AN317 Datasheet, PDF (1/3 Pages) STMicroelectronics – PROTECTION BY TRANSIL HOW TO ENSURE ABSOLUTE SAFETY

AN317
®
APPLICATION NOTE
PROTECTION BY TRANSIL
HOW TO ENSURE ABSOLUTE SAFETY
The function of a protection diode is to limit the voltage across the device being protected in case of
accidental overloads. The accidents (atmospheric overvoltages, switching on the mains, failures of the
equipment) are defined by standards. But selecting the ratings of a protection diode in accordance with the
standards does not always guarantee satisfactory safety. In certain cases accidental overloads higher than
those covered by the standards can destroy the protection diode. The user can accept this destruction due
to an exceptional accident, but he requires ABSOLUTE SAFETY, i.e. the equipment can stop operating but
in no case must it be destroyed. The protection diode should thus remain a short-circuit after the overload.
The purpose of this publication is to provide the designer with the elements necessary to define this
absolute safety. Figure 1 gives the current limits below which TRANSIL diodes cannot be made
open-circuits. By analogy with fuses, these limits can be characterized within the 10 µs to 1 s interval by I²t
(A²s).
Fig. 1 : Values of I²t for pulse durations less than a second.
DO-201
R6
DO-15
BEHAVIOUR OF TRANSIL DIODES IN CASE OF OVERLOADS
If an overload exceeds the limit Ipp specified for the Transil protection diode, it can be destroyed.
n Destruction always begins by an anode-cathode short-circuit.
n If a very high current then flows through the diode, the connections can melt and vaporize and the diode
becomes an open circuit.
Numerous tests have been performed at the STMicroelectronics characterization laboratory using current
generators (3 to 1800 A) to determine the limits below which the user can be sure that the diode will remain
a short-circuit after destuction of the silicon chip. The results of these tests are given for all TRANSIL
diodes in figure 1 and table 1.
Table 1 on page 2 gives the permanent short-circuit current ICP.
AN317/2004
1/3