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Flyback with LAN transformers

J

Joerg

Jan 1, 1970
0
Ok, most of us have used LAN tranformers for power transfers. However,
their datasheets are skimpy at best and usually contain no information
about the core. Of course it won't contain any regular air gap and you
cannot easily install one later because it's all potted up.

So, did anyone use these for a flyback design yet? Low power, something
well under 100mW.
 
F

Fred Bartoli

Jan 1, 1970
0
Joerg a écrit :
Ok, most of us have used LAN tranformers for power transfers. However,
their datasheets are skimpy at best and usually contain no information
about the core. Of course it won't contain any regular air gap and you
cannot easily install one later because it's all potted up.

So, did anyone use these for a flyback design yet? Low power, something
well under 100mW.

Which voltage from which voltage?
Do you really need flyback? I once did a funny self-oscillating one for
that kind of power, from 50V supply. Worked pretty well with about 60%,
maybe 70, efficiency IIRC.
I can dig up the drawing.

Anyway how about measuring your xformers? That's so easy...
 
J

Joerg

Jan 1, 1970
0
Fred said:
Joerg a écrit :


Which voltage from which voltage?
Do you really need flyback? I once did a funny self-oscillating one for
that kind of power, from 50V supply. Worked pretty well with about 60%,
maybe 70, efficiency IIRC.
I can dig up the drawing.

I've used oscillators as well but then there was no DC component and
everything was just fine. It's regular voltage levels, 12V, 24V etc.
Flyback is very easy from a topology point of view, and regulates very
fast. If there is no other way I'll just do the oscillator thing again.

Anyway how about measuring your xformers? That's so easy...

Sure. But you know how it goes after that. A couple of years later the
manufacturer "improves" the design and ... POOF.
 
F

Fred Bartoli

Jan 1, 1970
0
Joerg a écrit :
I've used oscillators as well but then there was no DC component and
everything was just fine. It's regular voltage levels, 12V, 24V etc.
Flyback is very easy from a topology point of view, and regulates very
fast. If there is no other way I'll just do the oscillator thing again.



Sure. But you know how it goes after that. A couple of years later the
manufacturer "improves" the design and ... POOF.

Then take enough safety margins. Given the power level it's probably not
difficult.
And then what can be improved?
Ferrite won't change much and evolves at a very slow pace, and
turns/geometry will stay the same or it'd be another part number.
 
J

John Larkin

Jan 1, 1970
0
Ok, most of us have used LAN tranformers for power transfers. However,
their datasheets are skimpy at best and usually contain no information
about the core. Of course it won't contain any regular air gap and you
cannot easily install one later because it's all potted up.

So, did anyone use these for a flyback design yet? Low power, something
well under 100mW.

We use the Tamuras as forward-mode dc/dc converters, driven p-p ct by
a pair of 2N7002's, in the 60 KHz ballpark. They are toroids,
ungapped, so probably don't store a lot of energy. They *are* cute and
cheap, and come with all sorts of interesting turns ratios.

John
 
J

Joerg

Jan 1, 1970
0
John said:
We use the Tamuras as forward-mode dc/dc converters, driven p-p ct by
a pair of 2N7002's, in the 60 KHz ballpark. They are toroids,
ungapped, so probably don't store a lot of energy. They *are* cute and
cheap, and come with all sorts of interesting turns ratios.

I'll check those out. Forward p-p would be a bit inconvenient here
because there is no high side drive capability. What power do you push
through? If it's more than a watt a flyback might be possible since even
p-p is never exactly 50.000% duty cycle.

At Digikey the Tamura xfmrs seem kind of pricey.
 
J

Joerg

Jan 1, 1970
0
Fred said:
Joerg a écrit :


Then take enough safety margins. Given the power level it's probably not
difficult.
And then what can be improved?
Ferrite won't change much and evolves at a very slow pace, and
turns/geometry will stay the same or it'd be another part number.

Well, I've seen it a few times that a manufacturer switched to another
source for the ferrite and problems arose. But you are right, if we had
something like 90% margin we'd be ok.
 
J

John Larkin

Jan 1, 1970
0
I'll check those out. Forward p-p would be a bit inconvenient here
because there is no high side drive capability. What power do you push
through? If it's more than a watt a flyback might be possible since even
p-p is never exactly 50.000% duty cycle.

At Digikey the Tamura xfmrs seem kind of pricey.

Oops, it's an Alfamag/Talema ISDN part I was thinking of, SMJ-140B. It
has 4 separate windings, 1:1:2:2, but you can get other ratios. We're
paying $1.18 at 1K. It's a nice cased sugar-cube surface-mount thing.

I think we're running about a watt, but it's probably good for more.
We're driving it with two open-drain 2N7002 fets into essentially a
center-tapped winding with the ct at +5 or +12, depending.

I posted a pic a while back, of a VME board with 16 of these.

It would probably work flyback, but a watt might be tricky. Just
guessing. It does look hunkier than the typical Ethernet xfmr.

John
 
P

Paul Mathews

Jan 1, 1970
0
Ok, most of us have used LAN tranformers for power transfers. However,
their datasheets are skimpy at best and usually contain no information
about the core. Of course it won't contain any regular air gap and you
cannot easily install one later because it's all potted up.

