ST AN1768 APPLICATION NOTE
AN1768
®
APPLICATION NOTE
ADMISSIBLE AVALANCHE POWER OF SCHOTTKY DIODES
D. JOUVE
INTRODUCTION
The design of Switch Mode Power Supply (SMPS) is subjected to ever increasing cost and efficiency
constraints.
Onewaytorespondto these aggressivespecifications is touse components closerto their intrinsiclimits.
The increasing use of Schottky diodes in the avalanche area is a good example of this evolution.
To help the designer to optimize the choice of the Schottky diode in a rectification application,
STMicroelectronics is proposing asimple tool to determine if agiven ST Schottky diode can withstandthe
avalanche energy fixed by the application conditions.
1. DESIGN RULES
The first step for the designer is to estimate, in the
worst-case conditions, the following parameters:
Operating junction temperature: Tj
n
n
Pulse duration of the avalanche current: tp
n
Avalanche energy by pulse generated by the
converter in the Schottky diode: E
AP
Fig. 2: Avalanche power derating over temperature range.
P (t ,T ) / P (tp, 25°C) versusT
1.2
1
ARM p j ARM j
STMicroelectronics guarantees for each Schottky
diode a reference avalanche power given at
tp=1µs and Tj=25°C: P
(1µs,25°C) (corre-
ARM
sponding to a rectangular current pulse ).
Table 1
gives P
(1µs,25°C) for some part
ARM
numbers.
Table 1: P
(1µs, 25°C) values for some ST
ARM
Schottky diodes.
P
(1µs; 25°C)
Part number
ARM
per diode
STPS1545D (2x7.5A) 2.7 kW
STPS2045CT (2x10A) 4 kW
STPS3045CT (2x15A) 6 kW
STPS20H100CT (2x10A) 10.8 kW
Derating curves
figure 2
and
figure 3
give the ad-
missible avalanche power versus tp and Tj.
(1µs, 25°C) for each part number as well as
P
ARM
the derating curves are given in the respective
datasheet.
The designer must ensure that the guaranteed
avalanche energy E
avalanche energy in the application E
(tp,Tj) is greater than the
ARM
AP
.
0.8
0.6
0.4
0.2
0
25 50 75 100 125 150 175
Tj (°C)
Fig. 3: Avalanche power derating over pulse dura-
tion range
P (t ,T ) / P (1µs, ) versus tp
ARM p j ARMTj
0.01
0.001
10
1
10.01 0.1
0.1
100 100010
tp(µs)
October 2003 - Ed: 1
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AN1768 - APPLICATION NOTE
2. DESIGN EXAMPLE
Let us consider the use of a STPS20H100CT (two
10A, 100V ST Schottky diodes in TO-220 package) used in a flyback converter (
figure 4
).
Fig. 4: Topology of a flyback converter.
Vdiode
Idiode
Vin
Figure 5
shows the corresponding current and
Vout
voltage waveforms through the two diodes.
Fig. 5: Current and voltage waveforms through
the two diodes.
= repetitive avalanche current)
(I
AR
I
Diode
tp
I
AR
V
R
corresponding energy
in the avalanche area
In a typical worst-case situation, the application
conditions are:
Operating junction temperature of the Schottky
n
diode:
Tj = 100°C
Pulse duration of the avalanche current:
n
tp = 10ns
Avalanche energy by pulse through the two
n
diodes connected in parallel:
= -130V, IAR= -1.5A, tp = 10ns
V
P
= 1.95µJ
⇒E
AP
Table 1 gives:
•
P
(1µs,25°C)
ARM
Figure 2 gives:
•
⇒P
⇒P
(1µs,100°C) = P
ARM
(1µs,100°C) = 4.86 kW
ARM
• Fig.3 gives:
⇒P
⇒P
(10ns,100°C) = P
ARM
(10ns,100°C) = 4.86 kW
ARM
STPS20H100CT
Ptp C
PtpC
PnsT
ARM j
PsT
ARM j
=10.8 kW perdiode
(, )
100
ARM
ARM
ARM
(,)
10
(,)
11µ
ARM
°
(, )
25
°
(1µs,25°C) x 0.45
=
(1µs,100°C)
=
.
045
Finally,
(10ns,100°C) = P
E
ARM
(10ns,100°C)x10ns
ARM
The maximum admissible avalanche energy of the
STPS20H100CT at 10ns and 100°C is:
E
(10ns,100°C) = 48.6µJ per diode
ARM
Consequently, as the guaranteed value
E
(10ns,100°C) (per diode) is higher than E
ARM
AP
measured through the two diodes connected in
parallel (48.6µJ > 1.95µJ), the STPS20H100CT
will withstand the avalanche energy generated by
the converter.
Informationfurnished is believed to be accurate and reliable. However,STMicroelectronicsassumes no responsibility for the consequences of
useof such information nor for any infringementofpatents or other rights of third partieswhich may result from its use. Nolicenseis granted by
implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to
change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
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