PSU Repair: A Case Study

Capacitor Postmortem

What sort of state were those capacitors in? Let's revisit them using a simple pulse generator and a voltage divider made of two 2kΩ resistors in series. I would expect a good capacitor to reach 63% of the applied voltage after one time constant where τ = RC in seconds. Another area of interest that can be observed with this simple test setup is the first microsecond of the charge curve. Any abrupt voltage change there would be indicative of significant series resistance. Since I am using a voltage divider, keep in mind that the Thevenin equivalent would be a source of half the voltage with 1kΩ series resistance.

First up is the dead 680µF cap from the 5VSB output. In principle, its time constant should be 680ms. But here, it reaches 63% of its final value near the 120ms mark, pegging its capacitance to somewhere in the neighborhood of 120µF. The capacitor reaches a final voltage of 5.3V, which is 300mV short from the applied voltage, implying a 300µA leakage current. Based on the typical 0.01CV leakage current formula, this is over three times worse than the 68µA expected.

Zooming in to see the first microsecond of the charge curve reveals a 60mV step, a telltale sign of significant series resistance: at 1µs, an ideal 100µF should be showing less than 56µV here, so the early voltage rise shown in the first 400ns or so is clearly dominated by ESR. Working through the voltage divider equation yields an equivalent series resistor value of about 11Ω. Based on Teapo's spec sheet, which appears to have an error on it since my 680µF cap here is 8x15 yet the sheet says 8x11, the ESR should be 0.11Ω. In other words, this old cap is two orders of magnitude out of spec on ESR.

I went back in the power supply to pull out the other 680µF cap I had to reuse and did quick checks for thoroughness' sake. The 63% time was spot-on for 680µF, leakage at 5V was 5µA and the initial step was 20mV, indicating an ESR of about 4Ω. While this is better than the first one, it is nowhere near good enough to call a definitive fix. If I put this supply into a PC and started pulling some current off 5VSB, the weak cap's ESR would make it overheat and fail in short order. The extra ripple this may cause on the auxiliary cap would also contribute to its premature failure.

Based on Teapo's specifications, these capacitors are only good for up to 640mA ripple current, which might be on the low side for a 2A flyback. The RMS ripple current on a discontinuous mode flyback converter can reach 80-90% of the DC output current. Had the 5VSB section been designed for long life at full load, the capacitor accepting energy from the transformer should have had at least a 1.6A RMS ripple current rating. Many people would be quick to blame “bad caps”, but even the best caps on Earth would still fail when operated at double or triple their rated ripple current. In this case, the capacitor used is almost certainly under-rated.

Having one dead and one dying cap on the 5VSB output certainly explains why the new cap was so effective at reducing 5VSB ripples, even while connected at the end of the ATX power cable.

Now, let's have a second look at my other two replaced caps.

Next is the auxiliary supply's 47µF cap. It reaches 63% in 140ns, putting its capacitance in the ballpark of 140pF, effectively the same as my multimeter once you subtract the 20pF or so worth of test leads and scope probe capacitances. Where leakage is concerned, I got less than 1µA. No surprise there from a capacitor that failed open.

Looking up Fuhjyyu's TN-series specifications turns up the following information about this 5x11mm capacitor: only 500 hours endurance rating and 82mA RMS current, 164mA if you keep it below 70 ºC, about 200mA for frequencies above 1kHz. These may seem like absurdly low specifications, but for a capacitor that does little more than feed 100mA or so to the main PWM chip for its operation and the MOSFET gate drive, it likely is adequate under normal circumstances. My FC replacement is rated for 615mA at 105 ºC, or about 520mA if derated to 1kHz, vastly exceeding the Fuhjyyu's specifications.

Also worth noting are the 30-40V peaks across the auxiliary cap in my first set of auxiliary capacitor voltage measurements with the old part in. This capacitor is only rated for 25V, which is already a tight margin for a 20V nominal output not counting switching transients. The tight voltage margin means its protection against flyback peaks depends entirely on the 5VSB capacitors.

Last is the PWM's 22µF bypass capacitor. Going by the 63% point, its capacitance falls in the neighborhood of 16µF, which is in line with the multimeter result. However, the charge curve has a 300mV step immediately after test voltage application, translating to a series resistance in the neighborhood of 57Ω. As far as leakage goes, this one is also under 1µA at 12V.

Going through the TN-series specs once again turns up that this capacitor has a nominal ripple rating of 52mA, or about 150mA after applying multipliers, which is likely also fine since this capacitor only needs to help deliver gate current drive pulses to turn the main MOSFET on. Here, my FC replacement is rated for 220mA after applying 1kHz derating, again more than a match.

When all of the capacitors on a flyback power supply's outputs have failed, there is little left standing in the way of flyback operation generating high voltage spikes and destroying components. From the state these caps were in, with peaks up to 40V showing up across the 25V auxiliary capacitor that were likely worse before I freshened the 5VSB caps, I am really surprised the magic smoke had not come out already. Also, depending on the one capacitor that failed open and another with sky-high ESR for gate drive current, they could have easily caused sluggish MOSFET turn-on and destroyed it. Few things ruin MOSFETs faster than spending microseconds in a half-on state with a few amperes flowing through them and 250V across their drain-source channel.

I happened to still have the original main output caps (they are the Fuhjyyus shown on the intro page), so I decided to give them a quick check, seeing if they fared any better on the main flyback converter's outputs. I had not done this before simply because my only decent meter until a few months ago only went up to 20µF. All but one were at least an order of magnitude off on capacitance. That “good” one came out dead on ESR at 90Ω. No survivors. I am really surprised this supply managed to power my Core 2 and Radeon HD 5770 through countless hours of WotLK raiding without tripping anti-surge until its main output caps were that far gone.

What Should A Good Cap Look Like?

A picture is worth a thousand words, so here is one for comparison's sake:


This is what 0.05O or so on a low-ESR cap should look like. There is some oscillation when the step is applied due to series inductance in the wiring and cap, then the capacitor voltage step settles at about 0.3mV above its initial voltage within nanoseconds. The step is so small, it is almost lost to the scope's noise floor.

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62 comments
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  • Nuckles_56
    An interesting read, it was interesting following the process you used to troubleshoot the problem
  • Crashman
    I used to do this for a living :)

    Don't tell my boss, I've managed to convince him that I'm only an expert at running benchmarks and writing about the results :)
  • epsiloneri
    Disclaimers won't help. The people who will likely hurt themselves trying this are the same who lack the reading comprehension and self awareness to understand those disclaimers are directed at them. I admire you courage in publishing this.
  • beetlejuicegr
    the truth is the paper clip and multimeter is all i can go in to psus. after all i haven't studied electricity or circuits or whatever.
    However i do hate to throw stuff earlier than it should, like you.
  • C12Friedman
    I like this article and I fully agree with the conclusion. I've repaired a few PSU's but, for the most part I scavenge them anymore since I can't put them in a new system (nor would I want to) IMO they aren't really worth anything other than for on a test bench.
  • Mr A
    Daniel, I know next to nothing about electronics, and yet I could not stop reading this article. Fascinating! Thanks very much!
  • Urzu1000
    This was a great article! It was informative, as well as interesting. Personally, I've only had one PSU fail on me so far. My brother-in-law's self-built computer had a really low-end Thermaltake PSU. 800W Bronze, and oh man, did that thing go out in a blaze of glory. Very loud popping, and smoke, and funny smells. When I ripped it out of the computer, there were burns inside the case. Miraculously, the other components remained unharmed, so I slapped in a new PSU (750W Gold Seasonic) and fired it up.

    Still working good, but I get black soot on my hands every time I open up that case. It's a black case, so it's hard to clean it off properly.
  • nukemaster
    Good read.

    Thanks for taking the time to document this repair.
  • Interesting article. I would have simply replaced the entire unit. You saw how to fix the failure, but how many units were damaged that you didn't see? A ticking time bomb that will eventually send some spike to your much more valuable hardware than a 10 year old PSU. Wasteful, yes. I get it. I don't like to waste either. And if it's on marginal hardware, fine. But on primary systems I'm not willing to take the risk. I'd rather throw away a 200$ part that has a 0.05$ repair solution, than risk frying 800$+ hardware.
  • kalmquist
    "Antec's manufacturer (Channelwell in this case) got the live and neutral wires backwards, which means that in the “off” position, the neutral line gets opened and everything on the primary side becomes live instead of neutral."

    That's really bad--I doubt it is even legal to sell a power supply wired like that. I've never bought a CWT (Channelwell) power supply, and based on this I wouldn't buy one, except perhaps for a high end model where you might gamble that the company would exercise a bit more care.
  • Daniel Sauvageau
    211508 said:
    Disclaimers won't help.

    They may not help the people who choose to ignore them but they help reduce the likelihood of legal issues if something blows up in their face or worse after they do so.

    2008047 said:
    Daniel, I know next to nothing about electronics, and yet I could not stop reading this article. Fascinating! Thanks very much!

    You're welcome. I try to put enough technical details in there to keep technically minded people interested but not so much as to make it inaccessible to more casual readers.

    723938 said:
    Interesting article. I would have simply replaced the entire unit. But on primary systems I'm not willing to take the risk. I'd rather throw away a 200$ part that has a 0.05$ repair solution, than risk frying 800$+ hardware.

    And a new PSU was the fix I initially implemented in my PCs too, though mainly because I could not be bothered to investigate at the time since I lacked the tools necessary to do so properly and with reasonable confidence.

    262841 said:
    That's really bad--I doubt it is even legal to sell a power supply wired like that. I've never bought a CWT (Channelwell) power supply, and based on this I wouldn't buy one, except perhaps for a high end model where you might gamble that the company would exercise a bit more care.

    The wires on the IEC plug filter board are likely inserted by hand. Without a few more units to compare it against, it is entirely possible that my inverted wiring was a one-off or otherwise uncommon manual assembly mistake. It likely happens to all other manufacturers too, the question being how often it gets through QC unchecked.
  • Daniel Sauvageau
    1526053 said:
    This was a great article! It was informative, as well as interesting. Personally, I've only had one PSU fail on me so far. My brother-in-law's self-built computer had a really low-end Thermaltake PSU. 800W Bronze, and oh man, did that thing go out in a blaze of glory. Very loud popping, and smoke, and funny smells. When I ripped it out of the computer, there were burns inside the case. Miraculously, the other components remained unharmed, so I slapped in a new PSU (750W Gold Seasonic) and fired it up.

    No need for miracles here: most spectacular catastrophic PSU failures occur on the primary side which is isolated from the outputs by the transformer. If the BJT/FET switching becomes too slowly due to weak gate drive, the junction or channel may exceed its SOA and blow up - I was surprised this had not happen to my SL300 here. If the switch stays on for too long or the transformer has a flux imbalance that causes it to move up its hysteresis curve, the transformer core eventually saturates and the low impedance destroys the switch. Either way, the amount of energy transferred to the secondary side from the fault should be negligible compared to the total capacity of output caps.

    The scary PSU failures are those involving the feedback circuits: if these fail open, you may end up with the primary side operating at its maximum duty cycle and outputting the maximum voltage the main outputs are capable of based on the transformer's turn ratio. This is basically what happened with the 5VSB output here: the auxiliary output was failing and caused the primary side of the 5VSB feedback loop powered by that auxiliary output to misbehave, allowing the 5VSB output to surge to about 8V not counting noise.

    I would be far more concerned with silent failures (like the 5VSB here) than spectacular ones.
  • chazking260
    Nice to revisit the technician aspect of troubleshooting. It's a dying art and cost prohibitive except in a lab environment. Great Problem reporting!
  • razor512
    I was able to repair a $20 power supply by replacing the dead caps, with some industrial 135C rated caps that I salvaged from some industrial equipment. (the caps likely cost far more than the power supply, but the repair worked.

    For PSUs I generally do not repair them beyond capacitors, unless it is something that is custom and cannot easily be replaced.

    After a repair, I always test power supplies with dummy loads. (mainly car headlights). (pack of 2, 55 watt headlights are generally around $7-8
  • nukemaster
    Hey I was just looking over this again in Spartan/Edge with reader mode and notice those "rubycon"caps are also fakes.

    They say rulycon

    Should have had the logo like this :)


    Instead it is.
  • shrapnel_indie
    120VAC is enough to kill some people, or at least interfere with their hearts enough to cause medical/physical issues. Some may just feel a pulsing "bite." Not worth being careless (or even intentionally) to find out which one you are. 240VAC will kill. I should be more precise though: The associated current does the damages, and each human body's electrical resistance can vary among others and/or with conditions.

    I do agree that neutral (white) and hot/live (black) wires get reversed quite often enough that it could be a problem. It's why decent outlet testers will cross-check the connections between hot, neutral, and ground.


    Quote:
    All capacitors have a tolerance rating that can range from -20% to as high as +80% for aluminium electrolytic’s affecting its actual or real value.
    - Electronic Tutorials, Capacitor Characteristics, http://www.electronics-tutorials.ws/capacitor/cap_3.html

    Sometimes caps do have other tolerance ratings, but they are usually marked when they do. The 600uF cap was more than likely within tolerance (min 544uF) while the 34uF cap was definitely not in tolerance and was bad. The 47uF cap measuring 112pF (min 37.6uF) was bad. The 22uF cap measuring 18uF was within tolerance. (min 17.6uF) (Mins given at -20% tolerance)

    Changing a voltage filter cap to a higher value has no harm other than it can have a higher "charging" current as it charges to the full voltage available to it. Changing it to a lower value will increase noise and ripple in the output voltage. Changing other caps, such as in timing circuits or frequency dividers with other values can change timings and potentially cause problems. It's usually not an issue (other than cost) to use a cap with a higher voltage rating than what is needed. It is important to observe polarity on polarized caps unless you want it to go BANG! Design, age of the design, and the materials called for by the design will affect size, leakage currents, and ESR of the caps, especially electrolytic caps.
  • Daniel Sauvageau
    35532 said:
    Hey I was just looking over this again in Spartan/Edge with reader mode and notice those "rubycon"caps are also fakes.

    In my old junk box which I trashed a few years ago, I also had "Lubycon", "Samxung" and a few other similar spoof-sounding names. The first time I came across a "Fuhjyyu" capacitor, I thought it was a spoof attempt on Fujitsu's name.

    Since many asian languages lack phonetics to clearly disambiguate those spelling variations, I bet many products ended up with these devices in them simply due to confusion or creative interpretation in the supply chain.
  • Crashman
    330834 said:
    I do agree that neutral (white) and hot/live (black) wires get reversed quite often enough that it could be a problem. It's why decent outlet testers will cross-check the connections between hot, neutral, and ground.
    I blame the difference between color codes of AC and DC circuits. Most DC circuits are "Negative Ground" and black is the "Negative" wire, so it gets connected to the chassis. Power supplies are both AC and DC, so the white wire is ground on one side and black is the ground on the other side, if you connected black to black you'd energize the case.

    I tried explaining this to an ELECTRICAL ENGINEER who couldn't figure out how to wire up a motorcycle. Seriously, he kept saying stuff about the stupid motorcycle company hooking up all the hot wires to ground and he was trying to fix it (he was actually converting it to positive ground and couldn't figure that out). The thing is, this guy designed control systems for manufacturing and it was all AC and wiring diagrams in his mind.
  • Daniel Sauvageau
    330834 said:
    Sometimes caps do have other tolerance ratings, but they are usually marked when they do.

    Technically, all components have tolerances on all parameters worth listing. Capacitors' nominal capacitance just happens to be notorious for having some of the widest tolerances in all of electrical engineering.

    In the case of the cap that still read 600µF on the multimeter, it turned out to have about 4 ohms of ESR in my step test when I revisited it. The reason it let so much noise through despite having a "good" capacitance is the ESR: the capacitor is no good at suppressing ripples at 10+ kHz if there is a 4 ohm ESR in front of it. If there is 1A of ripple current from the flyback, you get 4V worth of ripple instead of the expected 0.1V.
  • pjmelect
    I liked this article, It is important that the replacement capacitors are 105C type, as the normal 85C type capacitors would not last very long. I often repair power supplies and I use a ESR meter to test in circuit all of the electrolytic capacitors in the power supply which only takes a few minutes.
  • nukemaster
    Is low ESR caps wasted on normal 60hz power filtering(diode rectifier after a low voltage AC transformer)?
  • Someone Somewhere
    8708 said:
    330834 said:
    I do agree that neutral (white) and hot/live (black) wires get reversed quite often enough that it could be a problem. It's why decent outlet testers will cross-check the connections between hot, neutral, and ground.
    I blame the difference between color codes of AC and DC circuits. Most DC circuits are "Negative Ground" and black is the "Negative" wire, so it gets connected to the chassis. Power supplies are both AC and DC, so the white wire is ground on one side and black is the ground on the other side, if you connected black to black you'd energize the case. I tried explaining this to an ELECTRICAL ENGINEER who couldn't figure out how to wire up a motorcycle. Seriously, he kept saying stuff about the stupid motorcycle company hooking up all the hot wires to ground and he was trying to fix it (he was actually converting it to positive ground and couldn't figure that out). The thing is, this guy designed control systems for manufacturing and it was all AC and wiring diagrams in his mind.


    Oh... wow.

    I didn't realise the US used black for phase and white for neutral - I'd seen the wire colouring in the odd power cable etc. that comes with US gear, but...

    We use black for neutral here down under. Red (or yellow (was white until a decade or so ago, but UV turns everything white) or blue if three phase) is phase. Many appliances are built to European standards with brown phase and blue neutral.
  • Shankovich
    These articles, this is why I always love Tom's
  • Shankovich
    These articles, this is why I always love Tom's