Technical Field

The invention relates to a common mode choke comprising a magnetic core and a first and a second winding of insulated wire having the same number of turns wound on the magnetic core.

Background

Common mode chokes are widely used in industrial, electrical and telecom munications applications to remove or suppress common mode noise and related electromagnetic interference on power supply cables and signal lines.

Common mode noise, i.e. high frequency noise that is common to two or more data or power lines, may typically be radiated from sources such as unwanted radio signals, unshielded electronics, inverters and motors. If this noise is not suppressed, it can cause interference problems in electronics and electrical circuits. As an example, switched-mode power converters con nected to a mains AC supply net generate high frequency noise, which may typically be transmitted via the supply lines to the mains AC supply net. How ever, different regulations set limits on the amount of high frequency noise transmitted to the supply net. A common mode choke can be used to reduce or cancel the common mode noise and thus prevent it from reaching the mains AC supply net.

A common mode choke comprises two or more windings or coils of insulated wire wound on a single magnetic core such that common mode currents will flow in the same winding direction. The two windings have the same number of turns, and each winding is arranged in series with one of the supply lines. By common mode noise, currents flow in the same direction through each of the two windings, thus creating equal and in-phase magnetic fields, which add together. This means that to common mode signals, the windings act as inductors, which present a high impedance to these signals and reduces the high frequency common mode noise transmitted to the supply net. However by differential mode signals, including the desired signal such as the supply current delivered to the converter, the two currents will be equal and flow in opposite directions, and the magnetic flux produced in the core by these cur- rents will subtract and tend to cancel each other out since the windings are negative coupled. Thus, the common mode choke presents little inductance or impedance to differential mode currents. Thus, the common mode choke passes differential currents, which are equal but opposite, while it suppresses common mode currents.

Since the windings of the common mode choke act as inductors to common mode signals, the impedance of the choke increases proportionally with the inductance of the inductor as well as with the frequency of the signals applied to it. Normally, the common mode noise signals occur over a relatively broad frequency range, and thus, common mode noise signals in the higher end of the frequency range will be reduced more due to the higher impedance at these frequencies than the signals in the lower end of the frequency range.

In practice, however, a common mode choke has a more complex behaviour because of parasitic capacitance or stray capacitance between the individual turns of the windings due to their proximity to each other, and the inductor will act as though it includes a parallel capacitor. The parasitic capacitance changes the impedance of the choke by causing parallel resonance between the inductance and the capacitance at a resonance frequency, and thus the impedance of the common mode choke will show a resonance peak in its magnitude. Above the resonance frequency, the common mode choke actu ally has a capacitive reactance, and in this frequency range, the impedance will thus decrease, with a poor common mode noise reduction as the result. Therefore, the common mode choke should preferably be designed to have a relatively high resonance frequency, i.e. to keep the inductance and/or the capacitance in parallel to the windings low. On the other side, a high induct ance is desired to ensure a good common mode noise reduction in the fre quency range below the resonance frequency. The inductance of the common mode choke is proportional to the number of turns in the windings, and thus a high inductance requires a high number of turns. In other words, a certain number of turns of the windings is required to ensure sufficiently high impedance at the lower end of the frequency range.

Depending on the size of the magnetic core, this number of turns cannot al ways be arranged in one layer on the core, unless the size of the core is in creased. However, a larger core requires more space in the device in which the common mode choke is used, which again may result in a larger, i.e. less elegant, and more expensive enclosure of the device. Further, a larger core also increases the weight and the cost of the choke.

Therefore, the insulated wire may need to be wound in two or more layers. However, when the windings are wound in two layers, the impedance of the choke is changed significantly. The reason for this is that in addition to the parasitic capacitance or stray capacitance between neighbouring turns in each layer, there will now also be a parasitic capacitance between the layers, or more precisely between a given turn in one layer and its neighbouring turn in the other layer, i.e. the turn above or below that turn. This means that the total capacitance in parallel to each winding is increased considerably, when the windings are wound in two layers, which affects the resonance frequency and the impedance at the higher end of the frequency range negatively. Summary

Therefore, it is an object of embodiments of the invention to provide a com mon mode choke that can be wound in two or more layers in such a way that the increase in the capacitance in parallel to each winding caused by parasit ic capacitance between the layers of the windings is minimized.

According to embodiments of the invention the object is achieved in a com mon mode choke comprising a magnetic core; a first winding of insulated wire having a number of turns wound on said magnetic core; and a second winding of insulated wire having the same number of turns as the first wind ing wound on said magnetic core, wherein the turns of each winding are numbered consecutively; and the turns of each winding are arranged in at least two layers, of which a first layer of turns is wound directly on said mag- netic core and a second layer of turns is wound around a section of said first layer. The object is achieved when the two layers of each winding are ar ranged such that the difference between the highest number of a turn in the second layer and the lowest number of a turn in the section of the first layer on which the second layer is wound is less than or equal to 50 % of the num- ber of turns in the first layer of that winding.

When the difference between the highest number of a turn in the second lay er and the lowest number of a turn in the section of the first layer on which the second layer is wound does not exceed 50 % of the number of turns in the first layer of that winding, the effect of the parasitic capacitance between the layers is minimized, because the remaining turns of the first layer and the parasitic capacitances between them is still dominating in the determination of the resonance frequency and the impedance at the higher end of the fre quency range. Thus, the need for a larger core of the common mode choke can be eliminated, which may result in a smaller, more elegant and less ex pensive enclosure of the device in which the common mode choke is used.

In an embodiment, the first layer consists of turns having consecutive num bers from turn number one to the number of turns in the first layer; the sec- ond layer consists of turns having consecutive numbers higher than the number of turns in the first layer; and the section of the first layer on which the second layer is wound comprises the highest numbered turns of the first layer. By arranging the second layer at the end of the first layer, a practical solution is achieved.

