ST AN1768 APPLICATION NOTE

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AN1768

APPLICATION NOTE

ADMISSIBLE AVALANCHE POWER OF SCHOTTKY DIODES

October 2003 - Ed: 1

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.

0.2

0.4

0.6

0.8

1.2

25 50 75 100 125 150 175

Tj (°C)

P (t ,T ) / P (tp, 25°C) versusT

ARM p j ARM j

Fig. 2: Avalanche power derating over tempera-

ture range.

1. DESIGN RULES

The first step for the designer is to estimate, in the

worst-case conditions, the following parameters:

Operating junction temperature: Tj

Pulse duration of the avalanche current: tp

Avalanche energy by pulse generated by the

converter in the Schottky diode: E

STMicroelectronics guarantees for each Schottky

diode a reference avalanche power given at

tp=1µs and Tj=25°C: P

ARM

(1µs,25°C) (corre-

sponding to a rectangular current pulse ).

Table 1

gives P

ARM

(1µs,25°C) for some part

numbers.

Part number

ARM

(1µs; 25°C)

per diode

STPS1545D (2x7.5A) 2.7 kW

STPS2045CT (2x10A) 4 kW

STPS3045CT (2x15A) 6 kW

STPS20H100CT (2x10A) 10.8 kW

Table 1: P

ARM

(1µs, 25°C) values for some ST

Schottky diodes.

Derating curves

figure 2

and

figure 3

give the ad-

missible avalanche power versus tp and Tj.

ARM

(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 E

ARM

(tp,Tj) is greater than the

avalanche energy in the application E

P (t ,T ) / P (1µs, ) versus tp

ARM p j ARM

tp(µs)

10.01 0.1

0.1

0.001

0.01

100 100010

Fig. 3: Avalanche power derating over pulse dura-

tion range

AN1768 - APPLICATION NOTE

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2. DESIGN EXAMPLE

Let us consider the use of a STPS20H100CT (two

10A, 100V ST Schottky diodes in TO-220 pack-

age) used in a flyback converter (

figure 4

In a typical worst-case situation, the application

conditions are:

Operating junction temperature of the Schottky

diode:

Tj = 100°C

Pulse duration of the avalanche current:

tp = 10ns

Avalanche energy by pulse through the two

diodes connected in parallel:

= -130V, I

= -1.5A, tp = 10ns

⇒E

= 1.95µJ

•

Table 1 gives:

ARM

(1µs,25°C)

STPS20H100CT

=10.8 kW perdiode

•

Figure 2 gives:

Ptp C

PtpC

ARM

(, )

100

045

⇒P

ARM

(1µs,100°C) = P

ARM

(1µs,25°C) x 0.45

⇒P

ARM

(1µs,100°C) = 4.86 kW

• Fig.3 gives:

PnsT

PsT

ARM j

(,)

⇒P

ARM

(10ns,100°C) = P

ARM

(1µs,100°C)

⇒P

ARM

(10ns,100°C) = 4.86 kW

Finally,

ARM

(10ns,100°C) = P

ARM

(10ns,100°C)x10ns

The maximum admissible avalanche energy of the

STPS20H100CT at 10ns and 100°C is:

ARM

(10ns,100°C) = 48.6µJ per diode

Consequently, as the guaranteed value

ARM

(10ns,100°C) (per diode) is higher than E

measured through the two diodes connected in

parallel (48.6µJ > 1.95µJ), the STPS20H100CT

will withstand the avalanche energy generated by

the converter.

Vout

Idiode

Vdiode

Vin

Fig. 4: Topology of a flyback converter.

Diode

corresponding energy

in the avalanche area

Fig. 5: Current and voltage waveforms through

the two diodes.

= repetitive avalanche current)

Figure 5

shows the corresponding current and

voltage waveforms through the two diodes.

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