Issue link: https://resources.pcb.cadence.com/i/1180526
PSpice User Guide Convergence and "time step too small errors" October 2019 838 Product Version 17.4-2019 © 1999-2019 All Rights Reserved. limits of the numerics in PSpice. This example tries to approximate an ideal switch using the diode model: .MODEL DMOD(IS=1e-16 N=1e-6) The current through this diode is: I = 1e-16*e V/(N*.025) = 1e-16*e V/25e-9 Because the denominator in the exponential is so small, the current I is essentially zero for V < 0 and almost infinite for V > 0. Even if there are external components that limit the current, the "knee" of the diode's I-V curve is so sharp that it is almost a discontinuity. Note: Avoid unrealistic model parameters. Behavioral modeling expressions need extra care. Are the derivatives correct? The device equations built into PSpice include the derivatives, and these are correct. Depending on the device, the physical meaning of the derivatives can be small-signal conductance, transconductance or gain. Unrealistic model parameters can exceed the limit of 1e14, but it requires some effort. The main thing to look at is the behavioral modeling expressions, especially those having denominators. Discontinuities in models characteristics and their derivatives cause: 1. Ambiguity in calculation of derivatives at point of discontinuity. 2. Conductance calculated in nth iteration cannot become a good guess for next iteration. 3. Sudden switching of operating regions (example diode switching from off to on) and hence false convergence. Note: Transient analysis convergence failures are usually due to model discontinuities or unrealistic circuit, source, or parasitic modeling.