# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 411

399
Fig. 11.35
Exampleof the control of a single phaseZ-inverter
Taking the switching states illustrated in
as the basis and analysing the
processes in the Z-inverter starting at the time t = 0s, the following scenarios can be
obtained:
At the time t = 0s, the IGBTs T
1
andT
3
are on and the load current I
flows through T
1
and D
3
). The Z-inverter is in the zero state. Once IGBT T
4
is turned on, T
3
and T
4
make up the shorted state and the current flow through D
3
stops. The DC-bus
current I
DC
is then composed of the load current I
and the cross-current I
DC
– I
through IGBTT
3
The time span of the shorted state is called t
SS
. If t
SS
is set
in relation to the total period T of a switching cycle one get the short circuit duty cycle
D
SS
.
T
t
D
SS
SS
=
Eq. 11.21
The next step is to turn off T
3
will flow through T
1
and T
4
). The Z-inverter is now in the active state. Afterwards T
2
is switched on, which
then forms the shorted state together with T
1
. T
1
now carries the DC-bus current which
is increased above its previous state by the cross-current I
DC
– I
).
Finally, T
1
is turned off and the load current I
flows through D
2
and T
4
. This brings
about the zero state of theZ-inverter
).
Depending on the input voltage in relation to the desired output voltage, two general
operatingmodes of theZ-inverter areof importance:
The input voltage is higher than
2
times theRMS or peak value of the output
voltage. In this case, theZ-inverter can be operated likea standard inverter.
The input voltage is below
2
times the RMS or peak value of the output
voltage. In this case, the inherent boost function of the Z-inverter comes into
play.
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