Modeling in SPICE& a diode is not a trivial work. Although the operation of the diode is quite simple, extract a model from datasheet takes some time.Every component has its own syntax defined in SPICE , in the case of the diode:
.model ModelName D (par1=a par2=b………parn=x)
where par1 par2 …. parn are characteristic parameters of diode.
we can sum up the set of main parameters in the following table:
| Parameter | Description | Unit | Default value |
|---|---|---|---|
| BV | Reverse breakdown knee voltage | V | Infinite |
| CJO | Zero-bias p-n capacitance | F | 0 |
| EG | Bandgap voltage | eV | 1.11 |
| FC | Forward-bias depletion capacitance coefficient | no unit dimension | 0.5 |
| IBLV | Low-level reverse breakdown knee corrent | A | 0 |
| IBV | Reverse breakdown knee corrent | A | 1e-10 |
| IKF | High-injection knee current | A | Infinite |
| IS | Saturation corrent | A | 1e-14 |
| ISR | Recombination current parameter | A | 0 |
| M | p-n grading coefficient | no unit dimension | 0.5 |
| N | Emission coefficient | no unit dimension | 1.0 |
| NR | Emission coefficient for ISR | no unit dimension | 2.0 |
| RS | Parasitic resistance | Ohm | 0 |
| TT | Transit time | sec | 0 |
| VJ | p-n potential | V | 1.0 |
| XTI | IS temperature exponent | no unit dimension | 3.0 |
All these parameters are used by SPICE to describe the behavior of the diode in the different situations of signal, for example in direct polarization in DC that, forward current will be:
ID = IS*(e^(VD/(N*Vt))-1)
where VD is the forward voltage, Vt = k * T / q is the thermal voltage equal to 0.026 V at 27 degrees Celsius.
The so-called recombination current is instead calculated as
Irec = ISR*(e^(VD/(N*Vt))-1).
Other equations from the given parameters describing the capacitance of the junction, its evolution with temperature and more.
At this point we have to derive the various parameters from the datasheet of the component. Assume we want to model a silicon diode 1N4148. The extraction of the parameters of the table from the values reported in the datasheet, is not immediate for almost none of the parameters.look at the values of our interest in datasheet:
From the table we can get BV which is equal to VRM, in other cases reported as Vbr, or in the case of Zener diode Vz.
From this second table we see that the maximum leakage current at 25 degrees is Ir = 5 uA.We can take IBV as equal to 10 times Ir. Usually for this type of diodes the value of IBV is around 100uA. For Zener diodes Ir can be called Izk, or in other cases as Ibr.
CJO can be directly equal to the value specified in the datasheet as Cj or Ctot, in this case is 4pF.