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I'm using an AAT1217 for the boost converter in my circuit. The reference circuit I'm using is:

AAT1217 Reference Circuit

The datasheet suggests several inductors:

AAT1217 Suggested Inductors

It suggests using these fairly large inductors, the ones that you can see the coils. My question is why aren't the normal 0402/0603/0805 SMD inductor packages being used instead? For example, the MLZ series from TDK has a similar DC resistance and a fairly large DC current rating of 500 mA. The fine print does mention that "Current assumed when inductance ratio has decreased by 50% max". When does the inductance ratio change?

Also, I've run some experiments with both the CR43 and the MLZ inductor and they are both able to drive the servo motor (MG90S) fine. Am I missing anything? Or is it perfectly fine to switch to a smaller, more compact inductor? (the other thing that I am wondering is if the AAT1217 datasheet was written a while back when small inductor packages did not exist).

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The MLZ series of inductors is not designed for power conversion (buck, boost, etc.), but instead for power supply filtering. This means that a) the windings are not optimized for low skin losses, and b) the magnetic core is not optimized for low core loss. From a circuit design perspective, this means the boost regulator will be less efficient.

If efficiency is not what you're after -- if you simply care for functionality and compactness -- then I don't see any problem using MLZ (or other "ferrite choke") inductors, as long as your circuit doesn't overheat. In practice, this means sticking to low power levels. Just ensure the current rating covers your needs (with healthy margin), and evaluate the power supply thoroughly.

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    \$\begingroup\$ What are low skin loss and low core loss a function of? Or in other words, how do I determine if an inductor has low skin and core loss? The first parameter that comes to mind is the DC resistance but that seems comparable between the suggested inductors and the MLZ series. \$\endgroup\$ – rith87 Apr 7 '15 at 8:27
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    \$\begingroup\$ At DC, current flows with uniform density throughout the entire cross-section of the wire. As frequency rises, however, electrons repel each other (with increasing force), causing them to crowd near the edges of the wire -- hence, the "skin" effect. This means that, effectively, the wire cross section is reduced and therefore its resistance is higher. Further, with improper coil design, electrons may crowd to one side or the other of the wire, which also constricts the current flow. As for core loss, it's a function of core material and shape. Power inductors aim to minimize these losses. \$\endgroup\$ – Zulu Apr 8 '15 at 0:54
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    \$\begingroup\$ The most straightforward (and most useful) way to measure these losses is simply to measure the efficiency of the switching converter with the inductor installed. Exercise the converter over its \$V_{in}\$ and \$I_{out}\$ range for a more complete picture of what's going on. Whichever inductor yields best efficiency exhibits the least loss. \$\endgroup\$ – Zulu Apr 8 '15 at 0:56

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