Comparison Between Nanocrystalline and Common mode choke for RFI Filters

In most of the places of industrial and domestic power supplies, we meet nowadays some Switch Mode Power Supplies (SMPS) allowing tailored and volume reduced energy providers. As a consequence the occurrence of mutual electromagnetic interference between components strongly increase, leading to possible failures of electronic devices used in the neighborhood of each other such as computer, electronics for measurements and electrical machines. As well to control RF emission as to resist to interference, some standards such as EN 55011 provide standard values: peculiarly some frequency dependent limits of noise voltages

To control and calibrate conducted noise (<30MHz), the Radio Frequency Interference (RFI) Filters are used; these filters are based mainly on Common Mode Chokes (CMC) surrounded by capacitors and inductances. The noise spectrum from SMPS is generally concentrated from 20kHz to 200kHz, with discrete harmonic components, whereas the noise spectrum from inverters is spread from 10kHz to 30MHz and has to stay under limits depending on applications. The goal of CMC is to reduce the noise voltages below these limits, inside this frequency range.


A CMC is a magnetic core with two identical windings, wound in such a manner that the operating current which flows through these two windings provide for each one a field compensating in magnitude and direction the other one (same number of turns). Only a residual stray field can’t be avoided, inducing a differential mode magnetic flux which will be attenuated only slightly. On the contrary a RF noise ICM flowing in a winding in the opposite direction than I is not compensated at all. In this case the full core inductance Lc acts against the RF noise, providing the desired damping effect.

It comes thus:

                               (1)


with N the total number of turns of the CMC, S the iron cross section of the core, L the mean iron path length and µdyn the dynamic permeability of the core which depends strongly (see after) on the noise frequency f and the noise field strength N.ICM/L . To keep the maximum damping effect of Lc in working conditions, the design of the core has to prevent from magnetic saturation, which is to say B(f, N.I, N.ICM/L) < saturation.

It leads to the condition


Figure 2: symmetrical structure of 2 phase and 3 phase wound cores for Common Mode Choke

 
µdyn< Js.L / (N.ICM ) = Max(µdyn)           (2)
  

It comes out the rules for designing Common Mode Choke:

  •  To use a magnetic core with a high permeability µdyn to get high damping effect Lc (eq.1) in the required noise frequency spectrum; higher permeability promotes better noise attenuation characteristics over a wide frequency range: a good example is the Co-based amorphous alloys. 
  • To use a magnetic core with a high magnetic saturation Js which allows µdyn to be chosen higher (cf eq.2) and allows further increase of Lc; higher saturation directly raise the attenuation of high voltage pulse noise: a good example is Fe-based amorphous (high Js).
  • To fill the winding area at the center of the core and/or reduce the size of this area to get more compact component: increasing the number of turns N leads to both a beneficial raise of the damping effect (Lc) and to a lowering of the maximum accessible µdyn permeability (and then to a decrease of Lc). As a consequence a compromise in N is required, and the reduction in volume of the component is generally given by the length of wire diameters in one layer (see figure 2) which has to take its place on the core inner diameter.

The design optimization process will be explained in detail in the full paper.

Requirements for magnetic cores in Common Mode Chokes are the following ones:

  • high permeability µdyn in the range 10kHz – 10MHz.
  • high saturation Js to prevent saturation effects at high noise amplitude or at unbalanced currents.
  • high thermal stability up to 150°C
Core magnetic material Co-based amorphous Fe-based amorphous MnZn ferrite Fe-based Nanocrystalline
Max relative permeability µr (10kHz) ≥ 100,000  20,000 – 30,000 1000 – 15,000 20,000 – 200,000
Saturation induction Js (T) 0.6T 1.6 - 1.8T <0.5T 1.25T
Curie temperature (°C) 210°C   ≈ 700°C 220°C  ≈ 320°C
(amorphous phase)
Max. working temperature (°C)   90°C 100°C <120°C 150°C

Table 1: main characteristics of magnetic materials for Common Mode Choke

Recently the mergence of nanocrystalline materials in Power electronics applications move the choice of component designers toward more compact structure when comparison is made with previous magnetic materials for CMC such as Co-based or Fe-based amorphous, MnZn ferrite and 80%Ni permalloys. The main requirements for CMC are very well fulfilled with nanocrystalline materials in terms of high dynamic permeability, high saturation, high thermal stability (see table), providing strong ability to reduce substantially the CMC volume.