The turns of the second layer may be arranged with the lowest numbered turn wound around the highest numbered turn of the first layer and with num bers increasing in the direction away from this turn. Alternatively, the turns of the second layer may be arranged with the highest numbered turn wound around the highest numbered turn of the first layer and with numbers de creasing in the direction away from this turn. The effect of the parasitic capacitance between the layers can be further min imized, and the resonance frequency and the impedance at the higher end of the frequency range thus further improved, when the turns of each winding are arranged in at least three layers, wherein a third layer of turns is wound around at least a section of said second layer.

In an embodiment, the first layer comprises two sections of turns, a first sec tion consisting of turns having consecutive numbers below a first intermedi ate turn number and a second section consisting of turns having consecutive numbers above a second intermediate turn number, where said second in- termediate turn number is higher than said first intermediate turn number; the second layer consists of turns having consecutive numbers between said first intermediate turn number and said second intermediate turn number; and the section of the first layer on which the second layer is wound comprises the highest numbered turns of the first section of the first layer. In this way, the second layer of turns can be placed in an arbitrary position on the first layer.

In an embodiment, the section of the first layer on which the second layer is wound has the same number of turns as the second layer. The magnetic core on which the turns of the windings are wound may be a toroidal core. This type of core is available from many different suppliers. In that case, the windings may be arranged symmetrically on said toroidal core and separated from each other. This increases the leakage flux between the two windings, and the resulting leakage inductance will ensure that the com- mon mode choke will also have some differential mode impedance, which can provide a certain differential mode suppression as well at higher fre quencies. Brief Description of the Drawings

Embodiments of the invention will now be described more fully below with reference to the drawings, in which Figure 1 shows an example of a circuit in which a common mode choke can be used,

Figures 2a to 2c show some examples of common mode chokes using differ ently shaped magnetic cores,

Figures 3a and 3b illustrate the function of a common mode choke having two windings wound on a toroidal core,

Figures 4a and 4b show the ideal impedance of the common mode choke as a function of the frequency and the impedance having a resonance peak in its magnitude caused by parasitic capacitance between turns of the winding,

Figures 5a to 5c show a side view, a sectional view and an electrical diagram of a common mode choke having the turns of the windings wound in one lay- er,

Figures 6a to 6e show a side view, sectional views, an electrical diagram and the impedance as a function of the frequency of a common mode choke hav ing the turns of the windings wound in two layers according to prior art,

Figure 7 shows an example of a circuit in which two common mode chokes according to prior art are used to achieve sufficient common mode noise re duction, Figures 8a to 8h show side views, sectional views, electrical diagrams and the impedance as a function of the frequency of a common mode choke hav ing the turns of the windings wound in two layers according to the invention, Figures 9a to 9d show a side view, a sectional view, an electrical diagram and the impedance as a function of the frequency of another common mode choke having the turns of the windings wound in two layers according to the invention,

Figures 10a to 10d show a side view, a sectional view, an electrical diagram and the impedance as a function of the frequency of a common mode choke having the turns of the windings wound in two layers similar to Figures 9a to 9d and Figures 10a to 10d but with too many turns in the second layer,

Figures 1 1 a to 1 1 d show a side view, a sectional view, an electrical diagram and the impedance as a function of the frequency of a common mode choke having the turns of the windings wound in three layers according to the inven tion, and

Figures 12a to 12e show a side view, sectional views, an electrical diagram and the impedance as a function of the frequency of a further common mode choke having the turns of the windings wound in two layers according to the invention.

Detailed Description

Figure 1 shows a diagram of an example of a circuit 51 in which a common mode choke 52 can be used. In this example, the circuit 51 is a switched- mode power supply connected to a mains AC supply net and comprising a converter 53. Switched-mode converters exist in different circuit topologies, such as flyback, boost, buck, SEPIC and forward converters. The AC voltage from the mains supply (supplied via supply lines 54 and 55) is rectified in a bridge rectifier 56 and buffered in a capacitor 57, before it is fed to a pair of input terminals of the converter 53 as its input voltage. The output voltage of the converter 53 is delivered from a pair of output terminals to a load, which could be e.g. the electric motor of a linear actuator. The converter 53 generates high frequency noise, which may typically be transmitted via the supply lines 54 and 55 to the mains AC supply net. How ever, different regulations set limits on the amount of high frequency noise transmitted to the supply net. The high frequency noise generated by the converter 53 can be differential mode noise as well as common mode noise.

The differential mode noise causes a noise current to flow through one of the supply lines 54 and 55 from the converter 53, i.e. the source of the noise, to the mains AC supply net and in the opposite direction through the other one of the supply lines 54 and 55. The differential mode noise can be reduced by inserting an inductor in one (or both) of the supply lines 54 and 55 and a ca pacitor between them, thus creating a differential mode filter.

In contrast to the differential mode noise, the common mode noise causes similar noise currents to flow through both supply lines 54 and 55 in the same direction, e.g. from the converter 53 to the mains AC supply net. These cur rents will return to the converter 53 either through an explicit ground connec tion or through other means, such as parasitic capacitance through the air or other connections. The purpose of the common mode choke 52 is to reduce or cancel the common mode noise and thus prevent it from reaching the mains AC supply net.

A common mode choke comprises two (or more) windings of insulated wire wound on a single magnetic core such that common mode currents, when the common mode choke is arranged as in Figure 1 , will flow in the same winding direction. The two windings have the same number of turns, and each winding is arranged in series with one of the supply lines. The magnetic core can typically be a ferrite core, but also other core types, such as iron powder cores, can be used. Also the shape of the magnetic core may be dif- ferent. Figures 2a, 2b and 2c show some examples of common mode chokes using differently shaped magnetic cores. In Figure 2a, the two windings of a common mode choke are arranged on a toroidal core, while a rectangular shaped core is used in Figure 2b, but also cores shaped as a straight cylin- drical rod as shown in Figure 2c or other shapes can be used. In the follow ing, the toroidal core is used as an example.

The two windings can be arranged separately on the core as shown in Fig- ures 2a, 2b and 2c, but also other arrangements are possible. As an exam ple, the two windings can be arranged in two layers, so that one winding is wound in a first layer directly on the core, while the other is wound in a sec ond layer on top of the first layer. The two windings may also be only partly overlapping. As another example, the turns of the two windings may be inter- leaved with each other.

