Page 13 - Application Guide Semiconductor Fuse Link

Estimation of fuse life under cyclic overloads

The life of a fuse under cyclic loading is determined by :

(a) The highest peak-to-peak temperature excursion ( ) of the notches in the fuse
element. As increases, the peak-to-peak thermal strain increases, which in turn
reduces the notch life. For a given material and element design, the endurance in
cycles depends upon the thermal strain according to laws governing mechanical
fatigue.

(b) The average notch temperature. As the average notch temperature increases, the
resistance to mechanical fatigue decreases, which results in a reduction in notch life.

Consider the simple ON/OFF cycle shown in Fig.4. The cycle gives rise to a peak-to-peak temperature
fl uctuation ^0 as shown by the dotted line.

The average notch temperature is related to the r.m.s. value of the current, integrated over the
whole load cycle, (compared to the fuse rated current In) and depends on the ratio

puRMS = IRMS /In

Combining these relationships with the Manson-Coffi n law of mechanical fatigue gives the life of the
fuse (mean number of cycles to failure N) as the power-law
K
N
N = ---
x y
( ) ( puRMS)
Estimation of for a given duty cycle requires a knowledge of the transient thermal response
of the fuse. This information can be extracted from the time-current characteristic, from which a
transient thermal model can be constructed. Tests on fuses with various currents and duty cycles
enable the constants KN , x and y to be determined. KN is a constant which depends upon the
mechanical design of the fuse. The life is strongly dependent upon but the dependence on the
r.m.s. level is much weaker.

For applications with complex duty cycles with several time periods at different current levels, it is
necessary to estimate all the peaks and troughs of notch temperature within the cycle and count
the number of stress reversals. For a cycle with M blocks of current there is a maximum of M/2 such
reversals, and each reversal produces a fl uctuation which contributes to the fatigue process. The
life in cycles may then be estimated from
K
N
N = ------
x
x
( + + + ...) ( puRMS) y
x
1 2 3
The method requires advanced numerical analysis methods and a digital computer. It is the
method built in to the Select-A-Fuse / PE software (see section 18).

Selection of the correct fuse for applications with repetitive cyclic overloads is a complex procedure.
Contact Ferraz Shawmut Technical Services for assistance.

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