Buck small signal modelling

Discussion in 'The Projects Forum' started by bigm79, Oct 11, 2012.

  1. bigm79

    Thread Starter New Member

    Oct 11, 2012
    3
    0
    Hi all,

    I am trying to build a small signal model of a buck converter in order to design a voltage mode controller. I have it on good authority (Author: Mohan = First Course in Power Electronics) that I must use an element from the PSpice schematics library to simulate the switching power pole.
    This element is named Ideal_Transformer _CCM. The schematic and error messages I receive (in the message viewer and the probe windows respectively) are attached as nerdstuff file.

    It generally tells me that the transformer ports are not connected to anything and it looks on the schematic like it is. Though when the block is double clicked, symbols for each port within look unconnected and do not respond to any configuration attempted!:confused:

    I have also made the model in PSpice building my own ideal transformer but have not gotten satisfactory frequency response from this. The dB level is offset by approximately 25dB:(. I have attached the model and frequency response as aptly named files.

    If anyone can explain firstly how to make the model work with the ideal_transformer_CCM i would greatly appreciate it. However if an explanation of why the other model is offset would also be appreciated.

    Thanks! bigm79:)
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,226
    Don't understand why you would use a transformer, especially since transformers don't work with a DC input. :confused:

    Typically, for a small signal model, the PWM modulator is modeled as a linear gain block with gain equal to the PWM gain (PWM average output voltage divided by PMW input control voltage). Then you include the controller compensation circuit and the output LC filter in the loop to simulate the loop dynamics. That's how I did the Spice model for the buck converter I was building and the simulation was quite close to the actual circuit response.
     
    bigm79 likes this.
  3. bigm79

    Thread Starter New Member

    Oct 11, 2012
    3
    0
    Thankyou for the swift reply mate:). If your method works then perhaps I should change methods, because my method has caused me only trouble up to this point!

    I do believe that the "ideal" transformer would allow DC whereas real ones will not. The method is a prescribed one with a number of texts including Mohan (First Course in Power Electronics) and Erickson (Fundamentals of Power Electronics) citing its use.

    I will give your method a try and hopefully get better results than my obviously dodgy model:p.It would be of great help to me if you could send a screen shot of your model so perhaps I know I am on the right track please? I will give it a go right now regardless though. Thanks again mate!:)
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,226
    A transformer, "ideal" or otherwise, will not transfer a DC voltage across the windings so I don't understand their use of a "transformer" in this type of simulation.

    I don't have PSpice but I found a reference which stated that you can find it in the ABM library with the title "gain".
     
  5. bigm79

    Thread Starter New Member

    Oct 11, 2012
    3
    0
    ok thanks I found it anyway. think we will leave the transformer argument as I will not argue on behalf of text books! thanks again for your help mate :)
     
Loading...