Designer’s Guide Community

buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: stability & phase margin simulation Reply #15 - Jun 07^th, 2011, 8:29am Sorry, it was F. Wiedmann who has collected something about phase margin: http://sites.google.com/site/frankwiedmann/loopgain
Back to top	LvW (buddypoor: In memory of the great late Buddy Rich) IP Logged

newic

Senior Member

Offline

Posts: 138

Re: stability & phase margin simulation
Reply #16 - Jun 8^th, 2011, 6:42am

Hi all, thx for the sharing.

Regarding to my previous post to plot PSR simulation for that regulator.
I inject ac noise at the power supply and measure VF(vout) to obtain the PSR result. My question is do I need to break the feedback loop for this case? In my opinion, breaking the feedback path is not needed. Am I right?

Back to top

IP Logged

buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: stability & phase margin simulation Reply #17 - Jun 8^th, 2011, 7:14am Yes, that's correct. You must not open the loop for this case, since the PSRR is defined for the nominal operating conditions, of course. Opposite to this case, the phase margin is defined for the loop gain, which needs signal injection into the (open) loop.
Back to top	LvW (buddypoor: In memory of the great late Buddy Rich) IP Logged

newic Senior Member Offline Posts: 138	Re: stability & phase margin simulation Reply #18 - Jun 8^th, 2011, 7:18am Billions thx to buddypoor. to complete the discussion, the comparison with 3 cases are shown here. case1 ( incorrect LC method) case2 (stb) - case3 (correct LC method)
Back to top	regulator_3cases.png IP Logged

newic Senior Member Offline Posts: 138	Re: stability & phase margin simulation Reply #19 - Jun 8^th, 2011, 7:22am reupload
Back to top	regulator_3cases_001.png IP Logged

buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: stability & phase margin simulation Reply #20 - Jun 8^th, 2011, 9:20am What is the difference between cases 1 and 3?
Back to top	LvW (buddypoor: In memory of the great late Buddy Rich) IP Logged

newic

Senior Member

Offline

Posts: 138

Re: stability & phase margin simulation
Reply #21 - Jun 8^th, 2011, 6:19pm

case1 --> inject ac at the plus input of opamp
case3 -> inject ac in series with big cap at the minus input of opamp & ac ground the plus input (which is suggested by buddypoor)

case2 -> same as in the 2nd figure, the ac is injected at the plus input of opamp (using stb) ...

could someone comment on the case2 method. I am new to stb method.

from the comparison, they are almost alike at certain range. But the phase difference is relative difference in term of the starting phase

Back to top

IP Logged

buddypoor

Community Fellow

Offline

Posts: 529
Bremen, Germany

Re: stability & phase margin simulation
Reply #22 - Jun 8^th, 2011, 11:48pm

Hi newic,

just for my and your understanding:

When you start the simulation at very low frequencies (1mHz for example), you will notice that there is a difference at low frequencies between cases 1 and 3. Case 1 gives you not the loop gain (which must be negative) but the positive gain of the closed loop circuit.
However, because the inv. input has a large C to ground, both are identical in magnitude for medium and higher frequencies.
Nevertheless, case 1 does not comply with the phase margin definition, which applies only to the loop gain.

Back to top

LvW (buddypoor: In memory of the great late Buddy Rich)

IP Logged

newic Senior Member Offline Posts: 138	Re: stability & phase margin simulation Reply #23 - Jun 11^th, 2011, 8:51am just now i resimulate the case3. This round i got phase start at 180o instead of -180o...
Back to top	IP Logged

buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: stability & phase margin simulation Reply #24 - Jun 12^th, 2011, 5:55am newic wrote on Jun 11^th, 2011, 8:51am: just now i resimulate the case3. This round i got phase start at 180o instead of -180o... Is this a problem for you? I don`t think so if the phase becomes more negative for rising frequencies.
Back to top	LvW (buddypoor: In memory of the great late Buddy Rich) IP Logged

newic

Senior Member

Offline

Posts: 138

Re: stability & phase margin simulation
Reply #25 - Jun 12^th, 2011, 6:37am

sorry, i am bit slow.

In the case3, i got the phase starts at -180o and go toward -360o for higher frequency as shown in the overlapping figure.

This round, (not sure what i did), I got the phase starts at +180o and go toward 0o for higher frequency.

It puzzles me, how could I read the Phase Margin in these two cases? I think one of the results must be wrong right.

Back to top

IP Logged

buddypoor

Community Fellow

Offline

Posts: 529
Bremen, Germany

Re: stability & phase margin simulation
Reply #26 - Jun 12^th, 2011, 8:20am

Do you see any difference beteween plus and minus 180 deg?
Or between zero and -360 deg? Me not!

I think it (a) depends on your simulator if it prefers -180 deg or +180 deg, or (b) it depends on the lowest frequency you are simulating.
In any case, the phase margin is the difference (at the magnitude cross-over frequency) between the actual phase and zero deg (equivalent to -360 deg).

Back to top

LvW (buddypoor: In memory of the great late Buddy Rich)

IP Logged

newic Senior Member Offline Posts: 138	Re: stability & phase margin simulation Reply #27 - Jun 12^th, 2011, 7:21pm Hi buddypoor, Got it! Thank you very much!! :)
Back to top	IP Logged