Need help with switching power supply repair

[senator richards]

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy

[Arfa Daily]

"senator richards" <rnew...@gmail.com> wrote in message
news:dd3e3ab9-1f98-41b8...@u20g2000yqj.googlegroups.com...

The cap that filters the supply to the pwm chip on the primary side, maybe ?
It's pretty common on most designs of switcher. Work on the thing on an
isolation transformer if at all possible. They are potentially very very
dangerous if you are not fully competent with them

Arfa

[Jamie]

See if you have an optical coupler in there. I've seen these used as
part of the regulation and it's possible it isn't working any more.

Also check for bad caps, ripple could be causing a reg issue.

Jamie

[Phil Allison]

"senator richards"

>I am trying to troubleshoot a small SMPS that came from an A/V
> switcher. Input is 120vac and it is supposed to output + and - 15vdc
> at .8A. Currently it is outputting +17 on one output and somewhere
> between +22 and +30 on the other output.

** That is very odd.

You real sure of that measurement?

> The supply to the pwm chip is fluctuating between 7-15v,

** Means the supply for the IC is not holding up.

Check for open resistors or diodes in the supply chain to that IC.

Any yellow glue in sight ?


.... Phil

[Sjouke Burry]

Try to load both sides with about .1amp, low load on a switcher
can confuse the hardware sometimes.

[Franc Zabkar]

On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
<rnew...@gmail.com> put finger to keyboard and composed:

>I am trying to troubleshoot a small SMPS that came from an A/V
>switcher. Input is 120vac and it is supposed to output + and - 15vdc
>at .8A. Currently it is outputting +17 on one output and somewhere
>between +22 and +30 on the other output. My experience with SMPS has
>usually been shorted rectifiers or bad output filter caps so the first
>thing I did was check all the diodes and replace the output caps.
>Obviously that didn't fix the problem. The high voltage is about
>170vdc. The supply to the pwm chip is fluctuating between 7-15v, so

>i'm thinking this might be the problem ...

Just to add to what Arfa said, the supply to the PWM chip often starts
with a simple resistor feed from the +170V rectfied mains voltage, and
is then augmented by a regenerated supply from a tap on the switchmode
transformer. If this is the case, then there will be a small
electrolytic filter capacitor (~10uF) that often develops high ESR.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

[Phil Allison]

"senator richards"

> Thanks in advance for any help.

** FFS - get yourself an ESR meter.




.... Phil

[Kripton]

try to check the main capacitor with an esr meter, or change it !
build yourself easy an esr meter grab any you want on my web site below
regards,
--
----------
Kripton

the ESR Repository @ http://kripton2035.free.fr/esr-repository.html
the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html

[sci.electronics.repair]

On Sep 21, 8:08 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
> "senator richards" <rnewma...@gmail.com> wrote in message

Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5 which
appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R

[sci.electronics.repair]

On Sep 21, 8:34 pm, Jamie

There is an optical coupler. Its a CNX82A chip. Is there an easy way
to test it? I looked at Mouser and the part is discontinued and NLA.

I've checked all the caps for low ESR and they all check out OK.

Thanks.
-R

[sci.electronics.repair]

> Just to add to what Arfa said, the supply to the PWM chip often starts
> with a simple resistor feed from the +170V rectfied mains voltage, and
> is then augmented by a regenerated supply from a tap on the switchmode
> transformer. If this is the case, then there will be a small
> electrolytic filter capacitor (~10uF) that often develops high ESR.
>
> - Franc Zabkar

Hi Franc,

I found the 10uf cap you mention and measured the ESR. It was very
high (around 50), so i replaced it with another cap i had around. it
is not a low esr cap and only an 85 deg one, but figured it would be
ok for testing. it measures about 4.5 on the esr meter which is still
a little high (should be closer to 2). I still get the same result
when its hooked up.

-R

[sci.electronics.repair]

> try to check the main capacitor with an esr meter, or change it !
> build yourself easy an esr meter grab any you want on my web site below
> regards,

The main filter cap (68uf 400v) tests ok for ESR.

-R

[sci.electronics.repair]

I actually have one, but didn't have it with me the other day. I have
it now and have gone through all the caps. The only one that measured
bad was the 10uf 16v cap that Franc mentioned above. I changed it with
another cap and it didn't appear to fix the problem.

[Jeff Liebermann]

On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards

<rnew...@gmail.com> wrote:

>I am trying to troubleshoot a small SMPS that came from an A/V
>switcher.

A photo of the PS would be helpful. Mostly, I'm looking for what
manner of regulator chip is being used and whether there's a
transformer or optoisolator involved.

