Using soft ferrites for interference suppression

May 19th, 2015, Published in Articles: EngineerIT

 

In the field of electromagnetic compatibility several trends point to a growing necessity of EMC engineering. These trends, directed to functional upgrading or reducing cost, inevitably also contribute to an increasing level of electromagnetic interference (EMI) emissions.

The article covers general principles of EMC and EMC regulations, material specifications and EMI-suppression product lines. Applications and design considerations are also covered.

The trends are as follows: in signal processing  – change from analogue to digital (steep pulse edges, overshoot, ringing) and increase of clock frequencies.

In power conversion: change from linear to switch-mode supplies (high switching frequency, harmonics) and ncrease of switching frequencies.

Together with the increasing use of electronics this leads to a general EMC degradation and consequently EMC legislation is becoming stricter world-wide.

The most important regulations are the European Norms (EN) which are applicable in all European Union (EU) and European Free Trade Associated (EFTA) countries, FCC in United States and VCCI in Japan. The uniform legislation in the European Union is along the lines of the EMC directive 89/336/EEC. For every product to which no specific European norm applies, a general regulation is mandatory. These are the so called Generic Requirements (residential, commercial and light industry: EN 61000-6-3 for emissions and EN 61000-6-1 for immunity).

Of course the first step to avoid interference problems is a good design practice, to tackle the problem right from the start. This can be insufficient if the interference is directly related to the inherent operating principle and too late if the interference is detected not earlier than in the final design phase. In such cases extra suppression components are necessary, like ferrites, capacitors or shielding elements.

Ferrites provide a solution to many problems of conducted and (indirectly) radiated interference. They can be applied almost anywhere:

  • Shifted on wire or cable as beads, tubes or cable shields.
  • Mounted on PCB as beads-on-wire, wideband chokes, SMD inductors, multilayer suppressors or integrated inductive components.
  • Ring cores or U cores in mains filters, in the circuit, in a separate box or moulded in a connector.
  • Wideband chokes or coiled rod inductors in electrical appliances or motors.

No ground connections are necessary as ferrites are connected in series with the interfering circuit and not in parallel as in the case of a capacitor. The wideband, lossy impedance makes ferrites well-suited as RF suppressor componenst.

General principles of EMC

Regulations

Historically, all EMI regulations stated emission limits only. These define the maximum level of interference allowed  as a function  of frequency. In case of conducted interference it applies to the voltage on all inputs and outputs of the equipment, in case of radiated interference it applies to the field strength at a certain distance. Often two levels are stated:

  • Class A for commercial and industrial areas.
  • Class B for domestic and residential areas.

Class B is always stricter than class A. Also immunity is becoming subject of regulation. Taking into account the severity of the EMC problem, equipment must also be able to operate without functional degradation in a minimum EMI ambient. The difference between the actual level of emissions or susceptibility and the EMC limits is the required attenuation by filtering or shielding.

Sources and propagation

The source determines whether the interference is a transient or random variation in time (commutation motors, broadcast transmitters etc.) or a periodic signal (e.g. switched- mode power supplies).The frequency spectrum will be continuous in the first case and a line spectrum in the second. In practice, the minimum and maximum frequency involved are much more relevant and both types of sources can be broadband. Random variations are broadband if they are very fast, harmonic disturbances if the basic frequency is high or if the deviation from a sine wave is considerable.

Interferences can propagate as an electromagnetic wave in free space. Suppression then requires shielding with conductive materials. Also propagation occurs via conductive paths such as the mains network, to which the majority of electrical equipment is connected.

Below 30 MHz this is the main propagation mode. Suppression is done with a high impedance in series (inductor), a low impedance in parallel (capacitor) or a combination of both (filter).

Propagation via the mains can take place in two different modes: common and differential mode. Apart from phase and null which carry the supply current, there is the safety earth connection, which is generally taken as a reference.

Common-mode:

Phase and null interference voltages are equal. This is likely to occur if phase and null are close together and interference is coupling in from an external field (radiation or cross-talk).

