ayman@eecs.umich.edu (03/13/91)
I am trying to simulate a circuit in Hspice, where all diodes are modeled by
a switched voltage source in series with a resistor.
_______ \__________/\/\/\/\/\______
+ Rd | switches closes when
| Vd becomes greater
Vd ----+---- Vi than Vi
===
- |
-----------------------------------+
I tried the voltage-controlled resistor switch (Rd = 1M for Vd < Vi, Rd = 100
for Vd > Vi), but the dc solution didn't converge.
Any suggestions on how to make the solution converge? Or, are there other ways
to model the diodes to get a convergent solution?
Thanks.
--------------------------------------------
- Ayman I. Kayssi (ayman@eecs.umich.edu) -
- Advanced Computer Architecture Lab. -
- EECS Dept., Univ. of Michigan, Ann Arbor -
--------------------------------------------steveg@cadence.com (Steven Greenberg; x6231) (03/20/91)
In article <1991Mar16.211609.9149@zip.eecs.umich.edu> ayman@eecs.umich.edu writes: >I am trying to simulate a circuit in Hspice, where all diodes are modeled by >a switched voltage source in series with a resistor. > > _______ \__________/\/\/\/\/\______ > + Rd | switches closes when > | Vd becomes greater > Vd ----+---- Vi than Vi > === > - | > -----------------------------------+ > >I tried the voltage-controlled resistor switch (Rd = 1M for Vd < Vi, Rd = 100 >for Vd > Vi), but the dc solution didn't converge. >Any suggestions on how to make the solution converge? Or, are there other ways >to model the diodes to get a convergent solution? >Thanks. > >-------------------------------------------- >- Ayman I. Kayssi (ayman@eecs.umich.edu) - >- Advanced Computer Architecture Lab. - >- EECS Dept., Univ. of Michigan, Ann Arbor - >-------------------------------------------- Piece-wise linear circuits like this (especially with very coarse pieces) are not too friendly to Spice's Newton-Raphson non-linear equation solving algorithm. The problem is that the resistance in one segment is not a good predictor of the behavior of the component when it switches segments. Newton-Raphson depends on the predictive nature of the derivative (resistance). You might need to give Spice a large number of iterations to find the answer. Unfortunately, piece-wise-linear circuits can go into oscillations in the Newton-Raphson solver so that no amount of iterations would be sufficient. By the way, the switch as you have described it has about a microamp of negative current at zero volts. You might want to add a parallel current source to cancel out this current. This current may or may not matter in your circuit. /Steve