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Switching power supplies may be more complex than linear power supplies, yet they are much more common nowadays and irreplaceable when it comes to high power equipment.
Even though their design is much more complex than that of linear power supplies, their fundamental difference is that there are two rectification stages instead of one.
The operation principle of a switching power supply is not very hard to understand but it is a more complex design than linear power supplies nonetheless. Switching power supplies will not step down the AC voltage of the utility power grid; they will rectify it to DC voltage directly and then a switching transistor will invert it to AC voltage again but at a frequency thousands of times higher than that of the AC power grid.
The frequency of the power grid is 50Hz to 60Hz, depending on where on the planet you live in, and that is what linear power supplies use; typical switching power supplies change the transformer input frequency to anything between 100.000Hz and 2.000.000Hz. Then they will again rectify that high frequency AC voltage (or portion of it) to DC voltage of a desired value.
The higher the input frequency is, the smaller the electrolytic capacitor and the main transformer can be; a switching power supply is anywhere between 4 and 25 times smaller than a linear power supply of similar output specifications. The controllable high switching frequency of the transistor can also increase the electrical efficiency of the power supply dramatically by reducing the duty cycle to match the load's power demands, meaning that it can cut off portion of the input power and supply only the amount of power the load requires.
This approach is called pulse width modulation (PWM) and can double the power supply's efficiency; typical switching power supplies have an electrical efficiency of 75% or better.
Switching power supplies are not without disadvantages, which are many and critical. Remedying these disadvantages requires additional circuits (such as PFC controls, voltage regulation, etc) which further increases their cost and complexity. The high switching frequency of the main transistor is a serious source of electromagnetic interference (EMI) which needs to be supressed by using filters and shielding.
It also causes a lot of noise at the output, therefore using line filters is necessary to ensure the smooth operation of digital circuits and the noiseless operation of audio systems. Low quality products can cause similar noise at the input, interfering with other equipment connected on the same power line. Products without power factor correction will draw a lot of reactive power which is not used by the power supply but causes strain on the power lines and other electrical equipment, as well as making the power supply the source of strong harmonic distortions; this is why non-PFC corrected power supplies are actually prohibited by law in many countries.
The output voltage quality is actually worse than that of the linear power supplies, usually requiring the presence of voltage regulation circuits in order to reduce the voltage ripple enough to meet certain specifications. Switching power supplies also require a great in-rush current when they are turned on, meaning that momentarily they will draw an immense amount of power which can be disastrous to the utility power grid and electrical equipment, therefore yet another circuit is needed to limit that effect.
Finally, switching power supplies are less reliable by design; for example, a failure could cause a dramatic increase of the output voltage, damaging any connected equipment, or stress on the capacitors could cause them to explode, requiring careful designing and testing.
To combat these problems high quality power supplies integrate even more circuits, such as OCP (over current protection) and OVP (over voltage protection), making them even more complex and expensive.
Switching power supplies have a great deal of problems and their complexity makes designing and troubleshooting extremely difficult. Nevertheless, their much smaller size, much less weight and their high electrical efficiency makes the use of them into virtually all equipment a necessity. All computer power supplies of the past several decades are switching power supply designs, much like the power supplies of almost all consumer electronic equipment ever made.
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 !
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.
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.
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
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.
A hell of a lot of work in this Ironlaw. Thank you very much for making all this effort to educate us peasant mortals π
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
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).
With standard of content like this on a regular basis, you will go far ! top marks KG
Christ on a stick. lost for words. This is more for apprentice engineers !
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.
**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 ?!
Well that was surprising. very very good reading. bit complex for me in parts, but I picked up bits and pieces.
Ironlaw, thank you. very informative reading for a friday morning π now off to work to type in numbers.
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.
I feel educated, thank you π
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
I shall email Zardon for permission to use parts of this for an edu guide I am writing for college kids.
Brilliant piece of work. 10/10
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 !
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.
read it in detail – very good work indeed Ironlaw, you know your work π
My brain hurts.
Where do i find the for dummies version?
Nice work mate π
Cool and easy to understand