Looking for very flat SEPIC inductors

[Joerg]

Gents,

Does anyone know a source for dual winding shielded inductors that are a
little beefier than these but no higher than 0.1" (2.5mm), wider and
with almost twice the oomph?

http://www.cooperbussmann.com/pdf/733a2495-95a2-4812-a2f4-8b9c7c53359e.pdf

The SDQ25 10uH looks quite nice but my design would be too close to
saturation. Coilcraft only carries extremely flat ones that don't have
enough muscle. Their others are too tall.

Getting them will be a whole 'nother matter. Hopefully there's at least
some samples and then not too long leadtimes for reels. It's not
actually for a SEPIC but to add a negative rail to the output of a buck,
so it needs to be two windings on one core. Toroids would be ok as well.

 

[Joerg]

How much current do you need?

You could use a diode charge pump or my inductorless Groucho Marx
Generator.

Right now the peaks go to about 2.5A when the switcher screams along
pedal-to-metal.

It has to be low noise, there's imaging stuff going on in the same box.
Swinging hundreds of milliamps for the negative side would be a bit much
for inductorless transfer. I don't have hard numbers from the client yet
but I know for sure the Coiltronic ones won't cut it. If I find a
beefier one I can at least tell them "Ok, we can reach this goal but not
more than xxx mA out".

 

[Joerg]

Someone (Fair-Rite?) makes a core that you can snap around windows in
your PC board to make inductors. But the ones I saw were _much_ bigger
than these critters, and it makes less and less sense the further you
get away from a bazillion layers.

Two 5uH in series?

That's what I am trying right now. Series rings a bit but in parallel
would be ok. However, that becomes a sore spot for EMI. If there ain't
nothing else I'll have to do it :-(

 

[Joerg]

2 smaller inductors in series?

How about something like:
http://www.coilcraft.com/misc/lpr4012d.html

The 3015 and 4012 sizes in the PPD series is what I was looking at but
they are way too wimpy for this app. Even two would be marginal and I
really don't want to use more than two. Then I could as well design an
inverting switcher in addition.

If it has to be two I'll try the Coiltronics first because they max out
higher.

coilcraft has been very good at getting parts to me fairly quickly.
Using a non-1:1 ratio could let you get better use of the core.

Makes it tough to get a negative voltage equal to positive. Also, when I
looked at those the coil resistances weren't that mouth-watering ;-)

If you could live with single inductors, the rencousa RL-8300 fits in
very tights space. They aren't bashful about prices though.


You could look at adding LTC660 power inverters to the design.

or


-----/ o------------+----))))))------ +out
!
!
+---/ o---------- -out
!
!
\
o
!
GND

It requires some fancy logic to run the switches but it can make
good use of a single inductor.

Thanks, but doesn't work in my case because it's +/-12V. Charge pumps
are ok for a few ten milliamps but when you need more than 100mA it's tough.

 

[markp]

The 3015 and 4012 sizes in the PPD series is what I was looking at but
they are way too wimpy for this app. Even two would be marginal and I
really don't want to use more than two. Then I could as well design an
inverting switcher in addition.

If it has to be two I'll try the Coiltronics first because they max out
higher.



Makes it tough to get a negative voltage equal to positive. Also, when I
looked at those the coil resistances weren't that mouth-watering ;-)


Thanks, but doesn't work in my case because it's +/-12V. Charge pumps are
ok for a few ten milliamps but when you need more than 100mA it's tough.

Have you looked at Panasonic ELL4LG series? They are single inductors, 1.8mm
height and 800mA saturation for 10uH.

As an aside, under low load your buck might go into discontinous mode, in
which case there may not be enough energy to keep your aux charged if you
use a dual core inductor. You might need to provide a minimum load.

Mark.

 

[Joerg]

Have you looked at Panasonic ELL4LG series? They are single inductors, 1.8mm
height and 800mA saturation for 10uH.

Thanks, have to look for it. Seems a bit exoctic though since Digikey
only carries the ELL4LM singles. If it's 800mA per Winding for a grand
total of 1.6A I might just squeak by.

As an aside, under low load your buck might go into discontinous mode, in
which case there may not be enough energy to keep your aux charged if you
use a dual core inductor. You might need to provide a minimum load.

It's a synchronous buck, so in essence a gigantic push-pull driver
driving an "LC filter". The sims look great, so far. But the negative
will always be loaded much less.

 

[Joerg]

MooseFET wrote: [....]
The 3015 and 4012 sizes in the PPD series is what I was looking at but
they are way too wimpy for this app. Even two would be marginal and I
really don't want to use more than two. Then I could as well design an
inverting switcher in addition.

If it has to be two I'll try the Coiltronics first because they max out
higher.

Using a pair also can cause the radiated field to cancel if you care.

If they are close enough. But it makes a mess out of the trace routing.
Unless you can bury that it's like driving out the devil with Beelzebub.

Makes it tough to get a negative voltage equal to positive. Also, when I
looked at those the coil resistances weren't that mouth-watering ;-)

Cramming a lot into a small space takes some trade off.


[....]

Thanks, but doesn't work in my case because it's +/-12V. Charge pumps
are ok for a few ten milliamps but when you need more than 100mA it's tough.

You just need I/10mA swithers :>




The LTC660 is able to do more than 10mA. It is the highest current
down pumper
chip in the universe.

I don't have the final specs yet but I'll definitely have to deliver at
least 250mA on the negative rail.

 

[Joerg]

One regular choke, and one 100mA coupled choke.

That's an option, although that would add real estate. Thanks for the
Wuerth link but it seems 4mm is the smallest. But even 2.7mm would put
us a smidgen above, won't squeeze in.

 

[legg]

That's what I am trying right now. Series rings a bit but in parallel
would be ok. However, that becomes a sore spot for EMI. If there ain't
nothing else I'll have to do it :-(

Wurth goes to 2.7mm. WE-DD series.

http://catalogue.we-online.com/kata...ey=Inductors/Speicherdrosseln/Katalog/WE-DD_1

RL

 

[legg]

That's what I am trying right now. Series rings a bit but in parallel
would be ok. However, that becomes a sore spot for EMI. If there ain't
nothing else I'll have to do it :-(

One regular choke, and one 100mA coupled choke.

RL