Essential to the running any sort of simulation are the questions: What am I trying to model? How well does my model fit the actual device? For what range of what parameters is my model valid? It is difficult to get any meaning from a simulation unless you have some understanding of the model used. These notes attempt to help understand the diode model used in the `analog' component of the Chipmunk package. The equation used to model the diode is: Id = Is * (exp(Vd * lamda) - 1) * BLEND + (1 - BLEND) * ((Is * Vd) / Vcrow) * exp(lamda * Vcrow) where BLEND = 1 / (1 + exp(lamda * Vd) * exp(-lamda * Vcrow)) or = 1 / (1 + exp(lamda * (Vd - Vcrow))) lamda = 1 / (n * (k * T)/ q) Id is the current flowing through the diode Vd is the voltage difference between diode terminals The value (k * T)/q is a constant, generally taken to be about 25 / 1000. This leaves the variables n, Vcrow and Is to be explained. Before I do that, the model itself needs some explanation, particularly in the light of simple models such as Id = Is * (exp(Vd * lamda) - 1) which is the traditional model used. The traditional model does not take into account two aspects of static diode behaviour. The exponential rise predicted by exp(lamda * Vd) covers diode behaviour up until large voltages, which in our model are voltages above Vcrow. The exponential function also does not cover the reverse breakdown voltage. The large forward behaviour is modeled by our model somewhat better. Instead of exp(lambda * Vd) rising ever onwards and upwards, there is a kink in diode behaviour at Vcrow. Beyond this voltage, the current flowing through the diode behaves in a more resistor like manner. This voltage, Vcrow, is fixed for a particular diode. This kink is achieved in our model by the BLEND function. There are two distinct parts to the model, the part multiplied by BLEND and the part multiplied by (1 - BLEND). BLEND is controlled by exp(Vd - Vcrow). When Vd equals Vcrow, BLEND is 0.5. When Vd is much larger than Vcrow, the exp(Vd - Vcrow) becomes exponentially larger, making BLEND tend towards 0. In this case the first part of the model becomes insignificant, and the model tends towards ((Is * Vd) / Vcrow) * exp(lamda * Vcrow). Vcrow is a fixed value, as is Is and lambda, so this part of the model is simply a constant times Vd. When Vd is very much smaller than Vcrow, the exp(Vd - Vcrow) becomes exponentially smaller, with BLEND tending towards 1. In this case, the first part of the model dominates, with the second part dropping out, our model returning to the traditional model described above. The reverse breakdown voltage is not modeled by our system, (in common with the traditional model) so the model is only valid up until this reverse breakdown voltage. This still leaves us with the variables n and Is. The variable n is the `emission coefficient' or `ideality factor', with values around 2. This is related to the doping levels of the materials used in the pn junction. Later we'll see how to derive this number. `Is' is the leakage current, or reverse saturation current. How do we tie these variables to a particular diode's characteristics? I have a table listing VR, IF, VF @ IF, IR @ VR for different diodes. Crowbar voltages, n, and Is seem to be missing. For our model, the important values are the VF @ IF values, and the IR @ VR values. The IR gives the reverse saturation current, while the reverse voltage has not yet reached reverse breakdown. This IR value becomes our Is value directly. The VF @ IF values give the crowbar voltage (Vcrow) and we can derive the n variable using the current given. To derive n, at Vd = Vcrow our model becomes: Id = Is * (exp(lambda * Vcrow) - 1) (At this voltage, the -1 could be dropped. The C code has the dropped for the part of the model controlled by (1 - BLEND).) This becomes lambda = ln(Id / Is + 1) / Vcrow n * (k * T)/ q = Vcrow / ln (Id / Is + 1) n = 40 * Vcrow / ln (Id / Is + 1) In my table for an 1N4148, IR is 0.025 uA, IF is 1 volt and IF is 10 mA. Id = 10 * 10 ^ -3 = 10 ^ -2 Is = 0.025 * 10 ^ -6 = 25 * 10 ^ -9 Id / Is = (10 ^ -2) * (4 * 10 ^ 7) = 4 * 10 ^ 5 n = 40 / ln ((4 * 10 ^ 5) + 1) = 40 / 12.9 = 3.1 The value requested for nkT_q then would be 3.1 * 0.025 = 0.0775. (crowbar voltage would be 1, Is is 0.025 uA, as indicated above).

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