       This paper aims to detail the design of CMC, to compare actual competing solutions (ferrite and nanocrystalline) in terms of dynamic permeability, insertion attenuation and final component volume as a function of the technological choice made by designers (N, S,L). The main results are briefly presented hereinafter:

Insertion loss and the main designing features:
The insertion loss – also called insertion attenuation – represent the damping ability of the CMC, defined as Ins.Loss= 20log(V1/V2) where V1 is the voltage when the CMC was not inserted between the signal generator and the level meter, and V2 is the voltage when it was inserted between the signal generator and the level meter. The voltage ratio V1/V2 is directly connected to the impedance Z of the CMC, and then in the lower frequency range (0.01 – 1MHz) the insertion loss will be determined by the inductance of the CMC, whereas in the higher frequency range (1 – 100MHz) the insertion loss is mainly dependent on winding capacity. As a consequence at low frequencies, L and the damping effect are improved (see eq. 1) by an increase of the number N of turns (see figure 3 for nanocrystalline with a constant µr = 20 000) or by an increase of permeability (see figure 4 for ferrite and Nanocrystalline with N=50 turns)

Figure 3: Insertion loss versus frequency in the case of a Common Mode Choke for RFI filter, with a magnetic nanocrystalline wound core made of Nanocrystalline - µr = 20 000 – and three possible numbers of turns
It can be observed from Figure 4 when ferrite N30 and nanocrystalline - µr=20 000 are compared, that in spite of the higher permeability of the latter (dynamic permeability magnitude is going on the contrary than cut off frequency) this one (nanocrystalline) exhibits the advantage of a damping effect acting in a wider frequency band: this advantage is taken from large eddy currents effects in nanocrystalline materials and can be observed when capacitive effects remains at a rather low magnitude (case of low number N of turns) as in Figure 4. When N increases (figure 5), capacitive effects become rapidly predominant near resonance and then the exactly same insertion attenuation behavior is obtained for high frequencies.

Figure 4: Insertion loss versus frequency in the case of a Common Mode Choke with 50 turns for RFI filter, with a magnetic core made of nanocrystalline wound - µr = 20 000 or µr = 200 000 – or N30 ferrite -µr = 4600.

Figure 5: Insertion loss versus frequency in the case of a Common Mode Choke with 100 turns for RFI filter, with a magnetic core made of nanocrystalline wound - µr = 20 000 or µr = 200 000 – or N30 ferrite -µr = 4600.
These comparisons between CMC-solutions have been made to point out the different frequency behaviors of insertion loss, depending on nature of magnetic materials, permeability and number of turns; but till to now no requirement about non-saturation and volume reduction has been taken into account in such a description process. It’s the goal of the second part: to compare the ability of each material to reduce the core volume, taken into account the whole requirement of CMC

Compare the ability for each magnetic material to reduce the volume of the core: use of a designing tool:
A simple software is used as a tool to design industrial CMC starting from ferrite material and from nanocrystalline material; the design process describe the needs of non saturation under the low frequency load, of standing above minimum impedances at specified frequencies (typically 10 and 150kHz), reducing the volume of the core. A multi-variable computation is made to get the best technological and economical compromises between core volume and dimensions, number of turns, wire diameter, number of wire layer… From calculations, it comes out a proper comparison between ferrite and nanocrystalline as shown in table 2 in some industrial cases where the volume of the component is imposed and the volume of the core has to be minimized.
         It clearly points out that the volume of the core can be reduced by 50 to 80% when an appropriated ferrite is replaced by a well-chosen nanocrystalline core, as a result of its superimposed advantages of high and tailored permeabilities, high saturation and inductive behavior near CMC resonance.
Case Magnetic material – Commercial product Nominal current RMS  (in A) Max Common Mode current (in A) Insulated wire diameter (in mm) Imposed Diameter x height for CMC component µdyn at 1kHz Outer/ Inner diamete /Height (mm) Number of turns X Number of layer Core volume Reduction from ferrite
Case No 1 Nanocrystalline 10 A 
(2 phase) 		0.25 A 1.9 48x25 mm2 18,000 40/36/17 29x2 56%
Ferrite 3E25 6,000 40/30/17 27x2
Case No 2 Nanocrystalline 55.9 A   
(3phases) 		0.25 A 1.5 48x25 mm2 18,000 41/35/17 30x2 54%
Ferrite 3E5 77,500 41/26/18 21x2
Case No 3 Nanocrystalline 27.7 A
(2phases) 		0.4 A 3.1 35x30 mm2 18,000  25/22/17 13x2 67%
Ferrite N30 4,600  25/19/28 11x2
Case No 4 Nanocrystalline 10.6 A
(3phases) 		0.15 A 2 48x35 mm2 50,000  39/29/26 15x2 59%
Ferrite 3E5 7,500  39/29/64 15x2
Case No 5 Nanocrystalline 71 A
(3phases) 		0.4 A 5.14 80x45 mm2 50,000  59/53/24 9x2 79%
Ferrite 3E5 7,500 59/39/40 9x2

Table 2: Comparison between nanocrystalline and ferrite dedicated to final core volume after designing from imposed component volume

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