Differential-mode:

Phase and null interference voltages have opposite phase angle but equal magnitude. This is likely to occur in case of switching equipment connected to the mains. In general a combination of both types can be present.

Suppression with ferrites

At RF frequencies  a ferrite inductor shows a high impedance which suppresses unwanted interference. The resulting voltage over the load impedance will be lower than without suppression component, the ratio of the two is the insertion loss (Fig. 1.)

Fig. 2 Insertion loss of an inductor.

Fig. 1 Insertion loss of an inductor.

The insertion loss is expressed as :

The decibel (dB) as a unit is practical because interference levels are also expressed in dB. However insertion loss depends on source and load impedance, so it is not a pure product parameter like impedance (Z). In the application, source and load impedance generally are not 50 Ω resistive. They might be reactive, frequency dependent and quite different from 50 Ω.

Insertion loss is a standardised parameter for comparison, but it will not predict directly the attenuation in the application.

At low frequency, a ferrite inductor is a low-loss, constant self-inductance. Interferences occur at elevated frequencies and there the picture changes. Losses start to increase and at a certain frequency, the ferrimagnetic resonant frequency, permeability drops rapidly and the impedance becomes almost completely resistive. At higher frequencies it even behaves like a lossy capacitor. While for most applications the operating frequency should stay well below this resonance, effective interference suppression is achieved up to much higher frequencies.

The impedance peaks at the resonant frequency and the ferrite is effective in a wide frequency band around it. The material choice follows from the critical interference frequencies; ideally they should coincide with the ferrimagnetic resonance frequency, the top of the impedance curve. According to Snoek’s law, this resonant frequency is inversely proportional to the initial permeability, which gives us a guide for material choice. The higher the interference frequency, the lower the material permeability should be. The whole RF spectrum can be covered with a few materials if the right permeability steps are chosen.

At the resonant frequency and above, the impedance is largely resistive, which is a favourable characteristic of ferrites.

  • Firstly, a low-loss inductance can resonate with a capacitance in series (positive and negative reactance), leading to almost zero impedance and interference amplification! A resistor cannot resonate and is reliable independent of source and load impedances.
  • Secondly, a resistance dissipates interfering signals rather than reflecting them to the source. Small oscillations at high frequency can damage semiconductors or negatively affect circuit operation and therefore it is better to absorb them.
  • Thirdly, the shape of the impedance curve changes with the material losses. A lossy material will show a smooth variation of impedance with frequency and a real wideband attenuation. Interferences often have a wideband spectrum to suppress.

Current-compensation

Ferrite inductors inserted separately in both lines suppress both common and differential mode interference. However, saturation by the supply current can be a problem. Remedies are a low permeability material, a gapped or open circuit core type. Disadvantage is the larger number of turns required to achieve the same inductance, leading to higher copper losses. All this can be overcome with current-compensation. Phase and null supply currents are opposite and have equal magnitude. If both conductors pass through the same holes in the ferrite core, the net current is theoretically zero and no saturation occurs. In other words, these currents generate opposite fluxes of equal magnitude that cancel out.

In practice, some stray flux will occur. The stray flux paths will not coincide and these fluxes do not cancel out.

Examples of current-compensated inductors

  • A ring core with two windings with equal number of turns.
    The winding directions are such that the incoming current through one winding and the equally large outgoing current through the other generate opposite fluxes of equal magnitude. Current-compensation would be almost ideal with both windings along the total circumference, one over the other. But in practical cases each winding is placed on one half of the ring core because of insulation requirements.
  • A twisted wire inductor, which is wound with the twisted wire pair as if it were a single wire.
  • A tube or round cable shield shifted on a coaxial  cable.
  • A flat cable shield, shifted on a flat cable. Here the net current of all inductors together is zero.

In case of an I/O cable, such as coax or flat cable, the problem will not be saturation by high current. The reason for the current-compensation is now that the actual signal is also of RF frequency and it would be suppressed together with the interference. The current-compensated inductor  has one limitation: it is only active against common-mode interference. However the small leakage inductance will also suppress some differential-mode interference.

