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

Abstract

A DC common mode choke 100 comprises first and second coils 2, 3 wound around a magnetic core 1. A method of monitoring the magnetic saturation of the core comprises injecting an AC excitation current

Description

FIELD The present disclosure relates to a method and apparatus of monitoring 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. Therefore, it is critical to be able to dynamically monitor the core saturation due to the biasing DC magneto-motive forces. There is a need to provide a method and apparatus of monitoring the magnetic saturation of the core of a DC common mode choke with improved accuracy at high power densities or at least provides a useful alternative to known methods and apparatus for monitoring the magnetic saturation of the core of a DC common mode choke. SUMMARY According to a first aspect, a method of monitoring 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: injecting an AC excitation current into a first winding wound around the magnetic core; measuring the voltage induced by the AC excitation current in a second winding wound around the core; signal processing the measured voltage; and monitoring the magnetic saturation of the core based on the results of the signal processing. Aspects of the present disclosure are thus based on the idea to operate the core of a DC common mode choke as a transformer, wherein the first winding represents the transformer primary side and the second winding represents the transformer secondary side. According to the superposition principle, the field induced by the AC excitation current is additionally imprinted into the core magnetic field. By processing the voltage induced by an AC excitation current and measured in the second winding, the magnetic saturation of the core can be determined. For example, the percentage of saturation can be interpreted by inspecting the transformation ratio. Therefore, with the addition of a simple circuit, the magnetic saturation of common mode chokes can be accurately monitored, thus enabling DC common mode chokes to operate with high power densities. Furthermore the excitation current of the transformer primary side can be kept small due to a high permeability of the core in the linear region, thus minimizing the power rating of the excitation circuit. It is pointed out that the feature of injecting an AC excitation current into a first winding includes that an AC voltage is applied to the first winding, wherein the applied AC voltage leads to an additional current. For example, rectangular or trapezoidal voltage pulses may be applied to the first winding. Similarly, measuring the voltage induced by the AC excitation current in the second winding may include to measure a corresponding current. It is further pointed out that the saturation (meaning 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 AC excitation current is a magnetic H-field. However, for simplicity, the magnetic H-field caused by the AC excitation current as well as a magnetic H-fields produced by the first and second coils are simply referred to as magnetic fields. In some embodiments, the signal processing comprises monitoring the transformation ratio of the transformer formed by the first and second windings. The rationale behind such embodiments is as follows: when the core of the transformer begins to saturate, the transformation ratio decreases. This is because the core cannot effectively support additional magnetic flux changes in a saturated state, which leads to a reduction in inductance and therefore a lower voltage at the secondary coil. Put differently, the transformation ratio is determined by the relative permeability of the core material. Further, the signal processing may include rectification of the measured voltage to a DC voltage, wherein it is monitored if the rectified DC voltage falls under a predefined threshold. The rationale behind such embodiments is as follows: as a discussed above, when the core of the transformer begins to saturate, the transformation ratio decreases. This means that the measured (rectified) voltage at the second winding (detection coil) will drop as soon as the core reaches saturation, even though the voltage supplied by the excitation coil remains the same. Accordingly, saturation, and thus a change in magnetization, can be determined by monitoring the rectified voltage at the secondary winding. In particular, it may be monitored if the rectified DC voltage falls under a predefined threshold, which indicates a decreased transformation ratio and that the core has reached saturation. In some embodiments, the signal processing includes that the voltage induced by the AC excitation current in the second winding is bandpass filtered. Bandpass filtering ensures that only the induced voltage which is caused by the excitation current is admitted to signal processing, and not other voltages / signals. In some embodiments, the first winding is a first auxiliary winding wound around the core in addition to the first and second coils. Further, the second winding is a second auxiliary winding wound around the core in addition to the first and second coils. Accordingly, additional windings or coils are used for injecting the AC excitation current and measuring the induced voltage. This way, the auxiliary winding can be easily galvanically insulated from the first and second coils. In some embodiments, the first winding is formed by the first coil and the