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F1206A0R20FSTR Datasheet, PDF (20/23 Pages) AVX Corporation – Accu-Guard, SMD Thin-Film Fuse
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Accu-Guard®
SMD Thin-Film Fuse
DESIGNING FOR CURRENT PULSE SITUATIONS (CONT.)
3. Trapezoidal current pulse
The Joule integral for a trapezoidal pulse is
[ ] (Imin.)2 + Imin. x (Imax. - Imin.) + ( Imax-Imin)2 x t,
3
see Fig. 3a.
l max.
l min.
4. Lightning strike
A lightning strike pulse is shown in Figure 4a. After an initial
linear rise, the current declines exponentially.
0.51 max.
t0.5
l max.
t
Fig. 3a. Trapezoidal pulse parameters for Joule
integral calculation, example #3.
Thus, for current pulse in Figure 3b, the Joule integral is:
[ ] {(0.56A)2+0.56A x (1A-0.56A)+ (1A-0.56A)2 } x 3 x 10-3s = 1.9 x 10-3A2sec.
3
0.5 msec/div
0.5A/div
Fig. 4a. Lightning pulse parameters for Joule
integral calculation, example #4.
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
Imax.2 x t0.5 x (-1/2In 0.5) = 0.72Imax.2 x t0.5
Thus, for the sample lightning strike pulse in Figure 4b, the
total Joule integral is:
(25A)2 x 2 x 10-6sec/3+0.72 x (25A)2 x 10 x 10-6sec = 4.92 x 10-3A2sec.
10 μsec/div
5A/div
Fig. 3b. Trapezoidal pulse, example #3.
According to the I2t graph on page 6, the 0.5A fuse should
be chosen for this application, see Figure 3c.
FUSE PRE-ARCING JOULE INTEGRALS
vs. PRE-ARCING TIME
100 PRE-ARCING TIME l2t, A2 sec
10
1
10-1
10-2
10-3
0.50A
x
10-4
10-5
10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10
PRE-ARCING TIME, sec
Fig. 3c. Choice of 0.5A fuse, example #3.
Pre-arcing I2t
Maximum I2t design rule
X I2t for sample switch-on pulse
Fig. 4b. Lightning strike pulse, example #4.
For practical calculations, the duration of exponential decline
may be assumed to be 3t0.5, because within this time 98.5%
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
100 PRE-ARCING TIME l2t, A2 sec
10
1
1.25A
10-1
10-2
x
10-3
10-4
10-5
10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10
PRE-ARCING TIME, sec
Fig. 4c. Choice of 0.5A fuse, example #4.
Pre-arcing I2t
Maximum I2t design rule
X I2t for sample switch-on pulse
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