Method and apparatus of controlling the magnetic saturation of the core of a DC common mode choke

Abstract

A method of controlling the magnetic saturation of the core of a DC common mode choke comprising a magnetic core 1 and first and second coils 2, 3, comprises: determining the saturation percentage of

Description

FIELD The present disclosure relates to a method and apparatus of controlling the magnetic saturation of the core of a DC common mode choke. BACKGROUND With increased penetration of electrical systems and the progression towards full electric and hybrid propulsion systems, the use of energy storage systems and DC power distribution has gained increased use. Multiple loads and sources may be connected to a DC distribution network. In such DC network, adequate filtering functions such as for filtering noise produced by power electronic converters may be implemented using DC common mode chokes. A DC common mode choke generally is an electrical filter that blocks high-frequency noise common to two or more data or power lines while allowing the desired DC or low-frequency signal to pass. It typically comprises two coils wound around a magnetic core in different winding directions and arranged such that the common mode current creates a magnetic field that opposes any increase in common mode current. With DC common mode chokes, the magneto-motive forces from DC positive and negative rails ideally cancel each other due to the winding direction. However, in reality, there is no full canceling due to imperfections including winding structural asymmetries, eccentricity of the core materials or uneven asymmetrical thermal expansions. The result is a net DC magneto-motive force that tends to saturate the core beyond the effective operating point. However, as the core saturates, the inductance of the choke drops, which diminishes its ability to filter out high-frequency noise. There is a need to provide a method and apparatus of controlling the magnetic saturation of the core of a DC common mode choke that minimizes the net magnetomotive force in a DC common mode choke or at least provides a useful alternative to known methods and apparatus for controlling the magnetic saturation of the core of a DC common mode choke. SUMMARY According to a first aspect, a method of controlling the magnetic saturation of the core of a DC common mode choke is provided. The DC common mode choke comprises a magnetic core and first and second coils wound around the magnetic core. The method comprises the steps of: determining the saturation percentage of the core; injecting an auxiliary current into a winding of the choke that leads to a magnetic field that is in addition to any magnetic field produced by the first and second coils; determining again the saturation percentage of the core and a change in the saturation of the core caused by the injected auxiliary current; depending on the direction of the change in saturation, increasing or decreasing the injected auxiliary current, wherein the injected auxiliary current is increased if the saturation of the core has decreased, and the injected auxiliary current is decreased if the saturation of the core has increased; and repeating the two preceding steps until the saturation percentage of the core is at or below a predetermined value. Aspects of the present disclosure are thus based on the idea to compensate a magnetic saturation in DC common mode chokes which is caused by imperfections and asymmetries. The magnetic saturation compensation is provided for by determining the effects of an injected auxiliary current and the magnetic field that goes along with the injected auxiliary current on the saturation percentage of the magnetic core. One difficulty in compensating the magnetic saturation is to correctly identify the direction of the saturation, that is, whether the core is saturated in the first or the third quadrant of the B-H curve. The present disclosure implements a perturb-observe method, also known as hill-climbing method, to bring the core operating point to the origin. The method introduces a pre-defined auxiliary current (which is a compensation current), observes the variation in the saturation percentage, and then continues the compensation if the saturation percentage has decreased or decreases the auxiliary current if saturation percentage has increased. With simple injection of an auxiliary current and well-established algorithms, the size and weight of DC common mode chokes can be greatly reduced. The magnetic saturation may be compensated dynamically to cope with varying asymmetries through the mission profiles or product lifecycle. It is pointed out that the saturation percentage of the magnetic core may also be referred to as the magnetization of the core. The coils wound around the magnetic core may also