selecting inductors for buck converter

[daedalus]

Hi,

Im working on a little project right now that is quite cost sensitive, and need a small buck converter for the design. I am looking at using LM2574, as its pretty cheap, and has a low external component count. The problem is that the recommended inductors are rather expensive. Do I just need to match inductance, current rating, and core type, or is there more to the selection than this?

[nukie]

In this order if PCB real estate is unlimited

1) inductance
2) current rating
4) dc resistance
3) saturation current
4) inductance vs current

[Bored@Work]

Don't miss out on Jim William's writing about inductor selection. http://www.linear.com/docs/4129 The fun starts at page 22.

[fcb]

Can you be more specific on input/output voltages, current requirements etc...

There are some pretty low cost surface mount inductors available these days - and don't discount getting your own wound (or made in China).  I've done this in the past (Newport 1400X series replacement).

[Paul Price]

You can salvage toroidial inductors of high current capacity from discarded PC Desktop power supplies and rewind them, if necessary, to fit your need. You will find all the free parts and heatsinks inside definitely interesting. 

#define Free  = 0
#define True  = Free
byte Cost = 0;
byte Take_APaart_A_PC_Power_Supply= 0;

Main()
 {  if (Cost == Free)
      Take_Apart_A_PC_Power_Supply=True;
 }

Can you see the cost advantage in this?

[daedalus]

there are two buck regulators in the design, the first supply is powered from 18-36vdc, and has an output load of 100-160mA @ 5v. This is just running some 595 shift registers and a bunch of LEDs.

The second one is supplied by 18-30vdc, and is configured as an analog controlled current source using IS31LT3360, This is 120mA output at full scale. This is driving some LED strings.

The board its going on is a very low volume part, i'm only expecting to hand assemble 100 or so a year. I am trying to keep the total BOM down, but as I am hand assembling there is some benefit to lower part counts.

the LM2574 is about 1.87usd, this was the lowest cost part I could find at digikey that matched the spec
The IS31LT3360 is 0.60usd in singles, this was the first analog controlled buck I could find in a suitable package

im just trying to find inductors in the $1 region for them both, rather than the $5 region parts recommended.

EDIT:
forgot to mention, I will need to pass EMC testing, so I am looking at shielded type inductors.
My current thinking is that I need around 220uH for the IS31LT3360
and around 800uH for the LM2574

I am considering Panasonic ELL-6UH221M, which is rated at 410mA, 220uh and 0.81usd each in singles, and a higher inductance part in the same range for the lm2574

What I don't really get yet is what influence Rdc, Q and resonance frequency have on the choice?


PS: Thanks for the link Bored@work, looks like ive got some reading to do.

[Bored@Work]

PS: Thanks for the link Bored@work, looks like ive got some reading to do.

If you are looking for an inductor for a commercial product you will be disappointed. The reference gives some hints for selecting an inductor for some quick hack in the lab.

[fcb]

A couple of things.

Have you made a prototype?  I would loosely spec the inductor for current and buy a few similar types - and test them.

Q of an inductor is (2*PI*f*L) / R at some frequency.  The lower the Q, the higher the resistance - I wouldn't worry about it, it's unlikely that you will be going anywhere (frequency wise) that you will need to consider it.

You'll want the lowest resistance you can get and a core size that doesn't saturate - I guess you have used National's Simple Switcher program - it's now TI, but google it.

One other thing, 18-36v in, 5v@160mA is within the capability of the MC34063 - and it's very cheap and can be synchronized easily. The switching frequency maybe a little low for you though.

[szhighstar]

please supply below necessary information:
1.Inductance at 0A
2.Inductance at DC current
3.Dimension requested