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IGBT with Limited di/dt

Purpose

Dynamic IGBT model with finite current slopes during turn-on and turn-off.

Library

Electrical / Power Semiconductors

Description

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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.


Note 


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.

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The diagram below shows the collector current iC(t)   of the IGBT and the resulting collector-emitter voltage vCE(t)   during switching:

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Collector current and collector-emitter voltage

At t = toff   the gate signal becomes zero, and the device current iC   begins to fall. The current slope follows an aperiodic oscillation

             ⌊                              ⌋
                 2.4-(t-- toff)(             )
iC(t) = iC(toff)|⌈e-    tf      1+ 2.4(t--toff)  |⌉
                                    tf

where t
f   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 t
 f   .

At t = t
    on   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 I
C   and the rise time t
r   specified in the component parameters:

dimax      IC-
 dt  = 0.8⋅ tr

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.

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Parameters

Blocking voltage
Maximum voltage VCES   in volts (V) that under any conditions should be applied between collector and emitter.
Continuous collector current
Maximum dc current IC   in ampere (A) that the IGBT can conduct.
Forward voltage
Additional dc voltage Vf    in volts (V) between collector and emitter when the IGBT is conducting. The default is 0.
On-resistance
The resistance Ron   of the conducting device, in ohms (_O_  ). The default is 0.
Off-resistance
The resistance Roff    of the blocking device, in ohms (_O_  ). The default is inf. If multiple IGBTs are connected in series, the off-resistance must have a large finite value.
Rise time
Time tr   in seconds between instants when the collector current has risen from 10 % to 90 % of the continuous collector current IC    (see figure above).
Fall time
Time tf   in seconds between instants when the collector current has dropped from 90 % to 10 % of its initial value along an extrapolated straight line tangent to the maximum rate-of-change of the current (see figure above).
Stray inductance
Internal inductance Lσ   in henries (H) measured between the collector and emitter terminals.
Initial current
The initial current through the component at simulation start, in amperes (A). The default is 0.

Probe Signals

IGBT voltage
The voltage measured between collector and emitter.
IGBT current
The current through the IGBT flowing from collector to emitter.
IGBT conductivity
Conduction state of the internal switch. The signal outputs 0 when the IGBT is blocking, and 1 when it is conducting.