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Dynamic IGBT model with finite current slopes during turn-on and turn-off.
Switches
In contrast to the ideal IGBT model that switches instantaneously, this model includes collector current transients during switching. Thanks to the continuous current decay during turn-off, stray inductances may be connected in series with the device. In converter applications, the di/dt limitation during turn-on determines the magnitude of the reverse recovery effect in the free-wheeling diodes.
This IGBT model is used to simulate overvoltages produced by parasitic inductances in the circuit. Since the voltage and current transient waveforms are simplified, the model is not suited for the simulation of switching losses.
The behavior of this IGBT model is demonstrated with the following test circuit. The free-wheeling diode for the inductive load is modeled with reverse recovery.
The diagram below shows the collector current of the IGBT and the
resulting collector-emitter voltage
during switching:
Collector current and collector-emitter voltage
At the gate signal becomes zero, and the device current
begins to
fall. The current slope follows an aperiodic oscillation
where is the fall time specified in the component parameters. As illustrated in the
diagram, the maximum rate-of-change during turn-off is determined by
.
At a positive gate signal is applied. Unless the rate-of-change is limited
by other circuit components, the current rises linearly with constant di/dt. The
maximim di/dt depends on the rated continuous collector current
and the rise
time
specified in the component parameters:
The second diagram shows the collector current transients for different on-state currents. It can be seen that the fall time is independent of the on-state current. Since di/dt during turn-on is constant, the actual rise time is proportional to the on-state current. In a real IGBT, the rise time would only vary slightly with different on-state currents. Hence, assuming constant di/dt is a worst-case estimate in respect of the reverse-recovery current in the free-wheeling diode.