138
be found. Further, the load current in a range from 0A to
nom,C
I2
⋅
is shown as a set of
characteristics (similar as given in
to the right). If U
CE
is not stated at chip level
reference but for themodule terminals instead, it needs to be recalculated to chip level.
To do this, the voltage drop across the internal module resistance R
CC’+EE’
, which is
usually quoted in the datasheet, needs to bededucted from the valueof U
CE,Module
.
C 'EE 'CC
Module ,CE
Chip ,CE
I
R
U U
⋅
−
=
+
Eq. 3.22
The threshold voltageU
CE0
can be derived from the characteristics by placing a tangent
at I
C,nom
and extend it to the abscissa (Xcoordinate). The intersectionof the tangent with
the abscissa gives U
CE0
, as can be seen in
o the left. Also the forward slope
(differential) resistance r of the IGBT in forward direction can be derived from the
characteristics. Given these values, the forward losses of the IGBT at a certain current
cannowbe ascertained.
In a real application, however, very rarely will there be static conditions with constant
current. In drive applications for example the current through the IGBT changes
sinusoidally due to the control algorithm. Hence this variation (in current) has to be
consideredwhen calculating the forward losses. The average losses per one sinewave
period are delineated b
It is assumed here that the IGBT conducts for 50% of
the timeandblocks for the remaining time.
dt )t( )t(i)t( u
T
1
P
2
T
0
C CE
I,
cond
⋅ τ⋅
⋅
=
∫
Eq. 3.23
with
)t(i)T(r )T( U)t( u
C vj
vj
0CE
CE
⋅
+
=
and
)t
sin(
iˆ )t(i
C
C
ϖ ⋅
=
The turnon period
τ
(t) of the IGBT according to
is identically defined as the
diode turnon period. I
is entered into
nd the integer is resolved the
result is:
⋅
⋅
π
+ ⋅
ϕ⋅
⋅
⋅
+
π
⋅
=
2
C vj
C
vj
0CE
2
C
vj
C
vj
0CE
I,
cond
iˆ)T(r
3
1
8
iˆ
)T( U cos
4
iˆ
)T(r
iˆ
)T( U
2
1
P
+m
Eq. 3.24
To resolv
he following IGBTdata parameters areneeded:
•
Threshold voltage U
CE0
(T
vj
). As in
his can be taken from the I/U
diagram of the IGBT at different junction temperatures. Temperatures not
shown in the I/Udiagrammaybe derived by interpolation.
•
Onstate slope resistance r(T
vj
). This too can be taken from the I/U diagram as
detailed i
U
CE0
and r (and thus finallyU
CE
) are verymuch dependent on the voltage rating and the
technology of the IGBT. The reason for this is the doping concentration and the length
of the n

base of the IGBTwhich ismainly responsible for these values.
shows
an overview of the typical saturation voltages U
CEsat
at T
vj
= 125°C of Infineon