Electronics > Beginners
Why does flux walking in a magnetic core occur?
Andy Chee:
--- Quote from: mercurial on March 04, 2024, 11:13:52 am ---1. Does the flux accumulate over time primarily due to "hysteresis" property of the core or are there any other factors contributing to it?
--- End quote ---
The hysteresis property is not really a factor, but the permeability property is a major one.
--- Quote ---2. Could a simple drive circuit to a coil wound on the core cause ratcheting of flux.
--- End quote ---
Potentially yes.
--- Quote ---3. Does that mean that it is mandatory to always drive a transformer with a bipolar drive?
--- End quote ---
No.
Single-switch flyback, two-switch flyback, single-switch forward, and two-switch forward converters, are all unipolar. They are all limited to 50% duty cycle maximum, in order to allow the core to reset during the other 50%. Or if driven with low duty cycle 10%, the remaining 90% is plenty of time to allow the core to reset. If you go beyond 50% duty cycle, the core will not properly reset, resulting in eventual core saturation.
Bipolar drive permits high duty cycle beyond 50% (and commensurately higher power conversion), because the core is reset via polarity reversal. But unbalanced drive waveforms will eventually lead to core saturation as well, for example 90% in one direction and 91% in the other.
jonpaul:
Bonjour, bravo for the question.
As a power electronics designer since 70s, I have not hear this term "flux walk" , Iit is commonly called core saturation.
Any ferrous material has a non linear a B-H (flux vs exitation) relation. near linear at low flux and a flux limit at very high exitation.
Core losses due to heysterysis etc are also changng nonlinear with H.
For a perfect AC exitation, zero DC omponent, the cycle by cycle peak B is not changinbg.
In case a DC bias exists in exitation, eg now quite 50% DC in puch pull/bridge, the DC then causes a sucessive creeping cycle by cycle of the B in one dirsction.
As the B increases cycel by cleyc the magnetizing current increases as incermental L decreaes. See the B-H curves by core manufactureses, Arnold, Mag Mtls , Thomas for lams and tapewound iron, TDK, EPCOS, Siemens, etcv for ferrite.
Suggest you read a classic text on static electromagneti devices like Hunt and Stein, to lear about the reasons for ferrous material nonlinearities.
We never had these ussus: in 1970s various DC cancelling FB were used to even the V-S area plus/minus applies.
We avoid push pull, centertapped transformers.
If some DC is present use an airgap in the core.
Most modern topologies use a series cap to remove change of DB bias.
Bon chance,
Jon
MrAl:
--- Quote from: T3sl4co1l on March 04, 2024, 02:21:52 am ---As said, perfectly decomposable into "DC" (transient, exponential) and periodic (square/triangle) components.
That's a linear RL load, I assume--? The above can be proven analytically from the differential equation, without much effort (granted we're talking diff eq to begin with, lol).
I suppose, purely in the sense that, when dI/dt is positive or negative (read: ignoring by how much), and after each cycle of alternation, I is higher each time, it could be said to "walk" -- but it isn't much of a walk, if, after a while, it shuffles down to a crawl, right? Nor is it much of a "ratchet" (a mechanism with equal-spaced teeth to hold force/torque against). Pedantic perhaps -- but it shows the discrepancy between hand-waved descriptors and real behavior.
On the other hand, I suppose a real example might look more meaningful: with a saturating transformer, the per-cycle deltaI can be fairly constant (along just the initial slope of that exponential), until it hits saturation and inverter current explodes.
Put another way: the winding resistance will generally be quite low compared to magnetizing inductance, and saturation relatively close at hand (design Bpk might be 10-80% of Bsat, depending on Fsw, acceptable losses, etc.), so that not much imbalance needs to accumulate before saturation ensues (and saturation will be relatively hard and quick, because mu_eff will be fairly high in a power transformer).
Put still otherwise: a linear system might not be a good illustration, specifically because it can be decomposed as above; a nonlinear system however, while it admits the same decomposition before saturation, superposition is violated afterwards. (Within whatever approximation margin counts for "before" vs. "after".) The significance of this change is enough, and its occurrence in practice, to demand a signifier to represent it. Thus a meme is born; but as with many signifiers, the literal meaning of it is only loosely connected to its new meaning, and confusion ensues among those who haven't yet been initiated into that meaning. Which happens often, as the purpose of the signifier is to represent the thing without having to say what it is, and just explaining the thing would take longer by way of having to explain the phrase plus what it's talking about in the first place.
