Accu-Guard®
SMD Thin-Film Fuse
HOW TO CHOOSE THE CORRECT ACCU-GUARD® FUSE
FOR CIRCUIT PROTECTION
Correct choice of an Accu-Guard® fuse for a given application is fairly straightforward. The factor of pre-arc I2t, however, requires clarification. The proper design for pre-arc I2t is
presented by way of example.
DESIGN PARAMETERS
1. Operating Temperature
The Accu-Guard® is specified for operation in the temperature range of -55°C to +125°C. Note, however, that fusing
current is sensitive to temperature. This means that the fuse
must be derated or uprated at circuit temperatures other
than 25°C:
Environmental
Temperature
Accu-Guard®
Current Carrying Capacity*
F0402E,
F0603E
F0805B, F1206A, F0805B 2.50A
F1206B
& 3.00A
F0603C F0612D
-55°C to -11°C
1.07 x IR
1.07 x IR
1.07 x IR
-10°C to 60°C
IR
IR
IR
1.07 x IR 1.07 x IR
61°C to 100°C
0.85 x IR
0.93 x IR
0.90 x IR
0.90 x IR 0.80 x IR
101°C to 125°C
0.80 x IR
0.90 x IR
0.90 x IR
0.75 x IR 0.75 x IR
IR
IR
*As a function of nominal rated current, IR.
2. Circuit Voltage
Maximum Voltage: Accu-Guard® is specified for circuits of
up to rated voltage. Accu-Guard® will successfully break
currents at higher voltages as well, but over voltage may
crack the fuse body.
Minimum Voltage: Accu-Guard® cannot be used in circuits
with voltage of about 0.5V and less. The internal resistance
of the fuse will limit the fault current to a value which will prevent reliable actuation of the fuse (<2 x rated current).
3. Maximum Fault Current
5. Switch-on and Other Pulse Current
Many circuits generate a large current pulse when initially
connected to power. There are also circuits which are subject to momentary current pulses due to external sources;
telephone line cards which are subject to lightning-induced
pulses are one example. These current pulses must be
passed by the fuse without causing actuation. These pulses may be so large that they are the determining factor for
choosing the Accu-Guard® current rating; not necessarily
steady state current.
In order to design for current pulses, the concept of fuse
pre-arc Joule integral, I2t, must be understood. Fuse current
rating is defined by the requirement that 2 x IR will cause
actuation in <5 seconds. This rating does not indicate how
the fuse will react to very high currents of very short duration.
Rather, the fusing characteristic at very high currents is
specified by I2t-t curves (or I2t-I).
I2t expresses the amount of energy required to actuate the
fuse. Total I2t expresses the total energy which will be
passed by the fuse until total cessation of current flow.
Pre-arc I2t expresses that energy required to cause large
irreversible damage to the fuse element (Total I2t = pre-arc I2t
+ arc I2t). If the Joule integral of the switch-on pulse is
larger than the fuse pre-arc I2t, nuisance actuation will occur.
In order to choose the proper Accu-Guard® current rating for
a given application, it is necessary to calculate the I2t Joule
integral of the circuit switch-on and other current pulses and
compare them to the Accu-Guard® I2t-t curves. An AccuGuard® fuse must be chosen such that the pulse I2t is no
more than 50% of the pre-arc I2t of the prospective fuse.
Pre-arc I2t of the Accu-Guard® fuses is well characterized;
I2t-t and I2t-I graphs are in this catalog. The problem is calculating the I2t of the circuit current pulses. This concept is
not familiar to most engineers. Correct calculation of pulse
Joule integral and subsequent choice of Accu-Guard®
current rating is illustrated by way of the attached examples.
Accu-Guard® is fully tested and specified for fault currents
up to 50A. Accu-Guard® will successfully break currents
above 50A, but such over current may crack the fuse body
or damage the fuse terminations.
4. Steady-State Current
The Accu-Guard® current rating is based on IEC Specification 127-3. In accordance with this international standard,
Accu-Guard® is specified to operate at least 4 hours at rated
current without fusing (25°C). Engineering tests have shown
that F0805B and F1206A/B Accu-Guard® will in fact operate
at least 20,000 hours at rated current without fusing (25°C).
27
Accu-Guard®
SMD Thin-Film Fuse
DESIGNING FOR CURRENT
PULSE SITUATIONS
2. Triangular current pulse
The Joule integral for triangular pulse is
[(Imax.)2 x t]/3,
see Fig. 2a.
1. Sine wave current pulse
The Joule integral for sine wave pulse is
[(Imax.)2 x t]/2,
l max.
see Fig. 1a.
l max.
t
t
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)2 x 7.7 x 10-6 sec]/2 = 8.9 x 10-5 A2 sec.
Fig. 2a. Triangular pulse parameters for Joule
integral calculation, example #2.
Thus, for the current pulse in Figure 2b, the Joule integral is
[(1.5A)2 x 3 x 10-3 sec]/3 = 2.25 x 10-3 A2sec.
10 µsec/div
2 msec/div
0.5A/div
1A/div
Fig. 1b. Sine wave pulse, example #1.
Fig. 2b. Triangular pulse, example #2.
