APPLICATION NOTE

4/9

3.2.1. Difference of spike voltage between a

150V Schottky and 200V PN diode

used in §2) across the diode will be maximum, for

VV

n

n

V

R

INmax

s

p

out

=⋅

+

(cf Fig. 9)

generally an overvoltage spike at the turn-off of the

diode is observed (Fig. 9). It can be shown that

with a conventional bipolar diode, this spike is

more important for a Flyback converter working in

continuous mode than in a discontinuous mode.

In the case of a high spike voltage, the Maximum

defined in Fig. 9.

To limit this peak and to preserve a "guard band"

breakdown voltage), the designer places a

Generally, the "guard band" is such that the

maximum voltage reapplied to the diode does not

This spike voltage is due to the leakage inductance

recovery charge of the diode, which itself depends

on the diode technology: bipolar diode or Schottky

diode.

3.2.1.1 Turn-off behaviour for a PN diode

In the datasheet are specified the main turn-off

represented in Fig. 10:

The following oscillogram shows the turn-off

behaviour for a bipolar diode (STPR1620CT) with

snubber and without snubber, in a 24V/45W

Flyback working in continuous mode.

To observe the phenomenon correctly, it is

necessary to compensate the delay time between

the voltage and the current, (by temporal shift) due

to the measuring equipment Fig. 11.

V

D

V

s

I

D

V

in

n

p

n

s

V

P

V

o u t

V

D

V

Rmax

I

RM

I

D

Turn-off diode

R

s

C

s

V

RRM

V= V

RIN

+V

out

n

n

s

p

Fig. 9: Spike voltage across the rectification diode

I

d /dt

I F

t

d /dt

I R

Q = Q + Q

rr

rra

rrb

t = t + t

rrab

V

R

Q

rra

Q

rrb

I

RM

V

t

a

t

b

S =

t

t

b

a

V

Rma x

Fig. 10: Key parameters at turn-off for a bipolar

diode without snubber

dI /dt=600A/µs

R

I =4A

RM

V =42V

R

t0

t0

20V/div

2A/div

50ns/div

dV/dt

I

I

I

I

V

V

V

V

Tj=100°C

delay time

Compensative curve

R =22ohms

s

C =2.2nF

s

dI /dt=130A/µs

F

V

=250V

Rmax

V

=90V

Rmax

Fig. 11: Switching behaviour of a 200V bipolar

diode