Figures 3a and 3b illustrate the function of the common mode choke 52 of Figure 1 having two windings wound on a toroidal core 58. The four terminals of the choke are labelled a, b, c and d. One winding Wab is connected be- tween terminals a and b, while the other winding Wed is connected between terminals c and d.

By common mode noise, the two currents lab and led flow in the same direc tion through each of the two windings Wab and Wed, thus creating equal and in-phase magnetic fields, which add together as illustrated with the arrows in Figure 3a. This means that to common mode signals, the windings act as an inductor, which presents a high impedance to these signals and reduces the high frequency common mode noise transmitted to the supply net. By differential mode signals, including the desired signal such as the supply current delivered to the converter 3, the two currents lab and led will be equal and flow in opposite directions, and the magnetic flux produced in the core by these currents will subtract and tend to cancel each other out since the wind ings are negative coupled. Thus, the common mode choke presents little in- ductance or impedance to differential mode currents.

Flowever, it is noted that in practice, the common mode choke will have some leakage flux between the two windings Wab and Wed, and the resulting leak- age inductance will act in series with one (or both) of the supply lines 54 and 55. Thus, the common mode choke will also have some differential mode impedance, which can provide a certain differential mode suppression as well at higher frequencies. If desired, this differential mode noise filtering can be improved by inserting a capacitor between the supply lines 54 and 55 at the supply net side of the common mode choke 52, thus creating a differential mode filter.

As mentioned, each of the windings Wab and Wed acts as an inductor to common mode signals, and as it is well known, the impedance of an inductor increases proportionally with the inductance of the inductor as well as with the frequency of the signals applied to it. Normally, the common mode noise signals occur over a relatively broad frequency range, and thus, common mode noise signals in the higher end of the frequency range will be reduced more due to the higher impedance at these frequencies than the signals in the lower end of the frequency range. This is illustrated in Figure 4a, which shows the ideal impedance of the common mode choke as a function of the frequency. In practice, however, a common mode choke will have a more complex be haviour because of parasitic capacitance or stray capacitance between the individual turns of the windings due to their proximity to each other, and the inductor will act as though it includes a parallel capacitor. The parasitic ca pacitance changes the impedance of the choke by causing parallel reso- nance between the inductance and the capacitance at a resonance frequen cy fo, and thus the impedance of the common mode choke will show a reso nance peak in its magnitude, as it is illustrated in Figure 4b. Above the reso nance frequency fo, the common mode choke actually has a capacitive reac tance. Generally, the resonance frequency fo for a parallel resonance circuit having an inductor L in parallel with a capacitor C is given by The effect of the parasitic capacitance is explained in more detail below with reference to Figures 5a, 5b and 5c. Figure 5a is a side view of the common mode choke 2 corresponding to Figure 2a or 3a showing the two windings Wab and Wed, where the turns of each winding are arranged in one layer on the magnetic core 58. Figure 5b shows a corresponding sectional view illus trating how the turns of the windings may be numbered consecutively from 1 to N (where N in the figure is chosen to 28). Thus, each winding has N turns. In other words, the turns are numbered consecutively in the order they were wound during production of the choke. Figure 5c shows the corresponding electrical diagram of the choke, where the parasitic capacitance as an ap proximation is illustrated as a small capacitor connected across each turn of the windings.

As an approximation, the inductance LN of one of the windings Wab and Wed having N turns can be considered as being proportional to the expression N ² A/7, where A is the cross section area of the winding and l is the length of the winding. It is noted that since the windings of the common mode choke 52 are arranged in one layer, the length l is proportional to N, so that in this case, the inductance LN can be considered as being proportional to the num- ber of turns N.

Correspondingly, if the parasitic capacitance between two neighbouring turns is Cturn, the total capacitance in parallel to the winding is Ctum / (N-1 ). Thus, the resonance frequency fo for one of the windings Wab and Wed of the com- mon mode choke 52 in Figures 5a, 5b and 5c can be determined by

In Figures 5a, 5b and 5c, each winding is shown as having 28 turns as an example, and thus for this example, N = 28, and the inductance of one of the windings is L28, the total capacitance in parallel to the winding is Ctum / 27, and the resonance frequency fo will be

It was mentioned above, and it can also be seen from Figure 4b, that below the resonance frequency fo, the impedance of the windings of the common mode choke increases proportionally with the frequency of the noise signals. Thus, common mode noise signals in the higher end of this frequency range (i.e. below the resonance frequency fo) will be reduced more due to the high er impedance at these frequencies than signals in the lower end of the fre- quency range. The impedance also increases proportionally with the induct ance of the windings, and therefore, to ensure sufficiently high impedance at the lower end of the frequency range, the inductance must be sufficiently high. Since the inductance increases with the number of turns of the wind ings, a certain number of turns of the windings is required to ensure suffi- ciently high impedance at the lower end of the frequency range.

Depending on the size of the magnetic core 58, this number of turns cannot always be arranged in one layer on the core, and therefore, the insulated wire may need to be wound in two or more layers. Typically, the turns of a second layer are distributed over the full length of the first layer wound direct ly on the core, or the second layer is arranged near the middle of the first layer. An example of this is shown in Figure 6a illustrating a common mode choke 62, where each of the windings Wab and Wed has a total of N = 40 turns arranged with Ni = 28 turns in a first layer Li wound directly on the core and N2 = 12 turns in a second layer L2 wound on top of the first layer. A cor responding sectional view illustrating how the turns of the first layer Li are numbered consecutively from 1 to 28 and the turns of the second layer L2 are numbered from 29 to 40 is shown in Figure 6b. Again, it is seen that the turns are numbered consecutively in the order they were wound during production of the choke. The more general case with Ni turns in the first layer Li and N2 turns in the second layer L2 is illustrated in Figure 6c. However, the fact that the windings are now wound in two layers means that the impedance of the choke is changed significantly. The reason for this is that in addition to the parasitic capacitance or stray capacitance between neighbouring turns in each layer mentioned above, there will now also be a parasitic capacitance between a given turn in one layer and its neighbouring turn in the other layer, i.e. the turn above or below that turn. Thus, as an ex ample, it can be seen in Figure 6b that turn number 29 is arranged above turn number 20 in the first layer, and due to their proximity to each other there will be a parasitic capacitance between these turns having approxi- mately the same size as the parasitic capacitance Ctum between two neigh bouring turns in the same layer. Similarly, there will be a parasitic capaci tance between turn number 30 and turn number 19, between turn number 31 and turn number 18, and so on. Theoretically, there will also be a parasitic capacitance between e.g. turn number 29 and the turns 19 and 21 in the first layer, but due to the larger distance, these capacitances can be considered as negligible compared to the capacitance between turns arranged directly above each other.

