Transformers for switching power supplies - sources for stock parts.

[John Nagle]

Where do you find stock transformers for switching power supply design?
Apparently, for most commercial work, the transformer is at least semi-custom.
And no, I don't want to wind my own. I can model what I want in LTSpice,
but the transformer I've defined is not a standard part.

Digi-Key, surprisingly, isn't that helpful. Even the Pulse Engineering
site doesn't seem to help much.

What I'm looking for is a transformer with about 10uH on the primary,
and 140uH on the secondary. Nothing in the Digi-Key catalog seems to have
a turns ratio of more than 3:1, and I need more like 4:1. (I'm designing
a boost converter to boost 12V up to 120VDC at 6mA or so, using an
LT-3484 family IC, which is a camera flash capacitor charger. This
is part of my ongoing effort to drive antique teletypes with modern
electronics instead of boat-anchor power supplies.)

This is a little board-mount part, not a big power transformer,
probably about 1cc in volume. Sources?

John Nagle

 

[Don Klipstein]

Where do you find stock transformers for switching power supply design?
Apparently, for most commercial work, the transformer is at least semi-custom.
And no, I don't want to wind my own. I can model what I want in LTSpice,
but the transformer I've defined is not a standard part.

Digi-Key, surprisingly, isn't that helpful. Even the Pulse Engineering
site doesn't seem to help much.

What I'm looking for is a transformer with about 10uH on the primary,
and 140uH on the secondary. Nothing in the Digi-Key catalog seems to have
a turns ratio of more than 3:1, and I need more like 4:1. (I'm designing
a boost converter to boost 12V up to 120VDC at 6mA or so, using an
LT-3484 family IC, which is a camera flash capacitor charger. This
is part of my ongoing effort to drive antique teletypes with modern
electronics instead of boat-anchor power supplies.)

This is a little board-mount part, not a big power transformer,
probably about 1cc in volume. Sources?

1. It appears to me that such transformers are mainly custom rather than
off-the-shelf.

2. In this power range, I expect a "flyback design" to have good chance
of usefulness, and that means lower turns ratio is usable.

You did mention boosting 12V to 120V - why then should 4:1 be OK while
3:1 is not OK?

2a: If I need 120 VDC from 12 VDC, I don't need any bleeping turns ratio
at all - I can get that much from a mere inductor!

As for an extreme sidetrack - I designed a published xenon strobe
circuit providing 900 volts DC from 12 volts DC with a 1:2 transformer
(autotransformer-connected 1:1) with each winding having merely 16 turns,
for total turns count of 32.

http://repairfaq.cis.upenn.edu/sam/hss1sch.pdf

http://members.misty.com/don/samflash.html - though I am now seeing that
appropriate linkage to the relevant section from earlier on has a fair
chance of needing a repair.

Search for "HSS1 Inverter Transformer".

Looks like I can in a nice and reasonable manner get 450 volts (good
chance 500) with an inductor that can fit in a pingpong ball and that has
a mere 16 turns of wire.

As a result, I would advise looking into redesigning your boost
converter into something using an available transformer or an available
inductor.

120 VDC at 6 mA from 12V appears to me like something that an
appropriate derivative of my "HSS1 boost converter" can achieve with an
off-the-shelf inductor that Digi-Key stocks. Heck, I consider that doable
with even lower input voltage, as low as 4-5 volts.

So I give good chance of achieving a 12 ==> 120 VDC boost converter with
"magnetic component" being an off-the-shelf one of "sugar cube size" or a
little smaller.

- Don Klipstein ([email protected])

 

[Eeyore]

Where do you find stock transformers for switching power supply design?

Generally speaking you don't. Too many variables.

Graham

 

[[email protected]]

raid a old PC power supply, and use the isolation inductor/transformer
in reverse. Those are stock parts in Taiwan and China, and many of
them I've seen are the same,

Steve

 

[legg]

Where do you find stock transformers for switching power supply design?
Apparently, for most commercial work, the transformer is at least semi-custom.
And no, I don't want to wind my own. I can model what I want in LTSpice,
but the transformer I've defined is not a standard part.

You just answered your own question.

RL

 

[default]

raid a old PC power supply, and use the isolation inductor/transformer
in reverse. Those are stock parts in Taiwan and China, and many of
them I've seen are the same,

Steve

Roger that. find something in the same size range (Watt- amps) and
then use the same frequency, or near enough.
--

 

[default]

Where do you find stock transformers for switching power supply design?
Apparently, for most commercial work, the transformer is at least semi-custom.
And no, I don't want to wind my own. I can model what I want in LTSpice,
but the transformer I've defined is not a standard part.

Digi-Key, surprisingly, isn't that helpful. Even the Pulse Engineering
site doesn't seem to help much.

What I'm looking for is a transformer with about 10uH on the primary,
and 140uH on the secondary. Nothing in the Digi-Key catalog seems to have
a turns ratio of more than 3:1, and I need more like 4:1. (I'm designing
a boost converter to boost 12V up to 120VDC at 6mA or so, using an
LT-3484 family IC, which is a camera flash capacitor charger. This
is part of my ongoing effort to drive antique teletypes with modern
electronics instead of boat-anchor power supplies.)

This is a little board-mount part, not a big power transformer,
probably about 1cc in volume. Sources?

John Nagle

You don't find off the shelf parts like that. To design switching
supplies you must design the transformer for all practical purposes.

If it is one or two hobby items - compact fluorescent lamps will yield
some good cores. Then . . . you play to figure out how to do it.
There's so much to learn to become a swp designer - and it changes
daily.

It takes "arm wavin' and hed skratchin" beyond that.

Cruise the application notes - you aren't the first. Back-light, cold
cathode fluorescent supplies, for lap top computers, are in the range
you want.
--