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Switchmode transformer questions

A

Andrew VK3BFA

Jan 1, 1970
0
I am trying to recycle old computer power supplies, so need to rewind
them - and thanks to the people who responded with suggestions for
getting the ferrite core apart - mixed success, will file a report
when a batch of Toluene and MEK gets picked up tomorrow.

Started unwinding them, and got perplexed. And not being able to post
a diagram, bear with me.

The main primary winding seems to be centre tapped, but the centre tap
is not connected to any external components. On top of this are the
secondary windings, not a problem except for nutty turns ratios, vis
the 5v winding is 3 turns each side of Centre tape, the 12v winding is
4 turns each side of centre tap - doesnt make sense!

Another layer of tape, then a single turn of copper foil, one end
connected to primary winding. Another layer of tape, and then a 20
turn single layer (covers the bobbin area) of enamelled wire, one end
connected to the same end of the prinary as the copper foil, the other
end connected to the centre tap.

I can guess the copper foil is a shield to stop it radiating RFI, but
what does the top layer do? - and more to the point, if I rewind this
transformer in a conventional manner without it, will it affect
operation?

Any comments appreciated.

Andrew VK3BFA
 
R

R.Legg

Jan 1, 1970
0
The main primary winding seems to be centre tapped, but the centre tap
is not connected to any external components.

This is a split primary. The two sections are likely connected in
parallel, not in series. You must record the winding direction to
establish phase relationships of any layer. You must record wire guage
and insulation type in order to replace it. Inter-pin connections
made on the printed circuit board will reveal connections in the
functioning circuit.
secondary windings, not a problem except for nutty turns ratios, vis
the 5v winding is 3 turns each side of Centre tape, the 12v winding is
4 turns each side of centre tap - doesnt make sense!

The 12V windings are likely wound to connect in series with the 5V
winding, or the 12V output is stacked onto the 5V output, after
filtering.

The turns ratio is therefore 5V-3T 12V-7T. This is quite typical.
Consult the printed wiring.
Another layer of tape, then a single turn of copper foil, one end
connected to primary winding.

If you are attempting to duplicate this part, you will have to note;
the numbers of insulation layers, the material grade and type of
insulating 'tape', the dimension of margins present (to the bobbin
edge on each conductor layer), the method of maintaining the margin
that is being employed and any lead dressing and lead insulator type
used on all lead exits.
Another layer of tape, and then a 20
turn single layer (covers the bobbin area) of enamelled wire, one end
connected to the same end of the prinary as the copper foil, the other
end connected to the centre tap.

Again - dimensions, positioning and material thickness are relevent.

The screen is primary-connected to the quiet end or equipotential
point of the primary winding.

I can guess the copper foil is a shield to stop it radiating RFI, but
what does the top layer do? - and more to the point, if I rewind this
transformer in a conventional manner without it, will it affect
operation?

See separately posted comments re 'rewinding' an isolation
transformer. Guessing isn't good enough.

RL
 
A

Andrew VK3BFA

Jan 1, 1970
0
This is a split primary. The two sections are likely connected in
parallel, not in series. You must record the winding direction to
establish phase relationships of any layer. You must record wire guage
and insulation type in order to replace it. Inter-pin connections
made on the printed circuit board will reveal connections in the
functioning circuit.


The 12V windings are likely wound to connect in series with the 5V
winding, or the 12V output is stacked onto the 5V output, after
filtering.

The turns ratio is therefore 5V-3T 12V-7T. This is quite typical.
Consult the printed wiring.


If you are attempting to duplicate this part, you will have to note;
the numbers of insulation layers, the material grade and type of
insulating 'tape', the dimension of margins present (to the bobbin
edge on each conductor layer), the method of maintaining the margin
that is being employed and any lead dressing and lead insulator type
used on all lead exits.


Again - dimensions, positioning and material thickness are relevent.

The screen is primary-connected to the quiet end or equipotential
point of the primary winding.



See separately posted comments re 'rewinding' an isolation
transformer. Guessing isn't good enough.

RL


Mmm, thank you for your reply - will have to think about what you are
saying and try to make sense of it. But I would like to make a few
points in return.

I will dismantle another one, paying careful attention to phasing, no
of turns, etc. and let you have that info.

Are hf switchmode transformers inherently different from mains
frequency ones - I have wound heaps of the latter, all tested to
greater than 2.5KV isolation between windings. I use proper insulation
materials, between windings,between winding layers if HV, and more
between primary and secondary. I will not use the transformers as is,
because the current is inadequate due to conductor size. If I need to
know the correct conductor size, I look it up in a textbook. To get
the transformer ferrite bobbins apart, I soak them in toxic chemicals
- what that does to the original insulation on the winding wire, and
the insulation between windings, I do not know, but am not preapred to
take a punt on it.