This is illustrated in Figure 6d showing the corresponding electrical diagram of the choke 62, where the parasitic capacitances between neighbouring turns in the same layer as well as those between turns of different layers are shown. Also here, the turns are numbered consecutively from 1 to 40. All the shown parasitic capacitances are considered as having the capacitance val ue Ctum. For reasons of clarity and simplicity, only the winding Wab and the capacitors indicating the parasitic capacitances for this winding are shown. The diagram for winding Wed is similar. The presence of the additional para sitic capacitances between turns in the first layer and turns in the second lay er results in a large combination of series/parallel-coupled capacitors as shown in Figure 6d, which changes the impedance of the choke significantly, because the total capacitance in parallel to the winding of the common mode choke 62 is considerably increased compared to that of the choke 52 of Fig ures 5a-5c. When calculating the resulting capacitance of the large combination of se- ries/parallel-coupled capacitors, the capacitor of size Ctum indicating the par asitic capacitance between turn number 40 (i.e. the highest number of a turn in the second layer L2) and turn number 9 (i.e. the lowest number of a turn in the section of the first layer Li on which the second layer of turns is wound) will be the dominating capacitance. Actually, the resulting capacitance of the combination can be calculated to be approximately 4/3 Ctum. Thus, the total capacitance in parallel to the winding Wab of the common mode choke 62 is a series connection of eight capacitors of size Ctum and one capacitor of size 4/3 Ctum. This can be calculated to 4/35 Ctum, which is so close to 1 /9 Ctum that the resulting capacitance of the large combination of series/parallel- coupled capacitors could just as well be approximated to Ctum. This approxi mation will be used in the following. Of course, this increase in the total capacitance in parallel to the winding Wab of the common mode choke 62 compared to the choke 52 of Figures 5a-5c affects the resonance frequency. Each winding now has 40 turns, and be cause the length l of the windings is the same as the windings of the choke 52, the inductance l_4o of one of the windings of the choke 62 is l_4o = (40/28) ² L28, while the total capacitance in parallel to the winding as mentioned can be approximated to Ctum / 9. The resonance frequency fo for the choke 62 will thus be

This means that the common mode choke 62 has a resonance frequency around 0.40 times the resonance frequency of the choke 52. Figure 6e shows the impedance of the common mode choke 62 (thick line) as a func tion of the frequency compared to the impedance of the common mode choke 52 (thin line) from Figure 4a. It is clearly seen that due to the higher number of turns, the choke 62 has a higher impedance (and thus a better reduction of common mode noise signals) in the lower end of the frequency range, as it was also intended. However, on the other hand, it has a much lower impedance (and thus a poor reduction of common mode noise signals) in the higher end of the frequency range, which is due to the higher total ca pacitance in parallel to the winding. It is noted that the curves on Figure 6e are not drawn to scale, and thus the figure only intends to indicate the effects of arranging the turns of the windings on the magnetic core as described. This is also the case for Figures 8d, 9d, 10d, 1 1 d and 12d mentioned in the following. In other words, the common mode choke 52 with 28 turns in each winding has a good common mode noise reduction at higher frequencies and a poor common mode noise reduction at lower frequencies, while it is the other way round for the common mode choke 62 with 40 turns in each winding. To get a sufficient common mode noise reduction in the whole frequency range, it is therefore well known to arrange two such common mode chokes in series, as it is shown in Figure 7. In this way, the two impedance curves in Figure 6e are added together, and a good common mode noise reduction is achieved in a much broader frequency range. However, since this solution requires the use of an additional component in the form of the second common mode choke, the result is increased space requirements e.g. on a printed circuit board as well as increased costs.

Looking at Figure 6d, it can be seen that turns in the second layer L2 and turns arranged in the first layer Li below the second layer L2 (and thus cou- pled to each other with parasitic capacitances) may have a considerable dif ference between their consecutive numbers. As it was seen above, the re sulting capacitance of the large combination of series/parallel-coupled capac itors is determined by the difference between the highest number of a turn in the second layer and the lowest number of a turn in the section of the first layer on which the second layer of turns is wound. This is a reason for the large increase in the total capacitance in parallel to the winding Wab of the common mode choke 62. Thus, in the example above, the parasitic capaci tance between turn number 40 and turn number 9, which spans over 31 turns, is dominating in the resulting capacitance of the large combination of series/parallel-coupled capacitors, and only the eight parasitic capacitances between turns 1 to 9 are left to be in series with the combination. Therefore, as it is described in the following, it can be seen that the total ca pacitance in parallel to each winding can be reduced by arranging the two layers such that the difference between the highest number of a turn in the second layer l_2 and the lowest number of a turn in the section of the first lay er Li on which the second layer l_2 of turns is wound is kept as small as pos- sible.

Examples of how this can be done are illustrated in Figures 8a-8d and 9a-9d. Figures 8a-8d show a common mode choke 63 having 32 turns arranged with 28 turns in the first layer Li and 4 turns in the second layer l_2, while Fig- ures 9a-9d show a common mode choke 64 having 35 turns arranged with 28 turns in the first layer Li and 7 turns in the second layer l_2.