>Input is 120vac and it is supposed to output + and - 15vdc
>at .8A. Currently it is outputting +17 on one output and somewhere
>between +22 and +30 on the other output.

If you have +30VDC on an output circuit that is suppose to only
tolerate +15VDC, it's possible that the output caps are now dead,
especially if this PS has been run for quite some time. The output
caps are probably rated at 25WV, which doesn't apprecitate the 30V (or
more if there are voltage spikes on the output).

>My experience with SMPS has
>usually been shorted rectifiers or bad output filter caps

Yeah, that's the usual problem.

>so the first
>thing I did was check all the diodes and replace the output caps.
>Obviously that didn't fix the problem. The high voltage is about
>170vdc. The supply to the pwm chip is fluctuating between 7-15v,

Fluctuations like that are sometimes arcing in the xformer or spurious
oscillations in the regulator chip. High output voltage usually means
a failure in whatever is regulating or sensing the output voltage. As
others have suggested, get an isolation xformer before you kill
yourself. Then, attach a scope probe to the output lines, regulator
pins, and optoisolator, and see what's causing the fluctuations.
That's not normal.

--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[Jim Yanik]

Jeff Liebermann <je...@cruzio.com> wrote in
news:p8mm77dur3hb6af5n...@4ax.com:

determine what output rail the supply regulates from.
then check it's filter caps for ESR.

I've seen where the filter cap(s) on that rail had a high ESR,and the PWM
control tried to drive it back to the nominal voltage,and the OTHER output
rails climbed high because of that. And that caused high dissipation in
other circuits that eventually burned a hole in the PCB.

you could also have a bad electrolytic on the reference output on the PWM
chip. or a reference divider resistor changed value.

Find the datasheet for the PWM Ic and learn the inputs and what levels they
should be at,that will tell you how your supply is supposed to work.

--
Jim Yanik
jyanik
at
localnet
dot com

[sci.electronics.repair]

> determine what output rail the supply regulates from.
> then check it's filter caps for ESR.
>
> I've seen where the filter cap(s) on that rail had a high ESR,and the PWM
> control tried to drive it back to the nominal voltage,and the OTHER output
> rails climbed high because of that. And that caused high dissipation in
> other circuits that eventually burned a hole in the PCB.
>
> you could also have a bad electrolytic on the reference output on the PWM
> chip. or a reference divider resistor changed value.
>
> Find the datasheet for the PWM Ic and learn the inputs and what levels they
> should be at,that will tell you how your supply is supposed to work.
>

I've checked all the caps. i replaced all the output caps initially
before I had the ESR meter. Now with the meter i've gone through all
the caps (there are 8 total on this smps) and only one tested bad. It
was the 10uf 16v that Franc mentioned previously. Replacing it didn't
seem to fix the problem. I have the datasheet on the pwm. It is a
UC3842BN I checked the input voltage and it was erratic as mentioned
previously. I checked the two resistors, diode, and filter in the
supply and they all seem ok. Here is a link to some pictures I took
with my phone:

https://picasaweb.google.com/115370641579278903570/SMPS?authuser=0&authkey=Gv1sRgCJ2014unnrOWvgE&feat=directlink

[Arfa Daily]

"sci.electronics.repair" <rnew...@gmail.com> wrote in message
news:26b39f80-37e4-4dd2...@1g2000vbu.googlegroups.com...

A brand new cap might be a little better than that, but certainly right ball
park at 0.5 ohms, and would not be an issue at that figure. About the only
other thing that you could try, assuming that it uses a startup resistor
from the 170v, is to disconnect the self-feed diode from the switching
transformer. That way, you will force it to run from the startup supply
only, just in case the self-run voltage is fluctuating, and interfering with
your reading on the pwm chip's supply. Bear in mind though, that you can't
run it for too long on the startup resistor, as it will get quite hot, being
normally intended to supply current to the chip, only for as long as it
takes the supply to fully start up and settle. If the voltage supply to the
chip still jiggles around when it is only being fed by the resistor, then
after the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa

[Kripton]

I've never seen a chip 3842 dead...
but the small cap 10uF beside it has really often been solving the case !!!!!

[Phil Allison]

<Kripton>

> I've never seen a chip 3842 dead...
> but the small cap 10uF beside it has really often been solving the case
> !!!!!

** I have seen about 5 dead ones.

And I do few SMPS repairs.



.... Phil

[Jim Yanik]

"Arfa Daily" <arfa....@ntlworld.com> wrote in
news:bPQeq.11661$fR1....@newsfe24.ams2:

IIRC,the values on the DSE meter are MAX values,so IMO,if a cap is reading
what's listed,it's marginal.

