ST AN1453 APPLICATION NOTE

AN1453

®

NEW FAMILY OF 150V POWER SCHOTTKY

INTRODUCTION

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 andcost, 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.

APPLICATION NOTE

By F. GAUTIER

In the following examples, the conduction losses
between a 150V Schottky and a 200V bipolar
diode in a Flybackand aForward converter willbe
compared.

The conduction losses in the diode are calculated
from the classical formula:

P=VI+RI

cond T0 F(AV) d IF(RMS)

V :threshold voltage with V = V +R .I

t0 F(@IF) T0 d F

R : dynamic resistance with R = V / I

ddFF

where V

and Rdare calculated from the current

T0

⋅⋅

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

Figure 1 shows also, the typical current through
the rectification diode and thecorresponding I

2

and

I

IF(RMS)

:

Fig. 1: Typical current through a rectification diode

2

∆∆

F(AV)

This is why STMicroelectronics is introducing a
new family of 150V POWER SCHOTTKY diodes,
intendedfor12Vandmoresecondaryrectification,
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 thestatic and dynamic
parameters of the 150V Schottky will be detailed,
as well as their influence in this converter.

1.CONDUCTIONLOSSES&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 lossesin
the diode. They are directly linked to the curve of
forward voltage (V

) versus forward current (IF),

F

and obviously the best gain in efficiency will be
obtained with the lowest V

July 2001

.

F

I

D

I

ma x

I

min

0

α

I=(I I)

F(AV)

2

I(III

F(RMS)

R=

d

ID

+

max min

2

α

2

ID

=++

VV

max min2m

3

−

F(@Imax) F(@Imin)

II

−

max min

αI.T

D

I

⋅

)

ax min

VV

=

T0 F

NB:

-In the datasheet, the V
values given for I

and2IFat 125°C.

F

-In discontinuous mode I

and Rdare maximum

T0

=0.

min

T

(@imax) d max

RI−⋅

1/9

t

APPLICATION NOTE

1.1. Example 1: FLYBACK

The first example is a 24V/48W Flyback converter
working in continuous mode (Vmains=90V) with
the following conditions:

0.4, I 6.66A, I 3.33A, I 2A

== = =

α

ID max min out

ID ID

Fig. 2: Rectification diode in a Flyback converter

I

V

I

in

o u t

D

Calculations per diode give:

I = 1A and I = 1.6A

F(AV)per diode F(RMS)

per diode

We can now calculate the efficiency gain (∆η(%)=

- η) for this Flyback converter which has a

η

ref

reference (ref) efficiency of 85% with
STPR1020CT:

Fig. 3: Example of efficiency gain in Flyback
converter

V

P
V

out
out

=48W
=24V

T0

typ(V)

1.5A, 3A,
125°C

R

mΩ

d

P

cond

(W)

∆P

(W)

η=85

%

∆η%

STPR102CT
2x5A / 200V

0.58 46.5 1.4 0 (ref) 0 (ref)

PN diode

1.2. Example 2: FORWARD

In the following example,the conduction losses in
a 12V/96W Forward converter are simulated:

Fig. 4: Rectification diode in a Forward converter

I

I

o u

L

V

D1

in

D2

α

0.3, I 9A, I 7A, I 8A====

D1 Lmax Lmin out

Calculations per diode give:

==

I 2.4A, I 4.39A

F(AV)D1 F(RMS)D1

=

I 5.6A, I

F(AV)D2 F(R

MS)D2

The difference of efficiency between a
STPR1620CT (2x8A, 200V Ultrafast) and a
STPS16150CT (2x8A, 150V Schottky) for a 12V
output, are given in table Fig. 5:

Fig. 5: Example of efficiency gain in Flyback
converter

V

R

T0

d

mΩ

P
V

out
out

=96W
=12V

typ(V)

7A, 9A,

125°C

STPR1620CT

0.8 20 6.48 Ref Ref

P

(W)

cond

6.71A=

η=85

∆P

%

(W)

