flybacks & transformer leakage

[Adam. Seychell]

I'm building a 25W off line flyback SMPS, and wondering what are typical
leakage inductances for the gapped ferrite E core transformer. I'm
winding the secondary sandwiched between a split primary and getting
about 2.0% leakage. Without interleaved windings then leakage inductance
gets too embarrassing to mention.

Another question: Is flyback my best choice.

Can I ask why flyback is common for < 50W power levels ? Sure, flyback
converters eliminate the largish output inductors and use a single
switch. However its simplicity seems superficial because the designer is
challenged with high peak currents, EMI/ripple filtering, and magnetics
design.

The forward converter reduces these high peak currents but requires
output inductors. Energy stored in the transformer leakage inductance
still gets wasted.

Ok, so leakage inductance energy can be utilized with the two switch
forward converter, and additionally halving the peak voltages across the
switches. But this topology adds one gate drive transformer to the system.

With the need for a gate drive transformer, then seems almost no more
effort to go for half bridge topology and gain the benefits of even
better transformer core utilization. Comparing the half bridge to the
flyback converter in the 25W .. 100W range;

Flyback disadvantages:
* high peak currents and ripple
* large ferrite core with air gap
* high switch peak voltages

Full bridge disadvantages:
* needs a gate drive transformer
* needs output inductor
* an extra FET switch
* slightly more complex control circuitry


When does one topology over the other get more economical/easier to build ?







The half bridge seems simpler.
Adam

 

[Pooh Bear]

"Adam. Seychell" wrote:

I'm building a 25W off line flyback SMPS, and wondering what are typical
leakage inductances for the gapped ferrite E core transformer. I'm
winding the secondary sandwiched between a split primary and getting
about 2.0% leakage. Without interleaved windings then leakage inductance
gets too embarrassing to mention.

2% is just sweet. My current design has 3% IIRC ( non interleaved though ).

Another question: Is flyback my best choice.

At that power level almost certainly.

Can I ask why flyback is common for < 50W power levels ? Sure, flyback
converters eliminate the largish output inductors and use a single
switch. However its simplicity seems superficial because the designer is
challenged with high peak currents, EMI/ripple filtering, and magnetics
design.

It all comes out in the wash. It's a nice simple circuit with few components.

The forward converter reduces these high peak currents but requires
output inductors. Energy stored in the transformer leakage inductance
still gets wasted.

Ok, so leakage inductance energy can be utilized with the two switch
forward converter, and additionally halving the peak voltages across the
switches. But this topology adds one gate drive transformer to the system.

With the need for a gate drive transformer, then seems almost no more
effort to go for half bridge topology and gain the benefits of even
better transformer core utilization. Comparing the half bridge to the
flyback converter in the 25W .. 100W range;

I'm currently working on a half bridge forward converter for ~ 3kW.

So many more bits !

Flyback disadvantages:
* high peak currents and ripple
* large ferrite core with air gap
* high switch peak voltages

Full bridge disadvantages:
* needs a gate drive transformer
* needs output inductor
* an extra FET switch
* slightly more complex control circuitry

When does one topology over the other get more economical/easier to build ?

Well, actually you normally go flyback, single transistor forward, half bridge
forward, full bridge forward as the power goes up.

Flyback's ok up to a couple of hundred watts.

Graham

 

[Adam. Seychell]

Pooh Bear wrote:

2% is just sweet. My current design has 3% IIRC ( non interleaved though ).



At that power level almost certainly.

Thanks for the reassurance. When I was doing calculations for the RDC
snubber in order to clamp peak MOSFET Vds to 700V I get 1.2W dissipation
in the resistor ! Is this typical for 25W offline supply ?

How about the "two switch" discontinuous flyback ?
http://www.edn.com/contents/images/47173.pdf

 

[Pooh Bear]

"Adam. Seychell" wrote:

Pooh Bear wrote:


2% is just sweet. My current design has 3% IIRC ( non interleaved though ).



At that power level almost certainly.


Thanks for the reassurance. When I was doing calculations for the RDC
snubber in order to clamp peak MOSFET Vds to 700V I get 1.2W dissipation
in the resistor ! Is this typical for 25W offline supply ?

Not unsurprising.

Have you looked at a 200V TVS ? E.g. P6KE200. Gives an assured clamping voltage.

http://uk.farnell.com/jsp/endecaSearch/searchPage2.jsp?Ntk=gensearch&comSearch=true&select1=208174&paramSelectCount=5&showImages=true&st=parametricSelection&N=1005247+401&Ntt=p6ke&comSearch=true&Nto=tvs+&newSearch=true&paramSearch=true&st=parametricSelection&x=29&y=14

How about the "two switch" discontinuous flyback ?
http://www.edn.com/contents/images/47173.pdf

Too complex for 25W !


