While searching for a good inverting SMPS topology, I stumbled across the Cuk topology which suits me perfectly by all means. The only thing that worries me is that unlike more common SMPS topologies, like inverting buck-boost or flyback, Cuk topology seems to patented according to wikipedia and many other sources.

I am going to use the Cuk topology in a PSU for a eurorack synth I am working on and I was thinking about selling some PSUs made from leftover PCBs and parts.

So, my question is: realistically speaking, how safe is it to use the Cuk topology in commercial products today? What could be the effects of doing it? Both in case of niche, yet still commercial products and in case of more spreaded ones.


The wikipedia gives links to expired patents.

I am, however, pretty much sure that a number of other patents with slight changes could also be filed in more recent years (especially from IC manufacturers), but wasn't able to find any.

I am planning to sell the product mostly in Russian Federation, the patents are from the US. But still, theoretically speaking, what could happen in case if the product well be sold with worldwide shipping?

I do understand that this is more of a "lawyer" question than "electrical engineering" one, so the "electrical engineering" part of the question is "how do engineers deal with it".

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    \$\begingroup\$ Has the patent expired? \$\endgroup\$
    – Andy aka
    Oct 9, 2018 at 12:08
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    \$\begingroup\$ Check with a patent lawyer, chances are that such a patent won't hold since its an obvious extension of already existing technology. \$\endgroup\$
    – PlasmaHH
    Oct 9, 2018 at 12:10
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    \$\begingroup\$ In what countries do you want to make and sell the products? In which countries has the patent been filed? \$\endgroup\$
    – Jack B
    Oct 9, 2018 at 12:10
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    \$\begingroup\$ Given that a number of the major players in SMPS devices have application notes and reference designs in their datasheets implementing this topology without reference to any patents (which they normally do if such exists) I would be surprised if this is still covered (but IANAL). \$\endgroup\$ Oct 9, 2018 at 12:21
  • \$\begingroup\$ This is a legal question not an EE question. Voting to close. \$\endgroup\$
    – RoyC
    Oct 10, 2018 at 9:22

2 Answers 2


Disclaimer: I am not a lawyer and patent law is complicated, especially because it differs greatly between jurisdictions.

The patents you linked to have long expired. This means you can duplicate the converters covered in the patent. After all, that’s the point of patents. The inventor discloses the invention in return for a 20 year monopoly right. After the end of the 20 years everyone can use the invention.

If you find any new patents, they will cover only improvements that have not been disclosed in the prior art (old patents). Read the claims of the patent. They limit what is protected. For example, suppose there is some new capacitor X that improves the performance of buck converters. A claim will look like this:

  1. A buck converter comprising at least one capacitor X.

According to the above claim you can still use buck converters, if you do not include capacitor X.

What can happen if you infringe on a patent is that the assignee (owner of the property rights) can ask you to stop selling, using, manufacturing, or importing the invention. If you refuse, they can get a court order for the same.

Keep in mind that patents are regional rights. The only patents valid in your jurisdiction are the patents issued by your local patent office.

  • \$\begingroup\$ Can you address the "maintenance" issue discussed by @JonRB in their answer? Are there ever situations where a patent issued in one jurisdiction is enforceable in another? What if the product is sold or exported to another jurisdiction? \$\endgroup\$ Oct 9, 2018 at 13:49
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    \$\begingroup\$ @ElliotAlderson In order to ensure that patents without commercial application are not kept going for the full 20 years the patent office requires escalating maintenance fees be paid. I believe the first fee is due at the 3.5 year mark and then every 4 years for a total of three times in the U.S. A patent expires after 20 years or when a maintenance fee is not paid. \$\endgroup\$
    – user110971
    Oct 9, 2018 at 14:04
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    \$\begingroup\$ @ElliotAlderson A patent is always a regional right. You can obtain patent protection in more than one jurisdiction by applying at multiple patent offices either within one year or through the PCT (patent cooperation treaty). The local patent office needs to approve patentability, e.g. software patents are not allowed in Europe. If you patent an invention in a list of jurisdictions, everyone not resident at one of the jurisdictions can take your disclosure in the patent and exploit the invention. A patent prevents sale, use, manufacture, and import. You cannot avoid it by importing. \$\endgroup\$
    – user110971
    Oct 9, 2018 at 14:10
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    \$\begingroup\$ Another requirement for a patent is that the invention must be "novel", and must not be "obvious" to a reasonable practitioner of the field in question. For example, if it's well established that use of caps with low series resistance improves SMPS performance, and someone comes up with a cap whose ESR is 1/10 that of previous designs, using that cap in an SMPS in the same way as any other would likely not be "novel" even though nobody would had used used such caps before they were invented. Unfortunately, challenging patents because of lack of novelty is often difficult and expensive. \$\endgroup\$
    – supercat
    Oct 9, 2018 at 18:57

The patents associated with the Cuk topology

US Patent 4257087 filed 1979 https://patents.google.com/patent/US4257087 expired but maintenance fee appears to have been made

US Patent 4274133 filed 1979 https://patents.google.com/patent/US4274133 expired but maintenance fee appears to have been made

US Patent 4184197 filed 1977 https://patents.google.com/patent/US4184197 expired but maintenance fee appears to have been made

So essentially the way to determine if a patent has expired is


1) Determine if the patent was filed, or claims priority to a patent filed prior to 20 years before the date.

2) Determine if the maintenance fees have been paid.

20years have passed but checking the transaction history: http://portal.uspto.gov/external/portal/pair/ none have "Expired Patent"

What you will find is LT and other makers of Cuk based converter chips pay a licensing fee.

  • \$\begingroup\$ The patents expired in 1997 and 1999. The patents aren't extended renewed or restored by continuing to pay the maintenance fee. The fact that some continue to pay a licensing fee probably means that while the patent was still active, they agreed to pay the licensing fee, for a specific time (perhaps in perpetuity), or under certain conditions. I don't believe such a license is enforceable any longer (after the patent expires). More likely, continuing to pay the licensing fee entitles them to other benefits such as free or reduced fee support or consultation. \$\endgroup\$ Oct 10, 2018 at 9:38

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