From: keithl@klic.rain.com (Keith Lofstrom)
Newsgroups: sci.electronics
Subject: Repairing switchers (long)
Message-ID: <1992Aug9.235615.19656@klic.rain.com>
Date: 9 Aug 92 23:56:15 GMT
References: <1992Aug7.195705.18843@csx.cciw.ca>
Distribution: na
Organization: Keith Lofstrom Integrated Circuits
Lines: 276

>Also if you have any anecdotes about SPS repairs (typical components to
>go, cross references to commonly needed parts etc.) I'd like to hear
>from you.

ObDisclaimer:
***************************************************************************
WARNING!  DANGER!  WARNING!  DANGER!  WARNING!  DANGER!  WARNING!  DANGER!

Power supplies contain lethal voltages.  That means they can kill YOU.
Yes, YOU.  Unless YOU are especially cautious, YOU won't wake up tomorrow.
Pretend you are playing with a nuclear bomb.  A bomb can't kill you any
deader.

If you don't understand electronics, don't bother with power supplies.
This is analog circuitry, not cookbook logic stuff.  You need to have a
feel for currents and voltages and waveforms, or you won't be able to
figure out what's happening.  Find some circuit nerd (like me) who needs
some software written, and swap services.  Don't risk your life unless you
know what you are doing.

Power supply repair is a challenge, like mountain climbing.  Probably
about as dangerous.  Allow adequate time (3-4 hours, plus trips to the
parts store), don't attempt it while tired or distracted, and have the right
tools available.  Make sure somebody is around to haul you to the hospital
if you zap yourself.

If you DO hurt yourself, in spite of my warnings, I am NOT responsible.  I
never posted this, my signature is forged, the important parts got deleted
by your newsfeed, etc.  If you can't take responsibility for your own life,
don't repair power supplies.

WARNING!  DANGER!  WARNING!  DANGER!  WARNING!  DANGER!  WARNING!  DANGER!
***************************************************************************

Switchers - I've fixed about 40 of them.  The broken ones were all broken by
bad design or sloppy manufacturing - inadequate component choice, usually.
If you want to fix one properly, you will need to be able to find heftier
components that still meet the other design goals - made somewhat easier by
the advances in components.  A proper repair involves a little design.

You will need access to a semiconductor curve tracer, or else be ready to
build a lot of ad hoc kludges for pulse characterization of components.
Most of the power components that fail will have no easily available direct
replacements.  Looking at some components with DC sources can cook them.

You will also need an oscilloscope ( 5MHz bandwidth will do ), a voltmeter,
and access to a huge pile of power transistor and diode databooks.  National
Semiconductor, International Rectifier, Texas Instruments, Motorola books
will probably do; you may need SGS Thompson, Siemens, and Signetics for some
regulator chips.

Without a schematic, be prepared to spend some time tracing out circuits.
If you live in East Armpit, Nebraska, you probably won't be able to find the
components you need, so give up now.

An isolation transformer and a Variac are handy.  Also a pile of power
resistors from which you can build a test load.  A bench supply to externally
power the secondaries is useful, though I use the curve tracer for this.

------------------------------- rules ---------------------------------------

Rule number 0:  Put at least two switches between you and line.  Unplugging
counts as one switch.  You are human, and you will forget one switch from
time to time.  Forgetting shouldn't have to be lethal.  Take off watches and
metal jewelry.  Hell, get someone else to do it.  Life is too short anyway.
If you are a professor, find a gullable grad student :-).

Rule number 1:   Check the fuse. Keep plenty on hand - you may end up blowing
a dozen or so.  About a third of the time fuses blow for no reason at all.

Rule number 2:   Is everything hooked up and switched properly?  I've seen
a PhD spend 4 hours debugging a simple little box that "should work but
didn't".  When I innocently asked "what does this little switch do?" he
thought for about 10 seconds and turned the prettiest shade of red...

Rule number 3:   Good designs rarely fail.  If it is broken, it is because
of an idiot engineer, most likely.  Don't assume anything is designed right;
if a circuit you've traced out can't possibly work, check it again.  If it
simply LOOKS stupid, you have discovered a fundamental truth about some
power supply designers.  There are lots of good power supply designers out
there - I've rarely had a chance to explore their work, because I've never
had to fix their designs.

Rule number 4:  A toasted resistor means something else failed first.  Figure
out what could toast the resistor, or you will toast another one.

Rule number 5:  Buy extra replacement parts.  You will blow a few things up
during debugging.

