Bruce Archambeault, PhD
IEEE Fellow, IBM Distinguished Engineer
Inductance and Partial Inductance What's it all
mean?
Bruce Archambeault, PhD 2
Inductance Probably the most misunderstood concept in
electrical engineering Do not confuse inductance with inductors
Common Usage Self inductance Loop inductance Mutual inductance Equivalent inductance Partial inductance Partial self inductance Partial mutual inductance Apparent inductance
Bruce Archambeault, PhD 3
Inductance Current flow through metal =
inductance! Fundamental element in EVERYTHING Loop area first order concern Inductive impedance increases with
frequency and is MAJOR concern at high frequencies
fLX L 2=
Bruce Archambeault, PhD 4
Current Loop = Inductance
Courtesy of Elya Joffe
Bruce Archambeault, PhD 5
Inductance Definition
Faradays Law = SdtBdlE
tBAV
=V
B
Area = A
For a simple rectangular loop
Bruce Archambeault, PhD 6
Given the Definition of Inductance
Do these have inductance?
Ground Strap SMT CapacitorPCB Via
Not until return path for current is identified!
Bruce Archambeault, PhD 7
Self Inductance
Isolated circular loop
28ln
00 r
aaL
++++
++= 22
0 1121121
1ln2 p
ppppaL
Isolated rectangular loop
Note that inductance is directly influenced by loop AREA and less influenced by conductor size!
radiuswiresideoflengthp =
Bruce Archambeault, PhD 8
Mutual Inductance
1
221
1212
IM
IM=
=
Loop #1Loop #2 ( ) =
2212 dSrBS n
r
How much magnetic flux is induced in loop #2 from a current in loop #1?
Bruce Archambeault, PhD 9
Flux from Current in Loop #1
Bruce Archambeault, PhD 10
Flux from Current in Loop #1
Bruce Archambeault, PhD 11
0 200 400 600 800 10000
0.5
1
1.5
2
X: 24Y: 1.835
Spacing between the coils(mils)
M
u
t
u
a
l
I
n
d
u
c
t
a
n
c
e
(
n
H
)
Change in mutual inductance with spacing
X: 100Y: 0.7312
X: 500Y: 0.02507
X: 1000Y: 0.01955
The magnetic field drops off rapidly, so then does the mutual inductance
Bruce Archambeault, PhD 12
Mutual Inductance
Loop #1Loop #2 Less loop area in loop #2
means less magnetic flux in loop #2 and less mutual inductance
Loop #1Loop #2 Less loop area perpendicular to
the magnetic field in loop #2 means less magnetic flux in loop #2 and less mutual inductance
Bruce Archambeault, PhD 13
Partial Inductance
We now know that a loop of current has inductance
We now know that there must be a complete loop to have inductance
But where do we place this inductance in a circuit?
Bruce Archambeault, PhD 14
Zero-to-One TransitionWheres the Inductance Go??
Power Supply
And how could you possibly calculate it?Courtesy of Dr. Clayton Paul
Bruce Archambeault, PhD 15
Lp4
Lp1 Lp3
Lp2
Mp1-3Mp2-4
Total Loop Inductance from Partial InductanceL total=Lp1+ Lp2 + Lp3 + Lp4 2Mp1-3 2Mp2-4
Courtesy of Dr. Clayton Paul
Bruce Archambeault, PhD 16
Partial Inductance
Simply a way to break the overall loopinto pieces in order to find total inductance
L3
L4
L2
L1
L total=Lp11+ Lp22 + Lp33 + Lp44 - 2Lp13 - 2Lp24
Bruce Archambeault, PhD 17
Important Points About Inductance
Inductance is everywhere Loop area most important Inductance is everywhere
Bruce Archambeault, PhD 18
ExampleDecoupling Capacitor Mounting
Keep vias as close to capacitor pads as possible!
Height above Planes
Via Separation
Inductance Depends on Loop AREA
Bruce Archambeault, PhD 19
Via Configuration Can Change Inductance
Via
Capacitor Pads
SMT Capacitor
The Good
The Bad
The Ugly
Really Ugly
Better
Best
Bruce Archambeault, PhD 20
High Frequency Capacitors
Myth or Fact?
Bruce Archambeault, PhD 21
What is Capacitance?
Capacitance is the ability of a structure to hold charge (electrons) for a given voltage
VQC = CVQ =
Amount of charge stored is dependant on the size of the capacitance (and voltage)
Consider a capacitor as a bucket holding lots of electrons!