I guess the complaint comes down to this: if you object to phrases like "defund police" for facial / literal reasons ("defund... and then what?"), you should object to "flux walking" for the same reason ("walking...where? ...does it have legs now?").
(To be clear, the context of this post is mainly philosophical. I've already commented on the electronics, or physics, of this subject, and I'm not trying to reiterate that; nor has anything particularly contradictory been written since. What's new, I suppose, is I hadn't noted the above similarity explicitly before, which I guess is interesting enough to post? Perhaps others will find some insight in the philosophical or linguistic or thought-process given above.)
(I don't particularly care if you [the reader] use the term ["flux walking"], just realize it means something, not what is "written on the tin", and is really just a mimetic substitute for "DC imbalance", which is perfectly descriptive of what's happening, but I guess just too banal for the memes. Hence the former term continues to persist.)
Tim
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Hi there,
Well if you want to be perfectly 'real' then we have a ways to go yet.
First of all, the flux ratcheting does not have to be the same for each step just to call it ratcheting. But more exactly, the ratcheting tends to get worse and worse with bigger and bigger steps because the permeability starts to decrease with each step. That would make the more realistic picture look like a plot that is curving up rather than leveling out. The leveling out part would only come if the core did not get too saturated, and there was sufficient resistance to limit the amount it was able to go up.
This is kind of like everything else ... if we want to understand it we have to start with simpler explanations, then if the interest is still there we go with a more in-depth analysis.
I have some other plots around that show the path on the BH curve as the flux ratchets up. You can see the curve that encloses the origin get wider and wider and if there is an asymmetrical drive the whole thing works its way up with an offset too so it's not symmetrical about the origin anymore.
I'll see if i can find them.
MrAl:
--- Quote from: mercurial on March 04, 2024, 11:13:52 am ---Thanks to all for the amazing replies especially MrAl, ejeffrey, T3sl4co1l to name a few.
It was a highly enlightening experience reading thru all your essays, I really appreciate the time you'll put in to answer the question.
Some questions still bug me though.
1. Does the flux accumulate over time primarily due to "hysteresis" property of the core or are there any other factors contributing to it?
2. Could a simple drive circuit to a coil wound on the core cause ratcheting of flux.
3. Does that mean that it is mandatory to always drive a transformer with a bipolar drive?
--- End quote ---
Hi,
The hysteresis property means the core flux does not decrease even when the current goes to zero.
With an asymmetrical drive, the flux is higher in one direction than the other and that can cause the metal to be saturated in that one direction. As it gets higher and higher the permeability decreases and that decreases the inductance, and that causes more current to flow during each cycle. With more current each cycle the condition becomes worse. If the permeability did not change this would not happen as in an air core inductor.
If you could see the current during the time when it just starts to go into saturation it would be starting to go up sharply and then it starts to look like a spike. As the core becomes more and more saturated, it starts to look more and more like a short circuit which of course draws more current from the power source until something blows out.
Theory has advanced but it's still useful to think of the core as being made up of tiny domains where each domain acts like a tiny magnet. These magnets start out all randomly oriented. As current is applied, the magnets start to align with each other. This also creates a back EMF that counters the applied voltage, and that is the basis for the inductance. Since there is only so much bulk to any one core, there are only so many magnets. Once they become all aligned there are no more left to rotate and create a back EMF, so now the core behaves almost like there is nothing there.
We may still see a small inductance but because it becomes so much lower than the target design, the voltage pulses encounter an almost pure resistance that is very low and so we see a huge increase in current.
The plots I had shown previously show what happens when the core does not saturate completely as there is some external limit that holds the current down. In an actual core the plot would curve upward rather than level off and that's the more typical case because there is often very little to limit the current except for an active current limit mechanism. An active current limit mechanism will prevent the converter from blowing out.
[#3]
The main idea is that the core has to be reset. This means getting the flux back to zero. There are other ways to do this, but in something like a push pull circuit it is usually done with a symmetrical drive.
mercurial:
--- Quote from: jonpaul on March 04, 2024, 12:36:19 pm ---
We never had these ussus: in 1970s various DC cancelling FB were used to even the V-S area plus/minus applies.
We avoid push pull, centertapped transformers.
If some DC is present use an airgap in the core.
Most modern topologies use a series cap to remove change of DB bias.
--- End quote ---
1. Why center tapped transformers to be avoided? Don't center tapped also reverse the flux in the core?
2. What is wrong with push-pull why that needs to be avoided?
3. What is the V-S area.
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