### Inductor - Samwha Capacitor Group

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Introduction
Inductors and ferrite are dual element of capacitors. They work by increasing the loop
impedance of the circuit (Fig. 1), the device impedance, Z X = 2π f × L , must be
larger than the series combination of Z S + Z L .
L
ZS
Cp
Rs
ZL
L
Rp
Fig. 1. Inductor filtering circuit and equipment elements.
The cutoff frequency of such lowpass filter is given by
Foff =
1
2π R S L
Where RS = series resistance of Z S , Z L .
The attenuation at any frequency above ILdB is given by
ILdB = 20 log(1 +
ZX
)
ZS + ZL
If Z X >> ( Z S + Z L )
ILdB ≈ 20 log (
ZX
)
Z X + ZL
Again, as for capacitors, one must be careful to use the actual values of Z S , Z L at
the calculation frequency, if these are not pure resistances.
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Since inductors are dual elements of capacitors, they suffer a mirror kind of limitation:
While with capacitors a resonance is created with the leakage inductances of the
terminal leads, an inductor will have a leakage resonance produced by the interturn
capacitance of its winding.
The inductor being placed in series in a power supply and signal circuit, the device
must be:
Capable of carrying the functional current without overheating or
unacceptable voltage drop.
Unaffected, not driven into magnetic core saturation, by the through current.
Ferrite Inductor
Introduction
Ferrite inductors has been regarded as the miracle remedies that the EMC magician
pulls out of his sleeve, resolving in seconds a problem that was lingering for months.
(Fig. 2)
Magnetic flux
Φ = B × l (r1 − r2 )
r1
I (max) ( saturation ) =
r2
l
Bmax l (r2 − r1 )
L
Bmax (typ.) = 0.1 to 0.2 tesla
Fig. 2. Basic parameters of a ferrite core.
The most interesting characteristic of an EMC ferrite core is its impedance vs.
frequency curve. The impedance of the ferrite bead is a complex one, which can be
expressed as
Z bead = R + j 2 π f L
In the above equation, R, the resistive term, depends on the material resistivity
and is related to the eddy currents losses. Therefore, it is a frequency-dependent
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term. L, the inductive term, depends on µr, the relative permeability of the material.
It corresponds to the reactive behavior, which is the one primarily sought in magnetic
application below the megahertz level. It is given by
L = 0.2 N 2 µ r l ln(
Where
D1
)
D2
N = number of times passes through the ferrite bead
D1, D2 = outside and inside diameters
Quite often, Manufacturer catalogs provide the AL value for a given toroid size and
material. AL is measure, in nanohenries, for one turn such that, for N turns,
L = AL × N 2 ( nH )
This N2 dependency is a theoretical ideal that generally is not met except at nearzero current. More realistically, practitioners found that
L ≈ AL × N 1.5 or 1.8
Combining equations:
AL = 0.2 µr l ln (
D1
)
D2
The value of µr, in general, is more modest than for a purely magnetic component
such as a transformer or a choke. Typical values of µr for EMC ferrites are in the 50
to 1000 ranges. But, in contrast with magnetic materials used at 50/60Hz and up to
few tens of kHz, the µr for ferrites keeps a stable value across a very wide frequency
range, e.g., 0.1 to 10 MHz, and even beyond 100MHz for some materials. The upper
limit of ferrite bead impedance is reached by either core saturation (too many
ampere-turns) or too high an EMI frequency where leakage capacitance starts to
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As the current increases, inductance L, tends to decrease, but so does the “R”
term. Typically, for a small or medium size ferrite bead, this decrease starts for
current in the ampere range; with a 0.5 times decrease in bead impedance for a few
amperes of DC bias.
It is interesting to note that the upper region of the bead impedance vs. frequency
curve is dominated by the resistive term R; i.e., the upper-frequency portion of the
EMI spectrum is dissipated into heat.
The ferrite performs as a loss inductor, whose insertion loss is approximately.
A (dB) ≈ 20 log (1 +
)
Z S + Z L +ZW
where Z S , Z L = source and load impedance of the circuit
Z W = wire impedance between source and load
Conversely, if source and load close to each other, becomes simply.
A (dB) ≈ 20 log (1 +
)
ZS + ZL
These equations reveal several things that should be considered when choosing
the ferrite as a possible solution:
1. Ferrite has little effect in high-impedance circuits.
2. Increasing N, the number of turns, ideally will multiply bead impedance by N2.
3. If the wire impedance connecting source impedance and load impedance is
already large (very resistive conductor, high self-inductance), the attenuation
may be disappointing because of the wire impedance term.
