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AN836
APPLICATION NOTE
LOW FORWARD VOLTAGE SCHOTTKY DIODE
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:
V F ( IOUT )
η = -------------------------V OUT
η
:efficiency drop due to the diodes.
VF(IOUT)
:voltage drop at the output current
VOUT
:output voltage of the converter.
(IOUT) of the converter.
This formula shows that the influence of the forward voltage increases when the output voltage of the power supply decreases.
This parameter becomes very important for the new standard of 3.3V output voltage. Another key parameter is the leakage current which we have to take into account to develop high efficiency low forward voltage 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 chosen to have the best efficiency with a sufficient safety margin. This is to keep the reverse losses under
control (Tj max = 125°C).
REV. 2
May 2004
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AN836 APPLICATION NOTE
Table 1. Main Characteristics of Schottky
PART NUMBER
PACKAGE
IR
(125°C, 25V)
VF
(Io,125°C)
Io
A
typ
Max
Max
mV
mV
mA
(1)
(1)
2(1)
STPS125U
SOD6
1
STPS5L25B
DPAK
5
310
350
175
TO220AC
10
300
350
400
STPS15L25D/G
D2PAK/TO220AC
15
300
350
640
STPS20L25CT
TO220AB
2x10
300
350
400
STPS20L25CG
D2PAK
2x10
300
350
400
STPS10L25D
390
460
Note: 1. Main characteristics of the 25V low VF Schottky
OR-ING SCHOTTKY
To increase system reliability, power supplies are sometimes connected in parallel (Fig.1).
Figure 1. OR-ing Schottky in redundant power supply
IOUT/n
IOUT
POWER
SUPPLY
VOUT
3.3V, 5V or 12V
1
IOUT/n
POWER
SUPPLY
2
IOUT/n
POWER
SUPPLY
n
The Schottky diodes are connected in series with the output of each power supply. In the steady state a
continuous current (IOUT/n) flows in each Schottky. Obviously in this case there are no reverse losses.
When a power supply fails, the corresponding ORING diodes ceases to carry current and the system output voltage is not disturbed. The OR-ing diode then sees the reverse voltage VOUT.
In this application the most critical parameter is the forward voltage drop in order to maintain the best efficiency of the system. A breakdown voltage of 10V is sufficient for output voltage of 5V and 3.3.V.
The trade-off has been chosen to obtain a low forward voltage. In this application we can accept a high
leakage current because when the diode is blocked the reapplied voltage is low (3.3V or 5V), the junction
temperature is also low because there are only reverse losses. So it’s easy to keep reverse losses under
control.
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AN836 APPLICATION NOTE
Table 2 gives the main characteristics of the 10V and 15V schottky.
Table 2. Main Characteristics of 10V and 15V Schottky
PART NUMBER
PACKAGE
typ
Max
A
mV
mV
A
320
0.42
320
1.2
STPS15L10D
TO220AC
15
270
STPS80L10TV
ISOTOP
2x40
270
STPS20L15D
STPS40L15CW
IR
(100°C, 10V)
VF
(Io,105°C)
Io
TO220AC
20
TO247
2x20
0.31
(1)
0.31(3)
0.36
(1)
0.35(3)
Max
0.55(2)
0.55(2)
Per diode
Note: 1. I=19A
2. VR=12V
3. I=19A per diode
CONCLUSION
The equilibrium between efficiency and safety margin against thermal runaway has been best solved with
25V, 15V and 10V Power Schottky diodes. One being suitable for 3.3V supplies and the other for the redundant Powers supplies.
Moreover, the inclusion of Power Factor correction will significantly change the choice of the Schottky diode for the secondary Power supply rectification.
With PFC, the reapplied voltage across the output rectifiers will be lower. For example we can consider
using 25V Schottky in a 5V forward converter, whereas today a 45V Schottky diode is needed.
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AN836 APPLICATION NOTE
REVISION HISTORY
Table 3. Revision History
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Date
Revision
Description of Changes
August-1995
1
First Issue
10-May-2004
2
Stylesheet update. No content change.
AN836 APPLICATION NOTE
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by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
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authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
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