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DC/DC Buck's Phase Margin analysis using spectre ? (Read 106252 times)
Richard
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DC/DC Buck's Phase Margin analysis using spectre ?
Dec 08th, 2004, 11:31am
 
Hi,
 Has anyone use the spectre to analysis DC/DC buck converter phase margin ? We can use it in the switched-capacitor filter analysing the frequency response, so I think it can be used for loop-analysis in DC/DC converter.....if not possible, perhaps we can use the VERILOG-AMS in Cadence ?

Thanks.
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Eugene
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #1 - Dec 8th, 2004, 2:27pm
 
I've used SPICE for years to analyze the loop gain (gain and phase margin) of buck DC/DC converters and I am certain Spectre can do the same. However, I suspect you meant to ask about SpectreRF. To use Spectre, you would have to use a state space averaged model. The state space averaged model replaces the switch with an ideal DC/DC transformer. The more interesting approach, and probably the one you are asking about, is to apply SpectreRF directly to the time-varying model, perhaps even to the device level model. I think it would work for a pwm converter with a fixed switching frequency. I have always wanted to try it but never had the opportunity. I am not so sure about a resonant converter or any converter where the frequency can vary.
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Richard
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #2 - Dec 8th, 2004, 5:18pm
 
Thanks Eugene for the reply.
 I wish to have simulations (determining phase margin) directly on the transistor level schematics. We have good results using PSS analysis of spectre on switched-capacitor filter and PLL systems, why not on a PWM DC/DC converter ?
 Maybe you can say it being lazy.... Wink, but I try to avoid using State-Space model, as the system can be complex and I might overlook some critical parasitics.
 By the way, I am also interested and hope you can describe alittle more on how you use SPICE to analyse loop gain for buck ? do you construct a macro-model ? Do you use PSPICE or HSPICE ? How is the accuracy and any documentation to get me started ? such as an example ?

Thanks alot !!  :)
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Frank Wiedmann
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #3 - Dec 9th, 2004, 12:26am
 
I have not tried this, but I think you should be able to use the method described in the article http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=900125 by simply replacing the AC sources with PAC sources. The article explains how Spectre's stb analysis is working.

Ken, how about implementing a pstb analysis in SpectreRF?
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Ken Kundert
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #4 - Dec 9th, 2004, 1:14am
 
Frank,
   PSTB is a good idea. I'll pass on the request.

-Ken
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Eugene
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #5 - Dec 9th, 2004, 8:59am
 
Richard,
I hope this is readable. I had some trouble with the web page.

I agree with you that parasitics affect the analysis but I ususally do the state space averaged models anyway because: (1) the more complex the system, the more I learn about it by building a state space averaged model;(2) I rarely know the parasitics, especially those of external magnetics, accurately enough to justify skipping the state space averaged model.

I will post this message in pieces to circumvent some of the problems I'm having with this website.

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Eugene
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #6 - Dec 9th, 2004, 9:17am
 
In my previous life as a power electronics engineer, I implemented state space averaged models as macro models. If I were to build one today I would use VerilogA. If you do not have access to VerilogA, then you must resort to the old marcro models. For a buck converter, you replace the mosfet and catch diode with a nonlinear current-controlled-current source and a nonlinear voltage controlled voltage source. The voltage source sense the input filter capacitor voltage and a voltage numerically equal to the duty cycle. The voltage souce is nonlinear because the output voltage equals the product of the two sensed voltages. The voltage source drives the output inductor. The current source senses the output inductor current and the duty cycle signal and loads the input filter capacitor with the product of the two sensed signals. The current source may require a non-linear voltage controlled voltage source to perform the multiplication because if memory serves, there is no nonlinear current source that senses voltage and current in SPICE.
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Eugene
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #7 - Dec 9th, 2004, 9:26am
 
The above macro model is only good for the continuous conduction mode; it does not work if the inductor current periodically drops to zero amps. However, there are marcomodels that automatically switch between continuous and discontinuous state space averaged models on the fly.  I will list a reference by Chen and Rodriquez later.

For a simple voltage model converter, the duty cycle is proportional to the error amplifier output. For a current mode converter, duty cycle is linear combination of output voltage and inductor current.