In Figures 8a, 8b and 8c, turn number 29 is wound directly around turn num ber 28, and then turn number 30 around turn number 27, and so on. In other words, the second layer consists of turns number 29 to 32, and the section of the first layer on which the second layer of turns is wound consists of turns number 25 to 28. Thus, the second layer l_2 is arranged at the end of the first layer Li. With this arrangement of the second layer, the difference between the highest number of a turn in the second layer and the lowest number of a turn in the section of the first layer on which the second layer of turns is wound is kept small. Thus, also the total capacitance in parallel to each wind ing of the common mode choke is kept low, which results in a higher reso nance frequency. It is noted that with Ni = 28 turns in the first layer and N2 = 4 turns in the second layer, the fraction of the first layer that is covered by the second layer, i.e. the relation N2/N1, is equal to 1/7 or 14,3 %. The highest difference between consecutive numbers of turns in the two layers arranged on top of each other is 32-25 = 7, which corresponds to 25 % of the number Ni = 28 of turns in the first layer. Similarly to above, the capacitor of size Ctum indicating the parasitic capaci tance between turn number 32 and turn number 25 will here be the dominat ing capacitance of the combination of series/parallel-coupled capacitors, and the resulting capacitance of this combination can again be approximated to

Cturn.

Thus, the total capacitance in parallel to the winding Wab of the common mode choke 63 is a series connection of 24 capacitors of size Ctum and the combination of series/parallel-coupled capacitors, which as mentioned is also approximated to Ctum, i.e. the total capacitance can be calculated to 1 /25

Cturn.

Each winding of the common mode choke 63 has 32 turns, and the induct- ance L32 of one of the windings is L32 = (32/28) ² L28, while the total capaci tance in parallel to the winding as mentioned can be approximated to Ctum / 25. The resonance frequency fo for the choke 63 will thus be

This means that the common mode choke 63 has a resonance frequency that is around 0.84 times the resonance frequency of the choke 52. Figure 8d shows the impedance of the common mode choke 63 (thick line) as a func tion of the frequency compared to the impedance of the common mode chokes 52 and 62 (thin line). Compared to the common mode choke 52, the choke 63 has a slightly higher impedance in the lower end of the frequency range due to the higher number of turns, as it was also intended, and in the higher end of the frequency range, the impedance is only slightly reduced, since the total capacitance in parallel to the winding is only increased from to 1/27 Cturn tO 1/25 Cturn. However, if the four turns of the second layer had been arranged e.g. on the middle part of the first layer, as it was the case in Figures 6a-6e, the relation N2/N1 would still be equal to 1/7 or 14,3 %, but the highest difference be tween consecutive numbers of turns in the layers arranged on top of each other would now be 32-13 = 19, corresponding to 68 % of the number Ni = 28 of turns in the first layer, and the resulting capacitance in parallel to each winding would have been much higher with a corresponding effect on the resonance frequency and the common mode noise reduction. The more general case with a common mode choke having N turns arranged with Ni turns in the first layer Li and N2 turns in the second layer L2 is illus trated in Figure 8e. Here, the second layer consists of turns number Ni+1 to N1+N2 (=N), and the section of the first layer on which the second layer of turns is wound consists of turns number N1-N2 to Ni. Thus, the difference between the highest number (N1+N2) of a turn in the second layer and the lowest number (NI-(N2-1 )) of a turn in the section of the first layer on which the second layer of turns is wound is (NI+N2)-(NI-(N2-1 )) = 2N2Ί .

It is noted that the turns of the second layer, i.e. turns number 29, 30, 31 and 32, may also be arranged in the reverse order as that shown in Figures 8a-

8d. In that case, turn number 29 is wound around turn number 25, and then turn number 30 around turn number 26, and so on, as it is illustrated in Fig ures 8f-8h. Although the combination of series/parallel-coupled capacitors shown in Figure 8h is different from that of Figure 8c, the resulting capaci- tance of the combination can still be approximated to Ctum, so that also in this case, the total capacitance in parallel to each winding of the common mode choke is determined by the difference between the highest number (32) of a turn in the second layer and the lowest number (25) of a turn in the section of the first layer on which the second layer of turns is wound. Thus, the reso- nance frequency, the impedance and the common mode noise reduction will also be substantially unchanged. As mentioned, the common mode choke 64 shown in Figures 9a-9d has 35 turns arranged with 28 turns in the first layer Li and 7 turns in the second layer l_2. Again, turn number 29 is wound directly around turn number 28, and then turn number 30 around turn number 27, and so on. Thus, the second layer consists of turns number 29 to 35, and the section of the first layer on which the second layer of turns is wound consists of turns number 22 to 28. In this case, with Ni = 28 turns in the first layer and N2 = 7 turns in the sec ond layer, the relation N2/N1 is equal to 1/4 or 25 %, and the highest differ ence between consecutive numbers of turns in the two layers arranged on top of each other is 35-22 = 13, which corresponds to 46.4 % of the number Ni = 28 of turns in the first layer.

Similarly to above, the capacitor of size Ctum indicating the parasitic capaci tance between turn number 35 and turn number 22 will here be the dominat- ing capacitance of the combination of series/parallel-coupled capacitors, and the resulting capacitance of this combination can be approximated to Ctum.

Thus, the total capacitance in parallel to the winding Wab of the common mode choke 64 is a series connection of 21 capacitors of size Ctum and the combination of series/parallel-coupled capacitors, which as mentioned is also approximated to Ctum, i.e. the total capacitance can be calculated to 1 /22 Ctum.

Each winding of the common mode choke 64 has 35 turns, and the induct- ance L35 of one of the windings is L35 = (35/28) ² L28, while the total capaci tance in parallel to the winding as mentioned can be approximated to Ctum / 22. The resonance frequency fo for the choke 64 will thus be

This means that the common mode choke 64 has a resonance frequency that is around 0.72 times the resonance frequency of the choke 52. Figure 9d shows the impedance of the common mode choke 64 (thick line) as a func tion of the frequency compared to the impedance of the common mode chokes 52 and 62 (thin line). Compared to the common mode choke 63 (Fig ure 8d), the choke 64 has a further improved impedance in the lower end of the frequency range due to the higher number of turns. In the higher end of the frequency range, the impedance is a bit further reduced, since the total capacitance in parallel to the winding is increased from to 1/25 Ctum to 1/22 Cturn, but the common mode noise reduction is still at an acceptable level. Also here, the turns of the second layer l_2, i.e. turns number 29 to 35, may be arranged in the reverse order as that shown in Figures 9a-9c.

