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Convergence Issues in MC Transient simulation

Flyyn Rider

Hi,

I have a simple two diode connected npn bjt circuit where both the npns are biased thru their emitters. But when i run MC transient simulations for the difference in theri emitter base voltages I get convergence errors. Out of 200 runs only 3 are simulated. The problem is likely to be with the 'Transients Options' form. When I press 'skipdc' option to yes I get no convergence issues but a lot of transients at the DC. When I press 'skipdc' to no I get convergence issues. I have tried many things on the transient options form like 'Integration Methods Parameters', Iteratio' option, 'cmin' etc but to no avail. I need solution to this convergence issue. Output log file says to loosen tolerance like 'iabstol' on options paramter, but I can't find it anywhere.

I am not getting any convergence error for a single run normal Transient simulation but only for Monte Carlo (MC)! 

  •  

    Hi,

    I have a simple two diode connected npn bjt circuit where both the npns are biased thru their emitters. But when i run MC transient simulations for the difference in theri emitter base voltages I get convergence errors. Out of 200 runs only 3 are simulated. The problem is likely to be with the 'Transients Options' form. When I press 'skipdc' option to yes I get no convergence issues but a lot of transients at the DC. When I press 'skipdc' to no I get convergence issues. I have tried many things on the transient options form like 'Integration Methods Parameters', Iteratio' option, 'cmin' etc but to no avail. I need solution to this convergence issue.The image attached shows the transient simulation options I am using.

    Output log file says to loosen tolerance like 'iabstol' on options paramter, but I can't find it anywhere.

    I am not getting any convergence error for a single run normal Transient simulation but only for Monte Carlo (MC)! 

  • Here is the image:

    http://postimg.org/image/715s02zi5/

  • Why have you set method to "euler"? That's a very odd choice. euler is a heavily damped integration method and normally only used at discontinuities in the input waveform.

    iabstol is on the Simulation->Options->Analog form. Setting skipdc=yes is a bad idea.

    It's hard to give any real advice without seeing what the actual convergence problem is. If it's failing to converge on such a simple circuit, I suspect something is connected up incorrectly or there's a problem with the models. It would be best to contact customer support over this and then we can look at the data itself.

    BTW, you can attach pictures to the forum by adding them on the Options tab when you post. That's better than posting them elsewhere.

    Regards,

    Andrew.

  • Hi Andrew,

    I am attaching plots of MC simulations. I have added text with each plot to explain what is happening. I have a simple bjt temperature sensor circuit with current sources. The right bjt is carrying 5 times more current than left one. This difference in current densities is translated to difference in their Vbe. Now I use the formulae "diff of Vbe"=(kT/q)*ln(5) to estimate the temperature (which ofcourse turn out to be in V beacuse of the equaiton units!!). I am running simulations at 27, so my output must be around this number.

  • And the plots are uploaded here:

    http://postimg.org/image/jabisi1ab/

    http://postimg.org/image/fid8r3r4b/

    http://postimg.org/image/pznpzau7v/

    http://postimg.org/image/f0k3wjm91/

    Unfortunately we cant upload more than one image per post. Any way I need to know is there some better way to analyze process spread in your system? I mean other than MC and corner simulations.

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