# DC-DC Buck converter calculation

#### anishkgt

Joined Mar 21, 2017
523
Hey All,
I am trying to design a high current DC-DC buck converter using LT8390. The voltage source is a 12v 82A power supply salvaged from a Server.

In the datasheet from page 18 onwards, to calculate Inductance $L = 10*{vout}*R{sense}$. Then to calculate Rsense
$frac2*50{m}V/2*I{out}{(max)+\Delta I{L}}{(buck)}$ So to find delta IL(buck) = Vout x (Vin(max)-Vout)/f x L x Vin(max)

Every formula requires one variable from each other. Can somebody help me find what i've been missing ?

#### anishkgt

Joined Mar 21, 2017
523
So based on this formula on Page2 Section 3

L=Vout(Vin(max)-Vout)/Delta IL x fs x Vin(max), substituting
= 8.1(24-8.1)/(80 x 150000 x 24)
= 0.00000045. I presume the unit is Henry (H). Searching digikey i can't find any suitable inductor with that spec. Would anyone know how to ?

#### Papabravo

Joined Feb 24, 2006
13,967
You would have to make one from magnet wire and a toroid. 450 nH if I read your string of zeros correctly. Nobody writes strings of zeros like that. Use scientific notation like 4.5e-7 or engineering units like 450e-9. It greatly reduces the probability of error.

I didn't watch the whole thing, but this will set you on the right track.

#### eetech00

Joined Jun 8, 2013
1,947
So based on this formula on Page2 Section 3

L=Vout(Vin(max)-Vout)/Delta IL x fs x Vin(max), substituting
= 8.1(24-8.1)/(80 x 150000 x 24)
= 0.00000045. I presume the unit is Henry (H). Searching digikey i can't find any suitable inductor with that spec. Would anyone know how to ?
Use this formula:

L=(Vout*(VinMax-Vout))/Kind*Fsw*VinMax*Iout

Where Kind= ripple current coefficient and is relative to max output current. A good value is 0.2-0.4

It is from a TI document and is basically the same as LT equation.

https://www.ti.com/lit/an/slva535b/slva535b.pdf

#### anishkgt

Joined Mar 21, 2017
523
I was following that since LT had it in a different way. Thanks by the way

#### anishkgt

Joined Mar 21, 2017
523
Looking at this from Vishay seems a good match but the saturation current is lower than the RMS current ?

#### anishkgt

Joined Mar 21, 2017
523
Is that a bad choice since the core saturates before the rated current.

#### Papabravo

Joined Feb 24, 2006
13,967
Is that a bad choice since the core saturates before the rated current.
Saturating the core is NEVER good. It will absolutely kill your efficiency.