for example 400V output voltage and 100V input voltage, IGBTs with U
instead of 1.2kVmust be used. Only with a smaller gap from input to output voltage it
may be possible to use IGBTswith the sameU
rating as theVSI.
11.5 AC/AC converter
Voltage source inverters as described in chapter
together with an input rectifier
stage as described in chapter
ake up the majority of topologies in use. In this
configuration, the mains voltage is first rectified and then fed to a DC-bus. In a second
stage, the DC voltage is converted to an AC variable voltage and variable frequency.
Therefore two conversion steps are necessary to supply a load from the mains.
Alternatively, a topology can be used that allows direct conversion of themains voltage
into a voltage suitable for the consumer load. Such a topology is depicted in
as a Direct Matrix Converter (DMC). The basic idea is that, at any time, one of the
mains input phases (L1, L2, L3) will have a suitable voltage for the PWM control of the
output side (U, V,W). By appropriate control of the nine switches S
, the desired output
voltage can be derived. As the switches in this configuration have to conduct the current
bidirectionally, semiconductor devices as depicted in
a) to d) are required.
While the concepts a) to c) can use existing semiconductor types, the variant d) is a
special semiconductor type: RB IGBT. A reverse-blocking IGBT has a modified edge
structure that allows building up blocking voltage in both forward as well as reverse
The advantage of a matrix converter is its compact design, as the otherwise common
DC-bus circuit with its capacitors and bus bar can be eliminated. On the other hand, the
control of the power semiconductors is far more complex than in standard VSI. This
refers both to the software and the hardwarewith the needs of the18driver channels.
A disadvantage of theDMC existswhen the output frequency f
is identical to the input
. Often, this occurs at a 50/60Hz mains input, as this frequency is also the
nominal frequency of themotor load at the output (the rated speed is then obtained from
the number of motor pole pairs). In this state, the IGBTs do not alternate and there is a
steady state, which is known as 0Hz operation with a VSI. In this case, the thermal
resistance increases, since the current only flows through one third of the switches. The
controller has toavoid this case of synchronicity of f
A two-stage variant of the matrix converter is the so-called Indirect Matrix Converter
(IMC). Advantage of this variant is a simplified current commutation concept while
retaining theadvantages of theDMC such as:
Elimination of an energy storage (DC-bus capacitor in a VSI or the DC-bus
Sinusoidal mains currents.
Power factor close to 1.
Reduced cost of the input filter.
Reduced volumeand increased power density.