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I can understand how an isolation transformer can establish isolation; but since SOME SMPS supplies dont have transformers and they have rectifiers instead; I wonder how the AC and DC ends are isolated.

edit: Here an SMPS with no galvanic isolation: http://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/7516739/ Here is the datasheet(says there is no galvanic isolation): http://docs-europe.electrocomponents.com/webdocs/1072/0900766b810728dc.pdf How is that possible?

I'm asking because people here all agreed SMPS has small transformers which isolates the input and output power.

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  • \$\begingroup\$ It's simple: Without a transformer there can be no isolation. but since SMPS supplies dont have transformers and they have rectifiers instead; It doesn't work like that. You're comparing a mains transformer to rectifiers. These are completely different things ! A mains transformer lower the AC voltage and provides isolation, a rectifier makes DC from AC. \$\endgroup\$ – Bimpelrekkie Apr 20 '16 at 7:46
  • \$\begingroup\$ how bout here bridgre rectififer convertes mains ac to dc isnt it?: thinkisemi.com/fckeditor/editor/filemanager/connectors/php/… \$\endgroup\$ – user16307 Apr 20 '16 at 7:50
  • \$\begingroup\$ Yes, the same happens in the picture in Andy's answer. The way these supplies work cannot be compared to a solution using a mains transformer. They provide similar functions (lower voltage, isolation) but achieve that in different ways and for a different price. \$\endgroup\$ – Bimpelrekkie Apr 20 '16 at 7:55
  • \$\begingroup\$ @FakeMoustache Btw, is the galvanic isolation's only purpose or advantage is safety? \$\endgroup\$ – user16307 Apr 20 '16 at 8:01
  • \$\begingroup\$ Isolation is not only for safety, it can also help to separate and isolate sensitive parts of measurement equipment. Isolation prevents unexpected currents to flow. It makes design easier and safer as isolates parts can interfere less with eachother. Take 2 lab supplies of 30 V, 1A. You can connect them in series and have 60 V, 1A. This is possible because they're isolated. \$\endgroup\$ – Bimpelrekkie Apr 20 '16 at 8:13
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Sure they have transformers: -

enter image description here

enter image description here

The transformer looks different to a regular AC type but it's still the largest single component on the PCB but a whole lot smaller than it would be for 50/60 Hz operation. The transformer is the big yellow taped thing in front of the heat sink at the back/left.

SMPSs also use opto-isolators for feeding back a measure of the output voltage so that it can be regulated. This is another significant isolation feature thus, output and input remain galvanically isolated to several kV and are therefore "safe".

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  • \$\begingroup\$ But SMPS are small. How can that transformer in your diagram be small? Rectified AC is PWM switched and then fed to the transformer in your diagram. Is it possible to provide waveforms corresponding the diagram above? \$\endgroup\$ – user16307 Apr 20 '16 at 7:55
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    \$\begingroup\$ Because the frequency of the switching converter is much higher, the transformer can be much smaller. \$\endgroup\$ – Daniel Apr 20 '16 at 7:57
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    \$\begingroup\$ The trick is that the "oldfashioned" mains transformer needs to work at 50 or 60 Hz (the mains AC frequency) , for that it needs to be bulky. In an SMPS we can operate it at 100 kHz or more (note the switching transistor), this allows the transformer to be much cheaper, smaller and more efficient. \$\endgroup\$ – Bimpelrekkie Apr 20 '16 at 7:58
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    \$\begingroup\$ A transformer's size (for a given application and power transfer) is roughly inversely proportional to operating frequency. As an SMPS might operate at 100 kHz it's operating at about two thousand times higher than a conventional AC transformer hence it's tiny in comparison. It's all about avoidance of saturation. \$\endgroup\$ – Andy aka Apr 20 '16 at 8:00
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    \$\begingroup\$ The transformer is a transformer and some types of SMPS (fly back notably) it's sometimes referred to as a coupled inductor. The AC is rectified and smoothed producing a reasonable DC. The switching is done at anything from 10 kHz to 1MHz - there is an oscillator that is capable of being pulse width modulated. The PWM driver in my top picture runs at this higher frequency (nominally 100 kHz). \$\endgroup\$ – Andy aka Apr 20 '16 at 9:04
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I suppose this is a common misconception, among people who have studied DC-DC converters only. The truth is that transformers are often used in AC-DC SMPS, specifically when isolation is required.

