Page 21 - Application Guide Semiconductor Fuse Link

Surge withstand of semiconductor devices

Power diodes and thyristors

Semiconductor device manufacturers publish surge current withstand in terms of a half cycle surge
rating, characterized by the peak amplitude (IFSM) of a single sinusoidal half-cycle pulse which the
device can withstand. The duration of the test pulse (t0) is usually 8.33 ms or 10 ms, corresponding
to 60Hz or 50Hz half-cycles. Two withstand values are sometimes published : (a) with full rated
voltage re-applied to the device immediately after the surge has fi nished and (b) with zero reapplied
voltage. When fuse protection is provided, the fuse must clear the circuit current before the device is
damaged. Value (b) can normally be used, because after the fuse arcs have extinguished the residual
fuse resistance increases rapidly.

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Often the surge withstand is given as an I t value (“for fusing”). The r.m.s. value I0 for a half-sine wave
is given by
2 2 2
I0 = IFSM / 2 and the corresponding I t value is I t (for fusing) = I0 t0

However semiconductor devices in the 10000
ON state have very non-linear v-i forward
characteristics, and the instantaneous IFSM
(A)
power dissipation within the device is not
proportional to the square of the current.
1000
2
For this and other reasons the I t withstand
of the semiconductor device is not constant,
but decreases as the duration of the surge
becomes shorter. Fig.15 illustrates the
2
variation of IFSM and the corresponding I t 100
value for a typical thyristor. 0.001 time, s 0.01
10000

A simple model of single-cycle surge
withstand can be used to show that the I2t
(A2s)
N
withstand line can be represented by Io to
= constant where the “device exponent” 1000
N is approximately 3.0. Many device
manufacturers give IFSM at two different
times (often 8.33 ms and 1.5 ms) and when
such data is plotted in the form of Fig.15 100
the value of N for real devices is found to 0.001 time, s 0.01
lie within the range 2.5 4.0.

2
Fig.15 Variation of IFSM and I t for a thyristor
In the absence of better information a
device exponent of 3.0 should be used for
diodes and thyristors. In other words, the
2
3
device withstand is approximately a constant I t, and the device withstand I t should be adjusted by
3
using a constant I t for the actual fault duration. The use of the device exponent is illustrated in the
following example.

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