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Using properly the Analysis Tool

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Hi,

I have a problem on the visualization of the bode diagrams using Analysis Tool.

I created a simple Boost circuit with a certain switching frequency Fs=17.8kHz and then tried to find the bode diagram of the transfer function linking the duty cycle to the inductor current.

Using 'Impulse response' I found that the bode plot was inaccurate for frequencies above let's say 8kHz so I switched to 'AC Sweep' analysis which I set to produce 1000 logaritmically spaced points between 10Hz and 11000Hz, hoping in this way to get more resolution at those aforementioned high frequencies.

Soon I realized the 'AC Sweep' analysis chose to produce only 220 points.....(?!?!?!?!?) leaving me with only few points in the frequency range 8kHz-11kHz.

No matter how many points I ask for, the 'AC Sweep' will always shrink my number based on who-knows-why principle and leaving my bode plot with poor resolution where I need to get a clear look at the frequency response.

I tried 'Multitone Analysis' too but no success.

Someone has suggested to ''increase the switching frequency of the PWM'' !!!!! because his supposition was that PLECS analysis tool doesn't like to work too close to the switching frequency. I followed his tip and it worked, or at least the resolution of the bode plot increased sufficently.

Just for checking I then tried to launch an 'AC Sweep' analysis in the range 10-3000 Hz requesting 1000 points....I got only 679.

Yes, I also tried to adding specific frequencies in the 'Additional frequencies' field. No success, it still give me 679 points....(as you can see from the attached file)

Someone please explain to me what is happening down here!!!!

 

PS: if you want me to give you more images regarding all the things I've spoken about tell me.
asked Mar 12 by Castrol (12 points)

An applications engineer from Plexim will be giving a webinar on the small-signal analysis tools in two weeks. 

Description: 

Multiple small-signal analysis tools are built into PLECS, allowing users to quickly generate a Bode plot of a system for controller design and stability analysis. Using an isolated flyback converter, we will illustrate how to obtain a frequency response with PLECS to design a voltage controller, predict the system loop gain, and verify stability. 

Click to register for the webinar

1 Answer

0 votes

Quick Notes:

  • The bode plot is only meaningful to fs/2 in switching converters (sampled systems)
  • Using 1000 points sounds excessive to me. Might only be feasible in the impulse response analysis.
  • Try starting with a smaller model.
The "Buck Converter with Analysis Tools"  demo model is a good starting point. 
See attached model, where I modified the demo model.
answered Mar 13 by Falk Kyburz (233 points)
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