The pulse duration is 7.7µsec. We must find a fuse that can
absorb at least 8.9 x 10-5 X 2 = 1.8 x 10-4 A2sec Joule integral within 7.7 µsec without actuation. According to the I2t
graph on page 6, pre-arcing Joule integral is 2.3x10-4 A2sec
for the 0.5A fuse, which is slightly more than needed. The
next lower rating (0.375A), has only 6x10-5 A2sec, which is
not enough. Therefore, 0.5A fuse should be chosen for this
application, see Figure 1c.
The pulse duration is 3 msec. In the I2t graph on page 6, prearcing Joule integral for 3 msec pulse is 4 x 10-3A2sec for the
0.5A fuse (not enough) and 2 x 10-2 for the 0.75A fuse (more
than enough). Therefore, 0.75A fuse should be chosen for
this application, see Figure 2c.
FUSE PRE-ARCING JOULE INTEGRALS
vs. PRE-ARCING TIME
100
PRE-ARCING TIME
l2t,
A2
100
1
sec
10-1
10-2
1
10-3
10-1
10-4
10-3
PRE-ARCING TIME l2t, A2 sec
10
10
10-2
x
10-5
-7
-6
10
10
10-5 10-4 10-3 10-2 10-1
PRE-ARCING TIME, sec
0.75A
x
10-5
10-7 10-6 10-5 10-4 10-3 10-2 10-1
PRE-ARCING TIME, sec
0.5A
10-4
1
10
Fig. 1c. Choice of 0.5A fuse, example #1.
Pre-arcing I2t
Maximum I2t design rule
I2t for sample current pulse
X
28
FUSE PRE-ARCING JOULE INTEGRALS
vs. PRE-ARCING TIME
1
10
Fig. 2c. Choice of 0.75A fuse, example #2.
Pre-arcing I2t
Maximum I2t design rule
X I2t for sample switch-on pulse
Accu-Guard®
SMD Thin-Film Fuse
DESIGNING FOR CURRENT PULSE SITUATIONS (CONT.)
3. Trapezoidal current pulse
4. Lightning strike
The Joule integral for a trapezoidal pulse is
(Imin.)2 + Imin. x (Imax. - Imin.) + ( Imax-Imin)2 x t,
3
[
A lightning strike pulse is shown in Figure 4a. After an initial
linear rise, the current declines exponentially.
]
see Fig. 3a.
l max.
0.51 max.
l max.
l min.
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
t0.5
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
0.5A/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
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.
10
FUSE PRE-ARCING JOULE INTEGRALS
vs. PRE-ARCING TIME
1
10-1
10-2
10-3
0.50A
100
x
PRE-ARCING TIME l2t, A2 sec
10
10-4
1
10-5
10-7 10-6 10-5 10-4 10-3 10-2 10-1
PRE-ARCING TIME, sec
1
10
Fig. 3c. Choice of 0.5A fuse, example #3.
Pre-arcing I2t
Maximum I2t design rule
X
I2t for sample switch-on pulse
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
PRE-ARCING TIME, sec
1
10
Fig. 4c. Choice of 0.5A fuse, example #4.
Pre-arcing I2t
Maximum I2t design rule
X
I2t for sample switch-on pulse
29
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 a
5.1 x 10-3 A2sec Joule integral of 5 msec duration; the 0.75A
fuse will meet this requirement, see Figure 6b.
200 µsec/div
2 msec/div
2A/div
Fig. 5a. Complex pulse, example #5.
In Figure 5a, the Joule integral for the first triangle is
[(4.67A)2 x 294 x 10-6sec]/3=2.14 x 10-3 A2sec
and 0.75A fuse should meet this condition, see Figure 5b.
FUSE PRE-ARCING JOULE INTEGRALS
vs. PRE-ARCING TIME
100
Fig. 6a. Switch-on pulse and steady-state current,
example #6.
FUSE PRE-ARCING JOULE INTEGRALS
vs. PRE-ARCING TIME
100
10
10-1
10
10-2
1
10-3
10-2
10-3
PRE-ARCING TIME l2t, A2 sec
1
PRE-ARCING TIME l2t, A2 sec
0.75A
x
10-4
10-1
0.75A
x x
10-5
10-7 10-6 10-5 10-4 10-3 10-2 10-1
PRE-ARCING TIME, sec
10-4
10-5
10-7 10-6 10-5 10-4 10-3 10-2 10-1
PRE-ARCING TIME, sec
1
10
Fig. 5b. Choice of fuse, example #5.
Pre-arcing I2t
Maximum I2t design rule
X I2t for sample switch-on pulse
The Joule integral for the second triangle is
[(5.33A)2 x 269 x 10-6sec]/3 = 2.55 x 10-3 A2sec, 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 A2sec, and the total duration is 563 µsec. For the
0.75A fuse, the Joule integral is only 8.6 x 10-3 A2sec for this
pulse duration, so the 1A fuse should be chosen for this
application, see Figure 5b.
30
0.5A/div
1
10
Fig. 6b. Choice of 0.75A fuse, example #6.
Pre-arcing I2t
Maximum I2t design rule
X I2t for sample switch-on pulse
The steady-state current is 0.5A, and 1A fuse is typically recommended to meet the steady-state condition. Based on
steady-state current, the 1A fuse should be chosen for this
application.