IRF3808PbF
www.irf.com 7
Fig 15. Typical Avalanche Current Vs.Pulsewidth
Fig 16. Maximum Avalanche Energy
Vs. Temperature
Notes on Repetitive Avalanche Curves , Figures 15, 16:
(For further info, see AN-1005 at www.irf.com)
1. Avalanche failures assumption:
Purely a thermal phenomenon and failure occurs at a
temperature far in excess of T
jmax
. This is validated for
every part type.
2. Safe operation in Avalanche is allowed as long asT
jmax
is
not exceeded.
3. Equation below based on circuit and waveforms shown in
Figures 12a, 12b.
4. P
D (ave)
= Average power dissipation per single
avalanche pulse.
5. BV = Rated breakdown voltage (1.3 factor accounts for
voltage increase during avalanche).
6. I
av
= Allowable avalanche current.
7. ∆T = Allowable rise in junction temperature, not to exceed
T
jmax
(assumed as 25°C in Figure 15, 16).
t
av =
Average time in avalanche.
D = Duty cycle in avalanche = t
av
·f
Z
thJC
(D, t
av
) = Transient thermal resistance, see figure 11)
P
D (ave)
= 1/2 ( 1.3·BV·I
av
) = DT/ Z
thJC
I
av
=
2DT/ [1.3·BV·Z
th
]
E
AS (AR)
= P
D (ave)
·t
av
1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01
tav (sec)
1
10
100
1000
A
v
a
l
a
n
c
h
e
C
u
r
r
e
n
t
(
A
)
0.05
Duty Cycle = Single Pulse
0.10
Allowed avalanche Current vs
avalanche pulsewidth, tav
assuming
∆
Tj = 25°C due to
avalanche losses
0.01
25 50 75 100 125 150 175
Starting T
J
, Junction Temperature (°C)
0
100
200
300
400
500
E
A
R
,
A
v
a
l
a
n
c
h
e
E
n
e
r
g
y
(
m
J
)
TOP Single Pulse
BOTTOM 10% Duty Cycle
I
D
= 140A
