APPLICATION NOTE

6/4

If C is a low value of capacitance, the expression

leads to a first order differential equation:

VV

V

L

dI / dt

RD

R

f

R

max

==

+

⋅

with:

()

dI /dt

I

/t

and t

Q

/ I

/2

dI / dt

I

/ 2 Q

R

RM

b

b

rrb

RM

RRM

2

rr

==

=⋅

()

b

If the diode is of a snap-off type, we have

t0, Q

0

b

rrb

→→

and

is

very

high,

consequently V

to reach the breakdown voltage of the diode. (It is

not guaranteed by the manufacturer)

2) With a snubber, if it is supposed that

CC

C

S

j

Qrrb

>>

+

(more true for an ultrafast PN

by the second order differential equation:

dV

dt

2m

dV

dt

V. V

2

C

2

0

C

0

2

R0

2

R

SS

++

⋅

=

ωω

ω

where:

m: the absorption coefficient

m

R

2

C

L

SS

f

=

ω

0

fS

1

LC

=

⋅

There are 3 possible cases:

- m>1 behaviour without oscillation (over damping)

- m<1 behaviour with oscillation damped (under

damping), where the frequency oscillation can be

determined by:

ωω

r

2

1m

=−

0

- m=1 limit of behaviour without oscillation (critical

damping)

Like this, in this case, it is possible to suppress the

oscillation across the diode with m>1.

3.2.1.2 Turn-off behaviour for a Schottky diode

For an ideal Schottky diode, there is no recovery

has a capacitive type recovery Fig. 14.

I

I

0 and V

0

LRM

D

f

==

≈

Without a snubber circuit, the new solution of the

differential equation gives:

VV

2 V

DR

R

max

== ⋅

Unfortunately, it is difficult to realize a perfect high

voltage Schottky. The reason is the presence of a

parasitic bipolar diode in parallel with the Schottky.

If it is polarized, the recovery charge is added at

the turn-off. The phenomenon begins to appear for

a 100V technology.

In

the

same

conditions

as

before,

the

STPS16150CT is used:

dI /dt=250A/µs

R

I =1.6A

RM

V =42V

R

20V/div

2A/div

50ns/div

dV/dt

I

I

I

V

V

V

Tcase=100°C

R =22ohms

s

C =2.2nF

s

t0

t0

I

V

V

=120V

Rmax

V

=58V

Rmax

dI /dt=130A/µs

F

Fig. 15: Switching behaviour of a STPS16150CT

I

D

V

D

t

0

V

R

t

V

Rma x

Fig. 14: Turn-off for a perfect Schottky diode

without snubber