be referred to as windings. The magnetic core of the DC common mode choke may be a toroid core with circular, oval or rectangular shape. It is further pointed out that the magnetic field that is caused by the auxiliary current is a magnetic H-field. However, for simplicity, the magnetic H-field caused by the auxiliary current as well as a magnetic H-fields produced by the first and second coils are simply referred to as magnetic fields. It is further pointed out that decreasing the auxiliary current may lead to a situation in which the direction of the current is reversed (crossing zero), wherein this is not necessarily the case. In some embodiments, the winding into which the auxiliary current is injected is an auxiliary winding wound around the core. Accordingly, an additional winding or coil is used for injecting the auxiliary current. This way, the auxiliary winding can be easily galvanically insulated from the first and second coils. In some embodiments, however, the winding into which the auxiliary current is injected is one of the first and second coils. In such embodiments, one of the already existing first and second coils is used to inject the auxiliary current. For example, the auxiliary current is injected in a subset of the turns of the first or second coil. If the current is injected directly through one of the first and second coils, the structure design of the common-mode choke is retained, thereby realizing higher window area utilization of the core to increase inductance in the power line, wherein the window area is the inner opening of a toroid core. When using an auxiliary winding, such auxiliary winding consumes space in the window area and, thereby, limits space for the turns of the first and second coils. In both alternatives, the injected auxiliary current may be from a bidirectional current source such as a linear amplifier (e.g., a push-pull current amplifier), a bidirectional switched-mode power supply, or combination of them. The energy source for the current source can be, e.g., from a battery, low voltage DC bus or high voltage DC bus with an isolated DC-DC converter. In a variant of embodiments in which the current is injected directly through one of the first and second coils, a first electrical link connects one connection of the current source with one of the turns of one of the first or second coil, and a second electrical link connects the other connection of the current source with another one of the turns of respective coil. In this manner, injection of current in the first or second coil can be easily implemented. Generally, the injected auxiliary current is by a factor of, e.g., 100 or 1000 or more, smaller than the current in DC positive and negative power lines in which DC common mode currents shall be filtered out by the DC common mode choke. For example, the injected auxiliary current is in the range of milliamperes, including an order of magnitude of around 100 mA. In some embodiments, the DC common mode choke is operated at or near the origin of a B-H curve of magnetic hysteresis, wherein the steps of injecting auxiliary current are repeated until the saturation percentage of the core lies within the effective operating region, which is at or near zero in the optimal situation. The aforementioned method steps may be repeated in a continuing process during operation of the DC common mode choke. In other embodiments, the respective steps may repeat in predefined time intervals. According to a second aspect, an apparatus for controlling the magnetic saturation of the core of a DC common mode choke is provided. The apparatus comprises a DC common mode choke, the DC common mode choke comprising a magnetic core and first and second coils wound around the magnetic core. The apparatus further comprises a controller which is configured to effect the steps of: determining the saturation percentage of the core; injecting an auxiliary current into a winding of the choke that leads to a magnetic field that is in addition to any magnetic field produced by the first and second coils; determining again the saturation percentage of the core and a change in the saturation of the core caused by the injected current; depending on the direction of the change in saturation, increasing or decreasing the injected auxiliary current, wherein the injected auxiliary current is increased if the saturation of the core has decreased, and the injected auxiliary current is decreased if the saturation of the core has increased; and repeating the two preceding steps until the saturation percentage of the core is at or below a predetermined value. The advantages and functions of such apparatus are similar to those described with respect to the method of the first aspect. In some embodiments, the apparatus further comprises an auxiliary winding wound