The difference is that the transformer takes the place of the inductor of a regular DC-DC converter, and energy is transferred magnetically to one or many secondary coils.

Feedback is often achieved with an opto-isolator, but there are other methods of feedback as well.

Typically a first stage will be a non-isolated power factor correction stage, which passes the 400V DC to the regulator with transformer isolation.

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SMPS do have transformers. They are not quite the same as the 50Hz transformers you find in linear power supplies, because they work at a much higher frequency, (like ~100kHz), but they have one. Look inside a PC power supply: supply

The transformer is the block with the yellow tape and the writing on it, between the heat sinks.

Any AC-input supply also have rectifiers (usually as a bridge). They are placed before the transformer for SMPS, or after the transformer for a linear supply, but they don't replace the transformer.

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  • \$\begingroup\$ @Andyaka Here an SMPS with no galvanic isolation: uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/… Here is the datasheet(says there is no galvanic isolation): docs-europe.electrocomponents.com/webdocs/1072/… How is that possible? \$\endgroup\$ – user16307 Apr 20 '16 at 11:58
  • \$\begingroup\$ I edited my question. Apperantly some SMPS do not have transformers. So how re they isolated? \$\endgroup\$ – user16307 Apr 20 '16 at 12:55
  • \$\begingroup\$ @user16307 I believe that is a DC-DC SMPS/power supply supervisor. It looks at the output from two different supplies, and if one fails the other takes over, via this module. They simply added more regulation to it. And, as others have pointed out, many DC-DC SMPS are not isolated. \$\endgroup\$ – Brendan Simpson Apr 20 '16 at 13:08
  • \$\begingroup\$ DC TO DC?? uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/… It says input is AC here \$\endgroup\$ – user16307 Apr 20 '16 at 13:16
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    \$\begingroup\$ Well, it's simple: the SMPS that do not have transformers are not galvanically isolated. You can have unisolated switching supplies (they are usually called DC-DC converters rather than SMPS): they have and inductor instead of the transformer. But in this case, it's simply not isolated. \$\endgroup\$ – dim lost faith in SE Apr 20 '16 at 13:21
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To answer your original question, yes, you can have a non-isolated AC-DC SMPS that uses an inductor instead of a transformer. Generally you want isolation for safety, but for some applications isolation isn't required.

Some of the confusion is because the product page linked to in the question is all messed up. Although the page says the product is is a SMPS, the title is wrong. The page shows an inrush current limiter that takes AC input and outputs AC, limited to 10A. It doesn't provide galvanic isolation since it's kind of a surge protector for a SMPS, not a complete SMPS. The page describes the product as a "redundancy module", but that is a totally different product that can be added to an SMPS.

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  • \$\begingroup\$ Please don't make product recommendations on this site \$\endgroup\$ – Voltage Spike Dec 16 '16 at 6:43
  • \$\begingroup\$ I'm not recommending any products. My intent is to explain that the original poster's link doesn't describe an SMPS. But I'll edit my answer to take out the links if they are objectionable. \$\endgroup\$ – Ken Shirriff Dec 16 '16 at 6:54
  • \$\begingroup\$ Its not the worst thing to do, its just not within the guidelines. Better to provide links to a wiki or try and put the information in the post. \$\endgroup\$ – Voltage Spike Dec 16 '16 at 17:42
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I can understand how an isolation transformer can establish isolation; but since SOME SMPS supplies dont have transformers and they have rectifiers instead; I wonder how the AC and DC ends are isolated.

Simple: they aren't.

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