So, did anyone use these for a flyback design yet? Low power, something
well under 100mW.

Purpose designed miniature flyback coupled inductors ('transformers')
are readily available a equally low prices, so why bother?
Paul Mathews
 
J

Joerg

Jan 1, 1970
0
John said:
Oops, it's an Alfamag/Talema ISDN part I was thinking of, SMJ-140B. It
has 4 separate windings, 1:1:2:2, but you can get other ratios. We're
paying $1.18 at 1K. It's a nice cased sugar-cube surface-mount thing.

Nice, but most of my apps can stomach >$1. I've used some Asian ISDN
transformers for that (Murata?) but it's been a while and now it seems
ISDN isn't exactly flourishing anymore. Just like those cute modem
transformers that are probably going to almost vanish one day.

I think we're running about a watt, but it's probably good for more.
We're driving it with two open-drain 2N7002 fets into essentially a
center-tapped winding with the ct at +5 or +12, depending.

I posted a pic a while back, of a VME board with 16 of these.

It would probably work flyback, but a watt might be tricky. Just
guessing. It does look hunkier than the typical Ethernet xfmr.

That's the thing, they do not say how much DC they can take. I need less
than 100mW here but still, if it works on the bench that doesn't mean
it'll still work in three years because there ain't nothing about that
in datasheets and when I ask all I get is a shrug. Because it's the
first time someone asked.
 
J

Joerg

Jan 1, 1970
0
Paul said:
Purpose designed miniature flyback coupled inductors ('transformers')
are readily available a equally low prices, so why bother?


Where?
 
J

Joerg

Jan 1, 1970
0
Spehro said:
Is 1K the MOQ? Can you buy just 100? It has a nice high primary
inductance.

Question: Can someone teach them how to design a web site? And how not
to design one? It doesn't even let me get to the SMJ series and I don't
know which distributor sells their stuff. Because they don't say.

I hope they didn't hire the web designer from National...
 
J

John Larkin

Jan 1, 1970
0
Is 1K the MOQ? Can you buy just 100? It has a nice high primary
inductance.

And nice low dcr's too.

I think you can buy less, but they're made in India, so leadtime is
terrible. We have 1200 in stock if you need a few.

John
 
S

Spehro Pefhany

Jan 1, 1970
0
Oops, it's an Alfamag/Talema ISDN part I was thinking of, SMJ-140B. It
has 4 separate windings, 1:1:2:2, but you can get other ratios. We're
paying $1.18 at 1K. It's a nice cased sugar-cube surface-mount thing.

Is 1K the MOQ? Can you buy just 100? It has a nice high primary
inductance.


Best regards,
Spehro Pefhany
 
P

Paul Mathews

Jan 1, 1970
0
Where?

--
Regards, Joerg

http://www.analogconsultants.com- Hide quoted text -

- Show quoted text -

Some sources with very aggressive pricing on miniature flybacks
include Xfmrs, Inc, Premier Magnetics, Vishay/Dale, GB Int'l. Quantity
requirements for low prices are surprisingly low....you basicallly
have to cross their threshold for having the parts batch produced in
Asia. Prices in lower quantities are about 6x higher. We buy UR 1W
flybacks for about $.50 ea in 1K lots.
Paul Mathews
 
J

Joerg

Jan 1, 1970
0
Paul said:
Where?

--
Regards, Joerg

http://www.analogconsultants.com- Hide quoted text -

- Show quoted text -


Some sources with very aggressive pricing on miniature flybacks
include Xfmrs, Inc, Premier Magnetics, Vishay/Dale, GB Int'l. Quantity
requirements for low prices are surprisingly low....you basicallly
have to cross their threshold for having the parts batch produced in
Asia. Prices in lower quantities are about 6x higher. We buy UR 1W
flybacks for about $.50 ea in 1K lots.[/QUOTE]


$0.50/1k is expensive. Non-gapped is much less. But still witing for an
answer from Xfmrs, let's see. Not holding my breath though ;-)
 
P

Paul Mathews

Jan 1, 1970
0
$0.50/1k is expensive. Non-gapped is much less. But still witing for an
answer from Xfmrs, let's see. Not holding my breath though ;-)

--
Regards, Joerg

http://www.analogconsultants.com- Hide quoted text -

- Show quoted text -

Expect lower pricing for non UL-Recognized parts that don't have
inspection and creepage/clearance margins, taping, and related. What's
your target?
PM
 
J

Joerg

Jan 1, 1970
0
Paul said:
$0.50/1k is expensive. Non-gapped is much less. But still witing for an
answer from Xfmrs, let's see. Not holding my breath though ;-)

--
Regards, Joerg

http://www.analogconsultants.com- Hide quoted text -

- Show quoted text -


Expect lower pricing for non UL-Recognized parts that don't have
inspection and creepage/clearance margins, taping, and related. What's
your target?[/QUOTE]


Can't tell. But it doesn't have to be UL cert'd. However, the premium
versus a non-gapped core cannot exceed the cost of the parts we'd save
going from forward to flyback, maybe 10-15c.
 
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