Accu-Guard
®
SMD Thin-Film Fuse
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
-3
s = 1.9 x 10
-3
A2sec.
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
[
(I
min.
)
2
+ I
min.
x (I
max.
- I
min.
) + (
I
max
-
I
min
)
2
]
x t,
3
see Fig. 3a.
Fig. 3b. Trapezoidal pulse, example #3.
According to the I
2
t 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
0.50A
PRE-ARCING TIME l
2
t, A
2
sec
PRE-ARCING TIME, sec
100
10
1
10
-1
10
-2
10
-3
10
-4
10
-5
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
110
x
Fig. 3c. Choice of 0.5A fuse, example #3.
Pre-arcing I
2
t
Maximum I
2
t design rule
I
2
t for sample switch-on pulse
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.
2
x t
0.5
x (-1/2In 0.5) = 0.72I
max.
2 x t
0.5
Thus, for the sample lightning strike pulse in Figure 4b, the
total Joule integral is:
(25A)
2
x 2 x 10
-6
sec/3+0.72 x (25A)
2
x 10 x 10
-6
sec = 4.92 x 10
-3
A
2
sec.
Fig. 4a. Lightning pulse parameters for Joule
integral cal cu la tion, example #4.
Fig. 4b. Lightning strike pulse, example #4.
For practical calculations, the duration of ex po nen tial decline
may be assumed to be 3t
0.5
, 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
Fig. 4c. Choice of 0.5A fuse, example #4.
Pre-arcing I
2
t
Maximum I
2
t design rule
I
2
t for sample switch-on pulse
1.25A
PRE-ARCING TIME l
2
t, A
2
sec
PRE-ARCING TIME, sec
100
10
1
10
-1
10
-2
10
-3
10
-4
10
-5
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
110
x
