Forums

SMPS Long-Term Reliability

Started by Cursitor Doom April 26, 2015
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.
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: http://seventransistorlabs.com

"Cursitor Doom" <curd@notformail.com> wrote in message 
news:mhj2qh$7jg$2@dont-email.me...
> 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.
On Sun, 26 Apr 2015 11:44:10 -0500, the renowned "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>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
I have a Tek scope that uses a phase controlled (IIRC) triac in the power supply. Switching- sort of. --Spehro Pefhany
On Sun, 26 Apr 2015 11:44:10 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>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
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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote 
in message news:a29qja5tm93jsdfspfotpb1fpo4tvvhrv5@4ax.com...
> On Sun, 26 Apr 2015 11:44:10 -0500, "Tim Williams" > <tiwill@seventransistorlabs.com> wrote: > >>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 > > 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.
True. And I've done it with all of five* transistors; http://seventransistorlabs.com/Images/Deadbug_Sch.png *TL431 acts like it has a very accurate Vbe. So I'm cheating, so what. ;) 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: http://seventransistorlabs.com
 Cursitor Doom wrote:
> > > 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.
** 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
On Sun, 26 Apr 2015 21:45:22 -0700, Phil Allison wrote:

> ** 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.
LOL! Wish I had five bucks for every time you accused some poor, innocent soul here of being a troll, Phil. Seriously.
On Sun, 26 Apr 2015 19:00:02 -0500, Tim Williams wrote:
 
> 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.
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.
On 2015-04-26 9:13 AM, Cursitor Doom wrote:
> 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. >
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. -- Regards, Joerg http://www.analogconsultants.com/
On Mon, 27 Apr 2015 07:39:19 -0700, Joerg wrote:

> On 2015-04-26 9:13 AM, Cursitor Doom wrote: >> 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. >> >> > 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.
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.