13.3 Voltage ratings
When selecting components for an inverter, voltage levels arising under various
conditions need to be considered. These conditions include the nominal mains voltage
for mains operated inverters. Further, theDC-bus voltagewhich, in battery and
fuel cell operated inverters, is synonymous with the supply voltage U
. Also the
switching over-voltages occurring, as well as the maximum voltage permitted for a
maximum current inorder to staywithin the specifiedSOAof the components.
The DC-bus voltage is derived from the supply voltage either directly (e.g. with battery
operation) or indirectly (with input rectifier and DC-bus capacitor). For mains operated
inverters the mains variations must be included in the calculation. The over-voltages
that occur are typically in the region of 10% for low-voltage industrial applications,
around 15% for medium-voltage applications and 20% for traction applications. With
battery operated inverters an increase of 20% of the nominal value U
is a good
guideline. The following calculations for theDC-bus voltage apply, where s
is the safety
RMS , nom
Apart from other factors, the level of theDC-bus voltage also determines the failure rate
of the power semiconductor breakdown caused by cosmic radiation (chapter
well as the long term stability of the specifiedblocking currents.
As mentioned in the previous chapters, over-voltages occur at the power
semiconductors during the switching processes. These depend mainly, but not
exclusively, on the stray inductance of the commutation path.When selecting the power
semiconductors, the required blocking voltage of the components is estimated based on
theDC-bus voltage plus a safetymargin s
.With smaller stray inductances 50% can be
used as margin and with medium stray inductances 60%. "Small" and "medium" are
parameters, whichdependon the applicationunder consideration.
s 1 U U
Once the result exist from the calculation of
which is based on two-level
, select the component with the next higher available voltage rating.
exemplifies an overview of common supply voltages and derived DC-bus voltages, as
well as preferred voltage ratings for IGBTpower semiconductors.
For three-level inverters the required IGBT voltage rating canbehalved.