∆η%

STPR162CT
2x8A / 200V
PN diode

STPS10150CT
2x5A / 150V
Schottky diode

STPS16150CT
2x8A / 150V
Schottky diode

2/9

0.54 46.5 1.32 -0.08 +0.12

0.50 43 1.22 -0.18 +0.27

0.47 40 1.14 -0.26 +0.39

STPS16150CT

0.68 20 5.60 -0.95 +0.72

These two examples show that whatever the type
of converter, a significant efficiency gain can be
achievedonly by replacing a200V bipolar diode by
a 150V Schottky.

APPLICATION NOTE

2. REVERSE LOSSES AND T

JMAX

2.1. Reverse losses: Prev

The reverse losses can be determined by:

=⋅⋅−α)

PVI(1

rev R R

with:

): duty cycle when the reverse voltage (VR)is

(1-

applied
IR: leakage current versus VRand operating

junction temperature (T

V

: reapplied voltage accross the diode

R

)

j

Fig. 6 shows an example of reverse losses in a
Flyback converter with the following conditions:

()1−= = = °α0.4, V 80V, T 125 C

Rj

Fig. 6: Example of reverse losses in a Flyback
converter

STPS10150CT

per diode

I

Rtyp

100V, 125°C

130µA 4.2mW

P

rev

per diode

Thus, the reverse losses are very low due to the
low value of the leakage current.

The following paragraph will show that due to
these low values of reverse current, the thermal
runaway limit is only reached for high junction
temperature.

2.2. T

before thermal instability is reached

jmax

Remembering that the stability criterion is given
by:

dP

rev

<

dT1R

j th(j a)

−

with:

=−α)

P V .I .(1

rev R R(VR,Tjmax)

The above formulae give the critical value of the
leakage currentbefore the thermal runaway limit is
reached:

I

R(VR,Tjmax)

=

Vc.R

R th(j a)

The evolution of the leakagecurrent versus T

is given by:

V

R

IIexp

=

R(V ,Tj) R(V ,125)

RR

1

⋅⋅−

1()α

−

−

c(Tj 125)

and

j

From these physical laws, it can be deduced that:

Example:
Flyback converter with 2 diodes in parallel

()1−= = =α0.4, c 0.069, V 80V

R 2.4 C / W, R 7.6 C / W

th(j c)total th(c a)−−

Fig. 7: Example of T

For a dual
diode

STPS10150CT

=° =°

with STPS10150CT

jmax

I

Rmax

(80V,125°C)

1.3mA 45.28mA 176.5°C

R

I

R(VR,Tjmax)Tjmax

This example shows that in a typicalapplication, a
150V Schottky can be used up to 175°C.
STMicroelectronics specifies in the datasheet

at 175°C.

T

jmax

3. SWITCHING BEHAVIOUR

3.1. Turn-on behaviour

The behaviour at turn-on is characterized bya low
value of peak forward voltage (V
reverse recovery time (t

) (Fig. 8).

fr

) and forward

FP

Fig. 8: VFPand tfrfor STPS16150CT

=16A

I

F

/dt=100A/µs

dI

F

=25°C

T

j

t

fr

(ns)

V

(V)

FP

Per diode

STPS16150CT

These values depends mainly on the dI

100 2.2

/dt. The

F

switching losses at turn-on are always negligible.

3.2. Turn-off behaviour

The turn-off behaviour isa transitoryphenomenon
(ns), but repetitive depending on the switching
frequency. It is a source of spike voltage, noise
and for high switching frequency, ofnon-negligible
switching losses.

Inorder to illustrate thisphenomenon, the example
of a Flyback converter will be used once again.

The difference in behaviour between a 150V
Schottky and 200V bipolar diode will be compared
for the three following points: spike voltage, EMC
and switching losses.

T 125

=+⋅

jmax

1

In

c

I

R (V ,125 C)datashee

max R

I

R(V ,Tjmax)

R

° t

3/9