APPLICATION NOTE

5/9

Without a snubber, in this example the diode is

repeatedly in conduction because the oscillation is

very strong. Furthermore, the voltage is close to

the breakdown voltage. This means that the

system is no longer reliable and a snubber circuit is

necessary.

On these 2 oscillograms, we can see that the value

when the reverse voltage rises (typical behaviour

of a bipolar diode). At this time the voltage is not

fixed by the diode.

the datasheet. For example in Fig. 12, the

can be observed.

significantly increases with the temperature.

is fixed by the leakage inductance and the reverse

dI

dt

V

L

with V

n

n

VV

FR

f

R

s

p

IN

out

==

⋅

+

It is many time higher than in discontinuous mode

(lower than 1A/µs):

dI

dt

V

LL

withL

L

F

out

Sf

Sf

=

+

〉〉

Thus, with this curve we can see that, in

must evacuate a non-negligible charge, which

Fig. 11.

with a snubber circuit can be established:

Where:

V

n

n

V

S

s

p

IN

=⋅

equivalent

capacitance

modeling

the

reverse charge, necessary for the establishment of

the potential barrier, which supports the reverse

voltage.

I

I

and V

0

LRM

D

f

bipolar

=≈

The equivalent schematic can be used to define

VV

=

NB:

differential equation:

dV

dt

V

V

0 and

1 / L

C

2

C

2

CR

f

+⋅

+⋅

=

=

⋅

ωω

ω

0

2

0

2

0

2

I

I

and V

V

0

LRM

C

D

f0

==

=

In this equation, an approximation is made with C

the voltage applied.

The solution of the differential equation gives us:

VV

V

V

I

L

C

RD

R

R

2

RM

f

2

max

==

+

+

⋅













is very dependent on the

temperature.

STPR1620CG/CT

10

20

50

100

200

500

1

10

20

dIF/dt(A/µs)

IRM(A)

IF=IF(av) 90% confidence

Tj=125°C

Fig. 12: Peak reverse recovery current versus

V

D

V

D

V

s

L

s

n

p

L

P

R

s

C

s

C

Qrrb

C

j

C

s

R

s

L

f

I=I

L f

RM

n

s

V= V + V

R

S

o u t

V

o u t