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 IB and IC. 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:
The dynamics of the BJT are mainly the cause of the Miller feedback capacitance Cjc. The junction capacitors Cje and Cjcare constant in this model.
The Miller feedback capacitance Cjc is between the collector and basis of the BJT. The emittor capacitance Cje 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 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 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.
|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
|Cjc||10nF||CJC||Junction Collector Capacitance, between basis and collector.|
|Cje||10nF||CJE||Junction Emittor Capacitance, between internal junction and emittor.|
|Rth||1||N.A.||Thermal junction-case resistance.|
|Cth||0.5m||N.A.||Thermal junction-case capacitance.|
|Tth0||25||N.A.||Initial junction temperature.|