Switch mode power supplies

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I have a dead colour TV that I'm finally having a go at fixing - no
expert on these things though.

The chopper transistor on the switch mode supply has shorted out.
Replaced it and same again (very quickly fails), so there is a problem
somewhere.  I have a couple of questions:

1. Anyone know a good online description, in detail, of the operation
of these supplies.  For example, the feedback from the output side of
the supply that apparently controls the chopper transistor is through a
single four pin IC that I can't identify.  I suspect it may be an
optoisolator but not certain.  It carries markings OA . P621 . 4 G.,
The manufacturer logo looks like a "T", but don't recognise it (its not
an SGS Thomson logo).

2.  I can think of at least two possible generic causes of the failure:
    a) Failure somewhere in the actual TV circuit that is causing
excessive current drain and overloading the power supply, eg horizontal
output transistor (but this looks OK).
    b) Failure in the switchmode supply itself such that the chopper
transistor is forced permanently on, or is being driven at a much
higher duty cycle than it can handle.  Are there any other
possibilities, and testing strategies?

Cheers
Glenn


Re: Switch mode power supplies



----- Original Message -----
Newsgroups: aus.electronics
Sent: Thursday, August 24, 2006 8:57 PM
Subject: Switch mode power supplies


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Probably a Toshiba TLP621 opto isolator.
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Usually when the transistor goes short something else is gone, too.  Cause
and effect are usually hard to separate.  What else is gone depends on the
topology of the supply.  Is there a shottky diode that  may have upgraded
itself from semiconductor to full conductor, perhaps?

Cheers,
Alf



Re: Switch mode power supplies



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Its always worth checking if there's a regulation feedback optocoupler
(check for faulty or dry joints) often there's an error voltage smoothing
electrolytic - usually on the primary side and also smooths the control
chip's running supply (dried out or dry jointed. Shorted secondary diodes
also cause the chopper Tr to blow at switch on.



Re: Switch mode power supplies


Thanks for the tips.  The supply is all discrete components, no IC
(except for the suspected optocoupler, which is providing a feedback
signal to the primary side.  I've checked the two electros on the
primary side, and the main smoothing capacitor, but they look OK.  Also
done a look around and resolder for dry joints.  I'm attempting to
check every component on the primary side - so far, all are showing up
fine (except for the chopper).

Unfortunately, I have no circuit diagram so the job is a bit more of a
challenge.

Cheers
Glenn


ian field wrote:

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Re: Switch mode power supplies



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Possibly the 2 most common causes of SMPSU failure are dry joints and
electrolytics gone high ESR, its no good just measuring the capacitance as
this will often read acceptable within limits on a faulty cap. Dick Smith do
a good ESR meter.



Re: Switch mode power supplies



"ian field"  wrote

Possibly the 2 most common causes of SMPSU failure are dry joints and
electrolytics gone high ESR, its no good just measuring the capacitance as
this will often read acceptable within limits on a faulty cap.



***** Ian,your good advice is certainly wasted if we are to believe the OP:-

  "I've checked the two electros on the primary side, and the main smoothing
capacitor, but they look OK".


You understand,they must be in good working order because they LOOK OK!

Brian Goldsmith.


Re: Switch mode power supplies



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Very often the physical appearance of an electrolytic cap saves me the
bother of switching on the ESR meter - bulged top or shrivelled plastic
sleeve and out it comes, but I certainly agree not to rely on "they look
OK"!!!



Re: Switch mode power supplies


Thanks for all the posts and tips.  I need to be more careful how I put
things. "look OK" is my way of saying "tests OK", although that is a
basic test since I don't have an ESR meter.  Anyway, I have replaced
them just in case.  On more digging, I've found a couple of suspect low
power transistors that look like they are in the circuit driving the
base of the chopper.  I'll let you know if I have success.

The set doesn't owe me anything and would otherwise be carted off to
the tip.  So I don't have much too lose except a few hours of mucking
around.

Cheers
Glenn


ian field wrote:
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Re: Switch mode power supplies



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I also had no circuit diagram for the Zebra-Eltron PSU I mentioned in
my other post. Luckily it was fairly easy to work out the schematic so
I do now have a circuit.

I find the easiest way to do the shematic is to divide the schematic
into 3 functional blocks and draw each out on a separate piece of
paper. The first block I do, for example, is the secondary low voltage
side. Nxxt I do the primary high voltage side and lastly the mains
input rectifier part. Many PSU's are hard to work out because of the
number of heatsinks and or the desire to achieve highest packing
density and the only way to work out the schematic for these is to
remove the components which prevent you from reading values etc. An
arduous task but if you plan to work on the same type again the work
involved is invaluable.

Re: Switch mode power supplies



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I was asked last Friday if I could fix a single output 24V, 60W SMPS
for a Zebra-Eltron P310C plastic ID card printer. The local support
company wanted a mere $AU450 for a new one shipped to my door.

There appeared to be drastic failure of a number of components (turned
out that I had to replace 13 componets in all) with burnt resistors,
shattered output rectifier, shorted and cracked transorb, dried out
electro caps, leaky 1N4148, dead 3842 PWM chip, and of course the
power FET (STW13NB60A a TO247 device). There was nothing wrong with
the opto-coupler or the TL431 feedback regulator but the resistor
which determines the current through the opto diode was burnt so that
the value was unrecognisable. A bit of intuitive guesstimation showed
that 1.8K was about right and this proved to be so under load testing.

I couldn't see the need for the original specified 13A 600V FET for a
nominal 60W PSU because other similar supplies I had worked on used no
more than an 8A device. Anyway RS Components only had a couple of near
equivalents for the STW13NB60A at between $AU43 and $AU52 each - far
too much. I substituted a IRFIB6N60A, a TO220 device rated at 5.5A
600V. Under load testing I had no trouble getting 3.2A at 24 volts
(better than 72W) and the FET heatsink only got moderately warm. The
substitute FET was much cheaper at around $AU8.

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