IGBT Modules - Technologies, Driver and Application (Second Edition) - page 477

465
Using snubbers across the AC-terminals of the IGBT module changes the stray
inductance, which is reduced by the parts L
σ
,DC_ESL
and L
σ
,DC
. Therefore, with sufficiently
low stray inductanceof the snubber itself, it follows:
Module ,3
DC,2
ESL _DC,1
Module ,3
Snubber
,4
L
L
L
L
L
σ
σ
σ
σ
σ
+
+
<
+
Eq. 13.7
(
)
noSnubber
,
peak
Module ,3
Snubber
,4
DC
snubber
,
peak
U
dt
di
L
L U
U
< ⋅
+
− =
σ
σ
Eq. 13.8
The capacitance of the snubber can be roughly calculatedwith the following equation:
2
DC
peak ,CE
peak
total
,
Snubber
)U U(
I
L
C
σ
Eq. 13.9
L
σ
,total
: Total stray inductance (module, DC-bus, capacitors) [H]
I
peak
: Maximum peak current during switching [I]
U
CE,peak
: Maximum permitted collector-emitter voltageminus
safetymargin [V]
U
DC
:
DC-bus voltage [V]
The use of snubbers creates a damped resonant circuit, which generates a current
oscillation with a frequency f
osc
according to
uring the turn-on and turn-off
switching sequences.
Snubber
Snubber
,4
DC,2
ESL _DC,1
osc
osc
C)
L
L
L(
2
1
T
1
f
+
+
⋅π⋅
= =
σ
σ
σ
Eq. 13.10
Themaximum value of the oscillation (without the portion of theDC-bus voltage) can be
describedwit
Snubber
2
C DC,2
ESL _DC,1
peak , osc
C
I)
L
L(
U
+
σ
σ
Eq. 13.11
When selecting a suitable snubber capacitor, various aspects have to be considered.
Apart from the actual capacitance, assure a low self inductance of the capacitor (given
by the ESL value). Preferable are capacitors which have connections directly matching
the IGBT module
. This minimises the area covered by the leads and, hence, the
resulting inductance. In addition, the voltage rating of the capacitor has to match the
voltage rating of the semiconductor and theDC-bus voltage. The nominal voltage of the
capacitor should not be below 1kV if, in an application, 1.2kV IGBTmodules are used,
which are typically operated at a DC-bus voltage of less than 800V. It has to be noted
that this capacitor voltage is temperature dependent and it may lead to the selection of a
capacitor with more than 1kV when the de-rating is high due to high operating
temperatures. In order not to damage the capacitor during operation, one must not
7
Epcos, for example, manufactures capacitors with suitable connections for the IGBT modules of various
manufacturers such as Fuji Electric, InfineonTechnologies, Mitsubishi Electric, SemikronandToshiba.
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