Material specifications

There are different material categories:

  • Manganese-zinc ferrites (MnZn): These ferrites have a high permeability but also a low resistivity and are most effective at low frequencies. The ferrites 3S3 and 3S4 have a higher resistivity and are real wideband materials as well. 3S5 has been designed for high dc bias at high temperature.
  • Nickel-zinc ferrites (NiZn): These materials usually have a lower permeability but much higher electrical resistivity than the manganese-zinc ferrites and are effective up to 1000 MHz.  4S60 has the highest permeability and 4S3 was added for HF suppression.
  • Iron powder: Permeability of this material is also low but bandwidth is less than for nickel-zinc ferrites because of their low resistivity. Their main advantage is a saturation flux density which is much higher than for ferrites, so they are suitable for very high bias currents.

The main material parameters are given in the table in page 7 of the full article – link provided at end of this page, while the typical impedance curves are given in Fig. 2. For manganese zinc ferrites the frequency at which the impedance peaks, is given in Fig. 3.

Fig. 5 Impedance versus frequency for several ferrite materials. (measured on TN14/9/5 ring cores with 5 turns).

Fig. 2: Impedance versus frequency for several ferrite materials. (measured on TN14/9/5 ring cores with 5 turns).

 

Fig. 6  Frequency of impedance peak for some ferrite materials.

Fig. 3: Frequency of impedance peak for some ferrite materials.

Main material parameters

The impedance peak frequency versus permeability curve clearly confirms Snoek’s law. For the nickel zinc ferrites the same law is valid, but at high frequency the picture is more complex. Apart from resonant losses, eddy current losses will play an important role. They reduce the impedance at high frequencies for manganese zinc ferrites. For nickel zinc ferrites they are not very important below 100 MHz due to the much higher resistivity.

New materials

Some materials have been added in recent years :

  • Manganese-zinc ferrite 3S5: In order to meet the EMI regulations in the frequency range from 150 kHz up to 30 MHz, Ferroxcube has introduced its new 3S5 EMI suppression material. Although several ferrites  are available for this frequency range, hardly any material can keep its absolute value of complex permeability (defining the inductor’s impedance) when operating on a bias field (DC current) at high temperature. With the introduction of 3S5, Ferroxcube  is filling this gap. Applying 3S5 in an inductor  gives EMI suppression over the full frequency range and has the major benefit of sufficient permeability even when high bias currents  together with high temperature are applied.

Preferred applications

With the ever increasing demand of interference suppression, 3S5 can be applied in those applications where both high operating temperatures (140ºC) and high currents are involved e.g. power lines in industrial, but especially automotive environments.  Suppressing of interference signals along these lines can be achieved by inserting 3S5-based inductors.

  • Nickel-zinc ferrite 4S60: New EMI material  4S60 is the high permeability NiZn ferrite (µi = 2000) with high resistivity for EMI applications in the frequency range around 30 MHz. Due to its high permeability, 4S60 allows  reducing  size, if the upgoing slope of the impedance curve is important.  4S60 is recommended when wideband impedance is needed for noise filters, preferred applications are: line attenuation; current compensated chokes and common mode coils.
  • Manganese-zinc ferrite 3S3: Ferroxcube has introduced the high frequency EMI suppression material capable to attenuate unwanted interference up to 1 GHz, the 4S3 material.

With the ever increasing demand of EMI suppression materials for higher frequencies, the material 4S3 completes actual FXC EMI range materials providing  designers the capability of suppressing interference  up to 1 GHz. Beyond broadband impedance material 4S2, the 4S3 offer excellent impedance for higher frequencies being the attenuation optimum between 250 MHz and 1 GHz.

This article has been shortened.  For the full article see http://www.ferroxcube.com/FerroxcubeCorporateReception/datasheet/sfemisup.pdf

Contact Stuart Hanford, Arrow Altech Distribution, Tel 011 923-9600, shanford@arrow.altech.co.za

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