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May 2004

AN586

APPLICATION NOTE

USE OF TRANSILS AS RECTIFIER DIODES

REV. 2A - 3585

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.

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

TRANSIL

P6KE

1N5635A to

1N5665A

1.5KE

BZW50

Case

CB-417

DO 13

CB-429

AG

Rating of Equivalent Diode (A)

1

1.2

3

6

+

-