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I am currently designing a DC-DC Buck power supply and now that I have chosen values for all my resistors and capacitors, I am looking for actual components. I was wondering if the saturation current of the inductor is an important parameter and whether I should scale it to my desired Iout or to some other parameter?

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Off-the-shelf inductors for bucks usually only specify a maximum output current and temperature rise, and don't talk about \$B_{max}\$ or other magnetic parameters. This is for a few reasons:

  • A 'good' buck design usually has ripple current of 10-20% of the DC output level (it's operating well into continuous conduction mode at full load)
  • 10-20% ripple means the AC flux swing is small (a minor loop) so there isn't a lot of switching loss
  • Because the AC flux swing is small, the DC resistance of the winding is the major loss contributor
  • Because the AC flux swing is small, the permeability loss due to DC bias won't push the part into saturation

You can very easily get into trouble using one of these buck inductors in a non-buck application (one that has high AC ripple) - the magnetic parameters are often unknown and you're stuck empirically figuring out if the part will saturate out or not, by measuring the current and seeing if you're saturating out or not.

If you design your own inductor, you'll be picking your own core and will know all the magnetic parameters, which means you can analytically determine the risk of saturation in the end application.

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The saturation current of an inductor tells you at what point the inductance starts to roll off. In other words, operation above the saturation current will give you an inductance less than what you thought it would be. You want to avoid the situation of sourcing a higher current than your inductor saturation current(this could make your output have a very big ripple). Make sure your inductor is rated for the maximum current you will be supplying.

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    \$\begingroup\$ Your last sentence is not very clear. The main point is, the inductor's saturation current rating should be greater than the peak current you intend to put through it by a comfortable margin. \$\endgroup\$
    – Dave Tweed
    Commented Dec 12, 2012 at 19:30

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