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How computer power supplies work – KitGuru Guide

All switching mode power supplies, including of course computer PSUs, use one or more high frequency transformers to transform the high frequency AC input to DC. As we mentioned before the high frequency allows the use of a much smaller transformer which also is considerably cheaper.

Most power supplies have a single large transformer feeding power to each and every output line, except from the +5VSB. However there have been instances of special high output models using more than one primary transformer, such as the Corsair HX1000 which had two. A small transformer, usually found right next to the primary one, is responsible for the +5VSB line. Older units also had a second very small transformer, being used as an isolator for the PWM circuit.

Main transformer
PWM isolator and +5VSB transformers

The primary transformer separates the primary from the secondary side of the unit. The primary side is the one under AC voltage (the side where the rectifier bridge and main capacitor are) and the secondary side is the one under DC voltage (the side where the output cables are at).

The primary heatsink holds the number of diodes and transistors which are necessary to convert the incoming high voltage DC to high frequency AC. Other components may be found attached to it, such as several PFC components or even the rectifier bridge.

Primary heatsink

The secondary heatsink holds the Schottky diodes responsible for converting the DC output of the main transformer(s) to voltages usable by your computer parts. Depending on the power supply configuration Schottky diodes are being used to provide either a) only the +12V output and the lower voltages are being generated by DC to DC circuits, b) the +5V and the 12V output and the +3.3V voltage is generated by adding a voltage regulator to the +5V line, or c) even all three positive voltage outputs found in an ATX power supply and the +3.3V voltage line can be entirely independent or sharing its output with the +5V line. The negative voltages (-5V and -12V) are generated by simple diodes and are slowly being removed from ATX power supplies.

Secondary heatsink

The number and configuration of those components indicate the class (or topology or circuit methods) of the power supply. There are several different topologies currently in use, all with their advantages, disadvantages and operating limits. Manufacturers and engineers choose their product topology by taking into account the manufacturing cost and performance requirements of the product. End users should not be concerned about the topology of their unit because each and every design has its own operating range and its performance depends on the configuration and components used in the entire product. The actual performance and efficiency of a switching power supply depends on the design and quality of the entire product, not on the topology design alone.

On the secondary side of any switching power supply you will also notice induction coils and a large number of smaller capacitors. These are being used for filtering the DC output lines. There are other circuits too, depending on the class and design of the power supply. Some of them are the protection circuits (OVP, OCP, UVP etc), the PWM control, DC to DC conversion circuits and more.

Electrolytic capacitors

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25 comments

  1. Bloody Nora IronLaw, that certainly is detailed. I made it to page 2, now I need a glass of coke (cola not powder). I shall return later once I absorb the basics !

  2. I am one of the most demanding readers on the face of the planet. The standard of editorial content on this site is without a doubt leading the way on the net right now. Some of the news stories are very ermm interesting, but the reviews are fantastic.

  3. I have bookmarked this, I shall spend a few hours Sunday afternoon chilling and trying to work this out. I need to read this at a snails pace to get to grips with it all.

  4. Well this is taking power supplies to the next level. I got lost in parts, and im quite technical, but its well written and rather complex. Nice idea these articles, at least you guys expect the readers to know more than the usual tech site reader

  5. Thank you, thank you , thank you.; I always wanted something like this, with images and easy to understand text. I have struggled on page 4 and 5, but I have picked up more from this than any other PSU related article on the net.

  6. A hell of a lot of work in this Ironlaw. Thank you very much for making all this effort to educate us peasant mortals πŸ™‚

  7. brain overload………… ekkk.

    I am just too stupid to understand half of this, but thanks anyway ! the last page was as far as I made in understanding it all :p

  8. Stunning work IronLaw, technically way above my level, but I feel a little better for having read it. I love the PSU reviews on KitGuru too. at the same level as Hardocps (which are also excellent).

  9. With standard of content like this on a regular basis, you will go far ! top marks KG

  10. Christ on a stick. lost for words. This is more for apprentice engineers !

  11. Normally I see these guides and think, oh here we go again, some idiot reviewer ranting on about something stupid and trying to understand it himself.

    Not in this case. Ironlaw is a cut above! really really enjoyed this one.

  12. **claps** – I understood most of it, which is surprising as I only know the basics about power supplies. I hope to see more of these. I would like one explaining what exactly we need to buy when looking for a PSU, single rail 12v? multiple rails. and making sense of actual ratings. who needs 1200W psus for instance? do we ?!

  13. Well that was surprising. very very good reading. bit complex for me in parts, but I picked up bits and pieces.

  14. Ironlaw, thank you. very informative reading for a friday morning πŸ™‚ now off to work to type in numbers.

  15. lol, I just read something so badly written on bittech, then I come here to read this. its like comparing nickelodean to the discovery channel.

  16. I feel educated, thank you πŸ™‚

  17. For such a complex article it is very informative, a little complex for this geek, but im sure many people love all this about PSU’s. The last page was helpful :p

  18. I shall email Zardon for permission to use parts of this for an edu guide I am writing for college kids.

  19. Brilliant piece of work. 10/10

  20. I find power supplies totally confusing. I just buy one and get a good one after reading several reviews online. I actually ordered the thermaltake one after reading the review on kitguru. This was educational but honestly a little over my head for 70 percent of it. I learned a little, which is more than I normally do every day !

  21. Fantastic writing and very educational as other people have said. I read the power supplies reviews here in the last month and I prefer them to johnny guru. Johnny is great with PSUs but his writing style annoys me, jokes are fine, but half the review is him trying to be funny. The reviews on this site are just as detailed but more focused. I hope you get the corsair 1200w PSU to review, I want to order it, but its 260 here and im not sure if its worth it.

  22. read it in detail – very good work indeed Ironlaw, you know your work πŸ˜‰

  23. My brain hurts.

    Where do i find the for dummies version?
    Nice work mate πŸ˜‰

  24. Cool and easy to understand