> About the only other thing that you could try, assuming that it uses a
> startup resistor from the 170v, is to disconnect the self-feed diode
> from the switching transformer. That way, you will force it to run
> from the startup supply only, just in case the self-run voltage is
> fluctuating, and interfering with your reading on the pwm chip's
> supply. Bear in mind though, that you can't run it for too long on the
> startup resistor, as it will get quite hot, being normally intended to
> supply current to the chip, only for as long as it takes the supply to
> fully start up and settle. If the voltage supply to the chip still
> jiggles around when it is only being fed by the resistor, then after
> the 47uF cap, which may yet be faulty, but not in an ESR way, the next
> prime suspect would have to be the chip itself.
>
> Arfa
>
>

usually,the startup resistor doesn't supply enough current to keep the IC
running,just enough to start,that's why they have the housekeeping supply
from the XFMR. Otherwise,you get a "burst" or "chirp" mode of operation;the
PS starts,then dies because the IC isn't getting enough power.
Often,it's the housekeeping supply electrolytic cap that's gone bad,as you
said.

I've also seen where the start resistor goes high in value,even
open,preventing starting.(but not this guy's problem)

[Jim Yanik]

Kripton wrote in news:4e7c30a0$0$16475$426a...@news.free.fr:

>
> I've never seen a chip 3842 dead...
> but the small cap 10uF beside it has really often been solving the
> case !!!!!
>

I've had to replace a lot of 3842's.

it depends on the PS design,some designs leave the IC
vulnerable,particularly if the switcher FET/XSTR blows.

but the 3842 makes a nice PS circuit.

[Arfa Daily]

"Kripton" wrote in message news:4e7c30a0$0$16475$426a...@news.free.fr...

No ? There are a number of chips in the 35 and 38 series pwm controllers,
which are all pretty similar in function, and whilst I'm not certain that I
have specifically had a 3842 faulty, I have certainly replaced many of those
series over the years, usually for a dead symptom, due to the internal
voltage reference having failed. I agree that the caps decoupling the supply
and reference voltages are by far the commonest problem though, and that was
what I first suggested to the OP as a possibility for his problem. The
symptoms that he has, are pretty weird for a switcher. I, like others on
here, would initially have suspected the filter cap for the rail that the
regulation feedback is derived from, but the fact that he replaced all of
the secondary caps right off, seems to knock that one on the head.

Arfa

[Arfa Daily]

>> A brand new cap might be a little better than that, but certainly
>> right ball park at 0.5 ohms, and would not be an issue at that figure.
>
> IIRC,the values on the DSE meter are MAX values,so IMO,if a cap is reading
> what's listed,it's marginal.

"Approx worst case ESR values for new capacitors at 20deg C"

is what it actually says. Bob once told me that when he put that chart on
the front, he took it directly from a manufacturer's data sheet - Pan IIRC

I guess that there are two ways you can take that. I have always taken it on
the positive side, rather than the negative view that you take. As I'm sure
you well know from your own experience, using an ESR meter is a black art
anyway, as much magic as it is science, but given that, in the many years
that I have owned and used daily, a Bob Parker (DSE) meter, I have always
found the chart to be an excellent guide. From many years of experience with
SMPS's, and having found and replaced many pwm chip supply decoupling caps,
I would not feel the immediate need to replace one of that value and voltage
rating, that went 0.5 ohms. I would not consider it 'marginal' and
potentially the source of the problem, until it had got a lot closer to 1
ohm. Typically, caps that have failed in that position, will be up around
the 10 ohm mark, or open circuit.

>
>> About the only other thing that you could try, assuming that it uses a
>> startup resistor from the 170v, is to disconnect the self-feed diode
>> from the switching transformer. That way, you will force it to run
>> from the startup supply only, just in case the self-run voltage is
>> fluctuating, and interfering with your reading on the pwm chip's
>> supply. Bear in mind though, that you can't run it for too long on the
>> startup resistor, as it will get quite hot, being normally intended to
>> supply current to the chip, only for as long as it takes the supply to
>> fully start up and settle. If the voltage supply to the chip still
>> jiggles around when it is only being fed by the resistor, then after
>> the 47uF cap, which may yet be faulty, but not in an ESR way, the next
>> prime suspect would have to be the chip itself.
>>
>> Arfa
>>
>>
>
> usually,the startup resistor doesn't supply enough current to keep the IC
> running,just enough to start,that's why they have the housekeeping supply
> from the XFMR. Otherwise,you get a "burst" or "chirp" mode of
> operation;the
> PS starts,then dies because the IC isn't getting enough power.
> Often,it's the housekeeping supply electrolytic cap that's gone bad,as you
> said.
>
> I've also seen where the start resistor goes high in value,even
> open,preventing starting.(but not this guy's problem)
>
> --
> Jim Yanik

