AN586 Datasheet, PDF (1/4 Pages) STMicroelectronics – USE OF TRANSILS AS RECTIFIER DIODES

AN586
APPLICATION NOTE
USE OF TRANSILS AS RECTIFIER DIODES
INTRODUCTION
Transils are diodes especially designed to dissipate high peak power in avalanche operation. In direct con-
duction they have the properties of very good conventional diodes (with the possibility of handling very
high surge currents) and perform as well as rectifiers.
In general, there could be a voltage surge due either to the power supply or to the load. Power supply
overvoltages can be clamped by means of a Transil on the ac side : (see Figure 1).
Wherever possible, it is more attractive to use Transils directly as rectifiers instead of protecting separate
rectifiers with Transils since the number of components is reduced: (see Figure 2). The characteristics giv-
en in the data-sheets (IFSM, forward voltage drop characteristic) enable calculations to be made in this
mode of operation.
The following table gives the equivalence between Transils and common rectifiers.
Table 1.
TRANSIL
Case
Rating of Equivalent Diode (A)
P6KE
CB-417
1
1N5635A to
1N5665A
DO 13
1.2
1.5KE
CB-429
3
BZW50
AG
6
Neither of these two methods protects against overvoltages originating from the load. Fortunately, a study
of bridge rectifier operation shows that it is sufficient to limit the voltage on the dc side to protect the bridge
whatever the origin of the overvoltage : see Figure 3. In all cases, a single Transil is sufficient to protect a
single phase or a 3-phase bridge rectifier.
Figure 1. The diodes are not protected from overvoltages due to the load
May 2004
+
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REV. 2A - 3585
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