Flyback transformers, small, high power, where?

[Joerg]

legg wrote:

[...]

Got to be careful with organic binder. There have been disintegration
issues in the past, after a few years of service.

GF Nylon is glass in an organic binder, as are most bobbin materials.
Magnetic composite aging is more an issue because they're self-heating
with generally positive tempco of loss. And they can all be
misused....

Do you remember which literature that was in? Most of what I found was
in magazines such as "Power Electronics Technology", like this paper:

You may be in luck here, though links to such are not something I
bother to save, if in electronic format. These generally expire too
soon to be of any lasting use.

If I see a really good publication on the web I store the whole thing,
usually either in HTML or PDF. Very small price to pay. A few hundred kB
of hard drive space is really nothing these days compared to the usual
5-6MB per photo's from aunt Millie's birthday.

Following one of your own links yesterday, resulted in a pop-up for
one example of magnetics rises of this order:

http://www.ti.com.cn/cn/lit/ug/sluu866/sluu866.pdf

They're not uncommon, but you have to have your eyes open, before an
eybrow is likely to also rise.

Yup, nothing can beat a FLIR camera when it comes to power supply loss
determination.

 

[Joerg]

Maybe you should push the frequency up and deal with the EMI. Slow the
edges down a bit and give up a little efficiency.

That is option #2, once we've determined that no "stretch-limo" version
of a suitable E-core can be had or custom made. A few manufacturer
answers are still pending there.

Upping the frequency is technically easy but there are no flyback-PFC
chips that go above 150kHz, AFAICT. Then we may be looking at an
off-label use of a boost-PFC chip. A bit of a white-knuckle ride because
unless it's LTC it can't really be simulated, and LTC's boost-PFC chip
only goes to 300kHz.

 

[Nico Coesel]

That is option #2, once we've determined that no "stretch-limo" version
of a suitable E-core can be had or custom made. A few manufacturer
answers are still pending there.

Upping the frequency is technically easy but there are no flyback-PFC
chips that go above 150kHz, AFAICT. Then we may be looking at an
off-label use of a boost-PFC chip. A bit of a white-knuckle ride because
unless it's LTC it can't really be simulated, and LTC's boost-PFC chip
only goes to 300kHz.

Why cling on to LTSpice? You could model the transformer and drive
circuitry any way you like. IMHO the biggest problem will be finding a
high voltage MOSFET which can be switched fast enough and there is the
leakage inductance.

 

[Joerg]

Why cling on to LTSpice? You could model the transformer and drive
circuitry any way you like.

You can't do a full-blown SPICE analysis of the whole thing unless there
is a useful behavioral model. LTC has fairly good behavioral models
whereas other companies' models simulate every nook and cranny of a
chip, meaning just a few runs with ramp-up plus some simulated event can
take half a day. LTSpice is a major design help these days but, of
course, they kept the real modeling tricks to themselves.

I have to simulate events that take many seconds. With a switcher that
is quite painful in regular SPICE.

... IMHO the biggest problem will be finding a
high voltage MOSFET which can be switched fast enough and there is the
leakage inductance.

Those are the least of my concerns. Done it many times. You just have to
make sure the spikes always stay well below 70% of abs max and you don't
dissipate much heat due to spike muffling.

 

[Nico Coesel]

You can't do a full-blown SPICE analysis of the whole thing unless there
is a useful behavioral model. LTC has fairly good behavioral models
whereas other companies' models simulate every nook and cranny of a
chip, meaning just a few runs with ramp-up plus some simulated event can
take half a day. LTSpice is a major design help these days but, of
course, they kept the real modeling tricks to themselves.

I never simulate an entire SMPS. I just simulate whether the voltages
and currents aren't going crazy under several conditions and to get a
ball park on how much snubbing is required. Because there is always a
difference between te real world and the simulation I just make a
small PCB and measure & tune a real circuit before committing it to a
'final' PCB.

 

[Joerg]

I never simulate an entire SMPS. I just simulate whether the voltages
and currents aren't going crazy under several conditions and to get a
ball park on how much snubbing is required. Because there is always a
difference between te real world and the simulation I just make a
small PCB and measure & tune a real circuit before committing it to a
'final' PCB.

I have to follow a different philosophy most of the time. Often my
switchers go straight form the simulator to CAD, then layout. I must
simulate "in the system", meaning load changes, loop stability, input
surges and stuff like that. Occasionally the sim schematic already looks
like a family size pizza.