Flowever, if the number of turns in the second layer l_2 is further increased, so that the highest difference between the consecutive numbers of one of the turns in the second layer and one of the turns in the section of the first layer on which the turns of the second layer are arranged exceeds 50 % of the number Ni of turns in the first layer, it can be seen that the total capacitance in parallel to the winding will reach a level where the impedance in the higher end of the frequency range is too low and the common mode noise reduction no longer at an acceptable level. This is illustrated in Figures 10a-10d show ing a common mode choke 65 having 40 turns arranged with 28 turns in the first layer Li and 12 turns in the second layer l_2, i.e. the same number of turns as the common mode choke 62 of Figures 6a-6e. As in Figures 8a-8d and 9a-9d, turn number 29 is wound directly around turn number 28, and then turn number 30 around turn number 27, and so on. Thus, the second layer consists of turns number 29 to 40, and the section of the first layer on which the second layer of turns is wound consists of turns number 17 to 28. With Ni = 28 turns in the first layer and N2 = 12 turns in the second layer, the relation N2/N1 is equal to 3/7 or 42.9 %, and the highest difference between consecutive numbers of turns in the two layers arranged on top of each other is 40-17 = 23, which corresponds to 82.1 % of the num ber Ni = 28 of turns in the first layer. Here, the capacitor of size Ctum indicating the parasitic capacitance between turn number 40 and turn number 17 will be the dominating capacitance of the combination of series/parallel-coupled capacitors, and again the resulting capacitance of this combination can be approximated to Ctum. Thus, the total capacitance in parallel to the winding Wab of the common mode choke 65 is a series connection of 16 capacitors of size Ctum and the combination of se- ries/parallel-coupled capacitors, which as mentioned is also approximated to Ctum, i.e. the total capacitance can be calculated to 1/17 Ctum.

Each winding of the common mode choke 65 has 40 turns, and the induct ance L40 of one of the windings is l_4o = (40/28) ² L28, while the total capaci tance in parallel to the winding as mentioned can be approximated to Ctum / 17. The resonance frequency fo for the choke 65 will thus be

This means that the common mode choke 64 has a resonance frequency that is around 0.56 times the resonance frequency of the choke 52. Figure 10d shows the impedance of the common mode choke 65 (thick line) as a function of the frequency compared to the impedance of the common mode chokes 52 and 62 (thin line). It can be seen that the choke 65 has a further improved impedance in the lower end of the frequency range due to the higher number of turns. Thus, a good common mode noise reduction is achieved in this frequency range. However, since the total capacitance in parallel to the winding is now increased to 1/17 Ctum, the impedance in the higher end of the frequency range is reduced to a level around 63 % of the corresponding impedance level of the choke 52 from Figures 6a-6e, and the common mode noise reduction is no longer at an acceptable level in this fre- quency range. However, if a higher number of turns, e.g. the 40 turns used in the common mode chokes 62 and 65 shown in Figures 6a-6e and 10a-10d, is needed to achieve a sufficient common mode noise reduction in the lower end of the frequency range, the common mode noise reduction in the higher end of the frequency range can be improved by arranging the turns of the choke in more than two layers, so that the highest difference between consecutive numbers of turns in layers arranged on top of each other can still be kept below 50 % of the number Ni of turns in the first layer. This is illustrated in Figures 1 1 a-1 1 d showing a common mode choke 66 having 40 turns arranged with 28 turns in the first layer Li, 6 turns in the sec ond layer l_2 and 6 turns in a third layer l_3. Thus, the choke 66 has the same number of turns as the common mode choke 62 of Figures 6a-6e and the common mode choke 65 of Figures 10a-10d.

As in the previous figures, turn number 29 is wound directly around turn number 28, and then turn number 30 around turn number 27, and so on until turn number 34 that is wound around turn number 23. In the third layer, turn number 35 is now wound around turn number 34 in the second layer, and then turn number 36 around turn number 33, and so on until turn number 40 that is wound around turn number 29. With Ni = 28 turns in the first layer Li, N2 = 6 turns in the second layer l_2 and N3 = 6 turns in the third layer L3, the relations N2/N1 and N3/N1 are equal to 6/28 or 21.4 %. Thus, only 21.4 % of the first layer is covered by turns of the other layers. The highest difference between consecutive numbers of turns in layers arranged on top of each oth er is 40-29 = 1 1 and 34-23 = 1 1 , which corresponds to 39.3 % of the number Ni = 28 of turns in the first layer. This is well below 50 % of the number Ni of turns in the first layer, and as shown below, a good common mode noise re duction can be achieved.

The fact that the turns of the windings are now wound in three layers means that in addition to the parasitic capacitance or stray capacitance between neighbouring turns in each layer and between a given turn in the first layer and its neighbouring turn in the second layer, i.e. the turn above or below that turn, there will now also be a parasitic capacitance between a given turn in the second layer and its neighbouring turn in the third layer. This is illustrated in Figure 1 1 c. Theoretically, there will also be a parasitic capacitance be- tween a turn in the third layer and the turn in the first layer arranged below that turn, e.g. between turn number 35 in the third layer and turn number 23 in the first layer, but due to the larger distance between these turns, these capacitances can be considered as negligible compared to the capacitance between turns arranged directly above each other.

Here, the capacitors of size Ctum indicating the parasitic capacitances be tween turn number 40 and turn number 29 and between turn number 34 and turn number 23 will now be the dominating capacitances of the combination of series/parallel-coupled capacitors, and again the resulting capacitance of this combination can be approximated to Ctum. Thus, the total capacitance in parallel to the winding Wab of the common mode choke 66 is a series con nection of 22 capacitors of size Ctum and the combination of series/parallel- coupled capacitors, which as mentioned is also approximated to Ctum, i.e. the total capacitance can be calculated to 1 /23 Ctum.

