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Buck Converter (Step Down) Help

  1. Jul 29, 2016 #1
    1. The problem statement, all variables and given/known data

    See attached sheet

    Problem with the buck converter circuit as in the diagram attached. The PWM is fed from a OP AMP in comparator mode, can we find the values of R1 and Vout. We have been supplied with the values of R2, the Duty Cycle of the waveform is 30%, and Vref which is 1.25V. (Full question in the attachment.).

    2. Relevant equations



    3. The attempt at a solution

    I am having problems with part b of the question i have working on. For part a) I have managed to find that the circuit is a Buck Converter (Step down converter) and that duty cycle is controlled by a OP AMP in comparator mode which creates a error voltage and is fed into the oscillator. ( I have included more information about the two states of the circuit, when the transistor is on, and when the transistor is off, included the free wheel diode etc, and the inductors magnetic field collapsing).

    For Part b), i am getting stuck, i take it you have to start at the voltage divider and try and work out R2. I have started this with the equation


    which re-arranges to

    With Vcc being the voltage supply to the OP AMP which isn't supplied.

    I have read that the value of R2 can also be given by the


    With V_fb being the voltage to the non inverting input of the OP AMP, but also the current is not supplied for the equation. The current being the current going into R1 at the top of the voltage divider.

    I am a little bit stuck, if anyone can point me in the right direction. Hopefully it is something simple i am missing! Or if i haven't explained myself very well!

    Thanks for your help

    Attached Files:

    Last edited: Jul 29, 2016
  2. jcsd
  3. Jul 29, 2016 #2


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    I'm a bit rusty and might not be able to follow up any replies but..

    Hint: You haven't used the stated duty cycle yet.
  4. Jul 29, 2016 #3

    rude man

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    This problem can't be solved unless Vout is given.
    Once it is, use the fact that the net voltage across L is zero over 1 cycle (explain why).
    Or maybe Vin was given, then you can compute Vout the same way.
    Since the duty cycle is given, Vref, R1 and R2 are all meaningless.
    Last edited: Jul 30, 2016
  5. Jul 29, 2016 #4
    Hi CWatters,

    Ye, i have not used the duty cycle, which the equation is
    Vout = D*Vin
    But as either of the voltages have been supplied, I dont know where to go next!

    Hi rude man

    I thought the same, I wasn't sure if I was missing something simple!

    Thanks for your help
  6. Jul 29, 2016 #5

    rude man

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    Confront your instructor!
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