Issue link: https://resources.pcb.cadence.com/i/1180526
PSpice User Guide Convergence and "time step too small errors" October 2019 839 Product Version 17.4-2019 © 1999-2019 All Rights Reserved. Is the initial approximation close enough? Newton-Raphson is guaranteed to converge only if the analysis is started close to the answer. Also, there is no measurement that can tell how close is close enough. PSpice gets around this by making heavy use of continuity. Each analysis starts from a known solution and uses a variable step size to find the next solution. If the next solution does not converge PSpice reduces the step size, falls back and tries again. Incorrect initial estimates can cause convergence failure or even false convergence. Consider following scenarios: ■ Power electronic circuits may NOT require tight current/voltage tolerances. Setting the value of ABSTOL to 1u will help in the case of circuits that have currents that are larger than several amps. ■ Unless the circuit conducts kilo-Amperes of current, however, setting ABSTOL to a value that is greater than 1u will cause more convergence problems than it will solve. ■ PSpice does not always converge when relaxed tolerances are used. For example, setting the tolerance option, RELTOL, to a value which is greater than .01 can actually cause convergence problems ■ Setting GMIN to a value between 1n and 10n will often solve convergence problems. ■ Setting GMIN to a value, which is greater than 10n, may cause convergence problems. Diagnostics If PSpice encounters a convergence problem it inserts into the output file a message that looks like the following. ERROR -- Convergence problem in transient analysis at Time = 7.920E-03 Time step = 47.69E-15, minimum allowable step size = 300.0E-15