Bruce Archambeault, PhD 22
Comparison of Decoupling Capacitor Impedance100 mil Between Vias & 10 mil to Planes
0.01
0.1
1
10
100
1000
1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10
Frequency (Hz)
I
m
p
e
d
a
n
c
e
(
o
h
m
s
)
1000pF
0.01uF
0.1uF
1.0uF
Bruce Archambeault, PhD 23
0603 Size Cap Typical Mounting
Via Barrel 10 mils
60 mils
20 mils
10 mils*
9 mils9 mils
10 mils*
108 mils minimum
128 mils typical*Note: Minimum distance is 10 mils but more typical distance is 20 mils
Bruce Archambeault, PhD 24
0402 Size Cap Typical Mounting
Via Barrel 10 mils
40 mils
20 mils
10 mils*
8 mils8 mils
10 mils*
86 mils minimum
106 mils typical*Note: Minimum distance is 10 mils but more typical distance is 20 mils
Bruce Archambeault, PhD 25
3.2 nH3.7 nH4.2 nH100
3.0 nH3.5 nH3.9 nH90
2.8 nH3.2 nH3.6 nH80
2.6 nH3.0 nH3.4 nH70
2.3 nH2.7 nH3.1 nH60
2.1 nH2.5 nH2.8 nH50
1.9 nH2.2 nH2.5 nH40
1.6 nH1.9 nH2.2 nH30
1.3 nH1.6 nH1.8 nH20
0.9 nH1.1 nH1.2 nH10
0402 typical/minimum
(106 mils between via
barrels)
0603 typical/minimu
m(128 mils
between via barrels)
0805 typical/minimum
(148 mils between via
barrels)
Distance into board
to planes (mils)
Connection Inductance for Typical Capacitor Configurations
Bruce Archambeault, PhD 26
Connection Inductance for Typical Capacitor Configurations with 50 mils from Capacitor Pad to Via Pad
4.6 nH5.0 nH5.5 nH100
4.3 nH4.7 nH5.2 nH90
4.0 nH4.5 nH4.9 nH80
3.7 nH4.2 nH4.5 nH70
3.5 nH3.9 nH4.2 nH60
3.1 nH3.5 nH3.9 nH50
2.8 nH3.2 nH3.5 nH40
2.5 nH2.8 nH3.0 nH30
2.0 nH2.3 nH2.5 nH20
1.4 nH1.6 nH1.7 nH10
0402 (166 mils
between via barrels)
0603 (188 mils
between via barrels)
0805 (208 mils
between via barrels)
Distance into board
to planes (mils)
Bruce Archambeault, PhD 27
PCB Example for Return Current Impedance
Trace
GND Plane
22 trace
10 mils wide, 1 mil thick, 10 mils above GND plane
Bruce Archambeault, PhD 28
PCB Example for Return Current Impedance
Trace
GND Plane
Shortest DC path
For longest DC path, current returns under trace
Bruce Archambeault, PhD 29
MoM Results for Current DensityFrequency = 1 KHz
Bruce Archambeault, PhD 30
MoM Results for Current DensityFrequency = 1 MHz
Bruce Archambeault, PhD 31
U-shaped Trace InductancePowerPEEC Results
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08
Frequency (Hz)
i
n
d
u
c
t
a
n
c
e
(
u
H
)
Bruce Archambeault, PhD 32
Two Wires in Parallel
Reduce inductance by factor of two? NO!
ppp
pppParallel MLL
MLLL
221
221
+=
2
21
ppParallel
ppp
MLL
LLL+=
==
Only if parallel wires are FAR APART!Courtesy of Dr. Clayton Paul
Bruce Archambeault, PhD 33
Lets Apply this to Decoupling Capacitors
Equivalent inductance Two capacitors vs one capacitor Relative location of two capacitors Use via between planes as ideal capacitor
Bruce Archambeault, PhD 34
What Happens if a 2nd Decoupling Capacitor is placed near the First
Capacitor?
