Application of Soft Ferrite Material:

from EMC to RFID

26 April 2012

Alan Keenan

Industrial Electronics GmbH in partnership with

HF Technology & Fair-Rite Products Corp.

www.fair-rite.com www.ie4u.eu

Topics of Discussion

Today's presentation aims to give an insight into some of the lesser known facts

about the use of soft ferrite materials in Electrical / Electronic applications

1. What is a soft ferrite ?

2. The Bias effect of differential mode ferrite filtration. (EMC)

3. Multi-turn configurations. (EMC)

4. Cancellation effects of common mode chokes. (EMC)

5. What about Inductive applications ?

6. LF RFID (Inductive)

7. HF RFID (Inductive)

8. LW & MW Radio Balun Transformer. (Inductive)

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1. What is a soft ferrite ?

Soft ferrite – does not have a residual magnetic field after an external

magnetic field has been removed. (hard ferrite = permanent magnet)

The use of the term “ferrite” refers only to “soft ferrite” in this presentation.

Ferrite is a ceramic material with a cubic crystalline structure with the

chemical formula MO.Fe2O3 where Fe2O3 is iron oxide and MO refers to a

combination of two or more divalent metal oxides. (i.e: zinc, nickel,

manganese and copper) The addition of such metal oxides in various amounts

allows the creation of many different materials whose properties can be

tailored for a variety of uses.

Generally, we can consider 2 basic classes of ferrite, NiZn ferrites and MnZn

ferrites. NiZn ferrites tend to be used for EMI suppression applications and

MnZn ferrites tend to be used for Inductive applications.

The complex permeability of

ferrite.

The magnetic permeability of ferrite is complex, it has both real and imaginary

parts.

U’s is real and gives the ferrite its Inductive or energy storage properties.

U’’s is imaginary and gives the ferrite its resistive or lossy properties. Complex permeability v’s frequency of Fair-Rite 43 material.

How does a ferrite reduce EMI Signals

Ferrite has crystalline structure and within the crystalline structure there exists magnetic domains.

When a magnetic field is applied the magnetic domains align themselves in the same direction as the magnetic field. (the domains already aligned in such direction also exhibit domain wall expansion)

Energy barriers have to be overcome in order for domain alignment and domain expansion to happen.

These energy barriers cause “absorption” or “attenuation” of the applied magnetic field.

2. The Bias effect of differential mode ferrite filtration

2508056017y0

Typical Z vs. F response of diff mode chip bead impeder. Showing Impedance, Inductive Reactance and Effective Resistance

2. The Bias effect of differential mode ferrite filtration

Same part with various levels of test bias currents, notice three things, peak Z amplitude, peak Z frequency and Q.

2508056017Y0

What are the implications of this ?

Impedance will not be as expected when there is a DC or AC signal present – attenuation levels will not be as expected – another design iteration may be required.

What is the solution ?

Use the Bias graphs for the initial design stage. (as per catalog)

Take into account the signal current and how that will affect the Impedance of the Ferrite Chip Bead Impeder.

2. The Bias effect of differential mode ferrite filtration

3. Multi-turn configurations

Cable Cores and Multi-line PC Beads can be “wound”, in multi-

turn configurations.

This can result in an N2 increase in Impedance, however there

will also be a shift in peak Z.

3. Multi-turn configurations

The Graph shows the effects of adding additional turns to a

14 x 6 x 28mm core in 43 material, why does this happen ?

The equivalent circuit of the

ferrite can be expressed as a

parallel resonant circuit with a

resonant frequency given by

the formulae below. Adding

further turns increases the

Inductance and the Inter-

Winding capacitance of the

circuit. Thus if L & C increase

fp must reduce.

3. Multi-turn configurations

Consider the standard 2 wire differential circuit. The black

signals represent differential mode signal (the intended

signal) and the red signals represent common mode

signals or CM noise.

Thus the differential mode signals are 180 degrees out of

phase and the CM signals are in phase.

Now consider using differential mode filtering for this

circuit. (on either or both lines of the circuit) Using

differential mode filtering can in some instances lead to

saturation of the ferrite core. (The DM and CM signals

may be in phase)

Finally, consider a Common Mode Choke rather than one

or two DM Chokes.

In a CM Choke configuration the majority of the DM Flux

will be cancelled in the ferrite core due to the fact that

these fluxes are 180 degrees out of phase and that they

are equal in amplitude. Thus only the CM Flux will remain

in the ferrite core, saturation of that core would be very

unlikely.

4. Cancellation effects of common mode chokes.

The previous slides were discussing using soft ferrite in EMC

Applications – to attenuate unwanted EMI signals.

The next section is about using soft ferrite for Inductive

Applications, where we do not want to attenuate signals.

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5. What about Inductive Applications?

What if we want the ferrite to be efficient and NOT lossy, what material to

select ?

This comes down to the frequency of the applied signal – what frequencies

do we want to move through the ferrite core without being suppressed?

Lets look at a few typical applications.

LF RFID

HF RFID

LW & MW Radio balun transformer

6. LF RFID

In LF RFID we have a centre frequency of ~125kHz to 134kHz.

In many LF RFID applications you will find a wound ferrite rod as the

antenna of the RFID tag. The ferrite rod is used to concentrate the

magnetic flux in a smaller cubic area.

So what material to use?

We want a material that is efficient and not lossy at 125kHz.

Typically, RFID tag manufacturers would use a 2000 to 2300ui MnZn

ferrite material as the core of the antenna.

6. LF RFID

An ideal ferrite for LF RFID is 78 material from Fair-Rite Products Corp.

78 Material has a ui of 2300 and is a MnZn ferrite grade.

7. HF RFID

HF RFID has a centre carrier frequency of 13.56MHz, it also has sidebands of

some 423kHz on either side of the carrier. In this application we need a material where the U’ is dominant and the U’’ is low.

For standard “Q” applications, the ideal material is 61 material – however, it

should be noted that 61 material is also good for EMI suppression in the 400MHz

to 2GHz area.

7. HF RFID

For higher “Q” applications where a wider read / write distance is required, we

need to use a lower permeability material (also lower U”)

The ideal soft ferrite for such applications is Fair-Rite 67 material

It has to be noted that due to HF RFID having sidebands, the “Q” cannot be too

high so as to cut off those sidebands.

8. LW & MW Radio “balun”

transformer

LW Radio operates in the 30kHz to 300kHz range.

MW Radio operates in the 300kHz to 3MHz range.

Typical application would be a “balun” transformer to balance the output Z of the power amplifier with the input Z of the antenna so as to ensure max power transfer.

What material for LW ?

What material for MW ?

8. LW & MW Radio “balun”

transformer

What material for LW ? (30kHz to 300kHz)

What material for MW ? (300kHz to 3MHz)

For LW, we would typically select 78 material (depends on narrow band or broadband etc)

For MW, we could select 43 material or 61 material.

43 material for 300kHz to 1MHz

61 material for 800kHz to 3MHz

(both 43 & 61 materials can be used for

EMI suppression of higher frequency signals)

Questions ?