Dynamic IGBT model.

For high power systems and industrial drives, the IGBT replaces the Mosfet as the workhorse for power electronics. The model of the IGBT is build from a Mosfet and BJT. The input of the IGBT is modeled by the Mosfet, while the output is modeled by a BJT. Therefore the parameters are equal to these from the Mosfet and additionally there are parameters describing the on-state voltage drop of the BJT. The IGBT exhibits a tail current while turning off. This tail current is modeled using two extra parameters. Depending on the application where the IGBT is used, also the model should be chosen carefully. In many cases the ideal IGBT modeled by a on-state voltage drop in series with an on-off resistance controlled from a block diagram is sufficient.
The dynamics can be included by adding a RC or time delay to model the delay caused by the gate charging and some wire inductance to model the overvoltage during switching. The tail current is minimized in modern IGBT's
If more detailed wave forms are required, the complete non-linear IGBT model would be more appropriate. The non-linear capacitances of the mosfet finally determine the dynamic transient response during switching, while the tail current is typically modeled bya decaying current flowing throught the IGBT after turn off of the IGBT

What model is required
In the first place the static characteristic determines the static (mostly on-state) losses. If the IGBT is switching fast, the model can be approximated by a simple on-off switch with an on-state voltage in series with a resistance.
More detailed modeling would also include the dependency on the gate voltage, where the on resistance is modeled as a non-linear function of VCE and VGE. This is required if also the driver is modeled in detail and a more accurate simulation result is expected regarding the parasitic components surrounding the IGBT.


Static transfer function
The static transfer function for the IGBT is equal to the static transfer function of hte mosfet, but now the parameter KP is defined as the gain for the IGBT:

IGBTVC-VE=Vce+RCE·ICE
where
ICE is modulated by the drain current from the mosfet and approximates (KP/2) · (VGE-VTO)2
RCE is modeled by the parameter RD
VCE is given as a constant parameter VCEon


Dynamics
The gate capacitance CGS is constant in this model. The other capacitances are non-linear and are modeled by the parameters CGD, CDS, VJ, M and FC.

Tail current
The tail current is modeled by ITail_over_IForward and TailCurrentDecayTime. It is not modeled by a controlled current source but rather depending on the voltage over the IGBT. In this way the voltage over the IGBT influences the tail current.
ITail_over_IForward The parameter ITail_over_IForward defines the initial tail current as a fraction of the forward current. Set this value between 0 and 1. Default it is set to 0.1, meaning that the tail current wil start when the current ICE dropped to 0.1· IForward.
TailCurrentDecayTime The tail current will exponentially decay to zero with a time constant modeled by the parameter TailCurrentDecayTime


Integrated reverse conducting diode
The reverse conducting diode of the IGBT is always included in the model of the IGBT. The parameters can be set such to make it a very worse diode and also the reverse recovery of the diode can be included. The dynamic model for the diode is based on partly based on spice or on measurement parameters. A great advantage compared to the original Spice model is that the dynamic diode model can also simulate reverse recovery.
The reverse diode is modeled as an ideal diode, but including reverse recovery.

The reverse recovery in the diode model is based on stored charge during the conduction interval. Dependent on the way the diode is forced to turn-off, the reverse recovery current is provided by the stored charge in the diode. This can be modeled and parameterized in various ways:

  • Spice parameter based
  • Physical parameter based
  • Measured data based
Each method predicts the reverse recovery current during turn-off. Depending on the parameters provided, the model is parameterized. Leaving the remaining parameters equal to zero, cancels them.
  • Spice parameter based The original spice parameters are not that bad. The only problem is the model, that was originally designed for small signal diodes. However the parameter TT, modeling the transit time, can be used to model the reverse recovery behavior.
    The parameter TT can approximately be chosen as TT=40ns for a diode with a blocking voltage of 100V, to TT=4us for a diode capable of blocking 1000Volt. If TT is set to zero the transit time is approximated from the reverse breakdown voltage BV.
  • Physical parameter based The forward storage time in the diode is modeled by the parameter TT, modeling the transit time. The parameter is the same as in the original spice specification, so it can be used to model the reverse recovery behavior.
    The parameter TT can approximately be chosen as TT=40ns for a diode with a blocking voltage of 100V, to TT=4us for a diode capable of blocking 1000Volt. If TT is set to zero the transit time is approximated from the reverse breakdown voltage BV.
    The time constant with which the reverse recovery is ending is specified by the parameter τrr. If τrr (tau_rr)=0, the diode snaps off very fast. A value greater than zero defines the time constant by which the reverse recovery current decays from IRR towards zero.
  • Measured data based If measured data is available, the parameters IF, dIF/dt, QRR and TRR can be specified.
    IF The maximum forward current during the conduction of the diode. During conduction the total amount of charge is depending on this value.
    dIF/dtThe gradient of the diode current during turn of, measured at the zero crossing of the diode current. This value is depending on the load circuit connected to the diode and the parasitic inductance in series with the diode.
    QRRThe reverse recovered charge is taken from the specification in the data sheet and is specified for a typical forward current IF and turn off gradient dIF/dt of the forward current
    TRRThe reverse recovery time is taken from the specification in the data sheet and is specified for a typical forward current IF and turn off gradient dIF/dt of the forward current
    In the datasheet the parameters QRR and TRR are specified for a typical measurement, where IF and dIF/dt are the test circuit conditions.
To model the reverse recovery correctly, the step size dt for the simulation should be chosen such small, that the reverse recovery can be simulated in detail. This requires a small value of the step size dt, which leads to longer simulation times. However it will show the transients that will occur during the turn off of the diode and any unwanted effects due to a possible high reverse recovery current. Also the effects of the parasitic components surrounding the diode can be studied in more detail.

Losses and Thermal simulation
The IGBT model has a thermal connection that has to be connected to a heat sink model. The temperature rise due to the conduction, switching and reverse recovery losses is modeled on this connection. A heat sink is build from the components found in components/library/Heatsink The parameter Rth and Cth model the thermal model from junction to case. The initial temperature of the junction is modeled by the parameter Tth0. If a more detailed thermal model for the junction to case thermal path has to be build, Rth and Cth simply model the first chip-layer and the following layers are modeled by subsequent thermal models.

Overview of the parameters
The parameters for the IGBT are summarized in the following table. For the parameters that are compatible with the spice diode model the column Spice shows the spice parameter name. Parameters that do not exist in the spice model are indicated with N.A. Default values for the parameters are given.

IGBT Static Parameters
ParameterDefaultSpiceFunction
VTO3VTOGate threshold voltage
KP6.4KPGain for both the Mosfet and BJT
VCEon2N.A.On-state Collector Emittor voltage
RG1RGInternal Gate resistance
RD1uRDInternal Collector (Drain) resistance

IGBT Dynamic Parameters
ParameterDefaultSpiceFunction
CDS10pFCDSCollector Emittor Capacitance maximum value
CGD1000pFCGDGate Collector Capacitance maximum value
CGS640pFCGSGate Emittor Capacitance constant value
FC0.5N.A.Forward bias junction fit parameter
M 0.5N.A.Grading coefficient
VJ1 N.A.Junction Potential

IGBT Tail Current Parameters
ParameterDefaultSpiceFunction
ITail_over_IForward0.1N.A.Initial Tail current as fraction ofthe forward current. Set this value between 0 and 1.
TailCurrentDecayTime50nN.A.Time constant for hte exponential decay

IGBT Wire Parameters
ParameterDefaultSpiceFunction
LD10nHN.A.Collector wire inductance
LG5nH N.A.Gate wire inductance
LS12nHN.A.Emittor wire inductance

IGBT Reverse Diode Parameters
ParameterDefaultSpiceFunction
IS10-14 ISSaturation current.
BV106 BVReverse breakdown "knee" voltage.
N1.5NEmission coefficient.
TT0TTForward Storage Time (Transit Time).
RS1mOhm RSOn resistance.
τrr (tau_rr)0N.A.Decay time constant of the reverse recovery current after IRR. If this value is set to zero, the diode has a snappy recovery.
IF 0N.A.Measured maximum forward current.
dIF/dt 0N.A.Measured current gradient during turn off. (Measure at the zero-crossing)
QRR 0N.A.Measured reverse recovery charge.
TRR 0N.A.Measured reverse recovery time.

Thermal Parameters
ParameterDefaultSpiceFunction
Rth1N.A.Thermal junction-case resistance.
Cth0.5mN.A.Thermal junction-case capacitance.
Tth025N.A.Initial junction temperature.

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