# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 239

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very small gate resistor R
Gon
is chosen, which will be increased in value once U
GE(TO)
is
reached. This procedure is also called two-stage switching.
Another method is illustrated in
The gate is switched on with an increased
gate-emitter voltage and the gate current increases accordingly. After reaching the level
of U
GE(TO)
, the boost voltage (24V) is switched off and the IGBT capacitances are
charged by the nominal gate voltage (15V) andan reducedgate current.
6.3.1.8 Designof thegatedrives
The design of the gate drive focuses on the static and dynamic behaviour of the IGBT
but also the corresponding freewheeling diode. The fundamental aspects of the design
are considered below. Measures to influence the switching behaviour and integrated
protective functions are discussed in later chapters.
When selecting a driver stage, one important parameter is the maximum peak current
I
peak
, which is required to drive the IGBT. For this, the turn-on and turn-off currents are
considered separately. Even if inmany applications, turn-on and turn-off are treated the
same, both should be investigated separately and calculated with the smallest gate
resistor.
An estimation of themaximumpeak current canbedonebyusin
Gext
intG
min ,GE
max ,GE
minG
GE
peak
R R
U U
7.0
R
U 7.0 I
+
=
Eq. 6.5
I
peak
:
Peak current, which thediver stage has toprovide [A]
U
GE,max
: Positive gate voltage to switch the IGBTon [V]
U
GE,min
: Negative gate voltageor 0V to switch the IGBToff [V]
R
Gint
:
Internal gate resistor of the IGBT (if present) [
]
R
Gext
: External gate resistor [
]
If an additional external gate-emitter capacitor C
G
is used as part of the driver stage, in
good approximation this capacitance equals a short circuit of the internal gate resistor.
Hence, R
Gint
i
has tobe set to0
.
In practice, the correction factor of 0.7 is required for the peak current. The reason for
this is the internal driver impedance always present and parasitic effects of lead
resistances and inductances. The correction factor is derived using the following
considerations:
Assuming a constant internal capacitance C
GE
of the IGBT during the turn-on and turn-
off events, a parasitic inductance L
G
Gon
and L
Goff
, the
followingdifferential equation for a second order RLC circuit derives:
0 )t( i
C
1
dt
)t(di
R
dt
)t( idL
G
GE
G
G 2
G
2
= ⋅
+
+
Eq. 6.6
L: Sum of the inductances in thegate path [H]
R
G
: Sum of external and internal gate resistors [
]
i
G
(t): Time-dependent gate current [A]
Theminimumgate resistanceR
G,min
in the gate path, which does not lead to oscillations,
is:
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