Electronics > Beginners
Why does flux walking in a magnetic core occur?
MrAl:
--- Quote from: Andy Chee on March 04, 2024, 12:35:21 pm ---
--- 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?
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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.
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Potentially yes.
--- Quote ---3. Does that mean that it is mandatory to always drive a transformer with a bipolar drive?
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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.
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Hi,
I am not sure if you could say that bipolar drive permits duty cycles above 50 percent when the core is reset via polarity reversal. That's because in that case we would need a 50 percent one polarity and 50 percent the other polarity, or at least each half cycle would have to have the same pulse width.
It's hard to pin this down in simple terms though because even that isn't really the end of the story. For example, we could have one pulse that is 25 percent V+ and 75 percent V-, which is asymmetrical, but then the next pulse that is 75 percent V+ and 25 percent V-. The first and second pulses are asymmetrical, but together they balance out the core. This is as long as the initial inductance is high enough to accept that without saturating after the first pulse. This is the case with pure sine synthesized converters where there is a long train of varying pulse widths of the positive polarity followed by a long train of pulses of the negative polarity (or similar). After the second train of pulses the core is back to the original state.
mercurial:
--- Quote from: MrAl on March 04, 2024, 12:27:16 am ---Here are some plots that show how the core flux can increase over time with multiple pulses.
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What software was used to create those plots, is it a magnetic simulator of some sort?
MrAl:
--- Quote from: jonpaul on March 04, 2024, 12:36:19 pm ---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
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Hi,
Very surprised to hear you have never heard of the term "flux walk" maybe it is just a US thing.
All the engineers I have ever met that worked with power converters knew of this term, but that was in the US mostly.
As you probably know, the air gap allows some DC mainly because it lowers the permeability which then requires more wire turns. The net effect is it takes more offset to saturate the core.
It's also interesting that the air gap permeability is so low compared to the base core metal permeability that it can be very short and still affect the entire construction significantly.
MrAl:
--- Quote from: mercurial on March 04, 2024, 03:18:45 pm ---
--- Quote from: MrAl on March 04, 2024, 12:27:16 am ---Here are some plots that show how the core flux can increase over time with multiple pulses.
--- End quote ---
What software was used to create those plots, is it a magnetic simulator of some sort?
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Hi,
I created those plots with a circuit simulator like the LT Spice simulator which you can download for free.
I wanted to show the essence of what happens while staying away from the more complicated operation, but I have other plots that show a more complete picture which I will try to find now.
The second picture shows the BH curve when the flux is imbalanced and saturating on the positive part while not quite on the negative part.
mercurial:
Hi MrAl
For someone who worked in the power electronics industry for so many years it would be interesting to hear how did you really measure core saturation did you only use current as your eyes into the magnetics.
Did you use any special tools to measure the magnetic field while the transformer was operating?
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