IGBT Modules - Technologies, Driver and Application (Second Edition) - page 258

6 I
Fig. 6.42
Parasitic aspectswhen usinganexternal capacitor C
6.6.3 Gate lead inductance
When connecting the driver to the gate of the IGBT, inevitably there will be a parasitic
inductance present in series with the gate resistor. This results partly from the lead
inductances (regardless of whether the connection is a cable or a copper track on a
circuit board), the self-inductance of the gate resistor and the inductances of themodule
construction for the gate connection. Particularly high inductances arise when the
connection ismade via a cable, even if it is twisted.
The influence of the gate lead inductance is shown in
There are two
overlapping turn-on and turn-off sequences depicted with different inductances in the
gate path. The higher the inductance, the faster the IGBT turns on. When turning off,
however, the switching speed remains the same. Therewill only be a delay of the entire
switching process.
This behaviour only occurs during the turn-on sequencewith a bipolar gate voltage such
as -15V/15V.With an unipolar gate voltage of 0V/15V, on the other hand, the turn-on is
merely delayed. The switching speed remains unchanged. The reason for this is that
initially the inductance opposes the charging of the gate capacitance by the increasing
driver current. Once themaximum of the gate current is reached, the inductance tries to
maintain the current flow. It works briefly as a current source and supports charging the
gate capacitancewith an increased gate current (compared to a charge processwithout
or very low gate lead inductance). For the IGBT it is relevant when the threshold voltage
is reached. At a gate voltage of 0V/15V the maximum of the gate current, at
increased gate lead inductance, will only be reached once the gate voltage has already
passed the value U
. In this case only a faster gate charge after leaving the Miller
plateau towards the final value of 15V takes place. In contrast, at a gate voltage of -
15V/15V, the maximum of the gate current is reached before getting to U
. Thus,
from this point in time onwards, the gatewill already be charged with an increased gate
current due to thegate lead inductance.
More about the influence of the gate lead inductance can be found in chapte
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