IGBT Modules - Technologies, Driver and Application (Second Edition) - page 311

Fig. 7.7
Example to calculate theminimal dead time t
usingdatasheet values
7.5 Switching speeds
Basically it is desirable to switch with the highest possible speed as this will reduce the
switching times and, accordingly, the switching losses. However, some parameters
need to beobservedwhich, in the end, lead to an upper limit of the switching speeds.
The current gradient
di means the change in current per unit of time. Both for turn-on
as for turn-off it is advantageous to achieve high values in order to minimise the
switching losses. Basically the IGBT can achieve very fast turn-on speeds. In the real
application, this is only limited by the freewheeling diode commutating off (an exception
are the SiC freewheeling diodes). Due to the fast switching of the IGBT the reverse
recovery current peak of the diode increases. In addition, the tail of the diode reverse
recovery current increases. During this time, however, virtually the entire DC-bus
voltage is present across the diode. Accordingly, it may come to an avalanche
breakdown even below the voltage rating of the diode. This is due to the high
concentration of carriers and the high rate of change of the concentration and at the
same time the high electric field in the semiconductor. If the diode is not of sufficient
dynamic robustness and the commutation happens too fast for the diode it may come to
the destruction of the semiconductor. In the datasheet of the IGBT or IGBTmodule one
can usually find a guideline value for the turn-on current gradient. This value is usually
chosen such by the manufacturer that under the given test conditions no notable
oscillations or other operating points occur which could endanger the semiconductor.
The gate resistor mentioned in the datasheet should be interpreted along these lines,
i.e. this is the lowest possible recommended value under the given conditions according
to themanufacturer. Please note that these conditions usually include a negative control
voltage of -15V, a certain stray inductance and an unspecified impedance of the driver
output stage (which includes the gate lead inductance as described in chapter
. If
in the application there are different conditions then very different circumstances will
1...,301,302,303,304,305,306,307,308,309,310 312,313,314,315,316,317,318,319,320,321,...548
Powered by FlippingBook