Newton-Raphson Iterations — Ansoft Designer 7.0 在线帮助文档，Ansys Designer 教程

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Newton-Raphson Iterations

At each timestep, the simulator solves the discrete time approximation equations using the Newton-Raphson iterative method. Because the solution from the previous timestep is close to the solution for the current step, the number of Newton-Raphson iterations required to reach convergence at each time point is typically much less than the number of iterations needed for the initial DC operating point calculations.

You can set the maximum number of iterations to use with the maxiters option on the .OPTIONS statement. The default for transient analysis is 10 iterations.

The option limiting_step sets the maximum size for changes in node voltages at each step in the Newton-Raphson loop. Nexxim automatically identifies circuits that contain nonlinear elements. For large circuits with many nonlinear devices such as BJTs and MOSFETs, it is necessary to reduce the magnitude of the Δv_k update to aid in obtaining convergence. The Transient solver limits the size of voltage updates to a maximum value given by the limiting_step option. The default limiting_step for circuits with nonlinear devices is 0.3 Volts.

For circuits that contain only linear devices, Nexxim sets the default for limiting_step to a large number, effectively disabling it. A large limiting_step speeds the convergence of circuits that are mostly linear, while a smaller value may help nonlinear circuits to converge more reliably.

The maximum change in voltage per timestep is given by maxiters times limiting_step. In general, it is safer to change maxiters to adjust the size of the allowed voltage change per timestep, rather than changing the limiting_step.

The update_jacobian_period option can affect the overall speed of the Newton-Raphson process. By default, the Jacobian matrix is updated after every third iteration. Increasing the number of N-R iterations per Jacobian update can speed up the simulation, but is risky when the circuit contains nonlinear elements. Reducing the update_jacobian_period option to 2 or 1 slows the simulation but may be necessary for highly nonlinear circuits. The Jacobian update period is sometimes called the Šamanskii step size.


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