A disadvantage of direct gate drive in parallel connection that should be mentioned is
that no symmetrical compensating currents in the same direction in the auxiliary
emitters are possible. Due to themechanical design, it is basically impossible to achieve
symmetry in the stray inductances. The result is compensating currents of varying
magnitude in different directions in the auxiliary emitters. As a consequence, varying
voltages occur in different directions in the emitter resistors. This effect causes further
asymmetry when the IGBT is turned on or off. The effect is very clearly seen when two
IGBT modules are connected in parallel. A compensating current always produces a
positive voltage drop at one emitter resistor, and a negative voltage drop at the other
emitter resistor. It is also important that the connected conductors of the gate and the
auxiliary emitter be constructed symmetrically. If cables are to be used, ensure that all
the cables are the same length and the conductors are twisted identically (chapter
. The same rules apply for PCB layout: Symmetrical connection and as little gate
emitter inductanceas possible.
To lessen the disadvantages of using emitter resistors, a current compensated choke
can be integrated in the gate path instead of the emitter resistors
by using its inductance when different currents are flowing into the gate and auxiliary
emitter wires; itsmagnetic resistance also reduces the dynamic compensating currents.
If currents of the same rating flow into the gate and the emitter, the choke functions like
a low resistance. Large numbers of suitable SMD current compensated chokes are now
used in SMPS (Switched Mode Power Supply) and telecommunications and are ideal
for use in direct gate drives. For example, all current manufacturers of passive EMI
components have an extensive range of current compensatedSMD chokes.
The coupling capacitances of the IGBTmodule DCB to the heatsinks play a role
in the switching behaviour, especially for drive controls without galvanic isolation
between each of the auxiliary emitters and the gates (direct or indirect gate drive in
parallel connection). If the IGBT module baseplates do not have the same potential or
are isolated from earth, therewill be deviations in switching behaviour as the capacitive
behaviour of the IGBT modules changes. When commissioning gate drive circuits for
parallel connection, in particular, all of the surface area of the IGBTmodule baseplates
must have the same potential, just as it does in the final application. It is not sufficient
simply to connect it with a cable.
Gate and emitter resistorswith the smallest possible tolerances should be used in
parallel connection. Symmetrical positioning does not help at all if the gate resistors
have a 5% tolerance.