Spice compatible diode.

For compatibilty reasons a dynamic model for the diode based on Spice parameters is included in Caspoc. A great advantage is that the original parameter from spice can be used, but it also has some serious drawbacks.

No correct reverse recovery model
Compared to the diode model with reverse recovery, is that the dynamic diode model called DiodeReverseRecovery can also simulate reverse recovery. This spice compatible model simulates reverse recovery via a non-linear capacitance. The result of these spice simulations looks nice, but are completely wrong when it comes to simulating the reverse recovery. On the other hand, the spice compatible model comes in handy when modeling the static characteristic of a diode, or when only the spice parameters are known.

Import directly a .model parameter database
Since the spice parameters are equal in name to the original spice parameters, you can directly import these parameters by means of a .model parameter database. Connect a Mname block to the left input of the model and specifiy the name of the .model parameters.
The spice compatible diode model is found in components/library/Semiconductor/Diode.

Static characteristic
The static V-I characteristic based on spice parameters forms the basis for the static curve of the spice compatible diode model.

I_d=I_s(e^{q · Ud / N ·k ·T}-1) where

• q=1.6e-19
• k=1.38e-23

This equation is also temperature dependent. Compared to spice in Caspoc this temperature dependence is dynamic, so the losses from the diode heat up the heat sink and influence the parameters of the diode, generating more or less power loss. This closed circle is essential in correct efficiency simulations. In spice a fixed temperature has to be defined before the simulation starts.

Dynamics
The junction capacitance C_j0 is dependent on the voltage across the diode in this model. In the original spice model this capacitance is non-linear to model RF resonant oscillators in radio and TV systems. Later this non-linear capacitance is lso misused for modeling reverse recovery.
However to some extend you can model some dynamics with this non-linear capacitance and the forward transit time.

Reverse Recovery
The reverse recovery in the spice compatible model is based on stored charge during the conduction interval in a non-linear capacitance. Dependent on the way the diode is forced to turn-off, the reverse recovery current is provided by the stored charge in the diode. However since a non-linear capacitance is used for modeling this stored charge there is no good phyiscal relation between this capacitor and reverse recovery behavior To model the dynamics correctly, the step size dt for the simulation should be choosen 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 diode model has a thermal connection that has to be connected to a heat sink model. The temperature rise due to the conduction and reverse recovery losses is modeled on this connnection. A heat sink is build from the components found in components/library/Heatsink The parameter R_th and C_th model the thermal model form junction to case. The initial temperature of the junction is modeled by the parameter InitialTemp. 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.

Parameters
The parameter for the diode are summarized in the following table. Since the parameters 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 a N.A. Default values for the parameters are given.

Parameter	Default	Spice	Function
BV	109	BV	Reverse breakdown "knee" voltage.
Cj0	50pF	CJ0	Junction capacitance.
EG	1.11	EG	Activation Energy
FC	0.5	FC	Forward bias junction fit parameter
IS	10-14	IS	Saturation current.
IBV	10-10	IBV	Reverse breakdown current.
m	0.5	M	Grading coefficient
N	1.5	N	Emission coefficient.
RS	1mOhm	RS	Ohmic resistance.
Rth	1	N.A.	Thermal junction-case resistance.
TT	0	TT	Forward Storage Time (Transit Time).
Vj	1	VJ	Junction Potential
XTI	3	XTI	Saturation current temperature exponent