Graham

 

[Genome]

<cbarn24050@aol.com> wrote in message
news:1116318295.697854.51090@g49g2000cwa.googlegroups.com...

Hi Adam, leakage is relatively high in flybacks because the windings
conduct at different periods, this means that the fields of the 2
windings cannot cancel each other therefore split sandwiching doesn,t
help very much unlike in a forward transformer.

Ahem... Sandwiching the windings in a flyback converter has the same effect
on leakage inductance as it does in a forward converter.....

But, since as you say the windings do not conduct at the same time, it does
not halve the number of effective winding layers used to calculate proximity
effect.

DNA

 

[Pooh Bear]

cbarn24050@aol.com wrote:

< snip >

I gave up making supplies years ago as they are now so cheap, sometimes
cheaper than the cost of the parts, why are you bothering?

a) thay aren't actually cheap when you're talking about 1000's.

b) you can optimise the design for your application - show me any
consumer product that uses an Astec off-the-shelf power supply for example.

Graham

 

[Ken Smith]

In article <428977f4$0$5175$afc38c87@news.optusnet.com.au>,
Adam. Seychell <invald@invalid.com> wrote:

I'm building a 25W off line flyback SMPS, and wondering what are typical
leakage inductances for the gapped ferrite E core transformer. I'm
winding the secondary sandwiched between a split primary and getting
about 2.0% leakage. Without interleaved windings then leakage inductance
gets too embarrassing to mention.

At 5% you should start asking yourself whats wrong.

Another question: Is flyback my best choice.

Flybackers have lower parts count in total and less of the costly parts.

The flyback design primary side can be easily protected from shorts at
load because the short circuit protection doesn't have to act as quickly.

The forward converter reduces these high peak currents but requires
output inductors. Energy stored in the transformer leakage inductance
still gets wasted.

This doesn't have to be true. The snubbing and resetting circuits can
return a goodly portion of the leakage inductance energy back onto the
input capacitors.

Ok, so leakage inductance energy can be utilized with the two switch
forward converter, and additionally halving the peak voltages across the
switches. But this topology adds one gate drive transformer to the system.

You don't have to use a gate drive transformer at all if the smarts are on
the primary side and the feedback is either brought over by optoisolators
or you just sense the extra winding's voltage and lose some regulation.



--
--
kensmith@rahul.net forging knowledge

 

[analog]

cbarn24050@aol.com wrote:

Hi Adam, leakage is relatively high in flybacks because the windings
conduct at different periods, this means that the fields of the 2
windings cannot cancel each other therefore split sandwiching doesn't
help very much unlike in a forward transformer.

The leakage inductance of a flyback transformer is typically about
the same as that of any other topology's similarly sized transformer.
It is its own deliberately lowered magnetizing inductance (to which
its leakage inductance is proportionally larger) that makes leakage
inductance seem greater than it really is.

A flyback transformer is best thought of as an ideal inductor in
parallel with an ideal transformer (winding resistances and leakage
inductances are added outside this ideal core). The ideal transformer
part behaves like any ideal transformer, where (for two windings) the
windings only conduct simultaneous currents of equal and opposite-
ampere turns.

The ideal inductor part may conduct some ac current, or, if large,
may only conduct substantial dc. As a thought experiment, imagine
an ideal flyback transformer consisting of infinite inductor
(initialized to one amp) in parallel with an ideal 1:1 transformer,
all driven at 50 percent duty cycle. The inductor can pass no ac
and the transformer can pass no net dc. Think about it.

Regards -- analog

 

[Joerg]

Hello Cbarn,

I gave up making supplies years ago as they are now so cheap, sometimes
cheaper than the cost of the parts, why are you bothering?

For series production you can design them a lot cheaper than the modules
cost. I do that all the time.

Even for a onesie I occasionally whip one up, just because I don't have
to wait for the Digikey package to arrive. It also cuts down on the
amount of pet hair that flies around after our dogs cussed out the
delivery driver from behind the door ;-)

Regards, Joerg

http://www.analogconsultants.com

 

[Terry Given]

Paul Mathews wrote:

Another good reason to choose flyback is the easy implementation of
multiple secondaries.
All other things being equal, a good way to further reduce leakage
inductance is to use a core with a longer window and/or some way to
reduce the need for clearance/creepage margins. The overall goal is to
reduce the amount of volume of insulation between windings. Triple
insulated wire, provided that it is the very thin type, is one way to
achieve both of the above. Leakage inductance that is 1% of
magnetizing inductance is achievable using the above techniques.
Paul Mathews

It is also possible to reduce leakage using a pair of "flux cancelling"
windings, each with Nc turns. Wind Nc turns first on bobbin. Then wind
transformer. Then wind Nc turns (so one winding is "inside" the other
"outside"). Connect the windings in parallel. because the inner and
outer windings have different flux linkages due to leakage, current will
flow thereby creating a flux which counters the imbalance.

this trick gets used in HV transformers, where the secondary is a
triangular profile to minimise capacitance (and keep HV the hell away)
and a split primary is generally not used.

