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Accu-Guard
®
SMD Thin-Film Fuse
DESIGNING FOR CURRENT PULSE SITUATIONS (CONT.)
5. Complex current pulse 6. Switch-on pulse and steady-state current If the pulse consists of several waveforms, all of them should
be evaluated separately, and then the total Joule integral
should be calculated as well. In Figure 6a, the switch-on pulse is a triangle pulse with a5.1 x 10
-3 A
2 sec Joule integral of 5 msec duration; the 0.75Afuse will meet this requirement, see Figure 6b.
200 µ sec/div2A/div 2 msec/div0.5A/div
Fig. 5a. Complex pulse, example #5. Fig. 6a. Switch-on pulse and steady-state current,example #6.
2 x 294 x 10
-6 sec]/3=2.14 x 10
-3 A2secand 0.75A fuse should meet this condition, see Figure 5b. FUSE PRE-ARCING JOULE INTEGRALSvs. PRE-ARCING TIME
PRE-ARCING TIME l
2 t, A
2 sec
PRE-ARCING TIME, sec
10010110
FUSE PRE-ARCING JOULE INTEGRALSvs. PRE-ARCING TIME
PRE-ARCING TIME l
2 t, A
2 sec
PRE-ARCING TIME, sec
10010110
-1 -2 0.75A 10 x 10
-3 -1 10
-4 -2 0.75A 10 10
-5 -7 -6 -5 -4 -3 -2 -1 10
-3 xx 10 10 10 10 10 10 10 110 10
-4 10
-5 10
-7 10
-6 10
-5 10
-4 10
-3 10
-2 10
-1 110
Fig. 6b. Choice of 0.75A fuse, example #6.Pre-arcing I
2 tMaximum I
2 t design ruleI Fig. 5b. Choice of fuse, example #5.Pre-arcing I
X
2 2 t for sample switch-on pulse tMaximum I
2 t design ruleI
X
2 t for sample switch-on pulse The steady-state current is 0.5A, and 1A fuse is typically rec-ommended to meet the steady-state condition. Based on
steady-state current, the 1A fuse should be chosen for this
application. The Joule integral for the second triangle is[(5.33A)
2 x 269 x 10
-6 sec]/3 = 2.55 x 10
-3 A
2 sec, and 0.75A fuse is suitable for this case also, see Figure 5b.However, for the whole pulse, the Joule integral is 4.7 x 10
-3 A
2 sec, and the total duration is 563 µsec. For the0.75A fuse, the Joule integral is only 8.6 x 10
-3 A
2 sec for thispulse duration, so the 1A fuse should be chosen for this
application, see Figure 5b.
In Figure 5a, the Joule integral for the first triangle is[(4.67A)
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