Hello friends, I want to build a Buck converter using a Chinese XL1509 IC. For designing the buck converter, I have taken all the theoretical calculation formulas from the book Power Electronics written by L. Umanand. But the problem now is that the value of the inductor I’m getting using the formulas given in the book is significantly higher compared to the value given in the XL1509 IC datasheet.
For example, in the case of a 12V output, if the maximum input voltage is 40V and the minimum input voltage is 4.75V, then the inductance value I get is 352.5 µH, which is quite a bit higher than the value (150 µH) specified in the XL1509 datasheet!
Can anyone in this forum help reconcile the discrepancy between the book’s calculation and the datasheet values?

Here is the Theoretical Calculation from the power electronics book by L Umanand







Here is my Excel file for Calculation



[size=10pt]Here is my Excel file for calculation.
[/size]
https://docs.google.com/spreadsheet...ouid=104527294515503795614&rtpof=true&sd=true

 

For 40V in, 12V out and 200mA p/p ripple I get 280uH
Mark-space ratio is 30%, so the off time is 4.67us at 150kHz.
L=Vt/ΔI = 280uH.
Don't forget that:

• The tolerance on the inductor will probably be ±20%
• The inductance varies with current, temperature, and probably anything else that it could possibly vary with.
• The choice of ripple current is arbitrary (and it doesn't seem like XLSemi has specified it)

so any value between 150uH and 450uH would work fine.
More inductance = more expensive = less ripple (and goes into discontinuous mode at a lower load current)

 

I observed that changing the ripple current (diL\ΔiL) in the denominator of the inductance equation significantly affects the calculated inductance value. For example, when the ripple current is increased from 1% to 2% of the output current, the required inductance decreases from 352.5 µH to 176.25 µH.

 

I observed that changing the ripple current (diL\ΔiL) in the denominator of the inductance equation significantly affects the calculated inductance value. For example, when the ripple current is increased from 1% to 2% of the output current, the required inductance decreases from 352.5 µH to 176.25 µH.

Correct:
L=Vt/ΔI
Your text book says that the value of ΔI that the designer would allow for a given application.