I normally do what Paul suggested, and consider 1% leakage to be
acceptable (except in a planar transformer, where its crap). reducing
leakage helps quite a bit with snubber/clamp losses.

Cheers
Terry

 

[Adam. Seychell]

Paul Mathews wrote:

Another good reason to choose flyback is the easy implementation of
multiple secondaries.
All other things being equal, a good way to further reduce leakage
inductance is to use a core with a longer window and/or some way to
reduce the need for clearance/creepage margins. The overall goal is to
reduce the amount of volume of insulation between windings. Triple
insulated wire, provided that it is the very thin type, is one way to
achieve both of the above. Leakage inductance that is 1% of
magnetizing inductance is achievable using the above techniques.
Paul Mathews

I guess I'm just stuck with what I've got, at %2. If I could abandon the
use of safety margins and internal foil shielding it would go a long
way to lowering leakage inductance. The smaller the transformer the
worse things become because clearance margins must remain fixed at 5mm.

This is part of a one off hobby project, and also partly doing it to
learn something. It would be nice to be able to whip one up in an
afternoon, assuming I had PCB and all the generic at hand.
Can you actually buy part assembled DIY offline flyback converter kits ?

Adam

 

[Spehro Pefhany]

On Wed, 18 May 2005 10:08:48 +1000, the renowned "Adam. Seychell"
<invald@invalid.com> wrote:

Paul Mathews wrote:
Another good reason to choose flyback is the easy implementation of
multiple secondaries.
All other things being equal, a good way to further reduce leakage
inductance is to use a core with a longer window and/or some way to
reduce the need for clearance/creepage margins. The overall goal is to
reduce the amount of volume of insulation between windings. Triple
insulated wire, provided that it is the very thin type, is one way to
achieve both of the above. Leakage inductance that is 1% of
magnetizing inductance is achievable using the above techniques.
Paul Mathews


I guess I'm just stuck with what I've got, at %2. If I could abandon the
use of safety margins and internal foil shielding it would go a long
way to lowering leakage inductance. The smaller the transformer the
worse things become because clearance margins must remain fixed at 5mm.

With the triple insulated you can make things a lot more compact and
avoid stuff like having run tape up the inside of the bobbin to keep
the creepage high enough.

This is part of a one off hobby project, and also partly doing it to
learn something. It would be nice to be able to whip one up in an
afternoon, assuming I had PCB and all the generic at hand.
Can you actually buy part assembled DIY offline flyback converter kits ?

Adam

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Pooh Bear]

cbarn24050@aol.com wrote:

quite so poo bear, but is this guy making a consumer product? if so
what the hell is he doeing asking for advice here?

I had to start somewhere too !

Graham

 

[Pooh Bear]

Paul Mathews wrote:

Another good reason to choose flyback is the easy implementation of
multiple secondaries.
All other things being equal, a good way to further reduce leakage
inductance is to use a core with a longer window and/or some way to
reduce the need for clearance/creepage margins. The overall goal is to
reduce the amount of volume of insulation between windings. Triple
insulated wire, provided that it is the very thin type, is one way to
achieve both of the above. Leakage inductance that is 1% of
magnetizing inductance is achievable using the above techniques.

Good advice although processing very thin wire may be problematic.

I'm using TEW on my design btw. Solves all those margin problems.

Graham

 

[Pooh Bear]

"Adam. Seychell" wrote:

Paul Mathews wrote:
Another good reason to choose flyback is the easy implementation of
multiple secondaries.
All other things being equal, a good way to further reduce leakage
inductance is to use a core with a longer window and/or some way to
reduce the need for clearance/creepage margins. The overall goal is to
reduce the amount of volume of insulation between windings. Triple
insulated wire, provided that it is the very thin type, is one way to
achieve both of the above. Leakage inductance that is 1% of
magnetizing inductance is achievable using the above techniques.
Paul Mathews


I guess I'm just stuck with what I've got, at %2. If I could abandon the
use of safety margins

Use triple insulated wire..

http://www.furukawa.co.jp/makisen/eng/product/texe_feature.htm

You may be able to get a small sample enough for your needs.

and internal foil shielding it would go a long
way to lowering leakage inductance. The smaller the transformer the
worse things become because clearance margins must remain fixed at 5mm.

6mm surely ?

This is part of a one off hobby project, and also partly doing it to
learn something. It would be nice to be able to whip one up in an
afternoon, assuming I had PCB and all the generic at hand.
Can you actually buy part assembled DIY offline flyback converter kits ?

