How Much Inductance Do I Need?

How much inductance do you need for your buck converter to function nicely? I will show you how to figure that out without using any complex math.

Table of Contents

Formulas?

Just in case you are still in doubt about doing the calculations, there are Greek letters involved. For me, that a personal limit I will not cross, ever. 🙂

Field data

There are many buck controller ICs out there being produced by different manufacturers. And all those manufacturers have something in common. They provide a datasheet with their ICs and these datasheets have the information I need.

  • Maximum operating frequency
  • Reference circuit, with an inductor of a certain value

So I took note of a lot of these numbers, threw out the outliers and build the table you see here below.

Raw data

A list of operating frequencies and the range of inductance found in the circuit.

With this table, we can now see the inductance range for a certain switching frequency. For example, the 700 kHz. The lowest value encountered was 1 uH and the highest 10 uH.

Now let’s look at both values and assume we can get them both with the same size core.

Choosing the lower value will give you the highest current capability for your circuit, but with more voltage swing on the output.

The higher value will give you a much smoother output, but the core will saturate at a much lower current.

“Saturation of your inductor’s core is the moment your inductor stops being an inductor and just acts like a piece of wire.”

Pick one

At the end of the day, just pick one, preferably on the high side, just to be safe. High current testing is usually not the first thing we do when building a converter. You can always fine tune your inductor later on.

Low frequency

These buck controller ICs all operate at a frequency of 100 kHz or more. If you are using an Arduino or similar device, then chances are you end up in the 50 kHz range.

Below here are two graphs with the same data in it. Using the graph with the logarithmic scale, we can extend the lines and see what values we get.

For the 50 kHz range, that will be 50 uH on the low side and 250 uH on the high side.

Regular graph

A graph of operating frequencies and the range of inductance found in the circuit.

Logarithmic graph

A graph of operating frequencies and the range of inductance found in the circuit.

Building the inductor

If you also want to build the inductor yourself, then you really need to have a component tester.

My buck converter will be switching around 50 kHz, so I aim for an inductance of around 200 uH, which is near the top of the range.

Doing an inductance test with some jumper wire to get an estimate on the numbers of turn required.

I wound a piece of jumper wire around my inductor core as often as possible. 3.5 turns in my case. The component tester now tells me the inductance now is 0.11 mH → 110 uH.

If I did 6.5 turns instead of 3.5 I would expect to get 6.5 / 3.5 * 110 = 205 uH. Excellent!

Something cheap

Using rope instead of wire to get an estimate on the amount of wire required.

With this simple trick, I now know exactly how much copper wire I need to cut for my inductor.

Measuring the final inductance on my new inductor.

Whoops, apparently copper wire is much better at this than the jumper wire was. Well, at least I have the option of just removing a turn to get the induction lower.

Final result

Hand made inductor.

Excellent!


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