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I want to make an inductor for my buck converter since I cannot obtain the recommended inductors for the LM2596.

How do I calculate the saturation current for this inductor if I use a ferrite core?

Would using multi-stranded plastic padded wire for the windings be a good idea?

Specificationss:

  • buck converter frequency: 150 kHz.
  • maximum current: 2A - 2.5A.
  • output voltage: 5V.
  • input voltage: 12V.
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    \$\begingroup\$ Suppose you did have "the formula" but you don't know the exact magnetic properties of the core material. Note that not all ferrite core materials are the same. Then having that formula is still pretty useless. If you measure it, you will know for sure. \$\endgroup\$ Sep 21 at 11:54
  • \$\begingroup\$ The saturation current must be > 2.5 A, as that's the current you need to pass, and then some. The LM2596 recommendations will tell you by how much it should be more. Do you mean 'how do I design an inductor with a given minimum saturation current?' ? \$\endgroup\$
    – Neil_UK
    Sep 21 at 13:08
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    \$\begingroup\$ For a continuously operating converter, the saturation current may not be the most restrictive inductor parameter. The current swing will take it round a hysteresis loop, that will cause dissipation in the core. You may need to restrict the core field to less than Bsat due to core heating. \$\endgroup\$
    – Neil_UK
    Sep 21 at 13:13
  • \$\begingroup\$ @Neil_UK you may be right , but I haven't seen that mentioned in the datasheet, I've only seen which manufacturers and inductors are reccomended. \$\endgroup\$
    – A.H.Z
    Sep 21 at 14:07
  • \$\begingroup\$ @Neil_UK How about air core inductors then? \$\endgroup\$
    – A.H.Z
    Sep 21 at 14:08
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There are a few questions here:

Is there a formula for saturation current?

There are models, and some useful tools for some ferrite/powdered iron cores. There isn't a single 'formula', but those models are using some pretty well tested maths. But in the real world they only ever give you guidelines: the exact core material will vary, as will your turn spacing, air gapping, atmospheric moisture content, etc.

How do I calculate... if I use a ferrite core?

You can use a modelling tool to get a ballpark. Then you build a prototype and measure.

Would using multi-stranded plastic-insulated wire be a good idea?

No. There's a fair bit of peak current through your inductor. That means there's a fair bit of peak heating. Plastic insulation melts quite easily: that leads to shorts and more heating, and you have a runaway thermal situation pretty quickly. Which is why you'll only ever see enamel covered wire used in commercial PSUs.

I want to make an inductor... since I can't obtain the required inductors.

So you have a target! Play with a simulator for what you do have (if you can) and then do some experiments. Incidentally, depending on what power you need, you may be surprised what inductors you can 'get away with'.

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