# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 231

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signal turns on the transistors T
3
and T
2
in the example of
Accordingly, the
inverted signal turns onT
1
andT
4
.
When T
3
and T
2
are turned on, a positive supply voltage is present at the gate of the
IGBT. A current, limited by the gate resistor R
G
, charges the input and reverse transfer
capacitance of the IGBT. If the switches T
1
and T
4
are turned on, the positive supply
voltage is present at the emitter of the IGBT. This is equivalent to a negative gate-
emitter voltage U
GE
and the previously charged capacitances will be discharged. The
advantage of the H-bridge circuit is the unipolar supply voltage, i.e. no negative supply
voltage is needed. The disadvantage of theH-bridge circuit is themore complex control
of the transistors. Alsomore effort is required to attach a booster stage to increase the
peak current capability of the driver stage. Furthermore, it is not possible to use just one
common supply voltage for the driver stages of all bottom IGBTs in an inverter system.
EachH-bridge circuit requires a separate isolatedpower supply.
Fig. 6.17
IGBTgatedrivewithH-bridge circuit
6.3.1.2 Emitter follower in thegatepath
The circuit design of the (complementary) emitter follower is made of bipolar junction
transistors. For a positive input voltage U
in
the transistor T
1
operates as an emitter
follower
, which means that the output voltage U
out
is equal to the input
voltageminus the voltage drop of the base-emitter junction of T
1
. During this time T
2
is
switched off. The input and reverse transfer capacitance of the IGBT is then charged to
the voltage level of:
V7.0 U U U U
in
BE
in
out
− = − =
Eq. 6.4
As a consequence, to ensure the nominal turn-on voltage of 15V at the IGBT gate, the
applied input voltageU
in
has to be larger by the value of U
BE
, i.e. 15.7V. By control of T
2
with a voltageU
in
=U
EE
this transistor turns on and also operates as an emitter follower.
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