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Re: Boost Converter Efficiency Improvements
wrote:

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I do actually have quite a bit of space now. I've got rid of the big  
electrolytics and there're fewer parts than before.

I wanted to try this topology quickly as an experiment and the SRF1280
seems to be the largest dual inductor available from Farnell.

I'm just doing a proper PCB layout now, that should help reduce the  
RF noise. I can get rid of that big heatsink pour on the drain tab for
a start.


The pinout of those inductors leads to a realy nice layout; DC on top  
left, output to diode on top right, FET pulling down the bottom two  
pins.  

Do you think a "better" doide than the 35ns ES1J would help any? I can
get something like a Bourns CD214A-F1400 which is 25ns for a similar  
price.


With the added PWM resolution I can lower the inductance, which will  
lower the DC resistance.

I wonder how high I can go before the comparator in the PIC craps  
out...


Dave.


Re: Boost Converter Efficiency Improvements
David Jordan wrote:
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Farnell is often kind of limited in selection.


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That is always nice. But mind the "air gap" in the form of this
anthracite-colored epoxy up top. That's where magnetic field galore
spews out. You can test it this way: Take a 2nd SRF1280 and solder an
LED across one winding. Then run the converter under load and lower that
2nd SRF1280 inverted onto the one in the circuit. But slowly, like the
docking procedure at the space station, else the LED my go poof.


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Since you are operating in DCM every nsec helps. But if you run the sim
again with a smaller Schottky it becomes apparent that only around 10mW
can be gained. In your case 0.25%. As my old boss used to say, that's
not much to write home about.


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Those aren't exactly rockets but I believe they can do under 500nsec.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Boost Converter Efficiency Improvements
On Wednesday, 19 June 2013 05:58:13 UTC+10, Joerg  wrote:
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<snip>

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Of course, if you get you own coils wound, you can put them into RM cores, which
keep the magnetic field closely confined within the core outline. Toroids are
even better, but harder to get wound.

<snipped sensible stuff, but irrelevant to the narrow point I want to make>

--  
Bill Sloman, Sydney

Re: Boost Converter Efficiency Improvements
Bill Sloman wrote:
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But one has to mind core saturation. If it saturates in a switcher that
is usually accompanied by a loud band, flying debris, and occasionally
spattering molten solder :-)

The best thing to do when the EMI from the core is a concern is to plop
a Laird shield can over the whole thing. Laird is even in David's home
country.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Boost Converter Efficiency Improvements
On Wednesday, 19 June 2013 10:57:11 UTC+10, Joerg  wrote:
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Of course one has to pay attention to core saturation, power dissipation in
the windings (bearing in mind that the Curie temperature for ferrites is ap
preciably lower than  - say - the maximum junction temperatures for power t
ransistors

It's stuff like that that makes electronic design interesting, and keeps yo
u well paid. Iy kept me well paid too, when I was younger ...
  
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The best thing to do when EMI from the core is a concern is to design it ri
ght in the first place. Added extra shielding can be necessary if you have  
extraordinarily flux-sensitive components in the immediate vicinity, but it
's usually cheaper to make sure the flux sources are further away and point
ed in a different direction.

--  
Bill Sloman, Sydney

Re: Boost Converter Efficiency Improvements
Bill Sloman wrote:
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[...]

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When you have to have an air gap then leakage is inevitable. It needs to
be contained and orientation rarely helps.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Boost Converter Efficiency Improvements
On Thursday, 20 June 2013 03:46:31 UTC+10, Joerg  wrote:
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As long as ferrites don't have infinite permeability, there will always be  
some fringing field. Manufacturers gap RM cores by grinding only the centre
, so the outer shell is still an effective - if necessarily imperfect - scr
een.  

Non-progressively wound toroidal coils don't have any external magnetic fie
ld - a simple progressive winding produces an effective single turn in the  
plane of the toroid. The neater non-progressive winding schemes can cancel  
most of the external electric field as well, at the expense of higher self-
capacitance.

The pot core is the topological dual of the toroidal core. Leaving slots in
 it for the connection to the core makes it less than perfect, and the RM c
ore has bigger slots.

Orientation can help in some cases, if you think about it far enough in adv
ance.

--  
Bill Sloman, Sydney


Re: Boost Converter Efficiency Improvements
Bill Sloman wrote:
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Not with such simple cores like the SRF series. The gap is always up top
where the epoxy goes -> cheap. There is another series that's a tad
better but I'd have to dig that out. Used them in a sensitive app but
costs more. Tin-can style shields are so cheap, why the worry?


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There is a trick to muffle that via a loop-back half-turn. But toroids
are mostly out of league for switchers, on account of the cost.


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Pot cores are nice but mechanically a pain. They cost a lot, too.


