AN5381 Datasheet, PDF (1/6 Pages) Dynex Semiconductor – Case Non-rupture

The non-repetitive surge current
ITSM and the I²t value define the
limit of the electrical stress in the
forward direction of a thyristor
provided that it is triggered with
sufficient gate current. These
characteristics of the
semiconductor are used to design
short circuit protection, namely
fuses or circuit breakers. By
definition, this level of stress does
not destroy the thyristors or diodes.
If a thyristor becomes short circuit
in the forward direction and a
current flows which is greater than
the surge current limit, destruction
of the encapsulation will not
normally occur until this current is
substantially greater than the surge
current. This is because the
thyristor is effectively triggered on
by the fault current and normal
injection over a large area of the
silicon takes place.
AN 5381
Case Non-rupture
Current Ratings
Application Note
AN5381-2 February 2014 LN31349
Author: Colin Rout
If the thyristor becomes defective in
the reverse blocking state, a short
circuit current can flow in the
reverse direction. The cathode area
that remains undamaged does not
take part in carrying the current. A
small edge around the failure melts
and an arc develops in the case.
The intense heat generated by the
arc will lead to either cracking of
the ceramic case through thermal
shock or melting of the metal
flanges of the encapsulation. Hot
plasma then escapes through the
breach into the enclosure. In high
power installations where strong
magnetic fields exist, an equipment
short circuit or even burn down of
the equipment may be the
consequence.
Fig 1. Fault current flowing back
through a failed thyristor in a 3
phase bridge which shorts out two
phases of the supply.
Fig 2. A failed device in a parallel
application can experience a
current equal to the full forward
current through all parallel paths.
The case non-rupture current
rating is the value of the half
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