I will readily admit I cant do the maths on this subject, I am a
working tech not an engineer. But neither am I a complete idiot, I
would not still be alive if I was, I have a healthy respect for high
voltage and current.

Engineering should not be a "black art" - most things can be reduced
down to a few simple rules in application (design is totally different
- I am not trying to design the thing from scratch.) And as a hobbyist
I have been "stuffing around" all my life - some noteable success,
some spectacular failures.

You have given me reasons not to do this, I want some on HOW I can do
this.....

Andrew VK3BFA
 
K

Klaus Vestergaard Kragelund

Jan 1, 1970
0
Andrew VK3BFA said:
[email protected] (R.Legg) wrote in message


Mmm, thank you for your reply - will have to think about what you are
saying and try to make sense of it. But I would like to make a few
points in return.

I will dismantle another one, paying careful attention to phasing, no
of turns, etc. and let you have that info.

Are hf switchmode transformers inherently different from mains
frequency ones - I have wound heaps of the latter, all tested to
greater than 2.5KV isolation between windings.

Yes in a number of ways. First off, if this is a flyback trafo (coupled
inductor) there will most likely be an airgap. You need to measure the
inductance of a winding with a known no. of turns to find the AL value of
the xfrmr.

Moreover at HF switching, the way the xfrmr is wound is critical. If you
wind it wrongly you may wind the hot (switching) node close to the
secondaries effectively coupling lots of current to the secondary via
capacitive parasitics. All sorts of other effects are important, leakage
inductance, skindepth, proximity of layers.......

Cheers

Klaus
 
R

R.Legg

Jan 1, 1970
0
I will not use the transformers as is,
because the current is inadequate due to conductor size. If I need to
know the correct conductor size, I look it up in a textbook.

Be sure you're reading a text book that is applicable to the type of
application, ie HF switch-mode supplies. Good app notes were available
from the old Unitrode seminars, maintained on the web by TI. These
have recently been shifted so that old links no longer work. The new
links are:

http://focus.ti.com/docs/training/c...egory.jhtml?templateId=5517&navigationId=8455

Those specific to HF magnetics design:

http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014
To get
the transformer ferrite bobbins apart, I soak them in toxic chemicals
- what that does to the original insulation on the winding wire, and
the insulation between windings, I do not know, but am not preapred to
take a punt on it.

The bobbin, or ground insulation might be seriously effected also by
this method. The bobbin can only be recovered if it's reflow
temperature is higher than that of adhesives and varnishes employed in
the original insulation system - the application of simultaneous heat
and mechanical force being used to remove the cores. Some insulation
systems do not allow recovery of the ground insulation, under any
conditions, after thermosetting has occured once in the original
fabrication step.
I will readily admit I cant do the maths on this subject, I am a
working tech not an engineer. But neither am I a complete idiot, I
would not still be alive if I was, I have a healthy respect for high
voltage and current.

You won't know what you're capable of doing until you know what is
required of you. This is not neccessarily a physical constraint. The
previously listed tutorials give a good practical outline.

This area of electronics has been, up until recently, one which
required a considerable amount of design iteration, based on practical
results, and was an excellent field for technicians with practical
skills to excel in. Text-book engineering and accurate commercial
modelling was extremely slow on the uptake.

I am not an engineer. I have no interest in discussing the pros and
cons of formal education in this forum, and will not do so.
Engineering should not be a "black art" - most things can be reduced
down to a few simple rules in application (design is totally different
- I am not trying to design the thing from scratch.) And as a hobbyist
I have been "stuffing around" all my life - some noteable success,
some spectacular failures.

There are certain road blocks to the generation of safety-critical
circuits in a hobby environment, that have nothing to do with
practical common sense. If you own the facility in which these parts
are being employed (fire) and restrict their use to applications where
only knowledgable consenting individuals are at risk (a laboratory
environment), then there are potentially reduced liability
implications.

Your work could be used as a model to produce original, safe,
component designs, but should never be represented as such, until this
is proven. Recyling materials commercially is an art in itself, and
quite possibly may never be practical in situations involving
third-party liability for health and safety.
You have given me reasons not to do this, I want some on HOW I can do
this.....

I hope I haven't come across as being too negative. Re-using materials
is an obvious way to obtain the unobtainable quickly, in independant
small-scale product development or modification. I do it all the time.

