AN1453 datasheet(3/9 Pages) STMICROELECTRONICS

3 / 9 page

APPLICATION NOTE

3/9

2. REVERSE LOSSES AND TJMAX

2.1. Reverse losses: Prev

The reverse losses can be determined by:

rev

=⋅ ⋅ − α)

with:

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

applied

IR: leakage current versus VR and operating

junction temperature (Tj)

VR: reapplied voltage accross the diode

Fig. 6 shows an example of reverse losses in a

Flyback converter with the following conditions:

()

−=

0.4, V

80V, T

125 C

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. Tjmax before thermal instability is reached

Remembering that the stability criterion is given

by:

rev

th(j a)

−

with:

V .I

.(1

rev

R(VR,Tjmax)

=− α)

The above formulae give the critical value of the

leakage current before the thermal runaway limit is

reached:

Vc.R

R(VR,Tjmax)

th(j a)

⋅⋅ −

−

()

The evolution of the leakage current versus Tj and

VR is given by:

exp

R(V ,Tj)

R(V ,125)

c(Tj 125)

−

From these physical laws, it can be deduced that:

125

jmax

R(V ,Tjmax)

(V ,125 C)datashee

max

=+ ⋅

Example:

Flyback converter with 2 diodes in parallel

()

−=

0.4, c

0.069, V

80V

2.4 C / W, R

7.6 C / W

th(j c)total

th(c a)

−−

=°

This example shows that in a typical application, a

150V Schottky can be used up to 175°C.

STMicroelectronics specifies in the datasheet

Tjmax at 175°C.

3. SWITCHING BEHAVIOUR

3.1. Turn-on behaviour

The behaviour at turn-on is characterized by a low

value of peak forward voltage (VFP) and forward

reverse recovery time (tfr) (Fig. 8).

These values depends mainly on the dIF/dt. The

switching losses at turn-on are always negligible.

3.2. Turn-off behaviour

The turn-off behaviour is a transitory phenomenon

(ns), but repetitive depending on the switching

frequency. It is a source of spike voltage, noise

and for high switching frequency, of non-negligible

switching losses.

In order to illustrate this phenomenon, 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.

IRtyp per diode

100V, 125°C

per diode

STPS10150CT

130µA

4.2mW

Fig. 6: Example of reverse losses in a Flyback

converter

For a dual

diode

IRmax

(80V, 125°C)

IR(VR,Tjmax)

Tjmax

STPS10150CT

1.3mA

45.28mA 176.5°C

Fig. 7: Example of Tjmax with STPS10150CT

IF=16A

dIF/dt=100A/µs

Tj=25°C

Per diode

tfr

(ns)

VFP

(V)

STPS16150CT

100

2.2

Fig. 8: VFP and tfr for STPS16150CT

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