Boost converter wisdom

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Hi, all,

I'm doing a miniaturized version of my low noise diode laser controller  
for a big biomed customer.  It's supposed to replace an existing unit  
for retrofitting, as well as being designed in to their new products.

The wrinkle is that it has to run off a single +5V supply instead of our  
usual +24V.  Sooooo, it needs a boost converter.

To reduce the dissipation in the TEC driver, I'm going Class H there, as  
I mentioned in another thread, which means I also need a buck converter.  
  Bucks and inverting buck-boost converters I've done lots of, but I've  
never done a positive boost before.

There's also a possibility that +5V won't be enough headroom on the  
Class H stage.  Some TECs such as CUI's use much finer fingers and so  
get equivalent delta-T and heating capacity at higher voltages and lower  
currents.  Thus I'd like to have a population option to run the buck off  
the output of the boost.  This is of course fairly fraught, given the  
stability problems of boost converters running in CCM and the negative  
input resistance of all switchers.  Also, space is at a huge premium on  
this board.

Advice welcome!

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
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Stability isn't a problem in general.  Only specifically for peak current  
mode controllers.  This mode is pretty popular, but it isn't the only kind  
that's good.  CCM isn't required either -- for their market -- so they  
aren't very concerned about that...

If you must operate in CCM, consider an average current mode controller.  
Easy enough to do discrete, or there's probably a few you can find  
integrated?

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
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Re: Boost converter wisdom
On 08/24/2018 04:20 PM, Tim Williams wrote:


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Nice thing about CCM is that it avoids the free ringing of the  
suddenly-unloaded inductor, which reduces EMI by a lot.  That unstable  
zero is a pain, though.  Since most of the rest of the circuit is  
basically a linear regulator, I might just slow the loop way way down  
and use a big output cap to accommodate the buck regulator's transients.
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Thanks.  I'll revisit the math and see what comes out.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
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Is it really that bad?  The peak energy of that ringing is in the MF to HF  
band.  Easy to filter.  The hard switching transients, which are there  
regardless, carry the pernicious VHF content.  And it's a double-whammy,  
because DCM has a shorter falling edge (hard switch-on), while CCM has  
reverse recovery (if applicable) plus full height edge.

Buck has the same transients, of course, so if you can do that in your  
design already, boost is no problem at all.


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Have you considered doing the class H by modulating the SMPS directly -- in  
other words, a post-reg configuration?  Doesn't even need to be an LDO or  
pass device in that case; it could be your favorite C-mult if you like  
(given suitable adjustments for loop speed, that is). :-)

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
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Re: Boost converter wisdom
On 08/24/2018 09:18 PM, Tim Williams wrote:
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To some given level.  As I like to say, "there's SMPS quiet, and then  
there's instrument quiet."

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That's one reason I really like the TI Simple Switchers.  No sharp  
edges.  Schottky diodes are a lot better than synchronous rectifiers  
there.

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It's not boost vs. buck AFAICT, it's CCM vs DCM.

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That's how Class H works--the idea is to maintain just enough headroom
for the amp to work properly.

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The linear output stage helps a lot.

Cheers

Phil Hobbs


--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
On Sat, 25 Aug 2018 01:25:40 -0400, Phil Hobbs

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Of course, your filled-matchbook constraint adds some limits.

One nice topology is a resonant tank with a saturating switch that's
on only at low voltage. No spikes.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Boost converter wisdom
On Fri, 24 Aug 2018 15:10:00 -0400, Phil Hobbs

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Sometimes I buy a cheap isolated dc/dc converter brick and stack its
output on top of the incoming supply. Like if I have 12 and want 15,
just add 3.3 volts to the 12.

dc/dc things are so cheap that designing something only makes sense
for high volumes.




--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Boost converter wisdom
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Downside, they're noisy as sin.

