1/5

May 2004

AN836

APPLICATION NOTE

LOW FORWARD VOLTAGE SCHOTTKY DIODE

REV. 2

INTRODUCTION

In power supplies, the major portion of power losses is due to output rectifiers. The impact of these losses

on the efficiency can be expressed by:

η

:efficiency drop due to the diodes.

:voltage drop at the output current

(IOUT) of the converter.

:output voltage of the converter.

This formula shows that the influence of the forward voltage increases when the output voltage of the pow-

er supply decreases.

This parameter becomes very important for the new standard of 3.3V output voltage. Another key param-

eter is the leakage current which we have to take into account to develop high efficiency low forward volt-

age drop Schottky with the best trade-off.

TRADE-OFF

The 3 most important application characteristics of a Schottky are:

– forward voltage

– reverse leakage current

– reverse blocking voltage

Generally for a given application, the first step is to fix the reverse blocking voltage. We then study the

best trade-off (choice of a metal barrier) between the forward voltage and the leakage current.

A decrease of the forward voltage increases the efficiency of the converter but increases at the same time

the leakage current and limits operating range where we can keep the reverse losses under control. In the

datasheet this range is defined by Tj max.

To define the best trade off of a low forward voltage schottky we have to take into account its application

condition. STMicroelectronics has developed two families of low forward voltage Schottky well suitable to

two applications: the OR-ing Schottky and Schottky for 3.3V output power supply secondary.

SCHOTTKY DIODES FOR 3.3V SWITCHED MODE POWER SUPPLY

When a Schottky works in a switched mode power supply (forward, flyback,...), it sees during the same

switching period conduction losses and reverse losses.

In these configurations the trade-off between the forward voltage and the leakage current has to be cho-

sen to have the best efficiency with a sufficient safety margin. This is to keep the reverse losses under

control (Tj max = 125°C).

η

()

---------------------------

=