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how to circumvent :"Convergence problem in transient analysis"

Discussion in 'CAD' started by [email protected], Feb 9, 2006.

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  1. Guest

    Hi, I'm having that famous problem. Is there any way to cirumvent it
    (computing time, I have). Can anyone explain the reason for the error
    ? Can I go lower than the "minimum allowable step size" and what
    determines the "minimum allowable step size" anyway?


    The full error message is:
    "ERROR -- Convergence problem in transient analysis at Time =
    Time step = 125.0E-15, minimum allowable step size =

    ********SPICE DECK**********
    Vin 3 0 pulse (0 5 0 0.1ns 0.1ns 0.830us 4.150us)

    S1=2 3 3 0 Sbreak1
    S2=4 2 3 0 Sbreak2

    C1=2 0 0.119244376p IC=0
    C2=4 0 480nF IC=0

    ..model Sbreak1 VSWITCH Roff=1e12 Ron=1.0 Voff=4.9 Von=5.0
    ..model Sbreak2 VSWITCH Roff=1e12 Ron=1.0 Voff=0.1 Von=0.0
    ..OPTIONS CHGTOL=1.0e-16

    ..tran 0.0001ns 0.5s
    ********SPICE DECK**********
  2. Do you need so much ratio between Roff and Ron /1E12) and do you need
    so fast risetime on the pulse (0.1ns)

    If you can relax on any of those parameters then you should be albe to
    run without convergence problems


  3. Guest

    I've worked out analytical equations for the circuit and can model this
    thing in MATLAB but I'm trying to see if the two approaches give
    identical (or nearly identical) results.

    The large switch resistance I need to keep (although I have reduced it
    to 1000Mohms), and I can't sacrifice much w.r.t. the pulse parameter.
    Going from 0.1ns to 10ns allows the simulation to proceed, but I start
    seeing significant deviations from the MATLAB at this point. Note that
    my pulse wave form is high for 830ns then low for 4150ns, so a 10ns
    transition is starting to be significant ...

    Is there any way to have my cake and eat it too ?
  4. Are you using Orcad PSpice? If so - it has the possibility to shedule
    better resolution at specific points in time. So for example you would
    set the maximum step size at your pulse generator transitions (don't
    know about other programs).

    What about increasing the number of maximum iterations - perhaps it can
    get over the converging point after all. (I think they are name ITL1,
    ITL2, ITL4)


  5. qrk

    qrk Guest

    Looks like your running PSpice from the type of error message you got.
    This deck runs fine on LTspice. Download it, it's free and works
    fabulously. LTspice is also very compatible with PSpice syntax.
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