around the core, wherein the controller is configured to cause injecting the auxiliary current into the auxiliary winding. The auxiliary winding is galvanically insulated from the first and second windings. In some embodiments, the winding is one of the first and second coils, wherein the controller is configured to cause injecting the auxiliary current into the first or second coil. The apparatus may further comprise a bidirectional current source controlled by the controller, wherein the bidirectional current source provides the auxiliary current. As mentioned before, the bidirectional current source may be one of a linear amplifier, a bidirectional switched-mode power supply, or a combination thereof. The apparatus may further comprise first and second electrical links connecting the bidirectional current source with different turns of one of the first and second coils. In some embodiments, the controller is configured to operate the DC common mode choke at or near the origin of the B-H curve of magnetic hysteresis. In some embodiments, the first and second coils wound around the magnetic core each comprise a plurality of preformed conductor bars, wherein each conductor bar comprises an inner bar section arranged in the window area of the toroid core and an outer bar section arranged at the outside of the toroid core. In such embodiments, the traditional wire bent around a toroid core is replaced by preformed bars which do not need to be bent, this allowing for a simplified manufacturing process and high precision. In a variant of such embodiments, the conductor bars are U-shaped and connected at their ends to conductor plates which are integrated in a printed circuit board which allows to electrically connect the conductor bars in a simple manner. The U-shaped bars together with the conductor plates form two windings around the toroid core. In other embodiments, however, the first and second coils may be formed in a traditional manner by one or several wires of, e.g., circular or rectangular cross-section, which are wound and bent around the magnetic core. Generally, it is pointed out that the winding into which the auxiliary current is injected (such as the auxiliary winding) may also serve other purposes than those described so far. For example, it may serve to additionally measure a voltage induced by an AC excitation current that has been injected into a further auxiliary winding or one of the first and second coils. In such case, the core of a common mode choke is additionally operated as a transformer in order to determine the magnetic saturation of the core. The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and / or combined with any other feature or parameter described herein. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments will now be explained by way of example only with reference to the accompanying drawings in which: FIG. 1 is a first embodiment of an apparatus for controlling the magnetic saturation of the core of a DC common mode choke that comprises a core and first and second coils, wherein the apparatus comprises an auxiliary winding for injection of an auxiliary current; FIG. 2 is an embodiment similar to the embodiment of FIG. 1, wherein an auxiliary winding is wound around the core of the choke, wherein the choke is arranged on a printed circuit and wherein the first and second coils are implemented with U-shaped conductor bars; FIG. 3 is a top view on the printed circuit board of the DC common mode choke of FIG. 2; FIG. 4 is a side view of the DC common mode choke of FIGS. 2 and 3; FIG. 5 is a second embodiment of an apparatus for controlling the magnetic saturation of the core of a DC common mode choke having a core and first and second coils, wherein the apparatus comprises means for injection of an auxiliary current into one of the coils of the DC common mode choke; FIG. 6 is an embodiment similar to the embodiment of FIG. 5, wherein electrical links are provided to inject the auxiliary current into one of the coils, wherein the choke is arranged on a printed circuit and wherein the first and second coils are implemented as U-shaped conductor bars; FIG. 7 is a top view on the printed circuit board of the DC common mode choke of FIG. 6; FIG. 8 is a side view of the DC common mode choke of FIGS. 6 and 7; FIG. 9 shows schematically different embodiments of a bidirectional current source that provides for the auxiliary current; FIG. 10 is a flowchart of a method of controlling the magnetic saturation of the core of a DC common mode choke; FIG. 11 shows an example DC common mode choke having two windings wound around a toroid core, with the two winding opposing each other magnetically; and FIG. 12 depicts the B-H curve of magnetic hysteresis of a DC common mode choke. DETAILED DESCRIPTION Before discussing embodiments