Application of Soft Ferrite Material: from EMC to RFID

Transcription

1 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.

2 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)

3 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.

4 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.

5 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.

6 2. The Bias effect of differential mode ferrite filtration Typical Z vs. F response of diff mode chip bead impeder. Showing Impedance, Inductive Reactance and Effective Resistance y0

7 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 Y0

8 2. The Bias effect of differential mode ferrite filtration 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.

9 3. Multi-turn configurations Cable Cores and Multi-line PC Beads can be wound, in multiturn configurations. This can result in an N 2 increase in Impedance, however there will also be a shift in peak Z.

10 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?

11 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

12 4. Cancellation effects of common mode chokes. 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.

13 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.

14 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

15 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.

16 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.

17 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.

18 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.

19 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?

20 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?