Long-term reliability of off-line switchers - Page 1
Forum section: electronics / general

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 21, 2022, 08:34:12 pm
content: I'm no great expert here, but I think the vast majority of consumer-level off-line switching power supplies use an opto-coupler for feedback between the output and the live side? Like everything from the wall-warts for your inkjet to the 1500W power supply for your PC, let's say.I know optos have the behavior of changing their CTR property over the years.So how does this affect the 10,20, 30 year performance in terms of the output voltage going beyond or below its spec?Or do I misunderstand the issue?

author: Benta
author_rank: Super Contributor
timestamp: January 21, 2022, 08:58:08 pm
content: It doesn't.The optocoupler is in a feedback loop with gain, and any standard coupler will have at least a 1:3 spread in CTR, which the loop has been designed to compensate. And CTR drop is not dramatic over the years.I'd be much more worried about electrolytics succumbing to heat and ageing, as well as dust collecting at high-potential points.

author: richard.cs
author_rank: Super Contributor
timestamp: January 21, 2022, 09:03:05 pm
content: Also in some designs the opto runs in saturated mode where the feedback is a pulse width, in these CTR degradation doesn't influence loop gain. I have a few 40-year-life designs for a rail customer where we worried about it, but only after we'd engineered every single electrolytic capacitor out of the design.

author: tooki
author_rank: Super Contributor
timestamp: January 21, 2022, 09:51:37 pm
content: Quote from: Alex Eisenhut on January 21, 2022, 08:34:12 pmIt’s like BJT hfe: if your design relies on the CTR being precise, it’s a bad design.FYI, some PSU designs don’t use optocouplers, but instead use feedback windings on the transformer. I think this is mostly done in cheaper small PSUs (like USB chargers), but I don’t know how good this approach is.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2022, 12:01:54 am
content: Yeah, primary or aux sensing is popular with low cost, small supplies -- the regulation is ass, due to leakage inductance (which is on the poor side as well, due to the simple transformer windups they use, presumably for cost), but it's good enough for general purposes, phone charging, etc.Electrolytics really are the dominant failure mechanism; beyond that, probably transients, anything to do with marginal design ratings, insulation quality, etc. All of which can be designed for adequate margins, protection, filtering, etc., it just takes a little more work to do.For their part, semiconductors really are remarkable. A rock that's essentially unchanging, yet capable of highly dynamic and intricate behavior. They can last pretty much forever, as long as they're designed for it. (And for that, electromigration is probably the biggest barrier to long life of most commercial grade products? Older designs probably win here, as the wider conductors have less current density and thus susceptibility to electromigration.)There's plenty of examples of old consumer hardware that's survived despite its cheap design; sometimes you just get lucky. The challenge from a design standpoint is getting a 90% yield after say 50 years, rather than 0.1%.Transients bear some reflection. Keep in mind, we normally test (well, if/when we do..) to a standard level, and that's that. It's designed as an upper limit level, so that anything seen in practice likely won't exceed that, but there may still be cumulative damage, and there's still the outside chance that a worse event comes along, just too infrequently to matter with respect to the test. And that's good enough for commercial lifetimes, maybe a once-in-10-years transient exceeds that level and blows it up, and that's fine for commercial equipment, but if you're targeting longer life, you'd want to test to higher -- and use protective devices that don't wear (those stonking great mains TVSs), or at least much larger devices that wear less.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 22, 2022, 08:01:36 pm
content: Duh. That was a dumb question in retrospect. Of course there's a reference somewhere on the secondary side. For some weird reason, I thought the opto was a critical part in determining the output voltage... It's in a feedback loop...I have this 35 year old Sony Tektronix vectorscope with a bad supply, that's why I was wondering. It's an ugly supply visually too.

author: magic
author_rank: Super Contributor
timestamp: January 22, 2022, 08:06:10 pm
content: Quote from: tooki on January 21, 2022, 09:51:37 pmIt's common in miniature 5V only USB chargers.They have the interesting property that their output voltage at no load or any given load increases 2mV/°C because of the output rectifier's forward voltage driftThe newer chargers with variable output voltage tend to go back to secondary side regulation. Otherwise they would need to somehow communicate the requested voltage back to the primary side chip, at which point it's just easier to put regulation in the same chip which talks USB and use some dumb PWM on the primary.

author: tooki
author_rank: Super Contributor
timestamp: January 22, 2022, 11:18:20 pm
content: Quote from: Alex Eisenhut on January 22, 2022, 08:01:36 pmWell it's not wrong to say that the optocoupler (or an alternative like the auxiliary winding) is essential; without it, it couldn't work! It's only the specific characteristics that don't matter.

author: magic
author_rank: Super Contributor
timestamp: January 23, 2022, 12:05:28 am
content: Well, you could use the optocoupler's forward voltage itself as the reference.Actually, the absolutely cheapest and dodgiest PSUs simply connect the optocoupler in series with a zener to the output of the secondary. Somewhere near 7V or maybe more the combined thermal coefficient may even work out to zeroThey are still fairly insensitive to CTR, because current increases very fast with small increase in output voltage.