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Orientation only goes so far. Instead of disturbing the electronics over
here you now disturb the electronics over yonder instead.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Boost Converter Efficiency Improvements

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Hey, I have an app where a gapped pot core might be the thing to use. It's a
little speculative right now, but it might be worth a little research. Do you
know of anyone who could research available cores and do some math, to see if
what we need is even feasible? We need, basically, a couple of uH at high peak
current and low duty cycle (ie, low longterm RMS current), space constrained. We
have more height available than pcb footprint, and most commercial inductors
tend to be short.

We could even stack two pot cores on a common mounting bolt, to get more height
for available footprint. Wire in parallel to the PC board. Phase for leakage
field cancellation?


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

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Re: Boost Converter Efficiency Improvements
John Larkin wrote:
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We
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Off the top of my head I don't recall anyone specializing in this sort
of research. But "a couple of uH" and pot core are a somewhat odd
combination. Pot cores are nowadays (usually) only available in the very
high permeability ferrites. I guess that is because all the old telco
filter apps have become obsolete or gone DSP.

Of course, they will still bake you a custom core but that's going to cost.


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height
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If the air gap is in the middle as usual then the field cancellation
effect won't be that great.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Boost Converter Efficiency Improvements
Le Thu, 20 Jun 2013 08:44:36 -0700, John Larkin a écrit:

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How high is high?

I once worked on an IGBT bridge where the load was a mH order of  
magnitude/50kA (yep) air coil.
Wires did jump on the floor when doing the short circuit test.
Intimidating the first times.

--  
Thanks,
Fred.

Re: Boost Converter Efficiency Improvements
On 20 Jun 2013 18:56:56 GMT, Fred Bartoli

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The current is 130 amps maybe. Two inductors, each 2 uH at 65 amps, is
in the balpark. So each inductor has to store about 35 millijoules.

Height needs to be about 1" or less for the stack. Max diameter is
0.75", but preferably less.

I hate magnetics design. The part data sheets are all in gauss and
mean path lengths and cross-section areas and permeabilities. Why
can't they spec cores in electrical units like amp-turns and joules
and ohms of copper? You're lucky to get A-sub-l.


--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: Boost Converter Efficiency Improvements
John Larkin wrote:
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You could talk to these guys. They just designed a custom ferrite part
for one of my projects that has a highly unusual form factor and power
density, and numerous big players in the field had thrown in the towel
on that one:

http://www.rencousa.com/

The first video is pretty cool, especially at 1:37 min :-)

This series already seems to come quite close to what you need, and it's
mil-spec if that matters:

http://www.rencousa.com/sites/default/files/productfamilies/pdfs/RL-1324_0.pdf

They are extremely conservative with their Isat spec, I'd talk to them
to see how small you can go for 130 amps peak. But I don't know if it's
a pot core inside (could be more open).

Don't know the exact requirements in your application, they also have
toroidal swinging chokes that could possibly be stacked. Those lose
inductance in a controlled fashion when the big pulse comes, and
afterwards the nominal inductance returns.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Boost Converter Efficiency Improvements
Le Thu, 20 Jun 2013 13:19:19 -0700, John Larkin a écrit:

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Well,

L.I = n.B.Ae => n=L.I/(B.Ae)

Lets try the fatest RM core that roughly fit your needs (RM12)
From ferroxcube catalog:

Ae14%6mm^2
Say Bpeak = 200mT, then n=8.9 turns.
Then you aim for A_l=2u/9^2 = 25nH/T^2

OK, lets see if that's practical:
the A_l16%0nH/T^2 already has a 1.57mm gap. To get to 25nH/T^2 you'll  
need to gap it to 1.57*160/2510%mm
I guess you get the picture.

Another way to see that is:
almost all energy is stored in the gap.
W=1/2 L.I^2 = 1/2 v.B.H = 1/2 v.B^2/u0
  with v = gap volume.

You want 1uH@130A so W=8.5mJ which gives v=2.W.u0/B^253%4mm^3

which for 1mm thickness is 26mm dia. 2mm gap and your central leg is 20mm  
dia.

Pbs are the low Bsat, and the bad gap placement, which limit its volume.

You'd better use some micrometal toroid.

Let see:
T72-2 => A_l12%.8 => n=9 for 1.03uH
It has an Ae34%.9mm^2 which gives
B = L.I/(n.Ae) = 414mT

from http://www.micrometals.com/images/curves/0208BH.html we see that it  
fits with the 2 material.

Design done. Well almost...

--  
Thanks,
Fred.

Re: Boost Converter Efficiency Improvements
On 20 Jun 2013 21:58:57 GMT, Fred Bartoli

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I ran the Micrometals software and they want a T130 or bigger. One
solution is T130-8 with 6 turns. T130 is too big.




--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: Boost Converter Efficiency Improvements
Le Thu, 20 Jun 2013 16:00:22 -0700, John Larkin a écrit:

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Check my calcs, I may have made a mistake, but I don't think so and the  
T72-2 should go.
Maybe they derate Bsat, thanks to the unfriendly aging behavior of iron  
powdered cores, but in your case, if I understood well, this should not  
be a pb. Maybe contact them to have some figures, but the graph tells, it  
well I think.

