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Measuring uncompensated loop gain of an buck converter by PSS/PAC cadence spectr (Read 3557 times)
Ankit Shivhare
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Measuring uncompensated loop gain of an buck converter by PSS/PAC cadence spectr
Feb 03rd, 2016, 4:57am
 
I am designing synchronous buck converter with following specifications
fs=1MHz, L=1uH, C=200uF, Vin=5V, Vout=1.8V, R=3ohms, PWM Ramp VM=1V.
I am trying to do PSS/PAC analysis in cadence to find uncompensated open loop gain. I am assuming control voltage vc=0.36V which is applied to a comparator +ve terminal. -ve side of comaparator is connected with sawtooth(tr=999n,period=1000n)
This is transient output of vout(voltage across capacitor)

This is our PAC output
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Ken Kundert
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Re: Measuring uncompensated loop gain of an buck converter by PSS/PAC cadence spectr
Reply #1 - Feb 3rd, 2016, 11:36pm
 
Do you have a question?
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saralandry
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Re: Measuring uncompensated loop gain of an buck converter by PSS/PAC cadence spectr
Reply #2 - Nov 16th, 2016, 4:27am
 
Hi Ken,

I have a question and I do appreciate that if you help me. I don't know if this is Ankit Shivhare's problem or not. But regrading the figure he posted, I am guessing that he has a problem with PSS and PAC analysis. Me too.
C Would you please let me know how to do PSS and PAC analysis to extract the closed loop frequency response of a PWM modulator?
I attached the test bench. Two VCVS are used as comparators. (gain=1000, max=1, min=0)
Triangle wave is generated by Vpulse. P-to-P is 1 V. The carier frequency is 640 MHz. Vsource is also used as an input with pac mag=1 and pac phase=0.
Now I like to see the frequency response of the PWM. I think there is somthing wrong with the testbench since it is not essentially closed. Honestly speaking I have not got the concept to perform PSS and PAC analysis for this structure.
Would you please post a simple tutorial to perform such analyses for this circuit?
Clearly speaking, how can we extract the closed loop frequency response of the a simple standalone PWM genrator?


Thank you



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sheldon
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Re: Measuring uncompensated loop gain of an buck converter by PSS/PAC cadence spectr
Reply #3 - Nov 18th, 2016, 9:04am
 
Saralandry,

  Not sure what this circuit does, but it does not appear to be a PWM
modulator. In a DC-to-DC converter, the inputs to the PWM Modulator
are the feedback output voltage and the saw tooth wave. Here your
circuits converts the saw tooth waveform into a differential voltage.

                                                                                  Sheldon
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saralandry
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Re: Measuring uncompensated loop gain of an buck converter by PSS/PAC cadence spectr
Reply #4 - Nov 19th, 2016, 11:42am
 
Sheldon,

Thank you for the reply. If you are talking about the schematic posted by Ankit Shivhare, yes you're right.
What I posted is a complementary PWM generator. The sin input signal is converted to the diff mode. I sued two VCVCs with the gain of 1000, max=1 and min=0 as comparators. And a triangular wave.
I appreciate if you take a look at the Fig. 4 of this paper: "True Filterless Class-D Audio Amplifier". What I am looking for is to plot the frequency response of the PWM generator. Theoretically, it is supposed to be a low-pass form profile while the gain is defined by Gain=Vdd/Vtri ( Vdd, supply voltage, and Vtri is peak to peak voltage  of the triangular wave. I searched for a lot of solution but none of them has not worked for me.

I worked with verilog-A module of the comparator and I figured out it is not possible to do PSS/PAC analyses on verilog-A modules. Hidden state problem adds more difficulties.
What I want to know is what is the proper test bench with ideal (macro) models and how to properly set the parameters in the PSS/PAC analyses to extract the Fig. 4 of the paper.

Thank you


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deba
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Re: Measuring uncompensated loop gain of an buck converter by PSS/PAC cadence spectr
Reply #5 - Nov 19th, 2016, 9:18pm
 
Hi,
Those plots in Fig. (4) are the closed loop responses of the class-D amplifier. The circuit which you posted doesn't have the filter. If you have the complete loop setup, pss/pstb or pss/pac can give the transfer function.
Thanks
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