Issue link: https://resources.pcb.cadence.com/i/1180526
PSpice User Guide Convergence and "time step too small errors" October 2019 836 Product Version 17.4-2019 © 1999-2019 All Rights Reserved. Introduction In order to calculate the bias point, DC sweep and transient analysis for analog devices PSpice must solve a set of nonlinear equations which describe the circuit's behavior. This is accomplished by using an iterative technique—the Newton-Raphson algorithm—which starts by having an initial approximation to the solution and iteratively improves it until successive voltages and currents converge to the same result. In a few cases PSpice cannot find a solution to the nonlinear circuit equations. This is generally called a "convergence problem" because the symptom is that the Newton-Raphson repeating series cannot converge onto a consistent set of voltages and currents. The following discussion gives some background on the algorithms in PSpice and some guidelines for avoiding convergence problems. The transient analysis has the additional possibility of being unable to continue because the time step required becomes too small from something in the circuit moving too fast. This is also discussed below. Note: The AC and noise analyses are linear and do not use an iterative algorithm, so the following discussion does not apply to them. Digital devices are evaluated using boolean algebra; this discussion does not apply to them either. Newton-Raphson requirements The Newton-Raphson algorithm is guaranteed to converge to a solution. However, this guarantee has some conditions: 1. The nonlinear equations must have a solution. 2. The equations must be continuous. 3. The algorithm needs the equations' derivatives. 4. The initial approximation must be close enough to the solution. Each of these can be taken in order. Remember that the PSpice algorithms are used in computer hardware that has finite precision and finite dynamic range that produce these limits: ■ Voltages and currents in PSpice are limited to +/-1e10 volts and amps.