Convergence problems in time dependent simulations

Hello,

does anyone has experience how to solve convergence problem
in time dependent simulations ? I tried to produce square wave
input waveform from a sine wave input and using two diodes to
cut the waveform. This should be a straightforward but not necessary
the best method to produce square wave. The message I got "nonlinear
solver did not converge, ..., last timestep is not converged". Any specific
solver parameter sets which could help ? A better way to generate square
or PWM waveform ?