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
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Re: Boost converter wisdom
On Fri, 24 Aug 2018 15:25:04 -0500, "Tim Williams"

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Some are. The upside is that, managed properly, there is no ground
loop on the main board.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Boost converter wisdom
On 08/24/2018 04:22 PM, John Larkin wrote:
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This one is probably 10k-20k/year.   Mostly I need it to be quiet and to  
fit in a matchbox without taking the matches out. ;)

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
On Friday, August 24, 2018 at 4:23:10 PM UTC-4, John Larkin wrote:
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Anyone tried these recom ones?
https://www.recom-power.com/pdf/Innoline/Rxx-B.pdf

George H.
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Re: Boost converter wisdom
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Not those specifically, but they look worth a try.  RECOM is generally  
higher quality than others.  Sheesh, and you pay for those oddball voltages  
too, don't you...

Hmm, no EMC certs, that's maybe worrying?

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
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Re: Boost converter wisdom
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..

Cannot be answered without current, ripple, specs.
ebay is full of boost converters.
Made in China, 5$ a piece or so for a few amps.




Re: Boost converter wisdom
On 08/24/2018 10:33 PM, snipped-for-privacy@nospam.org wrote:
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I'm not looking for someone to design it for me--I'm interested in  
advice on what potholes to look out for.

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I have several.  Too big, too noisy, and the actual output current is  
about a third of the rating.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
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You still need to specify current and ripple.



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The one I have (for bit higher voltage performs very well.
Same for the bucks, and I have a boost/buck too.

It s not clear to me,
you have a 24 V unit, have only 5 V, add a box, put nice Chinese boost converter in it,
maybe if you ask them nicely they will supply it in a nice box with 'made in USA'? (not sure these days, and rightfully so),
pay them the 10 $, add a sticker on it 'Made By The Great Dr Hobbs' and charge a hundred,
and have a pizza.

Does not the great JL do the same and use Meanwell power supplies, wallwarts made in (whisper) C H I N A',
to evade all those mains power related requirements for his extremely high tech boxes?
I do the same, I use those Chinese (n-th time I mention China. oh well) to power all sort of stuff,
some have adjustable voltage and current limit with 10 turn pots, use it to charge batteries.
It is good stuff, I cannot make it for that, not even the board only, and no failures so far in ? 4 years?

Or reverse engineer, I have a circuit diagram from at least one somewhere.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR11.TRC1.A0.H0.X5V+to+24V+boost+converter.TRS0&_nkw=5V+to+24V+boost+converter&_sacat=0
starts at 0.76 $  with free shipping for 2A.

And now you do not have to re-design anything....
These seem to be the latest housing trend, not sure about the 5 V in.. more expensive, but 'box included' so to speak:
 https://www.ebay.com/itm/323360110515

Anyways, plenty on the market.

Re: Boost converter wisdom
On Sat, 25 Aug 2018 01:28:28 -0400, Phil Hobbs

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I've tested some CUI surface-mount dc/dc converters, the PDS1 types.
They seem happy at 2x rated load. That footprint is multi-sourced.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Boost converter wisdom
Phil Hobbs wrote...
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 I don't recall did you give us a set of specs?
 Output voltage(s) and maximum current(s)?


--  
 Thanks,
    - Win

Re: Boost converter wisdom
On 08/25/2018 10:00 PM, Winfield Hill wrote:
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A couple of amps at maybe +-3V, depending on the TEC.

Cheers

Phil Hobbs


--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
On Sat, 25 Aug 2018 23:41:47 -0400, Phil Hobbs

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That should be easily do-able with a 2MHz FSw, which would get around
your EMI problems (shouldn't be a big deal if it's laid out properly).
You should be able to get away with 2520 or maybe 4-5mm inductors, so
it should be pretty small.

Re: Boost converter wisdom
Phil Hobbs wrote...
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 3 volts, I thought you said it was a boost converter?
 BTW, if a split supply is required, a coupled inductor
 can be very useful.


--  
 Thanks,
    - Win

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