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Can you split Primary and Secondary Leakages


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Apr 12, 2024
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The subject should be 'Can you really move around primary and secondary leakage inductance' but I don't know how to edit subjects..

Being new to the subject and this board, I've been reading through old posts and stumbled on this subject. In this thread that is now closed Easy Peasy and Mtwieg had a brief single post each debate that I think may have been dropped too soon and its an issue I've been struggling with.

I've watched most of Sam Ben Yaakovs videos on magneticis several times. This one Demystifying the meaning of leakage Inductance is of particular interest to me. Then More on transformers, leakage, turns ratio, and voltage and current transfer ratio takes another bite at the subject with more simulation.

I believe mtwieg is making the case that SBY is stating in his videos. I think I saw the subject the same way as Easy Peasy and thought I had a clear picture in my head that these rules only applied when current was flowing, but then tested open, we should be able to distinguish which side the leakage inductance is on. I was unhappy with with the limited simulations that SBY had done with a coincidence result at the 2:1 ratio points he picked so I planned to do some odd ratio configurations and send him my model to make the point.

Well after many hours going over the attached LT spice file that did with an odd 7:3 ratio of Primary to secondary self inductance, I was further confused by seeming to confirm that even if you measure the transformer with infinite impedence you can't really determine where the leakage inductance is. I can 'see' it the way easy peasy describes it, but I think the simulation states otherwise.

Because many people reading this may not want to go through two 20 minute videos on the subject I added a pdf document that illustrates the comparative test I did with the top four transformer circuits in the attached LT Spice model where I have four transformations:

All the leakeage on the primary side with a transformer coupling factor of 1.0
No leakages, but using a coupling factor of .845
The leakages balanced left and right with a transformer coupling factor of 1.0
All the leakages on the secondary side with a transformer coupling factor of 1.0

I'm interested if someone can come up with the equations that allow different sharing of the leakage inductance besides the even split.

Also how would you change the simulation settings so the simulation will solve with the transformer operating with a much lower resistance, closer to a shorted transformer

Right now this discussion is only about the top four circuits in the attached LTSpice model, the lower four are the same circuits for driving the transformer from the left side instead of the right side as in the upper four. In all my testing so far, all four of those circuits have the same voltage gain and current response to load, but as Prof Sam Ben-Yaakov was stating in his video, the voltage ratios were not inverses and effecting by the load.



  • More On Transformers Video - 4 legs 7-3
    2.2 KB · Views: 26
  • Transformer Transformation Model Notes.pdf
    121.8 KB · Views: 47
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Yes you can split...say if you measure 1uH leakage (at pri with sec short) in transformer which is np/ns = 2:1, then you say you have 500nH in pri, and 500nH * (ns/np)^2 in the sec.
You can check this by putting into spice, shorting the sec, and seeing what is resonant frequency of pri with a known cap. You will see it corresponds to the above.
In fact, i have been brief with the truth, because the magnetising inductance does in fact come into it too, so i am a bit out, but as a first approximation thats it.
@decook1110 you can "decide" to apportion leakage to pri only, or sec only, or 1/4 ( effective ) on each wire for a two wdg transformer

( note that, for example, 5uH pri total leakage - i.e. all referred to pri - is converted by the turns ratio squared ( Nsec^2 / Npri^2 ) to get the sec side total referred leakage and vice versa when going the other way )

What you may not fully appreciate is that you can never access - in a practical way - a smaller part than the total leakage, whether you are driving from the pri or sec or whatever,

even in a flyback - when the main mosfet turns off - the total leakage ( and wiring loops ) determines how fast the current picks up on the sec side ( and falls on the pri side )

similarly for a full bridge, when the diagonal fets turn on for a power pulse - the total Lleak determines the max rate of change of currents in the pri and sec wdgs.

There are some very rare corner cases where leakage is not the same looking from one side to the other - but these are never encountered in practical power electronics.

--- Updated ---

Leakage inductance is a function of the flux in the pri wdg that does not couple to the sec side and vice versa - this effect is fully symmetrical - i.e. the energy in the leakage is the same whichever side is driven ( for equivalent currents - allowing for turns ratio ).
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Other previous Edaboard threads related to the topic

There are different possible transformer models. If you choose a blackbox model, leakage can't be unequivocally assigned to primary and secondary side. Turns ratio can be partly exchanged with leakage inductance distribution.

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