 

[Jasen Betts]

You wanting a 20W flyback transformer but wanting it in a small form
factor is the big joke here. How many HV designs have you done? Try one
negative 10W supply and one positive 10W supply and tie their returns
together. So two flybacks to achieve the power density you are after.
Two smaller form factor devices would fit your profile and likely allow
for off the shelf sourcing.

Something to think about.

not worth considering, if two 10W "transformers" will do the job,
you just need to wire them in series-parallel for a 20W converter,

the problem with fllyback is the "transformer" is really a coupled inductor
as it's an energy storage device and thus needs to be larger than the
transformer in a similarly rated forwards converter.

 

[legg]

legg wrote:

[...]

Got to be careful with organic binder. There have been disintegration
issues in the past, after a few years of service.

GF Nylon is glass in an organic binder, as are most bobbin materials.
Magnetic composite aging is more an issue because they're self-heating
with generally positive tempco of loss. And they can all be
misused....

There's another issue here. That GF Nylon example refers to filled
nylon at 5, 10 or maybe 15% filler by mass/volume. There are distinct
limits before the compound loses important physical properties that
permit it's use.

To maintain useful magnetic properties, the fill for dust has to be
much higher, so it's not a trivial exercise.

I'm not sure what mix has been achieved with ferrite, to date, but you
might expect something similar with other fillers. The ferrite
composites are all low temperature materials that are presented as
sheets.

There are some filled thermoplastics in effective service as
heat-transfer materials. Perhaps that technology is closer, though I
doubt it welcomes completely foreign metallic particles any better.

If I see a really good publication on the web I store the whole thing,
usually either in HTML or PDF. Very small price to pay. A few hundred kB
of hard drive space is really nothing these days compared to the usual
5-6MB per photo's from aunt Millie's birthday.

Of course you save the article, along with info on source. Will it
still be there tomorrow? This is just the sort of content that the
Wayback website doesn't archive.

'Good' is relative. How big do you want your 'stupid example' or
'microconverter follies' folder to get?

Yup, nothing can beat a FLIR camera when it comes to power supply loss
determination.

It's just a picture - you can bet that the interpretation of it's
content is explained quite differently (if at all), depending on who
does the 'splaining'.

There should always be bordering present in the legend for these
things - a colour that you can point to on the scale and say 'Well at
least nothing there got this hot.' It's a guarantee that the hottest
point isn't hotter than presented, and that the ambient is really
ambient.

RL

 

[legg]

Oxymoron?

Not mine. It's actually nanocrystaline, but googling either gets you
there.

RL

 

[SoothSayer]

not worth considering, if two 10W "transformers" will do the job,
you just need to wire them in series-parallel for a 20W converter,

It is worth considering, since the two transformer solution would be
more likely to be able to fit into the profile aspect. The pair would be
more reliable as well, each doing half the work (or less) than the 'fool
bore' solution.

the problem with fllyback is the "transformer" is really a coupled inductor
as it's an energy storage device and thus needs to be larger than the
transformer in a similarly rated forwards converter.

Then don't use a friggin flyback based driver.

I have made 180kV X-Ray supplies that get used at airports and sit
inside a lead box full of oil over a cubic foot in size. The series pair
method and c-w multipliers is a good choice here.

 

[Joerg]

I wasn't suggesting that you use these chips. I was giving you
examples of chips that could perform the pfc function without a
modulator, to demonstrate that you don't need one in your application.
Virtually any device that produces critical conduction in a flyback
can be made to do the same. You don't need a modulator, if you're
going to use a flyback circuit for isolated PFC, at 'low' power. You
don't need a modulator. You don't need a modulator. You don't need a
modulator.......

You must make the current follow the line voltage, else no good PF. This
does require a modulation, regardless of whether we call it modulator or
give it a different name.

Never the less, you might find the reading informative. First google
search return on LNK405.

First google search return on "LNK405". First google search return on
"LNK405". First google search return on "LNK405". First google search
return on "LNK405".

The datasheet link from Codico? Repeating it doesn't make it any more
useful. I know how to do PFC. What I need to know is where to get the
proper magnetics (without breaking the bank).

The reading isn't interesting because of the integrated switch, though
you might check out the size of those suckers. It's interesting
because of the shape and size of the parts used for a simple
commercial application at the same power level. The transformer may
not be your only issue. The linkswitches are not critical conduction
FM, so the transformers will be a bit larger, preventing their use.

Most of all their switching frequency is quite low, and the Rdson fairly
high. Big, toasty ...

