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Inductors are really not my strong point, and I have been getting a bit confused on the rated current vs saturation current.

I know the rated current is how much current the inductor can handle before the temperature increases too much, and all I know about saturation current is when that threshold is approached and exceeded, the core is filled with magnetic flux and the inductance starts to drop.

I have come across a chip inductor HERE which I am planning on using in a boost regulator circuit. It is cheaper and smaller than the current inductor I am using, with the same rated current. However, whereas my old one had a saturation current of 900mA, this one is very low at 250mA.

Why is there such a difference in saturation current in chip inductors? My boost regulator chip has recommended inductor values of 2.2uH to 4.7uH, so if my current peaks at 300mA for a few ms and the inductance drops somewhat, will this still be ok to use? And would this change in inductance affect the output stability in any way?

As much as I would love to save money and PCB space, I don't know enough about these parameters to make an informed decision. Any advice on how to proceed is appreciated.

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  • \$\begingroup\$ You are going to have to either calculate or use design software to find the Peak inductor current under the conditions you are operating on. Be sure to check the extreme edges of your operating ranges. For a boost converter, check peak current with maximum load current and minimum input voltage. \$\endgroup\$ – Dwayne Reid Feb 22 at 11:34
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Looking at the datasheet of the "chip inductor", page 2: Inductance over current, I conclude that only up to about 100 mA this inductor behaves "properly".

The other inductor has a similar plot, also on page 2, from which I would say that it is usable up to 600 mA.

That's a 6x difference!

Only if you are 100% sure that your application keeps the current through the inductor always below 100mA would I consider the chip inductor.

Note that depending if we're dealing with a buck or boost converter and what the input/output voltage ratios are, the momentary current through the inductor can be many factors larger than the (average) currents going into and coming out of the converter. In extreme cases the current through the inductor can be 10x as high or even more.

The chip inductor is physically smaller and has a different construction so it simply is less capable at storing magnetic energy.

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  • \$\begingroup\$ It is a boost converter, with a single AAA battery input (1.6V - 0.95V) and an output of 3V. The overall current (measured by an Agilent power analyser) is an average of 400uA, but during operation, spikes of 50mA for about 10-20ms occur, and potential peaks of 200-220mA for about 1-2ms. \$\endgroup\$ – MCG Feb 22 at 11:48
  • \$\begingroup\$ Thank you for the response, this has been the most useful answer to the question so far! \$\endgroup\$ – MCG Feb 22 at 11:49
  • \$\begingroup\$ What would be the biggest side effect if the saturation current was exceeded momentarily? I'm not too sure what happens to the actual boost regulator once the inductor saturates. I was going to experiment with my dev board, but thought it best to ask before I go potentially smoking components! \$\endgroup\$ – MCG Feb 22 at 11:57
  • \$\begingroup\$ @MCG When the coil saturates the inductance part of the coil basically becomes a short so you're left with the series resistance of the coil. That causes the current to increase even more and could potentially destroy the switching transistors. Also once saturated, the coil cannot store more energy so even if the converter keeps trying to charge the coil more that energy is lost. I would stay away from saturation. You can investigate it on a dev. board but use controlled circumstances for example to prevent currents from becoming too large. \$\endgroup\$ – Bimpelrekkie Feb 22 at 13:15
  • \$\begingroup\$ Lovely, that's what I was after, along with the points in the answer. Thanks for the help! \$\endgroup\$ – MCG Feb 22 at 13:35
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The general equation:

\$I_{Sat} = \frac{B_{max} \cdot N \cdot A_q}{L}\$

\$B_{max}\$ is given by core material. If you want higher \$I_{Sat}\$, but don't want to reduce inductance \$L\$, then you need more windings and/or core cross-section \$A_q\$, both resulting in a larger and more expensive inductance.

Dependent on core-material the indcutance might drop dramatically after saturation, and you might get very high current peaks at each switching. This will heat up your semiconductors and reduce their lifetime or immediately destroy them.

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  • \$\begingroup\$ That's good for general use, but what about with these particular components linked in the question? And in particular why do these chip inductors have a much less saturation current, but an equal rated current? Also, note i mentioned the peaks would only be for a few ms, so was interested in the output stability \$\endgroup\$ – MCG Feb 22 at 11:21
  • \$\begingroup\$ Rated current is related to thermal issues (wire properties), saturation current is related to core properties (core geometry, gap, turn number). Both are input parameters for an inductor design. \$\endgroup\$ – UweD Feb 22 at 11:30
  • \$\begingroup\$ There could be instabiltiy in the current control loop, depends on the controller design. \$\endgroup\$ – UweD Feb 22 at 11:33
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You are going to have to either calculate or use design software to find the Peak inductor current under the conditions you are operating on. Be sure to check the extreme edges of your operating ranges.

For a boost converter, check peak current with maximum load current and minimum input voltage. If the peak current does not exceed the saturation current of the smaller inductor, feel free to use it.

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