SMD Thin-Film Fuse

Thus, for current pulse in Figure 3b, the Joule integral is:

Fig. 3a. Trapezoidal pulse parameters for Joule

integral calculation, example #3.

4. Lightning strike

A lightning strike pulse is shown in Figure 4a. After an initial

linear rise, the current declines exponentially.

3. Trapezoidal current pulse

The Joule integral for a trapezoidal pulse is

3

see Fig. 3a.

t

l min.

l max.

0.5 msec/div

0.5A/div

Fig. 3b. Trapezoidal pulse, example #3.

be chosen for this application, see Figure 3c.

FUSE PRE-ARCING JOULE INTEGRALS

vs. PRE-ARCING TIME

0.50A

PRE-ARCING TIME, sec

100

10

1

110

x

Fig. 3c. Choice of 0.5A fuse, example #3.

X

X

DESIGNING FOR CURRENT PULSE SITUATIONS (CONT.)

Joule integral for the linear current rise is calculated as for a

triangular pulse, see example #2.

The Joule integral for the exponential decline is

I

max.

Thus, for the sample lightning strike pulse in Figure 4b, the

total Joule integral is:

0.51 max.

l max.

Fig. 4a. Lightning pulse parameters for Joule

integral calculation, example #4.

10 μsec/div

5A/div

Fig. 4b. Lightning strike pulse, example #4.

For practical calculations, the duration of exponential decline

may be assumed to be 3t

of the pulse energy is released. Thus, the total pulse duration

in this example is 30 μsec, and the 1.25A fuse should be

chosen for this application, see Figure 4c.

FUSE PRE-ARCING JOULE INTEGRALS

vs. PRE-ARCING TIME

Fig. 4c. Choice of 0.5A fuse, example #4.

1.25A

PRE-ARCING TIME, sec

100

10

1

110

x

3