 

[legg]

Thanks. It's a regular boost-mode chip though. Maybe it can be used as
flyback, should be possible. LTC doesn't have to offer much in that
area, AFAICT only up to 300kHz but at least that's a factor of three
versus they flyback-PFC chips. And the voltage control in those isn't
too great, would have to roll my own anyhow.

So yeah, that may be an idea. But first I want to find out what it takes
to get a xfmr that can be run under 150kHz.



These two have been obsoleted.

I wasn't suggesting that you use these chips. I was giving you
examples of chips that could perform the pfc function without a
modulator, to demonstrate that you don't need one in your application.
Virtually any device that produces critical conduction in a flyback
can be made to do the same. You don't need a modulator, if you're
going to use a flyback circuit for isolated PFC, at 'low' power. You
don't need a modulator. You don't need a modulator. You don't need a
modulator.......
Never the less, you might find the reading informative. First google
search return on LNK405.
First google search return on "LNK405". First google search return on
"LNK405". First google search return on "LNK405". First google search
return on "LNK405".
The reading isn't interesting because of the integrated switch, though
you might check out the size of those suckers. It's interesting
because of the shape and size of the parts used for a simple
commercial application at the same power level. The transformer may
not be your only issue. The linkswitches are not critical conduction
FM, so the transformers will be a bit larger, preventing their use.

RL

 

[notbob]

DOH!

I read it as 250mms, not mils, as in thousandths of an inch. Yer
right. That IS damn small! Nevermind.

nb

 

[Joerg]

DOH!

I read it as 250mms, not mils, as in thousandths of an inch. Yer
right. That IS damn small! Nevermind.

We hired a few British engineers and one of them handed a drawing to the
machine shop. A week and almost $900 in cross-charges later they gave
him the requested teeny tiny piece and his jaw dropped ... "WHAT?!"

 

[legg]

You must make the current follow the line voltage, else no good PF. This
does require a modulation, regardless of whether we call it modulator or
give it a different name.

A simple constant frequency discontinuous (or complete energy
transfer) flyback will, if completely unmodulated, produce credible
power factor corrected input current waveshapes, given an unfiltered
rectified AC input voltage. This is easily confirmed by observation,
calculation or basic power conversion reference material.

You would have to modulate duty cycle slowly (as in all PFC
regulators), in order to roughly regulate the output voltage without
introducing harmonic content. multiple input frequency current is
passed unaltered to the output filter.

You do not modulate it to obtain PFC. You cannot use a peak input
current control method, because it would be counterproductive to the
aim of PFC.

For a critical conduction mode circuit (which only differs by it's
introduction of FM), it's not as direct a relationship, but the power
throughput improvement, improved noise and reduced stress is enough to
make the alteration desirable. It is often enough to introduce
nonlinearities in the voltage modulator, to correct for the
predictable deviation that's introduced.

Critical conduction circuits don't give a damn what the peak current
is, though ovbiously switch current may be monitored to provide
protection, so long as it doesn't interfere.

RL

 

[Joerg]

A simple constant frequency discontinuous (or complete energy
transfer) flyback will, if completely unmodulated, produce credible
power factor corrected input current waveshapes, given an unfiltered
rectified AC input voltage. This is easily confirmed by observation,
calculation or basic power conversion reference material.

You would have to modulate duty cycle slowly (as in all PFC
regulators), in order to roughly regulate the output voltage without
introducing harmonic content. multiple input frequency current is
passed unaltered to the output filter.

And that's what I meant, you do need a modulator to do this because you
can't let the output voltage go haywire. I have tried using the regular
feedback for this, even on the LT chips, and it causes the PF to turn
horrid.

You do not modulate it to obtain PFC. You cannot use a peak input
current control method, because it would be counterproductive to the
aim of PFC.

For a critical conduction mode circuit (which only differs by it's
introduction of FM), it's not as direct a relationship, but the power
throughput improvement, improved noise and reduced stress is enough to
make the alteration desirable. It is often enough to introduce
nonlinearities in the voltage modulator, to correct for the
predictable deviation that's introduced.

That's how most flyback-PFC chips I looked at work, and they all have a
modulator.

Critical conduction circuits don't give a damn what the peak current
is, though ovbiously switch current may be monitored to provide
protection, so long as it doesn't interfere.

In my case there's two criteria: Holding the PF somewhere above 0.9,
using the smallest possible core and keeping the output voltage within a
+/-30% band. We will get there, but not with of-the-shelf ferrites.