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Q&A

Why do DC/DC switching controllers seem to favour the buck-boost topology over similar ones like Cuk, SEPIC and Zeta?

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I am looking at various DC/DC converter topologies for a power system I am designing. The most suitable topology for me is one that can perform both step-up and step-down functions, so I am looking into buck-boost and similar topologies like Cuk, SEPIC and Zeta.

While selecting the candidate ICs for the switching controller, I noticed a curios thing. On Digikey, where it is possible to filter by topology, the buck-boost & four-switch buck-boost topology occupies a large majority of the market. For instance, Digikey's catalog lists 471 active switching controller designs for a buck-boost topology, whereas for Cuk/SEPIC topologies there are only a handful of chips available (in the range of 15-20 chips).

Why is there such a preference for buck-boost topologies over Cuk and Sepic? Or is this just a shortcoming of Digikey's catalog?

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2 answers

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I think the point is simply that there are much more demand on the buck-boost topology than on the Cuk/Sepic ones. And there are GOOD reasons for that:

Several years ago, I asked a question on the SE forum: "where are Cuk converters". You can read the excellent answer there. The main point is that the Cuk/Sepic converters are of fourth order nature, which means that they regulate voltage much slower than 3d order regulators. What most of the people want is VOLTAGE REGULATION in addition to the buck/boost property. That's probably why there are fewer controllers of this type. I quote here the disadvantages of the SEPIC topology from Wikipedia:

  • Like the buck–boost converter, the SEPIC has a pulsating output current. The similar Ćuk converter does not have this disadvantage, but it can only have negative output polarity, unless the isolated Ćuk converter is used.
  • Since the SEPIC converter transfers all its energy via the series capacitor, a capacitor with high capacitance and current handling capability is required.
  • The fourth-order nature of the converter also makes the SEPIC converter difficult to control, making it only suitable for very slow varying applications.

My opinion is that the Cuk converter, with its remarkable property of "zero ripple" can still be useful if you only want to increase or decrease the voltage, and then REGULATE the voltage with a standard (non switching) voltage regulator. Then it should be possible to obtain a higher or lower regulated voltage, free of the usually important (and very difficult to filter) noise produced by switching converters.
This may be interesting for very low noise demanding applications.

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Thanks for the response. I read the SE question, it also is quite insightful. Mu3‭ 23 days ago

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I'm not sure there's a technical reason, except usually the offered parts are multi-topology and then they could be listed as buck-boost while they at the same time could as well be used as flyback, SEPIC etc.

This seems to be the case for TI and Maxim, which at a glance seem to call everything "buck/boost". Whereas AD, who's probably still the market leader, seems to list them separately and offer roughly as many buck/boost parts as flyback or SEPIC.

In the case of catalog companies specifically, they might have many reasons why they only list certain parts. Most notably the ever changing, ever tiresome "linecards" of silicon vendors that they are allowed to sell from.

For example LT went completely bonkers some 10 years ago and decided that they would only sell their parts through Arrow. Didn't work out too well for them... Digikey & others would probably have liked to sell their parts, but they weren't allowed to.

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Interesting, thanks for the answer. Yes, it seems that there are also multi-topology options, which does give me a wider range of chips to look at. I didn't know about LT not selling to Digikey - now I can find their chips there so I guess that debacle is over. Is there some sort of "chip database" where one could look up different ICs? Seems like it would be useful. Mu3‭ 27 days ago

@Mu3 I think the debacle ended when LT merged with AD. There's no such database of part markings AFAIK, though some manufacturers provide a cross ref service on their site. In practice this is about asking a veteran EE - "I need a buck/boost part, who should I check with?". Or you can ask this of an FAE from a component vendor, in which case you'll be restricted to whatever brands they have in their line card. Lundin‭ 26 days ago

Supply chain reason why I usually choose a four-switch buck-boost over a SEPIC. In my experience, inductors go out of stock not infrequently. A buck-boost uses a simple single winding inductor, while a SEPIC uses a coupled inductor. Off-the-shelf single winding inductors are more abundant than coupled inductors. Nick Alexeev‭ 15 days ago

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