76

11

Pulse Cycle Derating

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.

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.

Surface-mount Fuse Pulse Derating Curve

100

1000

10000

100000

100%

10%

Number of Pulses

Figure 5

Step 1 –

Determine Steady State

Fuse Current Rating

Step 2 –

Determine Pulse

Waveform by

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

Apply Standard Steady

State Derating (75%)

[I

fuse

Apply

Temperature Derating

[I

fuse

Steady State

Fuse Current

Rating

Figure 6

Selecting Surface-mount Fuses

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.

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.

1206/0603/0402 Series

Temperature Effect on Current Rating

2410 Series

Temperature Effect On Current Rating

-55

-55

-35

-15

5

25

45

65

85

105

125

-35

-15

5

25

45

65

85

105

145

125

105

100

95

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

110

105

100

95

90

85

80

75

70

65

60

55

50

Maximum Operating Temperature (°C)

Maximum Operating Temperature (°C)

Figure 4