Transcript 6.7 Ferrites and Common

Ferrites and
Common-Mode Chokes
• Magnetic field tend to concentrate in highpermeability(磁導率) 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.
• The quantity of reluctance(磁阻) R
depends on
– The permeability  of the
magnetic path.
– Cross-sectional area A
– Length l
• 用類比 lumped circuits 來分析
magnetic circuits
– Voltage  magnetomotive force
(mmf) NI
– Current  magnetic flux 
•High-permeability core: Rcore << Rair
–the majority of the flux confined to the core.
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.
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)
• 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.
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.
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.
A pair of wires carrying currents I1 and I2 are
wound around a ferromagnetic core.
Calculate the impedance
Consider common-mode currents (I1=IC, I2=IC)
 ZCM = p (L + M)
Consider differential-mode currents (I1=ID and I2=ID)
 ZDM = p (L  M)
If the windings are symmetric and all the flux
remains in the core
 L=M ; ZDM = 0
In the ideal case (L=M)
A common-mode choke
• has no effect on differential-mode current.
• but selectively places an inductance 2L in
series with the two conductors to commonmode currents.
Thus, common-mode choke can be effective
in blocking common-mode currents.
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.
Ferrite core materials have different frequency
responses of their permeability.
Typically: MnZn, NiZn
The impedance for a typical MnZn core
The impedance for a typical NiZn core
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 
6.8 Ferrite Beads
• Ferrite materials are basically
nonconductive ceramic(陶瓷) materials
• Ferrite materials can be used to
provide selective attenuation(衰減) of
high-frequency signals and not affect the
more important lower-frequency
components of the functional signal.
• 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.
• 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 differentialmode 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.
• 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
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.
From this result,
the equivalent circuit consists of
a resistance (dep. on frequency) in
series with an inductance (dep. on
frequency)
• 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.
END