
IR21141/IR22141SSPbF
Vcc/Vb
R DRp
I avg
C RES
flowing in R Goff and R DRn (see Fig. 22). If the voltage
drop at the gate exceeds the threshold voltage of the
IGBT, the device may self turn on, causing large
oscillation and relevant cross conduction.
R Gon
COM/Vs
Figure 21: R Gon Sizing
HS Turning ON
C RESoff
R Goff
OFF
dV/dt
where
R TOT = R DRp + R Gon
ON
R DRn
C IES
R Gon = gate on-resistor
R DRp = driver equivalent on-resistance
Figure 22: R Goff Sizing: Current Path When Low Side is
R DRp is approximately given by
Off and High Side Turns On
R DRp
? Vcc t SW
? I t
= ? o 1 + on 1
?
? ?
+
Vcc t SW ? t on 1
I o 2 + t SW
Vcc
I o 1 +
for t SW > t on 1
for t SW ≤ t on 1
The transfer function between the IGBT collector and
the IGBT gate then becomes:
V ge s ? ( R Goff + R DRn ) ? C RESoff
=
V de 1 + s ? ( R Goff + R DRn ) ? ( C RESoff + C IES )
(I O1+
,I O2+
and
t on1
from
“Static
Electrical
Characteristics”).
which yields to a high pass filter with a pole at:
Table 1 reports the gate resistance size for two
commonly used IGBTs (calculation made using typical
datasheet values and assuming V CC = 15 V).
1 / τ =
( R Goff
1
+ R DRn ) ? ( C RESoff + C IES )
Output Voltage Slope: The turn-on gate resistor
R Gon can be sized to control the output slope
(dV OUT /dt). While the output voltage has a non-
linear behaviour, the maximum output slope can be
As a result, when τ is faster than the collector rise time
(to be verified after calculation) the transfer function can
be approximated by:
approximated by:
dV out
dt
=
I avg
C RESoff
V ge
= s ? ( R Goff + R DRn ) ? C RESoff
V de
so that
Vcc ? V ge
C RESoff ?
V ge = ( R Goff + R DRn ) ? C RESoff ?
V th > V ge = ( R Goff + R DRn ) ? C RESoff
inserting the expression yielding I avg and rearranging:
*
R TOT =
dV out
dt
As an example, table 2 shows the sizing of gate
resistance to get dV out /dt= 5 V/ns when using two
popular IGBTs (typical datasheet values are used and
V CC = 15 V is assumed).
dV de
dt
in the time domain. Then the condition:
dV out
dt
must be verified to avoid spurious turn on.
Rearranging the equation yields:
NOTICE : Turn on time must be lower than T BL to avoid
improper desaturation detection and SSD triggering.
2.6 Sizing the Turn-Off Gate Resistor
The worst case in sizing the turn-off resistor R Goff is
R Goff <
V th
C RESoff
?
dV
dt
? R DRn
when the collector of the IGBT in the off state is forced
to commutate by an external event (e.g., the turn-on of
the companion IGBT). In this case the dV/dt of the
output node induces a parasitic current through C RESoff
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19
R DRn is approximately given by
? 2009 International Rectifier