Each winding of the common mode choke 66 has 40 turns, and the induct ance L40 of one of the windings is l_4o = (40/28) ² L28, while the total capaci tance in parallel to the winding as mentioned can be approximated to Ctum / 23. The resonance frequency fo for the choke 66 will thus be

This means that the common mode choke 66 has a resonance frequency that is around 0.65 times the resonance frequency of the choke 52. Figure 1 1 d shows the impedance of the common mode choke 66 (thick line) as a function of the frequency compared to the impedance of the common mode chokes 52 and 62 (thin line). It can be seen that compared to the common mode choke 52, the choke 66 has an improved impedance in the lower end of the frequency range due to the higher number of turns. Thus, a good common mode noise reduction is achieved in this frequency range. In the higher end of the frequency range, the impedance is now only slightly reduced, since the total capacitance in parallel to the winding is only increased from to 1 /27 Ctum to 1 /23 Ctum. This means that the common mode choke 66 has a good common mode noise reduction in the lower frequency range as well as the higher frequency range. Especially around its resonance frequency fo(66) the common mode choke 66 has a common mode noise reduction that is considerably better than it was the case for the previously described chokes.

Also here, the turns of the second layer l_2 and/or the third layer l_3, i.e. turns number 29 to 34 and/or turns number 35 to 40, may be arranged in the re verse order as that shown in Figures 1 1 a-1 1 d without changing the common mode noise reduction essentially.

As mentioned, the common mode choke 66 has 40 turns arranged with 28 turns in the first layer Li, 6 turns in the second layer l_2 and 6 turns in the third layer l_3, and the highest difference between consecutive numbers of turns arranged in two layers on top of each other is 1 1 , which corresponds to 39.3 % of the number of turns in the first layer. If a higher total number of turns is needed, the number of turns in the second and/or third layer can be in- creased. As long as the highest difference between consecutive numbers of turns arranged in layers on top of each other does not exceed 50 % of the number turns in the first layer, the common mode noise reduction in the higher end of the frequency range will be acceptable. Thus, as an example, the common mode choke 66 of Figures 1 1 a-11 d could be modified to have 7 turns in each of the second and third layers without exceeding the 50 % limit. In case the total number of turns should be further increased, the use of four or even more layers could be considered. In the embodiments described above and shown in Figures 8a-8e, 9a-9d and 1 1 a-11 d, the first layer Li consists of the turns consecutively numbered from 1 to 28 or more generally from 1 to Ni, while the turns having higher numbers are arranged in the second and/or third layers. This means that when the common mode choke is manufactured, the first Ni turns are wound directly on the core as the first layer, and then higher-numbered turns are wound as the second layer on top of those turns of the first layer having numbers clos est to Ni, and, if necessary, as a third layer on top of the second layer. How ever, the turns do not necessarily need to be arranged in this way. Other ar- rangements are possible, which will still ensure that the highest difference between consecutive numbers of turns arranged in layers on top of each oth er does not exceed 50 % of the number turns in the first layer, so that the common mode noise reduction in the higher end of the frequency range will be acceptable.

As an example of this, Figures 12a-12d show a common mode choke 67 having 35 turns arranged with 28 turns in the first layer Li and 7 turns in the second layer l_2, i.e. the same number of turns as the common mode choke 64 of Figures 9a-9d. However, here the turns consecutively numbered from 1 to 17 have first been wound as a part of the first layer Li directly on the core, but then, as a step backwards, the turns number 18 to 24 have been wound as the second layer l_2 around turns number 1 1 to 17 in the first layer, with turn number 18 being wound directly around turn number 1 1 , and then turn number 19 around turn number 12, and so on. Finally, the first layer has been continued with turns number 25 to 35. Thus, the second layer l_2 con sisting of turns number 18 to 24 is here arranged in the middle of the first layer Li consisting of turns number 1 to 17 and 25 to 35.

Figure 12c shows how the turns of the common mode choke 67 are capaci- tively coupled to each other. As it was the case for the common mode choke 64 of Figures 9a-9d, with Ni = 28 turns in the first layer and N2 = 7 turns in the second layer, the relation N2/N1 is equal to 1/4 or 25 %. The highest dif ference between consecutive numbers of turns that are arranged in layers on top of each other is 24-1 1 = 13, which corresponds to 46.4 % of the number Ni = 28 of turns in the first layer.

Similarly to above, the resulting capacitance of the combination of se- ries/parallel-coupled capacitors can be approximated to Ctum. Thus, the total capacitance in parallel to the winding Wab of the common mode choke 67 is a series connection of 21 capacitors of size Ctum and the combination of se- ries/parallel-coupled capacitors, which as mentioned is also approximated to Ctum, i.e. the total capacitance can be calculated to 1/22 Ctum.

Thus, each winding of the common mode choke 67 (as it was the case for the common mode choke 64) has 35 turns, and the inductance L35 of one of the windings is L35 = (35/28) ² L28, while the total capacitance in parallel to the winding as mentioned can be approximated to Ctum / 22. The resonance fre- quency fo for the choke 67 will thus be the same as for the choke 64, i.e.

This means that the impedance of the common mode choke 67 shown in Figure 12d is the same as that of the common mode choke 64 shown in Fig ure 9d. Thus, also the common mode noise reduction of the common mode choke 67 is at an acceptable level. Although the common mode choke 67 seen from the outside (see Figure 12a) may look very similar to the common mode choke 62 (see Figure 6a) because the turns of the second layer are arranged in the middle of the first layer, the order of the turns is very different, and so is the resulting common mode noise reduction.

Also here, the turns of the second layer L2, i.e. turns number 18 to 24, may be arranged in the reverse order as that shown in Figures 12a-12c.

