IGBT Modules - Technologies, Driver and Application (Second Edition) - page 405

Fig. 11.28
Three-level VSI in NPC/DCML/NCML and FCML designs (only one phase
Besides the selection of the IGBT voltage rating for this type of circuit topologies, the
insulation class (chapter
of the module housing also plays a crucial role as an
example shows i
For aDC-bus voltage of 3kV a three-level NPC circuit was
selected. Thus, generally half the DC-bus voltage, that is 1.5kV, appears across the
upper and lower branch of a half-bridge (plus a certain margin, resulting from a
deviation of the voltage balancing of the DC-bus capacitors and the regeneration of the
load possibly feeding back into the DC-bus). 3.3kV IGBTmodules are selected, based
on this "visible" voltage per individual switch. Depending onwhether the neutral point is
grounded or not, several different demands are put on the insulation rating of these
3.3kVmodule packages and their IGBT gate drive. If the neutral point is grounded, and
thus has the same potential as the heatsink on which the modules are assembled, the
result is an insulation requirement for 3.3kV modules. If the neutral point is not
grounded, however, the insulation requirement is referenced to the entirebus voltage (in
this case 3kV). As a consequence, the insulation requirements are those of 6.5kV
modules. To meet the different requirements in the various applications, manufacturers
therefore offer modules of the same voltage rating but with different insulation classes.
IGBT gate drives too are offered with DC/DC converters with increased insulation
properties. For example, the so-called B5 modules from Infineon Technologies should
be mentioned, and the Iso DC/DC converters by CONCEPT. These are special 3.3kV
modules and DC/DC converters that are designed for U
= 10.2kV instead for a
standard isolation voltage of U
= 6kV. Hence, they are suitable for three-level NPC
with non-grounded neutral point andFCML topologies up toamaximumDC-bus voltage
of approximately 4.5kV.
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