# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 413

401
Fig. 11.37
Equivalent circuit of theZ-inverter in the short circuit state
Therefore the diode D
5
is in blocking mode, as the voltage across the two capacitors
combined is at least twiceas high as the input voltage.
in
C
5D
DC
C L
U U2 U
V0 U
U U
− ⋅ =
=
=
Eq. 11.23
During the active and the zero state, the voltage U
DC
appears across the output bridge
as the sum of the input voltage plus the voltage across the inductors L
1
and L
2
. For the
periodT– t
SS
or 1 –D
SS
therefore applies:
in
in
SS
in
L
DC
UB U
D21
1
UU2 U
⋅ =
⋅ −
= + ⋅ =
Eq. 11.24
B is called the boost factor and denominates the ratio between the voltage gain of input
to theoutput side.
For a three-phase output bridge the following relation applies between the DC-bus
voltageU
DC
and the output voltage:
DC
out
U
2
m U
=
Eq. 11.25
m is themodulation factor.
Accordingly it follows fro
n
in
out
UB
2
m U
⋅ ⋅
=
Eq. 11.26
Small input voltages require a large boost factor B, assuming a constant output voltage.
This in turnmeans that the time t
SS
and thus the duty cycleD
SS
have to be chosen long.
Since the shorted condition should only be used for substituting the zero state (to avoid
distortion of the output current) a small modulation factor m is required. A small
modulation factor on the other hand causes the Z-inverter to work as a buck converter.
Accordingly, B must be increased. In consequence, the IGBTs will see high blocking
voltages in comparison with a conventional VSI (plus over-voltages during the current
commutation due to parasitic stray inductances). This means that in a real application,
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