SMD Thin-Film Fuse

presented by way of example.

DESIGN PARAMETERS

1. Operating Temperature

ture 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:

2. Circuit Voltage

currents at higher voltages as well, but over voltage may

crack the fuse body.

with voltage of about 0.5V and less. The internal resistance

of the fuse will limit the fault current to a value which will pre-

vent reliable actuation of the fuse (<2 x rated current).

3. Maximum Fault Current

above 50A, but such over current may crack the fuse body

or damage the fuse terminations.

4. Steady-State Current

tion 127-3. In accordance with this international standard,

current without fusing (25°C). Engineering tests have shown

at least 20,000 hours at rated current without fusing (25°C).

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 sub-

ject 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 puls-

es may be so large that they are the determining factor for

steady state current.

In order to design for current pulses, the concept of fuse

rating is defined by the requirement that 2 x I

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

passed by the fuse until total cessation of current flow.

integral of the circuit switch-on and other current pulses and

not familiar to most engineers. Correct calculation of pulse

current rating is illustrated by way of the attached examples.

FOR CIRCUIT PROTECTION

Environmental

Temperature

Current Carrying Capacity*

F0402E,

F0805B, F1206A,

F0805B 2.50A

F0603C

F0612D

F0603E

F1206B

& 3.00A

-55°C to -11°C

1.07 x I

R

1.07 x I

R

1.07 x I

R

1.07 x I

R

1.07 x I

R

-10°C to 60°C

I

R

I

R

I

R

I

R

I

R

61°C to 100°C

0.85 x I

R

0.93 x I

R

0.90 x I

R

0.90 x I

R

0.80 x I

R

101°C to 125°C

0.80 x I

R

0.90 x I

R

0.90 x I

R

0.75 x I

R

0.75 x I

R

*As a function of nominal rated current, I