SMPS Long-Term Reliability

C

Cursitor Doom

Guest
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.
Click to expand...
 
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
Click to expand...

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
Click to expand...

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.
Click to expand...

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.
Click to expand...

** 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.
Click to expand...

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.
Click to expand...

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 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.
Click to expand...

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.
 
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.
Click to expand...

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 Sun, 26 Apr 2015 19:00:02 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

--
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. ;)
Click to expand...

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

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
Click to expand...

...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.
 
Cursitor Doom wrote:

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
Click to expand...

** 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
 
On Sunday, April 26, 2015 at 4:58:09 PM UTC-7, Tim Williams wrote:

[on defects of early SMPS designs]
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.
Click to expand...

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.
 
"whit3rd" <whit3rd@gmail.com> wrote in message
news:429ff3de-106c-472a-ae5a-fc162b855983@googlegroups.com...
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.
Click to expand...

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

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com
 
On Mon, 27 Apr 2015 18:53:41 -0700, Phil Allison wrote:

Cursitor Doom wrote:

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


** 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.
Click to expand...

Get help, mate.
 
Cursitor Doom wrote:

Phil Allison wrote:

Cursitor Doom wrote:

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


** 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.

Get help, mate.
Click to expand...

** Get a life, troll.



.... Phil
 
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