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Accu-Guard
®
SMD Thin-Film Fuse
The Joule integral for triangular pulse is [(Imax.)
DESIGNING FOR CURRENTPULSE SITUATIONS
1. Sine wave current pulse
2 x t]/3,see Fig. 2a. The Joule integral for sine wave pulse is[(I
2 max. ) x t]/2, see Fig. 1a.
tl max. tl max. Fig. 2a. Triangular pulse parameters for Jouleintegral calculation, example #2. Fig. 1a. Sine wave pulse parameters for Joule integral calculation, example #1.Thus, for the current pulse in Figure 1b, the Joule integral is[(4.8A) Thus, for the current pulse in Figure 2b, the Joule integral is[(1.5A)
2 x 7.7 x 10
-6 sec]/2 = 8.9 x 10
-5 A
2 sec.
The pulse duration is 7.7µsec. We must find a fuse that canabsorb at least 8.9 x 10
2 x 3 x 10
-3 sec]/3 = 2.25 x 10
-3 A
2 sec.
10 µ sec/div1A/div 2 msec/div0.5A/div Fig. 1b. Sine wave pulse, example #1. Fig. 2b. Triangular pulse, example #2.The pulse duration is 3 msec. In the I
2 t graph on page 6, pre-arcing Joule integral for 3 msec pulse is 4 x 10
-5 X 2 = 1.8 x 10
-4 A
2 sec Joule inte-gral within 7.7 µsec without actuation. According to the I
-3 A
2 sec for the0.5A fuse (not enough) and 2 x 10
2 tgraph on page 6, pre-arcing Joule integral is 2.3x10
-2 for the 0.75A fuse (morethan enough). Therefore, 0.75A fuse should be chosen for
this application, see Figure 2c.
-4 A
2 secfor the 0.5A fuse, which is slightly more than needed. The
next lower rating (0.375A), has only 6x10
-5 FUSE PRE-ARCING JOULE INTEGRALSvs. PRE-ARCING TIME A
2 sec, which isnot enough. Therefore, 0.5A fuse should be chosen for this
application, see Figure 1c. FUSE PRE-ARCING JOULE INTEGRALSvs. PRE-ARCING TIME
2. Triangular current pulse
PRE-ARCING TIME l 2 t, A 2 sec
PRE-ARCING TIME, sec
10010110 PRE-ARCING TIME l 2 t, A 2 sec
PRE-ARCING TIME, sec
10010110 -1 -2 0.75A 10 10 -3 x -1 10 -4 10 -2 10 -5 -3 0.5A 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 110 10 10 -4 x 10 -5 Fig. 2c. Choice of 0.75A fuse, example #2.Pre-arcing I
10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 110 2 tMaximum I
2 Fig. 1c. Choice of 0.5A fuse, example #1.Pre-arcing I t design ruleI
2 tMaximum I
X 2 t for sample switch-on pulse
2 t design ruleI
X 2 t for sample current pulse
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