 Hitachi IGBT Module Application Manual
Table 3. Snubber Capacitor Values
Rated Collector
Current ( I
C
)
50 A class
75 A class
100 A class
150 A class
200 A class
300 A class
400 A class
600 A class
Snubber Capacitor
Values (
µ
F )
0.10 to 0.22
0.15 to 0.33
0.22 to 0.68
0.33 to 1.00
0.47 to 1.50
0.68 to 2.20
1.00 to 3.30
2.20 to 4.70
Depending on the inductance of the main circuit wiring, larger capacitance values than are listed in
Table 3 may be required. For a snubber capacitor, use a polyester film capacitor or an oil capacitor with good
frequency characteristics.
5.2.5 Snubber Resistance Selection
The resistor value varies according to capacitor value and the IGBT’s driving frequency. When voltage
V
overcharged to the snubber is used,
ε
S
n
generated when the current I is turned OFF becomes,
according to Equation 8:
Equation 8:
ε
S
n
= 0.5
!
C
S
!
(
V)
2
Most of this energy can be considered to be consumed by snubber resistance. Supposing that the output
current of a voltage inverter or something similar is a sinusoidal current with current I in Equation 7 as a peak
value, the energy of the snubber circuit can be considered to occur as a sine wave with
ε
S
n
in Equation 8
as a peak value. The average generated loss
P
SN
in the P-N snubber circuit, taking into account the
switching of the top and bottom arms becomes, according to Equation 9:
Equation 9:
P
SN
= (2/
π
)
!
ε
S
n
!
f
c
where
f
c
is a switching frequency.
Equation 10 indicates what resistance value must be selected to prevent oscillation of the collector
current at IGBT turn-ON.
Equation 10:
R
s
> 2
(
Lsn
)
-------------
-
Cs
where
L
sn
is the inductance of the snubber wiring.
Note:
R
s
represents resistance in discharging
V overcharged to C
s
,
so that the top limit must be noted.
5.2.6 Snubber Diode Selection
Select a snubber diode having the same class as the IGBT’s rated collector emitter voltage value. Use an
IGBT module with a current rating of 1/10 to 1/5 that of the IGBT used.
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