IGBT Modules - Technologies, Driver and Application (Second Edition) - page 140

128
Fig. 3.12
In static operation the diode losses ensue according t
(
)
F F vj
T vj
)TO(
F vj
FF D
I I)T(r )T( U I)T,I(U P
⋅ ⋅
+
= ⋅
=
Eq. 3.7
Usually the forward voltages of freewheeling diodes in IGBT modules are quoted in
datasheets at chip level and nominal current and for at least two different junction
temperatures (25°C and 125°C). For later chip generations even three temperature
values (25°C, 125° and 150°C) can be found. Further, the load current in a range from
0A to
nom,F
I2
is shown as a set of characteristics (similar as given in
to the
right). If U
F
is not stated at chip level reference but for the module 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 be
deducted from the valueof U
F,Module
.
F 'EE 'CC
Module ,F
Chip ,F
I
R
U U
=
+
Eq. 3.8
The threshold voltageU
(TO)
can be derived from the characteristics by placing a tangent
at I
F,nom
and extend it to the abscissa (X-coordinate). The intersection of the tangent with
the abscissa gives U
(TO)
, as can be seen
to the left. Also the forward slope
(differential) resistance r
T
of the diode in forward direction can be derived from the
characteristics. Given these values, the forward losses of the diode at a certain current
cannowbe ascertained.
In a real application, however, very seldomwill there be static conditions with constant
current. In drive applications for example the current through the diode changes
sinusoidally due to the control algorithm. So this changing (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 diode conducts for 50% of
the timeandblocks for the remaining time.
dt )t( )t(i)t(u
T
1
P
2
T
0
F F
D,
cond
⋅ τ⋅
=
Eq. 3.9
with
)t(i)T(r )T( U)t(u
F vj
T vj
)TO(
F
+
=
and
)t
sin(
iˆ )t(i
F
F
ϖ ⋅ =
1...,130,131,132,133,134,135,136,137,138,139 141,142,143,144,145,146,147,148,149,150,...548