IGBT Modules - Technologies, Driver and Application (Second Edition) - page 54

the p
-doped collector layer and holes following into the less strongly doped p-base
become more numerous. As the electron concentration flattens off, this flow of charge
carriers slowly comes to a standstill and the remaining charge carriers have to be
removed by recombination. The turn-off current in the IGBT can therefore be divided
into two phases: The first phase is determined by the turningoff of the inversion channel
and is characterised by a rapid decrease in current. The second phase lasts longer and
forms the tail current I
characteristic of an IGBT as shown in
. The first
phase is called theMOSFET portion and the second phase is the bipolar portion of the
current. The tail current therefore creates greater turn-off losses than inaMOSFET.
Fig. 1.32
Comparison of the main turn-on and turn-off behaviour of MOSFETs and
When it is turned on at the time t
, there is a charge carrier distribution in an IGBT
reducing from the collector to the emitter. If the IGBT is turned off, provided there is
inductive load, the component first takes up the voltage, before the current decays. The
process of turning off and the accompanying charge carrier distribution is illustrated
schematically in
Fig. 1.33
Charge carrier distributionof an IGBTduring the turningoff process
As the voltage has risen, charge carriers are removed from the area of the emitter
outwards up to the time t
because of the expanding space-charge region. Depending
1...,44,45,46,47,48,49,50,51,52,53 55,56,57,58,59,60,61,62,63,64,...548
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