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ML Converter Topologies 29
In a practical implementation, the three series connected switches in the outer path do not
need to be individual switches, but can be combined into one or two switches with higher blocking
capability. If one device with high blocking capability is used and one with low blocking capability,
the one with high blocking capability can be operated at fundamental frequency and only the switch
with low blocking capability is used for the high frequency modulation.
3.7 Topology comparison
The comparison of topologies is very application specific. The required operating range in
modulation depth and load angle ϕ, the converter voltage and power level, the type of switching
device, and the capacitor technology to be used all have a significant impact on any topology
benchmarking for a specific application. However, in a generic approach, these parameters are not
defined. A thorough comparison for a specific application is a big task on its own, as has been done
for example in [32]. A limitation to a set of generic criteria is therefore used in this chapter for a
basic topology comparison.
TABLE 8, CRITERIA USED FOR THE TOPOLOGY COMPARISON
Operating range regarding mod index and
cos(ϕ)
Total blocking voltage of active switches and
minimum number of active switches
Total blocking voltage of diodes and
minimum number of diodes
- Without considering disturbances
- Including scaling laws
- Including scaling laws
Number of parallel current paths - Including impact on maximum duty cycle and peak
current in any given device
Loss distribution - Qualitative statements on inherent loss distribution and
feasibility to control it
Number of input voltage levels - Generated by series connected capacitors
Number of flying capacitors - Including scaling laws
Total energy of flying capacitors - Based on different operational limitations
- Peak switching frequency limitation
- Average switching frequency limitation
- Given apparent output switching frequency
Modularity - Feasibility of building blocks
- Number of switches with different blocking voltage
- Number of capacitors with different operating voltage
It is impossible to include all possible variations of ML converters in the comparison. Only
converters with single supply (at least per phase) are considered, as other converters like cascaded