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a_k Junior Member Offline Posts: 17	Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC Reply #15 - Aug 06^th, 2015, 9:16am @Ken, Thank you for your reply.
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a_k Junior Member Offline Posts: 17	Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC Reply #16 - Aug 7^th, 2015, 6:41am @Ken, Thank you for your reply.
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a_k

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Posts: 17

Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC
Reply #17 - Aug 18^th, 2015, 11:36am

Hi Ken,

I implemented the VerilogA Comparator block discussed earlier with an opamp circuit (transistor level implementation).

The PSS analysis was performed in shooting mode with `q` [output of flip flop] as the oscillator node+. There was only one warning and is given below [2]. The PSS analysis results match well with `trans` analysis results. But the PAC analysis does not seem to work for this boost converter (operating as autonomous circuit). May be some parameter settings are wrong [1]. The voltage gain (Vo/Vi) and phase results are shown below and are incorrect.

Could you please tell me what might be the reason or any suggestion about how to run PAC analysis for this kind of circuit?

Please let me know if any further information is needed.

Thank you in advance.

[1] PSS and PAC settings:
Code:

pss �( �Vo �0 �) �pss �fund=161.23k �harms=0 �errpreset=moderate
+ � �tstab=249m �saveinit=yes �oscic=lin �method=gear2only
+ � �tstabmethod=gear2only �maxacfreq=200M �maxperiods=50 �annotate=status
+ � �maxiters=50
pac �pac �start=1m �stop=200M �maxsideband=0 �annotate=status

[2] Warning during PSS analysis:
Code:

Notice from spectre during IC analysis, during periodic steady state analysis `pss'.
 � �GminDC = 1 pS is large enough to noticeably affect the DC solution.
 � � � �dV(I33.MP1.m1:int_s) = -474.4 mV
 � � � �Use the `gmin_check' option to eliminate or expand this report.
 � �L0: Initial condition computed for node L0:1 is in error by 1 nA.
 � � � �Decrease `rforce' to reduce error in computed initial conditions. �However, setting rforce too small may result in convergence difficulties or in the matrix becoming singular.

DC simulation time: CPU = 300.954 ms, elapsed = 300.762 ms.

Using linear IC

Warning from spectre during periodic steady state analysis `pss'.
 � �WARNING: Linear IC: Fail to find out initial frequency. [ Early Reject ]

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a_k Junior Member Offline Posts: 17	Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC Reply #18 - Aug 18^th, 2015, 11:38am The Magnitude and phase plot obtained for Vo/Vi.
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Ken Kundert

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Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC
Reply #19 - Aug 18^th, 2015, 3:57pm

1. If trying to compute loop gain to determine small signal stability, you should be using the stb analysis rather than pac.
2. Because you are using an autonomous PSS analysis, PAC by default, sweeps frequency starting at the fundamental frequency rather than from DC. You should change this using 'sweeptype=absolute'.
3. You should reduce the frequency range on your sweep.

-Ken

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a_k Junior Member Offline Posts: 17	Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC Reply #20 - Aug 21^st, 2015, 2:39am @Ken, Thanks a lot. I got more or less proper magnitude and phase plots after changing the `sweeptype` to `absolute`. But I am confused about your first point. Should I use `stb` or `pstb` analysis to find the loop gain for this kind of circuit?
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Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley	Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC Reply #21 - Aug 21^st, 2015, 11:25pm PSTB
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a_k

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Posts: 17

Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC
Reply #22 - Sep 12^th, 2015, 11:52am

Hi Ken,

As my circuit (attached) has a low frequency pole (about 6Hz); in the PAC form I gave a frequency sweep range as 10mHz to 1MHz. For this frequency range, in the magnitude and phase plot I saw that the curves were dipping near 10mHz. So I decided to give a frequency range of 1fHz to 1MHz in PAC form. The magnitude and phase plot for this frequency range is attached.

There seems to be a very low frequency poles and zeros in the system, which are impractical. This seems to change the magnitude and phase plot characteristics.

PSS and PAC settings:
Code:

pss �( �q �0 �) �pss �fund=169.83k �harms=0 �errpreset=moderate
+ � �tstab=300m �saveinit=yes �oscic=lin �readic="readic.ic" �cmin=10f
+ � �writefinal="readic.ic" �method=gear2only �tstabmethod=gear2only
+ � �maxacfreq=200M �maxperiods=100 �annotate=status �maxiters=100
pac �pac �sweeptype=absolute �start=1f �stop=10M �maxsideband=0
+ � �annotate=status

Could you please tell me why the magnitude and phase plots have a strange behavior (curves were supposed to be flat) in the low frequency range (10^-7 to 10^-4 Hz) which cannot be related to realistic models?

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a_k Junior Member Offline Posts: 17	Re: Open loop pole-zero analysis of Boost Converters using PSS and PAC Reply #23 - Sep 12^th, 2015, 11:55am The magnitude and phase plot for 10^-15 to 10⁶ Hz range:
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