Dynamic BJT model.

The Bipolar Junction Transistor (BJT) used to be the workhorse of the low power electronics, but is now replaced by the Mosfet and IGBT. However it is still used and especially for modeling driver circuits, a BJT model is often required.
Most important in the BJT model is the gain between the I_B and I_C. Also the on-state voltage between Base, and Emittor and also between Collector and Emittor influence the performance of the BJT. For the dynamics the most important is the miller capacitance between Base and Collector.
The conduction and switching losses are exported to the thermal node.

What model is required
For the BJT there is basically only one model, including hte static and dyncamic effects. The static gain is required in most simulations where the BJT is applied. The dynamic model includes the static model and is therefore appropriate for modeling the static transfer function. The dynamic model is used in all cases and includes the Miller feedback capacitor.

Static transfer function
The static transfer function for the BJT is equal to:

H_fe=I_C/I_B The on-state Base-Emittor voltage V_be and the on-state Collector-Emittor voltage V_ce are static parameters. Notice that mostly V_ce < V_be.

Dynamics
The dynamics of the BJT are mainly the cause of the Miller feedback capacitance C_jc. The junction capacitors C_je and C_jcare constant in this model.
The Miller feedback capacitance C_jc is between the collector and basis of the BJT. The emittor capacitance C_je is between the internal junction and emittor connection.

Losses and Thermal simulation
The BJT 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 R_th and C_th model the thermal model from junction to case. The initial temperature of the junction is modeled by the parameter T_th0. If a more detailed thermal model for the junction to case thermal path has to be build, R_th and C_th simply model the first chip-layer and the following layers are modeled by subsequent thermal models.

Overview of the parameters
The parameters for the BJT are summarized in the following table. For the parameters that are compatible with the spice BJT 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.

BJT Static Parameters

Parameter	Default	Spice	Function
Hfe	70	HFE	Forward gain Ic/Ib
Vbe	1.5	VBE	Forward voltage drop from Base to Emittor
Vce	1	VCE	Forward voltage drop from Collector to Emittor

BJT Dynamic Parameters

Parameter	Default	Spice	Function
Cjc	10nF	CJC	Junction Collector Capacitance, between basis and collector.
Cje	10nF	CJE	Junction Emittor Capacitance, between internal junction and emittor.

Thermal Parameters

Parameter	Default	Spice	Function
Rth	1	N.A.	Thermal junction-case resistance.
Cth	0.5m	N.A.	Thermal junction-case capacitance.
Tth0	25	N.A.	Initial junction temperature.