SMPS Long-Term Reliability

Gentlemen,

I have a mountain of vintage scopes to fix up (mid 1970s onwards). Most if not indeed all seem to be suffering from problems with the power supply section. These were manufactured at around the time SMPS was taking over from linear, I would imagine, so before they ironed out the bugs in the early SMPS designs. My question is, what were the design flaws in the early power supplies that led to premature failure and have these issues now been successfully resolved in the switchers we see in test equipment today.

Thanks,

cd.

Reply to
Cursitor Doom
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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

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Seven Transistor Labs 
Electrical Engineering Consultation 
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Reply to
Tim Williams

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

--Spehro Pefhany

Reply to
Spehro Pefhany

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

1977-1987 time frame at GenRad Portable Products Division, Phoenix. ...Jim Thompson
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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson
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Seven Transistor Labs 
Electrical Engineering Consultation 
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Reply to
Tim Williams

** I see no point to such a vague, sweeping and presumptuous question.

It looks a awful lot like a troll.

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

... Phil

.... Phil

... Phil

Reply to
Phil Allison

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

Reply to
Cursitor Doom

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.

Reply to
Cursitor Doom

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

80's I have in the garage which is destined to go to E-Waste some day. The transistors might still be Ge-varieties and there is a phone number on the enclosure (!) where to order a set of new transistors.

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.

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Regards, Joerg 

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Reply to
Joerg

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.

Reply to
Cursitor Doom

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

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

** Your wish would get you nothing.

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

Reply to
Phil Allison
[on defects of early SMPS designs]

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.

Reply to
whit3rd

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

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Seven Transistor Labs, LLC 
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Reply to
Tim Williams

Get help, mate.

Reply to
Cursitor Doom

** Get a life, troll.

... Phil

Reply to
Phil Allison

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