New member seeking help.Urgent.

Buck Boost Converter

Hi,
I am a newbie here and I wish someone is able to help me.
I want to design a Buck Boost converter with specification as below;
Output power = 75W
Output voltage = 24V (constant)
Input voltage variable range from 10V to 26V
Frequency = 20kHz

From here, I got the value for L and C by equation below;
L = (1-D)^2 R / 2f
C = D / R.f.Vr (Vr = ripple voltage in %)
Am I right to use these equation?
Furthermore, based on these equation, i got the L and C value. But when I simulate the circuit, the output is incorrect as I varied the input and duty cycle.
Anyone out there is able to let me know whats wrong with my circuit?
Thanks for your help.

edmond_chan85

You don't show a circuit. That is what's wrong.

Here is the circuit that I tried to simulate.
Actually, I'm still very blur with my design. Or you may give me any idea from the start?

What value did you use for the ripple ratio? Here are all the formulas for the inductor assuming .4 ripple ratio.

Duty = (Vout - Vin) / Vout = 0.58333

I(inductor) = Iout / (1 - D) = 7.5A

I(peak) = I(inductor) * (1 + (r / 2)) = 9A

L = (Vin * D) / (r * I(inductor) * F) = 97.2uH

Vout = MAX output voltage
Vin = MIN input voltage
Iout = Designed output current
r = Ripple ratio
F = Switching frequency

Oh, and please use a better subject next time. One that actually explains what help you need.