Maintaining Impedance in
Split Core Ferrites for
Low-Frequency Applications
Rachael Parker
Vice President of Fair-Rite Products Corp.
This presentation is brought to you by:
Rachael Parker
Vice President
Fair-Rite Products Corp.
Joining Fair-Rite in 2014,
she holds a BS in Electrical Engineering,
an M.Eng in Electrical Engineering, and an
MS in Engineering Management.
Rachael spent her early career in product
development, project leadership, and
program management.
• Founded in 1952 – family owned and operated in US and China
• Providing ferrite components for the electronics industry
for over 60 years
• Wide product offering and materials for:
– EMI Suppression
– RFID-Antennas
– Power/Inductive
– Value Added Services (machining,
winding, assemblies)
• Visit our new website at
www.fair-rite.com
The challenge…
• So you’ve made it to the test lab to get your
product certified – but you fail!!
– The dreaded questions:
• What is the fix?
• How long will it take?
• How much will it cost??
Until now…
• To solve low-frequency EMC issues,
Snap-It ferrites were not a viable option
500
450
61
400
350
300
Impedance
(ohms) 250
200
43
Mind the Gap!
31
150
100
50
0
100 kHz
1 MHz
10 MHz
Frequency (Hz)
100 MHz
1 GHz
Review: How Ferrites Reduce Noise
Zs
Zs
Zsc
ZL
Zsc
ZL
= Source impedance
= Suppressor Core impedance
= Load impedance
Impedance Formula
Material permeability is a
frequency dependant
characteristic
10000.0
µ'
µ''
1000.0
Permeability
Ls = series inductance
Rs = series loss resistance
Lo = air core inductance
µ = material permeability
Permeability of 75-material
100.0
10.0
1.0
10,000
100,000
1,000,000
Frequency (Hz)
10,000,000
100,000,000
Impedance Formula
• Generally, as the initial permeability (µi) increases,
the impedance frequency range decreases
Material
µi (initial permeability)
Frequency Range
75
5000
150 kHz – 10 MHz
31
1500
1 MHz – 300 MHz
43/44
800/500
25 MHz – 300 MHz
61
125
200 MHz – 1 GHz
Initial permeability refers to µ when
magnetization levels extremely small
Impedance Formula
Ls = series inductance
Rs = series loss resistance
Lo = air core inductance
µ = material permeability
ID
O
D
h
[Dim– mm]
Ae = effective cross-sectional area
le = effective magnetic path length
µ0 = permeability of free space
N = number of turns
Impedance Formula
• Core performance also depends on geometric and
implementation parameters as part of Lo:
Parameter
Description
As parameter is
increased,
Z will…
N
Number of times the cable
passes through aperture the
core
Increase by N2
OD
Outer Diameter of the core
Increase linearly
ID
Inner Diameter of the core
Decrease linearly
H
Height of the core
Increase linearly
Putting it all together…
Impedance of 75-material solid core
0.000030
180
Ls
Rs
0.000025
180
160
160
140
140
120
120
100
0.000010
80
Magnitude (Ω)
0.000015
Rs (Ω)
Ls (H)
0.000020
100.00
Z(mag)
Z(phase)
60.00
100
40.00
80
20.00
60
0.00
60
0.000005
40
40
0.000000
-0.000005
10,000
80.00
Phase (Degrees)
Ls and Rs for 75-material solid core
20
100,000
1,000,000
Frequency (Hz)
10,000,000
0
100,000,000
-20.00
20
0
10,000
100,000
1,000,000
10,000,000
-40.00
100,000,000
Frequency (Hz)
• Low frequencies  Ls is main driver of impedance.
• High frequencies (> 1MHz) Rs is main driver of impedance
11
Solid Suppression Core Comparison
500
26--540002 size 14.3OD x 6.35ID x 28.6Length (mm)
450
61
400
350
300
43
Impedance
(ohms) 250
200
150
75
31
1 MHz
10 MHz
100
50
0
100 kHz
Frequency (Hz)
100 MHz
1
Challenges
Test facilities had implementation difficulties with solid cores.
Challenges
Performance was degraded in split cores due the
high permeability material (e.g., µi = 5000)
Impedance with an Air-Gap
• Air-gap in the magnetic circuit
reduces the inductance.
– le needs to be considered
separately.
• Core behaves as if it has
reduced permeability, known as
its effective permeability.
– Permeability is dependant on both
the magnetic path length and the
air gap.
lg
le
Ae
Solid Core
le
Ae
Split Core
Impedance with an Air-Gap
• Assuming gap length (lg) << magnetic path lengh (le)
tan(δ) refers to the magnetic loss
tangent and represents the
inefficiency of the system based on
the angle between the µ’ and µ’’.
Effect on Permeability
Initial versus Effective Permeability
10000
µ' - no gap
µ'' - no gap
µe' - std grind
1000
Permability
µe'' - std grind
100
10
1
10000
100000
1000000
Frequency
10000000
100000000
So what??
• For a high-permeability such as 75-material, even the
smallest gaps can degrade performance tremendously!
100%
Percent of No-Gap Impedance vs. Gap Length
(frequency = 1 MHz)
% of no-gap impedance
90%
80%
75 material (140 Ω)
70%
31 material (43 Ω)
60%
44 material (41 Ω)
50%
61 material (4 Ω)
40%
30%
20%
10%
0%
0.000
0.001
* Based on a specific core size
0.002
0.003
Gap Length (cm)
0.004
0.005
Minimizing the air-gap
Impedance of 75-material core with air-gaps
200
Z(mag) - no gap
180
Z(mag) - fine grind
160
Z(mag) - std grind
Magnitude (Ω)
140
120
100
80
60
40
20
0
10,000
100,000
1,000,000
Frequency (Hz)
10,000,000
100,000,000
The solution!
Design Considerations
• DC Bias
– Derating can occur at high current levels, so
ferrites are ideal for common-mode noise issues.
– When applicable, both current-carrying conductors should pass
through the core.
Impedance versus Magnetic Field Strength
120
Percent Original Impedance (%)
100
80
60
5MHz
3MHz
40
1MHz
0.5MHz
20
0
0.0
0.3
0.5
0.8
H(oersted)
1.0
1.3
Measured on an 18/10/6mm toroid.
1.5
Design Considerations
• Operating Temperature
– µi for 75-material can change 0.6% per degree Celsius in an
operating range from 20 - 70C.
– µi for 31-material can change 1.6% per degree Celsius in an
operating range from 20 - 70C.
Impedance versus Temperature (from 25°C)
Percent Original Impedance (%)
125
0.5MHz
100
1MHz
75
5MHz
3MHz
50
25
0
-40
-20
0
20
40
Temperature (C)
60
80
100
Measured on an 18/10/6mm toroid.
120
Thank You!
• Questions?