Buck Converter Design

You're coming really close to understanding this.

> In the first term, V1 is not multiplied by the voltage divider ( Rf2 / ( Rf1 + Rf2 )).
> But V1 is multiplied by "1 + etc." so you need to enclose "1 + etc." in another parenthesis so it all acts on V1.

I just tried this equation by adjusting different resistors values to get Gain_of_U3 to be 4, but it doesn't work.
I need to get Vo (U3) = V1 * Gain_of_U3. Optimal Gain_of _U3 need to be 4

Vo (U3) = ( V1 * ( 1 + (( R14 / ( R17 + (( Rf1 * Rf2 ) / ( Rf1 + Rf2 )))) * ( Rf2 / ( Rf1 + Rf2 )))) - (( R14 / ( R17 + (( Rf1 * Rf2 ) / ( Rf1 + Rf2 )))) * ( Rf2 / ( Rf1 + Rf2 )) * Vout )

Vo(U3) will only be = (V1 * Gain_of_U3) when Vout = 0.

I have highlighted an item which doesn't belong in this equation.
Vo (U3) = ( V1 * ( 1 + (( R14 / ( R17 + (( Rf1 * Rf2 ) / ( Rf1 + Rf2 )))) * ( Rf2 / ( Rf1 + Rf2 )))) - (( R14 / ( R17 + (( Rf1 * Rf2 ) / ( Rf1 + Rf2 )))) * ( Rf2 / ( Rf1 + Rf2 )) * Vout )

Does this help?

Yes it does, finally my circuit work.

Congratulations!

buck converter design

i am in hardware design of buck converter with MOSFET as switch...i have designed L and C values...i use R load....
input voltage=24v
output voltage=18v
L=1mH
C=100uF
R=100 ohm
switching frequency=48.828KHz
i use microcontroller program to drive pulse for MOSFET.....but it is very difficult for me to capture pulses and the output is very low....
can u just plz explain me what i have made mistake in my design?