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I just read about buck converters and boost converters and buck/boost converters. Great stuff.

But, why is a step-down converter called a buck converter?

I tried to research this myself. According to Google Book search, the phrase buck-boost transformer was in use at least as early at 1891 in a periodical called Architectural Review.

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    \$\begingroup\$ I suspect the term as applied in electric systems may have been created originally for transformers -- more specifically for autotransformers -- and relates to an idea that was commonly known back in the day. Logs were bucked (either under- or over- bucked) depending on from which end of the log sections were cut for milling. (Starting at the thicker base and working up or starting at the top and working down.) A bucking transformer can be seen as doing just that. And I suspect that's where the first author of the term got the idea since logging was a commonly shared experience in the 1800s. \$\endgroup\$ – jonk Aug 29 '19 at 0:14
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    \$\begingroup\$ I don't think it's opinion-based. Personally I don't like etymology questions, but if we don't want them anymore, we should rather have a meta discussion than closing a single question for the wrong reason. \$\endgroup\$ – Dmitry Grigoryev Aug 29 '19 at 7:48
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    \$\begingroup\$ BTW, I replaced "etymology" with "terminology" which has a much wider use on this site. \$\endgroup\$ – Dmitry Grigoryev Aug 29 '19 at 7:56
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    \$\begingroup\$ Awesome answer in comment above by jonk about wood logs is etymology. Answers below especially one by marcelm, are terminology. IMHO removing etymology tag is wrong if OP asked about the origins/history of expression (which is not clarified). Definitely not opinion based, and great question. \$\endgroup\$ – temoto Aug 29 '19 at 16:42
  • \$\begingroup\$ I am specifically interested in the etymology. \$\endgroup\$ – daveloyall Aug 29 '19 at 20:16
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It's the same sense as to "buck" a trend:

  1. to oppose or resist (something that seems oppressive or inevitable). "the shares bucked the market trend" synonyms: resist, oppose, contradict, defy, fight (against), go against, kick against "it takes guts to buck the system"

So you're "bucking" the input voltage to reduce the output voltage.

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    \$\begingroup\$ Which is more applicable to transformers than to regulators, but the idea of "bucking" as "reducing voltage" probably came from transformer usage, and got transferred to regulator usage. \$\endgroup\$ – TimWescott Aug 28 '19 at 23:27
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    \$\begingroup\$ Buck regulators use an inductor which opposes the current flow when the switch opens. So it's just as valid as the transformer use. \$\endgroup\$ – gbarry Aug 29 '19 at 3:08
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    \$\begingroup\$ @gbarry Right, we don't call linear regulators "buck" regulators \$\endgroup\$ – slebetman Aug 29 '19 at 9:13
  • \$\begingroup\$ There is a deeper root. It comes from the action of the buck converter being similar to a bull's or horse's bucking, which throws the rider down. \$\endgroup\$ – Motes Aug 29 '19 at 21:17
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I may be wrong(apparently there is no way to qualify any answer here as correct), but I had always assumed that "buck" referred to an action similar to a "bucking bronco or bull". A buck converter sends a voltage pulse only as often as it needs to in order to provide the rectified and filtered DC output required, just as a "bucking bronco or bull" will "buck" as often as he feels he needs to in order to eject the rider or loosen the strap.

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    \$\begingroup\$ Please, no downvotes because you dislike rodeos. Even though I understand where you may be coming from, it is a very plausible answer... \$\endgroup\$ – Hitek Aug 29 '19 at 2:32
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    \$\begingroup\$ (No one is downvoting?) I honestly thought it had to do with animals as well when I first heard the term but I knew that it probably wasn't correct. The verb "buck" could mean to be resistant against something. I think there could be a way to qualify an answer and that's looking it up in the dictionary :) \$\endgroup\$ – KingDuken Aug 29 '19 at 2:38
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    \$\begingroup\$ @KingDuken - I know, but right after I posted the answer I had visions of SJW bombardment, so I was just trying to preempt that :). If the term isn't derived from my suggested possible answer, than it does likely refer to resistance, as you say... \$\endgroup\$ – Hitek Aug 29 '19 at 2:42
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    \$\begingroup\$ Since most of the switching topologies work by making controlled pulses in response to operating conditions, it's hard to imagine that we'd take the "bucking" notion and specifically put it on the one that's a step-down. \$\endgroup\$ – gbarry Aug 29 '19 at 4:57
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    \$\begingroup\$ Let us continue this discussion in chat. \$\endgroup\$ – probably_someone Aug 29 '19 at 17:37
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Question probably belongs on english.stackexchange.com. It arises from bucking being an action an animal takes to throw riders off or down, so a buck converter "throws" the voltage down by a repetitive "bucking" mechanism.

buck (v.1)

of a horse, "make a violent back-arched leap in an effort to throw off a rider," 1848, apparently "jump like a buck," from buck (n.1). Related: Bucked; bucking. Buck up "cheer up" is from 1844, probably from the noun in the "man" sense.

(from etymonline.com)

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Step-down converters is really a subclass of DC-DC converters, while a buck converter is one specific topology ("brand") of step-down converter. In essence, a buck converter is a step-down converter, but not every step-down converter is a buck converter. In theory, anyway.

Let's look at an overview of the various non-isolated DC-DC converter topologies:

Step-down

Step-up/down

Step-up

As you can see, the subclasses are step-up, step-down, and step-up/down, and there are some topologies in each subclass. You can also see that there's only one topology in the step-down subclass: the buck converter. So:

But, why is a step-down converter called a buck converter?

Because buck converters are step-down converters, and in practice all step-down converters are buck converters. The names refer to different things, but in practice it doesn't matter, so they're used interchangeably.

The Texas Instruments book Power Topologies Handbook (by Markus Zehendner and Matthias Ulmann) also has a good overview of the various topologies (also including isolating converters).

If you want to know where the name "buck converter" comes from; I don't know, but the other answers try to address that.

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