Direct gatedrive inparallel connection
The emitter resistor is a key element in the direct gate drive system. Due to the fact that
all the components in a parallel connection show tolerances, the IGBTs will never turn
on or off symmetrically. One of the IGBTwill always be the first to turn on and therefore
the first to take up the load. This unbalanced switching is caused by tolerances in the
IGBT threshold voltage U
, differences in the stray inductances of the DC-bus and
the AC output path and ohmic und inductive tolerances in the gate path. The result is a
compensating current that flows through the auxiliary emitter. If no limit is applied to this
current, dynamic peak currents of up to several hundred ampsmay occur. The auxiliary
connections of the IGBTmodule are not usually designed for pulse current loads of this
magnitude, which inevitably leads to failure.
Basedonexperience, theemitter resistors are ratedas follows:
R R R
Unlike the gate resistor R
the emitter resistor R
must have pulse current capacity.
Therefore, to prevent early failures, the use of shunt resistors or metal film resistorswith
a high pulse current rating is recommended. In any case, the current in the emitter
resistors should bemeasuredand the design adjustedaccordingly.