The nonlinear state space averaged macro models simulate large signal transient behavior and SPICE automatically linearizes them for small signal AC analysis (gain and phase margin). To break the loop for gain and phase margin analysis, insert a DC voltage source in series with the duty cycle. Set the DC voltage to zero and make the AC magnitude unity. Run an AC analysis. The magnitude of the loop gain equals the magnitude of the ratio of the voltage on duty cycle side of the DC source to the voltage on the other side. The phase margin equals the phase of the same ratio.
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Eugene
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #8 - Dec 9th, 2004, 9:46am
 
As for references, the first paper on state space averaging dates back to the early 70s and was written by Drs. Wester and Middlebrook. Dr. Cuk developed a more formal mathematical framework for state space averaging shortly after. I believe the first paper on state space averaging for current mode converters was by R.B. Ridley. I believe there are more recent books in print but here's some older ones from my library:

Severns and Bloom, "Modern DC-to-DC Switchmodel Power Converter Circuits". Van Nostrand Reinhold.
This book discusses various topologies and state space averaged models.

Steven Sandler, "SMPS Simulation with SPICE3". McGraw Hill.

Papers on macro modeling:

V. Bello. "Computer Aided Analysis of Switching Regulators Using SPICE2". IEEE PESC, 1980 Record.

J. Chen and R. Rodriguez, "Duo-Mode Nonlinear State Space Averaged SPICE Model of a Current Mode Buck Converter", APEC 1988.
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Eugene
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #9 - Dec 9th, 2004, 10:02am
 
I forgot to comment on the accuracy of state space averaged models. The state space averaged models start to break down between 1/10 and 1/2 the switching frequency. Also, as you said, parasitics can cause inaccuracies. For example, the state space averaged model can not simulate the effects of glitches in the sensed inductor current caused by the catch diode's reverse recorvery transient.

There are a few papers on sampled data effects that extend the accuracy of the state space model right up to half the switching frequency but since it is risky to push the loop gain's cross over frequency out that far, such papers are mainly of academic interest.

Most of the time, I found the state space averaged models predicted phase margins to within 5 to 20 degrees of measurements, depending on how well I knew the true parameter values.
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Richard
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #10 - Dec 9th, 2004, 10:26am
 
Thanks Eugene and Frank.
 Frank : I am trying to use PAC for the sims, hopefully it will turn out okay. The question I have would be that we can see the gain response from PAC, can we also use it for phase response ?

 Eugene : thanks for the advice. One of the problem with state-space would be that depending on the designer's skill set on doing the model, we might not get correct answers. Also, customer(some of them are really dumb, it takes alot time ot explain State-space to them) usually would question the equivalent model and hope we use some standardized or commerical well-established tools. By the way, do you prefer the MATLAB or SPICE method ?  :)
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Eugene
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #11 - Dec 9th, 2004, 11:38am
 
For simulating power electronics I prefer SPICE. Matlab could do it but it is much easier to draw out the schematic for SPICE than to derive the circuit equations for matlab and re-invent the integration wheel.

As for state space averaging versus SpectreRF, I think it would be far easier to explain state space averaging than SpectreRF. I would think there are just as many ways, if not more, to get the SpectreRF simulation wrong as there are to get the state space averaged model wrong.  In the power electronics arena, state space averaging is a proven method of analysis that has been thoroughly documented over the last 30 years. I am not aware of any documentation about applying SpectreRF to power electronics. Ideally, one would use both state space averaging and SpectreRF and make sure the two results agreed. In my opinion, new methods should never make old methods obsolete, they should  instead provide an extra level of confidence. If I were the customer or technical reviewer, I would not trust an analysis from an engineer who simply plugged in the schematic, pressed the run button, then presented the results without some sort of check against classical methods.
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Frank Wiedmann
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #12 - Dec 9th, 2004, 11:37pm
 
[quote author=Richard  link=1102534266/0#10 date=1102616794]Frank : I am trying to use PAC for the sims, hopefully it will turn out okay. The question I have would be that we can see the gain response from PAC, can we also use it for phase response ?
[/quote]
I would suppose so (but, like I said, I have not tried it).
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Ken Kundert
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #13 - Dec 10th, 2004, 12:05am
 
One thing to keep in mind is that PAC will give you the small-signal gain. Stability will depend on the large signal characteristics. The large-signal characteristics may be the same as, or at least very similar to, the small-signal characteristics, but I don't know. Like Frank, I have never tried these simulations.

-Ken
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Richard
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Re: DC/DC Buck's Phase Margin analysis using spect
Reply #14 - Jan 4th, 2005, 4:11pm
 
Hi,
 I couldn't even do a gain-phase analysis on a linear feedback amplifier, let alone the switching DC/DC.
 I find that having the loop-cutter i.e. breaking the loop and subitute with a RC to pass the DC and block the AC really screw up the PSS analysis.
 I wonder if anyone has tried using the PSS for a simple case of feedback amplifer ?
Thanks. Smiley
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