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SIDACtor® Data Book and Design Guide

3. Because traces exhibit a certain level of inductance, keep the length of the ground trace

on the PCB 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:

V = L (di/dt)

L = 0.0051

r [loge 2 r/(t+w) +½ - logeG] in µH

where

r = length of trace

G = function of thickness and width as provided in Table 5.3

t = trace thickness

w = trace width

For example, assume circuit A is protected by a P3100SC with a VS equal to 300 V and a

ground trace one inch in length and a self-inductance equal to 2.4 µH/inch. Assume

circuit B has the identical characteristics as Circuit A, except the ground trace is five inches

in length instead of one inch in length. If both circuits are surged with a 100 A, 10x1000 µs

wave-form, the results would be as shown in Table 5.2:

Other practices to ensure sound grounding techniques are:

1. Cross signal grounds and earth grounds perpendicularly in order to minimize the field

effects of “noisy” power supplies.

2. 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 invariably is

connected first.

Table 5.2 Overshoot Caused by Trace Inductance

VL = L (di/dt)

SIDACtor device VS

Total protection level

(VL + VS)

Circuit A

VL = 2.4 µH (100 A/10 µs) = 24 V

300 V

324 V

Circuit B

VL = 12 µH (100 A/10 µs) = 120 V

300 V

420 V

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