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AN1453

APPLICATION NOTE

®

NEW FAMILY OF 150V POWER SCHOTTKY

July 2001

By F. GAUTIER

Nowadays, the Switch Mode Power Supply

(SMPS) is becoming more widespread as a result

of computer, telecom and consumer applications.

The

constant

increase

in

services

(more

peripherals) and performance, which offers us

these applications, tends to move conversion

systems towards higher output power.

In addition to these developments dictated by the

market, SMPS manufacturers are in competition,

their battlefield being the criteria of power density,

efficiency, reliability and cost, this last being factor

very critical.

Today, SMPS designers of 12V-24V output have

practically the choice between a 100V Schottky or

a 200V bipolar diode.

The availability of an intermediate voltage has

become necessary to gain in design optimization.

This is why STMicroelectronics is introducing a

new family of 150V POWER SCHOTTKY diodes,

intended for 12V and more secondary rectification,

in applications such as desktops, file servers or

adaptors for notebook.

Consequently, this application note will underline

the advantages of a 150V Schottky technology

compared to a 200V ultra fast diode.

In order to do this, the example of a Flyback

converter will be used, and the static and dynamic

parameters of the 150V Schottky will be detailed,

as well as their influence in this converter.

1. CONDUCTION LOSSES & EFFICIENCY GAIN

Schottky diodes are mainly used for output

rectification. In a typical SMPS working with a

switching

frequency

lower

than

100kHz,

conduction losses are generally the main losses in

the diode. They are directly linked to the curve of

and obviously the best gain in efficiency will be

In the following examples, the conduction losses

between a 150V Schottky and a 200V bipolar

diode in a Flyback and a Forward converter will be

compared.

The conduction losses in the diode are calculated

from the classical formula:

P= V

I

+ R

I

cond

T0

F(AV)

d

IF(RMS)

2

⋅⋅

V : threshold voltage with V

= V + R .I

t0

F(@ IF)

T0

d

F

R : dynamic resistance with R =

V / I

dd

F

F

∆∆

range of current view by the diode (Fig. 1), for

better accuracy.

Figure 1 shows also, the typical current through

and I

IF(RMS)

2

:

I=

(I

I

)

I(I

I

I

F(AV)

ID

max

min

F(RMS)

2

ID

max

min

2

m

α

α

2

3

2

+

=+

+

ax

min

d

F(@ Imax)

F(@ Imin)

max

min

T0

F

I

R=

VV

II

VV

⋅

−

−

=

)

(@ imax)

d

max

RI

−⋅

NB:

I

D

0

αI.T

D

I

min

T

t

I

ma x

Fig. 1: Typical current through a rectification diode

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