APPLICATION NOTE

7/9

It can be observed that this is not an ideal

Schottky. In fact, when the voltage rises at , we

identifiable because it is embedded in the

capacitive current.

Unlike a PN diode, we can see that with the 150V

distinctly lower.

a STPS16150CT is the same, with the lower initial

I

I

and V

0

LRM

D

f

schottky

=≈

the 200V bipolar and 150V Schottky diode.

Whatever

the

reverse

voltage,

the

junction

capacitance of the 150V Schottky is always higher

than for a PN diode. This justifies, the lower

observed with the Schottky diode.

In summary, when we compare the different

parameters with those of the bipolar diode, we

have:

()

II

CC

dI / dt

RM

RM

jj

R

bi

bipolar

schottky

bipolar

schottky

>

<

()

()

()

polar

R

schottky

bipolar

schottky

Rma

dI / dt

dV / dt

dV / dt

V

>

>

x

Rmax

bipolar

schottky

V

>

Thus, a different efficiency of the snubber circuit

with a bipolar diode and a Schottky diode is

observed. In most cases, we can say that the 150V

Schottky behaves better at turn-off, due to its

larger capacity and its softness recovery.

Model showed in Fig. 13 can be used to define the

snubber circuit.

NB:

In the case of a Forward converter with multiple

outputs (12V, 5V, 3.3V…) and cross regulation

with coupled inductor, the poor behaviour at the

turn-off with a bipolar diode on 12V output, will be

reflected on the other coupled outputs (that means

an overvoltage on rectification diode of 5V output).

A 150V Schottky will decrease the coupled effects.

3.2.2. EMC Comparison between a 150V

Schottky and a 200V bipolar diode

The better the behaviour at turn-off of the 150V

Schottky in comparison with a 200V bipolar diode,

the better performance in the EMC.

Fig. 16 shows the comparison of electromagnetic

disturbance conducted in a 45W/24V Flyback

converter

(in

continuous

mode)

between

a

STPR1620CT and a STPS16150CT with a

snubber circuit.

We can see near 30MHz, that there is a difference

of -10dB. This difference is partially explained by

the higher dV/dt with a bipolar diode at turn-off

than with a 150V Schottky. In fact, the lower

capacitance junction of the PN diode favors the

mode. ( i

C dV / dt, C

equivalent capacitance

junction-heatsink)

The other high dV/dt, which could take place due

to the strong oscillation, are suppressed by a good

choice of the snubber circuit.

F=1MHz,Tj=125°C

0.0

50.0

100.0

150.0

200.0

250.0

300.0

1

10

100

1000

VR(V)

STPR1620CT

STPS16150CT

STPR1620CT

STPS16150CT

Fig. 17: Electromagnetic disturbances conducted

between a STPR1620CT and a STPS16150CT