The dependency of the short circuit current on the positive gate voltage is contemplated
Appropriate protection circuitswere introduces in chapte
7.2.2 Negative control voltage andswitchingwith 0V
By applying a negative control voltage the IGBT is turned off. Just as with the positive
control voltage, a value of -20Vmust not be exceeded. Common is -15V which is also
shown in the datasheet as a characteristic value. In the application, however, where
cost plays a role aside from performance, it may not always be sensible to use this
value. Depending on the application then, turn-off voltages in a range of 0V to -15V can
be found, where the range of -5V to -10V has established itself in many applications.
Reasons for this are:
Lower required driver power, which is directly proportionate to the voltage lift
from the negative to the positive gate voltage.
Availability of driver IC. Many driver IC are developed on CMOS or BiCMOS
technology, which only shows a limited blocking capability of for example
maximum 30V between positive and negative supply voltage. Taking supply
voltage tolerances into account and sufficient safety margin to the maximum
voltage limits, the usual negative gate voltages in a rangeof -5V to -10V result.
Minor cost savings with the power supply to generate the negative gate
Low power applications often require a cost effective driver solution for the IGBT. To
waive the negative supply voltage simplifies the design of the driver power supply and is
hence very common in this power range. Parasitic turn-on needs to bementioned as a
disadvantage though when switching with 0V. For low power applications, a
compromise between the cost factor and parasitic turn-on might be acceptable. In
applications in the higher power range, however, parasitic turn-on is no longer tolerable,
and also can be handled by application relevant measures only with great difficulty or
not at all.
Parasitic turn-on describes a process in which an IGBT which has already turned off is
briefly turned on again. Typically, this happens in an IGBT configuration of a half-bridge
. Two different scenarios can now be described which bothmay lead
to the inadvertent turn-on of an IGBT:
Parasitic turn-on due to the feedback effect of the Miller capacitance. The
driving force in this is the voltage change
between collector and emitter.
Parasitic turn-on due to the feedback effect of the emitter stray inductance. The
driving force in this is the change in load current
BiCMOS combines bipolar andCMOS semiconductor technologies. Generally the input side is set up onCMOS
basis and the output stages are based on bipolar technology in order to providehigh driver power.