7 Switchingbehaviour in the application
The switching behaviour of an IGBT and the associated freewheeling diode is
influenced by many parameters in the application. Therefore, discrepancies from the
characteristics described in the datasheet such as turn-on and turn-off behaviour may
occur. Also certain rules need to be adhered to in order not to stress the power
semiconductors in use unnecessarily or even toprotect them fromdestruction.
The following will look in detail at aspects of IGBT implementation for many different
7.2 IGBT control voltage
An IGBT is turned on or kept in the on-state by a positive control voltage between gate
and emitter. Theoretically this voltage has to be at least at the level of the threshold
. To turn an IGBT off or keep it in the off-state a control voltage between
gate and emitter is required which is below the threshold voltage U
. As will be
shown below, these theoretical values are in practice rather useless and have to be
replacedby other voltagesmore relevant to theapplication.
7.2.1 Positive control voltage
When apositive control voltage is applied between gateandemitter, ormore precisely a
positive control voltage higher than the threshold voltage, then the IGBT will be turned
on. Due to the IGBT transconductance
the collector current I
is a function of the gate-
emitter voltage U
. There is also a dependency on the saturation voltage U
means: The higher the gate-emitter voltage the higher the possible collector current and
the lower the resulting saturation voltage. To achieve the lowest possible conduction
losses, which are determined by U
), it is thus desirable to work with a
rather high positive control voltage. It has to be born in mind though that a high gate-
emitter voltage will allow a high short circuit current in case of a short circuit. In the
application then, a compromise needs to be found between the conduction losses
during normal operation and the maximum short circuit current in case of a fault.
Common is 15V which is also shown in the datasheet as a characteristic value. The
absolute maximum shown and guaranteed by the manufacturer is typically 20V. This
should not be exceeded as otherwise, for example, dangerously high currents may
result in case of a short circuit asmentioned above. The real maximum thoughmay rise
to 60V or even 80V and is determined by the voltage blocking capability of the gate
For a typical oxide gauge of 100nm and a specific dielectric strength of
over can be expected at 100V. What is not considered here are possible peaks in the
fieldat the oxide edges, whichmay reduce the calculated dielectric blocking voltage.
The threshold voltageU
depends on IGBT andmanufacturer.
The transconductance g
defines the relation between an input voltage and an output current. The term
transconductance is the abbreviated form of ‘transfer conductance’. Conductance is here the effective
conductanceof anelectronic component.