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```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.
One way to respond to these aggressive specifications is to use components closer to their intrinsic limits.
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 a simple tool to determine if a given ST Schottky diode can withstand the
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:
n Operating junction temperature: Tj
n Pulse duration of the avalanche current: tp
n Avalanche energy by pulse generated by
the
converter in the Schottky diode: EAP
Fig. 2: Avalanche power derating over temperature range.
PARM(tp, Tj) / PARM(tp, 25°C) versus Tj
1.2
1
0.8
STMicroelectronics guarantees for each Schottky
diode a reference avalanche power given at
tp=1µs and Tj=25°C: PARM(1µs,25°C) (corresponding to a rectangular current pulse ).
Table 1 gives PARM(1µs,25°C) for some part
numbers.
Table 1: PARM(1µs, 25°C) values for some ST
Schottky diodes.
Part number
PARM(1µs; 25°C)
per diode
STPS1545D (2x7.5A)
2.7 kW
STPS2045CT (2x10A)
4 kW
STPS3045CT (2x15A)
6 kW
STPS20H100CT (2x10A)
10.8 kW
0.6
0.4
0.2
0
25
50
75
100
125
150
Tj (°C)
175
Fig. 3: Avalanche power derating over pulse duration range
PARM(tp, Tj) / PARM(1µs, Tj) versus tp
10
tp(µs)
1
Derating curves figure 2 and figure 3 give the admissible avalanche power versus tp and Tj.
PARM(1µs, 25°C) for each part number as well as
the derating curves are given in the respective
datasheet.
The designer must ensure that the guaranteed
avalanche energy EARM(tp,Tj) is greater than the
avalanche energy in the application EAP.
October 2003 - Ed: 1
0.01
0.1
1
10
100
1000
0.1
0.01
0.001
1/2
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
Vout
Vin
In a typical worst-case situation, the application
conditions are:
n Operating junction temperature of the Schottky
diode:
Tj = 100°C
n Pulse duration of the avalanche current:
tp = 10ns
n Avalanche energy by pulse through the two
diodes connected in parallel:
VP = -130V, IAR = -1.5A, tp = 10ns
⇒EAP = 1.95µJ
• Table 1 gives:
PARM(1µs, 25°C)STPS20H100CT = 10.8 kW per diode
• Figure 2 gives:
Figure 5 shows the corresponding current and
voltage waveforms through the two diodes.
Fig. 5: Current and voltage waveforms through
the two diodes.
(IAR = repetitive avalanche current)
PARM (tp ,100 °C )
= 0.45
PARM (tp ,25 °C )
⇒PARM(1µs,100°C) = PARM(1µs,25°C) x 0.45
⇒PARM(1µs,100°C) = 4.86 kW
• Fig.3 gives:
IDiode
PARM (10ns ,Tj )
=1
PARM (1µs ,Tj )
⇒PARM(10ns,100°C) = PARM(1µs,100°C)
⇒PARM(10ns,100°C) = 4.86 kW
tp
IAR
VR
corresponding energy
in the avalanche area
Finally,
EARM(10ns,100°C) = PARM(10ns,100°C)x10ns
The maximum admissible avalanche energy of the
STPS20H100CT at 10ns and 100°C is:
EARM(10ns,100°C) = 48.6µJ per diode
Consequently, as the guaranteed value
EARM(10ns,100°C) (per diode) is higher than EAP
measured through the two diodes connected in
parallel (48.6µJ > 1.95µJ), the STPS20H100CT
will withstand the avalanche energy generated by
the converter.
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