IGBT Modules - Technologies, Driver and Application (Second Edition) - page 122

IGBTmodules are usually designed for pollution degree 2 and very occasionally also for
pollution degree3.
Furthermore, an overvoltage class has to be specified, which is also divided into four
Overvoltage class 1
refers to systems connected to supplies in which
measures have been taken to limit brief voltage surges to a sufficiently low
Overvoltage class 2
applies to systems connected to a fixed supply, including
household appliances and portable equipment. The next class, overvoltage
class 3, applies to such appliances and equipment only if theywill be subject to
unusual demandswith regard to reliability and availability.
Overvoltage class 3
also applies to systems that are connected to a fixed
supply, but have special requirements with regard to reliability and availability,
such as industrial systems.
Overvoltage class 4
applies to systems such as electricity meters connected
to the supply on site.
Depending on how they are used in the application, IGBTmodules are usually grouped
into overvoltage class 2 (e.g. consumer applications) or 3 (e.g. industrial applications).
An impulse voltage can be derived from the relevant standards taking into account the
relevant system voltage for the application and the overvoltage class. For example,
according to IEC 60664-1, overvoltage class 2 and a system voltage of 300V give an
impulse voltage of 2.5kV. As a next step, the minimum clearances required can be
derived from the tables of the relevant standards.With an impulse voltage of 2.5kV and
a pollution degree of 2 it follows, providing that the electrical field is non-homogenous in
accordance with IEC 60664-1, that the clearance must be at least 1.5mm in order to
guarantee functional insulation. It should be noted that this applies only to a maximum
altitude of 2000m above sea level. If the system is used above this altitude, correction
factors must be taken into account, which increases the required clearance. In the
previous example, the clearance of 1.5mm increases by a factor of 1.48 to 2.22mm if it
is tobeused toamaximum altitudeof 5000m.
To determine theminimum creepage required, a third factor is required in addition to the
degree of contamination and associated voltages: The material group. Four material
groups are set out in the standards, graded according to creepage resistance or CTI
(comparative tracking index) value. The creepage resistance states the dielectric
strength of the surface of insulating materials, particularly under the influence of
humidity and contamination. It determines the voltage to which the base material does
not show any tracking if 50 drops of a standardised electrolyte solution are applied.
Measurement is carried out on an etched surface, with one drop falling between two
platinum electrodes every 30 seconds. The specific details of themethod for measuring
the CTI value are set out in the standard IEC 60112. The four material groups derived
from it are:
Material group I
, 600
Material group II
, 400
CTI <600
Material group IIIa
, 175
CTI < 400
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