Differential Mode and Common Mode Inductors
Inductors used for EMC purposes are low-loss, core-wound components, for use
typically up to a few MHz, beyond which self-resonance makes them progressively less
efficient. Their most typical application is for filtering the AC input of switch-mode power
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circuits, the output of switch-mode power supplies (SMPSs). Beside their use as standalone devices, they are also commonly incorporated in most EMI filters. Just like their
dual element, the capacitor, they can be used against DM or CM interference. (Fig. 3.)
IDM
L
IDM
L
Fig. 3-1 Differential Mode Choke
L
I
I
L
L
L
L
L
Fig. 3-2. Common Mode Choke
Fig. 3. Differential Mode and Common Mode Choke.
1. For DM suppression, there must be one inductor per wire; therefore the winding
carries all of the DC or low-frequency components and the undesired higher
frequencies. As a result, the magnetic core can be easily saturate and, for staying
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within a reasonable volume and cost, the value of L is rather limited, as soon as
the normal DM current exceeds a few amperes.
2. For CM suppression, the inductor has a double winding. The four terminals are to
be connected in such a way that the DM current produces opposing magnetic
fluxes in the core, hence the other name for these chokes.
To the contrary, as the CM currents flow in the same direction, their fluxes also
have the same direction, and the core behaves as a single inductance with “two-wiresin-hand”(so called CM chokes). Since the normal DM current does not saturate the
core, this allows more filtering capability (more L) in a given core size.
3. Combined DM + CM chokes. By letting a certain leakage flux take place in the
device, a CM inductance with a certain DM value is created Typically, with such
chokes, LDM = 1 to 5% LCM. (Fig. 3.)
< 0.5 μH
< 0.5 Ω
< 0.5 μH
< 0.5 Ω
300 μH
Fig. 3. Common mode choke with associated leakage inductances.
Indication method
1. For conducted EMI suppression (CM and DM) in all sorts of switch-mode power
circuits (SMPS, variable speed drives, DC/AC inverters, etc) on mains input side
2. For DM chokes, when the EMI sources is a low-Z, high-current (typically above
0.5A) circuit
3. For ripple and EMI suppression on the output side of switch-mode power circuits
Installation method
1. Select an inductor with a generously rated current handling. Selection based on
rms current can be misleading when the peak current has a high crest factor.
2. Install as close as possible to the switching source.
3. Check that the leakage resonance frequency, Fres, is sufficiently above the DM or
CM noise frequencies and definitely well above the SMPS switching frequency, Fs;
typically.
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Fres > 5 FS
4. Prefer the packaging style with terminal lugs on opposite sides rather than side by
side.
5. Do not install near susceptible circuits or components (e.g., sensors, A/D
6. Beware of inductive kick at power-off. It may require a voltage transient protection
device if there are fragile components on the same line.
7. Check that the resistive voltage drop (for CM and DM chokes) caused by the
current stays within acceptable limits.
Ground Chokes
A ground choke is a special, low-value inductor put in series in the safety wire to
artificially increase the ground-loop impedance, thus reducing the circulation of groundloop currents via the interconnecting cables. The choke must have a value small
enough to remain practically a short at power line.
Indication method
1. Use against EMI in the range of a few kHz to a few MHz when either of the
following is the case:
The current probe shows similar CM currents in interconnecting cables and
safety wire or ground straps.
Interconnected equipments are grounded to safety line at different points that
may not be equipotent.
2. Use when the building or system safety ground bus is very polluted.
3. Use to decrease ground currents injected by the first harmonics of a switch-mode
power supply.
4. Use when it is impossible to float either the PCB zero volt or the chassis from
ground-loop problem is in the kHz to MHz region.
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References
1. Mardiguian, M., EMI Troubleshooting Techniques, New York :
McGraw-HILL, 2000.
2. Mardiguian, M., Controlling Radiated Emissions by Design. New
York : Van Nostrand Reinhold, 1992. (Title acquired by Kluwer
Academic Publishers, Norwell, MA., in 1999.)
3. IEC Immunity Standards 1000/4-2, -3, -4, -5, and –6, and Mil-Std461,462.
4. White, D.R.J., M. Mardiguian, Electromagetic Shielding. Gainesville,
VA : Interference Control Technologies, Inc.
5. Vance, E., Coupling to Shielded Cables. New York : John Wiley &
Sons.
6. White, D.R.J., M. Mardiguian, EMI Control Methodology and
Procedures. Gainesville, VA : Interference Control Technologies, Inc.
7. Keenan, K., Digital Design for Specification Compliance. Pinellas
Park, FL : The Keenan Corporation.
8. Tsaliovich, A., Shielding Electronic Cables for Electromagnetic
Compatibility. New York : Van Nostrand Reinhold.
9. Goedbloed, J., Aspects of EMC at Equipment Level, Proc. 1997 IEEE
EMC Symposium, Austin, TX (tutorial notes).
SAMWHA ELECTRONICS CO., LTD
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