Rule number 6:  Before firing the supply up, figure out the primary circuit.
Typical switched 115/230V supplies have a full wave bridge and two filter
caps, resulting in about 340 volts DC on the primary.  Make sure there is
a discharge path through a power resistor - if not, temporarily solder one
on while you work on the supply.  A 20K ohm, 10W resistor will discharge a
1000 uF primary in about 1 minute.  You can switch in a smaller value resistor
for faster discharge, but you will probably forget to turn it off, and end up
blowing it up when you turn the supply back on.  In any case, get rid of the
primary side voltage somehow before you mess with things.

Rule number 7:  When building test loads, load all the supplies.  Some
switchers get unhappy if there isn't at least a trickle of current on each
supply.  A 20% load is often adequate.

Rule number 8:  When you think you have the supply fixed, run it up and down
with the Variac;  run it up to at least 1.2X normal line, if you can.  Note:
some switchers get very unhappy with 0.5X line - see note on stupid designers
above.  If you end up breaking it this way, at least you know what will
happen during brownouts.  If the result is overvoltage on the outputs (I've
seen this once) then throw the supply away, and kludge on a different
design if possible.  Overvoltage eats expensive equipment.

Rule number 9:  Anything can be fixed.  Sometimes it's too much work.  Be
willing to give up.

-------------------------------- what goes ------------------------------------

The things I have seen fail, in order, are:

1) Electrolytics.  Switchers put a lot of AC current into electrolytics,
many of which aren't designed for this treatment.  The capacitors cook,
and either short out or unweld themselves open.  The nastiest failure is
an "almost open" that opens and closes itself at line rates (I've encountered
a couple of those, murder to diagnose).

One capacitor failed on the input side of a pi filter on a secondary:

  --|>|------+----- Inductor ------+--------- 5V out
             |                     |
           -----   This one      -----
           -----   <---          -----
             |                     |

This was on the 5 volt supply of a 5V/12V switcher, the regulator worked off
the filtered 5V.  When the indicated capacitor opened, the inductor swallowed
most of the switching voltage from the transformer and diodes, resulting in
the regulator kicking in much harder to try to make 5V on the other side.
This put 35 volts on the allegedly 12V supply - as a result, all the 16V
tantalums on the 12V supply got blown out, as well as much of the stuff
hooked up to the 12V supply.

The reason for the pi filter in the first place was too much ripple on the
supply - the reason for too much ripple was the high ESR of the capacitors
used.  I replaced with better capacitors.

Replacement:  Don't just go for the same voltage and capacitance.  You need
low ESR, and if the first one failed, the second one will too.  One sorta
brain-damaged way to get low ESR is to use more capacitors - and possibly
more capacitance - wired in parallel.  You can get some idea of ESR with the
curve tracer - with a small voltage ( approx 200mV ) AC drive, a low ESR cap
looks like a circle, while a high ESR cap looks like an ellipse from the
resistive component.  Look for high temperature ratings, too - at least 105C.

If there are any power resistors sitting right next to an electrolytic, move
them enough to get airflow between.  Electrolytics are too damned temperature
sensitive - most are rated to only 85C.


2) The main switch transistor(s).  Usually a TO3 power transistor.  These
usually fail because something shorted things out (there should be a
current sense shutdown, but this involves buying another 5 cent resistor;
too expensive :-( ).  These also often fail from inadequate voltage rating.

These are usually the easiest component to check, assuming a curve tracer is
available, even if they are less likely to fail than one of the 10 or so
electrolytics.  I pull the transistor and check it before trying anything
else.  If the transistor works, write down some of the curve tracer
measurements.  You may accidentally fry it during repair.

You will want a transistor that can stand off at least 1.5X the peak primary
DC voltage, preferably 2X.  You will need to find what the current rating
of the transistor you pulled out is, and meet that with the replacement.
When you are looking for a replacement, find a new transistor with:

     Same type - usually NPN, non-darlington
     Same case
     Better voltage rating
     Better current rating
     Similar Beta/Hfe/Current Gain
     Same or Higher Ft
     Same or lower capacitance

Advances in components make finding an adequate replacement possible.
Curve trace the replacement.


3)  Secondary diodes:
I've lost a few of these.  The result of a diode failure is usually an
imbalance in secondary voltages.

Replace with:

     Same type - usually Shottkey diode
     Similar case (you may have some lattitude here)
     Better voltage rating
     Better current rating  (make sure with curve tracer)
     Lower capacitance


4)  Regulator ICs - these rarely fail, but a manufacturer can get a bad batch.
The result is a whole bunch of the same supply type failing.  There are so many
different ways a regulator can fail that you will just have to figure it out;
though a supply that goes tick ... tick ... tick may have a broken regulator
chip, if you can't find anything else wrong.  The ticking noise means the
regulation loop is broken somewhere.