second winding is formed by the second coil. Accordingly, the winding into which the AC excitation current is injected is the first coil and the winding in which the voltage induced by the AC excitation current is measured is the second coil. In such embodiments, the already existing first and second coils are used to implement the transformer. This allows to retain the structure design of the common-mode choke, 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 the toroid kinetic core. When using an auxiliary windings, such auxiliary windings consume space in the window area and, thereby, limit space for the turns of the first and second coils. In some embodiments, the frequency of the AC excitation current is chosen to be distinct from a switching frequency of switches of a power converter of an electrical circuit which comprises the DC common mode choke. In particular, the AC excitation current may be chosen to be distinct from the high common mode current peaks during choke normal operations to maximize the signal-to-noise ratio. In some embodiments, the frequency of the AC excitation current frequency is distinct from the switching frequency of the switches of a power converter in that it is lower than the switching frequency of the switches of the power converter but higher than the fundamental frequency. According to a second aspect, an apparatus for monitoring 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 first winding wound around the magnetic core, a second winding wound around the magnetic core, and a controller which is configured to effect the steps of: injecting an AC excitation current into the first winding; measuring the voltage induced by the AC excitation current in the second winding; signal processing the measured voltage; and monitoring the magnetic saturation of the core based on the results of the signal processing. 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 controller is configured to signal process the measured voltage to determine and monitor the transformation ratio of the transformer formed by the first and second windings. To this end, the controller may be configured to signal process the measured voltage by rectifying the measured voltage to a DC voltage. Further, the controller may be configured to monitor if the rectified DC voltage falls under a predefined threshold. In some embodiments, the controller is configured to signal process the measured voltage by bandpass filtering the measured voltage. In some embodiments, the first and second windings are located on diagonal positions of the magnetic core. This allows to accurately reflect the volumetric-average of the core saturation. In some embodiments, the first winding is a first auxiliary winding wound around the core in addition to the first and second coils, and / or the second winding is a second auxiliary winding wound around the core in addition to the first and second coils. Alternatively, the first winding is the first coil of the choke and the second winding is the second coil of the choke. The coils of the DC common mode choke of the present disclosure may be configured to accept a high electric current density such as a current density of 20 A / mm2 or more. Generally, it is pointed out that the first winding and / or the second winding may also serve other purposes than those described so far. For example, it may be provided that additionally a DC bias current is injected in the first winding or the second winding to compensate a magnetic saturation in DC common mode chokes which is caused by imperfections and asymmetries. 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 The present disclosure will be explained in more detail on the basis of exemplary embodiments with reference to the accompanying drawings in which: FIG. 1 is a schematic embodiment of an apparatus for monitoring the magnetic saturation of the core of a DC common mode choke, wherein the apparatus comprises first and second windings wound around the core and a signal processing unit configured to process a voltage induced by an AC excitation current injected into the first winding and measured by the second winding; FIG. 2 is a flowchart of a method of monitoring the magnetic saturation of the core of a DC common mode choke; FIG. 3 is a sequence of steps implemented for signal processing of the voltage measured in the second winding of the apparatus of FIG. 1; FIG. 4 depicts the frequency components of a DC common mode current flowing through a DC common mode choke during normal operation; FIG. 5 is an embodiment of an apparatus for monitoring 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 first and second windings which are provided by a first and second auxiliary coils; FIG. 6 is an embodiment of an apparatus for monitoring 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 first and second windings which are provided by the first and second coils of the choke; FIG. 7 shows schematically different embodiments of a current source that provides for an AC excitation current; FIG. 8 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. 9 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. 8 and 9. As shown by example in FIG. 8, 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. 