of the present disclosure, a few general remarks are provided with respect to DC common mode chokes with reference to FIGS. 11 and 12. As shown by example in FIG. 11, a DC common mode choke comprises a toroid magnetic core 1 and two coils / windings 2, 3 of opposite direction. The choke coils are each provided by a conducting wire wrapped around the single core. The wires are wrapped around the core in opposite directions. When common mode currents flow through the coils, flux is generated by the electromagnetic induction. However, as the direction of the generated flux is the same, both fluxes become stronger to increase their action as inductors. Conversely, differential mode currents flowing through the coil generate flux in opposing directions that cancel each other out. Common mode choke coils are thus filters that only act as inductors for common modes, and not against differential modes. Such DC common mode choke may be arranged between the DC positive and negative rails of a DC power distribution system used in full electric and hybrid propulsion systems. In differential mode, current travels on the DC positive rail in one direction from the source to a load, and in the opposite direction on the return DC negative rail. In common mode, the noise current travels on both lines in the same direction. Noise may be generated in such systems particularly by semiconductor switches which implements a power converter. In such systems, the magneto-motive force from the DC positive and negative rails cancels each other due to the winding direction arrangement. Such arrangement ensures that the magnetic cores used are not subject to saturation due to the high DC current and reacts to only the common mode current which is usually much lower in magnitude. The magnetic cores in DC common mode chokes mainly operate near the origin in the B-H curve as shown in FIG. 12 where the relative permeability is high. Beyond the outlined effective operating region OR, the relative permeability of the cores decreases drastically and the chokes will lose the functionality to limit common mode DC current. In reality, the DC magneto-motive forces hardly cancel each other completely due to imperfections including dimensional tolerance of the windings, eccentricity of the core materials, or even asymmetrical thermal expansions due to uneven cooling arrangement etc. The consequences are even worse in high power density DC common mode chokes where huge electric currents with a current density over 20A / mm2 are flowing in the DC rails. The result is a net DC magneto-motive force that tends to saturate the core beyond the effective operating region. The commonly adopted solution to this problem is to select magnetic cores with bigger size or wider effective operating region, which all result in a bulky and heavy DC common mode choke. The solution to this problem in accordance with the present disclosure will next be discussed with respect to FIGS. 1 to 10. FIG. 1 depicts an apparatus for controlling the magnetic saturation of the magnetic core of a DC common mode choke. The common mode choke 100 comprises a magnetic core 1, a first coil 2 and a second coil 3 which are both wound around the magnetic core 1. The magnetic core 1 is a toroid and comprises a window area 11 which is the central inner space of the toroid. The coils 2, 3 are depicted schematically. They each comprise a plurality of turns 20, 30 of a winding wire which together form the respective coil 2, 3. One of the coils 2 may be connected to a DC positive voltage rail and the other of the coils 3 may be connected to a DC negative voltage rail. The apparatus further comprises a bidirectional current source 5 and in auxiliary winding 4. Embodiments of such bidirectional current source are discussed with respect to FIG. 9. The bidirectional current source 5 provides for an auxiliary current I which is injected into the auxiliary winding 4. The auxiliary winding 4 is depicted schematically only and may comprise a small or high number of turns. The auxiliary winding 4 is separate from the coils 2, 3 and galvanically insulated from the coils 2, 3. The auxiliary current I injected into the auxiliary winding 4 is one or several orders of magnitude smaller than the current flowing in the DC positive and negative rails. The apparatus further comprises a controller 8. The controller 8 is configured to control operation of the current source 5 and of the auxiliary winding 4. The controller 8 is further configured to control and initiate a method of controlling the magnetic saturation of the core 11 of the choke 100, as is next discussed with respect to FIG. 10. Referring to FIG. 10, in step 101 it is determined an actual magnetic saturation percentage of the core 1. The magnetic saturation percentage is the magnetization of the core