Switching regulator

[LynneMac]

Hi
I am having problems with part b of the question.

3a) The circuit is a Buck Converter (Step down converter) and that duty cycle is controlled by an OP AMP in comparator mode.

For Part b) I have to started at the voltage divider and trying to work out R2.


which re-arranges to


Vcc being the voltage supply to the OP AMP which is unavailable on the schematic and question info.

I believe R2 can also be given by the


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

Any ideas how I can solve this?

Thanks

 

[Sunnysky]

Vout = (R1)/(R2)* Vref just like any Inverting Op Amp gain.
(edit)
%duty cycle = %Vout/ Vin when used in Continuous (current) Mode

see figure 6,7. https://en.m.wikipedia.org/wiki/Buck_converter

 

[LynneMac]

Vout = (R1)/(R2)* Vref just like any Inverting Op Amp gain.

%duty cycle = % Vin when used in Continuous (current) Mode

see figure 6,7. https://en.m.wikipedia.org/wiki/Buck_converter

Thanks for the reply.

Regarding Vout = (R1)/(R2)* Vref.

I could select any value from the table to make this formula work. Do you think the question is missing the Vout or Vin value in order to calculate correctly?

 

[Sunnysky]

oops. I left out important ratio

yes it seems 2 equations , 3 unknowns Vin, Vout, R2

if duty is 30% then Vo/Vin=30%
But Vo is unknown and controlled by R1/R2 *1.25V

hmm. I think your site is not a good reference for Buck regulators.