9 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 20 A / mm2 are flowing in the DC rails. The consequence due to the structural asymmetries or unequal thermal expansions will lead to a non-negligible saturation in the cores, and eventually shifting the cores out of the effective region. Therefore, it is critical to be able to dynamically monitor the core saturation due to the biasing DC magneto-motive forces. A monitoring method and apparatus in accordance with the present disclosure will next be discussed with respect to FIGS. 1 to 7. FIG. 1 depicts schematically 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. One of the coils 2 is connected to a DC positive voltage rail 81 and the other of the coils 3 is connected to a DC negative voltage rail 82. A load R is arranged between the positive voltage rail 81 and the negative voltage rail 82. The load R may comprise a power converter which includes a plurality of semiconductor switches. The apparatus further comprises a first winding 4 wound around the core 1 which, in the depicted embodiment, is a first auxiliary winding formed by a separate coil. As will be discussed with respect to FIG. 5, in other embodiments, the first winding 4 may alternatively be the first coil 2. Also, the apparatus comprises a second winding 5 wound around the core 1 which, in the depicted embodiment, is a second auxiliary winding formed by a further separate coil. Again, alternatively, the second winding 5 may be the second coil 3. The first auxiliary winding 4 is connected to an AC excitation current source 6 which injects an AC excitation current into the first winding 4. Accordingly, the first auxiliary winding 4 and the second auxiliary winding 5 form a transformer and the DC common mode choke 100 may be operated as a transformer. From that point of view, the first auxiliary winding 4 is an excitation coil and the second auxiliary winding 5 is a detection coil. Embodiments of the AC excitation current source 6 are discussed with respect to FIG. 7. The first and second auxiliary windings 4, 5 are provided with adequate galvanic isolation for the current source 6. The apparatus further comprises a signal processing unit 7 which receives a voltage signal induced by the AC excitation current in the second auxiliary winding 5. The voltage signal is received from a voltage measurement device 55 arranged in the second auxiliary winding 5. Alternatively, a current signal may be provided to signal processing unit 7. Generally, voltages and currents are used as synonyms in the present context. The signal processing unit 7 processes the measured voltage to monitor the magnetic saturation of the magnetic core 1, as will be discussed in more detail with respect to FIG. 3. The apparatus further comprises a controller 9. The controller 9 may be part of the signal processing unit 7 or vice versa. The controller 9 is configured to monitor the magnetic saturation of the core 1 of the DC common mode choke 100 and, to this end, is configured to control and initiate a method as is next discussed with respect to FIG. 2. Referring to FIG. 2, in step 21 an AC excitation current is injected into first winding 4 wound around magnetic core 1. The AC excitation current is provided by an excitation current source 6 which may be controlled by controller 9. The AC excitation current induces a voltage in the second winding 5 wound around magnetic core 1. This voltage induced in the second winding 5 is measured in step 22. The measurement may be by voltage measurement device 55. The measured voltage is then input into signal processing unit 7 in which the measured voltage receives signal processing. According to step 24, the magnetic saturation of the core 1 is monitored based on the results of the signal processing. In particular, the monitoring may be implemented by monitoring the transformation ratio of the transformer formed by the first and second windings. More particularly, the transformation ratio (voltage ratio) is defined as the ratio of the primary voltage to the secondary voltage. FIG. 3 provides additional details of how the signal processing in signal processing unit 7 may be carried out. In step 22 of FIG. 3, the detection coil output, i.e., the voltage output of the second winding 5 is measured / detected. Step 22 is similar to step 22 of FIG. 2. Subsequently, in step 231, the voltage signal is a bandpass filtered. Bandpass filtering serves to filter out frequencies in the measured voltage signal of the second auxiliary winding 5. This is schematically depicted in FIG. 4, which shows the frequency components of a DC common mode current. During normal operation, when the choke 100 is connected to the DC side of switching power converters, a DC common mode current with various frequency components as shown in FIG. 4 flows through the core 1 and, therefore, induces voltages of certain frequencies in the secondary winding 5. To maximize the effective signal-to-noise ratio, the AC excitation current frequency should be selected to be lower than the switching frequency of the power converters, but higher than the fundamental frequency. Further, the amplitude of the excitation current should be low enough to not incur magnetic saturations. FIG. 4 indicates a resonant peak D at the switching frequency of the power converters. The switching noise of such semiconductor switches shall be filtered by the DC common choke. The bandpass filtering, on the other side, serves to filter out frequencies such that only the voltage induced by the AC excitation current is admitted to the signal processing. In step 232, the filtered DC voltage signal is rectified in a rectifier. Rectification may be implemented as half-wave rectification or full-wave rectification with subsequent