Observation Point
Via #1Via #2 Moved in arc around Observation point while maintaining 500 mil distance to observation point
500 mils
distance
Bruce Archambeault, PhD 35
Second Via Around a circle
theta: angle as shown in the figure in degree
d2: distance between Port 2 and Port 3 in mil
d1: distance between Port 3 and Port 1 in mil
d: thickness of dielectric layer in mil
r: radius for all ports in mil
R: distance between Port 1 and Port 2 in mil
Courtesy of Jingook Kim, Jun Fan, Jim Drewniak
Missouri University of Science and Technology
Port 2
Port 1
( )yx,Port 3
( ) ( )( )
+
++
+++
rrdrRrd
drdrrdrRd
2
12
23
21
2
ln
ln
4ln
4
2sin22
1
RdRd
==
( )( )
++= 3
4
)2/sin(2ln
4 rrRrRd
1d
2d R
Port 2
Port 1
( )yx,Port 3
( ) ( )( )
+
++
+++
rrdrRrd
drdrrdrRd
2
12
23
21
2
ln
ln
4ln
4
2sin22
1
RdRd
==
( )( )
++= 3
4
)2/sin(2ln
4 rrRrRd
1d
2d R
Lequiv
Bruce Archambeault, PhD 36
Effective Inductance for Various Distances to Decoupling CapacitorWith Second Capacitor (Via) Equal Distance Around Circle
Plane Seperation = 35 mil -- Via Diameter = 20 mil
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
0 50 100 150 200
Angle (degrees)
I
n
d
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t
n
a
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e
(
p
H
)
250 mil
500mil
750 mil
1000 mil
Bruce Archambeault, PhD 37
Effective Inductance for Various Distances to Decoupling CapacitorWith Second Capacitor (Via) Equal Distance Around Circle
Plane Seperation = 10 mil -- Via Diameter = 20 mil
0
50
100
150
200
250
300
350
400
450
500
0 50 100 150 200Angle (degrees)
I
n
d
u
c
t
n
a
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e
(
p
H
)
500mil
250 mil
750 mil
1000 mil
Bruce Archambeault, PhD 38
Effective Inductance for Various Distances to Decoupling CapacitorWith Second Capacitor (Via) Equal Distance Around Circle
Plane Seperation = 5 mil -- Via Diameter = 20 mil
0
50
100
150
200
250
300
350
400
0 50 100 150 200Angle (degrees)
I
n
d
u
c
t
n
a
c
e
(
p
H
)
500mil
250 mil
750 mil
1000 mil
Bruce Archambeault, PhD 39
Understanding Inductance Effects and Proximity
1 via
10mm
2 via with degree 30
2 via with degree 90 2 via with degree 180
20cm
20cm
10cm
10cm
Bruce Archambeault, PhD 40
Current DensityA/m2
[m]
[m]
A/m2
[m]
[m]
A/m2
[m]
[m]
A/m2
[m]
[m]
Bruce Archambeault, PhD 41
Current Density in Planes8
8
8
8
8
8
16
16
1616
1624
24
24
24
32
32
32
40
40
40
48
48
48
56
56
56 646464
6472
72
8080
80
80
0.08 0.0850.09 0.095 0.1 0.1050.11 0.1150.120.08
0.085
0.09
0.095
0.1
0.105
0.11
0.115
0.12
8
8
8
8 8
8
16
16
16
16
16
24
24
24
24
32
32
32
32
40
40
40
4040
40
4848
48
48
48
48
56
56
5656
5656
6464
64
64
727272
72
728080
0.08 0.0850.09 0.095 0.1 0.1050.11 0.1150.120.08
0.085
0.09
0.095
0.1
0.105
0.11
0.115
0.12
8
8
8
8
8
816
16
16
16
1616
24
24
24 2432
32
32 32
40
40
40
48
48 4856
56
56
56
64
646 4
64 72
72
72
72
80
80
80
80
0.08 0.0850.09 0.095 0.1 0.105 0.11 0.1150.120.08
0.085
0.09
0.095
0.1
0.105
0.11
0.115
0.12
8
8
8
8
8
8
16
16
16
16
1624
24
24
2432
32
32
3240
40
40
40
40
40
48
4848
48
48
48
5656
56
56
56
56
64
64
64
6472
72
72
80 80
0.08 0.085 0.09 0.095 0.1 0.105 0.11 0.115 0.120.08
0.085
0.09
0.095
0.1
0.105
0.11
0.115
0.12
Bruce Archambeault, PhD 42
Effect of Plane width on Inductance
Case1 : 10 inches
Case2 : 5 inches
Case3 : 2 inches1 inch
Port1 Port2
Bruce Archambeault, PhD 43
Loop Inductance is Affected by Plane Width
~ 250pH
~ 330pH
~ 560pH
Case1 : 10 inches
Case2 : 5 inches
Case2 : 2 inches
Bruce Archambeault, PhD 44
Current Spreads in a Plane
Narrower planes means the multiple current paths are limited therefore effect of mutual inductance between
parallel paths increases!
Bruce Archambeault, PhD 45
Observations
Added via (capacitor) does not lower effective inductance to 70-75% of original single via case
Thicker dielectric results in higher inductance
Normalizing inductance to single via case gives same curve for all dielectric thicknesses
Bruce Archambeault, PhD 46
Summary Inductance has meaning only for current
loops Size of the loop has the most impact on
amount of inductance Current density also impact inductance Partial inductance is a very useful concept
to understand which portions of the loop have the largest impact on loop inductance