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CHAPTER 2: EXPERIMENTAL IMPLEMENTATION OF MATRIX CONVERTER DRIVE
Voltage [%]
Voltage [%]
1
0.5
0
−0.5
−1
1
0.5
0
−0.5
Venturini, q = 0.5
Input voltage envelope Output voltage reference
Venturini Optimized, q = 0.866
−1
0 0.005 0.01 0.015 0.02
time [s]
0.025 0.03 0.035 0.04
Figure 2.3: Output voltage comparison for modulation techniques
the input and the output to be of different nature: it should not have inductors both
at the input and output (imposed current) or capacitors (imposed voltage). Failing
this condition, the switch will have to close across two different sources of voltage or
currents, driven on each side at different values; this is an unacceptable condition that
will most probably cause the failure of the device. Assuming to have a motor as a load,
because of its inductive nature, every output must be connected to an input at any
given time, to prevent current transients.
Usually the chosen devices for the switches are IGBT because of their current capa-
bilities, however this devices are intrinsically unidirectional, thus every switch in Fig.
2.1 actually consists of two devices connected in anti-parallel, in common collector or,
more often, common emitter configuration. Taking this into account and consider-
ing the finite amount of time needed for a semiconductor to change state, it is clear
that to change configuration of the switches a special sequence must be introduced, to
overcome these problems [36][37]. To do so three commutation techniques have been
introduced: 2 step, 3 step or 4 step commutations. The two step commutation tech-
nique simply uses a single dead time between the switching off of a transistor and the
switching on of the other one, leaving to the clamp circuit the task of dealing with the
over-voltages generated when there is no path for the current. The four step commuta-
tion sequence is represented in Fig. 2.4, where each switch adopts the common emitter
configuration; the IGBT that creates a path for the current to flow from the source to
the load will be called forward IGBT and the one that allows the current to go from the
load to the source will be called reverse IGBT. This sequence implies a first step with
the switching off of the reverse IGBT (1); to control the appropriate IGBT the current’s
direction can be sensed and fed to the control. The second step will close the forward
20
(a)
(b)