Is A SEPIC Convertor Really All That Efficient ?

On Sunday, September 22, 2019 at 12:21:23 PM UTC+10, snipped-for-privacy@gmail.com wrot e:

good. I told him it would take me a LONG time to design one from scratch. B ut anyway I thought of something.

C running one of the screen voltage. They do pull current like a CRT.

om buck to boost seamlessly, right ? But it would never have to boost. Befo re that SMPS comes out of regulation it would probably shut down.

eed something to control those voltages but to have to boost it ? I know ho w plasma TVs work, so maybe they wanted control over this one voltage. But to have to be able to boost it ? Why ?

Maybe having two inductors to play with meant that they could get the compo nents into the space available.

Real designs are often constrained by non-electronic requirements - you can end up needed a really efficient converter because you can't get much cool ing air past the place you have got to squeeze the converter into.

In one system I worked, the start-up had spent a bundle on a carefully styl ed egonomic case, which made it tricky to get rid of the waste heat from th e Peltier junction we'd had to put in to get tighter temperature control at the sensing head than had been originally thought necessary.

--
Bill Sloman, Sydney

No idea. Situation dependent.

Maybe you hit the nail on the head, maybe it needs a zero-energy state while in standby? Or under startup conditions, it needs to start from zero (whereas a boost can't avoid the first bit of inrush)?

About splitting the inductor: the physical limitations Bill mentioned, are unlikely I would think. Not to say it can't happen, just that it likely will take a lot of conditions to be worthwhile.

I think it would bother me more, that I'd lose the ripple-saving property. No shared core means almost double the inductor volume, and higher ripple needs somewhat bigger capacitors.

It's still possible you find a pair of inductors which happen to fit just perfectly, whereas you can't find a dual that does -- that would be exactly such a case where it might win. (The offerings for large coupled inductors, off the shelf, is pretty pitiful.)

If custom windings are included (which they definitely would for a consumer plasma TV), it gets a lot harder to argue that.

SEPIC is also a fertile ground to tack on additional windings, for modest ratios (in the 1-3 range, say) and inversion (negative outputs). Though maybe they didn't need, or take advantage of, that in your example.

Last SEPIC I designed in, was for a 12V output, while accommodating an automotive voltage range (operational under cold cranking (6V) to load dump (>60V)). (Operation was not required during load dump, it just turned out that way; a feature only possible thanks to the tiny load current drawn by the module. It actually ended up rated a whopping 200V, making a MOV suitable to handle faster surges.)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/ 

 wrote in message  
news:4b8bfb95-3f6e-4749-aca4-98a0424114ce@googlegroups.com... 
It came up recently, friend of mine has an application where it would be  
good. I told him it would take me a LONG time to design one from scratch.  
But anyway I thought of something. 

Many years ago in the TV days I was working on a plasma and it had a SEPIC  
running one of the screen voltage. They do pull current like a CRT. 

Thing is, it had a well regulated SMPS. Tye SEPICs are for going from buck  
to boost seamlessly, right ? But it would never have to boost. Before that  
SMPS comes out of regulation it would probably shut down. 

So why did they use a SEPIC ? Are they THAT good ? I can understand you need  
something to control those voltages but to have to boost it ? I know how  
plasma TVs work, so maybe they wanted control over this one voltage. But to  
have to be able to boost it ? Why ?

IIRC the late Vladimir Vassilevsky refereed to them as "septic converters"

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afair it was a different goofy configuration he came up with and called and an anti-septic converter

It'a a nice topology.

That one adjusts from 0 to +75 volts. Output is essentially constant-power limited, about 7 watts, which is ideal for my load.

anything to avoid this rube-goldberg-type kind of loop control circuit would be nice:

I see lots and lots of papers analyzing it but none of them seem to come out and say "this is just a mess" directly?

Mine works fine. That LTC chip is great. I have the Spice model if anyone is interested. The slope compensation thing, controlled by R8, really helps.

I would be happy if you shared the Spice model for the whole circuit. I assume that LTC provides the LTSpice model for the LTC chip...

Clifford Heath.

On Mon, 23 Sep 2019 11:31:24 +1000, Clifford Heath wrote:

This is about it:

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SYMATTR Type ind SYMATTR SpiceLine Ipk=20 Rser=0.32 Cpar=2pF SYMBOL nmos 624 112 R0 WINDOW 0 110 45 Left 2 WINDOW 3 81 83 Left 2 SYMATTR InstName Q1 SYMATTR Value Si9420DY SYMBOL res 656 272 R0 WINDOW 0 89 24 Left 2 WINDOW 3 78 58 Left 2 SYMATTR InstName R1 SYMATTR Value 120m SYMBOL res 128 -64 R0 WINDOW 0 73 21 Left 2 WINDOW 3 60 56 Left 2 SYMATTR InstName R2 SYMATTR Value 4.99K SYMBOL res 1248 112 M180 WINDOW 0 -71 69 Left 2 WINDOW 3 -99 32 Left 2 SYMATTR InstName R3 SYMATTR Value 80.6K SYMBOL cap 1424 -80 R0 WINDOW 0 -61 25 Left 2 WINDOW 3 -64 58 Left 2 SYMATTR InstName C1

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On a sunny day (Sun, 22 Sep 2019 13:18:01 -0700 (PDT)) it happened Lasse Langwadt Christensen wrote in :

So I got bored reading the news and looked up this group. No matter what anybody says about SEPIC, I build this long ago as my lab power supply:

12V in 0-24V out or something, powered by some LED strip supply wallwart On 24/7 and no problems. I use some current transformers in the design and basically most parts are from the junkbox and the ringcores from old PC supplies, Programmed a PIC micro to drive it and measure voltage and current, do some basic math. Nothing wrong with SEPIC. I have this paper: slyt309.pdf from TI: 'Designing DC/DC converters based on SEPIC topology' google finds it.

Oh, great, another unreadable drawing.

--
 Thanks, 
    - Win

Are you sure you aren't doing plain old flyback instead? SEPIC is common ground. (Well, there's an isolated version as well, but with the coupling caps, it's not very useful.)

Anyway, you might not need the _Z components (that's if Vout > TL431_Vmax), R_LED or Rbias, C1, or C3.

TSM101 and friends may be of interest. (These controllers are oddball and hard to search for, unfortunately; you may find them under opamps, power management, reference, or special...) You still need all the resistors, because, what, would you expect a chip that coincidentally has precisely what voltage ratio, output current and compensation time constant(s) you need? So yeah, that's fine. Or perhaps you don't need such tight regulation and a zener is fine.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/