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I have tried to do Bode plot of my uncompensated plant by myself in LTspice 17.1, but the result looks strange to me. This topic is a follow-on to this one.. I have no clue why it comes with 290 degree phase margin because it’s always so close to 180 degrees.

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  • \$\begingroup\$ Do you know the difference between AC and transient analyses? \$\endgroup\$
    – Tim Williams
    Mar 1 at 12:05
  • \$\begingroup\$ Well honestly " I dont" but i think transient analyses is about circuit behavior when load is connected. We want as fast response as possible. And AC analyses is about injecting signal of various frequency and looking on gain and phase and the reason is to maintain negative feedback. We dont want circuit to oscilate at the output. But i am quite a newbie, sorry for missunderstoods. I dont have a teacher. \$\endgroup\$
    – MacIntoshCZ
    Mar 1 at 12:10
  • \$\begingroup\$ No no, not what you want from it. What SPICE is actually doing. It only knows what you've told it to do. And you can only tell it things it's been programmed to do. The "AC Analysis" option does a DC analysis (.op), linearization, and small-signal AC steady-state analysis. A highly nonlinear circuit like this will have a meaningless operating point. You must do a transient simulation, and average over many switching cycles, to get the control system response. Or use a simulator that can do a more advanced analysis mode (periodic steady state?). \$\endgroup\$
    – Tim Williams
    Mar 1 at 13:00
  • \$\begingroup\$ @TimWiliams Sorry I run simulation on "transient frequency response". I have added picture of it. Its new feature in Ltspice 17.1. I have also downloaded microcap 12, its maybe a better option, but i was more familiar with LtSpice. ez.analog.com/design-tools-and-calculators/ltspice/w/faqs-docs/… \$\endgroup\$
    – MacIntoshCZ
    Mar 1 at 13:40
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    \$\begingroup\$ Aha, so they added something like that. Seems I'm less familiar with LTspice than I thought! \$\endgroup\$
    – Tim Williams
    Mar 1 at 14:12

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