LTC3872 or LM3478 Inductor Wattage Requirement

Thread Starter

c.weaver

Joined Jun 19, 2018
9

AlbertHall

Joined Jun 4, 2014
12,347
You won't find a power rating for inductors. What you will find is a maximum current rating. Both those datasheets include formulae for calculating the inductor value and the resulting inductor currents. The inductor must be rated for at least that current with a safety margin otherwise the core will saturate and bad things will happen.

"Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper losses and preventing saturation.
Ferrite core material saturates “hard,” meaning that the
inductance collapses rapidly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequently, output voltage ripple.
Do not allow the core to saturate!"
 

Thread Starter

c.weaver

Joined Jun 19, 2018
9
You won't find a power rating for inductors. What you will find is a maximum current rating. Both those datasheets include formulae for calculating the inductor value and the resulting inductor currents. The inductor must be rated for at least that current with a safety margin otherwise the core will saturate and bad things will happen.

"Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper losses and preventing saturation.
Ferrite core material saturates “hard,” meaning that the
inductance collapses rapidly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequently, output voltage ripple.
Do not allow the core to saturate!"
You won't find a power rating for inductors. What you will find is a maximum current rating. Both those datasheets include formulae for calculating the inductor value and the resulting inductor currents. The inductor must be rated for at least that current with a safety margin otherwise the core will saturate and bad things will happen.

"Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper losses and preventing saturation.
Ferrite core material saturates “hard,” meaning that the
inductance collapses rapidly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequently, output voltage ripple.
Do not allow the core to saturate!"


Thank you so much. I'm an amateur and I still have problems understanding everything in a datasheet. Could you help me out and let me know what the max current could be?
 

AlbertHall

Joined Jun 4, 2014
12,347
The LTC3872 datasheet includes a detailed design and recommends an inductor on page 15.

The component chosen is a 2.2μH inductor made by
Sumida (part number CEP125-H 1ROMH)
 

ebp

Joined Feb 8, 2018
2,332
The late Jim Williams of Linear Technology (now part of Analog Devices) wrote a few ap notes on the generalities of switch mode power supplies that are helpful in understanding a lot of important aspects without getting heavily bogged down in math (too many writers on switchers want to reduce things to all-in-one equations that I don't think foster understanding). I recommend reading them.

Here's one:
Switching Regulators for Poets
A Gentle Guide for the Trepidatious

http://www.analog.com/media/en/technical-documentation/application-notes/an25fa.pdf

There is also lots of good info at TI, much of it from people like Lloyd Dixon of Unitrode (now part of TI). Mr. Dixon commented at a seminar I attended years ago that most engineers don't know much about inductors. He was also a good interpreter and teacher, rather than just a presenter of equations.
 
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