material. Determination of the saturation percentage can be performed in accordance with methods known to the skilled person. For example, the magnetic saturation percentage may be determined using pickup coils or Hall sensors. Another possibility to determine the magnetic saturation percentage is to survey current and voltage of the coils 2, 3. After determination of an initial saturation percentage of the core, and auxiliary current I is injected into the auxiliary winding 4 by means of the current source 5. The auxiliary current I may be positive or negative and provides for a perturbation. The auxiliary current I leads to a magnetic field that is in addition to any magnetic field produced by the first and second coils 2, 3. Accordingly, the injected current I changes the saturation percentage of the core. The change in the saturation of the core is determined / updated in step 103, wherein the saturation percentage (magnetization) of the core is measured again, which may be done in the same manner as with the initial measurement in step 101. It is then determined if the saturation of the core has decreased or increased, step 104. If the saturation of the core has decreased, the injected auxiliary current I is increased by an additional amount Al, step 106, as the injection of the auxiliary current I lead to a desired decrease of the saturation. If the saturation of the core has increased, the injected current I is reduced and reversed by adding an amount -Al, step 105. When the saturation of the core has increased, the effect of the injected auxiliary current I went into the wrong direction such that the injected current is then reversed (i.e., the polarity of the current is flipped). This is why the current source 5 of FIG. 1 needs to be bidirectional. The updated auxiliary current injection is executed in step 107. The steps are repeated until the saturation percentage of the core is at or below a predetermined value. In particular, it is desired to operate the DC common mode choke at or near the original of the B-H curve of FIG. 12, such that the steps may be repeated until the saturation percentage is at or close to zero. The process may be carried out continuously or in predetermined intervals. To control the process, the controller 8 comprises input lines 81 and output lines 82. The input lines 81 are connected to means that determine or participate in determining the saturation percentage of the core, such as Hall sensors. The input lines 81 may also receive current and voltage values of the coils 2, 3. The input lines 81 may further be connected to other surveillance and control devices. The output lines 82 comprise an output line which controls operation of the bidirectional current source 5. Other output lines may be connected to other controllers and to the means that determine or participate in determining the saturation percentage of the core to provide control signals to such means as well. The controller 8 may comprise one or several processors for executing instructions and a memory which is coupled to the processor(s) and in which instructions are stored which, when executed by the processor, cause the processor to perform the functions and steps shown in FIG. 10. The controller 8 may be a separate unit or may be integrated into a larger control unit. Also, the controller 8 may communicate with other control devices. FIGS. 2 to 4 show an implementation of the embodiment of FIG. 1. The common mode choke is arranged on a printed circuit board 7. A toroid core 1 with an inner window 11, a bidirectional current source 5 and in auxiliary winding 4 are depicted in a similar manner as in FIG. 1. The core 1 may have a rectangular or circular cross-section. The core 1 may be made of any magnetic material, such as a ferromagnetic or ferromagnetic material. The magnetic material may be a in iron-based alloy. The auxiliary winding 4 is wound around the core 1 and is powered by the current source 5. The controller 8 of FIG. 1 is present in the same manner, but not depicted. In the embodiment of FIGS. 2 to 4, the coils 2, 3 are formed in a particular manner in that the coils 2, 3 are formed by separate conductor bars 25, 35 which are each U-shaped. Each U-shaped conductor bar 25, 35 comprises an outer segment or section 252, 352 which extends at the outside of core 31, an inner segment or section 251, 351 which extends at the inside of core 1 and, accordingly, in the window area 11, and a top segment 253, 353 which connects sections 251,252, 351, 352 and which extends over the top end face of the core 1. As shown in FIG. 3, which is a top view onto the printed circuit board 7, the coils 2, 3 further comprise copper layers 70 which form conductor plates and are arranged in the printed circuit board 7, to which the U-shaped conductor bars 25, 35 are electrically