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Why does flux walking in a magnetic core occur?

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mercurial:
I'm finding it a bit difficult to grasp the concept of flux walking in a magnetic core.

I'm not sure how flux walking occurs but I was thinking of the BH curve of a material and its shown in the image below.

What I was thinking is at start the Flux starts at point 0 at this point lets say we give a square wave voltage pulse to a coil wound around the core.
As the current in the coil (wound around the core) increases the flux moves towards saturation at point 1 now as long as the pulse is high the core would stay at this point correct?.
Now let's say the pulse voltage starts dropping to 0 at this point the flux also would continue to drop till point 2 and stay there when the current drops to 0 (due to hysteresis of the core).
I think from now on for subsequent voltage pulses the flux should move from 2 to 1 and back to 2 so I'm not sure how flux walking would occur since the operating point is moving from 2 to 1 and back to 2?

MarkT:
From what I read flux walking is just having unwanted DC component in the voltage across the inductor, nothing to do with the magnetics per se.

mercurial:

--- Quote from: MarkT on March 01, 2024, 08:42:22 pm ---From what I read flux walking is just having unwanted DC component in the voltage across the inductor, nothing to do with the magnetics per se.

--- End quote ---

Wouldn't the dc component manifest as a small offset on the bh curve, why would the dc cause the flux to saturate?

T3sl4co1l:

--- Quote from: MarkT on March 01, 2024, 08:42:22 pm ---From what I read flux walking is just having unwanted DC component in the voltage across the inductor, nothing to do with the magnetics per se.

--- End quote ---

This.  It's another peculiar phrasing that, while not out-and-out wrong, is highly misleading as to the effect and mechanism, yet which persists, being repeated time and again without critical consideration.  It is a meme, reproducing freely and stably in a system of idea-passing, without being policed for its quasi-technical payload, that should invite rational scrutiny, but instead is smuggled through potential censors time and time again.

The other big one in magnetism being "collapse of the magnetic field"; no, you're just switching the coil to a high-impedance state, EMF goes up (constrained by terminal capacitance or other impedance), and discharge is relatively rapid (higher V <--> higher dI/dt).  "Collapse" implies something entirely catastrophic, no doubt in part leading newbies to put oversized diodes in more places than is necessary, or indeed wise.  (I'm always amused to see a 1N4007, rated 30A peak and probably breaking down around 1600V, applied to a 5V 100 ohm relay coil; not that it hurts anything, and not that it costs much -- it's probably one of the cheapest diodes out there -- but the juxtaposition of a small-signal driver transistor like BC847, with a rectifier diode like 1N4007, is just that: amusing.  On the more serious side, I've seen newbies propose SMPS circuits with diodes shorting the primary; those precipitate a rather faster realization. :) )

--- Quote from: mercurial on March 01, 2024, 09:11:02 pm ---
--- Quote from: MarkT on March 01, 2024, 08:42:22 pm ---From what I read flux walking is just having unwanted DC component in the voltage across the inductor, nothing to do with the magnetics per se.

--- End quote ---

Wouldn't the dc component manifest as a small offset on the bh curve, why would the dc cause the flux to saturate?

--- End quote ---

It does.  But permeability is high, so it doesn't take much magnetization (A/m) to saturate.  A typical ferrite-core transformer say of 200W scale, saturates in a couple to tens of At.

When DC can be relieved, such as with a coupling capacitor, it's fine, and so you have many half-bridge applications that do this.  When it can't, as in a push-pull converter, the inverter's fixed DC offset (determined by comparative timing of the two switches) is applied directly to the low-resistance primary, and currents can be quite high, comparable to reflected load current for example.

This feels different enough to the... Idunno, the less technical or in-depth, the more casual or informal? mind, that it seems to deserve its own term.  Or perhaps the phrase originated with some author looking for a catchier title than "DC offset", and other authors found it similarly catchy enough to include, despite its shortcomings.  Whatever the case, we're stuck with it, it isn't going away from historical references any time soon, and the best I can do in a post-"flux walking" world, is to hopefully innoculate readers against it, by explaining it, stopping to perform that critical analysis that's so often left without.

Besides balanced timing (which you can only do so well before design variance is dominated by gate driver and MOSFET parameters), reducing duty cycle is a viable strategy: leaving the inverter open-circuit for a fraction of the cycle, effectively gives it a higher average drive impedance over the cycle.  Mechanically what's happening is, the rising or falling edges are faster or slower depending on current at that phase, therefore there is some current-to-flux squishiness.  This is further compromised in CCM, where load current clamps secondary voltage to zero during off-cycles, unless (secondary-referred) magnetization current exceeds load current (at which point you're probably running the transformer very close to or banging into saturation already?).  In DCM, the transformer is allowed to be open-circuit (free ringdown) for a fraction of the cycle and therefore flux is always reset (give or take how much ringdown occurs, but if it's not banging into the supply rails, it's going to be a tiny fraction of load current).

Two-switch forward, or one-switch with reset winding and catch diode, ensures reset by limiting duty cycle below 50%.  With equal switch-on and reset voltages, this is exactly BCM; in practice, switch and diode voltage drops will take in less flux during the on-time and (potentially) deliver more flux during off-time, so there is guaranteed margin, even if the duty isn't limited to exactly 50%, but say 52 or 55% or something, might still be perfectly fine.

Tim

mag_therm:
I don't recall the term "walking" in the old days, but it was a problem.
Adding blocking capacitors to MW inverters was costly and used a lot of enclosure volume.
We used lower quality cores, I recall specifying motor grade lams instead of GOSS in 50 Hz inverters
We used airgaps. I even recall addition of a delay pot after the D flop on one side to null of the offset!
It might be one reason the push pull inverters went mostly obsolete.
One way to avoid the problem is to use a half bridge with split capacitors.- Another is  asymmetrical core excursions as used these days.