Temperature Derating
A fuse is a temperature sensitive device. Therefore, operating temperature will have an effect on fuse performance and lifetime.
Operating temperature should be taken into consideration when selecting the fuse current rating. The Thermal Derating Curve for
surface mount fuses is presented in Figure 4. Use it to determine the derating percentage based on operating temperature and
apply it to the derated system current.
Figure 4
1206/0603/0402 Series
Temperature Effect on Current Rating
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
-55
110
105
100
95
2410 Series
Temperature Effect On Current Rating
% De-rating
% De-rating
-35
-15
5
25
45
65
85
105
125
145
90
85
80
75
70
65
60
55
50
-55
-35
-15
5
25
45
65
85
105
125
Maximum Operating Temperature (°C)
Maximum Operating Temperature (°C)
Pulse Cycle Derating
Once the I
2
t value for the application waveform has been
determined, it must be derated based on the number of cycles
expected over the system lifetime. Since the stress induced by the
current pulse is mechanical in nature, the number of times the
stress is applied has significant bearing on how much derating
must be applied to the fuse rating. Figure 5 presents the current
pulse derating curve for our surface-mount chip fuses up to
100,000 cycles.
Figure 5
Surface-mount Fuse Pulse Derating Curve
100%
% of Minimum I
2
t
10%
100
1000
10000
100000
Number of Pulses
Selecting Surface-mount Fuses
11
Fuse selection seems straightforward, in that, you pick one which has a current rating just a bit higher than your worstcase system
operating current. Unfortunately, it’s not that simple. There are derating considerations for operating current and application
temperature. Turn-on and other system operations (like processor speed changes or motor start up) cause current surges or
spikes that also require consideration when selecting a fuse. So selecting the right fuse for your application is not as simple as
knowing the nominal current drawn by the system.
Fuse Selection Flowchart
However, the basic considerations for fuse selection are shown in the flowchart presented in Figure 6. Following this flow chart
will help you select a fuse best suited for your application conditions.
Figure 6
Step 1 –
Determine Steady State
Fuse Current Rating
Apply Standard Steady
State Derating (75%)
[I
fuse
≥
I
sys
/0.75]
Apply
Temperature Derating
[I
fuse
≥
I
sys
/0.75/K
temp
]
Steady State
Fuse Current
Rating
Step 2 –
Determine Pulse
Waveform by
Calculating I
2
t
Step 3 –
Apply Pulse
Cycle Derating
Step 4 –
Apply Pulse
Temperature
Derating
Step 5 –
Apply Derating
for Variance in
the Circuit
Step 6 –
Select Fuse Current
Rating for Pulse
Environment
Step 7 –
Select Fuse Current Rating
(use higher value between Step 1 and Step 6)
Step 8 –
Check Voltage Rating
76
MACOM [ Tyco Electronics ]
