Buck converter (Step Down) circuit help

Discussion in 'Homework Help' started by Barry1234, Jul 28, 2016.

  1. Barry1234

    Thread Starter New Member

    Jun 13, 2016
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    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 indoctors 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

     V_{REF} = (\frac{R2}{R1 + R2} ) * V_{cc}

    which re-arranges to
     R2 = (\frac{V_{REF}}{V_{cc}} - 1) * V_{cc}

    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

     R2 = \frac{V_{fb}}{I_{r1/2}}

    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 BuckConv.JPG
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    I fear there is some information missing. Does it give the value of Vout anywhere?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Unless I'm missing something, that appears to be either a bad question or a trick question.
    There does not seem to be enough info to determine the answer to b).
    Without knowing the answer to a least one other variable (Vout), the answer can be any value for R2.
    In other words, any arbitrary value for R2 can be solved for a Vin that satisfies there being a 30% duty-cycle with Vref=1.25V.
    Edit: Albert beat me to it.
     
  4. Barry1234

    Thread Starter New Member

    Jun 13, 2016
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    Hi crutschow and AlbertHall,

    I thought the same thing, and i can't find anything else which helps in this kind of problem. Thats all the info i have got unfortunately.

    Thanks for your help
     
  5. Bangersandmash

    New Member

    Jun 10, 2016
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    Hi Barry you are missing the E24 Series table upload a screenshot of that.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    That doesn't solve the problem of insufficient information in the question to generate a unique answer.
     
    AlbertHall likes this.
  7. MrAl

    Well-Known Member

    Jun 17, 2014
    2,439
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    Hello,

    I get these two, this one:
    R2=50000/(4*Vout-5)
    and this one:
    R2=250000/(6*Vin-25)

    One is in terms of Vout and one in terms of Vin, but because we know neither Vin or Vout we cant solve it for a single number, just a function. That means we cant look in any table of preferred values and pick a single one. We can generate another function that does this for us automatically, but that is most likely not what the question is asking for. It is more likely that something is missing from the problem specification, namely the output voltage. If we knew that we could solve both questions. Since we dont know that, we can only write functions like those above.
    There are many cases where the problems require solutions in the form of one or more functions, but this doesnt look like that.
     
  8. Bangersandmash

    New Member

    Jun 10, 2016
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    The missing value is........ Vout=7.5V
     
  9. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Oh good, you've had your crystal ball repaired.
     
  10. Bangersandmash

    New Member

    Jun 10, 2016
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    Is Barrys formula correct? What would the value of Vcc be?
     
  11. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    1. The first one is OK.
    2. In that first equation Vcc is Vout.

    But you still can't get R2, Vout, or Vin.
     
  12. Bangersandmash

    New Member

    Jun 10, 2016
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    Should it not be Vout=vref*(R1+R2/R2)

    Vout=1.25(10+2/2) = 7.5 meaning R2=2?
     
  13. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    You can't just put in a value for R2 and then claim to have calculated it.
    Vout=1.25((10+R2)/R2)=??
     
  14. Bangersandmash

    New Member

    Jun 10, 2016
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    7.5V is the true value that was missing off the question no guess work or trial and error involved. Using the formula and through substitution it gives 7.5V the correct Vout value so R2 must be 2.
     
  15. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    You went from:
    To:
    In your second equation you have substituted the value of R2 with the number '2'. You are imposing the condition that R2 is equal to 2, so it is no surprise that that is the value that pops out of the formula.
     
  16. Bangersandmash

    New Member

    Jun 10, 2016
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    Im just working backwards using an equation I found to determine Vout. So I now know Vout=7.5V as this was missing off the question (mistake of the lecturers). How else could you determine R2 now that Vout is known?
     
  17. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Your first equation:
    Vout=vref*(R1+R2/R2) OK, except there should be brackets around the 'R1+R2' bit

    Your second equation:
    Vout=1.25(10+2/2) Here you have put in the value for R1 as 10, OK
    But you have also put in a value of 2 for R2, hence it is no surprise that vaue pops out for R2.
    What would happen if you put in 3 instead of 2?
     
  18. Bangersandmash

    New Member

    Jun 10, 2016
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    Then I would not get the correct 7.5 value for Vout
     
  19. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    And you know 7.5V is correct because?
     
  20. Bangersandmash

    New Member

    Jun 10, 2016
    26
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    The teachers left the value out of the question by accident. They just realised their mistake.
     
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