Micro-Cap simulation

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Tenderfoot:
I decided to learn how to use Micro-Cap and the first project that came to mind was a linear bench PSU, variable voltage and variable current limiting. Maybe this was a bad choice. What I have found is that at various settings the simulation output is nice and stable, but alter the settings a little, or alter a component value a little and the output goes mad. I'm aware that PSU design is trickier than it might look, so am I asking too much of Micro-Cap (or any simulation for that matter)?

Doctorandus_P:
It is common for control loops to be stable in some configurations, and unstable in others. Modifying a single resistor, inductor or capacitor can be the difference between stable or unstable.

There is a lot of math involved, and simulators are a part of the toolbox to figuring out whether the math is applied correctly, or just to experiment with how to get a control loop to be stable.

If you were building the circuit in real life you may experience the same. Change the value of a small capacitor, and suddenly your whole circuit goes bonkers. Because of this, breadboards are not suitable for a lot of circuits. They always add capacitance due to the way they are designed, and this is not always within acceptable limits.

tggzzz:

--- Quote from: Tenderfoot on May 27, 2024, 05:08:45 pm ---I decided to learn how to use Micro-Cap and the first project that came to mind was a linear bench PSU, variable voltage and variable current limiting. Maybe this was a bad choice. What I have found is that at various settings the simulation output is nice and stable, but alter the settings a little, or alter a component value a little and the output goes mad. I'm aware that PSU design is trickier than it might look, so am I asking too much of Micro-Cap (or any simulation for that matter)?

--- End quote ---

That is too vague for diagnosis, however...

The first thing you need to understand is the mathematics of feedback and control theory. Those models will give you a solid framework for understanding the kinds of thing that can happen in general systems.

After that, simulation can be useful doing the arithmetic for specific examples.

It is always worth remembering that all mathematical models are wrong, but some are useful.

nctnico:

--- Quote from: Tenderfoot on May 27, 2024, 05:08:45 pm ---I decided to learn how to use Micro-Cap and the first project that came to mind was a linear bench PSU, variable voltage and variable current limiting. Maybe this was a bad choice. What I have found is that at various settings the simulation output is nice and stable, but alter the settings a little, or alter a component value a little and the output goes mad. I'm aware that PSU design is trickier than it might look, so am I asking too much of Micro-Cap (or any simulation for that matter)?

--- End quote ---
What I don't like about Micro-Cap is that it continues to simulate the circuit even if you alter it to a state it won't work. Maybe that is your problem. I have not looked into configuring Micro-Cap to do a single shot simulation though.

Where it comes to simulation a control loop, you'll need to use techniques to do a frequency sweep over the control loop in order to find the loop's frequency response and phase margin. Especially current limiting can be tricky.

thm_w:
Maybe you can post the circuit and files here, so specific advice could be given.