And it's easy to give it a try...

--  
Thanks,
Fred.

Re: Boost Converter Efficiency Improvements
Le Thu, 20 Jun 2013 23:15:48 +0000, Fred Bartoli a écrit:

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Just checked with their tool.

Either 1x T72-2 with 9 turns => 1u and inductance is still 90% at 130A
or 2 paralleled T72-2 with 13 turns => 1.97uH @ 65A
or 2 stacked T72-2 with 7 turns => 1.13uH @ 130A


--  
Thanks,
Fred.

Re: Boost Converter Efficiency Improvements
On Friday, 21 June 2013 00:18:55 UTC+10, Joerg  wrote:
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The SRF series from Bourns

http://www.bourns.com/data/global/pdfs/SRF0703.pdf

may use ferrite cores, but it doesn't look as if they incorporate anything  
as sophisticated as a two-part RM core.

They may be cheap, but they are certainly nasty. My point throughout this t
hread has been that they aren't actually all that much cheaper than a prope
rly designed coil wound onto an off-the-shelf former by a cottage coil wind
er near you, and since you get to choose the the off-the-shelf ferrite core
 that you wrap around the coil, you've got a lot more control over the indu
ctor you use.

Because you can tailor the coil and the ferrite to you application, you wil
l normally end up using less copper and less ferrite than you would if you  
settled for the nearest available off-the-shelf core, and there's a real ch
ance that you will end up with a cheaper product, as well as a better one.
  
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Machines for winding toroidal cores exist, and are cute, but since you've g
ot to wind the copper on to the - gappable - dispensing drum before you win
d it off again onto the toroid, they are relatively slow. This hasn't stopp
ed iron-cored toroidal mains transformers from pretty much taking over the  
market for consumer electronics - the flat transformers give you nice flat  
pieces of kit to stick on your shelves.
  
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They've been replaced by RM cores, which di seem to be produced in serious  
volumes.
  
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So? Every improvement you make to electromagnetic compatibility only goes s
o far. The trick is to make enough of them to make your problem small enoug
h to live with. Lots of small improvements don't impress like one big impro
vement, but they do get the job done. That's engineering versus showmanship
.

--  
Bill Sloman, Sydney

Re: Boost Converter Efficiency Improvements
Bill Sloman wrote:
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I think you haven't talked to cottage coil winders in a few decades. The
last I knew was in the German town I used to live in as a teenager. That
closed in the early 80's and became ... <gasp> ... a BP gas station :-(

The current custom magnetics houses are larger places with CNC machines
and the whole nine yards. There, you either have to cough up a
substantial chunk of money for NRE or place a large minimum order.


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Unless you are talking thousands per month, this is only wishful thinking.


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That problem has been licked decades ago. But as I said earlier, it's
now done by sizeable custom manufacturers and if you order only 2-3
pieces you have to first put a big check on the table.


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Those are more leaky, not suitable for some applications.


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Well, in my cases the most important metric is usually time, due dates,
first customer shipments. If the success of a whole big machine depends
on my electronics then I don't dilly-dally with inductor orientations.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Boost Converter Efficiency Improvements
On Friday, 21 June 2013 09:56:31 UTC+10, Joerg  wrote:
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You may not have talked to a cottage coil-winder in a few decades, but I go
t some prototype coils wound at

http://www.acewikkeltechniek.nl/ace-wikkeltechniek-bv/engineering-ontwikkel
ing  

in Horst in the Netherlands last year. They wouldn't appreciate being calle
d a cottage coil-winder but it didn't look like a huge place when I dropped
 in to deliver my formers and winding specifications. Horst is no metropoli
s.

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I got a couple of pairs of coils wound for about 200 euro - not exactly a h
uge NRE, or a huge minimum order. I save a bit by putting on the double scr
eening myself - it's a bit fiddly, and doesn't exactly need a coil winding  
machine.
  
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.

I beg to differ.

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That's the simplest of the astatic - non-progressive - schemes. Rayner and  
Kibble list a few more in section 4.2.1 of their "Coaxial AC Bridges" ISBN  
0-85274-389-0, if you can find a copy.  

(Amazon.com product link shortened)

As far as I know, it's out of print. For a while the British National Physi
cal Laboratory at Teddington would print copies on demand (under license fr
om Adam Hilger) but they'd stopped that when I last looked, and their curre
nt web-site is perversely opaque.

"Ayrton-Perry" is better

http://en.wikipedia.org/wiki/Ayrton-Perry_winding

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I didn't - see above.
  
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Granting the broad-line distributor stocking patterns, RM core would seem t
o be suitable for a lot more applications than pot cores.

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Unless it's obvious and easy.

--  
Bill Sloman, Sydney

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