Hitachi IGBT Module Application Manual
Figure 24 shows current and voltage waveforms of the snubber diode D
S
when the snubber circuit
functions.
Figure 24. Snubber Diode Current and Voltage Waveforms
Values associated with the labeled portions of Figures 23 and 24 can be determined using Equations 3,
4, 5, and 6:
Equation 3:
T
S
= 2
π
!
(
LstxCs
)/ 4
V = I
C
!
(
Lst
)
------------
Cs
Equation 4:
Equation 5:
V
f
= L
sn
!
di
c
/ dt + V
fr
Definitions:
I
C
L
sn
IGBT turn-OFF current
Snubber circuit inductance as viewed from IGBT’s collector
and emitter terminals
Forward recovery voltage (typically about 50V)
V
fr
di
c
/ dt
Current change ratio in IGBT’s fall period.
T
n
which is defined as the time required to discharge 95% of the overcharge
voltage of
C
s
is approximated using Equation 6:
Equation 6:
T
n
= 3
!
C
s
!
R
s
Here,
R
s
must be set to a value such that
T
S
+
T
n
<
1 / f
c
.
Note:
In the above equation, for the case of a three-phase circuit, the value
for
T
S
in Equation 3 must be multiplied by
3
, and the
V
in
Equation 4 must be multiplied by 1/
3
.
5.2.4 Collector Current Class and Snubber Capacitor Values
The snubber circuit capacitor value can be calculated using Equation 7:
Equation 7:
C = L
st
( I /
V )
2
Table 3 gives approximate guidelines for snubber capacitor values.
21
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