SMPS Long-Term Reliability

The most glaring omission is usually current-mode control (which is absurd if you think about it: the heart of a switching supply is the current through the energy storage inductor, wherever that might be), but if the control loop hasn't cacked in that time, it's probably the same thing as anything else, electrolytics.

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

I have a Tek scope that uses a phase controlled (IIRC) triac in the power supply. Switching- sort of.

--Spehro Pefhany

You can't necessarily draw such a conclusion about loop control. I was using current-mode control with discrete components in the

-- Seven Transistor Labs Electrical Engineering Consultation Website:

True. And I've done it with all of five* transistors;

But given the selection of early controller chips (like SG3524 and related PP types, or MC34063 -- which to be fair, is mostly current controlled in its usual implementation, but.. ??!), the chances of finding a "good" one in period equipment seems unlikely.

Hmm, speaking of blocking oscillators, there was one of those "famously" in the Apple II. That's a little new against the OP's timeline, but shows the approach was definitely not unknown.

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

It looks a awful lot like a troll.

Convince me I am wrong - go ahead make my day.

... Phil

.... Phil

... Phil

LOL! Wish I had five bucks for every time you accused some poor, innocent soul here of being a troll, Phil. Seriously.

There is trace evidence on the boards I've seen where charred components have been replaced - mostly diodes and power resistors that weren't of sufficient rating, presumably. They've damaged the surrounding pcb traces as well. I'm tempted to rebuild on fresh boards from scratch, just re- using the transformers, inductors and whatnot from the original, ditching everything else and upgrading the ICs to modern ones. There's no money in this, of course, but since I'm just a hobbyist and this is what passes for fun for me, I'm not bothered.

Lack of current mode control is one thing as Tim mentioned. Then they pressed semiconductors that didn't want to into heavy duty switching service. Instead of hard limiters there were often just snubbers where it can take just one adverse event and ... *PHUT* ... here goes a transistor or a diode. Input transient protection? Wot's dat? And so on.

A classic example is a fat 12VDC -> 115VAC converter from the 70's or

The very first things I'd eye with a great deal of suspicion are all electrolytic capacitors in there. Including small ones in low power paths. Sometimes they just dry up over time.

Thanks for that, Joerg. I note on the one in front of me right now they've used a TO-3 cased BJT instead of a mosfet for the chopper device. No inrush current limiting to speak of, either.

TL431, my favorite jelly-bean part. In your PNG, acting as an OpAmp... did that sort of thing many times.

...Jim Thompson

You did not make my day, so you are trolling as usual.

FYI:

Posting dopey questions to amuse and draw attention to yourself is trolling.

... Phil

And, I STILL see copies of the Apple II design. The last one I had to fix, had no bad components. But the startup was so sensitive/flaky that it had to reform its capacitors before it would work. And, it had to work before it would reform its capacitors.

The fix: leave it on overnight. It was working fine in the morning.

On the upside, it's usually the kind of thing that's perfectly okay with that sort of operation (slow start, short protection, etc.). Pretty impressive for ~3 transistors.

The basic design has (had?) also seen considerable longevity in home media devices (VCRs, then DVD players), for some reason. Is that a design culture sort of thing? Can't say I've ever designed a VCR, I have no idea.

Tim

Get help, mate.

... Phil