Why Does An ATX Power Supply Have Two Main Smoothing Capacitors? How ATX PSU Works Tutorial
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- Опубліковано 6 жов 2024
- Have you ever wondered why an ATX Power Supply has two main smoothing capacitors after the bridge rectifier? Did you ever think they could just fit one larger capacitor instead? Well now you can find out how this circuit works, no maths, no waffle, no fluff just the facts you need to know to understand and fix this stuff!
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Thank you
Richard
I am always impressed by your ability to draft up a circuit, in realtime, with annotations, explanatory notes, colour keyed current flows, and graphs. All while running a narrative. Your brain works in ways mine doesn't, and we all benefit from your desire to teach. Thank you!
Thank You 😊
I mentioned your post to my wife and she want's to point out that in her opinion I'm (like all men apparently ) hopeless at multitasking 😂😂😂
@@LearnElectronicsRepair and thats why your wife loves you sir!
@@LearnElectronicsRepair Good explanation but would it be important to have two capacitors rather than one, and also is it really important to change the squar wave for a sinewave? Because the circuit get bigger and bigger and the purpose of it seems strange as the square wave can just as do the job???
@@democracyforallWell yes, it's important to have two capacitors. If you had only one capacitor, you would have no Junction between the two capacitors; and it's important to have the LLC circuit for more efficiency.
I have learnt more in 45mins with you than any other presenter
You have missed your calling, you should have been an instructor. You are to clever to be playing around with flea market old stuff. Your knowledge is immense and you put it over well.
In schools and universities, you have to follow study plans, deal with students who don't even want to be there. On UA-cam, it's different. You were probably looking for a video on the topic of power supplies and you have an interest, and that's why you learn. And don't get me wrong, I also believe that the video's author has an incredible teaching ability.
the effort put here is so impressive.
Universities must include this to better their lecture
Your explanations are gold. The fact that you explain what happens to the wave while it passes through the components and why the components are where they are, makes all the difference. Cheers.
I didn’t realize the video was 42 minutes long until it was over. Well done.
Fan fook in fantastic! That was brilliant - I was losing it a bit up until the bell analogy came in - the summary brought it all into perspective. Lots learned - thanks very much - 40yrs guessing made easier watching this 👍
This was an excellent presentation on how this type of PSU operates, the bell analogy for resonance is the best I've seen.
The LLC circuit explanation is great. I'd often wondered about that, as it seemed logical that sending an analogue device like a transformer a square wave would not be doing the system much good from a noise perspective. The bell analogy will stick in the head nicely, thanks!
the best explanation in my 20 year line, even if 1 capacitor is bad the smps won't work, have 3 cooler master smps 2 with active pfc, trying to repair since ages, no 5Vvsb present in all.
You are the best teacher I never had.
I learned more about circuits and electronics in 42min. than 4 years in the University.
I'm just 7 minutes into the video and this is the most I have learned about power supplies.
agree
The thoroughness and pace is very appreciated.
Thanks for another great vid. I'm an electrician with limited electronics knowledge (I dabble at fixing gear for myself, friends + family, mostly because I love fixing instead of replacing). I already knew a lot of what you explained but I still learned a lot. Every day's a school day. p.s. Parents: go easy on kids who take their toys apart - they may be studying electronics early but you don't realise it. :)
Loved the bell . Always thought about the resonance but genius to think of the pendulum as a square wave . No matter how quickly or slowly you hit the bell with the pendulum, the bell still rings at its resonant frequency . Genius .
@Dave T , Yeah exactly that! The pendulum/hammer/clanger injects🙂 energy into the system but the resonance does the work
Dear. Rich, bell analogy to explain LLC topology is simply...fantastic! Never thought that way...regards from Italy.
Saluti 🙂
Since discovering your videos I can't seem to watch any other electronics repair guys, they bore me. I'm still learning and you're making it enjoyable.
practically explained perfectly ... the bell was the obsolute example for a resonator 😊😊👍👍👌👌
Thank you for explaining this along with the LLC part of it. I never quite understood the way it worked, but the way you explained it nicely. It smooths the sharp edges of the square wave.
Of several youtube videos on this subject, your video was by far the best explanation. Thank you!
Very helpful to my understanding of PSU. The fact you add a sketch to explain things is great.
That makes sense to me now why the transistors went short on the other video when one of the filter caps was open circuit. Thank you for these videos it really does help me understand.
I really enjoyed watching this video. So much so I'll be watching it again and maybe a third time. Your analogy with the bell was a good one. Brilliantly done. 😁😁
Cheers Edwin, I was really chuffed with the bell when I thought of it
Thanks so much Richard. You pretty much answered any questions I had. I have a few on the bench currently, an ATX psu from a customer computer that came in. swapped for a good one now I'll look at this one. I've got a bazillion of them here to play with.
Really enjoyed this explanation mate.
Thanks for taking the time to put such effort into this video.
Addicted to your videos now, so glad I found this channel, I have learnt so much and find your description of circuits very clear and easy to understand. Thank you sir
Great video! I had some prior understanding how ATX power supplies work but this video explained some details I wasn't previously aware of.
This is the best power supply explanation on AT I have seen ! Thank you very much. Raj (India)
I learned a lot! I always thought transistors were low voltage devices. I can see now that I could fry my oscilloscope if I were to randomly probe transistors inside one of these supplies!
Be careful connecting your oscilloscope to the collector or drain of a FET switching transistor. I have done it but don't recommend it. Best to use a high voltage probe to prevent damage to your scope.
A video that could be helpful:
ua-cam.com/video/xaELqAo4kkQ/v-deo.html
I appreciate the clear explanations, as well as saving power supplies from the e-waste bin.
Very well explained, in fact, clear as a bell.
Yeah I never saw a quality bell that was glass.
Thank you. I thought the bell was the best part 🙂
Brilliant Explanation! I don't recall a great explanation in my electronic classed where theory meets reality. I really enjoy your videos. Keep em on.
Thank you for making the operation of that circuit easy for me to understand,. I really enjoyed yout explanation.
As always, a video full of well dispensed knowledge.
Side note : not all LLC circuits are resonant. They do transform square waves into sinusoidal waves but it is not what it is called resonance.
In a LLC circuit and basically all RLC circuits, resonance occurs at a certain frequency. The phenomenon is a noticeable voltage therefore current increase in the circuit due to combined characteristics of all components.
Often, accidental resonance means destruction of the circuit.
The same phenomenon applies to physical oscillators. That's how wind or troops walking at pace can destroy bridges.
Great explanation I was wondering why there's two caps instead of the usual 400V caps, now it makes sense. Thank you
A good simple explanation, of a complicated circuit. Thank you.
Excellent video ... worth watching several times to internalize. Thx!
I learned so much from this video, I feel like I owe you money now 😂. Thank you so much for this. You have a new subscriber.
Very well explained as always. Thanks Richard
Brilliant job , one of your best yet. Thanks.
So , if you did change the value of BOTH of the large caps to a lower capacitor value (not voltage rating) it would still work but at a lower max output .
Very informative, feel like i am back in school, new suber after watching this. Ty for your effort and time.
Much obliged for your amazing work in educating us!!!
Excellent explanation of LLC resonator circuit for higher efficiency. Thank you very much.
Nice job. Thanks for the description, very visual, loved the bell analogy, I will use that.
On older supplies they have a 110/240v selector, somehow the selector changes the capacitor config to a voltage doubler
Yeah, we can have a look at that. One of the PSU I rejected when making this video (as over complicating things) has the 240V/120V selector and it simply shorts two points on the PCB when set to 120V
For 120V, the center of the 2 caps is tied to the AC neutral. The diode bridge charges one cap on the positive half cycle, the other on the negative half cycle. It's basically two half-wave rectifiers, AKA a voltage doubler, to get 320V DC from 120V AC.
Great explanation Richard. Love the Bell idea, it makes sense. Many thanks
I am fixing an old Iwatsu scope's power supply and it has dual diodes for 12V, -12V and 5V rails and the anodes are actually connected together. And a single 4ns rectifier diode for the 54 volt rail. Not sure if it was done for added current handling, but the specs for rectifier diodes were hard to meet with newer components when I looked for replacements. The 54volt rail consumes the least current so they used an ultrafast 1 amp diode for it, but other rails are higher current.
I ended up replacing the 1 amp diode that had burned with a 4 amp rated diode that had a matching forward voltage/current curve all the way to 1 amps. And I only found one that had matching speed, capacitance and forward voltage when I looked up from Mouser :)
GOLD. The bell made it much easier to understand it.
this video tied up so much about what I learned from you in past videos - thx!
Thanks for the recap on Bridge rectifiers, hadn't seen that since 1st or 2nd year Electrical engineering degree.
a good video. Most of the power supplies I have worked on with 2 smoothing caps have either been as you describe here or they are 110 240 switchable or even autosensing using a thyristor between midpoint and bridge for 110 240 selection, of course, most modern PSU's have PFC so no need for the selection.
I was waiting for him explain the clever way of allowing a 120/240 voltage selector for free in the design as well. like this i.stack.imgur.com/1jiaD.png
Brilliant! Devastatingly useful! Thank you! Liked and Subscribed.
Thanks! Another brilliantly explained subject.
Fascinating - great design and explanation.
Thank you for commenting and attaching this video, it's a new to me, but I've watched it. I mean the video :D cause I didn't have LIKE on it till now. But, I will use it for my apprentices and technicians as it's a GREAT resource for them to learn. Like a all in one :).
The world's best teacher thanks sir
Brilliant! Many, many thanks! I have to say, all of your tutorials are simply amazing. I have learnt so much. Thank-you again!
gave you a thumbs up especially for the bell analogy
Thanks for the very educational video. LLC is a new thing (at least it is to my old brain when thinking about SMPS LOL).
You really do make a great teacher! Some of the others just run through everything g way to fast at least for me. Not enough time to think about what they are saying, but you are great!!! Thank you! I said this before but you should build a single to 3 phase converter. I think it would get a lot of views
Explained very well. Thank you.
Thank you, thank you, thank you! I learned a lot of very helpful information and now understand this stuff better!
Quite a good presentation. Liked the bell analogy.
I'm honing my understanding of the functions of and the relationships between these components and the resulting outcome. With your help. I really appreciate it.
Would it be I were just approaching career age level of life, but that is not the case. Strangely, I never feel like an 'old dog'. Never have felt fully 'grown up'. I suppose my inate sense of curiosity multiplied by a strong desire to understand things, (and then put that to use)divided by my age , in years√ has helped to learn new tricks, everyday, which we all know, an old dog cannot.
There may be an algebraic formula in the preceding lines, and it may need to be revised, and I would absolutely and happily accept any input from you all out there in comment land. 👍
Its true to say the transistors are driven in antiphase but you have to remember that there needs to dead time where they are both off. As any overlap In the switching pulses will short the power rail out.
You are totally correct but let's not over-complicate this when I'm trying to teach the basics of what is going on here yeah? 😉
True - but luckily the SMPS controller chip is working it out for us !
This deadtime ensures the one transistor is completely off before the other one is on.
The controller always put in some 4% dead time minimum, but it can be adjusted on a dedicated function pin.
This is by the way used as an extreme start up condition using a capacitor on this pin, which makes for a long dead time (pause) at initial startup.
Also called a softstart, as the transistors are only briefly turned on for the first few cycles.
@@LearnElectronicsRepair that actually keeps it simpler by keeping it in high detail.
That was great, thanks for not getting bogged down in the resonant imaginary components!
Maybe couple videos about intermittent failures? How to diagnose them, some tips'n tricks.
Very interesting watched pretty much the whole thing, wish there was more on what's driving the two power transistor bases though .
Brilliant work. Really clear. Thank you!
Thank you for this very detailed and easy to understand video. And yes, i did like the bell analogy! :)
Why rectify the AC, only to convert to +/- square wave to put through a transformer? Why not stick the AC straight into the transformer to derive the lower voltage rails, then rectify only on the secondary sides? Especially if sine waves are more efficient at driving transformers than square waves? Just discovered your channel -- you're the first person I've seen explain ATX power supply topologies in this much detail. Thank you!
Thanks for this explanation. You drew the transistors as BJTs was that intentional or a mistake?
@fixfaxerify Intentional. In this circuit the transistors are almost always BJT. In fact I've never seen ATX use mosfet here, though the 5V STB supply is usually mosfet driven
Its not the only reason...: Other reason is that if they are in parallel the internal resistance will be smaller and together they will be more effective than only one. Next reason is that if one fails then the other can keep it going...(if one of them has enough capacitance). Also there will be shared current so one of them dont get so hot (charge and discharge current will produce internal heat because of internal resistance)
ding dong.............. ive never heard it put like that before. it really makes the point. thank you richard
Great video, very well explained.
Nice explanation Richard but I am a bit confused about the negative voltage pulses you have drawn on the first full wave bridge rectifier.... I believe that line would be a steady 0V ....
Subbed and liked..👍
That's actually a good catch.
I suppose if you used the mains neutral as your reference you'd see negative pulses on the DC ground but I'd expect it to be a half wave instead of the rectified wave as drawn.
Great Stuff ! Thank you, Richard.
Great analogy! Love the video
Thank you again!
I kind of got it, well I thought I got it, until it hit me... Why take an AC signal and convert it into DC, just to turn it back into AC? Seems like a waste of energy and components. Silly question? I'm really trying to understand this, and your help is invaluable.
Not a silly question actually. So the mains is AC but a high voltage. We need a low voltage DC. Yeah you are correct in wondering about this, because in many cases the power supply has active PFC, which basically does the conversion 240C AC --> 380-400V DC --> low voltage AC --> low voltage DC. I'll think about how best to explain this but the answer is most probably it is the most efficient way to do it.
@@LearnElectronicsRepair I think the easiest way is to say that for higher power applications it's much more efficient, much smaller and much less expensive than a linear transformer PSU at 50/60Hz - DC is required so it can be 'chopped up' into a much higher frequency AC which can work with relatively tiny transformers, the main downside being noise. Probably not the best explanation but it's what's in my head :)
@@LearnElectronicsRepair But I'm referring to the pulsed DC on the high voltage side, at timestamp 28:00. So, with the answer from @budgiefish, it means that the sole purpose is to speed up the frequency?
Amazing ability to tech complex ATX PSU. Pls make some latest ATX PSU anatomy.
Go Ahead.
Love this video Richard! Thank you!
Good explanation, thank you.👌🍒
thanks for your teaching you do a great service for the people who like the electronics. long live with the god's blessings....
Brilliant, very well explained 👍
28:20 You forgot 1uF polypropylene capacitor in series with the winding. This is critical. Actually it holds the charge.
Great explanation, always enjoy your videos...
really enjoyed the video
and learnt something new
what an awesome explanation :)
When this PSUs are 120V/220V switchable they pull an extra trick, at 120 circuit is more like a capacitor doubler circuit, so V+ is still 300V or so. At 220V is just a full bridge with the two caps in series and V+ is 311.
Had a go at troubleshooting an LG 55" TV last night. Everyone kept saying the PS was defective, because it would not turn on, just had a pulsing red light. Nah. Checked all the voltages, checked the caps and no faults found. Seems it has a faulty motherboard, which is way beyond my capabilities. Yet. Incidentally, the PS was effectively the same as this one, but with slightly different voltages, 3.5V, 12V, 24V.
Normally capacitors are defective on the power supply
Disconnect the LED back lights (assuming it is an LED TV) and see if it stops pulsing. Often there are two or four strings of LEDs int he back lighting and if they don't all draw the same current (caused by one or more defective LEDs in one or more strings) then the TV will likely behave as you describe. Really you need a backlight tester to check each string out - if one has a different voltage to the other, using the tester, then that is your fault.
@@LearnElectronicsRepair Thanks for the tip! I disconnected the backlights and it made no difference, unfortunately. There's only one connector for the backlight so I assume there is a distribution board "deeper in the sandwich" somewhere. I'll just wait for the new parts now and test them out. Much appreciated, regardless.
@Mr Guru I actually thought along those lines, so I have plans to add a fan to the bga heatsink, provided there's room. The little bit of extra noise won't bother me. :)
@@trone32 They seemed to be fine for me, I removed the larger ones and tested them without finding any issues, and the PS gives all the correct voltages to the main board. It will be interesting to see: I've ordered a new main board for it now.
Great explanation ! 🥂
Great explanation I loved it '👍
Very good Richard. Many thanks.
the video was very informative , thanks
I'm the first time on this channel and the explanation was excellent. Was wondering for a while how these power supply work. Also what I would like to know is how the driving square wave selected? Why is it in MHz range? Does the frequency of the square wave changes based on load, if so, how is the load balanced on different voltage taps (3.3V, 5V, 12V) ?
Greetings:
The time it takes for the bell clanger to swing from one side to the other provides the "off" time between pulses to the output transformer allowing for the sinewaves to ring out.
Very nicely explained !!!
Thanks!!!
Yes i did learn, apreciate it.
The green wire you said recieves 0V (dunno how it can recieve nothing) actually gives out 5v. I know this as a fact, just measure it with a meter. It's a well known fact that connecting the green wire to black (ground) will allow you to switch on an aTX supply without a motherboard being connected.
Does it float high at 5v, and pulling it to ground (low) fire up the psu? I always knew about shorting green but didn't know exactly what all is going on behind the scenes.
You short the green wire to ground pulling the 5V down to 0V. The psu technically receives a 0V signal to start but it is probably better to say it is ground as it is a voltage in reference to another. Remember the unplugged state of the psu would also be 0V on the green however no other voltages are present so the psu cannot start.
"Receives 0v" translates to "is pulled low" or "is connected to ground"
Yes, he could have phrased it more accurately but your comment comes across as pedantic considering your familiarity with the function of the green wire.
@@speedycpu yes exactly that
@@speedycpu yes exactly how it works
Hi I have been watching you for awhile. I love your informative videos. Power Circuits are my favorites. You may have already answered the question. But what happens to the Drive Transistors if the neutral side is not connected? Does the High side MOSFET get the entire 320V load?
When the ups turns on, it needs 2 sec before the power reaches its maximum, the capacitors supply some power during this time gap. When motoric devices turn on, power reduces for some seconds.