You may have a heck of a time finding a replacement.  An exact replacement
is required.  Socket the replacement.  Use a good socket that grips the chip
tightly - thermal excursions can walk an IC out of its socket.

-------------------------- Techniques ---------------------------------------

Check the fuse.

Dust the damn thing, with kleenex and Qtips.  Of course you've never changed
filters on the fan.  Dust kills power supplies.  After this, you'll change
them.

Look at a working version of the supply, if any are available.  Get a schematic
if you can.  Apple and Sun are lousy for schematics, HP is good.  Clones
are impossible.

Look for all the usual visual stuff, open traces, shorts, burned components.

Disconnect the supply, writing down somewhere where all the connectors go
and which direction ( if I need to be telling you this stuff, you aren't
qualified).  Don't use the regular circuit as a dummy load, though you may
want to find out how much current it draws, to help you build test loads.
Use a bench supply for testing the regular circuit.

Power up the secondaries - one at a time, with an external voltage source -
first.  Look for shorts.  Drive capacitors with curve tracer AC.  Look for
ESR or opens.

Load the outputs at about 20% of full load.  Power up the primary with curve
tracer in AC mode, slowly, to about 40VAC.  You should be able to watch the
primary capacitors charge up.  This finds primary shorts and capacitor opens.

If you bring the supply up to full input and nothing happens, check for
primary voltage - usually 340 volts.  If the supply is 115 volt only, and
there is only one capacitor, there is probably only 170 volts across it.

If there is voltage, AND the primary is separately isolated with an isolation
transformer, locate the primary common, usually the lowest voltage on primary
side (You traced the circuit, remember?).  Hook the scope signal input to
this point (at 50V / div, line sweep and sync ), there will probably be a
significant AC signal here.   Try connecting this point to oscilloscope
ground with a 100 Kohm resistor - if the signal doesn't diminish, you aren't
properly isolated.  If it does diminish, power down, disconnect, and discharge,
ground the scope here, and power back up.

Check for oscillations around the regulator chip.  If none, check for voltage.
Switcher regulators are sometimes powered with their own separate little
supply, or off a big power resistor from primary voltage.  Sometimes they are
powered with a little kickstart circuit from the primary, then a separate
winding off the transformer.  These are good for the tick...tick...tick
type of failures.  You may have to power the regulator separately, with a
bench supply, to get things started.

From here on out, it's measurement, and debugging, and tinkering, and such.
Let us know what you find wrong.  Perhaps we can embarrass some manufacturers
into doing a better job on their supplies.

******************************************************************************
Again - and I repeat - don't repair power supplies unless you are a pretty
good analog circuit tinkerer, can act safely around high voltage, and have
the right tools.  Take the thing to a TV repair shop, instead.  It would
spoil my day to find out somebody with more ambition than sense took the
above notes and hurt themselves trying to do something they weren't ready for
yet.
******************************************************************************

Good Luck;
--
Keith Lofstrom         keithl@klic.rain.com       Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Power ICs
(These are additions I sent to the author - great job!)

	PLEASE REPLY TO fmgst@unix.cis.pitt.edu - BORROWING ACCOUNT

	Greetings. I realize that this is basic but add to your list
	that people should check the solder joints around power
	resistors (mostly in cheezy TV power supplies) and around
	any power Xsistors and diodes. Sometimes this is the only
	problem (open joint) and I hate to admit but it got me
	cussing once :-)

	Also, add that it's a damn good idea to just "redo" all
	the solder connection on power "stuff" ESPECIALLY if the
	PCB is discolored (say, BROWN) around these points.

	Also, continuing on your theme of idiot designers, I've seen
	PSes that had traces that carried MUCH more current than
	they were designed to carry. If people see a trace that has
	BROWN PCB around it, tell them to scrape that green coating
	and solder in a tinned wire from origin to destination.
	Or, if space limits, just scrape the ceating, flux, and
	tin the trace with liberal solder...

	Also, it's usually a good idea to drill (YES, I said DRILL)
	a few holes around any power components IF, AND ONLY IF there
	are no other leads or components in the way. I usually find
	that Power Supplies use one sided boards (even cheaper :-)
	and that my holes (even tiny ones) do wonders...

	Also (last one, promise :-), and I'm suprised no one mentioned
	this in the "green coating" thread: I use your typical
	red nail polish (don't tell my sister :-) Just warn people
	not to use the stuff with the metallic flicks in it :-)

	Take care.

	P.S. Thanks for your post!!! I hope all the morons and those
	damned accountants (after all, they make the final decision!)
	are listening................................................
