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stability analysis with step response and AC loop analysis

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chang830

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Hi,
I am doing the stabilty analysis. The step analysis showed that the ciruit is not stable, because we found it has some ringing even oscillation. But the AC loop analysis showed that the phase margin is enough. I was puzzled by it.

Anyone has ever encountered the similar things?

Thanks
 

If you use cadence system, a stb analysis can be used. Perhaps the answer is there.
 

AC analysis makes solution only around dc operation point. During large signal transient the circuit parameters is changed and it's probably at some point the stability condiition don't realise.
AC analysis serves only for turn circuit parameters to meet stability criteria. And transient analysis for checking because it is more correct.
If u have [long damped] oscillations it means that ur circuit isn't stable or near to this.
 

Re: stability analysis with step response and AC loop analys

Hello,
to me, I never see exception of this Bode digram theory
as long as the oscillation starts in small signal region.

What is needed here,
a closer and careful examination node by node
at what's really going on in the circuit near oscillation frequency.

one way to do might be,
try to open the loop, inject a small sinewave at osc. freq.
see does outcome voltage get amplified and its phase / waveform...
 

A mistake I made before was I ended sim when open-loop phase margin > 60 , but when close-loop things changed and the loading cap was removed.
 

Re: stability analysis with step response and AC loop analys

And I want to know how to analyze the stability of the loop?
how to break it to do simulation?
thx!
 

hi there,

i think there might be some mistake when u r breaking the loop for stability analysis. Can u post ur cir file in which u r doing this.. then probaly i may help u.

Ur dc operating point of circuit should not change when u r doing an AC analysis. what r u doing for this ???
 

Break the loop at the gate of the power fet and apply the signal there and see the open loop response. but make sure that the biasing of the circuit is still maintained.
Generally the stability simulation is done by modeling few components. Becoz ac analysis is generally done for unsaturated blocks whereas i buck converter we have blocks operating in saturated mode.
 

There is no meaning of suggesting many things, when you don't show us your actual circuit schematic. We can give better suggestion if you state correctly your schematic and its application environment. Step function always consists of odd harmonics. Oscillation at the rising and falling edge of the pulse are no wonder. And its always see how much your pulse is dampened. When you have 45 degree of PM it tells critically damped and though it is stable still consist of more than one oscillation at the pulse edge.
 

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