Coupled inductor sepic design as for flyback design?

[Flyback]

Hello,

The following article discusses sepic converter design when a coupled inductor is used.

https://www.electro-tech-online.com/custompdfs/2013/09/slyt411.pdf

Why does this article singularly fail to declare that the design of a sepic with coupled inductor should be based on the design of a flyback with equal primary and secondary inductances as occurring in the coupled sepic inductor?

In a coupled inductor sepic, the FET and diode currents are exactly the same as for a flyback.

It is the primary and secondary currents which usually differ between sepic and flyback (and obviously the flyback doesn’t feature the sepic capacitor current, as it has no sepic capacitor)

In fact, in a coupled inductor sepic, the primary and secondary current waveforms are virtually the same as in a flyback if the sepic capacitor is low value and the windings are closely coupled.
However, it is not usual to use low value sepic capacitors in closely coupled sepic inductors, as the low value sepic capacitor rings excessively with the leakage inductance.

In the coupled inductor sepic, when the sepic capacitor value is made higher value, then the primary and secondary currents do then differ from those in a flyback….they get lesser in RMS value than the flyback. –Also, the peak primary and secondary current desreases.-However, the FET and diode currents remain the same as in a flyback.

(obviously the flyback needs a prmary RCD clamp which the sepic does not….so the drain turn off voltage rises up to the clamp level in the flyback, so this is different to that in a sepic…however, the design is best completed with the analogy to the flyback borne in mind…..why does the above article fail to mention this?

 

[ericgibbs]

Why does this article singularly fail to declare that the design of a sepic with coupled inductor should be based on the design of a flyback with equal primary and secondary inductances as occurring in the coupled sepic inductor?

Would it be more proper to contact the John Betten the Author of the article and ask him why.?

Have you tried.?

 

[Flyback]

I don't know Mr Betten's email, or even if he still works at ti.com.
I believe if one of the worlds biggest semiconductor companies writes an public article for use by its customers to produce commercial products for sale, then that article has been publicly declared, and should be open to public scrutiny if such is required. At the end of the day, my post only enhances and augments what Mr Betten has said......making it more likely that people will buy the ti.com products referred to in that article.
I don't think this should be about personalities, this should be about the common good, the common goal of engineering companies to provide effective solutions to customers.

 

[jpanhalt]

Why does this article singularly fail to declare that the design of a sepic with coupled inductor should be based on the design of a flyback with equal primary and secondary inductances as occurring in the coupled sepic inductor?

The article seems to be about comparing coupled-inductor to non-coupled-inductor sepic designs:

Source = Title
Benefits of a coupled-inductor
SEPIC converter

I assume the author felt that comparison to flyback topology was: 1) known; and 2) not related to the subject he wanted to explore.

John

 

[Flyback]

The article fails to state that in the coupled inductor sepic, the actual value of the sepic capacitor current depends on the Faradic value of the sepic capacitor, and this is a crucial point in coupled inductor sepic design. Similarly, the rms primary and secondary currents and peak currents in a coupled inductor sepic again depend on the Faradic value of the sepic capacitor, and this again is a crucial point that was missed. As the value of the sepic capacitor changes, so too does the rms primary and secondary currents...........because of this variable nature of the coupled inductor sepic, it is best to approach it from the start point of a flyback design, and then later on factor in the sepic capacitor value and its effects.
In any case, whatever the sepic capacitor value in a coupled inductor sepic, the fet and diode currents are exactly the same as in the flyback. The article states that with coupled inductor sepics, one often introduces extra leakage inductance in order to reduce the resonant frequency of the sepic capacitor and the leakage inductance. Yes ok , this is true, but what one is actually doing is making the switching frequency go above the ringing frequency of the sepic cap and the leakage inductance. This isn't only achieved by introducing extra leakage inductance, but can be achieved by increasing the sepic capacitor's capacitance, or increasing the switching frequency, or a combination of these things.

 

[jpanhalt]

The document you linked to elsewhere (https://www.electro-tech-online.com/custompdfs/2013/09/snva168e.pdf) by Dongbing Zhang presents similar information as John Betten's and likewise does not mention flyback topology. Notably, that document has a more recent revision date (April, 2013) and has a hot link for feedback.

Maybe the most productive approach would be to make your concerns known directly to TI via that link. If the documents are in error, I am sure TI will want to correct that. If instead, the documents simply omit discussion of flyback topology, but are otherwise correct, I suspect there will be no changes. In any event, that approach may be more productive than discussing them in the third person without involving either author.

John

 

[MrAl]

Hi,

When some of these papers are written the object is to get a certain amount if information across and that's it. If they appear too brief for your taste that's really the way it goes. It doesnt matter how crucial a given point is, if they author didnt feel like writing about it they didnt feel like writing about it and there could be a ton of reasons for this.

Many app notes are like this too, especially from the past. One little circuit with very very little design info as they assume the reader knows how to go over the circuit and figure out the design criteria.

But if you do feel that strongly that something more should have been included, then yes you really have to get back to the original author.

If i was going to gripe about articles written and posted on the web i'd be here all day. I still do gripe from time to time though. But the worst part is not that they leave out related information, it's that they leave out details of the ideas they are trying to get across, details that take hours to find elsewhere sometimes even when they are simple concepts that involve maybe one or two small formulas. That gets really frustrating because then you are down to reinventing the wheel yourself because it's often not even found on the web.

And second to that is accuracy. Sometimes the formulas are not even correct so again you have to reinvent the wheel...go over every aspect of the theory to find the equation you need. Takes hours sometimes!

So be happy if you find information that is at least all there and accurate. And be happy they didnt just throw a couple circuits at you with little captions that hardly provide any information at all.

 

[Flyback]

By the way, there is another serious fault seen in the article mentioned above (and repeated below)

On the top left of the 4th page of this article (below and in first post) the reader is recommended to add a heavy RC snubber in order to mitigate the low-ish frequency ringing seen in the uncoupled inductor sepic.............this is bad advice..do you agree?

The only reason you would suffer such a bad ringing (in an uncoupled inductor sepic) is if your sepic converter's feedback loop crossover frequency was too near the resonant frequency of the sepic capacitor and the two uncoupled sepic inductors. You should not use a snubber, but instead should arrange it that the feedback loop crossover frequency is not near the aforementioned resonant frequency.

https://www.ti.com/lit/an/slyt411/slyt411.pdf