As mentioned above, in the common mode choke 67 shown in Figures 12a- 12c the second layer L2 consisting of turns number 18 to 24 is arranged in the middle of the first layer Li consisting of turns number 1 to 17 and 25 to 35, so that the turns of the second layer l_2 are wound around turns number 1 1 to 17. However, the impedance and thus also the common mode noise reduction of the common mode choke 67 will be unchanged if the turns of the second layer are not arranged in the middle of the first layer. Thus as exam ples, the second layer could consist of turns number 13 to 19 wound around turns number 6 to 12 or turns number 25 to 31 wound around turns number 18 to 24. More generally, as it is illustrated in Figure 12e, the first layer Li can here be considered as comprising two sections, i.e. a first section consisting of turns numbered from 1 to a first intermediate turn number N _a and a second section consisting of turns numbered from a second intermediate turn number Nb to the total number of turns N, which is also here equal to N1+N2. Naturally, the second intermediate turn number Nb is higher than the first intermediate turn number N _a . The second layer L2 thus consists of the N2 turns from number N _a +1 to N _a +N2, and it is wound around the highest numbered turns of the first section of the first layer, i.e. turn number N _a and the turn numbers just below turn number N _a .

The common mode chokes 63, 63a, 64, 66 and 67 shown in Figures 8a-8h, 9a-c, 1 1 a-c and 12a-c have their windings (W _ab ; Wed) arranged on a toroidal core (58), but as mentioned above, the windings may also be arranged on differently shaped magnetic cores, such as a rectangular-shaped core (as shown in Figure 2b), cores shaped as a straight cylindrical rod (as shown in Figure 2c) or other shapes.

The two windings W _ab and Wed may also be arranged differently in relation to each other instead of being arranged symmetrically and opposite to each other as shown in Figures 8a-8h, 9a-c, 1 1 a-c and 12a-c. Thus, they may be arranged closer to each other, or they may be partly or fully overlapping. The turns of the two windings may also be interleaved with each other. Further, it is noted that the common mode choke may also have more than two wind- ings, e.g. if the choke is designed to be used with a three-phase mains AC supply net. In all cases, the turns of each winding should be arranged as de scribed above. Further, in the common mode chokes 63, 63a, 64, 66 and 67 shown in Fig ures 8a-8h, 9a-c, 1 1 a-c and 12a-c, the turns of the layers are wound with the same (or opposite) pitch. Thus, if there are e.g. seven turns in the second layer, there will also be seven turns in the section of the first layer on which the second layer is wound, which will often be the case. Flowever, this is not a requirement for the invention. The layers may just as well be wound with different pitches. As an example, in the choke 63 of Figures 8a-8c the second layer l_2 consisting of the four turns number 29 to 32 could be wound with a higher pitch than the first layer Li, so that the section of the first layer on which the second layer is wound consists of the eight turns number 21 to 28. As long as the difference between the highest number of a turn in the second layer and the lowest number of a turn in the section of the first layer on which the second layer is wound is less than or equal to 50 % of the number of turns in the first layer, the common mode noise reduction will be acceptable. In other words, there is disclosed a common mode choke 63; 63a; 64; 66; 67 comprising a magnetic core 58; a first winding Wab of insulated wire having a number of turns wound on said magnetic core; and a second winding Wed of insulated wire having the same number of turns as the first winding wound on said magnetic core, wherein the turns of each winding Wab; Wed are num- bered consecutively; and the turns of each winding Wab; Wed are arranged in at least two layers, of which a first layer Li of turns is wound directly on said magnetic core and a second layer l_2 of turns is wound around a section of said first layer. The two layers of each winding Wab; Wed are arranged such that the difference between the highest number of a turn in the second layer l_2 and the lowest number of a turn in the section of the first layer Li on which the second layer l_2 is wound is less than or equal to 50 % of the number of turns in the first layer Li of that winding. When the difference between the highest number of a turn in the second lay er and the lowest number of a turn in the section of the first layer on which the second layer is wound does not exceed 50 % of the number of turns in the first layer of that winding, the effect of the parasitic capacitance between the layers is minimized, because the remaining turns of the first layer and the parasitic capacitances between them is still dominating in the determination of the resonance frequency and the impedance at the higher end of the fre quency range. Thus, the need for a larger core of the common mode choke can be eliminated, which may result in a smaller, more elegant and less ex- pensive enclosure of the device in which the common mode choke is used.

In an embodiment, the first layer Li consists of turns having consecutive numbers from turn number one to the number of turns Ni in the first layer Li; the second layer l_2 consists of turns having consecutive numbers higher than the number of turns Ni in the first layer Li; and the section of the first layer Li on which the second layer l_2 is wound comprises the highest num bered turns of the first layer Li. By arranging the second layer at the end of the first layer, a practical solution is achieved. The turns of the second layer l_2 may be arranged with the lowest numbered turn Ni+1 wound around the highest numbered turn Ni of the first layer Li and with numbers increasing in the direction away from this turn. Alternative ly, the turns of the second layer l_2 may be arranged with the highest num bered turn N1+N2 wound around the highest numbered turn Ni of the first layer Li and with numbers decreasing in the direction away from this turn.

The effect of the parasitic capacitance between the layers can be further min imized, and the resonance frequency and the impedance at the higher end of the frequency range thus further improved, when the turns of each winding are arranged in at least three layers, wherein a third layer L3 of turns is wound around at least a section of said second layer L2. In an embodiment, the first layer Li comprises two sections of turns, a first section consisting of turns having consecutive numbers below a first interme diate turn number N _a and a second section consisting of turns having con secutive numbers above a second intermediate turn number Nb, where said second intermediate turn number Nb is higher than said first intermediate turn number N _a ; the second layer l_2 consists of turns having consecutive num bers between said first intermediate turn number N _a and said second inter mediate turn number Nb; and the section of the first layer Li on which the second layer l_2 is wound comprises the highest numbered turns of the first section of the first layer Li. In this way, the second layer of turns can be placed in an arbitrary position on the first layer.

In an embodiment, the section of the first layer Li on which the second layer l_2 is wound has the same number N2 of turns as the second layer L2.

The magnetic core 58 on which the turns of the windings W _ab ; Wed are wound may be a toroidal core. This type of core is available from many different suppliers. In that case, the windings W _ab ; Wed may be arranged symmetrically on said toroidal core 58 and separated from each other. This increases the leakage flux between the two windings, and the resulting leakage inductance will ensure that the common mode choke will also have some differential mode impedance, which can provide a certain differential mode suppression as well at higher frequencies. Although various embodiments of the present invention have been described and shown, the invention is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims.