lifetime of an offline psu

[treez]

Hello,
Can you confirm, that for a 100W, 90-265VAC, Offline flyback with a preceeding boost PFC stage, the main things which may cause eventual end-of-lfe are….

1..Inrush current at startup
2..Power line transients wearing the MOV out over time until eventually it blows the fuse.
3..Electrolytic capacitor degradation
4..Feedback optocoupler degradation in CTR.
5..Heat to the semiconductors eventually wearing them out.
6..Inrush NTC eventually giving up.
7..Resonance at switch on in emi filter which overvoltages the x caps, and if the mov is placed after the emi filter and before the bridge, then the mov will wear out aswell due to this.


The PSU is in a totally enclosed metal case with no fan in it.

 

[betwixt]

All of those are possibilites but you have to consider then in terms of likelyhood.

1. inrush current itself causes no harm, it is the stress it places on the other components that can reduce their longevity.
2. that would depend greatly on how many transients and how big they are. Personally, in 50+ years, I have never seen an MOV die like that.
3. would be my main concern, natural outgassing and environmental leakage gives them a slow path to failure no matter how you treat them.
4. yes, but consider the degradation takes many years (decades maybe) and being in a feedback loop means the degradation is to some degree compensated.
5. yes, but again they may last as long it not longer than any other component.
6. most stressed but designed to take it.
7. theoreticaly possible but with the damping effects of the line and rectifiers surounding it, probably not a real concern.

One of the most common failures I see is high value resistors going open circuit, especially in high voltage situations, even though the powe dissipation is below their rating.

Brian.

 

[treez]

Thanks,
I am surprised that you dont think that MOVs wearing out due to transients is a too significant failure mode in offline smps...the following website believes that power line transients blowing movs up is the main reason that offline switch mode led drivers fail quickly

QUESTION: Is the Isotera system suitable for use with led bulbs fitted into traditional lamp sockets?
Answer: No, LED bulbs have a small AC/DC converter built inside and are incompatible with the Isotera system. Small power converters tend to be vulnerable to spikes on the mains, which makes them susceptible to premature failure. So, although LED's may operate for 10s of thousands of hours, these small converters may not. That is why the Isotera system is based on centralised power conversion comprising effective filtering of transients on the mains.

here is the link for the above quote...its the first "FAQ"...

**broken link removed**

Concerning outgassing of electrolytics with age, i would love to find a reference from a manufacturer confessing this kind of thing, may i ask do you know of one?

Sorry , by "environmental leakage" in electrolytic capacitors, i take it you mean the increase of leakage current over time?

 

[betwixt]

Don't forget you are reading Isotera's sales literature, it isn't in their interest to praise the reliability of other products! It begs the question - where do Isotera's "centralised power conversion" source originate - I bet it's an AC SMPS with a MOV in it somewhere!

What I meant by environmental leakage is the natural, if unintentional drying out of the electrolyte. It has to go somewhere over time and the obvious route is around the seal and wires as they pass through the rubber bung. In situations where the temperature cycles between hot and cold, there will be expansion of the gasses inside the can and they will carry some of the evaporated liquid electrolyte. Inevitably, when hot and under pressure it will try to escape and after cooling it will try to draw atmosphere back in again. Over time, maybe many years this migrates "vents" the electrolyte and causes reduction in capacitance and increase in ESR.

I was a little surprised when moving equipment back in to my new workshop a few weeks ago to note the date on the electrolytics in my 12V/20A power supply. They were new when I built it and the date code on them is 1978! I have not measured their ESR and I don't have their original specification to compare it with but the PSU is still spot on output voltage and ripple is < 20mV under full load. Not bad for 37 year old caps!

Brian.

 

[1/2dime]

Google "MTBF Calculation". Each part is rated differently (a carbon film resistor has a higher failure rate than a metal film, which has a higher failure rate than a wire-wound), failure rates are added up, averaged, multiplied, divided and guessed at. E