Buck converter (Step Down) circuit help

Thread Starter

Barry1234

Joined Jun 13, 2016
6
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
 

crutschow

Joined Mar 14, 2008
23,787
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.
 

Thread Starter

Barry1234

Joined Jun 13, 2016
6
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
 

MrAl

Joined Jun 17, 2014
6,814
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.
 
You can't just put in a value for R2 and then claim to have calculated it.
Vout=1.25((10+R2)/R2)=??
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.
 
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.
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?
 

AlbertHall

Joined Jun 4, 2014
8,810
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?
 
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?
Then I would not get the correct 7.5 value for Vout
 
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