# 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
6
0
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

2. ### AlbertHall Well-Known Member

Jun 4, 2014
2,261
448
I fear there is some information missing. Does it give the value of Vout anywhere?

3. ### crutschow Expert

Mar 14, 2008
13,482
3,370
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
6
0
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
26
0
Hi Barry you are missing the E24 Series table upload a screenshot of that.

6. ### crutschow Expert

Mar 14, 2008
13,482
3,370
That doesn't solve the problem of insufficient information in the question to generate a unique answer.

AlbertHall likes this.
7. ### MrAl Distinguished Member

Jun 17, 2014
2,553
515
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
26
0
The missing value is........ Vout=7.5V

9. ### AlbertHall Well-Known Member

Jun 4, 2014
2,261
448
Oh good, you've had your crystal ball repaired.

10. ### Bangersandmash New Member

Jun 10, 2016
26
0
Is Barrys formula correct? What would the value of Vcc be?

11. ### AlbertHall Well-Known Member

Jun 4, 2014
2,261
448
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
26
0
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
2,261
448
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
26
0
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
2,261
448
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
26
0
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
2,261
448
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
26
0
Then I would not get the correct 7.5 value for Vout

19. ### AlbertHall Well-Known Member

Jun 4, 2014
2,261
448
And you know 7.5V is correct because?

20. ### Bangersandmash New Member

Jun 10, 2016
26
0
The teachers left the value out of the question by accident. They just realised their mistake.