Ah,,,,the mystery of magnetics, such as inductors and transformers: known about by all electronic engineers, but really understood by very few. That's understandable, of course, there are lots of others things to we have to know about in circuit design, and there is only so much time in the day.
I was reminded of this a few weeks ago in an informal conversation with Tony Armstrong, power products marketing manager at Linear Technology Corp. Tony was discussing an upcoming dc/dc converter which operates at a switching frequency from below a megahertz up to over 5 MHz. I asked him how many users would consider using this part at the higher end, and he said, "very, very few at this point."
And why was this so? To paraphrase Tony, there's a sweet spot of operation around two MHz, where you get good efficiency, small footprint, manageable EMI, and just as important, many suppliers of suitable inductors. He added that as you approach three and four MHz, you have only a handful of suppliers; at five MHz, there is only one credible supplier, which is never a good situation in the real bill-of-materials world. And while most engineers know the primary parameters of inductors, such as nominal inductance value, dc resistance, and Q, it's those second-and third-order parameters which make or break a leading-edge, high-frequency design.
Ironically, the inductor is among the oldest electrical components, part of the passive trio that also includes the resistor and capacitor. In the 1840s, Michael Faraday and Joseph Henry did seminal investigations of electricity, magnetism, inductors, and electromagnetism (we take these as basic facts, but it was a real surprise back then), well before "electronics" as we now know it existed.
So, why do we use the designation L on schematics for inductors, when we use the more logical R for resistor and C for capacitor? Any serious research to find this out with certainty is a big undertaking (by research, I mean more than a web search; it requires looking for original notes and documents). One theory is that the L is used in honor of Henrich Lenz, who formulated the law of induction in 1834.
That's possible, of course. Whatever the reason, the inductive time constant is quite long, you might say.
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