Too exotic I reckon for such a thing.

Graham

 

[Adam. Seychell]

Pooh Bear wrote:

Paul Mathews wrote:


Another good reason to choose flyback is the easy implementation of
multiple secondaries.
All other things being equal, a good way to further reduce leakage
inductance is to use a core with a longer window and/or some way to
reduce the need for clearance/creepage margins. The overall goal is to
reduce the amount of volume of insulation between windings. Triple
insulated wire, provided that it is the very thin type, is one way to
achieve both of the above. Leakage inductance that is 1% of
magnetizing inductance is achievable using the above techniques.


Good advice although processing very thin wire may be problematic.

I'm using TEW on my design btw. Solves all those margin problems.

Graham

Since this is a hobby project, and the transformer is not getting wound
professionally then I'm predicting it will be difficult sourcing small
quantities of triple insulated wire. I've got nearly 10 kg of various
sizes of the standard enamel winding wire at my disposal.

Adam

 

[Joerg]

Hello Cbarn,

Well yes , series production can be cheaper but this guy is making 1.
Also bear in mind that Digikey and their like have a 100% + mark up.
I'd admire anyone who can whip 1 up in an afternoon, even more so if
they can get it to work as well. Another aspect for series production
is reliablity, 1 error that comes to light after you have 200+ units in
the field can cost you dearly. ...

True. But after a couple decades or so you build up a "repertoire" and
then most such designs become deja-vu. It's mostly a matter of having
the right toroid cores and stuff in the lab supplies. I always make sure
there is a pound or two of #77 in stock.

...Then there are the regulations to comply
with, if you make your own you have to cost all that in as well.

True as well. Then again, claims by module mfgs that theirs are
"compliant with xyz" sometimes turn out to be, well, a bit of a stretch.
At the end of the day the whole system must pass anyways. In the medical
world even the isolation requirements have to be dealt with separately
because it is often next to impossible to meet EMI plus low leakage
without an iso transformer.

Regards, Joerg

http://www.analogconsultants.com

 

[Terry Given]

Adam. Seychell wrote:

Pooh Bear wrote:

Paul Mathews wrote:


Another good reason to choose flyback is the easy implementation of
multiple secondaries.
All other things being equal, a good way to further reduce leakage
inductance is to use a core with a longer window and/or some way to
reduce the need for clearance/creepage margins. The overall goal is to
reduce the amount of volume of insulation between windings. Triple
insulated wire, provided that it is the very thin type, is one way to
achieve both of the above. Leakage inductance that is 1% of
magnetizing inductance is achievable using the above techniques.



Good advice although processing very thin wire may be problematic.

I'm using TEW on my design btw. Solves all those margin problems.

Graham


Since this is a hobby project, and the transformer is not getting wound
professionally then I'm predicting it will be difficult sourcing small
quantities of triple insulated wire. I've got nearly 10 kg of various
sizes of the standard enamel winding wire at my disposal.

Adam

just ask your local transformer manufacturer for some. He (or she, or
it) will probably just give you some.

Cheers
Terry

 

[Joerg]

Hello Terry,

just ask your local transformer manufacturer for some. ...

Is there still such a thing? None around here. Where I lived before
there used to be a "motor rewinding" business. There I could purchase
all sorts of wires, cores etc. The guys were very knowledgeable. In the
mid 80's this old barn was suddenly razed and they built a gas station.
Sigh...

Regards, Joerg

http://www.analogconsultants.com

 

[Pooh Bear]

Terry Given wrote:

Adam. Seychell wrote:
Pooh Bear wrote:

Paul Mathews wrote:


Another good reason to choose flyback is the easy implementation of
multiple secondaries.
All other things being equal, a good way to further reduce leakage
inductance is to use a core with a longer window and/or some way to
reduce the need for clearance/creepage margins. The overall goal is to
reduce the amount of volume of insulation between windings. Triple
insulated wire, provided that it is the very thin type, is one way to
achieve both of the above. Leakage inductance that is 1% of
magnetizing inductance is achievable using the above techniques.



Good advice although processing very thin wire may be problematic.

I'm using TEW on my design btw. Solves all those margin problems.

Graham


Since this is a hobby project, and the transformer is not getting wound
professionally then I'm predicting it will be difficult sourcing small
quantities of triple insulated wire. I've got nearly 10 kg of various
sizes of the standard enamel winding wire at my disposal.

Adam

just ask your local transformer manufacturer for some. He (or she, or
it) will probably just give you some.

Cheers
Terry

In the UK there's bugger all useful little local businesses like that anymore.

The rot set in during the 80s when Margaret Thatcher's conservative government
decreed that the UK didn't need manufacturing and we could survive on the
service economy. It's been downhill ever since.

Graham