SIDACtor
Data Book
PCB Layout
Teccor Electronics
(972) 580-7777
5 - 15
T
greater than one-fourth of a wavelength.
Finally, because traces exhibit a certain level of inductance, the length of the ground
trace on the PCB should be kept as short as possible in order to minimize its voltage
contribution during a transient condition. In order to determine the actual voltage
contributed to trace inductance, use the following equations:
1. V=L (dI/dt)
2. L=0.0051
ρ
[log
e
2
ρ
/(t+w) +
- log
e
G] in μH
ρ
= length of trace
G = is a function thickness and width as is given below
t
= trace thickness
w = trace width
As an example, assume circuit A is protected by a P3100SC with a V
S
equal to 300V
and a ground trace one inch in length and a self-inductance equal to 2.4μH/inch.
Assume circuit B has the identical characteristics as the first, except the ground trace
is 5 inches in length instead of 1 inch in length. If both circuits are surged with a 100A,
10x1000μs wave-form, the results would be as follows:
Other practices to ensure sound grounding techniques are:
1. Always cross signal grounds and earth grounds perpendicularly in order to
minimize the field effects of
“
noisy
”
power supplies.
2. Always make sure that the ground fingers on any edge connector extend farther out than
any power or signal leads in order to guarantee that the ground connection is invariably
connected first.
Values Of Constants For The Geometric Mean Distance Of A Rectangle
Notes: Sides of the rectangle are t and w. The geometric mean distance R is given by:
log
e
R = log
e
(t+w)-1.5+log
e
G. R=K(t+w), log
e
K = -1.5+log
e
G.
V
L
= L (di/dt)
SIDACtor V
S
Total protection level
(V
L
+ V
S
)
Circuit A
V
L
=2.4 μH (100A/10(s) =24V
V
L
=12 μH (100A/10(s) =120V
300V
324V
Circuit B
300V
420V
t/w or w/t
K
Log
e
G
t/w or w/t
K
Log
e
G
t/w or w/t
K
Log
e
G
.0
0.22313
.0
0.350
0.22366
0.00236
0.70
0.22355
0.00187
0.025
0.22333
0.00089
0.400
0.22364
0.00228
0.75
0.22354
0.00184
0.050
0.22346
0.00146
0.450
0.22362
0.00219
0.80
0.22353
0.00181
0.100
0.22360
0.00210
0.500
0.22360
0.00211
0.85
0.22353
0.00179
0.150
0.22366
0.00239
0.50
0.22360
0.00211
0.90
0.22353
0.00178
0.200
0.22369
0.00249
0.55
0.22358
0.00203
0.95
0.223525
0.00177
0.250
0.22369
0.00249
0.60
0.22357
0.00197
1.00
0.223525
0.00177
0.300
0.22368
0.00244
0.65
0.22356
0.00192
.0
.0
.0