smoothing of the output as known to the skilled person. The rectified voltage has a height, which may be the height of the smoothed signal or the height of the half waves. Subsequently, in step 232, the output is provided to a microcontroller which monitors if the height of the rectified voltage falls under a predefined threshold. Such microcontroller may be included in signal processing unit 7 may be part of controller 9. Alternatively, such monitoring may be implemented by means of analog circuits. The monitoring is based on interpreting the saturation from the transformation ratio, which is determined by the relative permeability of the core material. When the core 1 begins to saturate, the transformation ratio (ratio of the primary voltage to the secondary voltage) decreases because the core cannot effectively support additional magnetic flux changes in a saturated state, which leads to a reduction in inductance and therefore a lower voltage at the secondary coil. This means that the rectified voltage drops as soon as the core reaches saturation, even though the voltage supplied by the excitation coil remains the same. Accordingly, saturation can be determined by monitoring the rectified voltage. It may be monitored if the rectified DC voltage falls under a predefined threshold, which indicates that the core has reached saturation. To effect and control the process of FIGS. 2 and 3, the controller 9 comprises input lines 91 and output lines 92. The input lines 91 may receive information from signal processing unit 7. 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 excitation current source 6. Controller 9 may also provide control signals on output lines to signal processing unit 7 and voltage measurement device 55. Other output lines may be connected to other controllers. The controller 9 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 effect / perform the functions and steps shown in FIGS. 2 and 3. The controller 9 may be a separate unit or may be integrated into a larger control unit. Also, the controller 9 may communicate with other control devices. Also, as mentioned, signal processing unit 7 may be integrated into the controller 9. For example, the controller 9 may comprise a signal processing chip for signal processing of the measured voltage. FIG. 5 a depicts an example embodiment of another apparatus for controlling the magnetic saturation of the 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 first auxiliary winding 4 which forms an excitation coil and a second auxiliary winding 5 which forms a detection coil. Each are depicted schematically by a single turn winding. However, this is to be understood as a schematic depiction and both the first auxiliary winding 4 and the second auxiliary winding 5 may comprise a plurality of turns. The primary auxiliary winding is connected to an AC excitation current source (not shown) such as AC excitation current source 6 of FIG. 1. The AC excitation current source 6 provide for an AC excitation current in the first auxiliary winding 4 which leads to an induced voltage in the second auxiliary winding 5 which is detected and receives signal processing in the same manner as a discussed with respect to FIGS. 2 and 3. The first auxiliary / primary and second auxiliary / secondary windings 4, 5 are located on the diagonal positions of the core 1 to accurately reflect the volumetric-average of the core saturation. FIG. 6 depicts an alternative implementation in which the apparatus is realized without introducing dedicated auxiliary windings but rather utilizes the high voltage winding coils 2, 3 of the choke itself. Either the positive or the negative of the DC windings can be used as the primary winding while the remaining can be the secondary winding. The excitation circuit and the detection circuit are isolated according to the functional insulation requirements. More particularly, as shown in FIG. 6, an AC excitation current is directly injected in one of the first or second coils 2, 3, wherein one electrical cable connected to one terminal of an AC excitation current source 6 (see FIG. 1) is connected with one of the turns 20, 30 of the respective coil and another electrical cable connected to the other terminal of the AC excitation current source 6 is connected with another one of the turns 20, 30 of the respective coil. A voltage induced by the AC excitation current in the respective other coil 3, 2 is then measured in the other coil 3, 2. The signal processing is in the same manner as discussed with respect to FIGS. 2 and 3. FIG. 7 shows some possible configurations to generate the AC excitation current. The excitation current source 6 can be provided by using (a) a linear amplifier, such as Howland current pump, (b) a switched-mode power supply, or (c) a combination of them. The energy source 60 for the current source 6 can be from (d) a battery, (e) low voltage DC bus or (f) 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 monitoring 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 coli (2) and a second coils (3) wound around the magnetic core (1), the method comprising the steps of:injecting (21) an AC excitation current into a first winding (2, 4) wound around the magnetic core (1);measuring (22) the voltage induced by the AC excitation current in a second winding (3, 5) wound around the core (1);signal processing (23) the measured voltage; andmonitoring (24) the magnetic saturation of the core (1) based on the results of the signal processing.

