of the claims.

Claim 1:
A transformer (<NUM>) comprising:
a core (<NUM>) comprising a perimeter portion (<NUM>) joined to a central intervening portion (<NUM>),
wherein the perimeter portion (<NUM>) has an open interior,
wherein the central intervening portion (<NUM>) is disposed in the open interior of the perimeter portion (<NUM>) creating a first opening and a second opening in the space between the central intervening portion and the perimeter portion;
a primary winding (<NUM>) wound around the central intervening portion (<NUM>);
a secondary winding (<NUM>) wound around the central intervening portion (<NUM>),
wherein the primary winding (<NUM>) is capable of electromagnetic interaction with the secondary winding (<NUM>); and
a first ferrite member (<NUM>) and a second ferrite member (<NUM>) disposed between the primary winding (<NUM>) and the secondary winding (<NUM>),
wherein the first ferrite member (<NUM>) is positioned adjacent to the central intervening portion (<NUM>) outside of the open interior on a first side of the core (<NUM>) and the second ferrite member (<NUM>) is positioned adjacent to the central intervening portion (<NUM>) outside of the open interior on a second side of the core (<NUM>) opposite the first side and parallel to the first ferrite member (<NUM>) to increase a series inductance with the primary winding,
wherein the first ferrite member and the second ferrite member have at least one of a width and a height smaller than a width and a height of the open interior of the perimeter portion.