Ferrites and Common-Mode Chokes

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Ferrites and Common-Mode Chokes
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• Magnetic field tend to concentrate in
  high- permeability(???) materials.

e.g. The magnetic flux ? was confined to the
ferromagnetic core.

• Some of the flux leaks out and completes the
  magnetic path through the surrounding air.

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• The quantity of reluctance(??) R depends on
• The permeability ? of the
• magnetic path.
• Cross-sectional area A
• Length l
• ??? lumped circuits ??? magnetic circuits
• Voltage ? magnetomotive force
• (mmf) NI
• Current ? magnetic flux ?

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• High-permeability core Rcore ltlt Rair
• the majority of the flux confined to the core.

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• The reluctances of the path
• ? the permeablities of the path.
• The portion of the total flux that remains in the
  core
• ? the ratios of the relative permeablities of
  the two paths.

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• Common-mode Differential-mode current

Consider the pair of parallel conductors
carring current I1 and I2. Decompose with
differential-mode current ID and common-mode
current IC.

I1 IC ID I2 IC ? ID
ID 0.5 ( I1 ? I2) IC 0.5 ( I1 I2)
?
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• The differential-mode currents
• are equal in magnitude but oppositely directed in
  the two wires.
• The common-mode currents
• are equal in magnitude and are directed in the
  same direction.

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• The differential-mode current are oppositely
  directed.
• ? The resulting electric field will also be
  oppositely directed.

Two conductors are not collocated. ? The fields
will not exactly cancel. ? It will subtract to
give a small net radiated electric field.
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• The common-mode currents are directed in the same
  direction.
• Their radiated fields will add giving a much
  larger contribution to the total radiated field
  than will the differential-mode current.

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• A pair of wires carrying currents I1 and I2 are
  wound around a ferromagnetic core.

• Calculate the impedance

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Consider common-mode currents (I1IC, I2IC) ?
ZCM p (L M)
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Consider differential-mode currents (I1ID and
I2ID) ? ZDM p (L ? M)
If the windings are symmetric and all the flux
remains in the core ? LM ZDM 0
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• In the ideal case (LM)
• A common-mode choke
• has no effect on differential-mode current.
• but selectively places an inductance 2L in series
  with the two conductors to common-mode currents.
• Thus, common-mode choke can be effective in
  blocking common-mode currents.

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• Ferromagnetic materials
• ''saturation effect'' at high currents
• Their permeabilities tend to deteriorate with
  increasing frequency.
• The functional or differential-mode current ID
  are the desired currents and usually large in
  magnitude.
• The common-mode choke
• Fluxes (due to high differential-mode currents)
  cancel in the core.
• No saturation.

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Ferrite core materials have different frequency
responses of their permeability.
Typically MnZn, NiZn
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• The impedance for a typical MnZn core

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• The impedance for a typical NiZn core

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• The frequency response of the impedance of
• a inductor (formed by winding five turns of 20
  gauge wire on two toroids)

1 MHz 60 MHz MnZn 500? MnZn 380 ? NiZn 80
? NiZn 1200 ?
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6.8 Ferrite Beads
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• Ferrite materials are basically
• nonconductive ceramic(??) materials
• Ferrite materials can be used to
• provide selective attenuation(??) of
  high-frequency signals and not affect the more
  important lower-frequency components of the
  functional signal.

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• The most common form of ferrite materials is a
  bead.

• The ferrite material is formed around a wire, so
  that
• the device resembles an ordinary resister.

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• The ferrite bead can be inserted in series with a
  wire or land, and provide a high-frequency
  impedance in that conductor.
• The ferrite bead affects both differential- mode
  and common-mode currents equally.
• If the high-frequency components of the
  differential-mode current are important from a
  functional standpoint, then the ferrite bead may
  affect functional performance of the system.

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• The current produces
• magnetic flux in the
• circumferential direction.
• This flux passes through the bead material
• producing an internal inductance.
• The inductance
• ? the permeability of the bead material
• Lbead ?0?rK
• K const, dep. on the bead dimension

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• The bead material is characteristized by
• a complex relative permeability
• ?r ?'r(f) ? j ?"r(f)
• The real part ?'r
• is related to the stored magnetic energy in the
  bead material.
• The imaginary part ?"r
• is related to the losses in the bead material.
• ?'r ?"r both are functions of frequency.

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• From this result,
• the equivalent circuit consists of
• a resistance (dep. on frequency) in series with
  an inductance (dep. on frequency)

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• Typical ferrite beads give impedances of order
  100? above 100MHz.
• Multiple-hole ferrite beads can be used to
  increase the high-frequency impedance.

• The impedance of ferrite beads is typically used
  in low-impedance circuits.
• Ferrite beads and the other uses of ferrites are
  susceptible to saturation when used in circuits
  that pass high-level, low-frequency currents.

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END