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ML Converter Topologies 23
N >= 2, M >= 2
N M
Generalized ML
ANPC
A higher number of output levels are available. The
smallest step possible at the output is smaller than the
lowest capacitor voltage. Such small steps can only be
achieved by simultaneous commutations. Non
optimum modulation schemes have to be used in the
case of single commutations.
3.5.3 Modular multi level converter M 2 LC
The M 2 LC [17] applies two converter branches per phase leg, one connecting to the positive
supply and one connecting to the negative supply. Both branches are made of switched capacitors
that can either be series connected or bypassed, thus generating a controllable DC voltage. Both
branches can generate the same output voltages and are applied simultaneously. If full bridges
instead of half bridges are used in the two branches, an AC input voltage can be applied. Direct ML
AC/AC conversion based on such cascaded H-bridges has been proposed by Erickson [23], and by
Glinka and Marquardt [24].
Different ways of implementing the individual branches are possible as shown in Figure 27.
Graph (a) and (b) show two possible implementations with 2-L half bridges, the AC output of
every half bridge connecting to either DC+ or DC- of an adjacent cell. Graph (b) shows one cell at
the output shared by both branches, which reduces parts count, but increases the losses in this cell.
Graph (c) connects outputs of adjacent cells and also makes direct series connection of two cell
capacitors. This may have an impact on the design but is exactly equivalent from operational point
of view.
2
1
(a) (b) (c)
Figure 27, different implementations of the M 2 LC
The M 2 LC capacitors can be operated in steady state by maintaining the same average current
in all capacitors, independently of absolute current and output voltage. Consequently, all capacitors