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Can I feed DC into an SMPS to get a regulated output? If yes, why? and What are the advantages and disadvantages?

For example, I would like to input 200VDC to an SMPS with an input range of 100-230VAC. The SMPS output rating may 48VDC.

A basic topology I consider for SMPS:

Image courtesy: Electronics Coach - Switch Mode Power Supply

enter image description here

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    \$\begingroup\$ Depends on the SMPS. What does the manual say? \$\endgroup\$
    – Justme
    Apr 6, 2023 at 14:14
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    \$\begingroup\$ It highly depends on the topology of the SMPS. There are many DC - DC types, as well as AC - DC types. SOME of the AC - DC types could potentially take DC as an input. \$\endgroup\$ Apr 6, 2023 at 14:24
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    \$\begingroup\$ Most of them should be fine. 50 Hz is quasi-DC to them anyways, so they already run on "DC". But power handling might have to be derated. Ones with more control logic might enter some fault mode and not work though \$\endgroup\$
    – tobalt
    Apr 6, 2023 at 14:27
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    \$\begingroup\$ All is usually well until you have a short on the primary side and the AC rated fuse have no possibility to extinguish the arc inside it and you have a fire. Here are some examples of what it would look like: youtube.com/watch?v=Zez2r1RPpWY \$\endgroup\$
    – winny
    Apr 6, 2023 at 14:28
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    \$\begingroup\$ @winny: nice demonstration. I'm mostly a designer of itty bitty clean things, that's a nice demonstration of what happens when you don't treat the great big dirty things with proper respect. \$\endgroup\$
    – TimWescott
    Apr 6, 2023 at 14:42

2 Answers 2

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All SMPS that work from a connection to an AC mains supply have a fuse. The fuse prevents the SMPS from catching fire should it fail. The fuse is an important safety device. Using the image provided by the OP, I've added the fuse: -

enter image description here

I would like to input 200VDC to an SMPS with an input range of 100-230VAC

Generally, fuses that are rated at 250 volts AC will have DC voltage rating that is about 50% to 75% of the AC voltage rating. 200 volts is at the 80% level and it's unlikely that an installation like this would be regarded as safe.

Quote from How To Avoid Incorrect Use Of AC Fuse In DC Circuit: -

A fuse rated at 1000V AC may be rated at 500V DC or 750V DC, dependant on its construction. As a general rule of thumb, a standard AC fuse will need to be derated by 50 per cent, that is, 1000V AC would be rated at 500V DC to be safe.

An incorrectly rated fuse is a showstopper.

What are the advantages and disadvantages?

"Showstoppers" trump examining other pros and cons.

So, why does a fuse have a voltage rating and, why is the AC voltage rating higher than its DC voltage rating?

  • Fuses have voltage ratings so that if an electrical arc begins when the fuse "blows", the fuse has sufficient dimensions to extinguish this arc
  • On AC, the waveform naturally passes through 0 volts and this helps extinguish an arc that may have formed.
  • DC does not have the arc-extinguishing features of AC because it's always present.

If you added an external fuse that was correctly rated you can turn the showstopper into a viable solution. I expect this will work for many SMPS brands but, some won't work because they rely on AC to power low voltage drive circuits using capacitive droppers (for example).

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    \$\begingroup\$ I would add my own personal rule of thumb: if I have a choice between something (like a fuse) that is rated for a specific task (like DC at a certain voltage), and applying a rule of thumb (like derating an AC fuse for the task) then I'll go with the part that's specified for the task. How much extra I'll pay for this depends on a lot of things, but for a part that costs a small fraction of the total system cost it could be two or three times the price of the unspecified alternative, and I'd still be happy. \$\endgroup\$
    – TimWescott
    Apr 6, 2023 at 14:46
  • \$\begingroup\$ @Andy aka . Okay. So shall we conclude that the SMPS may fail at the component level if we supply DC? \$\endgroup\$
    – Nithin
    Apr 6, 2023 at 14:54
  • \$\begingroup\$ @Nithin for those devices that operate correctly, I don't think there's any more chance of one failing on DC than AC. \$\endgroup\$
    – Andy aka
    Apr 6, 2023 at 15:04
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    \$\begingroup\$ @Nithin It's not that it will fail, it's just a big difference in consequences if it fails. I agree with Andy, the SMPS will be as happy with DC as AC otherwise. \$\endgroup\$
    – winny
    Apr 6, 2023 at 15:11
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    \$\begingroup\$ @Nithin: You may be missing an important point: there is no fundamental reason why it can't work. So there's a good chance that you can find one as a catalog item, that's rated for your input. It'll be serving a smaller market so it'll cost more, but it may cost less than either starting a fire when it fails, or having to roll your own. \$\endgroup\$
    – TimWescott
    Apr 6, 2023 at 19:51
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Many SMPS will have no trouble operating with DC input. In many cases, the first part of a line powered SMPS is a rectifier - they operate on DC.

As an example, have a look at these circuits using the Viper series from ST:

enter image description here

All of them convert the AC to DC before going into the switcher part.

You'll want to use a DC voltage that approximates the peak AC voltage that the circuit is expecting. For a 230VAC input, you'd want to use around 300VDC.

A lot of switching power supplies will operate far below their rated input voltage. The German computer magazine C'T ran some tests years ago, and had no trouble running 230VAC rated devices using DC voltage as far down as 50VDC.

At lower voltages, the power supply will have to draw more input current to meet the output power demands. Using one far below its rated input voltage may cause overheating leading to premature failure.

For a personal project, I'd be willing to use a switching power supply on DC below the rated AC voltage. If you need it for a professional product then you really ought to check with the manufacturer. They can tell you if it is OK, or maybe recommend a better power supply.

There are other AC input switching power supplies that can't work with a DC input. They may have an AC coupled input, or they may use a capacitive dropper to power the high voltage side. Those won't work at all on DC.

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    \$\begingroup\$ Previous Q&As about feeding DC into a device that normally rectifies AC have pointed out heating in 2 of the 4 power diodes in a bridge being a possible problem: all the current goes through one pair instead of dividing equally. If they were over-specced, they might get close to overheating if you run the power-supply close to its full load. \$\endgroup\$ Apr 7, 2023 at 8:26

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