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SEPIC article is not correct?

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grizedale

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Hello,

Do you realise that the following article about leakage inductance of SEPIC inductors is nonsense?


Power Tip 32: Beware of circulating currents in a SEPIC coupled-inductor –Part 1
By Robert Kollman , Texas Instruments

**broken link removed**


It is for sure that there is NO BENEFIT of a SEPIC inductor with higher leakage inductance, so why are they writing this article?

If anything the SEPIC inductor with higher leakage brings about more unwanted resonations between itself and the SEPIC capacitor.



The following articles make NO mention about the danger of well-coupled SEPIC inductors in SEPIC converters……….

Inductor selection for SEPIC designs
By Coiltronics

**broken link removed**


Bottom left of the first page of the following article states how GOOD coupling in the SEPIC inductor is actually BENEFICIAL……………..

Selecting coupled inductors for SEPIC applications
By Coilcraft.
http://www.polyscope.ch/dlCenter/ps/2011_4/04_11.01.pdf




The following is an LTspice simulation of SEPIC with 1:1 coupling of the inductor, and as expected, these so called “circulating currents” are NOWHERE to be seen.

SEPIC LTspice simulation........just put in .asc doc and run.....

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TEXT 160 -496 Left 2 !K L1 L2  1
 

First of all, the article is explicitly referring to converters operating in CCM. Your simulation is operating in DCM. Second, it also explicitly states that a large C_AC will mitigate the effect, and you do have a large C_AC.

I modified your file to force CCM, and lowered C_AC to 1uF. I also added slope compensation, and also changed the FET (since I didn't have the model for yours).

Now when you change K from 1 to 0.9, you see significant reduction in ripple current on both inductors, and on C_AC.

You should really read articles more thoroughly before you dismiss them like that.
 

Attachments

  • grizedale sepic modified.zip
    2.6 KB · Views: 167
Right i will go and run your sim.

I hope your findings dont mean that in CCM SEPIC, the sizing of the fet source sense resistor depends on the amount of coupling in the sepic inductor, because most sepic inductor datasheets dont even state the degree of coupling...eg...

Sepic inductor datasheet

https://www.bourns.com/data/global/pdfs/SRF1280.pdf
 
Expectable coupling of industry standard SEPIC inductors is an interesting question. The fact, that it's rarely specified in datasheet obviously won't mean, that it's 1.0. Some manufacturers are even offering low and high leakage inductance types without specifying numbers. That's really absurd.

The said manufacturer (WE-online.com) however suggest "high" (whatever it is) leakage inductance for SEPIC and "low" leakage inductance for flyback applications. The k=0.9 number assumed by mtwieg sounds like a good first guess. I tend to translate "low" leakaga inductance with k >= 0.95 and high with k at least < 0.9, may be considerably lower. If you want to know more exactly, you should measure inductors of your choice yourself. Technically according to the core shape of typical SEPIC inductors, very low low leakage inbductance isn't feasible.

I'm not presently working with SEPIC designs, thus I didn't yet evaluate the pros and cons of different coupling factors.
 
Yes, the coupling often isn't given, and can't be extracted from the information they give (though it could be measured fairly easily). With small inductance values (a few uH or less) I'd expect them to be pretty low; like 0.8 at most.
 

thanks mtwieg i ran your sim.

youre right there is less primary inductor ripple with the greater leakage.

Thats nice to know.

However, its too expensive to get sepic inductors custom wound with high leakage.

Therefore, one has to make do with the multitude of off-the-shelf sepic inductors which have a clearly stated good coupling of >0.96.

....there are plenty of these around so obscelescence wont be an issue.

So, sepic with high leakage is good......but not "that" much better than a sepic with low leakage.

So i would think that for a sepic , just pick one of the many sepic inductors with coupling >0.96 and be done with it.

Would you agree?
 

Depends completely on the application... If you're going to be operating in DCM, then a leaky inductor probably isn't worthwhile. Even when operating heavily in CCM, the leaky inductor won't make a huge difference in efficiency. But as the article states, it will significantly reduce input and output ripple, and reduce EMI.

As to what the effect on variation in coupling is, I don't know. I doubt it affects stability in any significant way, but only a detailed analysis would say for sure...
 

Where did you get the k > 0.96 number? I didn't see it any of the linked datasheets?

P.S.: I found, that Coilcraft gives coupling for nearly all coupled inductors. It's in fact mostly above 0.95, often 0.99. Apparently, they don't see a problem yet to suggest these parts for SEPIC. So your initial question is justified.
 
Last edited:

FvM:

K>0.96 is the general number that i saw from the same coilcraft datasheets that you have seen.

-As you rightly say, its easy to get good coupling with a sepic inductor, you just wind the coils bifilar.
 

I think most power electronics engineers who saw the white paper realized straight away that when you couple the inductors on a sepic, you get something that is not a pure sepic any more; rather it now has a large resonant element to its behavior. If you take the switching frequency up much higher then you operate well inside the resonant half cycle and the behavior returns to more like pure sepic. This can also be seen if you let the switching freq go very low - and you can clearly see the resonant ringing inside a switching cycle.
Increasing the leakage has the effect of pushing the resonance lower in freq which makes the switching time a small part of the resonant half cycle - thus back to more like pure sepic operation.
 
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