# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 333

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optimised for low switching frequencies and high stray inductances. Also, additional
improvements can bemade by using driver stages optimised for the application (chapter
. Snubbers should thereforeonly beused as anexception.
Not covered in this chapter were short circuitswith IGBTs and snubber networks. These
can no longer be measured easily with an U
CEsat
monitor due to the oscillations of the
RCD-network between IGBT and snubber diode. Thus, theymust be evaluated explicitly
when implementing a snubber network.
7.11.2Resonant switching
To use IGBTs in high frequency applications is possible by keeping one of the two
parameters – the current I
C
or the voltageU
CE
– at or near zero at the time of switching.
To switch with I
C
at or near zero is called zero current switching (ZCS), while switching
with U
CE
at or near zero is then called zero voltage switching (ZVS). Topologies based
on this concept are often integrated into LC-networks and operate at or near the
resonance point of these LC-combinations. This explains the classification of such
circuits as "resonant topologies".
exemplifies the design of a circuit with a series-connected resonant network
in the loadpath (the exampleof a parallel-connected resonant network is detailed i
. The resonant frequency of the LC-combination results from
and is set
at 25kHz.
r
r
0
CL 2
1
f
⋅π⋅
=
Eq. 7.10
If the IGBT switching frequency equals the resonant frequency of the LC-circuit exactly,
then the IGBT will switch at each zero crossing of the load current
). If the
switching frequency is higher than the resonant frequency, then the load side will show
inductive behaviour, i.e. the current lags the voltage. In this case, the IGBT turn on
passively at the zero crossing. Turn-off, however, happens actively at a current above
0A
). If the switching frequency is below the resonant frequency, then the
load side will show capacitive behaviour, i.e. the current leads the voltage. Under these
operating conditions the IGBT turn on at a current above 0A. Once the current crosses
zero the IGBT turns off passively
).
When the IGBT turns on passively, please note that it may come to a brief boost of the
IGBT collector-emitter voltage as soon as the current starts to flow in the IGBT. The
reason for this is that once a positive gate voltage is applied, the IGBT is not yet flooded
with carriers and at the same time there is no space-charge region left in the IGBT (the
blocking voltage present equals the forward voltage of the associated freewheeling
diode). As soon as it takes up current, the conductance of the IGBT is still low and – in
connection with the existing stray inductances – causes a voltage drop greater than the
nominal IGBT saturation voltage, which would normally have been expected. Once the
conductance increases due to the flooding of the IGBT drift zone with carriers, the
voltage overshoot decreases, until it has dropped to the appropriate saturation value
. In an application, therefore, which features a U
CEsat
monitor, it may come to
an erratic trip when the voltage overshoot exceeds the reference voltage of the
monitoring circuit (chapte
.
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