Toroidal Inductors, Current Compensated Chokes, Common Mode Chokes and Beads

Coils wound on toroids have due to the closed magnetic circuit, a high inductance and thus a good attenuation effect. If they are connected into a mains network in order to block or suppress interference of any frequency the choice of inductor must be determined by the kind of interference that applies. This article discuss construction and its impact of toroidal inductors, current compensated chokes, common mode chokes and beads.

Interference Background

We have to deal with either common mode / asymmetrical interference or differential mode / symmetrical interference.

If we have a noise or interference source in a network that produces interference currents via short circuiting parasitic capacitance in, for example, the load, the two interference types may be illustrated with Figure 1.

The cure for this kind of interference current might be use of 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 so called 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-operate and produce 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 have to be used.

Current Compensated Chokes

Saturation effects caused by high signal currents, or DC current super imposed on the signal, reduce the effectiveness of the choke. The use of standard inductors in the signal path adversely impairs the useful signal. Current compensation circumvents these disadvantages. In current compensation, the “useful return current” must be passed through the choke. 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, above all, 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.

Special construction of the current-compensated chokes allow the reduction of undesirable parasitic effects. Careful selection of core/winding relationship 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 to be insulated components. In most cases, the use of this design, however, unproblematic as insulated components, such as capacitors, provide the necessary separation.

Multi-chamber Current-Compensated Power Line Chokes

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.

Attenuation Beads

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 application, a double or multiaperture 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 afterwards 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 [1] below.

Example: µ = 750, h = 2.5 mm, D = 6.3mm, d=3.8 mm gives A_L = 750 x 0.2 x 2.5 x ln(6.3/3.8) = 190 nH.

Ferrite beads also are manufactured as chips. Examples of ferrite chip characteristics of two different materials are stated in the following table 1.

Note how the flux density B has been reduced considerably at 100 °C, though the Curie temperature TC is situated considerably higher.
How the different materials may depend on frequency is shown in following diagram.

Note how lower permeabilities improve the impedance at higher frequencies as well as how DC load, increasing the flux density B, lowers the impedance. In order to cover a broader frequency band it might be necessary to connect in series ferrite beads with different materials.

Finally we should observe that ferrites have a certain conductance. In sensitive applications it might be necessary to use isolated/lacquered beads.