Yes, agreed, but you've got to admit that the supply to the chip fluctuating
between 7 and 15 volts is a pretty bizarre state of affairs, and there's not
actually too much bar the startup R and it's decoupling cap, connected to
that pin. Of course, the self-powering supply, also connected to this point,
might be interfering with the reading, which is why I suggested
disconnecting it. As to whether the supply will run in this condition,
depends on the value of the startup resistor, but running or not, it might
at least give an indication of where next to look, depending on whether the
voltage on the chip side of the resistor is now steady, indicating that the
fluctuating voltage is coming back via the self-powering supply, and in
which case is a red herring, or still varying as a result of the chip
drawing current erratically. I might have felt inclined to think that the
fluctuation was the startup bleed via the startup resistor at 7 volts,
followed by the self-power coming on line at 15 volts as the supply gets
going, followed by the supply shutting back down and so on, due to some
secondary side problem. However, that then doesn't stack up with the
secondary side voltages being steady, but wrong, and worse than wrong,
unbalanced.

It is an odd state of affairs to be sure, but of course, as we all know,
some problems on switchers can be very obscure.

Arfa

[Jamie]

Talking about switching supplies. Today I decided to look at a supply
removed from an allen Bradley Panel computer, this is the second time
the supply has quick, we were able to buy exact replacement supplies so it
was cheaper to do that at the time.

This time I decided to investigate the issue on one of the broken
supplies. I found the main cap on the HV side to be shorted, it was
something like 230Uf 450VDC. This cap did not cause any components to
open that I could see other than a varistor in series to it for the
inrush currents. Any way, I could not get that damn cap out of the
board, I don't know what type of solder they used on that cap but the
only thing I had that would melt the solder was a 140 watt soldering
pistol, and that didn't do me a lot of good since I needed to get to the
other side of it.

My regular station set to 850F would not melt this solder but items
around it had no issues breaking down with that temp.. I guess If I
really want to fix this thing, I will have to use my air iron and some
added heat with another iron on the area to blow the solder out the back
side.

Oh well.

Jamie

[Archon]

I find adding fresh solder to difficult melts and sucking it out
repetitively eventually will fill the joint with regular solder. Of
course if theres a heavy plane of copper, its a problem.
JC

[Franc Zabkar]

On Fri, 23 Sep 2011 16:52:09 +0100, "Arfa Daily"
<arfa....@ntlworld.com> put finger to keyboard and composed:

>However, that then doesn't stack up with the
>secondary side voltages being steady, but wrong, and worse than wrong,
>unbalanced.

As has already been mentioned, there could be something wrong with the
OP's measurements. He states that "currently it is outputting +17 on

one output and somewhere between +22 and +30 on the other output."

One would expect that the +/-15V supplies would have identical
windings. Moreover, since they would be wound over the same core, then
one would expect that the two outputs should track each other
reasonably well, allowing for slight load differences.

Maybe there is a clue in the fact that the OP's measurements are both
positive. Could it be that he is uing the wrong 0V reference?

For example, if he is mistakenly using the -15V rail as the 0V
reference, then he would be measuring +15V for ground, and +30V for
the +15V rail, if the supply were working correctly.

[Arfa Daily]

"Franc Zabkar" <fza...@iinternode.on.net> wrote in message
news:obeq77t7ek0ovdfbs...@4ax.com...

Hmmm. That's a very valid point, Franc. Although possibly not specifically
in switchmode power supplies, I got the impression that the OP was
reasonably savvy in electronic repairs in general, so did not even consider
that the measurement technique could be in error. To be honest, I never even
noticed that he was declaring both to be positive ... :-\

Arfa

[Geo]

On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
<rnew...@gmail.com> wrote:

>I am trying to troubleshoot a small SMPS that came from an A/V
>switcher. Input is 120vac and it is supposed to output + and - 15vdc
>at .8A. Currently it is outputting +17 on one output and somewhere
>between +22 and +30 on the other output. My experience with SMPS has
>usually been shorted rectifiers or bad output filter caps so the first
>thing I did was check all the diodes and replace the output caps.
>Obviously that didn't fix the problem. The high voltage is about
>170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
>i'm thinking this might be the problem, but maybe its something else.
>In case its not obvious, i'm fairly new to tinkering with these
>things. Thanks in advance for any help.
>

I don't see that a fluctuating supply would cause a polarity reversal
on the -15v rail.