connected, such that together a continuous winding 2, 3 is formed. The advantage of an inductor winding implemented in such manner lies in that preformed conductor bars may be implemented which do not require bending and which may be formed in a simple manner with high precision. Further, the winding can be electrically contacted and controlled through a printed circuit board. The side view of FIG. 4 also indicates the U-shaped structure of the conductor bars 25, 35. In other embodiments, however, traditional wires may be wound and bent around the core 1 to form coils 2, 3. FIG. 5 depicts another embodiment of an apparatus for controlling the magnetic saturation of the core of a DC common mode choke. Regarding the core 1, the coils 2, 3, the bidirectional current source 5 and the controller 8, the embodiment of FIG. 5 is identical to the embodiment of FIG. 1 such that reference is made to the description of FIG. 1 in this respect. The difference of the embodiment of FIG. 5 over the embodiment of FIG. 1 lies in that an auxiliary current I is injected not into an auxiliary winding, but directly into one of the first and second coils. This way, additional wiring in the window area 11 of the core 1 is avoided. For injecting the auxiliary current I, electrical links 61, 62 are provided (such as cables or PCB traces / layers), wherein one electrical link 61 connects one terminal of the current source 5 with one of the turns 30 of coil 3 and the other electrical link 62 connects the other terminal of current source 5 with another one of turns 30 of coil 3. The method implemented by controller 8 and depicted in FIG. 10 is the same as in FIG. 1. It is pointed out that, by connecting the wires 61, 62 to specific of the turns 30, it may be provided that the auxiliary current is injected into a subset of the turns 30 of coil 3 (or alternatively into a subset of coils 20 of coil 2). FIGS. 6 to 8 show an implementation of the embodiment of FIG. 5. The common mode choke is arranged on a printed circuit board 7. A toroid core 1 with an inner window 11 and a bidirectional current source 5 are depicted in a similar manner as in FIG. 5. The core 1 may have a rectangular or circular cross-section. The core 1 may be made of any magnetic material, such as a ferromagnetic or ferromagnetic material. The magnetic material may be a in iron-based alloy. The controller 8 of FIG. 5 is present in the same manner, but not depicted. In the embodiment of FIGS. 6 to 8, the coils 2, 3 are formed by separate conductor bars 25, 35 which are each U-shaped. Each U-shaped conductor bar 25, 35 comprises an outer segment or section 252, 352 which extends at the outside of core 31, an inner segment or section 251, 351 which extends at the inside of core 1 and, accordingly, in the window area 11, and a top segment 253, 353 which connects sections 251, 252, 351, 352 and which extends over the top end face of the core 1. In this respect, reference is also made to the description of FIGS. 2 to 4. FIG. 7 is a top view onto the printed circuit board 7 and depicts copper layers 70 which form conductor plates and are arranged in the printed circuit board 7, to which the U-shaped conductor bars 25, 35 are electrically connected, such that together a continuous winding 2, 3 is formed. The side view of FIG. 8 also indicates the U-shaped structure of the conductor bars 25, 35. As can be seen particularly in FIGS. 6 and 8, and auxiliary current I is injected into one of the coils (in the depicted embodiment in coil 3) in that one of the electrical links 61 connected to current source 5 is electrically connected to the top segment 353 of one of the conductor bars 35 and the other one of the electrical links 62 connected to current source 5 is electrically connected to the top segment 353 of another one of the conductor bars 35, such as the neighboring conductor bar. FIG. 9 depicts schematically embodiments of the bidirectional current source 5. The bidirectional current source 5 may be a linear amplifier, such as a push-pull current amplifier, or a bidirectional switched-mode power supply, or a combination of them. An energy source 50 for the current source 5 can be from a battery, low voltage DC bus or high voltage DC bus with an isolated DC-DC converter. It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in anyway. Also, those skilled in the art will appreciate that other aspects of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. Various features of the various embodiments disclosed herein can be combined in different combinations to create new embodiments within the scope of the present disclosure. In particular, the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. Any ranges given herein include any and all specific values within the range and any and all sub-ranges within the given range.

