potential. Since the reverse blocking capability of IGBTs is generally not specifie
values are definitely tobe avoided.
In the application the commutating speed
of the current is defined by the turned off
IGBTand the inductanceof the commutationpath.
The turn-on losses of the diode are defined by
When considering that during
the entire turn-on sequence either low currents are multiplied and integrated with high
voltages or high currents with low voltages, the resulting turn-on losses may be
disregarded under practical aspects. Particularly when they are brought into relation to
the turn-off losses, whichwill be considered below.
3.3.2 Diode turn-off
To characterise the diode turn-off behaviour the test circuit from
s used again.
After the diode D
is turned on at the time t
, according to the process in chapter
the inductance pushes the current through the diode (disregarding the losses here) until
is turned on again at the time t
and the current commutates from the diode
to the IGBTT
. This constitutes the turn-off moment of the diode.
Before the diode can return to blocking any voltage after turn-off all the carriers have to
be removed from the drift region first. Only after that can the depletion layer establish
itself in the diode. The removal of the carriers occurs by recombination (chapter
Carriers which could not be removed by recombination are taken out of the drift region
by a negative current. Current slopes, current peaks and their total duration are critical
factors when selecting the diode, as their behaviour determines the turn-on behaviour
and the turn-on losses of the IGBT on the one hand and may create EMI problems in
the applicationon the other hand.
Schematic of diode turn-off process
Because IGBTs are operated in almost all applications with an anti-parallel freewheeling diode connected, the
reverseblocking capability of IGBTs is not specifiedby themanufacturers.