# New member seeking help.Urgent.

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#### edmond_chan85

##### New Member
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?

edmond_chan85

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#### mneary

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

#### edmond_chan85

##### New Member
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?

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#### smanches

##### New Member
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.

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