[Franc Zabkar]

On Fri, 23 Sep 2011 01:56:37 +0100, "Arfa Daily"

<arfa....@ntlworld.com> put finger to keyboard and composed:

>About the only
>other thing that you could try, assuming that it uses a startup resistor
>from the 170v, is to disconnect the self-feed diode from the switching
>transformer. That way, you will force it to run from the startup supply
>only, just in case the self-run voltage is fluctuating, and interfering with
>your reading on the pwm chip's supply. Bear in mind though, that you can't
>run it for too long on the startup resistor, as it will get quite hot, being
>normally intended to supply current to the chip, only for as long as it
>takes the supply to fully start up and settle.

If the feed is coming directly from the +170V rail, then I can't see
that there will be much difference in the power dissipation of the
startup resistor after the power supply has settled into run mode.

For example, if we assume that the chip's supply voltages are 5V at
startup and 15V in run mode, the the voltage across the resistor will
vary between 165V and 155V.

[Arfa Daily]

"Franc Zabkar" <fza...@iinternode.on.net> wrote in message

news:ma7s775bkq5h737pg...@4ax.com...

Typically, the startup supply doesn't come *directly* from the high voltage
rail, It is normally fed via a diode, and the self-powering rail is arranged
to be a higher voltage than the startup supply. This results in the diode
feeding the startup supply, to become reverse biased once the supply fully
starts, which then results in the current draw from that supply reducing to
as good as zero, and thus likewise dropping the power dissipation in the
startup resistor to virtually zero.

Arfa

[Arfa Daily]

"Geo" <nhhu...@dea.spamcon.org> wrote in message
news:vp2s77dq27rf3h4is...@4ax.com...

No, you're right, it wouldn't, which is why Franc and I elsewhere in the
thread, have now entered into the discussion of how exactly the OP is taking
his measurements i.e. what point he is using as his ground, for a start.

Arfa

[Franc Zabkar]

On Sun, 25 Sep 2011 01:29:05 +0100, "Arfa Daily"

<arfa....@ntlworld.com> put finger to keyboard and composed:

>Typically, the startup supply doesn't come *directly* from the high voltage
>rail, It is normally fed via a diode, and the self-powering rail is arranged
>to be a higher voltage than the startup supply. This results in the diode
>feeding the startup supply, to become reverse biased once the supply fully
>starts, which then results in the current draw from that supply reducing to
>as good as zero, and thus likewise dropping the power dissipation in the
>startup resistor to virtually zero.
>
>Arfa

Your comment took me by surprise. Just to make sure I wasn't having a
brain fart, I consulted an application note:
http://www.nalanda.nitc.ac.in/industry/AppNotes/Unitrode/slua143.pdf

I confess that I haven't really thought about this subject before, but
according to page 4 of the PDF, there is no back-biased diode. AISI, a
diode that is fed from a 170VDC supply cannot become reverse biased by
a much lower bootstrap voltage.

The app note suggests that the UC3842 will remain in the off state
until the capacitor on its Vcc pin charges up to the UVLO (under
voltage lockout) turn-on voltage of 16V. During this time the IC draws
only 1mA.

After the UVLO turn-on threshold is reached, the IC turns on and
pulses the chopper. The bootstrap winding then generates the Vcc
supply for the IC and prevents the capacitor from discharging below
the IC's UVLO turn-off threshold of 10V.

Fig 31 on page 13 of the same document has an application circuit for
an isolated +/-12V supply. The 56K resistor (R2) can pass about 3mA.
This means that 2mA goes toward charging the capacitor. The UC3842
datasheet specifies a typical operating current of 14mA.

http://www.elektronik.sk/datasheet/UC3842.pdf

If you were to disconnect the bootstrap winding, then the output
voltage will only remain alive for as long as it takes the Vcc
capacitor to discharge from 16V to 10V.

I = C . dV/dt

so ...

dt = C x dV / I

= 100uF x (16V - 10V) / (14 - 3)mA
= 55 msec

[Arfa Daily]

"Franc Zabkar" <fza...@iinternode.on.net> wrote in message

news:ehet77tj8mdnnuam8...@4ax.com...

Hmmm. OK. I'll buy that. I must admit that I haven't taken too much notice
of how that bit of the circuitry works in recent years. Perhaps I'm just
going back to the early days when I was first taught about these things on
manufacturers' service courses. Maybe there used to be a zener on the end of
the startup resistor, and then a diode. The diode could then be reverse
biased by the self-powering supply, and the current in the startup resistor
would decrease to that of the zener's draw only. I'll have to have a look
back on some of the older schematics for VCRs and DVDs etc

Arfa