Sidactor Protection Thyristor Products Page 21 SIDACtor Protection Thyristor Products

SIDACtor Protection Thyristors 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 02/23/17 Table 1.1 Overshoot Caused by Trace Inductance V L = L (di/dt) SIDACtor device V S Protection Level (V L + V S ) Circuit A V L = 2.4 H (100 A/10 s) = 24 V 300 V 324 V Circuit B V L = 12 H (100 A/10 s) = 120 V 300 V 420 V 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 1.2 Values of Constants for the Geometric Mean Distance of a Rectangle t/w or w/t K Log e G 0.000 0.22313 0.0 0.025 0.22333 0.00089 0.050 0.22346 0.00146 0.100 0.22360 0.00210 0.150 0.22366 0.00239 0.200 0.22369 0.00249 0.250 0.22369 0.00249 0.300 0.22368 0.00244 0.350 0.22366 0.00236 0.400 0.22364 0.00228 0.450 0.22362 0.00219 0.500 0.22360 0.00211 0.550 0.22358 0.00203 0.600 0.22357 0.00197 0.650 0.22356 0.00192 0.700 0.22355 0.00187 0.750 0.22354 0.00184 0.800 0.22353 0.00181 0.850 0.22353 0.00179 0.900 0.22353 0.00178 0.950 0.223525 0.00177 1.000 0.223525 0.00177 Note: 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. Grounding Although often overlooked, grounding is a very important design consideration when laying out a protection interface circuit. To optimize its effectiveness, several things should be considered in sequence: 1. Provide a large copper plane with a grid pattern for the Ground reference point. 2. Decide whether to use a single-point or a multi-point grounding scheme. A single-point (also called centralized) grounding scheme is used for circuit dimensions smaller than one-tenth of a wavelength ( l = 300,000/frequency) and a multi-point (distributed) grounding scheme is used for circuit trace lengths greater than one-fourth of a wavelength. 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 [log e 2 r /(t+w) + - log e G] in H where r = length of trace G = function of thickness and width (as provided in Table 1.2) t = trace thickness w = trace width For example, assume circuit A is protected by a P3100SCL with a V S equal to 300 V and a ground trace one inch in length and a self-inductance equal to 2.4 H/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 s wave-form, the results would be as shown in Table 1.1: Telecommunications Protection (continued)

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