2. The method of claim 1, wherein the signal processing (23) comprises monitoring the transformation ratio of the transformer formed by the first winding (2, 4) and the second winding (3, 5).

3. The method of claim 2, wherein the signal processing (23) comprises rectification (232) of the measured voltage to a DC voltage.

4. The method of claim 3, wherein monitoring (233) if the rectified DC voltage falls under a predefined threshold.

5. The method of any preceding claim, wherein the signal processing (23) comprises bandpass filtering (231) of the voltage induced by the AC excitation current in the second winding.

6. The method of any preceding claim, wherein the first winding is a first auxiliary winding (4) wound around the core (1) in addition to the first coil (2) and the second coil (3).

7. The method of any preceding claim, wherein the second winding is a second auxiliary winding (5) wound around the core (1) in addition to the first coil (2) and the second coil (3).

8. The method of any one of claims 1 to 5, wherein the first winding is the first coil (2) of the choke.

9. The method of any one of claims 1 to 5 and 8, wherein the second winding is the second coil (3) of the choke.

10. The method of any preceding claim, wherein the frequency of the AC excitation current frequency is chosen to be distinct from a switching frequency of switches of a power converter of an electrical circuit which comprises the DC common mode choke.

11. The method of claim 10, wherein the frequency of the AC excitation current frequency is distinct from the switching frequency of the switches in that it is lower than the switching frequency of the switches but higher than a fundamental frequency.

12. 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);a first winding (2, 4) wound around the magnetic core (1);a second winding (3, 5) wound around the magnetic core (1); anda controller (7, 9) configured to effect the steps of:injecting (21) an AC excitation current into the first winding (2, 4);measuring (22) the voltage induced by the AC excitation current in the second winding (3, 5);signal processing (23) of the measured voltage; andmonitoring (24) the magnetic saturation of the core (1) based on the results of the signal processing.

13. The apparatus of claim 12, wherein the controller (7, 9) is configured to signal process the measured voltage to determine and monitor the transformation ratio of the transformer formed by the first winding (2, 4) and the second winding (3, 5).

14. The apparatus of claim 13, wherein the controller (7, 9) is configured to signal process the measured voltage by rectifying the measured voltage to a DC voltage.

15. The apparatus of claim 14, wherein the controller (7, 9) is further configured to monitor if the rectified DC voltage falls under a predefined threshold.

16. The apparatus of any one of claims 12 to 15, wherein the controller (7, 9) is configured to signal process the measured voltage by bandpass filtering the measured voltage.5 17. The apparatus of any one of claims 12 to 16, wherein the first winding (2, 4) and thesecond winding (3, 5) are located on diagonal positions of the magnetic core (1).

18. The apparatus of any one of claims 12 to 17, wherein the first winding is a first auxiliary winding (4) wound around the core (1) in addition to the first coil (2) and the 10 second coil (3), and / or that second winding is a second auxiliary winding (5) wound around the core (1) in addition to the first coil (2) and the second coil (3).

19. The apparatus of any one of claims 12 to 17, wherein the first winding is the first coil (2) of the choke and / or that the second winding is the second coil (3) of the choke.1520. The apparatus of any one of claims 10 to 19, wherein the first coil (2) and the second coil (3) of the DC common mode choke (100) are configured to accept a current density of 20 A / mm2 or more.15

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