Claims

1. A method of controlling the magnetic saturation of the core of a DC common mode choke, wherein the DC common mode choke (100) comprises a magnetic core (1) and a first coil (2) and a second coil (3) wound around the magnetic core (1), the method comprising the steps of:determining (101) the saturation percentage of the core (1);injecting (102) an auxiliary current (I) into a winding (2, 3, 4) of the choke (1) that leads to a magnetic field that is in addition to any magnetic field produced by the first coil (2) and the second coil (3);determining (103) again the saturation percentage of the core (1) and a change in the saturation of the core (1) caused by the injected auxiliary current (I);depending on the direction of the change in saturation, increasing or decreasing the injected auxiliary current (I), wherein the injected auxiliary current (I) is increased (106) if the saturation of the core (1) has decreased, and the injected auxiliary current (I) is decreased (105) if the saturation of the core (1) has increased; andrepeating the two preceding steps until the saturation percentage of the core (1) is at or below a predetermined value.

2. The method of claim 1, wherein the winding is an auxiliary winding (4) wound around the core (1), wherein the auxiliary current (I) is injected into the auxiliary winding (4).

3. The method of claim 2, wherein the auxiliary winding (4) is galvanically insulated from the first coil (2) and the second coil (3).

4. The method of claim 1, wherein the winding is one of the first coil (2) and the second coil (3), wherein the auxiliary current (I) is injected into the first coil (2) or the second coil (3).

5. The method of claim 4, wherein the auxiliary current (I) is injected in a subset of the turns (30, 35) of the first coil (2) or the second coil (3).

6. The method of any preceding claim, wherein the injected auxiliary current (I) is from a bidirectional current source (5).

7. The method of claim 6, when dependent on claim 4 or 5, wherein a first electrical link (61) connects one connection of the current source (5) with one of the turns (30, 35) of one of the first coil (2) or the second coil (3) and that a second electrical link (61)connects the other connection of the current source (5) with another one of the turns (30, 35) of respective coil (3).

8. The method of any preceding claim, wherein the injected auxiliary current (I) is in the range of milliamperes.

9. The method of any preceding claim, wherein the DC common mode choke (100) is operated at or near the origin of a B-H curve of magnetic hysteresis.

10. The method of any preceding claim, wherein the steps are repeated in a continuing process during operation of the DC common mode choke.

11. An apparatus for controlling the magnetic saturation of the core of a DC common mode choke, the apparatus comprising:a DC common mode choke (100), the DC common mode choke comprising a magnetic core (1) and a first coil (2) and a second coil (3) wound around the magnetic core (1); anda controller (8) configured to effect the steps of:determining (101) the saturation percentage of the core (1);injecting (102) an auxiliary current (I) into a winding (2, 3, 4) of the choke that leads to a magnetic field that is in addition to any magnetic field produced by the first coil (2) and the second coil (3);determining (103) again the saturation percentage of the core (1) and a change in the saturation of the core (1) caused by the injected auxiliary current (I);depending on the direction of the change in saturation, increasing or decreasing the injected auxiliary current (I), wherein the injected auxiliary current (I) is increased (106) if the saturation of the core (1) has decreased, and the injected auxiliary current (I) is decreased (105) if the saturation of the core (1) has increased; andrepeating the two preceding steps until the saturation percentage of the core (1) is at or below a predetermined value.

12. The apparatus of claim 11, wherein an auxiliary winding wound (4) around the core (1), wherein the controller (8) is configured to cause injecting the auxiliary current (I) into the auxiliary winding (4).

13. The apparatus of claim 11 or 12, wherein the auxiliary winding (4) is galvanically insulated from the first coil (2) and the second coil (3).

14. The apparatus of claim 11, wherein the winding is one of the first coil (2) and the second coil (3), wherein the controller (8) is configured to cause injecting the auxiliary current (I) into the first coil (2) or the second coil (3).

15. The apparatus of any one of claims 11 to 13, wherein a bidirectional current source (5) controlled by the controller (8), wherein the bidirectional current source (5) provides the auxiliary current (I).

16. The apparatus of claim 15, wherein the bidirectional current source (5) is a linear amplifier, a bidirectional switched-mode power supply, or a combination thereof.

17. The apparatus of claim 15 or 16, when dependent on claim 14, wherein a first electrical link (61) and a second electrical link (62) connect the bidirectional current source (5) with different turns (30, 35) of one of the first coil (2) and the second coil (3).

18. The apparatus of any one of claims 11 to 17, wherein the controller (8) is configured to operate the DC common mode choke (100) at or near the origin of the B-H curve of magnetic hysteresis.

19. The apparatus of any one of claims 11 to 18, wherein the first coil (2) and the second coil (3) each comprise a plurality of preformed conductor bars (25, 35), each conductor bar (25, 35) comprising an inner bar section (251, 351) arranged in a window area (11) of the core (1) and an outer bar section (252, 352) arranged at the outside of the core (1).

20. The apparatus of claim 19, wherein the conductor bars (25, 35) are U-shaped and connected at their ends to conductor plates (70) which are integrated into a printed circuit board (7).15

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