Equivalent circuit of theZ-inverter in the short circuit state
Therefore the diode D
is in blocking mode, as the voltage across the two capacitors
combined is at least twiceas high as the input voltage.
U U2 U
− ⋅ =
During the active and the zero state, the voltage U
appears across the output bridge
as the sum of the input voltage plus the voltage across the inductors L
. For the
or 1 –D
= + ⋅ =
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
and the output voltage:
m is themodulation factor.
Accordingly it follows fro
Small input voltages require a large boost factor B, assuming a constant output voltage.
This in turnmeans that the time t
and thus the duty cycleD
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,