IGBT with Limited di/dt
Purpose
Dynamic IGBT model with finite current slopes during turn-on and turn-off
Library
Electrical / Power Semiconductors
Description
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
Due to the small time-constants introduced by the turn-on and turn-off transients a stiff solver is recommended for this device model.
If multiple IGBTs are connected in series, the off-resistance may not be infinite.
The behavior of this IGBT model is demonstrated in Fig. 210. The free-wheeling diode for the inductive load is modeled with reverse recovery.
Fig. 210 Test circuit for IGBT with Limited di/dt
The diagram in Fig. 211 shows the collector current \(i_\mathrm{C}(t)\) of the IGBT and the resulting collector-emitter voltage \(v_\mathrm{CE}(t)\) during switching.
Fig. 211 Collector current and collector-emitter voltage
At \(t=t_\mathrm{off}\) the gate signal becomes zero, and the device current \(i_\mathrm{C}\) begins to fall. The current slope follows an aperiodic oscillation
where \(t_\mathrm{f}\) 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 \(t_\mathrm{f}\).
At \(t=t_\mathrm{on}\) 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 maximum di/dt depends on the rated continuous collector current \(I_\mathrm{C}\) and the rise time \(t_\mathrm{r}\) specified in the component parameters:
The diagram in Fig. 212 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.
Fig. 212 Collector current transients for different on-state currents
Parameters
- Blocking voltage
Maximum voltage \(V_\mathrm{CES}\) in volts \((\mathrm{V})\) that under any conditions should be applied between collector and emitter.
- Continuous collector current
Maximum dc current \(I_\mathrm{C}\) in amperes \((\mathrm{A})\) that the IGBT can conduct.
- Forward voltage
Additional dc voltage \(V_\mathrm{f}\) in volts \((\mathrm{V})\) between collector and emitter when the IGBT is conducting. The default is
0.- On-resistance
The resistance \(R_\mathrm{on}\) of the conducting device, in ohms \((\Omega)\). The default is
0.- Off-resistance
The resistance \(R_\mathrm{off}\) of the blocking device, in ohms \((\Omega)\). The default is
1e6. This parameter may be set toinfunless multiple IGBTs are connected in series.- Rise time
Time \(t_\mathrm{r}\) in seconds \((\mathrm{s})\) between instants when the collector current has risen from \(10\,\%\) to \(90\,\%\) of the continuous collector current \(I_\mathrm{C}\) (see figure above).
- Fall time
Time \(t_\mathrm{f}\) in seconds \((\mathrm{s})\) 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 \(L_\sigma\) in henries \((\mathrm{H})\) measured between the collector and emitter terminals.
- Initial current
The initial current through the component at simulation start, in amperes \((\mathrm{A})\). 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.