- Posted by doEEEt Media Group
- On September 12, 2022
Coils wound on toroids have, due to the closed magnetic circuit, high inductance and thus a good attenuation effect. If they are connected to a mains network to block or suppress interference of any frequency, the choice of inductor must be determined by the kind of interference that applies. This article discusses the construction and its impact on toroidal inductors, current compensated chokes, common mode chokes, and beads.
We have to deal with either common mode / asymmetrical interference or differential mode / symmetrical interference.
Suppose we have a noise or interference source in a network that produces interference currents via short circuiting parasitic capacitance in, for example, the load. In that case, the two interference types may be illustrated in Figure 1.
The cure for this kind of interference current might be using chokes combined with X- and Y- safety capacitors, i.e., capacitors intended for interference suppression in the mains. Asymmetrical interference is best rectified by a current compensated choke, where two counteracting windings cause the load current to create two opposing magnet fluxes. Those two fluxes cancel each other out. Any saturation risk is out of the question.
This method allows high permeability ferrites and subsequent high inductance. For asymmetric interference signals, the inductance of the two windings co-operates and produces approximately 3.5 times higher inductance than a separate winding.
Note that the opposing fluxes from the load current create certain magnetic stray fluxes around the coils. This is illustrated in following figure 3.
When there is a symmetrical interference (Figure 4.), we can’t use a current compensated choke. The fluxes of the load and interference currents will co-operate inside the toroid. In order not to come near magnetic saturation, ferrites with low to medium-high permeability must be used.
Current Compensated Chokes
Saturation effects caused by high signal currents, or DC currents superimposed on the signal, reduce the effectiveness of the choke. Using standard inductors in the signal path adversely impairs the useful signal. Current compensation circumvents these disadvantages. The “useful return current” must pass through the choke in current compensation. In this way, the useful current does not contribute to the magnetization of the core. See Figures 2. and 5.
Current-compensated chokes can be manufactured with different ferrite geometries; the best known are toroidal ring core and ribbed core. Different core materials enable their use in various frequency ranges. A very well-known component, but one not designed as a common mode choke, is the snap ferrite or the split ferrite sleeve.
The effect of current-compensated chokes on coupled interference is used for data and signal lines. They are often the only option to avoid the interference suppression component affecting the useful signal.
Current Compensated Choke Types
There is a wide range of current compensated choke types. Here is an overview of the most common types.
SMD Common Mode Noise Suppressor
The SMD Common Mode Noise Suppressor is usually not based on a ring core in size 0805 or 1206. Only for this reason is it possible to achieve such a compact current-compensated component. However, the stray field remains negligibly small because it is a closed ferrite material system. Typical applications for the SMD common mode noise suppressors are USB, Firewire, or High-Speed Data Lines.
NiZn Core Current-Compensated SMD Filter
The NiZn current-compensated SMD line filters offer high common mode impedance with a smaller footprint than the MnZn types. NiZn ferrite base material provides a wider working temperature range.
At the same time, the leakage inductance is lower, so the signal is less affected. The NiZn core current-compensated SMD line filters can also be used at high signal frequencies.
MnZn Current-Compensated SMD Filter with Separated Construction
The MnZn current-compensated SMD line filters can also be prepared with separated construction and both sectional and bifilar winding technology.
The separated construction of the sectional winding allows both the attenuation of high-frequency symmetrical frequency components and the suppression of asymmetrical interference components. However, if the quality of the useful signal is too greatly affected, the original bifilar winding technology should be chosen.
High-Frequency MnZn Current-Compensated SMD Filter
A special high-frequency design of manganese-zinc allows the frequency band in the single and double-figure megahertz range to be covered.
The special construction of the current-compensated chokes reduces undesirable parasitic effects. Careful core/winding relationship selection allows a very high current for a comparable footprint.
However, if conducting components or packaging parts are placed in the immediate vicinity, the required safety separation must be ensured on some types, as enameled wires are not considered insulated components. In most cases, this design’s use is unproblematic as insulated components, such as capacitors, provide the necessary separation.
Multi-chamber Current-Compensated Power Line Chokes
Current-Compensated SMD filter
In contrast to SMD common mode noise suppressors, the current-compensated SMD line filters include ring cores. As a result, stray fields can almost be excluded. The different geometries and, above all, the very flat package heights of the various types, offer potential solutions for every application. High current ratings, as used in low-voltage applications, are also available.
Despite their compact construction, the current-compensated SMD line filters can offer 4x current-compensated versions.
MnZn Current-Compensated SMD Filter
The MnZn current-compensated SMD line filter (such as the Würth Elektronik WE-SL1 series) excels by its low space requirement, both in terms of package height and as footprint.
The manganese-zinc basic material provides adequate balance attenuation values.
High-Density MnZn Current-Compensated SMD filter with Separated Construction
The MnZn current-compensated SMD line filter with separated construction can also be made in a high-density version that represents an advancement of the MnZn current-compensated SMD line filters. Despite the halved package height, almost the same performance can be attained, at least for low inductance values. A 3x current-compensated version has been developed, mainly used for low voltages.
Current Compensated Choke for Mains Voltage Applications
Multichamber current-compensated choke has roughly twice the leakage inductance relative to comparable toroidal chokes. The effect on symmetrical interference increases without having to use an additional inductor. Parasitic parallel capacitance is reduced as a result of the construction with a multi-chamber coil body.
The impedance profile is raised at higher frequencies. At the same time, resilience against burst and surge pulses is improved.
A special variant of the toroidal inductor is the so-called attenuation bead that becomes effective in the MHz range. It consists in its most simple form of a miniature toroid in low or high permeability ferrite or sometimes of iron powder. In alternative designs, the “bead” becomes a tube or, depending on the application, a double or multi-aperture ferrite core. In all these variants, the lead passes through the toroid. In designs where all space in the construction is consumed, and we find afterward that interference suppression measures are needed, a small ferrite bead may be the solution.
It increases the inductance of the lead/wire in the RF range and functions as an energy absorber, for example, transients. A simple formula for
estimating the induction factor AL of the bead is Equation  below.
AL= µ x 0.4π/c » µ x 0.2 x h x ln(D/d) 
h is height, D is outer diameter, and d is the inner diameter of the toroid; the measures are expressed in mm.
Example: µ = 750, h = 2.5 mm, D = 6.3mm, d=3.8 mm gives AL = 750 x 0.2 x 2.5 x ln(6.3/3.8) = 190 nH.
Ferrite beads also are manufactured as chips. Examples of the ferrite chip characteristics of two different materials are stated in Table 1.
Note how the flux density B has been reduced considerably at 100 °C, though the Curie temperature TC is considerably higher.
The following diagram shows how the different materials may depend on frequency.
Note how lower permeabilities improve the impedance at higher frequencies and how DC load, increasing the flux density B, lowers the impedance. To cover a broader frequency band, connecting in series ferrite beads with different materials might be necessary.
Finally, we should observe that ferrites have a certain conductance. In sensitive applications, it might be necessary to use isolated/lacquered beads.
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