I recall only one situation, where new materials were not available on
a critical commercial delivery schedule, that the recovery of core
materials (only) from scrapped parts (that were responsible for the
shortage)was justified. In this case intimate knowledge of the
product, its components, and the physical implications of the specific
recovery and re-use was available. Only the cores were recycled - all
other materials remained as scrap.

These parts were employed with the strictest controls possible, to
ensure that no damage resulted from the increased fab steps to which
they were being subjected. The costs involved were obviously outside
normal budgeted expense, but were less than potential penalties of
breaking contractual obligations of a delivery schedule.

This situation was avoided subsequently by the adoption of more
responsive material supply and handling procedures, and by moving the
individual responsible for the original error temporarily to another,
less demanding, occupation, while the causes for the error were
investigated and prophylactic solutions were developed and put into
place.

RL
 
L

legg

Jan 1, 1970
0
If I duplicate the existing primariy windings (no turns/wire
size/location on bobbin) and rewind the secondary to get more current,
it should work - or have I got that wrong. As a hobbyist, I tend to
err on the side of caution, and use more/better insulation than the
original, so that "should" be OK as well.

Depending on what the current is you're trying to get out of what
layer, there is usually room for improvement in a single hand-wound
part. A production device is usually underfilled intentionally, to
avoid machine-winding difficulties that could easily be averted by an
attentive winder.

The real difficulty is improving the current capacity of the primary,
if the actual output power is expected to be increased. There are
natural plateaus that occur for any winding breadth, into which just
so many turns of a certain guage of wire will fit.

Once wound, check that leakage inductance hasn't increased
dramatically (primary inductance with secondary shorted) and that
magnetizing inductance remains high (primary inductance secondary open
- checks for clean core mating and internal shorts). Primary
inductance should not fall dramatically when the part is installed
into the printed wiring (indicates phasing or termination errors)

If the total power is to increase, ensure that secondary snubbers,
primary coupling capacitors(half bridge) , voltage snubber (single
ended forward) , freewheeling diodes (two transistor forward) and
current limiting circuitry are up to it. Most other overstresses are
temperature detectable.
I have worked out the mods to the TL494 regulator thingo by just using
the existing windings on one supply and transfering the voltage
sensing etc functions to the 12v winding instead of the original 5v
winding.

A good start if you are aiming at a 12V regulated output. You may find
the output to be overcompensated and sluggish if the same frequency
compensation parts are used. Some simpler optocoupled feedback
circuits rely on the relationship between the opto led emitter
position - being powered off the 5V output - to provide inherently
fast response to 5V load changes. A TL431 is often found there.
Shifting this could produce unpredictable changes, and possibly even
affect the circuit's ability to start.

You made no mention of a housekeeping power winding on the
transformer. It may be important to determine what start-up power is
expected in the control sections and when, so that your mods do not
interfere with generation of this critical feature and prevent
start-up. It's been a long time since I've seen a commercial supply
with an auxiliary 50/60Hz transformer used for this.
Other people in response have mentioned the air gap on the E-I cores -
as far as I can tell from a very detailed examination of the cores,
there is none in this particular case - I can tell by looking at the
glue marks after the core is dismanted - the mating faces of the core
are clean.

No gap is needed in a forward or half bridge circuit, which are the
topologies most likely used above 75W.
Anyway, the next step is to get some wire to rewind the little buggers
- dont want to re-use the original. Your point about not knowing if
the dismantling process has caused problems with the bobbin is well
made, all I can go on is they "look" ok - no sign of any mechanical
deformation.

No deformation, discoloration or cracking are fair signs of integrity.
Can you suggest a test procedure when I have the thing up and running
to see if there are any lurking long term problems (short term test is
"Hey, it didnt catch fire/fuse the mains/explode loudly/ and the volts
are right..." medium term is "...its not too hot to touch...")

Transient load testing will reveal a multitude of sins.

It might make sense to test an unmodified unit first, so that you get
a fair idea of what is intended to BE normal, and what protection
features ARE present that might be affected by the new configuration.

Protection functions are not always provided by dedicated identifiable
circuitry, but may be acieved only through synergistic accident, in
the original circuit configuration. This may be hard to maintain or
avoid. ie overload or short circuit behaviour.

RL
 
Y

Yzordderex

Jan 1, 1970
0
Andrew,

Don't know what tips others offered, new to board.

Try using heat to seperate xfmrs. I usually heat up part around 250f
for about 1/2 hour. Gently pry apart with a couple pair channel lock
or large pliers.
Keyword: Gentle

Hint is courtesy Phil Barbara - Formally of Renco Transformer. Don't
know where he is now.

Top layer may be feedback winding. FB winding usually outermost.

73
Bob
 
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